THE EFFECT OF COMPOSITION, ANNEALING TEMPERATURE AND
THICKNESS ON MAGNETORESISTANCE OF FERROMAGNETIC
MULTILAYER STRUCTURES
NAZIMAH BINTI KHAMIS @ SUBARI
A thesis submitted in fulfilment
of the requirements for the award of the degree of
Master of Science (Physics)
Faculty of Science
Universiti Teknologi Malaysia
FEBRUARY 2010
iii
Specially dedicated to my beloved husband, Norazam Ahmad, and to my daughters,
Nuraishah Maisarah and Nurul Hidayah.
iv
ACKNOWLEDGEMENTS
‫ اﻠﺤﻤﺪﻠﻠﮫﺮﺐاﻟﻌﻠﻣﯿﻦ‬. Kupanjatkan syukur kepada-Mu Ya Allah, atas kurniaan
kekuatan dan kesihatan untuk menyelesaikan kajian ini.
I would like to express a special appreciation to my supervisor, Professor
Yussof Wahab, for his beneficial guidance, assistance and advice throughout the
completion of this study.
And thousand thanks to my co-supervisor, Professor
Samsudi Sakrani, for his fruitful suggestion and opinion during this M.Sc journey.
To fellow postgraduates students at Physics Department especially those in
IRPA SET group, your presence always inspires me to give the best I could to finish
this project. Thank you very much to Encik Nazari Kamarudin and Puan Fazilah
Lasim for their unforgettable support in the experimental work at Vacuum Lab,
Physics Department. Not forgetting Puan Wani and Encik Faizal from Ibnu Sina
Institute, thank you for the kind assistance in finishing this research. Thank you also
to Encik Azmi Ibrahim from Telekom R&D for the XRD analysis and Encik
Hishamudin Abdul Rahim for AFM analysis.
A very deep gratitude to Ministry of Science, Technology and Innovation
(MOSTI), for providing financial assistance through research funding IRPA 09-0206-0057-SR005/09-06, and RMC, UTM for short term research funding number
75120.
To my husband, thanks for the never ending love and support, and finally to
my daughters, I left this path to be followed someday, Amin.
v
ABSTRACT
Giant Magnetoresistance (GMR) refers to a phenomenon of a considerable
drop in electrical resistance in ultrathin ferromagnetic/non-magnetic layers structure,
when a sufficiently high magnetic field is applied to the structure. Since the first
time it was observed in late 1980’s, this area of research has attracted a very strong
interest due to the deep fundamental physics and its promising technological
potential especially in data storage industry. This dissertation discusses the
magnetoresistance of ferromagnetic/non-magnetic multilayers fabricated using
electron beam evaporation technique. The magnetoresistance value is studied
towards the effect of compositional variance, a range of post deposited annealing
temperature, and multilayer thickness in terms of number of repetition of the bilayer
as well as the non-magnetic layer thicknesses. The results on compositional variance
show as deposited Co/Cu/Ni gives the largest magnetoresistance value of 8.75%
followed by Ni/Cu/Ni (5.85%), Co/Ag/Co (2.64%) and Ni/Ag/Ni (1.48%). These
four ferromagnetic multilayers were given heat treatment with a range of elevating
temperature. The temperature ranges between 200°C to 350°C, resulting the
magnetoresistance of the above structures increases to 14.25%, 10.68%, 7.45% and
4.38% respectively. However, the thermal stability for each multilayer systems were
degraded after further annealing at a temperature called the blocking temperature.
From the investigation, it was found that the blocking temperatures (or the optimum
annealing temperature) for Co/Cu/Ni, Ni/Cu/Ni, Co/Ag/Co and Ni/Ag/Ni were
280°C, 290°C, 310°C and 280°C respectively. The study of thickness dependence
GMR, focus only on Co/Cu/Co structures. It was shown that in [Co/Cu/Co]n
structures, the magnetoresistance increase as n increases from 1 to 20. Meanwhile,
for Cox/Cuy/Cox structures, the magnetoresistance increases as y decreases from 15
nm to 1 nm with x is fixed at 6 nm. The X-ray diffraction study for the optimum
structures shows peaks of Co, Cu, Ni and Ag which describe the multilayers are in
the crystalline form. The atomic force microscopy (AFM) image analysis for Co and
Cu thin films with elevated annealing temperature reveals that the higher the
annealing temperature, the larger the grain size of the multilayer. The EDX spectrum
analysis confirmed the presence of Co, Cu, Ni, and Ag in the multilayers.
vi
ABSTRAK
Kemagnetorintangan raksaksa (GMR) merujuk kepada satu penyusutan
rintangan ketara dalam lapisan ferromagnet/bukan magnet ultra nipis apabila medan
magnet yang cukup kuat dikenakan terhadap struktur tersebut. Semenjak pertama
kali ditemui pada tahun 1988, bidang penyelidikan ini telah menarik minat yang kuat
di kalangan penyelidik disebabkan oleh asas fiziknya yang mendalam dan potensi
teknologinya terutama terhadap industri penyimpanan data.
Tesis ini
membincangkan magnetorintangan dari lapisan ferromagnet/bukan magnet yang di
fabrikasi menggunakan kaedah penyejatan alur elektron. Nilai magnetorintangan
dikaji terhadap tiga parameter utama iaitu kesan terhadap komposisi berlainan, kesan
terhadap suhu sepuh-lindap selepas pendepositan, dan kesan ketebalan multilapisan
tersebut dalam bentuk bilangan ulangan dwilapisan dan ketebalan lapisan bukan
magnet.
Keputusan dalam kajian variasi komposisi menunjukkan selepas
pendepositan multilapisan Co/Cu/Ni memberikan nilai magnetorintangan tertinggi
iaitu 8.75%, diikuti oleh Ni/Cu/Ni (5.85%), Co/Ag/Co (2.64%) dan Ni/Ag/Ni
(1.48%). Keempat-empat struktur multilapisan ini kemudian diberikan rawatan
suhu. Julat suhu adalah dari 250°C hingga 350°C. Hasilnya menunjukkan
magnetorintangan struktur di atas masing-masing meningkat kepada 14.25%,
10.68%, 7.45% dan 4.38%. Walau bagaimanapun, kestabilan terma untuk setiap
struktur multilapisan menurun pada suhu sepuh lindap yang lebih tinggi. Suhu ini
dipanggil suhu sekatan. Daripada kajian, didapati suhu sekatan (ataupun suhu sepuh
lindap optimum) untuk struktur Co/Cu/Ni, Ni/Cu/Ni, Co/Ag/Co dan Ni/Ag/Ni
masing-masing adalah 280°C, 290°C, 310°C dan 280°C. Kajian tentang kesan
ketebalan terhadap GMR difokuskan kepada struktur Co/Cu/Co sahaja. Pada
struktur [Co/Cu/Co]n, didapati magnetorintangan bertambah dengan meningkatya
nilai n dari 1 hingga 20. Bagi struktur CoxCuyCox pula, magnetorintangan bertambah
apabila y menyusut dari 15 nm kepada 1 nm. Kajian belauan sinar-X terhadap
terhadap struktur multilapisan optimum menunjukkan kehadiran puncak-puncak Co,
Cu, Ni dan Ag, yang mana menunjukkan multilapisan berstruktur hablur. Analisis
imej mikroskop imbasan daya atom terhadap filem nipis Co dan Cu bagi suhu yang
meningkat menunjukkan pada suhu sepuh lindap yang lebih tinggi, saiz butiran
multilapisan adalah lebih besar. Analisis spektrum serakan tenaga sinar-X
mengesahkan kehadiran unsur-unsur Co, Cu, Ni dan Ag dalam multilapisan yang
dikaji.
vii
TABLE OF CONTENTS
CHAPTER
TITLE
DECLARATION
ii
DEDICATION
iii
ACKNOLEDGEMENTS
iv
ABSTRACT
v
ABSTRAK
vi
TABLE OF CONTENTS
vii
LIST OF TABLES
x
LIST OF FIGURES
xi
LIST OF ABBREVIATIONS
xiv
LIST OF SYMBOLS
xv
LIST OF APPENDICES
1
2
PAGE
xviii
INTRODUCTION
1
1.1 Background of Research
1
1.2 Introduction to Giant Magnetoresistance
2
1.3 Research Objectives
5
1.4 Research Scopes
5
1.5 Research Problem Statement
6
1.6 Thesis Outline
8
THEORY AND LITERATURE REVIEW
9
2.1 Introduction to Ferromagnetism
9
2.1.1
Physical Origin of Ferromagnetism
10
2.1.2
Domain and Domain Walls
12
viii
2.1.3
Hysteresis
13
2.1.4
Curie Temperature and Annealing Temperature
13
2.2 Introduction to GMR Theory
2.2.1
Mott’s Two Current Model
17
2.2.2
Resistor Model
19
2.2.3
Spin-dependent Scattering
22
2.2.4
Spin-dependent Conduction in Ferromagnet
25
2.3 Band Structure
3
15
26
2.3.1
Spin-orbit Coupling
27
2.3.2
Exchange-split d Bands
28
2.3.3
The Conductivity of 3d Metals
31
2.3.4
The Present of Interfaces
32
2.3.5
Band Matching in Magnetic Multilayer
32
2.3.6
CASTEP
33
2.3.7
Density Functional Theory
34
2.4 Fabrication of Ferromagnetic Multilayer Structures
34
2.5 Development of GMR Research
35
2.6 Spintronics : GMR Future Technology Application
37
RESEARCH METHODOLOGY
39
3.1 Sample Fabrication
39
3.1.1
Substrate and Source Material
39
3.1.2
Substrate Pre-clean and Preparation
40
3.1.3
Electron Beam Evaporation Technique and
Principles
40
3.1.4
Annealing Process
42
3.1.5
Sample Features and Structures
44
3.2 Thickness Control and Measurement
46
3.2.1 Film Thickness Monitor (FTM)
46
3.2.2 Surface Profiler
48
3.3 Magnetoresistance Measurement
3.3.1
Four Point Probe Sample
48
50
ix
3.3.2
Magnetoresistance Measurement Set-up
3.4 Structural Analysis of Ferromagnetic Thin Films
4
53
3.4.1
X-Ray Diffractometer Analysis
53
3.4.2
Energy Dispersive X-Ray Analysis
54
3.4.3
Atomic Force Microscope Analysis
55
3.5 Band Structure Analysis
56
RESULTS AND DISCUSSION
57
4.1 Introduction
57
4.2 Thickness of Films
58
4.3 Compositional Dependence Magnetoresistance
59
4.3.1
Band Structure Factor
62
4.3.2
Lattice Matching Factor
67
4.4 Annealing Temperature Dependence Magnetoresistance
69
4.5 Thickness Dependence Magnetoresistance
77
4.5.1
Non-magnetic Layer Thickness Dependence
78
4.5.2
Number of Multilayer Dependence
81
4.6 Spectroscopy and Surface Morphology Analysis
4.6.1
84
X-Ray Diffractometer/ Energy Dispersive X-Ray
Analysis
4.7 Surface Morphology Analysis
5
52
85
93
SUMMARY AND CONCLUSIONS
100
5.1 Summary and Conclusion
100
5.2 Suggestions
103
REFERENCES
106
APPENDIX
117
x
LIST OF TABLES
TABLE NO
TITLE
PAGE
2.1
Curie temperature of ferromagnetic elements.
14
3.1
List of annealed samples.
43
3.2
List of compositional varying samples (C-series) and
non-magnetic thickness varying samples (N-seies).
44
3.3
List of number of multilayer varying samples (R).
45
4.1
Difference in thickness measurement observed by Film
Thickness Monitor and Surface Profiler.
58
4.2
Lattice parameter for ferromagnetic metals (Co, Ni, Fe,
Cr) and non-magnetic metal (Cu,Ag).
67
4.3
The average lattice mismatch and the GMR for
Co/Cu/Ni, Ni/Cu/Ni, Co/Ag/Co and Ni/Ag/Ni
multilayer.
69
4.4
Sample description for XRD scans and followed by
EDX scans.
85
xi
LIST OF FIGURES
FIGURE NO
TITLE
PAGE
1.1
Schematic representation of the GMR effect, after
Tsymbal et.al, 2001.
2.1
Schematic illustration of electron transport in a
multilayer for parallel (a) and antiparallel (b)
magnetization of the successive ferromagnetic layers.
24
The electronic band structure (left panels) and the
density of states (right panels) of Cu (a) and fcc Co for
the majority-spin (b) and the minority-spin (c) electrons.
30
2.2
Beam
Evaporation
4
3.1
Edwards Electron
Photograph.
System
42
3.2
The FTM Display connected to quartz crystal inside the
e-beam chamber
47
3.3
Four point probe measurement of sheet resistance.
49
3.4
Four point sample for magnetoresistance measurement.
51
3.5
Magnetoresistance measurement set-up. Dotted arrows,
indicating the magnetic field direction, whether from A
to B or from B to A.
52
4.1(a)
Magnetoresistance response towards external magnetic
field for Co(6.5)/Cu(4.0)/Ni(5.5) multilayers.
59
4.1(b)
Magnetoresistance response towards external magnetic
field for Ni(7.5)/Cu(4.0)/Ni(7..5) multilayers.
60
4.1 (c)
Magnetoresistance response towards external magnetic
field for Co(6.0)/Ag(5.0)/Co (6.0) multilayers.
60
4.1 (d)
Magnetoresistance response towards external magnetic
field for Ni(7.0)/Ag(4.0)/Ni(7.0) multilayers.
61
4.2
Electronic band structures (left panels) and density of
states (right panels) of a) Co and b) Cu.
63
4.3
Electronic band structures (left panels) and density of
64
xii
states (right panels) of a) Cu and b) Ni.
4.4
Electronic band structures (left panels) and density of
states (right panels) of a) Co and b) Ag.
65
4.5
Electronic band structure (left panels) and density of
states (right panels) of a) Ni and b)Ag.
66
4.6 (a)
Magnetoresistance curve for Co(6.5nm)/Cu(4.0nm)/Ni(5.5nm),
annealed at different temperatures.
70
4.6 (b)
Magnetoresistance curve for Ni(7.5nm)/Cu(4.0nm)/Ni(7.5nm),
annealed at different temperatures.
71
4.6 (c)
Magnetoresistance curve for Co(6.0nm)/Ag(5.0nm)/Co (6.0nm),
annealed at different temperatures.
72
4.6 (d)
Magnetoresistance curve for Ni(7.0nm)/Ag(4.0nm)/Ni(7.0nm),
annealed at different temperatures.
73
4.7
Annealing temperature versus magnetoresistance ratio
for Co(6.5nm)/Cu(4.0nm)/Ni(5.5nm), Ni(7.5nm)/Cu(4.0nm)/Ni(7.5nm),
Co(6.0nm)/Ag(5.0nm)/Co (6.0nm) and
Ni(7.0nm)/Ag(4.0nm)/Ni(7.0nm).
77
4.8
The magnetoresistance effect on Cu thicknesses for
Co/Cu/Co multilayers
79
4.9
Cu thicknesses versus magnetoresistance of Co/Cu/Co
multilayers in experiment fitted with the expected result
based on equation 4.1.
81
4.10
Number of multilayers repetition versus
magnetoresistance for Co/Cu/Co structures.
82
4.11
Two theta XRD patterns for [Co/Cu]n , with n = 2,5,16
and 20. All samples were annealed at 270°C for two
hours.
83
4.12
(a) The two-theta XRD pattern , (b) The EDX spectrum
for Co(6.5)/Cu(4.0)/Ni(5.5).
86
4.13
(a) The two-theta XRD pattern, (b) The EDX spectrum
for Ni(7.5)/Cu(4.0)/Ni(7.5)
88
4.14
(a) The two-theta XRD pattern, (b) The EDX spectrum
for Co(7.0)/Ag(4.0)/Co (7.0)
90
4.15
(a) The Two-theta XRD pattern, (b) The EDX spectrum
for Ni(6.0)/Ag(5.0)/Ni (6.0)
91
4.16
AFM topography images of as deposited Co film (a),
and Co films annealed at 200°C (b), 250°C (c), 280°C
(d) and 350°C (e).
95
4.17
RMS roughness and average grain diameter of Co thin
96
xiii
films as a function of annealing temperature.
4.18
AFM topography images of as deposited Cu film (a),
and Cu films annealed at 200°C (b), 250°C (c), 280°C
(d) and 350°C (e).
98
4.19
RMS roughness and average grain diameter of Cu thin
films as a function of annealing temperature.
99
xiv
LIST OF ABBREVIATIONS
OMR
-
Ordinary magnetoresistance
AMR
-
Anisotropic magnetoresistance
GMR
-
Giant magnetoresistance
CMR
-
Colossal magnetoresistance
TMR
-
Tunnelling magnetoresistance
AFM
-
Atomic force microscope
FESEM
-
Field emission scanning electron microscope
XRD
-
X-Ray diffractometer
EDX
-
Energy dispersive X-Ray
HDD
-
Hard disk drive
CIP
-
Current-in-plane
MBE
-
Molecular beam epitaxy
DC
-
Direct current
RF
-
Radio Frequency
MRAM
-
Magnetic random access memory
MTJ
-
Magnetic tunnel junction
FTM
-
Film thickness monitor
DFT
-
Density functional theory
FM
-
Ferromagnetic
NM
-
Non-magnetic
RMS
-
Root mean square
xv
LIST OF SYMBOLS
Ag
-
Argentum
B
-
Magnetic flux density
C
-
Quartz constant
Co
-
Cobalt
Cu
-
Copper
d NM
-
Non-magnetic layer thickness
d FM
-
Ferromagnetic layer thickness
d
-
Thickness, film
d
-
Plane spacing in atomic lattice
E
-
Electric field
e
-
Charge of electron
F
-
Correction factor
f
-
Frequency
∆f
-
Frequency change
H
-
External magnetic field
Hs
-
Saturation magnetic field
I
-
Current
i ,
-
Intrinsic resistivities for spin channels
Jn
-
Exchange constant
K
-
Breadth constant
kF
-
Fermi’s momentum
xvi
L
-
Crystallite length
lNM
-
Mean free path of conduction electron
MR
-
Magnetoresistance
MR%
-
Magnetoresistance ratio
m
-
Mass of electron
Ni
-
Nickel
n
-
Avogadro number
n
-
Integer, determine by order
n (Ef)
-
Density of states
N
-
R
-
Number of four layer unit cells
withimultilayermultilayer multilayer
Resistance
R
-
Resistance, at certain magnetic field
R AP
-
Resistance for anti-parallel alignment
RP
-
Resistance for parallel alignment
Rmin
-
Saturation magnetoresistance
R ,
-
Resistance for two spin channel
Rs
-
Semiconductor sheet resistance
Si
-
Silicon
Si3 N 4
-
Silicon nitride
T
-
Temperature
Tc
-
Curie temperature
V
-
Voltage
Zi
-
Height value of each point
Zave
-
Average surface height

