Post on 13-Jan-2016
description
Andreas Scholl,1 Marco Liberati,2 Hendrik Ohldag,3 Frithjof Nolting,4 Joachim Stöhr3
1Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA2Department of Physics, Montana State University, Bozeman, MT 59717, USA
3Stanford Synchrotron Radiation Laboratory, Stanford, CA 94309, USA4Swiss Light Source, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland
The role of planar and vertical domain walls and uncompensated interface spins in exchange bias.
X-RAY SPECTROSCOPY & IMAGING OF ANTIFERROMAGNETS VERTICAL DOMAIN WALLS
PLANAR WALLS UNCOMPENSATED INTERFACE SPINS
Malozemoff, PRB 35, 1987
Random field problem:
Competition between domain wall and interface energy:
Meiklejohn & Bean, Phys. Rev. 102 1956
Ideal interface:
Experiment:
AFM
FM
INTRODUCTION
Observations about exchange bias:
• Exchange bias is an interface effect.
• Exchange bias is a result of symmetry breaking.
• Exchange bias appears in a variety of materials.
Basic physics understood – Exchange interaction
X-ray microscopy and spectroscopy bring forth a microscopic understanding.
Co domain switching
LaFeO3Local loops
-223 Oe 92 Oe
103 Oe 223 Oe
XMCD
XMLD
[100]
[010]
10 mm-200 -100 0 100 200
Field (Oe)
1 2
12
+30 Oe
-30 Oe
LaFeO3
F. Nolting et al., Nature 2000
Co/LaFeO3
Co
A. Scholl et al., APL 2004
Domain area distributionLocal exchange bias
Bias field vs. domain area
Local, remanent hysteresis loops of Co/LaFeO3 show
a dependence of the variance of the local bias field
with the domain size. Large domains show a small
variance. For small domains a large width of the
domain size distribution was observed. The
sample was measured as-grown and did not
possess a macroscopic bias, explaining that both
directions of the bias occurred with equal
probability. An approximately linear dependence of
the width of the bias distribution with the inverse
domain diameter is in accordance with predictions
of the model proposed by Malozemoff.
CONCLUSIONS
Coferromagnetic
NiO interfaceferromagnetic
NiOantiferromagnetic
XMCDXMCDXMLD
NiO Co
[010]
[100]
~1 monolayer
E
Ohldag et al. PRL 2001
-3k -2k -1k 0 1k 2k 3k
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
Applied Field (Oe)
-15
-10
-5
0
5
10
15
Mn
Co
XM
CD
Asy
mm
etry
(%
)
0.2 0.3
0.1
0.2
3nmCo/NiO
2nmCo/IrMn
2nmCoFe/PtMn[a]
1nmCoFe/PtMn[a]
3nmCoFe/PtMn[b]
Spinned (mBohr)
Inte
rfac
ial E
nerg
y (m
J/m
2 )
Ohldag et al. PRL 2003
Redox reaction atCo/NiO interface
AFM Interface loop compared with FM loop
Coupling energy scales with density of pinned spins
NiO(001)
NiO/Si
Stöhr et al., PRL 1999Scholl et al., Science 2000Ohldag et al., PRL 2001
s
XMCDCoL3/L2Co
LaFeO3
E
LaFeO3
B/AXMLD
Ferromagnet
Antiferromagnet
[100] [110]
X
h
H
rightE
leftE
NiO XMLD
Co XMCD
5 m
[011]
[011]
HAB
FM
A FM
H
FM
A FM
A B
H
H
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
0.85
0.90
0.95
1.00
1.05
1.10
1.15
1.20
1.25
L 2 ra
tio
Field (T)
HE H||E
0
10
20
30
40
50
Wall Angle Fit Error
An
gle
(°)
[110]
[100]
X
h
H
horE
verE
Y
Z
PEEM imaging shows that the exchange
coupling of Co to a NiO(001) substrate
results in a uniaxial anistropy. Two classes
of Co domains possess easy axes along in-
plane <011> directions. A magnetic field
applied along one <011> direction leads to
switching of one class of domains at low
field (A) and rotation of the other class of
domains at high field (B).
