Metallic Thin Films Revisited: Fe,
Co, Ni Multilayers
Spintronics
A. Kellou and H. Aourag
Metallic Thin Films Revisited: Fe, Co, Ni Multilayers
Spintronics
Spintronics: To Control a Spin of Electrons, not a Charge
Magnetic Nanostructures for Spintronics
 Magnetic Multilayers
 Magnetic Wires
 Magnetic Quantum Dots
Applications of Magnetic Nanostructures
 Reading Heads, Magnetic Field Sensors, MRAM
 Field Effect Transistor, Spin-Valve Transistor
 Quantum Computer
Basic Structure
The prototype device that is already in use in industry
as a
read head
memory-storage cell
is the
giant-magnetoresistive (GMR) sandwich structure
which consists of alternating
ferromagnetic and nonmagnetic metal layers.
Basic Structure
Depending on the relative orientation of the magnetizations
in the magnetic layers,
the device resistance changes from
small (parallel magnetizations)
to large (antiparallel magnetizations).
This change in resistance (also called magnetoresistance)
is used to sense changes in magnetic fields
Basic Structure
Basic Structure
two different approaches:
1) existing GMR-based technology
- developing new materials with
larger spin polarization of electrons
-
making improvements or variations in the existing device
that allow for better spin filtering.
2) finding novel ways of both generation and
utilization of spin-polarized currents.
Basic Structure
Problems:
existing metal-based devices do not amplify signals
(although they are successful switches or valves),
whereas semiconductor based spintronic devices
could in principle provide amplification and serve,
in general, as multi-functional devices.
spin polarizers and spin valves
Magnetic Random Access Memory
(MRAM)
Low Resistance
Reversible
High Resistance
Issues in Magnetic Multilayers
 Fabrication of Ordered Nanostructures on a Surface
 A detailed understanding of the various atomic processes
that occur during the formation of nanosized islands on surfaces
 Surfaces are not simply a static media onto which the
deposited atoms and diffuse
Deposition and nucleation on a surface is important
29
III. Applications:
ii) binary alloys
FeCr, CoCr, and NiCr:
Structural and magnetic properties
-4647,27
-4647,28
-4888,61
NM
FM
AF
FeCr
-4888,62
-4647,29
-4888,63
-4647,30
-4888,64
NM
FM
AF
CoCr
-5143,32
NM
FM
AF
NiCr
-4647,31
-4647,32
-4647,33
Energy (Ry)
Energy (Ry)
Energy (Ry)
-5143,34
-4888,65
-4888,66
-5143,38
-4888,67
-4647,34
-5143,36
-4888,68
-5143,40
-4647,35
120
130
140
150
160
170
180
130
3
140
150
160
170
180
120
130
140
150
160
170
3
Volume (a.u. )
3
Volume (a.u. )
Volume (a.u. )
a
B
E
MX
MCr
M
FeCr
Theory nm
fm
af
5.324
5.381
5.377
216
231
232
-4647.3167
-4647.3330
-4647.3390
1.68
0.37
2.01
CoCr
Theory nm
fm
af
5.360
5.409
5.425
189
224
179
-4888.6493
-4888.6847
-4888.6576
1.25
-
1.59
-
2.98
-
NiCr
Theory nm
fm
af
5.453
5.489
5.511
213
239
217
-5143.3630
-5143.3951
-5143.3953
0.60
2.39
3.29
180
190
200
30
III. Applications:
iii) Ternary alloys
Semi-Heusler alloys
•Half-metallic materials possess 100% electron polarization at the Fermi
energy.
•New class of magnetic materials displaying metallic character for one
electron spin population and insulating character for the other.
•Technological interest as potential pure spin sources for use in spintronic
devices, data storage applications, and magnetic sensors.
•Difficult to confirm experimentally the half-metallicity charcter (clean
stoichiometric surfac).
To known if the intermettallic alloys based on a ferromagnet -Ti -Cr can lead
to a half-metallicity behavior.
31
III. Applications:
iii) Ternary alloys
Semi-Heusler alloys
Ground states from total energy
calculations
• FeCoTi, CoTiCr, NiTiCr, and FeCoNi are predicted ferromagnetic.
