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

Thank you!

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