Post on 05-Jan-2016
description
Spin reorientation at the antiferromagnetic NiO(001) surface in response to an adjacent ferromagnetH. Ohldag, A. Scholl, F. Nolting, S. Anders, F.U. Hillebrecht, and J. Stöhr
Exchange bias
Unidirectional anisotropy in FM adjacent to AFMdiscovered 1956 by Meiklejohn and Bean
H
M
Exchange-bias field (size depends on many parameters)
step grain / domain boundary
AFM
FM
Spin-flopInterfacial spinsMeiklejohn, Bean (1956)Takano et al. (1997)
Koon (1997)
•High Spatial Resolution•Elemental Sensitivity•Ferromagnetic Contrast•Antiferromagnetic Contrast•Surface/Interface Sensitivity
PEEM
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.
Parallel domain correlation at interface
Ni XMLD Co XMCD
[010]
[100]
[010]
[100]
E
What causes correlation - exchange coupling at interface ?
AFM FM
[±110] [±110]
Coferromagnetic
NiO interfaceferromagnetic
NiOantiferromagnetic
XMCDXMCDXMLD
Nickeloxide interface magnetism
NiO Co
[010]
[100]
~1 monolayer
E
Summary
• 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
NiO Co
Domain structure of Co/NiO(001)
Co/NiONiO
NiO interface structure is not bulklike!
•(±110) walls disappear after Co deposition
•AFM axis rotates in-plane along [±110]
[100][010]
Bare NiO(001) Co/NiO(001)
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)
[100]
[001]
[010]
One of 4 "T" domains
3 possible "S" domains
<011>
_
<110>_
<101>
_
<211>_
<121>_
<112>
_
x'
Single crystalline NiO (001)sample preparation :
cleaved ex-situannealed at 380 K in PEEM chamberCo (< 2 nm) deposited in-situ