• FeNiTi, FeNiCr, FeTiCr, and FeCoCr and are predicted antiferromagnetic.
• FeCoCr and FeNiCr are nonmagnetic.
-7040,030
-7434,15
-7294,750
NM
FM
FeCoTi
-7040,035
-8373,96
NM
FM
FeCoCr
NM
FM
FeNiTi
-7294,755
-7434,16
NM
FM
FeCoNi
-8373,98
-7294,760
-7040,050
-7040,055
Energy (Ry)
Energy (Ry)
Energy (Ry)
-7294,765
-7040,045
-7294,770
-7294,775
-7294,780
-7434,17
-7434,18
-7434,19
-7294,785
-7040,060
Energy (Ry)
-7040,040
-7294,795
220
250
260
270
280
290
300
310
320
230
240
330
270
280
290
300
260
270
280
290
210
220
230
240
250
260
270
280
290
300
3
Volume (a.u )
Volume (a.u )
310
250
3
3
260
-8374,04
-8374,08
-7434,20
-7040,070
-8374,02
-8374,06
-7294,790
-7040,065
-8374,00
Volume (a.u. )
3
Volume (a.u )
-7688,85
FeTiCr
-6354,85
Energy (Ry)
Energy (Ry)
-6596,195
-6596,200
-6596,205
-6596,210
-6596,215
-6354,86
-6354,87
270
280
290
300
310
320
3
Volume (a.u. )
330
340
350
250
260
270
280
290
300
310
3
Volume (a.u. )
320
330
340
NM
FM
-7688,87
-7688,88
-6850,896
-6850,904
-6850,912
-6850,920
-6850,928
-7688,91
-6354,88
260
NiTiCr
-7688,90
-6596,225
-6850,880
-6850,888
-7688,89
-6596,220
250
NM
FM
-7688,86
NM
FM
Energy (Ry)
CoTiCr
FeNiCr
-6354,84
NM
FM
Energy (Ry)
-6596,190
210
220
230
240
250
260
3
Volume (a.u. )
270
280
290
260
270
280
290
300
310
320
3
Volume (a.u. )
330
340
350
32
III. Applications:
iii) Ternary alloys
Semi-Heusler alloys
Total DOS
• All alloys are polarized except FeNiCr and CoTiCr.
• FeCoTi, FeNiTi, and NiTiCr have a majority spin in a deep minimum right
the Fermi level, leading to a pseudo-gap which is responsible for 100%
electron polarization.
80
80
40
0
-40
-80
-120
0,2
0,3
0,4
0,5
0,6
0,7
Up
160
FeCoNi
Up
0
-40
-80
-120
Dn
0,1
0,2
0,3
Energy (Ry)
0,4
0,5
0,6
0,7
0,8
40
0
-40
-80
-120
FeTiCr
Up
160
120
0,2
0,3
0,4
0,5
0,6
0,7
160
Up
120
120
80
80
-80
-120
-160
Dn
-200
0,1
0,2
0,3
0,4
0,5
0,6
Energy (Ry)
0,7
0,8
0,9
40
0
-40
-80
-120
-160
Dn
-200
0,2
0,3
0,4
0,5
Energy (Ry)
0,6
0,7
0,8
0,3
0,4
0,5
0,6
0,7
160
CoTiCr
Up
40
0
-40
-80
-120
NiTiCr
Up
120
80
40
0
-40
-80
-120
-160
-160
Dn
Dn
-200
0,1
0,1
0,2
0,3
0,4
0,5
0,6
Energy (Ry)
0,8
200
-200
0,1
0,2
Energy (Ry)
DOS (states/spin)
-40
DOS (states/spin)
DOS (states/spin)
DOS (states/spin)
0
Dn
0,1
0,8
80
40
-80
-120
-200
Dn
0,1
FeNiCr
0
-40
-200
Energy (Ry)
160
40
-160
-160
Energy (Ry)
Up
80
80
-200
0,8
FeCoCr
120
120
40
-160
Dn
0,1
FeNiTi
DOS (states/spin)
120
160
200
160
Up
DOS (states/spin)
FeCoTi
120
DOS (states/spin)
DOS (states/spin)
160
0,7
0,8
0,9
0,2
0,3
0,4
0,5
Energy (Ry)
0,6
0,7
0,8
33
III. Applications:
iii) Ternary alloys
Heusler alloys
•
Stoichiometric composition X2YZ
• Electronic structure can range from
metallic to semi-metallic or
semiconducting behavior.
• Half-metallic ferromagnetism, in which
the bandstructure for majority electrons
is metallic while the bandstructure for
minority electrons is insulating.
• Anomalous peak in the yield stress and
high temperature strength and excellent
oxidation and corrosion resistance.
34
III. Applications:
iii) Ternary alloys
Heusler alloys
•
All alloys are ferromagnetic, except Co2AlTi and Ni2AlTi (paramagnetic).
• Large magnetization in Cr alloys .
a
(paramètre du reseau)
MX
MAl
MX’
Fe2AlTi
nm
fm
11.005
11.014
11.115a
0.62
-0.01
-0.19
Co2AlTi
nm
fm
11.019
11.005
11.058b
0.37
0.34b
-0.00
-0.05
-0.05c
Ni2AlTi
nm
fm
11.136
11.136
10.926a
+0.00
+0.00
-0.00
Fe2AlCr
nm
fm
10.685
10.684
0.08
0.00
-1.04
Co2AlCr
nm
fm
10.758
10.797
11.134b
0.69
0.78b
-0.03
1.61
1.60e
Ni2AlCr
nm
fm
10.841
10.960
0.26
-0.03
2.39
35
III. Applications:
iii) Ternary alloys
Heusler alloys
Lattice parameters and bulk modulii
• Cr has induced a volume contraction although Z(Ti) < Z(Cr).
• This fact is due to changes in bonding.
• Cr has allso induced large bulk modulii except ofr Ni2AlCr (large magnitzation,
hgh volume)
11.3
Ti
Cr
11.2
240
Ti
Cr
a (Angöstrom)
a (Angöstrom)
220
11.1
11.0
10.9
10.8
10.7
10.6
200
180
160
140
Fe
Co
Element
Ni
Fe
Co
Element
Ni
36
III. Applications:
iii) Ternary alloys
Heusler alloys
Total DOS
• Cr has induced Fermi displacement to the right (anti-bonding states) with a
prounounced half-metallicity character in Fe2AlCr and to the left in Co2AlCr and
Ni2AlCr.
120
120
100
100
Fe2AlTi
80
40
20
0
-20
-40
-60
-80
-100
80
60
DOS (States/Spin)
DOS (States/Spin)
60
100
40
20
0
-20
-40
-60
0.0
0.2
0
-20
-40
-60
-80
-100
-120
-0.6
-0.2
20
-100
-120
-0.4
40
-80
-120
-0.6
Ni2AlTi
60
DOS (States/Spin)
80
120
Co2AlTi
-0.4
0.4
-0.2
0.0
0.2
0.4
-0.6
-0.4
E-EF (Ry)
E-EF (Ry)
-0.2
0.0
0.2
0.4
E-EF (Ry)
120
100
80
100
Fe2AlCr
80
DOS (States/Spin)
DOS (States/Spin)
60
40
20
0
-20
-40
-60
120
Co2AlCr
100
60
80
40
60
DOS (States/Spin)
120
20
0
-20
-40
-60
-80
-80
-100
-100
-0.6
-0.4
-0.2
E-EF (Ry)
0.0
0.2
0.4
40
20
0
-20
-40
-60
-80
-100
-120
-120
Ni2AlCr
-0.6
-0.4
-0.2
E-EF (Ry)
0.0
0.2
-120
-0.6
-0.4
-0.2
E-EF (Ry)
0.0
0.2
37
III. Applications
i) Transition element family
ii) Binary systems
iii) Ternary systems
iv) Layered structures



Clean V(001), Cr(001) and Fe (100) surfaces
TM/5Cr(001) (TM = Ti, V, Cr, Mn, Fe, Co, Ni)
Fe/Cr(001) systems
38
III. Applications:
iv) Layered structures
properties (GMR, MAE, high local moments …) when
ferromagnetic and antiferromagnetic transition elements are layered.
 Interesting
 Determination of interlayer exchange coupling (IEC).
 Effect of magnetism in surface, interface, and superlattices phenomena
 Ferromagnetic substrates are well studied: Cu(001), Ag(001), Au(001),
but not antiferromagnetic Cr !!!
Fe(001), Co(001) …
Z
Vacuum
Vacuum
The unit cell in film calculations.
39
III. Applications:
iv) Layered structures
Clean V(001), Cr(001), and Fe(001) surfaces
 Surface magnetism in the (001) direction: nonmagnetic V, antiferromagnetic
Cr, and ferromagnetic Fe. 5-layers of V(001), Fe(001) and Cr(001) in repeated
slab structure.
Magnetism occurs in V and is enhanced in Cr and Fe (001) surfaces
because of the lying bonds (coordination number).
M3 (Surface)
M2 (Sub-surface)
Z=0
M1 (Central)
M1
M2
M3
BULK
V(001)
-0.17
-0.08
0.67
0.00
Fe(001)
2.53
2.42
3.02
2.26
Cr(001)
1.21
-1.56
2.62
± 0.77
40
III. Applications:
iv) Layered structures
TM on 5-Cr(001) layers (TM = Ti, V, Cr, Mn, Fe, Co, Ni)
 Several theoretical and experimental studies were devoted to the surface
properties of the magnetic 3d transition metal grown on noble metal (Cu, Ag,
and Au) and ferromagnetic (Fe, Co, and Ni) but not Cr(001).
 Study of total and surface energies of Cr(001) films, magnetic, and
electronic properties of 3d transition-metal (Ti, V, Cr, Mn, Fe, Co, Ni)
monolayer on Cr(001), with two opposite spin orientations leading to
ferromagnetic and antiferromagnetic configurations.
TM
Cr(1)
Cr(2)
Cr(3)
(a) 3-Cr(001)
(b) 5-Cr(001)
(c) TM/5-Cr(001)
41
III. Applications:
iv) Layered structures
TM on 5-Cr(001) layers (TM = Ti, V, Cr, Mn, Fe, Co, Ni)
Difference in total energy
TM
Cr (S)
Ti, V, Cr
Fe, Co, and Ni
ferromagnetic
coupled
antiferromagnetic
coupled
Nothing about Mn (ferrimagnetic coupled ???!)
15.0
E=EFM-EAFM (mRy/atom)
Antiferromagnetic
14.7
14.4
14.1
2
1
0
-1
-2
Ferromagnetic
Ti
V
Cr
Mn
Fe
Element
Co
Ni
42
III. Applications:
iv) Layered structures
TM on 5-Cr(001) layers (TM = Ti, V, Cr, Mn, Fe, Co, Ni)
Transition metal and total magnetic moment
 TM’ s magnetic moment increases from Ti to Mn and decrease from Mn to
Ni, in both ferromagnetic and antiferromagnetic configurations.
 Mn deposition induces the highest value, followed by Fe, Co, and Ni.
Total magnetic moment has the same behavior as TM magnetic moment.
3
FM
AFM
3
FM
AFM
2
2
1
M (B)
MTM (B)
1
0
-1
0
-1
-2
-2
-3
-3
Ti
V
Cr
Mn
Element
Fe
Co
Ni
Ti
V
Cr
Mn
Element
Fe
Co
Ni
43
III. Applications:
iv) Layered structures
TM on 5-Cr(001) layers (TM = Ti, V, Cr, Mn, Fe, Co, Ni)
Spin Density Waves in Cr thin films
The periodic nature the oscillations in 7-Cr(001) is strongly related to the
itinerant linear Spin-Density Waves (observed in Cr multilayers, bulk Cr and
its alloys.
Cr thin films need SDW to have antiferromagnetic ground state.
3,0
2,5
2,0
1,5
1,0
M ()
0,5
0,0
-0,5
-1,0
-1,5
-2,0
-2,5
-3,0
Cr4
Cr3
Cr2
Cr1
Layer
Cr2
Cr3
Cr4
44
III. Applications:
iv) Layered structures
TM on 5-Cr(001) layers (TM = Ti, V, Cr, Mn, Fe, Co, Ni)
 Several theoretical and experimental studies were devoted to the surface
properties of the magnetic 3d transition metal grown on noble metal (Cu, Ag,
and Au) and ferromagnetic (Fe, Co, and Ni) but not Cr(001).
