Magnetic thin films: from basic research to spintronics Christian Binek 11/18/2005 Physics 201H Why...
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Transcript of Magnetic thin films: from basic research to spintronics Christian Binek 11/18/2005 Physics 201H Why...
Magnetic thin films: from basic research to spintronics
Christian Binek11/18/2005
Physics
201H
Why thin films
Length (and time) scales determine the physics of a system
all macroscopic properties
Electronic states
Quantum mechanics tells us: Confinement of electrons by lowering dimensions affects the electronic states
3D bulk 2D film 1D wire 0D quantum dotsartificial atoms
Size matters
11/18/2005
Physics
201HWhen can films considered to be thin
or
thin with respect to what
Thin in comparison with the characteristic length scale
Examples:
-Superconducting thin film thickness correlation length
-optical thin film like dielectric mirrors Length scale /4 500nm/4
dcharacteristic length
d
d
11/18/2005
Physics
201H-Magnetic thin films approach the ultimate extreme
ferromagnet
ferromagnet
spacernonmagnetic
Spacer thickness d in # of atomic layers
thickness quantum mechanical exchange interaction length a few atomic layers
d=8 monolayer
J(d=8)>0
Ferromagneticcoupling
d=10 monolayerJ(d=10)<0
Antiferromagneticcoupling
Exchange J(d)
How to grow magnetic heterostructures
?
> 250 000
Molecular Beam Epitaxy
•Thin film growth @ low deposition rate•Ultra high vacuum condition
)Pa(mbar 810 1010
deposited material
Important growth modes in heteroepitaxy
Reflection High-Energy Electron Diffraction RHEED
Layer-by layer (Frank van der Merwe)
3D islands (Volmer weber)
Monolayer followed by 3D islands (Stranski Krastanov)
Electron gunup to 50 keV
3o
sample RHEEDscreen
Eyecamera
specific free energy
B A
B A substrate
interface
What are the magnetic heterolayers good for
Basic components of modern spintronic devices
•Conventional electronics has ignored the spin of the electron
•Advantages using spin degree of freedom:
magnetic field sensors M-RAM
?
Spin-transistor
semiconductor
Quantum-information
1960 1970 1980 1990 2000 201010-3
10-2
10-1
100
101
102
103
104
105
Are
al d
ensi
ty [M
b/in
2 ]
Year
Superparamagnetic effect
inductive read head
Magnetoresistive heads
GMR
Evolution of magnetic data storage on hard disc drives
•Impact of GMR based field sensors on magnetic data storage
rotating sensorlayer FM1
fixed layer FM2
How to pin FM2 while the sensor layer FM1 rotates?
Exchange Bias!Pinning of the ferromagnet
by an antiferromagnet
-40 -20 20 40
-10
-5
5
10
m [1
0-7A
m2]
0H [mT]
TN
AF
T
H
Hfc
field cooling: from T>TN
to T<TN
FMFM0
FMAFEB t M
SJS:H
Meiklejohn Bean: uniform magnetization reversal of a pinned FM
coupling constant: J
FM interface magnetization: SFM
MFM :saturation magnetization of FM layer
tFM
KFM, H
MFM
0AF F
EM
FM FM
J S SH
M t
Exchange bias field:
AF interface magnetization: SAF
20 FM FM FM FMF - M t cos K t s nH i
A FMF S S-J cos
Stoner-Wohlfarth AF/FM-interface coupling
F2
0 FM FM FM M FAF MF -( M t J ) coH s K t sinSS
0 FM FMM tFM F
AF
M
FMHSJ
M t
S
Cr2O3(0001)/Pt0.67nm/(Co0.35nm/Pt1.2nm)3/Pt3.1nm
Electric control of the Exchange Bias
Investigated multilayer system:
perpendicular magneticanisotropy
tPt=1.20nm
tCo=0.35nm
FM thin film with
EαM II
Magnetoeletric effect of Cr2O3
MagnetizationM=m/V
electric field E=U/d
Cr2O3 (0001)
U
Co
Co
Co
Pt
Pt
Pt
Cr2O3 (0001)
Cr2O3: Magnetoeletric AF, TN=308K
U
FMFM
FMAFE0 tM
SJSH
E M contributes to SAF
Idea:
SQUID-magnetometry @ T=290K *
*A. Hochstrat, Ch.Binek, Xi Chen, W.Kleemann, JMMM 272-276, 325 (2003)
M
-300 0 300
-0.04
0.00
0.04
(µ
0HE)
[mT
]
E [kV/m]
CoPt
Cr2O3 (0001)
U=Ed
Change of the exchange bias field as a function of the electric field at T = 150K