-
Spin asymmetry parameter
β
-
Breadth of peak phase

-
Independent conductivity

-
Conductivities for up-spin

-
Conductivities for down-spin
 Drude
-
Drude’s conductivity
xvii
ρ↑
-
Spin-up
ρ↓
-
Spin-down
-
Resistivity of non-magnetic layer
ρq
-
Quartz density
ρf
-
Thin film density
F
-
Fermi’s velocity

-
Velocity in spin channel

-
X-ray wavelength

-
Mean free path

-
Pai

-
Relaxation time of electrons
-
Planck constsnt

-
Micro

-
Angle
δRMS
-
Root mean square roughness
2θ
-
Diffraction angle (two-theta)
Δ
-
Delta
θ
-
Angle
<
-
Less than
 NM
ħ
xviii
LIST OF APPENDICES
APPENDIX
A
TITLE
Photographs of Characterization Instruments
PAGE
117
CHAPTER 1
INTRODUCTION
1.1
Background of Research
The phenomenon of magnetism, that is the mutual attraction of two pieces iron
or iron ore had been known since 4000 years ago. The ancient Greeks have been
reported to experiment with this mysterious force but the application were limited at that
time, until scientific research was made by William Gilbert (1504-1603) 400 years ago.
He wrote the book ‘De Magnet’ which explained his research findings on magnetic
properties. Before 19th century, the only important application of magnet is Compass.
Hans Christian Oersted (1775-1851) observed that magnetic field interact with
electric current in 1820. Inspired with this, he creates the first electromagnet in 1825
based on the principle that electric current produce magnetic field. This was a scientific
breakthrough in magnetism and since that, researches towards magnetic material were
rapidly grown and its applications were widely increased.
2
Magnetoresistance is the properties of some materials to lose or gain electrical
resistance when an external magnetic field is applied to them. The effect was first
discovered by William Thomson in 1857, but he was unable to lower the electrical
resistance by more than 5%. This effect was later called ordinary magnetoresistance
(OMR). Ordinary magnetoresistance arises from the effect of Lorentz force on the
electron trajectories due to the applied magnetic field.
It does not saturate at the
saturation magnetic field and usually small in metals which is less than 1% in fields of 1
Tesla (Tsymbal and Pettifor, 2001).
More recent researches discovered materials
showing anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR),
colossal magnetoresistance (CMR) and tunneling magnetoresistance (TMR).
The discovery of giant magnetoresistance phenomena in 1988 was a
breakthrough in magnetic material and thin film magnetism. Since then, researches in
thin film magnetism are looking forward to obtain larger magnetoresistance ratio
because in principal, larger magnetoresistance ratio ascribe bigger data storage volume.
1.2
Introduction to Giant Magnetoresistance
Giant Magnetoresistance or GMR is a quantum mechanical effect observed in
thin film structures composed of alternating ferromagnetic and non-magnetic layers.
The effect is a significant decrease in electrical resistance in the presence of magnetic
field. In the absence of an applied magnetic field, the direction of magnetization of
adjacent ferromagnetic layers is antiparallel due to a weak anti-ferromagnetic coupling
between layers, and it decreases to a lower level of resistance when the magnetization of
3
the adjacent layers align due to an applied external field. The spins of the electrons of
the non-magnetic metal align parallel or antiparallel with an applied magnetic field in
equal numbers, and therefore suffer less magnetic scattering when the magnetizations of
the ferromagnetic layers are parallel.
Figure 1.1 shows the schematic representation of GMR effect.
The upper
diagram (a), shows change in the resistance of the magnetic multilayer as a function of
applied magnetic field. The middle diagram (b), is the magnetization configurations
(indicated by the arrows) of the multilayer at various magnetic field. The diagram
shows that the magnetizations are aligned antiparallel at zero field whereas the
magnetizations are aligned parallel when the external field H is larger than the saturation
field Hs.
multilayer.
The lower diagram (c), shows the magnetization curve of the magnetic
4
Figure 1.1: Schematic representation of the GMR effect, after Tsymbal and Pettifor,
2001.
5
1.3
Research Objectives
This dissertation was carried out to reach the following objectives:
i.
To fabricate and design the GMR structures by electron beam
evaporation method.
ii.
To study the effect of fabrication parameters and post-annealing
parameters towards the GMR values.
iii.
To investigate the dependencies of composition, annealing temperature
and thickness on the GMR value.
1.4
Research Scopes
This dissertation emphasizes on deposition and characterization of different
ferromagnetic multilayer structures.
A ferromagnetic multilayer structure basically
consists of ferromagnetic layer separated by a thin conducting non-magnetic layer.
Ferromagnetic materials used in this study are Cobalt (Co) and Nickel (Ni).
conducting materials used are Copper (Cu) and Argentum (Ag).
The
The multilayer
structures studied are namely Co/Cu/Ni, Ni/Ag/Ni, Co/Ag/Co and Ni/Cu/Ni. These
ferromagnetic multilayer structures were deposited using electron beam method, which
were then followed by annealing.
The Magnetoresistance Ratio (MR%) for each
structures is measured, focusing on effect of different fabrication and post-annealing
parameters. The following parameters are studied: Composition difference of
6
ferromagnetic materials and conducting materials; annealing temperature ranged from
200°C to 350°C; conducting layer thickness vary from 1 to 15 nm, and the number of
the repetition of the bilayer from 1 to 20 bilayer. Structural characterizations using
methods of Atomic Force Microscopy, Field Emission Scanning Electron Microscopy
and X-Ray Diffractometry are also carried out as supportive evidences of the result
obtained.
Atomic Force Microscope (AFM) was used to probe the surface morphology of
the magnetic and non-magnetic thin film layer.
Field Emission Scanning Electron
Microscope (FESEM) was used to see the surface structure of the magnetic and nonmagnetic thin film layer. X-Ray Diffractometer (XRD) was used to provide information
about the crystallographic structure of the magnetic and non-magnetic layer.
1.5
Research Problem Statement
In the early 1980s, thin film magnetism was applied to higher-density to
nonvolatile random access memory by Honeywell in their development of anisotropic
magnetoresistance (AMR) memory. Later, a new path leading to the integration of
magnetic devices into computer technology began to emerge with the discovery of giant
magnetoresistance (GMR). GMR heads are more sensitive compared to the previous
anisotropic magnetoresistive (AMR) heads.
7
Different from GMR heads, an AMR heads employs a special conductive
material that changes its resistance in the presence of magnetic field. As the head passed
over the surface of the disk, this material changes its resistance as the magnetic fields
changes corresponding to the stored patterns on disk. A sensor is used to detect these
changes in resistance, which allows the bits on platter to be read. While the older AMR
heads typically exhibit about 2% of resistance change when passing from one magnetic
polarity to another, for GMR heads the value is between 5% to 8%. This means GMR
heads can detect much weaker and smaller signals, which is the key to increasing areal
density.
Magnetic disk drive products have had their areal density increased by factor of
35 million since the introduction of the first disk drive, in 1957. Since 1991, the rate of
increase has accelerated to 60% per year, and since 1997 it has accelerated further to an
incredible 100% per year (Wolf et. al, 2006). The acceleration was the result of the
introduction of AMR read heads in 1991 and GMR read heads in 1997. Today, nearly
all disk drives in the industry incorporated the GMR read-head design, which is faster in
speed and smaller in size.
The achievements of switching from AMR read head
technology to GMR read head technology would not be possible without a detailed
understanding of the physics of GMR.
To further understand GMR, a deeper insight into ferromagnetic multilayer
structures is pertinent. The technique of fabrication, fabrication parameters such as
material composition, number of bilayer, non-magnetic layer thickness and also the post
annealing treatment, produces a variety of spin transport behavior in ferromagnetic
multilayer structures. All these parameters significantly influence the GMR effect in the
structures.
8
Study on magnetic and non-magnetic material composition can provide better
understanding on magnetic and non-magnetic coupling in GMR phenomenon. Detailed
information about band structure and lattice matching could be associated. From the
study on post annealing treatment, the GMR response on a range of annealing
temperatures could be observed. Study on the number of bilayer could enhance the
understanding of spin dependent transport where supportive evident to resistor model
could be obtained. The GMR response towards increasing non-magnetic layer thickness
could be provided from the non-magnetic layer thickness study.
1.6
Thesis Outline
A general background and brief introduction to GMR effect are discussed in
Chapter 1. This is followed by objectives, scope of the study, significances of the study
and the outline of the dissertation. In Chapter 2, an overview of GMR properties in
ferromagnetic multilayer structures is presented.
Chapter 3 discussed about the
methodology which is focused on fabrication procedures, measurement technique and
characterization technique of ferromagnetic multilayers. A general overview of the
measurement and characterization technique is discussed. Results of all the experimental
work such as effects on the GMR towards composition difference, various annealing
temperature, various conducting layer thickness, number of the repetition of the bilayer
and also structural characterization are presented in Chapter 4. Chapter 5 summarizes
and concludes the findings obtain in the earlier chapter, and also provides suggestions
for future work in this area of research.
CHAPTER 2
THEORY AND LITERATURE REVIEW
2.1
Introduction to Ferromagnetism
Ferromagnetic materials are generally used in the making of GMR sensors
because of two advantages. First, ferromagnetic thin film has high resistance that is
pertinent for obtaining a high magnetoresistance value.
Second, the anisotropic
characteristic or in other words the homogeneity in all direction in ferromagnetic thin
film can be made uniaxial subjected to external magnetic field strength applied to the
thin film (Chen, 2005).
Ferromagnetism is the common and usual form of magnetism as we are familiar
with, as exhibited in horseshoe magnets and refrigerator magnets. It contributes to most
of the magnetic behavior encountered in everyday life.
10
Since the early time of magnetic study, the term ‘ferromagnet’ was used for any
material that could exhibit spontaneous magnetization that is a net magnetic moment in
the absence of external magnetic field. This general definition is still in common use
nowadays. Recently, however, different classes of spontaneous magnetization have
been identified when there is more than one magnetic ion per primitive cell of the
material. This leads to a stricter definition of ferromagnetism.
In particular, ferromagnetism can be ascribed to materials where all of its
magnetic moments/ions add to a positive contribution to the net magnetization. If some
of the magnetic ions subtract from the net magnetization (partially anti-aligned), then the
material is ferrimagnetic. If the magnetic ions are completely anti-aligned so as to have
zero net magnetization, despite the magnetic ordering, it is an antiferromagnet.
2.1.1
Physical Origin of Ferromagnetism
The property of ferromagnetism is due to the direct influence of two effects from
quantum mechanics that are the spin and the Pauli Exclusion Principle. Pauli Exclusion
Principle is a quantum mechanical principle that states no two identical electrons may
occupy the same quantum state simultaneously.
The spin of an electron when combined with its orbital angular momentum
results in a magnetic dipole moment and creates a magnetic field. In many materials
11
however, especially those with a completely filled electron shells, the total dipole
moment of all the electrons is zero, or in other words the spins are in up/down pairs.
Only atoms with partially filled shells (for example unpaired spins) can experience a net
magnetic moment in the absence of the external magnetic field.
A ferromagnetic
material has many such electrons, and if they are aligned they create a measurable
macroscopic field. For this reason Fe, Co, Ni are ferromagnet.
These permanent dipoles (or spins) tend to align in parallel to an external
magnetic field, an effect called paramagnetism. A related but much smaller effect is
diamagnetism, due to the orbital motion induced by an external field, resulting in a
dipole moment opposite to the applied field. Ferromagnetism involves an additional
phenomenon; the dipoles tend to align spontaneously, without any applied field. This is
a purely quantum-mechanical effect.
According to classical electromagnetism, two nearby magnetic dipoles will tend
to align in opposite direction (which would create an antiferromagnetic material). In a
ferromagnet however, they tend to align in the same direction because of the Pauli
Exclusion Principle. According to Pauli, two electrons with the same spin state cannot
lie at the same position, and thus feel an effective additional repulsion that lowers their
electrostatic energy.
This difference in energy is called the exchange energy and
induces nearby electron to align.
12
2.1.2
Domains and Domain Walls
At long distances, after many thousands of moments/ions, the exchange energy is
overtaken by the classical tendency of dipoles to anti-align.
This is why, in an
equilibrated (non-magnetized) ferromagnetic material, the dipoles in the whole material
are not aligned. They are organizing into magnetic domains or also known as Weiss
domain.
The domains are aligned (magnetized) at short range, but at long range
adjacent domains are anti-aligned. The transition between two domains, where the
magnetizations flips, is called a Domain Wall. It is called whether a Bloch wall or Neel
wall depending upon whether the magnetization rotates parallel or perpendicular to the
domain interface. This transition is a gradual transition on the atomic scale which
covers a distance of about 300 ions for iron (Tsymbal and Pettifor, 2001).
Generally, an ordinary piece of iron has little or no net magnetic moment.
However, if it is placed in a strong enough magnetic field, and the moments will remain
re-oriented when the field is tuned off, therefore creating a ‘permanent’ magnet. This
magnetization as a function of the external field is described by a hysteresis curve.
Although this state of aligned domains is not a minimal-energy configuration, it is
extremely stable and has been observed to persist for millions of years in seafloor
magnetite aligned by the Earth’s magnetic field whose poles can thereby be seen to flip
at long intervals. The net magnetization can be destroyed by heating and then cooling
(annealing) the material without an external magnetic field.
13
2.1.3
Hysteresis
Hysteresis is well known in ferromagnetic materials. When an external magnetic
field is applied to a ferromagnet, the atomic dipoles align themselves with the external
field. Even when the external field is removed, part of the alignment will be retained
and the material has become magnetized. The relationship between magnetic field
strength (H) and magnetic flux density (B) is not linear in such materials.
If the
relationship between the two is plotted for increasing levels of field strength, it will
follow a curve up to a point where further increases in magnetic field strength will result
in no further change in flux density. This condition is called magnetic saturation.
If the magnetic field is then reduced linearly, the plotted relationship will follow
a different curve back towards zero field strength at which point it will be offset from
the original curve by an amount called the remanent flux density or remanence. If this
relationship is plotted for all strengths of applied magnetic field the result is sort of Sshaped loop. The thickness of the middle bit of the S describes the amount of hysteresis,
related to the coercivity of the material.
2.1.4
Curie Temperature and Annealing Temperature
The Curie point in ferromagnetic material is the temperature above which it
losses its characteristic ferromagnetic ability. At temperatures below the Curie point,
the magnetic moments are partially aligned within magnetic domains in ferromagnetic
materials. As the temperature is increased towards the Curie point, the alignment
14
(magnetization) within each domain decreases. Above the Curie point, the material is
purely paramagnetic and there are no magnetized domains of aligned moments.
While annealing process improved the microstructure of the magnetic multilayer
thin film, it does have a threshold limit. Annealing for a temperature extending 400°C
decreasing the magnetoresistance value of a magnetic multilayer ( Malkinski et.al,2000).
Another group reveals that annealing at 350°C decrease the magnetoresistance value and
further annealing up to 650°C diminishing the magnetoresistance signal (Hecker et. al,
2002).
Some other researchers annealed their magnetic multilayers below the
temperature of 400°C (Hall et.al, 1996; Suzuki et.al, 1997). It is expected that the
threshold annealing temperature for magnetic multilayer thin film range between 350°C
and 400°C.
At temperatures above the Curie point, an applied magnetic field has a
paramagnetic effect on the magnetization, but the combination of the paramagnetism
with ferromagnetism leads to the magnetization following a hysteresis curve with the
applied field strength. The destruction of magnetization at the Curie temperature is a
second-order phase transition and critical point where the magnetic susceptibility is
theoretically infinite. Table 2.1 below shows the Curie temperature for ferromagnetic
elements.
Table 2.1 : Curie temperature of ferromagnetic elements.
Material
Curie Temperature, Tc
Iron (Fe)
768°C (1043K)
Cobalt (Co)
1121°C (1388 K)
Nickel (Ni)
354°C (629 K)
15
While the Curie point of bulk Iron, Cobalt and Nickel are high, the Curie point
for thin layers of such elements had to be much lower due to lesser number of magnetic
moments in the layers. Due to this, the range of annealing temperature for Iron, Cobalt
and Nickel layers need to be decided prior to temperature dependence study for the
layers. The GMR value of the magnetic layers after annealing is been made, can be used
as indication of their magnetic ordering. The GMR value for magnetic layers or
multilayers should increase with the increasing annealing temperature as the magnetic
alignment decrease. When the Curie point is reached, the magnetization is completely
disordered and this degrades the GMR value.
2.2
Introduction to GMR theory
In understanding the transport properties in metallic superlattice, we should
consider that the transport behaviours are affected by many inherent complexities of the
material.
Many possible complications arise in these types of material such as
interfacial interdiffusion at various lateral length scales, bulk defects, and structural
changes as a function of an individual layer and overall thickness, different length scales
affecting
the
structures,
magnetism
and
transport,
and
differences
in
the
magnetotransport along the different directions in the superlattices (Warda et al, 2004).
However, to make it simple to understand, GMR can be qualitatively understood
using Mott model, which was introduced in 1936 to explain the sudden increase in
resistivity of ferromagnetic metals as they are heated above Curie temperature (Mott,
1964). Mott proposed two main points. First, the electrical conductivity in metals can
16
be described in terms of two largely independent conducting channels that are the upspin and down-spin electrons.
These electrons are distinguished according to the
projection of their spins along the quantization axis.
The probability of spin-flip
scattering processes in metals is normally small as compared to the probability of
scattering processes in which the spin is conserved. This means that the up-spin and
down-spin electrons do not mix over long distances and therefore the electrical
conduction occurs in parallel for the two spin channels.
Second, in ferromagnetic metals the scattering rates of the up-spin and downspin electrons are quite different, whatever the nature of the scattering center is.
According to Mott, the electric current is primarily carried by electrons from the valence
sp bands due to their low effective mass and high mobility. The d bands play an
important role in providing final states for the scattering of the sp electrons.
In
ferromagnets the d band are exchange-split, so that the density of states is not the same
for the up-spin and down-spin electrons at the Fermi energy.
The probability of
scattering into these states is proportional to their density, so that the scattering rates are
spin-dependent which is different for the two conduction channels.
17
2.2.1
Mott’s Two Current Model
At low temperature, the current is carried in parallel by spin-up and spin-down
electrons. The introduction of impurities generates a residual resistivity ρ↑(0) and ρ↓(0)
for each spin direction. The overall resistivity is then:
ρ (0) =
 (0)  (0)
 (0)   (0)
(2.1)
Two processes that are known to mix the momentum from one spin channel to
the other one are the elastic scattering of electrons by spin waves and mutual spin-flip
process between two interacting electrons.
However the description of these
mechanisms is beyond the scope of this section.
If we denote the average velocity in each spin channel by  and the electric field
is E, then we can write
-
1
e
1
E =  +
(   )
m

