Ultrathin films and preparation
of films
1.Ultrathin films
Surface anisotropy, Magnetisation, M(T) behavior,
Domains
1.Film preparation
(a) Vacuum evaporation
(b) Magnetron sputtering
(c) Laser abrasion
(d) Molecular beam epitaxy
(e) Self-assembled monolayer
3. Measurements
E.Bauer, Growth of thin films, J.Phys: Condens. Matter 11(1999)9365
Interesting aspects:
(1) For studying new phases of materials
such as, fcc cobalt and fcc iron grown on Cu (001)
(2) Two dimensional features which not encountered in
bulk specimens.
(3) Whether any dead layer appears ?
(1)Perpendicular anisotropy
N=(P↑-P↓)/ (P↑+P↓)
Spin polarization, p (in%), of photoelectrons from Fe(100) on Ag(100),
versus magnetic field along the surface normal (Stampanoni et al.,PRL
59(1987)2483).
Temperature dependence of the saturation polarization of a 3.5 ML
thick epitaxial bcc Fe film on Ag(001) and a 3 ML fcc Fe on Cu(001).
Insert: thickness dependence of the Cuire temperature of the bcc Fe
films.
Fcc Fe on (001)Cu
Polarization P(H) of a 5 ML fcc Fe film on Cu(001),
(a) for sample temperature T=215, 267, and 375 K,
(b) at T=30 K, H is perpendicular to the film plan.
Polarization P(H) measured at T=30 K, (a) for 3 ML
fcc Fe on Cu(001), (b) for 1 ML. H is perpendicular to
the film plane.
Temperature dependence of the reduced polarization P/Po for 1, 3,
and 5 ML films of fcc Fe on Cu(001). Po is the saturation polarization
at low temperature. The Curie temperature are 230K for 1ML film,
390 K for 3 and 5 ML films.
Fcc Co on Cu(001)
PRL 58(1987)933
Normalized polarization P/Po as a function of the externally applied field
perpendicular to the film plane. Data are given for five film thickness at
T=300 K.
Temperature dependence of the spin polarization for a 1 ML film
measured in saturation. Applied field 15 KOe
3M 286(2005)405
Fe(100) on V(100)
PRB 70(2004)214406
Magnetisation loop of 2.2 nm thick Ni(111) on Cu(111), coated by
Cu(111), measured by TOM. The films show a perpendicular anisotropy (PA) between 1.0 – 2.5 nm (Gradmann, Ann. Phys.17(1966)91).
J Phys: Condens Matt
86(2005)s573
From the above two figures, the following facts are evident:
(1) Films of 1ML or thicker of bcc Fe/Ag(001) is ferromagnetic, Tc
~400K;
For >5ML bcc Fe/Ag(001) Its Tc approaches to bulk;
3.5ML and 5ML layer thickness Fe(100) on Ag(100) show
perpendicular anisotropy.
(2) For fccFe/Cu(001) 1, 3 and 5 ML Tc=220, 360 and 410K,
respectively, 3ML Fe/Cu appears P anisotropy at 30K.
(3) Fcc Co/Cu(001) 1ML is F and Tc ~400K, Perpendicular anisotropy
appears at room T.
(4) 1-2.5 ML Ni(111)/Cu(111) appears perpendicular anisotropy.
(5) Orbital Moment appears in ultra-thin Co film (<2ML).
In short : > 1ML F appears, Perpendicular anisotropy appears in ultrathin films, Orbital Moment appears in ultra-thin Co film.
Magnetisation loops of Pd/Co multilayers, taken at 300 K, with the
field in the film plane (dashed curves) or along the surface normal
(full lines) (Carcia APL 47(1985)178).
Total anisotropy Kt for evaporated (111) texturized polycrystalline
Co/Pd multilayers versus thickness t of Co films.
Co/Pt Multilatersa
Magnetic hysteresis loops at 20 oC.
Keff d = 2ks + (kV -2πMs2)d
Effective anisotropy times Co thickness versus cobalt
thickness for [Co/Pt] multilayers (Engle PRL 67(1990)1910).
Surface Magnetic Anisotropy ?