Magnetoelectric Switching of Exchange Bias*:2 Control via field-cooling*P. Borisov, A. Hochstrat, Xi Chen, W. Kleemann and Ch. Binek, PRL 94 117203 (2005)
Magnetic Field Cooling (MFC)
cooling from T>TN in 0Hfr = +0.6 T and Efr=-500 kV/m
Magneto
Field
Electric
Cooling
(+,-)
cooling from T>TN in 0Hfr =+0.6 T and Efr=+500 kV/m
Magneto
Field
Electric
Cooling
(+,+)
Magneto-optical Kerr measurements @ T = 298 K after cooling from T>TN in 0Hfr = 0.6 T
The sign of the Exchange bias follows the sign of EfrHfr
-0.2 0.0 0.2
-1.0
-0.5
0.0
0.5
1.0
M /
MS
0H [mT][T]
(+,-)EfrHfr<0
(+,+)EfrHfr>0
H
R
Spintronic applications*
ME
FM 1
FM 2V V
ME
FM 1
FM 2
*Ch. Binek and B. Doudin, J. Phys.: Condens. Matter 17 (2005) L39–L44
ME
V
ME
FM1
FM2
NM
V
FM1
FM2
NM
U
H
R
U
-He-Hi He-Hi
H
R
Voltage
Input
X:=0
1
+H
-H
Exclusive Or
magn. fieldY:=
-V
+V
1
0
Output
R high
R low
0
1
Example:0+V
-H 0
x | y | xORy 0 | 0 | 00 | 1 | 11 | 0 | 11 | 1 | 0
finite anisotropy KAF≠0
22
333
08 AFFMFMAF
FMAF
FMFM
FMAFe
ttMK
SSJtMSSJ
H
Basic research with magnetic heterostructures
generalized Meiklejohn Bean approach
J :coupling constant
SAF/FM :AF/FM interface magnetization
tAF/FM :AF/FM layer thickness
MFM :saturation magnetization of FM layer
Experimental check of advanced models
understanding the basic microscopic mechanism of exchange bias
Exchange bias is a non-equilibrium phenomenon
new approach to relaxation phenomena in non-equilibrium thermodynamics
reduction of the EB shift upon
subsequent magnetization reversal
of the FM layer
Training effect:
- origin of training effect
- simple expression for
0 EBH vs. n
The training effect: a novel approach to study relaxation physics
Relaxation towards equilibrium
Landau-KhalatnikovF
SS
:phenomenological damping constant
Training not continuous process in time, but triggered by FM loop
discretization of the LK- equation
AF AFAF
S (n 1) S (n)S
Discretization:
LK- differential equation difference equation
1st& 9th hysteresis of NiO(001)/Fe
Comparison with experimental results on NiO-Fe
NiO
12nm Fe
(001) compensated
experimental data
recursive sequence
232 eEB 0 EB EB EB
n
f H (n) H (n) H H (n 1) min.
f0
and
0.015 (mT)-2
0 EB
f, 0
( H )
e
0 EBH 3.66 mTe
Magnetic NanoparticlesCollaborations
25nm25nm25nm25nm
self-assembled Co clusters
~5nm
Transmissionelectron microscopicimage
I thermally decompose metal carbonyls in the presence of appropriate surfactants
You want to know what I am doing?
Fundamental questions
Which magnetic interactions dominate the system
What kind of magnetic order can we observe
For large particle distances the dipolar interaction will dominate
Here is a real fundamental question:
Do dipolar systems still obey extensive thermodynamics
What does this mean:
Magnetic moment ,T,H = 2 Magnetic moment ,T,H
Simulations suggest:
Yes: for a 2 dimensional array of dipolar interacting particles
but
No: for a 3 dimensional array of dipolar interacting particles
Modifications of conventional thermodynamics required
Summary
MBE is a technology at the forefront of
modern material science
magnetic heterolayers are basic ingredients for
spintronic applications
magnetism of thin films and nanoparticles
provides experimental access to fundamental questions in statistical physics
25nm25nm25nm25nm
V(X)
x
x
dVF
dx
equilibriumdV
0dx
Mechanical analogy
equilibrium dF0
d
F()
eq-eq
xeq
Damped harmonic oscillator:
mmx x Dx 0
2dV d 1Dx
dx dx 2
Solution for:
2 22 2
0 01 1t t t
2 22x(t) e A e B e
with0
x(0) 0
x(0) x
22
0
1 D:
2 m
22
0
1
2
22 2
0 0
1 1...
2 2
00 02
20
x1A x x
2 12
2
002
20
x1B x
2 12
2
0
A 0
B x
20 t
0x e
22
01t t
220x(t) x e e
20 t
0x(t) x e
also derived fromintegration of:
dVx
dx where
m/ dV
Dxdx
0 5 10 15
0,0
0,2
0,4
0,6
0,8
1,0
X(t
)
t
Temporal evolution of X with increasing damping:
20 t
0x(t) x e
0
x t
x 0
dx Ddt
x
0 dVx x
dx m
Near earth outer space:610 (100 )P mbar Pa
384,400 km
1110 (1 )P mbar nPa