2  
(2.2a)
and
-
1
e
1
E =  +
(   )
m

2  
(2.2b)
where  is the relaxation time of the electrons, e is the charge of electron, m is the mass
of electron.
18
Then for each spin channel, we define intrinsic resistivities as:
i , = ne  ,
(2.3a)
or
 , =
m
ne  ,
(2.3b)
2
where n is the Avogadro number.
The resistivity is then calculated as :
 (T ) 
E
i  i 
(2.4)
Equation (2.4) hence gives

     4   (      )
      4  
(2.5)
and each spin resistivity can be written as :
 , (T )   , (0)   i, (T )
(2.6)
where  i, is the intrinsic resistivity of each spin channel of the pure metal.
19
2.2.2
Resistor Model
Current-in-plane GMR or CIP GMR can be qualitatively understood using the
very simple resistor model (Edwards et al., 1992). According to the resistor model, each
metallic layer (and each interface) is treated as an independent resistor. Within each
spin conduction channel the resistors are added in parallel or in series depending on the
relationship between the mean free path and the layer thickness. If the mean free path is
short compared to the layer thickness, then each layer conducts the electric current
independently and the resistor should be added in parallel. Under this circumstance, it is
obvious that the resistance of the parallel and antiparallel are the same, making the GMR
is zero. These observations indicate that to obtain non-zero GMR the mean free path
should be sufficiently long. This is in agreement with the qualitative picture of GMR
(Figure 2.1 a), which is based on the possibility for the electrons to propagate across the
non-magnetic layer freely sensing the magnetizations of the two consecutive
ferromagnetic layers. If the length of the mean free path is longer compared to the layer
thickness, the probability of scattering within the multilayer is the sum of scattering
probabilities within each layer and each interface. Thus, within a given spin channel the
total resistance is the sum of resistances of each layer and each interfaces or in other
words the resistors are connected in series.
Magnetic multilayers exhibiting giant
magnetoresistance is good example for this limiting case.
In building up the resistor network for the multilayer, consider a unit cell which
consist of four layers, two ferromagnetic and two non-magnetic as shown in Figure 2.1
(a) and 2.1 (b). Assume the global spin-quantization axis is the same with magnetization
direction.
In each ferromagnetic layer the electron spin can be either parallel or
antiparallel to the magnetization direction.
If the electron spin is parallel to the
magnetization direction, the electron is locally a majority-spin electron. If the electron
spin is antiparallel to the magnetization direction, the electron is a minority-spin
electron. The majority and minority spin resistivities of the ferromagnetic layer are
20
different and denote as ρ↑ and ρ↓ respectively. The resistance of the bilayer which
consists of the ferromagnetic layer and the non-magnetic layer for the two spin channel
is equal to:
R , =  NM d NM   , d FM
(2.7)
where  NM is the resistivity of non-magnetic layer, d NM is the thickness of non-magnetic
layer, d FM is the thickness of ferromagnetic layer.
For simplicity, we assumed the interface resistance between the ferromagnetic
and non-magnetic layer has been omitted. Using the resistances which are defined in
Equation 2.7, the equivalent network of resistors for the parallel and antiparallel
magnetizations are shown in Figure 2.1 (c) and 2.1 (d) respectively. The total resistance
of the parallel-aligned multilayer is given by:
RP = N
R R
R  R
(2.8)
where N is the number of the four layer unit cells within the multilayer.
The total resistance of the antiparallel-alignment multilayer R AP is then equal to:
R AP = N
R  R
2
(2.9)
Therefore, the magnetoresistance ratio can be determined by a simple expression which
is:
21
( R  R ) 2
R R AP  RP

= 
R
RP
4 R R
(2.10)
By using Equation 2.7 and 2.10 we can pinpoint the main factors which
determine the GMR. By assuming that the resistance of the spacer layer is small
compared to the resistance of the ferromagnetic layer, the expression for GMR is:
2
R (      )
(  1) 2

=
R
4   
4
(2.11)
where  is the spin asymmetry parameter and is equal to   /   .
From Equation 2.11 it is obvious that the magnitude of the GMR is strongly
dependent on the asymmetry in the resistivity between the two spin conduction channels
in ferromagnetic layers. Large asymmetry where  >>1 or  <<1 is an important
requirement for obtaining high values of GMR. If there is no spin asymmetry in the
resistivity of the two spin channels or in other words   1 , the GMR will be zero.
The finite resistance of the spacer layer is given by:
R
(  1) 2

R

d  d
4   NM 1  NM
d FM 
d FM




(2.12)
22
where  is equal to  NM /   . It is clear that for any given value of  , the GMR will
decrease with decreasing d NM / d FM .
Using the resistor model, few hypotheses can be made. Firstly, in stacking
multilayers, the GMR are higher than in single multilayers because stacking the
multilayers is equivalent to adding up more resistors in series to a circuit, thus give
higher resistance. Secondly, in single multilayers (magnetic/non-magnetic/magnetic),
the magnetization of the magnetic layers needs to be in anti-parallel state to obtain
higher resistance and higher GMR. And thirdly, in single multilayers (magnetic/nonmagnetic/magnetic), in order to obtain higher GMR value, a low resistivity of a nonmagnetic spacer layer is needed.
2.2.3
Spin-dependent Scattering
Using Mott’s argument and model it is straightforward to explain GMR in
magnetic multilayers. Consider a collinear magnetic configuration as shown in Figure
2.1, and assume that the scattering is strong for the electron with the spin antiparallel to
the magnetization direction, and is weak for electrons with spin parallel to the
magnetization direction. This is supposed to reflect the asymmetry in the density of
states at the Fermi level, in accordance with Mott’s second argument. For the parallelaligned magnetic layers (Figure 2.1 (a)), the up-spin electrons pass through the structure
almost without scattering, because their spin is parallel to the magnetization of the
layers. On the contrary, the down-spin electrons are scattered strongly within both
magnetic layers, because their spin is antiparallel to the magnetization of the layers.
23
Since conduction occurs in parallel for the two spin channels, the total resistivity
of the multilayer is determined mainly by the highly-conductive up-spin electrons and
appears to be low. For the antiparallel-aligned multilayer which is depicted in Figure
2.1 (b), both the up-spin and down-spin electrons are scattered strongly within one of the
magnetic layers, because within one of the layers the spin is antiparallel to the
magnetization direction. Therefore, the total resistivity is high.
Wang and Li, (1995), suggested when the ambient temperature is lower than the
Curie temperature of magnetic materials, the direction of polarized spins of different
magnetic atoms are parallelly ordered, however, to different magnetic clusters, they are
disordered, therefore the interface layers can be described as superparamagnetic layers
and the exchange coupling between the ferromagnetic films can be omitted.
24
Fig. 2.1 : Schematic illustration of electron transport in a multilayer for parallel (a) and
antiparallel (b) magnetization of the successive ferromagnetic layers.
In Figure 2.1, the magnetization directions are indicated by the arrows. The solid
lines are individual electron trajectories within the two spin channels. It is assumed that
the mean free path is much longer than the layer thicknesses and the net electric current
flows in the plane of the layers. Bottom panels show the resistor network within the
two-current series resistor model. For the parallel-aligned multilayer as in Figure 2.1
(a), the up-spin electrons pass through the structure almost without scattering, whereas
the down-spin electrons are scattered strongly within both ferromagnetic layers. Since
conduction occurs in parallel for the two spin channel, the total resistivity of the
multilayer is low.
25
For the antiparallel-aligned multilayer as in Figure 2.1 (b), both the up-spin and downspin electrons are scattered strongly within one of the ferromagnetic layers, and the total
resistivity is high.
Figure 2.1 (c) and 2.1 (d) show the resistor network within two current series
resistor model. For the parallel aligned multilayer, as in Figure 2.1 (c), resistance for the
up-spin electron is low and resistance for the down-spin electron is high results in low
total resistivity. For the antiparallel aligned multilayer, both up-spin and down-spin
electrons experience high resistance, which resulting in high total resistivity.
2.2.4
Spin-dependent Conduction in Ferromagnet
According to Mott’s first argument, the conductivity of a metal is the sum of the
independent conductivities for the up-spin and down-spin electrons, which can be
expressed by:
   
(2.13)
Within each conduction channel the conductivity is determined by number of factors
which Drude expressed them as follows:
2
 Drude
e2 kF


