Interfacial Moments In Co/NiO
Origins Of Antiferromagnetic/Ferromagnetic Coupling
mF
MPI Halle
Hendrik Ohldag, Tom Regan, Andreas Scholl, Frithjof Nolting, Elke Arenholz, Franz Ulrich Hillebrecht and Joachim Stöhr
Co And NiO
Giant Magneto
Resistance:
FM 1
W
FM 2
AFM
Co MCD
Line shape in x-ray absorption
used to identify metal-oxide
transition:
(GMR)-read head for
hard disks
M  MxOy
Magnetization(a.u.)
1.0
If a ferromagnet is
coupled to an
antiferromagnet
0.5
“Real System”
5mm
0.0
Co/NiO 
Co/CoNiOx/NiO
NiO MCD
-0.5
-1.0
-1500 -1000
-500
0
500
1000
0.02
NiO
PEEM to image AFM domains only at
NiO(001) surface.
0.01
0.00
868
0.10
Co
CoNiO
Free Interfacial moments:
- Exchange coupling
- Uniaxial anisotropy
- Coercivity increase
- Unidirectional anisotropy
-
e
-
FeO
Fe
Electron Yield (a.u.)
e
800
900
- Loop shift
Photon Energy (eV)
Fe
40
e
Electron Yield (a.u.)
e
-
0
704
Fe+ Ox
706
710
708
Photon Energy (eV)
Co L 2
3.0
2.5
2.0
795
1.0
Absorption of linear polarized x-rays (linear dichroism)
 spin anisotropy / antiferromagnetic (AFM) contrast
-
0.7
400
0.3
0
0.0
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
-1000
0
1000
2000
Applied Field (Oe)
no Fe
PEEM to image AFM and FM domains
in Co/ NiO(001)
XAS and dichroism  link between interface polarization and
magnetic properties
AFM REORIENTATION UPON Co
DEPOSITION
“FREE” CoNiOx SPINS CAUSE UNIAXIAL ANISOTROPY
(EXCHANGE COUPLING)
PARALLEL COUPLING
“PINNED” CoNiOx SPINS MAY CAUSE UNIDIRECTIONAL
ANISOTROPY (EXCHANGE BIAS)
UNIAXIAL ANISOTROPY
NiO L 2
300
Electron Yield (a.u.)
e
800
800
Photon Energy (eV)
-
Fe3+
1.5
790
e
1200
1.3
-2000
20
Absorption of circular polarized x-rays (circular dichroism)
 spin orientation / ferromagnetic (FM) contrast
-
- Exchange bias
3
60
780
Photon Energy (eV)
NiO XMCD Asymmetry [%]
Electron Yield (a.u.)
Fe2+
700
778
Hysteresis of Uncompensated Interfacial Ni spins in 3nm Co/NiO
6
600
L3-edge
0
10
20
30
40
Annealing tim e at 600K (m ins)
Pinned Interfacial moments:
500
Co
CoO
Co/NiO
Model
Co DEPOSITION CREATES AND POLARIZES CoNiOX
INTERFACE
Absorption of monochromatic tuneable x-rays
 elemental and chemical contrast
Fe
874
Co
776
NiO
Spatial resolution ~ 50-100nm
Ni
872
0.00
Interface thickness (nm )
SURFACE WALLS ARE NOT MIRROR
PLANES LIKE BULK WALLS
A Photoemission Electron Microscope
(PEEM) magnifies the local electron yield
onto a screen.
-
870
X-ray absorption shows chemically active interface
PEEM reveals its polarization.
The Method:
e
B
0.05
The Solution:
-
Ni
NiO
Co/NiO
Model
1500
Co
e
A
L2-edge
Applied Field (Oe)
Chemical and magnetic
characterization of all layers and
their interface.
Ni
H. Ohldag, A. Scholl et al., Phys. Rev. Lett. 86(13), pp 2878 (2001).
200
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).
100
876
879
882
Photon Energy (eV)
Coercivity (Oe)
- Loop shift
(UNIDIRECTIONAL
anisotropy) - Loop widening
(UNIAXIAL anisotropy) -
CoNiOx Interface
Electron Yield (arb. units.)
The Unsolved Origin Of:
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
References