Linear dichroism spectroscopy (XMLD)
measures the rotation of the NiO anti-
ferromagnetic axis in response to the
rotation of the Co magnetization in an
external field. Spectra show no spin-flop
of NiO without Co cap layer but evidence
of a rotation of the surface magnetic
moments in the presence of a Co layer.
The layer acts like a lever that twists the
magnetic structure of the AFM. A planar domain wall parallel to
the interface is wound up. The data was fitted applying a model
developed by Mauri et al. Fitting parameters are the interface
exchange stiffness A12 and the antiferromagnetic domain wall
energy EAFM. For Co/NiO(001) we find EAFM = 0.66 mJ/m2 and A12 =
1.52·10-13 J/m.
AFM domain wall energy Interface energy
FM anisotropy Zeeman energy
Uncompensated spins at the
surface of the NiO single
crystal are magnetically
coupled to a Co layer,
indicated by the identical FM
domain images of Ni and Co.
The uncompensated spins
are the result of a chemical
reaction at the Co/NiO interface leading to a reduction of the
NiO surface and partial oxidization of the Co layer.
Uncompensated interface spins were also observed for other
combinations of ferromagnets and ferromagnets, for example
CoFe/PtMn and Co/IrMn. Element-specific hysteresis loops of
the uncompensated moments at the surface of the
antiferromagnet showed a pinned fraction, which was aligned
with the bias direction of the material. Rotation of the bias
direction led to a clear vertical loop shift. The amount of
pinned uncompensated spins in several materials was found
to be proportional to the exchange bias interfacial energy,
suggesting that pinned moments are responsible for the bias.
NiO/MgO(001)LaFeO3
PEEM-2 at BL 7.3.1.1 Octupole magnet at BL 4.0.2
PEEM images of
Antiferromagnets
X-ray absorption spectroscopy (XAS) is an element-specific technique that
measures the chemical state and the electronic structure of materials. The
magnetization of a ferromagnet relatively to the x-ray propagation direction can be
determined using circularly polarized x-rays (X-ray Magnetic Circular Dichroism).
Sum rules quantitatively determine the spin and orbital moment. The magnetic axis
of antiferromagnets can be sensed using linearly polarized x-rays (X-ray Magnetic
Linear Dichroism). In combination with microscopy techniques like Photoemission
Electron Microscopy (PEEM), x-ray techniques are uniquely capable of visualizing
domain structures of ferromagnets and antiferromagnets at high spatial resolution.
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
0.0
0.1
0.2
0.3
0.4
0.5
0.6
NiO(001) NiO/Ag(001) poly-NiO/Si RT poly-NiO/Si 400K
XM
LD
(re
l. to
1)
Field (T)
The domain wall rotation of different NiO materials in contact
with 2.5 nm Co is compared. A strong rotation is found in a
NiO single crystal. An epitaxial NiO film on Ag(001) shows an
intermediate rotation while a polycrystalline NiO film on Si
shows a very small rotation. Only the polycrystalline film
possesses a significant exchange bias. The decreasing
Mauri-Model:
-200 -150 -100 -50 0 50 100 150 200
-10
-5
0
5
10
L3
XM
CD
(a
.u.)
Field (mT)
poly-NiO/Si RT 400 K
structural quality going from a single crystal over an epitaxial film to a
polycrystalline film results in a higher defect density and better pinning
of domain walls in the soft, low-anisotropy antiferromagnet NiO. On one
hand this leads to a greatly reduced planar wall rotation, on the other
hand to much better biasing properties. We learn that good exchange
bias materials are characterized by a strong anisotropy or a high defect
density to prevent erasure of the bias state by the generation of a planar
wall. A planar wall will likely play no role in high-anisotropy
antiferromagnets but needs to be taken into consideration in soft
antiferromagnets, like NiO.
• Exchange bias is mediated by uncompensated spins at the surface of the antiferromagnet.
• Uncompensated spins are randomly distributed and lead to an enhanced bias in small,
lateral domains in accordance with Malozemoff’s model.
• Planar walls appear in soft, structurally perfect antiferromagnets but likely play no role in
hard, polycrystalline antiferromagnets with good exchange bias properties.
Scholl et al., PRL 2004
The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
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