 6
(2.14)
26
where  Drude is Drude conductivity per spin, e 2 /  is approximately equal to 0.387 x
10-4 is the spin quantum conductance, k F is the Fermi momentum and  is the mean
free path.
The mean free path can be expressed by:
   F  (2.15)
(2.15)
where  F is Fermi velocity and  is the relaxation time.
2.3
Band Structure
The electronic band structure of the multilayer is believed to be the most
important property which determines the spin dependent conduction and the GMR. In
most experimental works, 3d transition metal that are Co, Fe and Ni, and their alloys are
used in combination with non-magnetic spacer layer such as Cr or the noble metals like
Cu, Ag and Au. The electronic band structure of these metals is characterized by several
similar features that are spin-orbit coupling, exchange-split d bands in 3d metals, the
conductivity of d bands, the present of interfaces in a magnetic multilayer and band
matching.
27
2.3.1
Spin-orbit Coupling
Spin orbit coupling in 3d transition metals is very weak and the electronic
structure for the up-spin and down-spin electron can be considered independently. 3d
elements are characterized by the presence of the 4s, 4p and 3d valence states which are
distinguished by their orbital momentum. A dispersive sp bands is similar to freeelectron band is created by 4s and 4p states (Tsymbal and Pettifor, 2001). The sp
electrons have a high velocity, a low density of states and consequently a long mean free
path. These features are believed to be mainly responsible for the conductivity in 3d
metals. Contradict to these features, the d band is localized in a relatively narrow energy
window and is characterized by a high density of states and a low velocity of electrons.
In the interval of energy where the sp and d bands cross, electrons cannot be considered
as independent because of the strong sp-d hybridization, which modify substantially the
band structure. Hybridization changes dramatically the properties of sp electrons which
is reflected in the band bending and results in a reduced velocity associated with the sp
band.
Jaya et al. (2002), evaluated the interlayer exchange coupling of two local
moment ferromagnetic sublayers in a magnetic/non-magnetic/magnetic multilayer at
different spacer (non-magnetic) thickness. The interlayer exchange coupling was found
to have an oscillating behaviour with respect to the non-magnetic layer thickness and it
oscillates between ferromagnetic and antiferromagnetic configurations.
Significant
influences of the electron correlation effects on the interlayer exchange coupling and the
correlation effects were found to change the nature and magnitude of the exchange
coupling. The oscillation period of the interlayer exchange coupling was also found to
depend on the band occupation.
28
Gubbiotti et al. (2005), found the presence of bilinear and biquadratic exchange
coupling in Co/Ru/Co trilayers. The trilayers effective uniaxial anisotropy on the top of
Co film thickness has been estimated. This biquadratic coupling, usually referred to as
‘orange peel coupling’ or ‘Neel coupling’ depends on the roughness of the layers and
favors the parallel alignment of the layers magnetic moments so it enters as a positive
contribution to bilinear exchange coupling.
2.3.2
Exchange-split d Bands
In ferromagnetic 3d metals, the d band is exchange-split. Due to the localized
nature of the d electrons, two d electrons experience a strong Coulomb repulsion
provided that they have antiparallel spins and occupy the same orbital. To reduce the
energy it is advantageous for the d electrons to have parallel oriented spins because the
Pauli’s Exclusion Principle does not permit two electrons with the same spin to approach
each other closely that is occupy the same orbital. Hence, the Coulomb interaction is
reduced. Therefore, the Coulomb repulsion in conjunction with the Pauli’s Exclusion
Principle leads to the ferromagnetic exchange interaction and favors the formation of
spontaneous magnetic moment.
However putting all the electrons into states with the same spin direction
increases the total kinetic energy. The increase is being larger in two cases. Firstly,
when the d band is wide, and secondly, when the d band’s density of states is low.
Therefore there are two competing tendencies that have to be balanced in order to find
whether the ferromagnetic ordering is favored. The condition which has to be satisfied
29
for the appearance of ferromagnetism is the famous Stoner criterion Jn (Ef) > 1. J is the
exchange constant which is approximately 1eV for transition metals, n (Ef) is the density
of states for a given spin at the Fermi energy.
The Stoner criterion is satisfied for bcc Fe, fcc Co and fcc Ni. Due to the
exchange splitting of the d band, the number of the occupied states is different for the
up-spin and down-spin electrons and giving rise to the non-zero magnetic moments of
2.2 μB, 1.7 μB and 0.6 μB for Fe, Co and Ni respectively.
In order to distinguished between the high and low-occupied spin states, the
terms ‘majority-spin electrons’ and ‘minority-spin electrons’ are usually used. The band
structure of Co, as a representative of the ferromagnetic 3d metals is shown in Figure 2.2
(b) and 2.2 (c).
30
Figure 2.2 : The electronic band structure (left panels) and the density of states (right
panels) of Cu (a) and fcc Co for the majority-spin (b) and minority-spin (c) electrons.
After Tsymbal and Pettifor, 2001.
31
2.3.3
The Conductivity of 3d Metals
The conductivity is determined by the position of Fermi energy with respect to
the d bands. In the case of Cu, the d bands are fully occupied and the Fermi levels lies
within the sp band as in Figure 2.2 (a). Electrons within the sp band have high velocity
and low density of states which lower the probability of scattering. Therefore, the mean
free path is long and Cu becomes a very good conductor. The same case goes for another
noble metal like Ag and Au.
On the other hand, for ferromagnetic metal like Co the majority d band is fully
occupied whereas the d majority band is only partly occupied as depicted in Figure 2.2
(a). This is the result of exchange splitting. The Fermi level lies within the sp band for
the majority spins but within the d band for the minority spin. The exchange splitting of
the spin bands lead to crucial difference in the conductivity between the majority- and
minority- spin electrons. For the majority spins the conductivity is governed by sp
electrons and is high. This is the same case like Cu. However for the minority spins, the
conductivity is not entirely determined by the sp electrons. Because of the strong sp-d
hybridization which mixes the sp and d states the contribution of both the sp and d
electrons become important. The minority bands represent hybridized spd bands which
are not dispersive and have a high density of states. As a result, the mean free paths
associated with these bands are relatively short and the minority-spin conductivity is
low. These arguments, which are based on the spin polarized band structure, explain the
strong spin asymmetry in the conductivity of bulk Co.
32
2.3.4
The Present of Interfaces
The present of interfaces in magnetic multilayer adds a new important feature
towards the spin dependent transport in bulk elemental ferromagnets. Two adjacent
metals creating the interface have different band structures. This will lead to a potential
step at the interface, and results in the transmission probability across the interface being
less than 1. If the interface separates the ferromagnetic and non-magnetic metals, the
transmission will be spin-dependent due to the spin dependence of the band structure of
the ferromagnetic layer.
2.3.5
Band Matching in Magnetic Multilayer
Another important aspect to be considered in magnetic multilayer is band
matching. Band matching plays important role in spin-dependent interface scattering
due to the intermixing of atoms near the interfaces. By ignoring the change in the
chemical state of the atoms that is by assuming that their energy levels and magnetic
moment are identical to those in the bulk of the adjacent layers, the intermixing at the
interfaces produce a random potential which is strongly spin-dependent. This spin
dependency is a direct consequence of the good band matching for the spins in Co/Cu,
which implies a small scattering potential and the poor band matching for the minority
spins in Co/Cu, which implies a large scattering potential. A similar behavior takes
place in Fe/Cr multilayer. Thus the matching or mismatching of the bands between the
ferromagnetic and nonmagnetic metals results in spin-dependent potentials at disordered
interfaces which contribute to GMR.
33
2.3.6
CASTEP
CASTEP is a commercial and academic software package which uses Density
Functional Theory with a plane wave basis set to calculate electronic properties of
solids. CASTEP programme is a first principle quantum mechanic (ab initio) code for
performing electronic structure calculations. Within the density functional formalism, it
can be use to simulate a wide range of material including crystalline solids, surfaces,
molecules, liquids and amorphous materials. Those are the properties of any material
that can be thought of as an assembly of nuclei and electrons can be calculated with the
only limitation being the finite speed and the memory of the computers being used. In
Physics Department, Faculty of Science UTM, CASTEP was bought for use on parallel
computers by researchers and was located at Computer Instrumentation Laboratory.
CASTEP is a fully featured first principles code and its capabilities are quite
numerous. The basic quantity is the total energy from which many other quantities are
derived. For example the derivative of total energy with respect to atomic positions
results in the forces and the derivative with respect to cell parameters give stresses.
These are then use to perform full geometry optimizations and possibly finite
temperature of molecular dynamics.
Furthermore, symmetry and constraints (both
internal and external to the cell) can be imposed in the calculations, either as defined by
user, or automatically using in-built symmetry detection.
34
2.3.7
Density Functional Theory (DFT)
Density Functional Theory (DFT) is a quantum mechanical theory used in
physics and chemistry to investigate the electronic structure (principally the ground
state) of many body systems, in particular atoms, molecules, and the condensed phases.
With this theory, the properties of many-electrons system can be determined by using
functionals, that is functions of another function, which in this case the spatially
dependent electron density. DFT is among the most popular and versatile methods
available in condensed-matter physics, computational physics and computational
chemistry.
2.4
Fabrication of Ferromagnetic Multilayer Structures
The first multilayer that demonstrated GMR effect was using molecular beam
epitaxy (MBE) technique (Baibich et.al,1988). This method is considered expensive
and time consuming. Less expensive method compared to MBE are direct current (DC)
magnetron sputtering method (Dieny et.al, 1990; Bernabe et.al,19991; Peterson et.al,
2003); radio frequency (RF) sputtering method (Spizzo et.al, 2003) and ion beam
sputtering method (Colis et.al, 2000).
Other technique reported including thermal
evaporation method (Marszalek et.al, 2004).
Among various types of thin film deposition techniques, electron beam
evaporation method was chosen in this study because of several factors. Electron beam
evaporation method is chosen in this study because of its suitability for depositing high
melting point ferromagnetic metals (Zeltser and Smith,1996). This method also have
35
been reported to produce uniform GMR (Kenane et al., 2006). Karcher and Kolesnikov,
2005, reported of less intermixed interfaces and smaller grain sizes of films deposited
using electron beam technique compared to sputtered films.
This technique allows the production of thin film coatings from pure elements,
including most metals, as well as numerous alloys and compounds. Electron beam
evaporation offers several advantages over competing processes including precise
control of low or high deposition rates, excellent material utilization, co-deposition and
sequential deposition systems and a uniform low temperature deposition (Anklam et al.,
1995). Electron beam offers higher evaporation rates, freedom from contamination,
precise rate controls at very low deposition levels, precise film composition and cooler
