Exchange Bias and the Devices Based on Exchange Bias
Xiaozhe Zhang
06/24/2016
Exchange Bias
The interface between a ferromagnetic (FM) and an antiferromagnetic (AFM) material
Curie temperature (TC) of the FM phase is higher than the Neel temperature (T𝑁) of the AFM phase
Cooled from a temperature T (TN < T < TC) to temperature T < TN in a static magnetic field
The M-H hysteresis loop of the system at T < TN, after the field cooled operation, manifests a shift along the field direction
FM
AFM
Curie temperature (TC)
Neel temperature (T𝑁)
Field cool FM
AFM
(TN < T < TC) T < TN
Exchange Bias
In the temperature range, TN < T < TC, if a magnetic field is applied to an FM-AFM bi-layer system, the spins of the FM layer will align themselves along the direction of the applied field, while the spins in the AFM layer will remain random.
The hysteresis loop shifted towards the opposite direction to the applied field direction
Exchange Bias
However, due to sizeable AFM anisotropy, the orientation of AFM spins will remain unchanged. As a result, the AFM spins at the interface will exert microscopic torque on the FM spins in order tomaintainferromagnetic coupling.
As the temperature falls below TN, an application of field will align the spins of the AFM layer at the interface, parallel to the spins of FM layer, while the spins adjacent to the interface in the AFM layer will retain their AFM arrangements.
Exchange Bias
Additional field would be required to rotate the FM spins in order to compensate the microscopic torque.
The system acts as if there is an additional biasing field, whichshifts the hysteresis loop along the field direction, which is manifested by exchange bias, HE
This shift of the hysteresis loop is termed as exchange bias (HE)
Devices Base on Exchange Bias
Hysteresis loop along (a) [100] direction of CoFeB (5 nm)/Si layer, (b) [100] direction, (c) [100] direction, and (d) [010] direction of CoFeB (5 nm)/BFO (35 nm)STO bilayer.
Applied Physics Letters,89,24,242114,2006.
Room temperature magnetic properties of CoFe/BFO heterostructure showing improvement of coercivity and exchange bias
Nano Letters, vol. 8, no. 7, pp. 2050–2055, 2008.
Devices Base on Exchange Bias
YbFO/Fe3O4 multilayer
YbFO
Fe3O4Curie temperature (950K)
Neel temperature (~150K)
(TN < T < TC)
Room temperature
T < 150 K
Field cool
Expect
-3000 -2000 -1000 0 1000 2000 3000
-4.760
-4.755
-4.750
-4.745
-4.740
Kerr
angle
(a.u
.)
H(Oe)
60 K
EB=(-1089+887)/2=-101 Oe
-3000 -2000 -1000 0 1000 2000 3000
1.0
0.5
0.0
-0.5
-1.0
Ma
gn
eti
za
tio
n (
a.u
.)
Applied Magnetic Field (Oe)
MOKE for YbFO/Fe3O4 multilayer
-6 -4 -2 0 2 4 6
2.848
2.850
2.852
2.854
2.856
2.858
2.860
2.862
C
A
C
-6 -4 -2 0 2 4 6
2.584
2.586
2.588
2.590
2.592
2.594
2.596
H
A
H
-1000 Oe
1250 Oe
-3000 -2000 -1000 0 1000 2000 3000
2.850
2.855
2.860
left m
agnetic fie
ld
A
left magnetic field
right magnetic field31K
MOKE for YbFO/Fe3O4 multilayer
-1000 Oe
1250 Oe
-6 -4 -2 0 2 4 6
2.500
2.502
2.504
2.506
2.508
2.510
2.512
2.514
2.516
2.518
2.520
C
A
C
-6 -4 -2 0 2 4 6
2.548
2.550
2.552
2.554
2.556
2.558
2.560
2.562
H
A
H
-3000 -2000 -1000 0 1000 2000 3000
2.55
2.56
left m
agnetic fie
ld
A
left magnetic field
right magnetic field61K
MOKE for YbFO/Fe3O4 multilayer
-6 -4 -2 0 2 4 6
2.525
2.530
2.535
2.540
2.545
2.550
C
A
C
-6 -4 -2 0 2 4 6
2.546
2.548
2.550
2.552
2.554
2.556
2.558
2.560
H
A
H
-1000 Oe
1250 Oe
-3000 -2000 -1000 0 1000 2000 3000
2.545
2.550
2.555
2.560
2.565
left
mag
ne
tic fie
ld
A
left magnetic field
right magnetic field100K
MOKE for YbFO/Fe3O4 multilayer
-3000 -2000 -1000 0 1000 2000 3000
0.00000
0.00002
0.00004
0.00006
Mom
en
t (e
mu)
Field (Oe)
-3000 -2000 -1000 0 1000 2000 3000
-0.000020
-0.000015
-0.000010
-0.000005
0.000000
0.000005
0.000010
0.000015
0.000020
0.000025
Mom
en
t (e
mu)
Field
-10000 -5000 0 5000 10000
-0.00005
0.00000
0.00005
Mom
en
t (e
mu)
Field (Oe)
10 K 100 K
300 K
SQUID for YbFO/Fe3O4 multilayer at different temperature
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