 Study of total and surface energies of Cr(001) films, magnetic, and
electronic properties of 3d transition-metal (Ti, V, Cr, Mn, Fe, Co, Ni)
monolayer on Cr(001), with two opposite spin orientations leading to
ferromagnetic and antiferromagnetic configurations.
TM
Cr(1)
Cr(2)
Cr(3)
(a) 3-Cr(001)
(b) 5-Cr(001)
(c) TM/5-Cr(001)
45
III. Applications:
iv) Layered structures
Fe/Cr(001) systems
 Study of the diffusion, the surface alloy formation, and the magnetic properties in
Fe/Cr(001) systems and magnetic properties of Fen/Crn(001) superlattices.
 Fe/Cr multilayer exhibit interlayer exchange coupling (IEC), giant magnetoresistance (GMR), …etc.
Experimental results, obtained by similar techniques, often contradict each another
and theoretical calculations also demonstrated a very complex behavior and solutions
with close energies.
Cr
Fe
I3
I2
I1
(a) 4Cr(001)
(b) 1Fe/3Cr(001)
(c) 2Fe/2Cr(001)
I
(d) Fe50Cr50/3Cr(001)
(e) 1Fe/Fe50Cr50/2Cr(001)
Fig. 5.24 Upper half-slab of the unit cell in: (a) 4Cr(001), (b) 1Fe/3Cr(001), (c)
2Fe/2Cr(001), (d) Fe50Cr50/3Cr(001), and (e) 1Fe/Fe50Cr50/2Cr(001). The first layer (I)
corresponds to central layers.
46
III. Applications:
iv) Layered structures
Fe/Cr(001) systems
Total energies and total and partial magnetic moments
bulk
bulk
E f  EStruct  (nFe EFe
 nCr ECr
)
Ef (Ry/atom)
M1 (B)
M2 (B)
M3 (B)
M4 (B)
M (B)
Bulk Fe
-
+2.32
-
-
-
+2.25
Bulk Cr
-
+0.77
-0.77
-
-
+0.00
2Cr(001)
++
+-
79.07
79.09
-1.82
+1.84
+2.84
-2.82
-
-
+4.52
-4.56
3Cr(001)
+++
+-+
47.22
47.24
+1.15
+1.17
-1.54
-1.53
+2.64
+2.62
-
+2.94
+3.94
1Fe/2Cr
++/+
+-/+
++/+-/-
43.61
42.78
43.05
43.07
+0.28
+0.85
-0.65
-0.47
+0.31
-0.81
+0.54
+0.54
+2.54
+2.51
-2.52
-2.60
-
+6.19
+4.45
-4.14
-4.67
1Fe/3Cr
+-+/+
+-+/-
29.46
29.72
-1.07
+0.83
+1.05
-0.84
-0.94
+0.80
+2.55
-2.52
+4.39
-4.56
2Fe/2Cr
+-/++
+-/++-/-+
+-/--
24.62
37.35
37.86
24.69
+0.62
+0.67
-0.35
-0.18
-0.74
-0.53
+0.16
+0.27
+2.03
+1.68
-1.51
-1.98
+2.95
-2.75
+2.77
-2.96
+9.06
-2.91
+2.81
-9.05
47
III. Applications:
iv) Layered structures
Fe/Cr(001) systems
Bilayer formation against the monolayer formation
E MB  E1Fe / 3Cr ( 001)
1
 ( E 2 Fe / 2Cr ( 001)  E 4Cr ( 001) )
2
 This energy is positive (+0.54 mRy/unit cell) in the ferromagnetic
state and negative (-8.10 mRy/atom) in the nonmagnetic state.
 This means that magnetic moments allow BL formation (2Fe/2Cr(001)),
whereas nonmagnetic state favors ML formation (1Fe/3Cr(001)).
 This result contradicts the description which was discussed for Cr (ML)
on Fe(001) substrate, where ML formation is preferred for the ferromagnetic
configuration.