substrate temperatures. The materials used for evaporation are available in near limitless
shapes and form, the most common being pellets, slugs and disks.
2.5
Development of GMR research
GMR is the most fascinating discovery in thin film magnetism. The discovery of
GMR in 1988 lead to the invention of IBM’s record breaking 16.8 gigabyte of hard disk
drive for desktop computers using their special GMR structure over a decade later. First
GMR based Magnetic Random Access Memory (MRAM) consist of mixture of nickel,
iron and cobalt provided a nine fold improvement in read-access time. By assessing the
microstructure of GMR materials, this yield was enhanced. Recent research include
developing new technologies for making layers that are uniform and can withstand high
temperature to prolonged the durability of material layers.
36
Since the invention of the hard-disk drive in 1956, the technology of the
magnetic head sensors has never ceased to evolve. Today’s sensors are drastically
different from those used in those early heads; they can detect and transmit information
from recorded data at densities greater than 200 Gbit/in2 and data rates approaching 1
GHz according to Childress and Fontana Jr., (2005).
Numerous advances in
nanomagnetics, magnetic ultrathin films, magneto-electronics as well as device
processing, have fueled the remarkable progress of the GMR technology.
Besides revealing new vision in data storage industry, GMR sensors are used in
traffic and army. The sensors reduce the size and power needs of traffic monitoring
system and may help save lives and money in weapons-detection system. Other than
that, GMR smart shock absorbers are used in high performance mountain bike. Other
applications are as diverse as solid state compass and detection of landmines.
37
2.6
Spintronics : GMR Future Technology Application
In semiconductor devices, taking advantage of the charge of electrons is most
essential. On the other hand, magnetic materials are exploited on the basis of electron
spin. To develop new electronics, it is interesting to combine both features, the charges
and the spin of electrons, because in this case, the spin of electrons that carries the
information can be used as an added degree of freedom in novel electronic devices. Thus
the development of functional ferromagnetic semiconductors is a key to the development
of spintronics, which will certainly be the devices of the future (Hong, 2006).
Ten years after the GMR discovery, all hard disk drives included GMR-based
read heads. This has led to a significant improvement in the storage density and has
demonstrated the potential of applications based on spintronics. Nonvolatile random
access magnetic memories, including magnetic tunnel junctions, (MTJs) as the storage
element could be another great application of spintronics. Actually, their low power
consumption, their high scalability and their nonvolatility place them in a good position
on the random access memory market with respect to semiconductor-based memories
(Schuhl and Lacour, 2005).
Current efforts in designing and manufacturing spintronic devices involve two
different approaches. First, perfecting the existing GMR based technology by either
developing new materials with larger spin polarization of electrons or making
improvement or variations in the existing device that allow for better spin filtering.
Second, finding novel way of both generation and utilization of spin polarized current.
These include investigations of spin transport in semiconductors and looking for ways in
which semiconductors can function as spin polarizers and spin valves.
38
The importance of these efforts lies in the fact that the existing metal-based
devices do not amplify signals (although they successful switches or valves), whereas
semiconductor-based spintronics devices could in principle provide amplification and
serve as multi functional devices. Perhaps even more importantly, it would be much
easier for semiconductor-based spintronics devices to be integrated with traditional
semiconductor technology.
CHAPTER 3
RESEARCH METHODOLOGY
3.1
Sample Fabrication
Sample fabrication processes include the selection of substrate and source
material, substrate pre-clean and preparations, deposition using electron beam method
and post deposit annealing process.
3.1.1
Substrate and Source Material
The corning glass slides (CORNING Micro Slides model no: 2947) were used as
substrate. Cobalt (Co), Nickel (Ni), Copper (Cu) and Silver (Ag) granules manufactured
by FLUKA with purity 4N (99.99%) were used as target materials.
40
3.1.2
Substrate Pre-clean and Preparation
The existence of contaminants such as particles, metals, dust and watermarks on
the surface of the substrate can cause poor adhesion between the film and the substrate
and this will affect the structure of the samples grown. Therefore substrates needed to
be cleaned with degreasing solvent such as chromic acid before samples deposition.
Substrates were submerged in chromic acid solution and then immersed in an
ultrasound bath for 30 minutes. Then the substrates were taken out from the bath and
rinsed with distilled water for two minutes. The substrates were then blow-dried using a
dryer.
3.1.3
Electron Beam Evaporation Technique and Principles
Electron beam evaporation is a form of physical vapor deposition in which a
target anode is bombarded with an electron beam given off by a charged tungsten
filament under high vacuum condition. The electron beam causes atoms from the target
to transform into gaseous phase. These atoms then precipitated into solid forms, coating
the substrate in the vacuum chamber with a thin layer of the anode (target) material.
Edwards Auto 306 Evaporation System (Auto 306), which is shown in Figure 3.1
is used to fabricate ferromagnetic multilayer structures.
The system consists of a
tungsten filament, a shuttle, graphite crucibles, a voltage and current sources, a rotary
pump, a turbo pump, film thickness monitor (FTM) and a control panel. The deposition
41
chamber is evacuated to 10-6 Torr using both the rotary and the turbo pumps. First the
rotary pump was used to pump down the pressure of the chamber to 10-2 Torr. Then the
turbo pump assisted the rotary pump to pump down the pressure down to 10-6 Torr. The
following steps were taken to fabricate ferromagnetic multilayer structures.
i.
The chamber was pumped down to 10-2 Torr using the rotary pump.
ii.
The vacuum chamber was opened and the corning glass substrate was placed on
top of the graphite crucibles. The target source was then put into one of the
crucible. The vacuum chamber was then closed.
iii.
The chamber was pumped down to 10-6 Torr by using the turbo pump. This
procedure took 2 hours.
iv.
When the vacuum system reached the pressure of 10-6 Torr, degassing step was
taken by supplying approximately 70% of current for a period of 15 minutes.
This step was to ensure the residual gas that may exist in the chamber from the
previous deposition process was being removed out.
v.
A 3 kV voltage was set. The current was then raised slowly until the voltage
reading started to dropped. Usually in the case of cobalt, nickel, copper and
silver deposition the current reading ranges from 1.5 to 2.0 mA.
vi.
The voltage source, current source and the height of the crucible were adjusted to
obtain the suitable deposition rate that is 0.01 nm/s. The deposition rate reading
was shown on the film thickness monitor (FTM).
42
vii.
When the appropriate thickness reached, the shutter was then used to block the
evaporation from reaching the substrate. The current and voltage source was
then stopped.
Figure 3.1 : Edwards Electron Beam Evaporation System Photograph.
3.1.4
Annealing Process
Annealing is a heat treatment in which a sample is exposed to an elevated
temperature for an extended time and then slowly cooled. Annealing heat treatment is
largely characterized by induced structural changes which are ultimately responsible for
altering the sample properties. In magnetoresistance studies, annealing process may
alter the magnetic properties of the thin film which may affect the magnetoresistance
value (Carbucicchio et al., 1999).
43
Table 3.1 below summarizes the annealed samples structures where the
annealing temperatures range from 280°C to 380°C.
Table 3.1 : List of annealed samples
Sample structure
sample label
annealing
annealing
temperature
time
Co(6.5)/Cu(4.0)/Ni(5.5)
T1(a)
as deposited
2 hour
Co(6.5)/Cu(4.0)/Ni(5.5)
T1(b)
280°C
2 hour
Co(6.5)/Cu(4.0)/Ni(5.5)
T1(c)
290°C
2 hour
Co(6.5)/Cu(4.0)/Ni(5.5)
T1(d)
300°C
2 hour
Co(6.5)/Cu(4.0)/Ni(5.5)
T1(e)
310°C
2 hour
Co(6.5)/Cu(4.0)/Ni(5.5)
T1(f)
350°C
2 hour
Co(6.5)/Cu(4.0)/Ni(5.5)
T1(g)
380°C
2 hour
Ni(7.5)/Cu(4.0)/Ni(7.5)
T2(a)
as deposited
2 hour
Ni(7.5)/Cu(4.0)/Ni(7.5)
T2(b)
280°C
2 hour
Ni(7.5)/Cu(4.0)/Ni(7.5)
T2(c)
290°C
2 hour
Ni(7.5)/Cu(4.0)/Ni(7.5)
T2(d)
300°C
2 hour
Ni(7.5)/Cu(4.0)/Ni(7.5)
T2(e)
310°C
2 hour
Ni(7.5)/Cu(4.0)/Ni(7.5)
T2(f)
350°C
2 hour
Ni(7.5)/Cu(4.0)/Ni(7.5)
T2(g)
380°C
2 hour
Co(6.0)/Ag(5.0)/Co (6.0)
T3(a)
as deposited
2 hour
Co(6.0)/Ag(5.0)/Co (6.0)
T3(b)
280°C
2 hour
Co(6.0)/Ag(5.0)/Co (6.0)
T3(c)
290°C
2 hour
Co(6.0)/Ag(5.0)/Co (6.0)
T3(d)
300°C
2 hour
Co(6.0)/Ag(5.0)/Co (6.0)
T3(e)
310°C
2 hour
Co(6.0)/Ag(5.0)/Co (6.0)
T3(f)
350°C
2 hour
Co(6.0)/Ag(5.0)/Co (6.0)
T3(g)
380°C
2 hour
44
Ni(7.0)/Ag(4.0)/Ni(7.0)
T4(a)
as deposited
2 hour
Ni(7.0)/Ag(4.0)/Ni(7.0)
T4(b)
280°C
2 hour
Ni(7.0)/Ag(4.0)/Ni(7.0)
T4(c)
290°C
2 hour
Ni(7.0)/Ag(4.0)/Ni(7.0)
T4(d)
300°C
2 hour
Ni(7.0)/Ag(4.0)/Ni(7.0)
T4(e)
310°C
2 hour
Ni(7.0)/Ag(4.0)/Ni(7.0)
T4(f)
350°C
2 hour
Ni(7.0)/Ag(4.0)/Ni(7.0)
T4(g)
380°C
2 hour
3.1.5
Samples Features and Structures
Tables 3.2 and 3.3 summarize the other sample properties and sample structures
deposited using electron beam evaporation system.
Table 3.2 : List of compositional varying samples (C-series) and non-magnetic
thickness varying samples (N-series).
Sample type
Sample label
Sample features/ structures
C1
Co(6.5)/Cu(4.0)/Ni(5.5)
C2
Ni(7.5)/Cu(4.0)/Ni(7..5)
C3
Co(6.0)/Ag(5.0)/Co (6.0)
C4
Ni(7.0)/Ag(4.0)/Ni(7.0)
N1(a)
Co(6)/Cu(1)/Co (6)
thickness ranging from N1(b)
Co(6)/Cu(3)/Co (6)
1 nm to 15 nm
N1(c)
Co(6)/Cu(5)/Co (6)
N1(d)
Co(6)/Cu(9)/Co (6)
Compositional vary
Non-magnetic layer
45
N1(e)
Co(6)/Cu(13)/Co (6)
N1(f)
Co(6)/Cu(15)/Co (6)
N2(a)
Ni(7)/Ag(1)/Ni(7)
N2(b)
Ni(7)/Ag(3)/Ni(7)
N2(c)
Ni(7)/Ag(5)/Ni(7)
N2(d)
Ni(7)/Ag(9)/Ni(7)
N2(e)
Ni(7)/Ag(13)/Ni(7)
N2(f)
Ni(7)/Ag(15)/Ni(7)
Table 3.3 : List of number of multilayer varying samples (R).
Sample type
Sample label
Sample features/ structures
Number of repetition of the
R1
[Co(6)/Cu(3)/Co (6)]1
multilayer ranging from 1
R2
[Co(6)/Cu(3)/Co (6)]2
to 20 multilayer.
R3
[Co(6)/Cu(3)/Co (6)]3
R4
[Co(6)/Cu(3)/Co (6)]4
R5
[Co(6)/Cu(3)/Co (6)]5
R6
[Co(6)/Cu(3)/Co (6)]6
R7
[Co(6)/Cu(3)/Co (6)]7
R8
[Co(6)/Cu(3)/Co (6)]8
R9
[Co(6)/Cu(3)/Co (6)]9
R10
[Co(6)/Cu(3)/Co (6)]10
R11
[Co(6)/Cu(3)/Co (6)]11
R12
[Co(6)/Cu(3)/Co (6)]12
R13
[Co(6)/Cu(3)/Co (6)]13
R14
[Co(6)/Cu(3)/Co (6)]14
R15
[Co(6)/Cu(3)/Co (6)]15
R16
[Co(6)/Cu(3)/Co (6)]16
R17
[Co(6)/Cu(3)/Co (6)]17
R18
[Co(6)/Cu(3)/Co (6)]18
46
3.2
R19
[Co(6)/Cu(3)/Co (6)]19
R20
[Co(6)/Cu(3)/Co (6)]20
Thickness Control and Measurement
Thickness measurements in fabricating ferromagnetic multilayer structures in
this study were done by using two complementary methods. During the deposition
process, the thickness of the magnetic or non-magnetic layer was controlled using Film
Thickness Monitor (FTM) which attached to the electron beam evaporation system. The
same layer thickness was then measured using Surface Profiler to confirm the thickness
measured by the FTM.
3.2.1
Film Thickness Monitor (FTM)
The Film Thickness Monitor (FTM) is used to monitor the thickness of film
deposited on the substrate. When the expected thickness is reached, the shutter is then
used to block the evaporated materials from reaching the substrate. The Film Thickness
Monitor (FTM) is a microprocessor based frequency counter capable of converting
frequency changes into deposition rates and thickness information for a range of
deposition materials. It is used in conjunction with a quartz crystal, which is placed in
the deposition field whose output frequency is controlled by crystal.
47
Figure 3.2 : The FTM Display which connected to quartz crystal inside the e-beam
chamber
Quartz crystal is capable of measuring film thickness and deposition rates during
the deposition process. It is based on the frequency change in the quartz crystal upon the
mass of the material being deposited. Thickness d and frequency change ∆f can be
express as :
d
where
 f .C   a
f 2   f




(3.1)
f is the resonans frequency (5 or 6 Mcs-1), ∆f is the frequency change in quartz
crystal, C is constant (1670 Kcs-1), ρq is the density of quartz and ρf is the density of
thin film.
The negative sign is because frequency reduces from larger value to smaller
value. When f = 5 Mcs-1, ∆f = 1 cs-1
which is equal to the increase of mass of the
material being deposited, m = 1.8 x 10-8 gcm-2. ∆f can be calibrated to give the exact and
correct d value. Linearity of d and ∆f is restricted with the mass of the material being
48
deposited, because m which can occupy a maximum value. After the maximum value,
the quartz crystal needs to change with a new one for further measurement.
3.2.2
Surface Profiler
Another method of measuring film thickness is by using surface profiler. To
confirm the thickness measured by FTM, a single layer of cobalt, copper, silver and
nickel layer was measured using Dektak 3 Surface Profiler and the result was as shown
in Table 4.1. The surface profiler consists of a stylus with a 25 microns tip diameter. It
probes the surface of the samples and measure step heights from as low as 1 nm to
maximum 50 microns.
Thickness measurements are made electromechanically by
moving the sample beneath the diamond-tipped stylus, according to a user programmed
scan length and speed. Prior to measurement, a step was created on the sample to
provide surface variations on the sample surface.
3.3
Magnetoresistance Measurement
Magnetoresistance measurement in this study was made using four point probe
method. According to Chen et. al, (unpublished), in a sheet resistance measurement,
several resistances need to be considered as shown in Figure 3.3 (a). The probe has a
probe resistance Rp. It can be determined by shorting two probes and measuring their
49
resistances. At the interface between the probe tip and the film, there is a probe contact
resistance, Rcp. When the current flows from the small tip into the semiconductor and
spreads out in the semiconductor, there will be a spreading resistance, Rsp. Finally the
film itself has a sheet resistance Rs.
The equivalent circuit for the measurement of sheet resistance by using the fourpoint probe is shown in Figure 3.3 (c). Two probes carry the current and the other two
probes sense the voltage.
Each probe has a probe resistance Rp, a probe contact
resistance, Rcp and a spreading resistance Rsp associated with it. However these parasitic
resistances can be neglected for the two voltage probes because the voltage is measured
with a high impedance voltmeter, which draws very little current. Thus the voltage
drops across these parasitic resistances are insignificantly small. The voltage reading
from the voltmeter is approximately equal to the voltage drop across the sheet resistance.
Figure 3.3: Four point probe measurement of sheet resistance.
50
By using the four-point probe method, the sheet resistance can be calculated:
Rs  F
V
,
I
(3.2)
where V is the voltage reading from the voltmeter, I is the current carried by the two
current-carrying probes, and F is a correction factor. For collinear or in-line probes with
equal probe spacing, the correction factor F can be written as a product of three separate
correction factors:
F= F1 F2 F3
(3.3)
F1 corrects for finite sample thickness, F2 corrects for the finite lateral sample
dimension, and F3 corrects for the placement of the probes with finite distances from the
sample edges. For very thin samples with the probes being far from the sample edge,
Chen et. al, (unpublished), estimated F2 and F3 are approximately equal to one, and the
expression of the semiconductor sheet resistance becomes:
Rs 
3.3.1
 V
ln 2 I
(3.4)
Four Point Probe Sample
The deposited multilayer was silver pasted with a copper wire at four point
contact to make it a four point probe sample. The schematic diagram of four point probe
sample is as shown in Figure 3.4. By using the four point probe resistance measurement
method, the thin film resistance can be calculated with the common formula of
resistance that is:
51
R
V
I
(3.5)
where V is the voltage reading from the voltmeter and I is the constant current supplied
to the sample. The four point probe resistance measurement method can eliminate the
effect by the probe resistance, probe contact resistance and spreading resistance
(Schroeder,1998).
non-magnetic
layer
voltmeter
A
B
copper wire
constant current
source
silver paste
copper wire
silver paste
magnetic layer
Figure 3.4 : Four point sample for magnetoresistance measurement.
52
3.3.2
Magnetoresistance Measurement Set-up
The four point probe sample which was connected to a voltmeter and a constant
current source (DPS 175) was then fitted into the magnetoresistance measurement setup. This set-up consists of two magnetic poles (EMU 75 magnetic field generator), a
digital gauss meter (DGM-102) and a magnetic field indicator. These components were
manufactured by Roorkee Scientific Equipment. The schematic diagram that represents
the magnetoresistance measurement set-up is as shown in Figure 3.5. The direction of
the applied magnetic field was changed by changing the current direction using a switch.
≈ 5cm
magnetic pole
A
B
magnetic pole
sample
digital gauss meter
magnetic field
indicator
Figure 3.5 : Magnetoresistance measurement set-up. Dotted arrows, indicating the
magnetic field direction, whether from A to B or from B to A.
53
The magnetoresistance (MR) is calculated using the formula below:
MR  │
R  Rmin
│ x 100%
Rmin
(3.6)
where R is the resistance at certain applied magnetic field strength and Rmin is the
resistance at where saturation magnetization takes place, at 2000 Gauss.
3.4
Structural Analysis of Ferromagnetic Thin Films
Structural studies of ferromagnetic thin film were done by X-Ray Diffraction
analysis (XRD), Atomic Force Microscope analysis (AFM), and Field Emission
Scanning Electron Microscope analysis (FESEM).
3.4.1
X-Ray Diffractometer Analysis
Diffraction refers to various phenomena associated with the bending of waves
when they interact with obstacles in their path.
It occurs with any type of wave
including X-rays. When X-rays hit an atom, they make the electronic cloud move. The
movement of these charges re-radiates waves with the same frequency which is known
as Rayleigh scattering.
The scattered waves can themselves be scattered but this
secondary scattering is assumed to be negligible. A similar process occurs upon
54
scattering neutron waves from the nuclei or by a coherent spin interaction with an
unpaired electron.
These re-emitted wave fields interfere with each other either
constructively or destructively producing a different pattern on a detector film. The
resulting wave interference pattern is the basis of diffraction analysis.
X-rays
wavelengths are comparable with inter-atomic distances and thus are an excellent probe
for this length scale.
3.4.2
Energy Dispersive X-Ray Analysis
The scanning electron microscope (SEM) is a type of electron microscope that
creates various images by focusing a high energy beam of electrons onto the surface of a
sample and detecting signals from the interaction of the incident electrons with the
sample’s surface.
In a typical SEM electrons are thermoionically emitted from a
tungsten or lanthanum hexaboride (LaB6) cathode and are accelerated towards an anode.
Alternatively, electrons can be emitted via field emission (FE). Field emission is the
emission from the surface of a condensed phase into another phase due to the presence
of high electric fields. In this phenomenon, electrons with energies below the Fermi
level tunnel through the potential barrier at the surface, with the high electric field
sufficiently narrows for the electrons to have a non-negligible tunneling probability.
55
3.4.3
Atomic Force Microscope Analysis
The Atomic Force Microscopy enables not only to image the surface in atomic
resolution but also to measure the force at nano-newton scale.
In Atomic Force
Microscope (AFM), an atomically sharp tip is scanned over a surface with feedback
mechanisms that enable the piezo-electric scanners to maintain the tip at a constant force
(to obtain height information), or height (to obtain force information) above the sample
surface. Tips are typically made from Si3 N 4 or Si , and extended down from the end of
a cantilever. AFM head employs an optical detection system in which the tip is attached
to the underside of a reflective cantilever.
A diode laser is focused onto the back of a reflective cantilever. As the tip scans
the surface of the sample, moving up and down with the contour of the surface, the laser
beam is deflected off the attached cantilever into a dual element photodiode. The
photodetector measures the difference in light intensities between the upper and lower
photodetectors, and then converts to voltage. Feedback from the photodiode difference
signal, through software control from the computer, enables the tip to maintain either a
constant force or constant height above the sample. In the constant force mode the
piezo-electric transducer monitors real time height deviation. In the constant height
mode the deflection force on the sample is recorded.
56
3.5
Band Structure Analysis
Band structures of ferromagnetic and nonmagnetic metal were obtained by
simple modeling using CASTEP modeling software. CASTEP incorporates the Density
Functional Theory (DFT) in its calculations. CASTEP exploits DFT mechanical code to
simulate the properties of solids, interfaces and surfaces for a wide range of material
classes including ceramics, semiconductors and metals. In this study CASTEP were
used to simulate the electronic band structure and density of states for metals involved
that are Copper (Cu), Nickel (Ni), Cobalt (Co) and Argentum (Ag).
CHAPTER 4
RESULTS AND DISCUSSION
4.1
Introduction
This chapter describes the experimental results to the magnetoresistance of the
samples which were fabricated using electron beam evaporation technique. The effect
of several fabrication parameters such as composition, annealing temperature and
thickness on magnetoresistance was studied. The parameters are material composition,
temperature treatment and sample thickness.
The structural characterization of the
samples using characterization method discussed in chapter three are also presented.
58
4.2
Thickness of Films
The thicknesses of the deposited film were measured using Film Thickness
Monitor (FTM) during the deposition process. The thickness is then reconfirmed by
using DEKTAK 3 Surface Profiler. Table 4.1 below shows the thickness measurement
obtained by Film Thickness Monitor and using Surface Profiler.
Table 4.1 : Difference in thickness measurement observed by Film Thickness
Monitor and Surface Profiler.
Thickness
measured
using Surface
Profiler (nm)
15.2
Deviation
R1, [Co(6)/Cu(3)/Co (6)]1
Thickness
observed
using FTM
(nm)
15
R2, [Co(6)/Cu(3)/Co (6)]2
30
30.5
1.7%
R3, [Co(6)/Cu(3)/Co (6)]3
45
45.7
1.6%
R4, [Co(6)/Cu(3)/Co (6)]4
60
60.8
1.3%
R5, [Co(6)/Cu(3)/Co (6)]5
75
76.2
1.6%
R6, [Co(6)/Cu(3)/Co (6)]6
90
91.2
1.3%
R7, [Co(6)/Cu(3)/Co (6)]7
105
106.6
1.5%
R8, [Co(6)/Cu(3)/Co (6)]8
120
121.9
1.6%
Sample structure
1.3%
Comparing the thicknesses from FTM measurements and Dektak 3 Surface
Profiler measurements for eight samples (R1-R8), gives the average variance of 1.5%.
This results show that the thickness being measured by FTM during the deposition
process only slightly differs from counter measurement made using Surface Profiler.
This confirms that the thickness measurements are precise and acceptable.
59
4.3
Compositional Dependence Magnetoresistance
The magnetoresistance as has been discussed in Chapter 3, is ascribed to a
considerable drop in the percentage of resistance of a sandwiched magnetic/nonmagnetic layers when a sufficiently high magnetic field is applied to the sample (Sakrani
et al., 2006). Figure 4.1 (a), (b), (c) and (d) below show the magnetoresistance curve of
Co/Cu/Ni, Ni/Cu/Ni, Co/Ag/Co and Ni/Ag/Ni respectively.
The curves show the
changes in the resistance of the samples in the vicinity of applied magnetic field. The
resistance change is described as; when there is no magnetic field applied, magnetization
of the ferromagnetic layers ( Co-Ni, Ni-Ni, and Co-Co) are anti-parallel with each other.
Based on spin-dependent scattering principle, in this condition, the resistivity is high
thus the magnetoresistance is also high. When the applied magnetic field is increased, it
will align the magnetization of the ferromagnetic layers gradually until they become
completely parallel with each other. In this condition, the resistivity is low and as a
result, the magnetoresistance is low.
10
9
Magnetoresistance (%)
8
7
6
5
4
3
2
1
0
-2500
-2000
-1500
-1000
-500
-1 0
500
1000
1500
2000
2500
Magnetic Field, Gauss
Figure 4.1 (a) : The external magnetic field versus the magnetoresistance response for
Co(6.5)/Cu(4.0)/Ni(5.5) multilayers.
60
6
Magnetoresistance (%)
5
4
3
2
1
0
-2500
-2000
-1500
-1000
-500
0
500
1000
1500
2000
2500
Magnetic Field, Gauss
Figure 4.1 (b) : The external magnetic field versus the magnetoresistance response for
Ni(7.5)/Cu(4.0)/Ni(7..5) multilayers.
2.5
Magnetoresistance (%)
2
1.5
1
0.5
0
-2500
-2000
-1500
-1000
-500
0
500
1000
1500
2000
2500
Magnetic Field, Gauss
Figure 4.1 (c) : The external magnetic field versus the magnetoresistance response for
Co(6.0)/Ag(5.0)/Co (6.0) multilayers.
61
1.6
Magnetoresistance (%)
1.4
1.2
1
0.8
0.6
0.4
0.2
0
-2500
-2000
-1500
-1000
-500
0
500
1000
1500
2000
2500
Magnetic Field, Gauss
Figure 4.1 (d) : The external magnetic field versus the magnetoresistance response for
Ni(7.0)/Ag(4.0)/Ni(7.0) multilayers.
From Figure 4.1, it was found that as deposited Co/Cu/Ni multilayer shows
magnetoresistance value of 8.77%, as deposited Ni/Cu/Ni multilayer shows
magnetoresistance value of 5.28%, as deposited Co/Ag/Co shows magnetoresistance
value of 2.27% and as deposited Ni/Ag/Ni show magnetoresistance value of 1.48%
respectively. All the measurements were made at room temperature with magnetic field
of -2000 to 2000 gauss applied in plane with the sample.
Since GMR was discovered in 1988, a large number of magnetic multilayer
structures which display GMR effect were discovered. The highest published values of
magnetoresistance to date are 220% in Fe/Cr multilayers (Schad et al., 1995) and 120%
in Co/Cu multilayers (Parkin et.al, 1991). In comparison with the results obtained in
Figure 4.1 (a), 4.1 (b), 4.1 (c) and 4.1 (d), fairly large values of GMR were obtained
which
62
are 22% of Co/Ag at room temperature (Araki, 1993) 28% of Ni/Ag at 4.2K (Rodmacq
et al., 1993) and 9% of Ni/Cu at 4.2K (Sato, et al., 1994)
Ferromagnetic 3d metals have a pronounced spin asymmetry in their
conductivity due to the presence of exchange split d bands and therefore the
magnetoresistance should be high. But however, many findings reported that some
multilayers are highly magnetoresistive whereas the others are not, although they
contain ferromagnetic 3d metals. It looks like that spin asymmetry in the band structure
is a necessary but not sufficient condition for high GMR values. Parkin, 1994, after
tested some 30,000 different multilayer combinations of different element and
dimensions at IBM proposed a hypothesis that GMR to a great extent is determined by
ferromagnetic metal/non-magnetic metal pair, rather by an individual material
considered separately. To support this explanation GMR was found to be much lower in
Co/Cr and Fe/Cr. A 3% GMR in Co/Cr has been reported (Parkin et. al, 1990), and
5.5% GMR in Fe/Cu (Monchesky et.al., 1999) as compared to Fe/Cr and Co/Cu
multilayers.
4.3.1
Band Structure Factor
There are two factors which are crucial for obtaining high values of GMR in
terms of composition dependence. These are the band matching and lattice matching
between the ferromagnetic and nonmagnetic metals (Tsymbal and Pettifor, 2001). Rao
and Freeman, 1998, and Gebele et al., 2000, explained that a good band matching for
one spin orientation between a ferromagnetic metal (FM) and a nonmagnetic (NM)
63
metal implies high transmission for this spin across the FM/NM interface. On the other
hand, a large band mismatch for the other spin orientation implies that the transmission
is poor. In Co/Cu/Ni multilayer the band structure of Co/Cu and Cu/Ni collaborate to a
good GMR value compare to Ni/Cu in Ni/Cu/Ni multilayer, Co/Ag in Co/Ag/Co
multilayer and Ni/Ag in Ni/Ag/Ni multilayer.
Figure 4.2 : Electronic band structures (left panels) and density of states (right panels)
of a) Co and b) Cu.
From Figure 4.2, the band structure for non-magnetic Cu shows fully occupied d
bands and the presence of a dispersive sp band at the Fermi energy, which results in high
conductivity of Cu. The band structure of ferromagnetic Co shows the exchange-split d
bands and the Fermi energy lies within the sp bands which also lead to high spin