•The reduced symmetry at the surface (Neel 1954);
•The ratio of Lz2 / (Lx2 + Ly2) is increased near the surface
•Interface anisotropy (LS coupling)
[1] J.G.Gay and Roy Richter, PRL 56(1986)2728, [2] G.H.O. Daalderop et al.,
PRB 41(1990)11919, [3] D.S.Wang et al., PRL 70(1993)869.
PRL 88(2002)217202
Surface Magnetisation
Ferromagnetism in fcc Fe(111) on CuAu (111). Magnetic moment
µFe in the Fe film versus the mean lattice parameter aCuAu or Au
concentration cAu in the substrate.
Variation of magnetic moment calculated by layer in an 8 ML Ni/Cu
(001) film. The calculated bulk and surface moments are 0.56 µB/Ni
and 0.74 µB/Ni (bulk moment 0.6 µB/Ni) (Tersoff PRB 26(1982)6186).
The interior, bulklike layers
(layers 3-6 from Cu)
The surface-like layers
(layers 7 and 8 from Cu)
Spin-resoled density of state for 8 ML Ni(001) film on Cu(001)
Ms (T) behaviour
48Ni/52Fe (111) films
on Cu(111)
Curie temperature of 48Ni/52Fe(111) versus number of atomic
layers DM. The experiments is from Gradmann (Phys. Status Solidi
27(1968)313. Green-function theory from Brodkorb 16(1966)225.)
Domain in ultrathin films
Calculated spin distribution in a thinn sample containing
A 180O domain wall.
(a) Domain pattern as measured by
MFM above the surface of an eiptaix
Cu/200nmNi/Cu(100) film.
(b) Vibrating sample magnetometry
M-H loop of the sample
Schematic drawing of the evaporation chamber.
Thermal evaporation and the
uniformity of deposits
• The simplest technology, raising the temperature of the
source materials;
• An open boat, suspended on a wire;
• The boat or wire is a high temperature material, such
as W or Mo and must not react with the evaporant;
• The substrate hold should be rotated in order to get
uniform deposits;
• The deposition rate is determined by the source area
temperature and the distance between the source
and substrate as well as the evaporant itself;
• Electron beam deposition for high temperature materials
or materials which interact with the crucible.
Binary alloy evaporation
dZA / dZB = (MB/MA )1/2 exp[-(ΔHA- ΔHB)/RT] (CA/CB)
= K (CA/CB).
MA, MB are the mass of A and B element; ΔHA and
ΔHB the evaporation heat, CA : CB is the atomic
ratio.
(the ratio of A:B)
(K)
The variation of the ratio of evaporated A and B
element in binary alloy with time
Sputtering and ion beam assisted
deposition
Sputtering (Ion beam assisted deposition (IBAD), Ion beam
sputter deposition (IBSD)) provides the better quality deposits:
• at low substrate temperature, thus avoiding large scale
interdiffusion,
• adhere well to the substrate,
• to realize a reactive sputtering.
Schematic picture of magnetron sputtering
The sputtering rate for the different element
(using 500 eV Ar+).
Pulsed laser
width 10-20 ms,
Density 1-5 J/cm2
Schematic picture of Laser ablation
• A continuum NY81-C Nd:YIG laser
• The wavelength, pulse frequency and pulse width are
355 nm, 10Hz and 10ns, respectively
• The focused laser beam with the energy density of 3-4
J/cm2
• Ceramic target
• The distance between the target and substrate is 55 mm
• 3x10-5mTorr before introducing pure O2
• O2 gass flow of 60 sccm at a pressure of 75 mTorr
• After deposition, the amorphous film is post annealed for
2 minutes at 650oC in air
Bi2.0Dy1.0Fe3.5Ga1.0O12
Summary
(1)The chemical composition of the film is the same as that of
target
(2) The polycrystalline films on ceramic glass substrate have
easy magnetization axis normal To the film surface,
nanometer size grain and very smooth surface
(3) The film shows high squareness of Faraday hysteresis loop
(4) Magnetization of the film at temperature range from 240K to
340K is almost temperature independent.
The special points of Pulse Laser
Ablation
The advantages
• The ablated sample with the same composition as the
target composition;
• High energy particles is beneficial for the film growth and
realizing a chemical reaction on substrate;
• Reaction deposition;
• Multilayers growth and thickness control precisely.
The disadvantages
• Forming small particle, 0.1-10 µ m,
• thickness deposited is not uniform
Advantages of MBE
(1) Growth under controlled and monitored conditions with in
situ analysis of film structure and composition (RHEED,
LHEED, XPS, AES).
