CoNiO x Interface

CoNiOx Interface

Interfacial Moments In Co/NiOOrigins Of Antiferromagnetic/Ferromagnetic Coupling

Hendrik Ohldag, Tom Regan, Andreas Scholl, Frithjof Nolting, Elke Arenholz, Franz Ulrich Hillebrecht and Joachim Stöhr

MPI Halle

Co And NiO

H. Ohldag, A. Scholl et al., Phys. Rev. Lett. 86(13), pp 2878 (2001).

F.U. Hillebrecht, H. Ohldag et al., Phys. Rev. Lett. 86(15), pp 3419 (2001).

H. Ohldag, N.B. Weber et al., Synch. Rad. News 13(6), pp. 25 (2001).

T.J. Regan, H. Ohldag et al., Phys. Rev. B, in print (2001).

Or on the web: http://www-als.lbl.gov/als/als_news/news_archive/vol.178_060601.html

X-ray absorption shows chemically active interfacePEEM reveals its polarization.

Co DEPOSITION CREATES AND POLARIZES CoNiOX INTERFACE

776 778 780

0.00

0.05

0.10 Co CoO Co/NiO Model

Ele

ctro

n Y

ield

(ar

b. u

nits

.)

Photon Energy (eV)

868 870 872 874

0.00

0.01

0.02

L3-edge

L2-edge

BA

Co

Ni Ni NiO Co/NiO Model

Line shape in x-ray absorption used to identify metal-oxide

transition:

M MxOy

“Real System”

Co/NiO Co/CoNiOx/NiO

5m

Co MCD

NiO MCD

NiO

Co

NiO

CoCoNiO

The Solution:

Chemical and magnetic characterization of all layers and their interface.

The Unsolved Origin Of:

-1500 -1000 -500 0 500 1000 1500

-1.0

-0.5

0.0

0.5

1.0

Mag

netiz

atio

n(a.

u.)

Applied Field (Oe)

FM 1

FM 2AFM

- Loop shift (UNIDIRECTIONAL anisotropy) - - Loop widening(UNIAXIAL anisotropy) -

If a ferromagnet iscoupled to anantiferromagnet

Giant Magneto Resistance:

(GMR)-read head for hard disks

PEEM to image AFM and FM domains in Co/ NiO(001)

AFM REORIENTATION UPON Co DEPOSITION

PARALLEL COUPLING

UNIAXIAL ANISOTROPY

PEEM to image AFM domains only at NiO(001) surface.

SURFACE WALLS ARE NOT MIRROR PLANES LIKE BULK

WALLS

References

The Method:

A Photoemission Electron Microscope (PEEM) magnifies the local electron yield onto a screen.

Spatial resolution ~ 50-100nm

790 795 800

1.5

2.0

2.5

3.0 Co L2

Ele

ctr

on

Yie

ld (

a.u

.)

Photon Energy (eV)

e-e-

876 879 882

100

200

300 NiO L2

Ele

ctr

on

Yie

ld (

a.u

.)

Photon Energy (eV)

e-e-

Absorption of monochromatic tuneable x-rays elemental and chemical contrast

Absorption of circular polarized x-rays (circular dichroism) spin orientation / ferromagnetic (FM) contrast

Absorption of linear polarized x-rays (linear dichroism) spin anisotropy / antiferromagnetic (AFM) contrast

500 600 700 800 900

3

6

Ele

ctr

on

Yie

ld (

a.u

.)

Photon Energy (eV)

e-

Ni Fe

e-

e-

FeO

Fe

e-

0

20

40

60

704 706 708 710P hoton Energy (eV)

Ele

ctr

on

Yie

ld (

a.u

.)

Fe

Fe+O x

e

30o

no FeFe3+

Fe2+

XAS and dichroism link between interface polarization and magnetic properties

“FREE” CoNiOx SPINS CAUSE UNIAXIAL ANISOTROPY (EXCHANGE COUPLING)

“PINNED” CoNiOx SPINS MAY CAUSE UNIDIRECTIONAL ANISOTROPY (EXCHANGE BIAS)

0 10 20 30 400.0

0.3

0.7

1.0

1.3

Inte

rfa

ce

th

ickn

ess (

nm

)

Annealing time at 600K (mins)

0

400

800

1200

Co

erc

ivity

(Oe

)

Free Interfacial moments:

- Exchange coupling

- Uniaxial anisotropy

- Coercivity increase

Pinned Interfacial moments:

- Exchange bias

- Unidirectional anisotropy

- Loop shift

-2000 -1000 0 1000 2000

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

Hysteresis of Uncompensated Interfacial Ni spins in 3nm Co/NiO

NiO

XM

CD

Asy

mm

etry

[%]

Applied Field (Oe)