conductivity. Both spin conductivity contribution from Co and Cu are believed to be
responsible for high GMR value in Co/Cu/Ni multilayers as shown in Figure 4.1 (a).
64
Figure 4.3 : Electronic band structures (left panels) and density of states (right panels)
of a) Cu and b) Ni.
Figure 4.3 shows that Ni band structures are quite the same as Cu. Fermi energy
for both Cu and Ni lies within the sp bands. Due to the high velocity of electrons within
the sp bands and the low density of states with resultant low probability of scattering, the
mean free path is long and thus making Cu and Ni a good conductor. Spin conduction
contribution from Cu and Ni, results in high GMR value as in Figure 4.1 (b).
65
Figure 4.4 : Electronic band structures (left panels) and density of states (right panels)
of a) Co and b) Ag.
The electronic and atomic structure for Ag is similar to Cu. Nevertheless, Ag is
not as good band matching for 3d ferromagnets compared to Cu (Tsymbal and Pettifor,
2001). Although Ag is a very good conductor, which is characterized by the Fermi
energy that lies within the sp bands as depicted in Figure 4.4 (b) , due to a relatively
poor band matching with Co, provide Co/Ag multilayer with a relatively poor spin
transmission across the Co/Ag interface. This resulted to a relatively low GMR value
for Co/Ag multilayer, as shown in Figure 4.1 (c) when compared to Co/Cu and Ni/Cu
(Figure 4.1 (a)).
66
Figure 4.5 : Electronic band structure (left panels) and density of states (right panels) of
a) Ni and b)Ag.
Ni is strong ferromagnet with entirely filled majority-spin d bands. However
poor band matching with Ag leads to poor spin transmission across the Ni/Ag interface.
The band mismatch for Ni and Ag is shown in Figure 4.5. Comparing the density of
states for both Ni and Ag, the occupied energy level for sp bands shows a significant
different, which resulting in low GMR value in Ni/Ag multilayers. Figure 4.1 (d) shows
a lowest GMR value for Ni/Ag/Ni multilayer compared to Co/Cu/Ni, Ni/Cu/Ni and
Co/Ag/Co as depicted in Figure 4.1 (a), (b), (c) respectively.
67
4.3.2
Lattice Matching Factor
Another factor that influence the GMR value is lattice matching. According to
Tsymbal and Pettifor, 2001, lattice matching of the subsequent layers is very important
factor for GMR as lattice mismatched leads to the formation of misfit dislocation and
other structural defects at the interfaces. Scattering by this defect in the nonmagnetic
spacer layer is spin-independent, resulting in a reduction of GMR.
Although the
scattering by defects in a ferromagnet layer could be spin dependent, the spin asymmetry
in the scattering potentials will vary depending on structural details. The presence of
various types of defects will make the average of the scattering potential only weaklydependent on the spin, which can lead to reduce values of GMR. Table 4.2 below
summarizes the lattice parameter a, for Co, Cu, Ni , Ag, Fe and Cr which explain how
lattice matching and lattice mismatch contributes to GMR value in Co/Cu/Ni, Ni/Cu/Ni,
Co/Ag/Co, Ni/Ag/Ni, Co/Cu/Co and Fe/Cr/Fe.
Table 4.2 : Lattice parameter for ferromagnetic metals (Co, Ni, Fe, Cr) and nonmagnetic metal (Cu,Ag).
Thin film
Lattice
structure
material
parameter, a (Å)
Co
3.54
Face centered cubic, fcc
Cu
3.61
Face centered cubic, fcc
Ni
3.52
Face centered cubic, fcc
Ag
4.09
Face centered cubic, fcc
Fe
2.87
Face centered cubic, fcc
Cr
2.88
Face centered cubic, fcc
68
Thin films of Co, Cu, Ni and Ag grow in fcc structure with lattice parameter
3.54Å, 3.61Å and 3.52Å and 4.09Å respectively. In Co/Cu/Ni multilayer, the Co/Cu
lattice mismatch is 1.98% and the Cu/Ni lattice mismatch is 2.56% and give the average
lattice mismatch is 2.27%. The GMR value for Co/Cu/Ni is 8.77% as shown in Figure
4.1 (a).
In Ni/Cu/Ni multilayer the lattice mismatch is 2.56% and the GMR value of
5.28%, as shown in Figure 4.1(b). With the same principle applies to Co/Ag/Co and
Ni/Ag/Ni, lattice mismatch are 15.54% and 16.20% thus gives the GMR value of 2.27%
and 1.48% as the results are shown in Figure 4.1 (c) and 4.1 (d) respectively. From the
overall results obtain in Figure 4.1, it appears that small lattice mismatch gives higher
magnetoresistance value. In order to make better comparison for each multilayer, the
associated interface, lattice mismatch, and GMR are summarized in Table 4.3.
69
Table 4.3 : The average lattice mismatch and the GMR for Co/Cu/Ni, Ni/Cu/Ni,
Co/Ag/Co and Ni/Ag/Ni multilayer.
Multilayer
Interface
Co/Cu/Ni
Co/Cu
Lattice Mismatch
3.54  3.61
3.54
Cu/Ni
3.61  3.52
3.52
Ni/Cu/Ni
Ni/Cu
3.52  3.61
3.52
Cu/Ni
3.61  3.52
3.521
Co/Ag/Co
Co/Ag
3.54  4.09
3.54
Ag/Co
4.09  3.54
3.54
Ni/Ag/Ni
Ni/Ag
3.52  4.09
3.52
Ag/Ni
 100  1.98%
Average
Lattice
Mismatch
GMR
2.27%
8.77%
2.56%
5.28%
15.54%
2.27%
16.20%
1.48%
 100  2.56%
 100  2.56%
 100  2.56%
 100  15.54%
 100  15.54%
 100  16.20%
4.09  3.52  100  16.20%
3.52
4.4
Annealing Temperature Dependence Magnetoresistance
Thermal treatment or annealing is a common procedure for producing structural
changes.
Structural changes would change the magnetic properties of the multilayers.
Chen, 2003, indicated that the optimum time for annealing Co/Cu multilayer is about 2
hour. By assuming Co/Cu/Ni, Ni/Cu/Ni, Co/Ag/Co and Ni/Ag/Ni have almost similar
70
features like Co/Cu, the annealing time chosen was 2 hours.
All the samples had
undergone annealing temperature between 200°C to 350°C.
Magnetoresistance
measurement was then carried out and the results were shown in Figure 4.6 (a), 4.6 (b),
4.6 (c) and 4.6 (d).
16
as deposited
14
200 deg
Magnetoresistance (%)
12
250 deg
10
280 deg
8
290 deg
6
310 deg
350 deg
4
2
0
-2500
-2000
-1500 -1000
-500
-2
0
500
1000
1500
2000
2500
Magnetic Field, Gauss
Figure 4.6 (a) : Magnetoresistance curve for Co(6.5nm)/Cu(4.0nm)/Ni(5.5nm), annealed at
different temperatures for 2 hours.
As deposited Co(6.5nm)/Cu(4.0nm)/Ni(5.5nm) multilayer shows GMR of
8.77% (as
dotted line in Figure 4.6 (a)). Annealing at 200°C, 250°C and 280°C increased the GMR
to 10.68%, 11.36% and 14.25% respectively. Further annealing at 290°C, 310°C and
350°C however, lower the GMR to 12.75%, 6.37% and 3.05% respectively.
The
blocking temperature for this system which was indicated by the highest GMR
percentage curve is approximately 280°C.
71
12
as deposited
Magnetoresistance (%)
10
230 deg
260 deg
8
290 deg
300 deg
6
310 deg
4
330 deg
2
0
-2500
-2000
-1500
-1000
-500
0
500
1000
1500
2000
2500
-2
Magnetic Field, Gauss
Figure 4.6 (b) : Magnetoresistance curve for Ni(7.5nm)/Cu(4.0nm)/Ni(7.5nm), annealed at
different temperatures for 2 hours.
Figure 4.6 (b) shows that for Ni(7.5nm)/Cu(4.0nm)/Ni(7.5nm) multilayer, the as
deposited GMR was 5.86%.
Annealing temperature of 230°C, 260°C and 290°C
increased the GMR to 7.97%, 8.98% and 10.68% respectively. Further annealing at
elevated temperatures of 300°C, 310°C and 330°C cause the decreased of the GMR to
6.25%, 4.10% and 1.93 respectively. It seems that the degradation of GMR starts at
around 290°C.
72
8
as deposited
Magnetoresistance (%)
7
230 deg
6
260 deg
5
290 deg
4
310 deg
3
320 deg
350 deg
2
1
0
-2500
-2000
-1500
-1000
-500
0
500
1000
1500
2000
2500
Magnetic Field, Gauss
Figure 4.6 (c) : Magnetoresistance curve for Co(6.0nm)/Ag(5.0nm)/Co (6.0nm), annealed at
different temperatures for 2 hours.
Figure 4.6 (c) shows the as deposited Co(6.0nm)/Ag(5.0nm)/Co
(6.0nm)
was 2.63%.
Then the GMR were enhanced to 3.74%, 4.37%, 5.87% and 7.45% when the multilayer
was annealed at 230°C, 260°C, 290°C and 310°C respectively. Higher temperature of
320°C and 350°C reduced the GMR to 6.98% and 1.83% respectively. The blocking
temperature for Co(6.0nm)/Ag(5.0nm)/Co (6.0nm) was observed at around 310°C.
73
5
4.5
as deposited
Magnetoresistance (%)
4
200 deg
3.5
230 deg
3
260 deg
2.5
280 deg
2
290 deg
1.5
320 deg
1
0.5
-2500
-2000
-1500
-1000
0
-500 -0.5 0
500
1000
1500
2000
2500
Magnetic Field, Gauss
Figure 4.6 (d) : Magnetoresistance curve for Ni(7.0nm)/Ag(4.0nm)/Ni(7.0nm), annealed at
different temperatures for 2 hours.
The as deposited GMR for Ni(7.0nm)/Ag(4.0nm)/Ni(7.0nm) shows a value of 1.48% as
shown in figure 4.6 (d). Annealing at 200°C, 230°C, 260°C and 280°C increased the
GMR to 1.88%, 2.13%, 3.97% and 4.38% respectively. Further annealing at 290°C and
320°C decreased the GMR to 3.21% and 2.76% respectively. The blocking temperature
for this structure occurs at around 280°C.
Generally, for Co/Cu/Ni, Ni/Cu/Ni, Co/Ag/Co and Ni/Ag/Ni multilayers, the as
deposited GMR was enhanced by annealing up to a temperature called the blocking
temperature where beyond this temperature limit, the GMR were dramatically decreased.
The increasing GMR in Co/Cu/Ni multilayer were caused by the diffusion of Cu along
74
the Co and Ni grain boundaries (May et al.,1998 and Wilhelm et al., 1999). Kuch et al.
(1999) observed diffusion behaviour in Ni/Cu/Co trilayers under photoelectron emission
spectroscopy. In the same agreement, Anna et al. (1999) also suggested the same
diffusion behaviour to Co/Ag/Co multilayer, while Loloee et al., (1995), observed the
diffusion behaviour to Ag/Co and Cu/Co interfaces. Therefore, the same argument is
associated with the increasing GMR when annealing the Ni/Cu/Ni and Ni/Ag/Ni .
As diffusion of Cu into Ni matrix, Ag into Co matrix and Ag into Ni matrix were
strongly believed to increase the GMR in multilayers mentioned earlier, the term
‘diffusion’ need to be clarified.
In magnetic multilayer, diffusion refers to a net
transport of molecules from a region of higher concentration to one of lower
concentration by random molecular motion. The result of diffusion is gradual mixing of
material.
Siritaratiwat et al., (2000), explained that the interlayer (non-magnetic)
diffusion into ferromagnetic grain boundaries or matrix creating intra-layer magnetic
discontinuities in ferromagnetic layers which elevate interlayer antiferromagnetic
coupling and consequently, causes the increment of GMR.
The degradation of GMR when annealing temperatures exceed the blocking
temperature is associated with the intermixing of nonmagnetic atoms (Cu and Ag) with
the ferromagnetic atoms (Co and Ni). Hecker et al., (2003), suggested this intermixing
resulted in the decrease of magnetic homogeneity for ferromagnetic/non-magnetic
interfaces. If diffusion is the key factor of enhancing GMR value, intermixing is the key
factor for the degradation of GMR value for temperature treated ferromagnetic
multilayer structures.
75
According to Dinia et al., (2002), intermixing refers to the mixing of two
magnetic and non-magnetic atoms from two neighboring boundaries after diffusion
takes place. The intermixing resulted in a new type of combined magnetic and nonmagnetic atoms. Hecker et al. (2002) & Kim et al. (2002) suggested the temperature
range between diffusion and intermixing to take place is called the ‘thermal stability’ for
a certain multilayers. Intermixing of atoms promotes significant decay of the transport
properties and magnetic properties of the multilayers. In other words, the alloying
tendency of ferromagnetic and nonmagnetic atoms which occurs above the blocking
temperature cause the decay of GMR and the changes in the magnetic properties of the
multilayers investigated.
Belozrov et al., (2003), explained that the increasing magnetoresistance is due to
a small atomic diffusion at the origin of smothered interfaces. In Co/Cu, Cu/Ni, Co/Ag
and Ni/Ag interfaces the recrystallization process of Co, Cu, Ni and Ag during the early
stage (0°C -300°C) of annealing increased the GMR for each samples. However, when
the annealing temperature exceed to 350°C, GMR for each samples dropped to around
20%. This is because at higher temperature, Co, Cu, Ni and Ag species are weakly
soluble to each other. While the solubility increases with the increasing annealing
temperature, however, at temperature of about 350°C and above, the solubility become
weaker.
As the annealing temperature is increased, Co and Ni atoms are gradually
precipitated from the Cu and Ag matrix and form Co and Ni clusters (Yu et al., 1995).
Their bodies and interfaces with Cu and Ag matrix become the conduction electron spindependent scattering centers. Very small Co and Ni particles or atoms may loose their
magnetic moment due to interaction with the nonmagnetic Cu or Ag matrix, which
76
adversely affects the GMR. On the other hand, when the annealing temperature is
increased to a large value i.e >350°C, Co and Ni particle become large, thus directly
cause the appearance of a ferromagnetic interaction between larger Co and Ni particles
or in other words, antiparallel alignment for better GMR does not occur in portions of
the samples.
It was interesting to note that the systems which exhibit high values of GMR
such as Fe/Cr, Co/Cu, Co/Ag are all immiscible (Gijs & Bauer, 1997).
This fact
indicates that intermixing at the interfaces is not favorable to GMR. One of the reasons
for this might be a reduction in the magnetic moments in the intermixed region which
negatively affects GMR. In addition, intermixing may result in misaligned spins, which
are weakly coupled with the ferromagnetic layer, or magnetically ‘dead’ layers.
Figure 4.7 shows the magnetoresistance effect on annealing temperature for
multilayers discussed previously. It appears that beyond the blocking temperature limits
the sudden and significant decreases of magnetoresistance occurs to all the studied
multilayers.
77
Figure 4.7 : Annealing temperature versus magnetoresistance for
Co(6.5nm)/Cu(4.0nm)/Ni(5.5nm), Ni(7.5nm)/Cu(4.0nm)/Ni(7.5nm), Co(6.0nm)/Ag(5.0nm)/Co (6.0nm) and
Ni(7.0nm)/Ag(4.0nm)/Ni(7.0nm).
4.5
Thickness Dependence Magnetoresistance
When considering the dependence of GMR on thickness of the multilayer, there
are three thickness parameters need to be taken into account. Firstly, the magnetic layer
thickness dependence, secondly, the non-magnetic layer thickness dependence and
thirdly, the number of the repetition of the multilayer. Results in this section focus only
on the thickness effect on GMR exhibit by Co/Cu/Co multilayer because the high value
of GMR reported by other researchers (Parkin et al., 1994; Marrows et al., 1994;
Vavassori et al., 2003; and Turilli et al., 1999) and the simplicity of producing it rather
than Co/Cu/Ni multilayer.
78
This section would not further discussing the effect of magnetic layer thickness
towards the GMR of Co/Cu/Co multilayer. This is because of the previous work by
Chen, 2005, reported that the optimum thickness of Co magnetic layer is 6 nm. This
was supported by work done by Vavassori, 2003, and Turilli, 1999. The results obtained
in Section 4.5.1 and 4.5.2 used the assumption that the optimum magnetic layer
thickness dependence to produce a good GMR value was 6 nm.
4.5.1
Non-magnetic Layer Thickness Dependence
A number of Co(6nm)/Cu(x
nm)/Co(6nm)
multilayers with x = 1 to 15 nm were
prepared using electron beam evaporation method, same as the method used in the
previous section. As discussed in Section 4.4 which shows better GMR value obtained
upon annealing, all of the samples were annealed at 270°C for 2 hours. The GMR
versus Cu thicknesses for each sample is then plotted as in Figure 4.8.
79
Figure 4.8 : The magnetoresistance effect on Cu thicknesses for Co/Cu/Co multilayers.
Figure 4.8 shows the value of GMR decreases monotonically with the increasing
of the non-magnetic (Cu) layer thicknesses. According to Dieny et al., (1991), this
decrease can be qualitatively associated with two contributing factors that are scattering
probability and the shunting current effect. Shunting current refers to current that pass
around another direction in the multilayers. In other words, it is defined as unwanted
short circuit between non-magnetic layer and magnetic layer and this is caused by Cu
layer itself.
According to Childress and Fontana Jr. (2005), the non-magnetic Cu layer
become a major source of shunting current effect unless it is kept as thin as possible
because of its high conductivity. However, as the non-magnetic layer is made thinner,
coupling between the two magnetic layers must be well controlled for proper
magnetoresistance operation. Thus the interfaces between the non-magnetic and
80
magnetic layers, as well as the non-magnetic layer itself need to carefully engineered to
minimize this problem.
With the increasing Cu thicknesses, the probability of scattering increases as the
conduction electron traverses in the Cu layer, which reduces the flow of electrons
between the ferromagnetic layers and consequently reduces the GMR value.
The
increasing thickness of the non-magnetic layer enhances the shunting current within the
non-magnetic layer, which also reduces the GMR.
Dieny et al. (1994) suggested these two contributions to GMR can be described
by the following expression:
R  R 


