Download Exchange bias NiO Co

Spin reorientation at the antiferromagnetic NiO(001)
surface in response to an adjacent ferromagnet
H. Ohldag, A. Scholl, F. Nolting, S. Anders, F.U. Hillebrecht, and J. Stöhr
Parallel domain correlation at interface
Exchange bias
Unidirectional anisotropy in FM adjacent to AFM
M
AFM
Ni XMLD
discovered 1956 by Meiklejohn and Bean
Co XMCD
FM
[010]
H
s
E
Exchange-bias field (size depends on many parameters)
The exchange bias effect is exploited in magnetic data storage devices, though the
origin of this effect is still poorly understood. Different models do exist, most of
them assume that the magnetic structure at the surface is the same as for the bulk.
Interfacial spins
Spin-flop
Meiklejohn, Bean (1956)
Takano et al. (1997)
[010]
•High Spatial Resolution
•Elemental Sensitivity
•Ferromagnetic Contrast
•Antiferromagnetic Contrast
•Surface/Interface Sensitivity
AFM
grain / domain boundary
[100]
What causes correlation - exchange coupling at interface ?
Nickeloxide interface magnetism
Co / NiO(001)
• Antiferromagnet, Néel temperature 523 K
• Spins lie in ferromagnetic {111} sheets along
<112> directions
• {111} sheets are coupled antiferromagnetically
• Strong rhombohedral compression along <111>
axes (10-3)
One of 4 "T" domains
[001]
NiO
antiferromagnetic
E
NiO interface
ferromagnetic
s
s
[010]
3 possible "S" domains
[100]
XMLD
XMCD
NiO
x'
[010]
NiO
Co
NiO interface structure is
not bulklike!
•(±110) walls disappear after
Co deposition
•AFM axis rotates in-plane
along [±110]
[100]
Bare NiO(001)
Co/NiO
XMCD
Co
~1 monolayer
Summary
Domain structure of Co/NiO(001)
NiO
Co
ferromagnetic
[010]
[100]
Single crystalline NiO (001)
sample preparation :
cleaved ex-situ
annealed at 380 K in PEEM chamber
Co (< 2 nm) deposited in-situ
[±110]
[100]
PEEM
Koon (1997)
FM
step
[±110]
•
Surface magnetic structure can differ from bulk structure
=> bulk methods are of limited value for the understanding
of interface effects
•
Clear evidence for parallel coupling between AFM and FM
in Co/NiO, no indication of spin canting, ruling out spin-flop
models
•
Exchange coupling is related to about 1 ML of
uncompensated spins at the AFM surface
Co/NiO(001)