48
III. Applications:
iv) Layered structures
Fe/Cr(001) systems
Diffusion and surface alloy formation against phase separation
 Fe do not diffuse to Cr bulk layers.
 No magnetism favors phase separation or clustering, whereas magnetism
favors formation of Fe50Cr50/3Cr(001) followed by Fe/Fe50Cr50/3Cr(001)
ordered surface alloys (confirmed in recent experimental study).
Ef
M (B)
(Ry/atom)
1Fe/3Cr
nm
fm
1Cr/1Fe/2Cr nm
fm
2Cr/1Fe/1Cr nm
fm
39.36
29.46
51.45
37.36
51.92
41.11
E SA  E alloy  ( xECOV  (1  x) EUNC )
ESA
+4.39
M (B)
(mRy/atom)
+7.96
Fe50Cr50/3Cr(001)
nm
fm
-1.30
+3.80
+5.39
+0.19
1Fe/Fe50Cr50/2Cr(001)
nm
fm
-0.11
+2.51
+14.38
50
III. Applications:
iv) Layered structures
Fe/Cr(001) systems
Fen/Crn(001) superlattices
 The formation energy is stabilized after n = 4.
 The total magnetic moment is growing with the number of Fe and Cr
layers.
 Total energies favor the following spin alignments: +/+, ++/--, +++/+-+,
++++/-+-+, +++++/-+-+-.
15
E
M
9,0
10
7,5
M ()
5
4,5
3,0
0
1,5
-5
0,0
1
2
3
n
4
5
E (mRy/atom)
6,0
51
V. Conclusion
We have given additional results to structural, electronic, and magnetic
properties the selected transition materials (Ti, V, Cr, Mn, Fe, Co, and Ni)
and their related systems; binary alloys, ternary alloys in Half-Heusler and
Heusler structures, thin films and superlattices.
We have shown the importance of d-states in the ground state properties
in these systems.
We have also studied the equilibrium parameters and the stability
mechanism from the different formation energies and from the position of
the Fermi level in the density of states.
The new form of the GGA approximation is adequate for transition metals
and their related alloys.
The obtained structural properties are in good agreement with
experimental data and more efficient than LDA ones.
52
V Conclusion
Binary alloys
In the binary systems XTi and XCr (X=Fe, Co, Ni),
effects of magnetism is studied and related to the
structural and electronic structures.
The martensitic transformation (MT) phenomena of
NiTi have been studied and optimized lattice
parameters for B19’ were given.
The different roles of d-states were highlighted and
are totally responsible for unexpected and
controversial behaviors.
53
V Conclusion
Ternary alloys
Structural parameters, formation energies, magnetic moments, and
electronic properties of XYZ Half-heusler and X2AlX’ Heusler alloys (X=Fe,
Co, Ni; X’=Ti, Cr) were presented.
The obtained results of lattice parameters and local magnetic moments
agree very well with the experimental results.
Cr sites carry large magnetic moments and the moments at the X sites are
usually small, when compared to Ti substitution.
All the densities of states are marked by a pseudogap left the Fermi level,
except for Fe2AlTi where the pseudogap is right EF.
Among the selected materials, the Fe2AlCr and Co2AlCr alloys present a
pronounced half-metallicity character.
54
V Conclusion
Layered structures
The existence of itinerant linear Spin-Density Wave (SDW) is responsible
for antiferromagnetic coupling between two adjacent Cr layers in Cr(001).
Mn overlayer induces the highest magnetic moments and relies between
two opposite spin alignments in TM/Cr(001). Ferrimagnetic (FI) coupling
can occur. Further investigations within the c(2x2) unit cell are necessary.
Ti, V, and Cr overlayers are antiferromagnetically coupled to the Cr subsurface layer; Mn, Fe, Co and Ni are ferromagnetically coupled.
Fe layers are always antiferromagnetically coupled to Cr layers in Fe/Cr
systems.
Fe atoms prefer to be deposited as an overlayer rather than being diffused
in the Cr layers with formation of an ordered surface alloy.
Magnetism is responsible for the BL formation and ordered surface
alloying in Fe/Cr (GMR, Colossal RM)
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