R  R o
  d NM 

exp
l
 NM  ,
d 
1   NM 
 d0 
(4.1)
where dNM is the non-magnetic layer thickness, lNM is related to the mean free path of
the conduction electron in the spacer layer, do is the effective thickness which depends
on the conductance of the system in the absence of the non-magnetic layer and (∆R/R)o
is the normalization coefficient.
The exponential factor represents the probability that the electrons is not
scattered within the non-magnetic layer. The factor in the denominator describes the
shunting effect due to the non-magnetic layer. The actual result obtained from the
experiment, which is Figure 4.8 was fitted with the result based on Equation 4.1,
81
as shown in Figure 4.9. Both the actual results (dotted line) and the expected result
(straight line) shows an exponential behavior, thus it proved that the result is in good
agreement with work done by Dieny et al. (1994).
20
Magnetoresistance (%)
18
16
14
12
Experiment
10
Equation 4.1
8
6
4
2
0
0
2
4
6
8
10
12
14
16
Cu thicknesses (nm)
Figure 4.9: Cu thicknesses versus magnetoresistance of Co/Cu/Co multilayers in
experiment fitted with the expected result based on Equation 4.1.
4.5.2
Number of Multilayer Dependence
A number of [Co(6nm)/Cu(3nm)]n were prepared with n ranging from 1to 20
multilayer(s). After being annealed at 270°C for two hours, the GMR for each sample
was measured and the result is as shown in Figure 4.10.
82
Figure 4.10 : Number of multilayers repetition versus magnetoresistance for Co/Cu/Co
structures.
From Figure 4.10, it shows increasing trend of GMR as the number of multilayer
repetition increases from 1 to 20. Generally, the improvement in the structural quality
for the thicker multilayers is believed to contribute to the increasing GMR. This is
clearly shown in Figure 4.11 where X-Ray diffraction (XRD) pattern for [Co/Cu]n
multilayers for n = 2, 5, 16 and 20 were compared.
83
Figure 4.11: Two theta XRD patterns for [Co/Cu]n , with n = 2,5,16 and 20. All
samples were annealed at 270°C for two hours.
When the number of the multilayer is increased, the Co (111), Cu (200) and Cu
(220) peaks were found to be narrower. This means that the increase of n leads to the
increase of grain size (X-H, Xu et. al. 2004). The grain size was estimated by the
Scherrer formula from the Co (111) peak width of XRD patterns. The grain size
increases from 7.71 to 16.56 nm as n increases from 2 to 20 as shown in Figure 4.11.
Thus, the increase of n cause the grain size (Co grain) to be larger where larger grain
size leads to the better structural properties of Co/Cu multilayer which contributes to
higher GMR value.
Apart from the improvement of structural properties of Co/Cu multilayer, the
major factor which is responsible for the behavior of GMR versus number of multilayers
is the presence of diffuse scattering at the outer boundaries of the multilayer. The outer
boundary scattering is a very important characteristic that increasing GMR for Co/Cu/Co
84
multilayers because their thickness is comparable to the mean free path of the
conduction electrons in the conduction channel (Plaskett and McGuire,1993). However,
when the multilayer is thick enough (n reaches 100) the GMR seem to saturate and then
decrease.
Plaskett and McGuire found at this thickness and above, diffuse outer
boundary scattering reduces the conductivity of the good conduction channel and hence
negatively effect the GMR.
When the number of n increased, the number of interface is consequently
increased. Since interface scattering is more significant than bulk scattering (Dong,
1999) it is responsible for the total resistivity of Co/Cu multilayer.
The interface
scattering at each Co/Cu interface sum up to the total resistivity of samples which
according to Smadar and Nathan, (2001), increases with the increasing number of Co/Cu
bilayers. When the resistivity of a particular multilayer is high, this resulted to a higher
GMR value because resistivity is directly proportional to GMR value as expressed in
GMR formula.
4.6
Spectroscopy and Surface Morphology Analysis
This section presents the spectroscopy analyses that were done using XRD and
EDX method while surface morphology analysis were done using AFM method. These
analyses were made to compliment the previous results in which only comparing the
magnetoresistance value towards composition, temperature and thickness parameters.
85
4.6.1
X-Ray Diffractometer/ Energy Dispersive X-Ray Analysis
Samples of highest GMR value in temperature dependence analysis were
scanned using XRD technique. After XRD measurement, the samples were undergone
EDX analysis. The samples involved are as listed in Table 4.4 below:
Table 4.4 : Sample description for XRD scans and followed by EDX scans.
Sample no.
Sample structure
Annealing
Annealing
temperature
time
T1(b)
Co(6.5)/Cu(4.0)/Ni(5.5)
280°C
2 hour
T2(c)
Ni(7.5)/Cu(4.0)/Ni(7..5)
290°C
2 hour
T3(e)
Co(6.0)/Ag(5.0)/Co (6.0)
310°C
2 hour
T4(b)
Ni(7.0)/Ag(4.0)/Ni(7.0)
280°C
2 hour
The XRD technique is the primary tool for investigating the structure of the
crystalline material while EDX is a technique use for identifying the elemental
composition of the specimen. Figure 4.12, 4.13, 4.14 and 4.15 show the XRD pattern
and the EDX spectrum for sample T1 (b), T2 (c), T3 (e) and T4 (b) respectively.
86
Figure 4.12 : (a) The two-theta XRD pattern , (b) The EDX spectrum for
Co(6.5)/Cu(4.0)/Ni(5.5).
From the X-ray diffraction (XRD) analysis of Co(6.5)/Cu(4.0)/Ni(5.5), the XRD
pattern as in Figure 4.12 (a) shows significant peaks of Co (111) at 44.22°, Cu (200) at
50.43° and Ni (220) at 51.85°. Those peaks indicates that the Co(6.5)/Cu(4.0)/Ni(5.5)
87
multilayer was of a crystalline structure. As Copper is one of the materials that highly
prone to oxidization, the doubt of CuO being formed in the multilayer disappeared as the
pattern shows no CuO peak which usually appear at 38.72°.
The EDX spectrum for Co(6.5)/Cu(4.0)/Ni(5.5) as in Figure 4.12 (b) shows Co, Ni
and Cu peaks at 6.92 keV, 7.48 keV and 8.08 keV respectively. The percentage of Co,
Ni and Cu are 4.43%, 2.77% and 3.72% respectively. The other elements found are Mg,
Al, Na came from the substrate.
88
Figure 4.13 : (a) The two-theta XRD pattern, (b) The EDX spectrum for
Ni(7.5)/Cu(4.0)/Ni(7.5)
The XRD pattern of Ni(7.5)/Cu(4.0)/Ni(7..5) shows peaks of Ni (111) at 44.5°, Cu
(200) at 50.43° and Ni (220) at 76.37° as can be observed in Figure 4.13 (a). No copper
oxide peak was detected. The multilayer shows a crystalline structure.
89
The EDX spectrum for Ni(7.5)/Cu(4.0)/Ni(7..5) as in Figure 4.13 (b) shows Cu peak
observed at 7.48 keV and Ni peak at 8.08 keV. Strong peak of silicon and oxygen
observed at 1.71 keV and 0.41 keV come from the corning glass which Silicon Oxide is
the major component, together with Na, Mg, Al, K and Ca.
Figure 4.14 (a) shows the XRD pattern for Co(7.0)/Ag(4.0)/Co (7.0) . Peaks of
Co (111) and Ag (220) appear at 44.2° and 64.43° respectively. The sample show a
crystalline structure and no oxidized layer were formed because neither Cobalt Oxide
nor Argentum Oxide observed in the pattern.
From the EDX spectrum for Co(7.0)/Ag(4.0)/Co (7.0) as depicted in Figure 4.14 (b),
Ag peak observed at 2.98 keV and Co peak observed at 6.92 keV. The percentage of Ag
was 2.71% and Co was 9.47%. A relatively higher percentage of Co observed compared
to Ag because the total thickness of Co in the sample was 14 nm while Ag thickness was
only 4 nm.
90
Figure 4.14 : (a) The two-theta XRD pattern, (b) The EDX spectrum for
Co(7.0)/Ag(4.0)/Co (7.0)
91
Figure 4.15 : (a) The Two-theta XRD pattern, (b) The EDX spectrum for
Ni(6.0)/Ag(5.0)/Ni (6.0)
The EDX spectrum for Ni(6.0)/Ag(5.0)/Ni (6.0) as in figure 4.15 (b) shows Ni peaks
observed at 0.93 keV and 7.48 keV while Ag peak observed at 2.98 keV.
percentage of Ni and Ag are 9.73% and 3.57% respectively.
The
92
Generally, samples that exhibit GMR shows quite a crystalline structure. This is
indicated by significantly low and broad peaks on the two-theta XRD pattern in Figure
4.12 (a), 4,13 (a), 4.14 (a) and 4.15 (a). These patterns of spectra are common for thin
films. When the number of the multilayer increased, the total thickness of the thin film
also increased and the GMR should be higher according to the resistor model. In this
condition, the thin film properties would eventually change into bulk material properties
and the peaks should be eventually higher and narrower. As a supporting evidence to
this statement, in Figure 4.11, the two-theta XRD pattern for [Co(6nm)/Cu(3nm)]20 shows
higher and narrower Cu and Co peaks compared to the two-theta XRD pattern for
[Co(6nm)/Cu(3nm)]2.
Thus, sample with higher number of multilayer shows better
crystalinity and higher value of GMR.
The EDX spectra as demonstrated in Figure 4.12 (b), 4.13 (b), 4.14 (b) and 4.15
(b), show relatively small percentage of Co, Cu, Ni and Ag compared to Silicon and
Oxygen. This is due to the electron beam’s deep penetration properties. Electron
penetration distance into corning glass substrate, which is greater than the multilayer
thickness give strong signals or peaks of corning glass materials which are Silicon
Oxide, Na, Mg, Al, K and Ca. If the number of the multilayer increased, according to
resistor model, the GMR should increase. The increasing number of multilayer would
increase the Co, Cu, Ni and Ag percentage in the spectra.
93
4.7
Surface Morphology Analysis
The surface morphology analysis was done using Atomic Force Microscope
(AFM).
The purpose of the analysis was to study the surface morphology of
ferromagnetic material and nonmagnetic material on increasing annealing temperature.
Cobalt (Co) represented as ferromagnetic material while Copper (Cu) represented the
non magnetic material in surface morphology studies of ferromagnetic multilayer
structure. Since AFM probe onto the sample surface, the analysis was carried out by
probing only on single layer of Co and Cu.
The roughness of the film in terms of root mean square (RMS), indicating the
mean of the root for deviation of a surface with respect to a perfect surface. The RMS
roughness, RMS,  RMS , is defined as:
N
 RMS 
 (Z
i 1
i
 Z ave ) 2
N
(4.1)
where Z i is the height value of each point, Z ave is the average surface height and N is the
number of points on the indicated surface (Sans et. al., 2004). The mean diameter of
grain was obtained by assuming that the grain takes a circular shape.
Figure 4.16 shows the three dimensional AFM topographic images of Co thin
film. It appears that the surface roughness of Co thin film increase from the as deposited
film stage as the annealing temperature increased up to 350°C. The surface roughness
was 0.568 nm at as deposited stage. The value increases to 1.973 nm, 2.151 nm, 2.525
nm and 4.317 nm when the Co films were annealed to 200°C, 250°C, 280°C and 350°C
respectively. The mean grain diameter at the as deposited stage was 5 nm. The mean
94
grain diameter increases to 29 nm, 34 nm, 37 nm, and 42 nm as the annealing
temperature increased to 200°C, 250°C, 280°C and 350°C respectively. The average
grain diameter and the surface roughness were depicted in Figure 4.17. As multilayers
containing Co as ferromagnetic layer show high GMR value when the annealing
temperature exceeded 280°C, as shown in Figure 4.6 (a) and Figure 4.6 (c), this
suggested that the optimum surface roughness and the optimum grain diameter for
systems containing Co were 2.225 nm and 37 nm respectively.
However, it has been shown by Joyce et al. (1998) that RMS roughness is not a
satisfactory indication of the interfacial structure. Joyce suggested that the interfacial
structure must be examined more closely by taking diffuse reflectivity into account.
In a different method, Rozenberg et al. (1999) in the X-Ray investigation of
sputtered Co/Cu specimens found that a simple geometrical factor, namely, the
difference in mean distance between the magnetrons and the surface of the samples,
leads to noticeable changes in their structural parameters that are bilayer period,
individual thickness of Co and Cu layers, and thickness fluctuations. The increase
interfacial roughness may induce local contacts of neighboring Co layers, which
decrease drastically the absolute value of magnetoresistance.
95
a)
c)
b)
d)
e)
Figure 4.16 : AFM topography images of as deposited Co film (a), and Co films annealed
at 200°C (b), 250°C (c), 280°C (d) and 350°C (e).
96
Figure 4.17: RMS roughness and average grain diameter of Co thin films as a function
of annealing temperature.
Figure 4.17 shows the surface roughness and the average grain diameter of Co
thin films when the annealing temperature increases. The surface roughness increased
with increasing annealing temperature. However, referring to previous results in Figure
4.6 (a) and (c), when the GMR dropped from the maximum values which occur at
around 280°C-310°C there is no significant relation between the surface roughness and
the degradation of GMR.
Figure 4.18 shows the three dimensional AFM topographic images of Cu thin
film. The surface roughness of Cu thin film increases from the as deposited stage as the
annealing temperature increase up to 350°C. Generally the increasing roughness and
grain diameter follow the same trend as Co thin films. The surface roughness was 0.676
nm at as deposited stage. The value increases to 2.132 nm, 2.435 nm, 2.861 nm and
97
4.617 nm when the Cu films were annealed to 200°C, 250°C, 280°C and 350°C
respectively, depicted as in Figure 4.19. The mean grain diameter at the as deposited
stage was 6 nm. The mean diameter increases to 27nm, 36 nm, 44 nm, and 40 nm as the
annealing temperature increases to 200°C, 250°C, 280°C and 350°C respectively.
Figure 4.18 shows the AFM topography images of Cu films and Figure 4.19
shows the mean grain diameter and surface roughness for Cu thin films. These results
are associated with multilayers containing Cu as the nonmagnetic layer which are
Co/Cu/Ni and Ni/Cu/Ni.
Co/Cu/Ni multilayers show high GMR value when the annealing temperature
reached 280°C while Ni/Cu/Ni multilayers show highest GMR value at 290°C. This
suggests for multilayers containing Cu as non-magnetic layer, approximately 280°C to
290°C were the optimum annealing temperature. From Figure 4.19, it suggested the
optimum surface roughness and average grain diameter for Co/Cu/Ni and Ni/Cu/Ni
multilayer are 3.04nm and 44nm respectively. As the surface roughness and grain
diameter increases with increasing annealing temperature, however, there is no
significant relation between the degradation of GMR at temperature above 290°C with
surface roughness and grain diameter.
e)
a)
a)
98
a)
b)
c)
d)
e)
Figure 4.18 : AFM topography images of as deposited Cu film (a), and Cu films annealed
at 200°C (b), 250°C (c), 280°C (d) and 350°C (e).
99
Figure 4.19: RMS roughness and average grain diameter of Cu thin films as a function
of annealing temperature.
CHAPTER 5
SUMMARY AND CONCLUSION
5.1
Summary and Conclusion
Ferromagnetic multilayer structures were successfully fabricated using the
electron beam evaporation method. The magnetoresistance of the multilayer structures
with different ferromagnetic and nonmagnetic composition, different layer thicknesses
and different annealing temperatures were also successfully being carried out.
The compositional study of ferromagnetic multilayer structures reveals that band
matching and lattice matching are two important factors need to be taken into account
prior to match a ferromagnetic material with a non-magnetic conducting material.
Further insight into material band structure and density of states are important to
determine whether or not the establishment of a good band matching between a
ferromagnetic material and non-magnetic material. Generally a good band matching
101
between ferromagnetic and non-magnetic metal lead to high spin transmission across the
ferromagnetic/nonmagnetic interface and a large band mismatch between ferromagnetic
and non-magnetic metal implies a poor spin transmission. A good band matching
between two materials (in this case between ferromagnetic material and non-magnetic
material interface) is determined when this two materials have almost the same
electronic band structure and density of states properties.
Lattice matching between two adjacent ferromagnetic and non-magnetic
materials plays important role in obtaining high GMR value. A good lattice matching
favors high spin transmission at the interface while a poor lattice matching (lattice
mismatch) leads to the structural defects due to interface misfit dislocation.
The
formations of structural defects at the interface causes a poor spin transmission and thus
lower the GMR value.
The key parameter need to be taken into account for the
ferromagnetic and non-magnetic materials is the lattice parameter, a. A perfect lattice
matching is when the lattice parameter of two adjacent ferromagnetic and non-magnetic
layers is almost or the same value.
Temperature dependence study proves that heat treatment or annealing to
ferromagnetic multilayer structures provides better GMR value if compared to as deposit
multilayers.
The GMR increases with increasing annealing temperature until the
temperature reaches a point called the ‘blocking temperature’. When the annealing
temperature exceeds the blocking temperature, the GMR shows a dramatic decrease.
Multilayers involved in these studies which are Co/Cu/Ni, Ni/Cu/Ni, Co/Ag/Co and
Ni/Ag/Ni reached their blocking temperature at 280°C, 290°C, 310°C and 280°C
respectively. Diffusion and intermixing at the ferromagnetic/non-magnetic interface are
two main reasons of the dramatic degradation of GMR.
102
Thickness dependence study in ferromagnetic multilayer structures is divided
into two thickness parameters which are non-magnetic layer thickness dependence and
number of multilayer dependence. In non-magnetic layer thickness dependence study,
the results obtain shows GMR decreases in an exponential behavior as non-magnetic
layer thickness increases. The decreases in GMR were associated with the scattering
probability and the shunting current effects.
When the number of multilayer was increased, the GMR value was also
increased. In stacking multilayers, according to the resistor model, the GMR are higher
than in single multilayers because stacking the multilayers is equivalent to adding up
more resistors in series to a circuit, thus give higher resistance.
In addition, the
improvements in structural quality in thicker multilayers contributed in increasing GMR
value. Apart from structural improvement and the resistor model, another important
factor that contributes to enhance the GMR was the presence of diffuse scattering at the
outer boundaries of the multilayer.
Confirmation analyses were made using EDX, XRD and AFM technique. The
EDX study for Co/Cu/Ni, Ni/Cu/Ni, Co/Ag/Co and Ni/Ag/Ni multilayer proves the
presence of Co, Cu, Ni and Ag elements in the EDX spectra. The XRD analyses also
show the presence of Co, Cu, Ni and Ag peaks in the XRD spectra. The surface
morphology analysis using AFM shows the surface roughness and grain diameter of Co
and Cu increased as the annealing temperature increased.
In conclusion, the objectives of this study were successfully achieved. It was
shown that ferromagnetic multilayer structures fabricated using electron beam
evaporation method exhibit considerably high GMR values after taking into
103
consideration the composition effects, the annealing temperature effects and the
thickness effects prior to measuring the GMR.
5.2
Suggestions
This thesis has initiated the effects of composition, annealing temperature, nonmagnetic layer thickness
as
well
as
the
overall
multilayer
thickness
on
magnetoresistance of ferromagnetic multilayer structures namely Co/Cu/Ni, Ni/Cu/Ni,
Co/Ag/Co and Ni/Ag/Ni. Magnetic materials involved are Co and Ni while Cu and Ag
are the non-magnetic materials. However, a more comprehensive study remains to be
carried out.
As a suggestion for future studies, instead of using single ferromagnetic material
like Co and Ni as the magnetic layer, using alloy such as permalloy (Ni80Fe20) as
magnetic layer could be carried out. A promising increment in annealed permalloy
multilayers had been carried out by Kitada Masahiro and Yamamoto Kazuhiro, (1995).
Permalloy has a high magnetic permeability, low coercivity, near zero magnetostriction
and significant anisotropic magnetoresistance. The low magnetostriction is critical for
industrial applications, where variable stresses in thin films would otherwise cause a
ruinously large variation in magnetic properties.
104
GMR measurement in this study involved the application of external magnetic
field of maximum 2000 gauss only. For further investigation, a stronger magnetic field
in the order of Tesla could be applied to the sample. Large magnitude of external
magnetic field probably could enhance the reversible or irreversible magnetization of
magnetic layers and would probably enhance the magnetoresistance or otherwise.
Since the magnitude of GMR is related to the asymmetry in the scattering rates
within the two conduction channels (within magnetic/non-magnetic interfaces), it was
expected that modifying the spin-dependent scattering by introducing appropriate
impurities either at the interfaces or in the bulk of the ferromagnetic layers would
enhance GMR. Further investigation on introducing appropriate impurities into the
multilayers should be carried out in order to study the impurity dependence towards
GMR behavior. Significant interactive effect between impurities and the GMR of CoFe/Cu multilayers were revealed by Masakiyo Tsunoda et.al, (2002).
As the thickness of non-magnetic layers (non-magnetic with high conductivity
metals, referring to Cu and Ag in this research) play important role in obtaining high
GMR value, careful and precise control to produce a thin layer with minimum roughness
need to introduced. For future work, controlling the non-magnetic layer properties can
be done through oxygen plasma exposure on the top of the reference layer and/or lowpressure oxygen exposure during the growth of the non-magnetic layer. Peterson et.al,
(2003), demonstrated a better non-magnetic layer quality with oxygen assisted growth in
magnetic multilayers.
105
It was interesting to observe ‘ageing magnetoresistance’ in iron-bismuth thin
films as reported by Hsu, (2005). According to Hsu, due to precipitation process, the
magnetoresistance characteristics change with time. Ordinary magnetoresistance was
observed initially but the magnetoresistance evolves into a negative and isotropic
magnetoresistance, like GMR.
As another suggestion for future work, the time
dependence magnetoresistance should be carried out to find out how the magnetic
properties in Co/Cu/Ni, Ni/Cu/Ni, Co/Ag/Co and Ni/Ag/Ni change with time.
By implementing the suggestions above, hopefully it would increase the
understanding of spin dependent scattering in ferromagnetic multilayer structures. The
physics of GMR in metallic layered structures is so multifaceted that it will undoubtedly
remain the subject of great interest in near future.
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APPENDIX A
Photographs of Characterization Instruments
1. Dektak3 Surface Profiler
2. Carl Zeiss Supra 35VP Field Emission Scanning Electron Microscope/EDX
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3. PANanalytica X’pert PRO Materials Research X-ray Diffractometer
3. Seiko SPI 3800N Atomic Force Micrope