(2) A key advantage of MBE is that it enables growth of the
layered structure along specific crystalline direction;
(3) Lattice-matching between the seed film (prelayer) and
substrate can be achieved by appropriate choice of materials
and the growth axis of the magnetic structure selected.
Magnetic hysteresis loops for oriented Co-Pt superlattice recorded by MO effect.
Schematic representation of the
three growth modes (a) Island (b)
layer-plus-island (c) layer by layer.
Substrate
The change of AES peak with
The deposition
deposite
ML
Two arrangements for four deposited atoms in the same
phase epitaxy
7 AA bonds
8 AA bonds
(stable state)
In the case of the same phase epitaxy, the stable state is oneLayer-arrangement, namely, two demitional growth.
For the different phase epitaxy
(a)
(b)
-4uAB – 12uAA
-8uAB-10uAA
If uAA > 2uAB the case of (a) is beneficial for the reduction of
energy
The condition for double layers arrangement (Island):
(a) N=8, uAA>2uAB, (b) N=18, uAA>1.5uAB, (c) N=32,
uAA>1.33uAB, (d) N=50, uAA>1.25uAB, (e) N=72, uAA>
1.24 uAB.
Other factors should be considered
(a) The size of the epitaxy atoms
If the size of A atom (epitaxy) is larger than that of B
(substrate), a compressive strain appears in the epitaxy
layers, conversely, tensile force appears;
(b) The strain increases with the increase of epitaxy
thickness and finally dislocation could exist;
(c) Island appears if the size A atom is largely different from
B atom (substrate).
Electron-based techniques for examining
surface and thin film process
AES (Auger electron spectroscopy)
LEED (Low energy electron diffraction)
RHEED (Reflection high energy electron microscopy)
TEM (Transmission electron microscopy)
REM (Reflection electron microscopy)
STM (Scanning tunneling microscopy)
AFM (Atomic force microscopy)
PEEM (Photoemission microscopy)
SEM (Scanning electron microscopy)
SNOM (Scanning near field optical microscopy)
XPS (X-ray photoemission spectroscopy)
UPS (Ultra-violet photoemission spectroscopy)
Auger Electron Spectoscopy (AES)
Si KL1L2,3 transition
Si KLL Auger scheme (Chang Surface Sci., 25(1974)53).
High resolution AES spectrum of Ge LMM for 5KV incident energy.
The strongest peaks, within the L2M4,5M4,5 series at 1145 and
L3M4,5M4,5.
The surface AES
of Fe
The integrating spectroscopy, N(E), of the surface AES,
and N’(E)=dN(E)/dE.
Photoelectron Spectroscopies:
XPS and UPS
After the electron at inner shell or valence electron absorb
photon energy, they leave atom and become photo-electron,
Ek = hv – Eb, where, hv photon energy,
UPS uses ultra-violet radiation as the probe and collects
electrons directly from the valence band, XPS excites a core
hole with X-rays and collect binding energy of the electrons
at the inner cells.
XPS
The electron energy spectrum on Ni obtained by
bombardment of 1.25Kev photon.
Scanning Tunneling Microscopy
(STM)
• The tunneling current is measured by W needle
• The distance between the tip and sample surface is below
1 nm; resolution along vertical is 0.01nm and in transverse
is 0.1nm
• The tip is applied a few voltage and the tunneling current
is 0.1 to 1.0 nA
• The current is related not only to the height of atom on the
surface, but also to the atomic density (density state)
Atomic Force Microscopy (AFM)
STM is only applied to observe
surface for conductor or semiconductor, while AFM is an
appropriate tool for all samples.
The reflect light place is 3-10nm
after the height of tip changes
0.01nm.
Three operation models of AFM:
(1) contact (2) non-contact (3)
tapping model.
Transmission electron microscopy (TEM)
(1)With TEM one can obtain diffraction patterns and images
of the sample, revealing microstractural defects such as
dislocation, grain-, twin- and antiphase boundaries
(2) In order for the electrons to pass through the specimen,
it has to be electron transparent (hundreds of nm)
(3) High resolution than a light microscope
Atoimic resolution TEM image of a Co doped TiO2
film. No segregation of impurity phases was observed in the film.