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