Exchange Bias from Double Multilayer Structures
C. H. Marrows, P. Steadman, M. Ali, A. T. Hindmarch, and B. J. HickeyDepartment of Physics and Astronomy, University of Leeds, Leeds.
LS2 9JTS. Langridge, R. Dalgliesh and S. FosterISIS Facility, Rutherford Appleton Laboratory, Didcot, Oxon. OX11 0QX
Introduction
• Exchange Bias – AF/F bilayer interaction.
• Double Multilayer Structures – model system for AF/F studies.
• Modelling – can predict magnetic structure.
• Neutron Reflectometry – depth sensitive vector magnetometry.
Exchange Bias
Problems with Meiklejohn-Bean:•Predicted exchange bias orders of magnitude too large.•Coercivity enhancement is not predicted.•Temperature dependence is not predicted.
Antiferromagnet
Ferromagnet
Two ModelsDomain Wall Formation inAntiferromagnet (Mauri) Interface Roughness
(Malozemoff)
D. Mauri, H. C. Siegmann, P.S. Bagus and E. Kay
J. Appl. Phys. 62, 3047 (1987)
A. P. MalozemoffPhys. Rev. B 35, 3697 (1987).
Ferromagnet
Antiferromagnet
Double Multilayer Structure
}}
Ferro
mag
netica
lly
Couple
d
Multila
yer
Antie
rrom
agn
etica
lly
Couple
d M
ultila
yer
Ta (75Å)
{Co (60Å)/Ru (10Å)}10
{Co (35Å)/Ru (15Å)}10
Ta (75Å)Si (001)
X-ray Reflectivity
dA
F
dF
2/dAF
2/dF
Magnetisation Data
}}Antiferromagnet
Ferromagnet
Double Multilayer
Modelling the spin structure.Energy per unit area = Zeeman + Anisotropy +
Coupling
N
i
N
iiiiiiiiiii JtKHtm
1
1
111,
20 )cos(coscos
•Layer index i
•Layer moment m
•Layer thickness t
•Applied field H
•Anisotropy constant K
•Interlayer Coupling Constant J
Minimise energy by varying moment orientations as field is swept – trace out hysteresis loop with full magnetic configuration known at each point.
Monte-Carlo Algorithm.
Neutron Reflectometry with Polarisation Analysis
Hµn Qkin
kou
t
Non-spin flip scatteringµn
M
Spin flip scatteringµn
Mµn
µn
n
Neutron Reflectometry
H=6kOe
Q(Å-1)
0.04 0.06 0.08 0.10 0.12 0.14
INTE
NSI
TY
10-6
10-5
10-4
10-3
10-2
Col 1 vs 54420.uu Col 4 vs 54420.dd Col 7 vs 54420.ud Col 10 vs 54420.du Col 10 vs uusim Col 10 vs ddsim Col 17 vs udsim Col 19 vs dusim
Saturation (6 kOe)Spin-flop phase (600 Oe)Exchange Spring (160 Oe)
µn
kin
kout
•No spin-flip scattering•No AF peak
•Spin-flip scattering•AF peak
•Decrease of spin-flip scattering•AF peak
2nd order
AF peak
2nd orderAF peak
Hysteresis CycleH=6kOe
Q(Å-1)
0.04 0.06 0.08 0.10 0.12 0.14
INTE
NSI
TY
10-6
10-5
10-4
10-3
10-2
Col 1 vs 54420.uu Col 4 vs 54420.dd Col 7 vs 54420.ud Col 10 vs 54420.du Col 10 vs uusim Col 10 vs ddsim Col 17 vs udsim Col 19 vs dusim
Saturation: 6 kOeSpin-flop Phase: 600 Oe
Exchange Spring: 160 Oe •Generate spin-structure from calculation.
•Pass to PNR simulation code (Polly).
•Fit PNR data using simulated annealing (changes <10°).
New Double Multilayer•Previous multilayers did not have exchange bias.•Introduce anisotropy into antiferromagnetic layer by adding Pt to magnetic layers.
{Co (56Å)/Ru (5Å)} 10
{CoPt (60Å)/Ru (10Å)} n
Si (001)
}}
MOKEn=5 n=10
-1000 -500 0 500 1000
-1.0
-0.5
0.0
0.5
1.0
H (Oe)
M/M
0
Hex=-46 Oe
Hex=-20 Oe
-1000 -500 0 500 1000
-1.0
-0.5
0.0
0.5
1.0
H (Oe)
M/M
0Antife
rrom
agnetica
lly
Couple
d M
ultila
yer
Ferro
mag
netica
lly
Couple
d
Multila
yer
dA
F
dF
Sensitive only to upper layers (~200Å)
Polarised Neutron Reflectometry
-2000 -1000 0 1000 2000
-0.185
-0.180
-0.175
-0.170
-0.165
H (Oe)
M/M
0
0.1
10-7
10-6
10-5
10-4
10-3
Inte
nsit
y (A
rb. U
nits
)
Q (A-1)
35 Oe
0.1
10-7
10-6
10-5
10-4
10-3
10-2
Inte
nsit
y (A
rb. U
nits
)Q (A
-1)
2.5 kOe
0.110
-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
Inte
nsit
y (A
rb. U
nits
)
Q (A-1)
3.0 kOe
0.110
-7
10-6
10-5
10-4
10-3
Inte
nsit
y (A
rb. U
nits
)
Q (A-1)
950 Oe
Co (60Å)
{CoPt (60Å)/Ru (10Å)} 10
Hex=250 Oe
Summary• A large anisotropy in the
antiferromagnet is necessary for exchange bias.
• A Mauri type exchange spring exists in the exchanged biased multilayers – model system for perfect interface.
• Planar wall confined to AF layers.
