1. Magnetic thin films: from basic research to spintronics Christian Binek
Physics
201H
11/18/2005
Why thin films
Size matters
Length (and time) scales determine the physics of a system
Quantum mechanics tells us: Confinement of electrons by lowering dimensions
affects the electronic states
Electronic states
3D bulk
2D film
1D wire
0D quantum dots
artificial atoms
all macroscopic properties

2. Physics 201H When can films considered to be thin dor
thin with respect to what d
11/18/2005
dcharacteristic length
Thin in comparison with the characteristic length scale
Examples:
thickness  correlation length
-Superconducting thin film
Length scale  /4  500nm/4
-optical thin film like dielectric mirrors

3. Physics 201H -Magnetic thin films approach the ultimate extreme
11/18/2005
thickness  quantum mechanical exchange interaction length  a few atomic layers d
Exchange J(d)
ferromagnet
spacer
nonmagnetic
ferromagnet
Spacer thickness d in # of atomic layers
d=8 monolayer
J(d=8)>0
d=10 monolayer
J(d=10)<0
Ferromagnetic
coupling
Antiferromagnetic
coupling

4. How to grow magnetic heterostructures?
>

5. Molecular Beam Epitaxy
Thin film
low deposition rate
Ultra high vacuum condition

6. Important growth modes in heteroepitaxy
specific free energy
Layer-by layer (Frank van der Merwe)
substrate
deposited
material
interface
Monolayer followed by 3D islands (Stranski Krastanov)
3D islands (Volmer weber)
Reflection High-Energy Electron Diffraction
RHEED
Electron gun
up to 50 keV
sample
RHEED
screen
Eye
camera

7. 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:
Quantum-
information
magnetic field sensors
M-RAM
Spin-transistor
semiconductor

8. Impact of GMR based field sensors on magnetic data storage
Evolution of magnetic data storage on hard disc drives
Superparamagnetic effect
GMR
Magnetoresistive heads
inductive read head

9. rotating sensor
layer FM1
fixed layer FM2

10. Pinning of the ferromagnet by an antiferromagnet
 How to pin FM2 while the sensor layer FM1 rotates?
Exchange Bias!
Pinning of the ferromagnet by an antiferromagnet
field cooling:
from T>TN
to T<TN

11. AF/FM-interface coupling
Meiklejohn Bean: uniform magnetization reversal of a pinned FM
FM interface magnetization: SFM
MFM
tFM
coupling constant: J
AF interface magnetization: SAF
KFM, H
MFM :saturation magnetization of FM layer
Exchange bias field:
Stoner-Wohlfarth
AF/FM-interface coupling

12. Electric control of the Exchange Bias
Investigated multilayer system:
Cr2O3(0001)/Pt0.67nm/(Co0.35nm/Pt1.2nm)3/Pt3.1nm
tPt=1.20nm
FM thin film
with Pt
tCo=0.35nm
perpendicular
magnetic
anisotropy Co Pt Co Pt Co
Cr2O3: Magnetoeletric AF, TN=308K U
Magnetoeletric effect of Cr2O3
T=290K *
electric field E=U/d
Cr2O3 (0001)
Magnetization
M=m/V U
Idea:
Cr2O3 (0001)
E M contributes to SAF
*A. Hochstrat, Ch.Binek, Xi Chen, W.Kleemann, JMMM , 325 (2003)

13. Change of the exchange bias field as a function of the electric field at T = 150KCo Pt
Cr2O3 (0001)
U=Ed

14. Magnetoelectric Switching of Exchange Bias*:2
Magnetoelectric Switching of Exchange Bias*:
Control via field-cooling
*P. Borisov, A. Hochstrat, Xi Chen, W. Kleemann and Ch. Binek, PRL (2005)
Magneto-optical Kerr T = 298 K after cooling from T>TN in 0Hfr = 0.6 T
[T]
Magnetic Field Cooling (MFC)
cooling from
T>TN
in 0Hfr = +0.6 T
and
Efr=-500 kV/m
(+,-)
EfrHfr<0
(+,+)
EfrHfr>0 M a g n e t o E l e c t r i F i e l d C o l i n g
(+,-)
cooling from
T>TN
in 0Hfr =+0.6 T
and
Efr=+500 kV/m M a g n e t o E l e c t r i F i e l d C o l i n g
(+,+)
The sign of the Exchange bias follows the sign of EfrHfr

15. Spintronic applications*
*Ch. Binek and B. Doudin, J. Phys.: Condens. Matter 17 (2005) L39–L44 V ME
FM 1
FM 2 V
FM 2 ME
FM 1 H R

16. V V
FM2
FM2 NM NM
FM1
FM1 ME ME R
-He-Hi
He-Hi H

17. R H +V x | y | xORy 0 | 0 | 0 -V 0 | 1 | 1 1 | 0 | 1 1 | 1 | 0 +H -H
Exclusive Or +V
x | y | xORy
0 | 0 | 0
0 | 1 | 1
1 | 0 | 1
1 | 1 | 0
X:=
Voltage
R high -V 1
Input
Output +H 1
Y:=
magn. field -H
R low 1 R
Example: +V -H H

18. Basic research with magnetic heterostructures
generalized Meiklejohn Bean approach
finite anisotropy KAF≠0
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

19. The training effect: a novel approach to study relaxation physics
reduction of the EB shift upon
subsequent magnetization reversal
of the FM layer
- origin of training effect
- simple expression for

20. Relaxation towards equilibrium discretization of the LK- equation
Landau-Khalatnikov
:phenomenological damping constant
Training not continuous process in time, but triggered by FM loop
discretization of the LK- equation
Discretization:
LK- differential equation  difference equation

21. Comparison with experimental results on NiO-Fe
1st& 9th hysteresis of NiO(001)/Fe
(001) compensated
NiO
12nm Fe

22. experimental data
recursive sequence
min. e
0.015 (mT)-2
and
3.66 mT e

23. Magnetic Nanoparticles
Collaborations
self-assembled Co clusters
I thermally decompose
metal carbonyls
in the presence of
appropriate surfactants
You want to know
what I am doing?
Transmission
electron microscopic
image
~5nm

24. 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

25. = 2 Here is a real fundamental question:
Do dipolar systems still obey extensive thermodynamics
What does this mean:
Magnetic moment
,T,H
Magnetic moment
,T,H
= 2
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

26. 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

29. Mechanical analogy V(X) x F()  equilibrium equilibrium eq -eq xeq
Damped
harmonic oscillator:

30. Solution for:
with

31. Temporal evolution of X with increasing damping:
where
also derived from
integration of: m
Temporal evolution of X with increasing damping:

32. 384,400 km
Near earth outer space: