Huge magneto-crystalline anisotropy of x-ray linear dichroism observed
  on Co/FeMn bilayers
Huge magneto-crystalline anisotropy of x-ray linear dichroism observed on Co/FeMn
bilayers
W. Kuch∗
Freie Universität Berlin, Institut für Experimentalphysik, Arnimallee 14, D-14195 Berlin, Germany
F. Offi,† L. I. Chelaru,‡ J. Wang,§ K. Fukumoto,¶ M. Kotsugi,∗∗ and J. Kirschner
Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, D-06120 Halle, Germany
J. Kuneš
Theoretical Physics III, Center for Electronic Correlations and Magnetism,
Institute of Physics, University of Augsburg, D-86135 Augsburg, Germany. and
Institute of Physics, Academy of Sciences of the Czech Republic,
Cukrovarnická 10, 162 53 Praha 6, Czech Republic.
(Dated: 23.03.2007)
We present an x-ray spectromicroscopic investigation of single-crystalline magnetic FeMn/Co
bilayers on Cu(001), using X-ray magnetic circular (XMCD) and linear (XMLD) dichroism at the
Co and Fe L3 absorption edges in combination with photoelectron emission microscopy (PEEM).
Using the magnetic coupling between the ferromagnetic Co layer and the antiferromagnetic FeMn
layer we are able to produce magnetic domains with two different crystallographic orientations of
the magnetic easy axis within the same sample at the same time. We find a huge difference in the
XMLD contrast between the two types of magnetic domains, which we discuss in terms of intrinsic
magneto-crystalline anisotropy of XMLD of the Co layer. We also demonstrate that due to the high
sensitivity of the method, the small number of induced ferromagnetic Fe moments at the FeMn–Co
interface is sufficient to obtain magnetic contrast from XMLD in a metallic system.
PACS numbers: 75.70.Ak, 68.37.-d, 75.50.Ee
I. INTRODUCTION
The recent interest in the magnetic coupling between
antiferromagnetic (AF) and ferromagnetic (FM) mate-
rials is motivated by the quest for fundamental insight
into the phenomenon of exchange bias.1 This effect, the
discovery of which dates back to the 1950’s,2 manifests
itself in a shift of the magnetization curve along the field
axis. Nowadays the exchange bias effect is employed in
a variety of devices, such as sensors or hard disk read
heads, based on magnetic thin films.3,4
Only few methods can be used to study the spin struc-
ture of ultrathin antiferromagnetic films. While neutron
diffraction and Mössbauer spectroscopy have been suc-
cessfully employed to explore the spin structures of many
bulk antiferromagnets already decades ago, both meth-
ods suffer from a lack of signal if films of a few atomic
layers are to be investigated. X-ray magnetic linear
dichroism (XMLD) in the soft x-ray absorption, on the
other hand, is a method with sub-monolayer sensitivity.
XMLD refers to the difference between x-ray absorption
spectra for the plane of x-ray polarization aligned parallel
and perpendicular to the atomic moments.5 By symme-
try the XMLD signal does not depend on the orientation
of magnetic moments but only on their axial alignment,
and is thus suitable for the investigation of ferromagnetic
as well as collinear antiferromagnetic spin structures, an-
tiferromagnetic thin films in particular. Linear dichroic
signal is also encountered in case of structural, not mag-
netic, reduction of the symmetry, as is commonly the
case in the direction along the film normal due to the
presence of interfaces.6,7 This can be eliminated if mea-
surements are compared in which only the direction of the
spin axis is varied, while the lattice geometry is fixed. In
the following we will use the acronym “XMLD” for this
situation only.
Unlike x-ray magnetic circular dichroism (XMCD),
which in 3d metallic systems essentially measures inte-
gral quantities, namely the spin and orbital magnetic
moments, the size and shape of XMLD depend also on
the details of the electronic structure. Although in-
tegral sum rules have been put forward for XMLD,8
which relate the integral over the XMLD signal to the
magneto-crystalline-anisotropy energy, this integral is
usually much smaller than the amplitude of the plus–
minus feature in the XMLD spectrum. Theoretical cal-
culations predict that the latter may vary significantly
with the crystallographic orientation of the magnetic
moments.9 This is what we call the magneto-crystalline
anisotropy of XMLD.
In this paper we study the magneto-crystalline
anisotropy of XMLD of a thin Co layer. We use the AF–
FM coupling between the FeMn and Co layers to manipu-
late the orientation of the Co moments, namely the obser-
vation that Co moments in a Co/FeMn bilayer align along
〈110〉 directions when in contact with a magnetically dis-
ordered (paramagnetic) FeMn layer (above its Néel tem-
perature), while they prefer 〈100〉 directions when the
FeMn layer magnetically orders (antiferromagnetic).10,11
We take advantage of the fact that the Néel temperature
depends on the thickness of the FeMn layer.12 Growing
wedge-shaped samples we are thus able to study both
〈110〉- and 〈100〉-oriented domains at the same time.
We use a photoelectron emission microscope (PEEM),
as described in Refs. 13,14,15 for the microscopic laterally
resolved detection of the x-ray absorption cross section of
the FeMn/Co bilayers. In combination with x-ray mag-
netic circular dichroism (XMCD) in the soft x-ray absorp-
tion as a magnetic contrast mechanism, PEEM is rou-
tinely used for the element-resolved observation of mag-
netic domain patterns in multilayered structures.13,16,17
XMLD can equally serve as the magnetic contrast mech-
anism for PEEM if linearly polarized x-rays are used.
Here, a combined XMCD and XMLD spectromicroscopic
investigation of single-crystalline FeMn/Co bilayers on
Cu(001) is presented. We find that the XMLD signal
of the FM Co layer exhibits a strikingly different behav-
ior when in contact with a paramagnetic and an anti-
ferromagnetically ordered FeMn layer, while the XMCD
contrast does not differ appreciably. We compare our ob-
servations to ab initio calculations of the L3 XMLD in
bulk fcc Co for different crystallographic orientations of
the magnetic moments.
Investigation of the influence of spin and electronic
structure on the XMLD requires single-crystalline sam-
ples with well characterized AF–FM interfaces. Be-
cause of the small lattice mismatch (0.4%),18 Fe50Mn50
films (FeMn in the following) on a Cu(001) single crys-
tal are ideal candidates for such investigations. Epitax-
ial, virtually unstrained FeMn films can be grown in a
layer-by-layer mode by thermal deposition on Cu(001)
at room temperature.12 This provides an opportunity
to study the magnetic properties of an AF/FM system
in single crystalline FeMn/Co and Co/FeMn bilayers on
Cu(001).10,12,19 Scanning tunneling microscopy revealed
atomically smooth interfaces with islands or vacancies
of single atomic height.20 Based on XMCD-PEEM in-
vestigations of FM/FeMn/FM trilayers and on XMLD
spectroscopy experiments of Co/FeMn bilayers, we con-
cluded previously that a non-collinear three-dimensional
spin structure is present in the ultrathin FeMn layers,
possibly similar to the so-called 3Q spin structure present
in bulk FeMn.21 Combination of the Kerr magnetometry
and XMCD-PEEM imaging showed that the magnetic
coupling across the interface is mediated by step edges
of single atom height, while atomically flat areas do not
contribute.22
II. EXPERIMENT
All experiments were performed in-situ in an ultrahigh
vacuum system with a base pressure below 10−8 Pa. The
disk-shaped Cu(001) single crystal was cleaned by cycles
of 1 keV argon ion bombardment at 300 K and subse-
quent annealing at 873 K for 15 minutes. The surface
exhibited a sharp (1× 1) low energy electron diffraction
pattern. No contaminations were detectable by Auger
electron spectroscopy (AES).
The films were grown by thermal evaporation on the
clean substrate at room temperature in zero external
magnetic field. Fe and Co were evaporated by electron
bombardment of high purity wires (99.99% purity) of 2
mm diameter, while a rod (99.5% purity) of 4 mm di-
ameter was used for Mn. FexMn1−x films of equiatomic
composition (x = 0.50 ± 0.02) were obtained by simul-
taneous evaporation of Fe and Mn from two different
sources. During the deposition the pressure in the cham-
ber was kept below 5 × 10−8 Pa. A typical evaporation
rate was 1 ML per minute. The composition of the FeMn
films was estimated from the evaporation rates of the two
sources, determined by medium energy electron diffrac-
tion (MEED), and cross-checked by Auger electron spec-
troscopy peak ratios. No indication of segregation of Cu
into or on top of the FeMn layers was found. The thick-
ness of the films was determined by MEED, which shows
pronounced layer-by-layer oscillations.12 The FeMn layer
was grown in the form of small wedges of 200 µm width,
using the method described in Ref. 23.
The experiments were performed at the UE56/2-
PGM1 helical undulator beamline of the Berlin syn-
chrotron radiation facility BESSY, which can be set to
deliver circularly polarized radiation of either helicity
with a degree of circular polarization of about 80%, or
linear vertical or horizontal polarization of > 97%.24 The
set-up of the electrostatic PEEM was identical to that de-
scribed in Refs. 13,14,15. The light was incident at an
angle of 30◦ with respect to the sample surface. Rotation
of the sample about the surface normal allowed to take
images for different x-ray azimuthal angles of incidence.
Parameters were set to a lateral resolution of 350 nm,
and a field of view of 60 µm.
The XMCD images represent a grayscale-coded ab-
sorption asymmetry for opposite helicities of the circu-
larly polarized x-rays at the L3 absorption maximum
(777.5 eV),
AXMCD =
I+ − I−
I+ + I−
, (1)
i.e., the difference of absorption images acquired with
opposite helicities divided by their sum. For the quan-
titative analysis, background images acquired at lower
photon energy (5 eV below the L3 maximum) were sub-
tracted.
For XMLD, the maximum contrast was determined
from a series of images acquired with 0.2 eV photon en-
ergy step around the maximum of the Co L3 absorption
peak of a 6 ML Co/Cu(001) film, using p-polarized light.
Maximum contrast was found between images taken at
photon energies E1 = 776.5 eV and E2 = 777.9 eV. Since
the acquisition time necessary to observe XMLD contrast
at the Fe L3 edge in FeMn/Co/Cu(001) bilayers was of
the order of hours, no such photon energy sweeps were
undertaken for the Fe L3 edge (maximum at 707.5 eV);
instead, the same relative photon energies as determined
for the Co L3 edge were tentatively used (E1 = 706.5 eV
FIG. 1: Magnetic domain images of FeMn/Co/Cu(001) struc-
ture obtained at the L3 edges of (a), (b) Co, (c), (d) Fe, and
(e) Mn. The thickness of the FeMn layer, increasing from
the top to the bottom of each image, is shown on the ver-
tical axis. Crystallographic orientation of the Cu substrate
and azimuthal angle of incidence are shown in panels (a) and
(b), respectively. The left and right columns represent the
XMCD and XMLD contrast, respectively. Arrows indicate
the domain magnetization.
and E2 = 707.9 eV). Images at the two photon energies
were taken using s and p polarized x-rays. Because of
the 30◦ incidence, XMLD from in-plane magnetization is
larger by a factor of 4/3 for s polarized excitation. Tak-
ing into account the opposite sign of the effect for the
two polarizations we used the following formula for the
XMLD contrast
AXMLD =
Is(E2)− Is(E1)
Is(E2) + Is(E1)
Ip(E2)− Ip(E1)
Ip(E2) + Ip(E1)
The images thus reproduce quantitatively the difference
between images taken at the higher photon energy minus
images taken at the lower photon energy for s polarized
x-rays, divided by the sum of these images.
III. RESULTS AND DISCUSSION
This section is divided into three parts. First, we
demonstrate the performance of the spectromicroscopic
domain imaging and present the contrast obtained on the
L3 edges of Co, Fe and Mn. Next, we study the depen-
dence of the XMCD and XMLD contrast at the Co L3
edge on the azimuthal angle of incidence. Finally, we
discuss the quantitative difference between the XMLD
signal obtained from 〈110〉 and 〈100〉 domains.
In Fig. 1 typical XMCD and XMLD images obtained
at the L3 edge of Co are shown together with those ob-
tained at the L3 edges of Fe and Mn. A wedge-shaped
FeMn/12 ML Co bilayer was used. The Co XMCD im-
age (a) shows micron-sized magnetic domains. While the
domain magnetization lies along 〈110〉 directions in the
upper half of the image, only domains with magnetiza-
tion along 〈100〉 can be seen the lower half. This be-
havior was observed previously,10,11 and is related to the
fact that for less than about 10 ML FeMn thickness is
paramagnetic at room temperature, while thicker FeMn
layers develop AF order.12
Identical domain patterns with strongly reduced
XMCD contrast are observed at the Fe (c) and Mn (e) L3
edges. The contrast arises due to the induced moments
in the FeMn layer.19 The Mn image is a negative of the Fe
and Co images, indicative of an antiparallel orientation
of the Mn moments with respect to the magnetization
direction of the FM Co layer.
Images of the same spot obtained with XMLD as mag-
netic contrast mechanism are shown in the right column
of Fig. 1. The magnetic domain pattern is clearly visible
at the Co L3 edge (b). Note that domains with oppo-
site magnetization direction cannot be distinguished by
XMLD (compare the right lower parts of images (a) and
(b)). The XMLD contrast at the Fe L3 edge is much
weaker than at the Co L3 edge. Only after averaging over
images with about 170 minutes total acquisition time we
were able to recognize at least the magnetic domains in
the top part of the image.
The images of Fig. 1 are presented on quite different
grayscale ranges: While the full contrast from saturated
white to saturated black in the Co XMCD image (a) is
20%, it is amplified to 8% for the Fe (c) and 1.5% for Mn
(e) XMCD images, and amounts to 3% in the Co XMLD
image (b), and only 0.7% in the Fe XMLD image (d).
Our previously published XMLD spectra,21 in which
the XMLD signal at the Fe L3 edge is below the noise
level, supported a non-collinear antiferromagnetic ar-
rangement of Fe spins in the AF FeMn layer. In this
case only the small induced ferromagnetic moment in
the FeMn layer, the XMCD signal of which corresponds
to about 30% of the Fe atoms in the interface atomic
layer,19 leads to an XMLD signal. Although XMLD has
been successfully applied in the past to image antifer-
romagnetic domains in PEEM,25,26,27,28 no attempt was
made on metallic antiferromagnets, and it is commonly
believed that the reduced crystal field splitting of the
electronic states in metals9,29,30 is prohibiting the use of
XMLD for magnetic imaging. Fig. 1 (d), however, shows
that it is possible to image the XMLD signal, even of
the comparably low number of the induced moments, by
PEEM.
Quantitative estimates support the interpretation of
the contrast observed in Fig. 1 (d): In the top part of the
image, the Fe XMLD is about a factor of 6 weaker than
the Co XMLD at the same position. This is about the
same ratio as between the respective XMCD contrasts in
panels (a) and (c) at about 6 ML FeMn thickness. This
size of induced ferromagnetic alignment is consistent with
our earlier investigation of FeMn/Co bilayers.19 Further-
more, the Fe XMLD originating from induced moments
at the interface decreases with increasing total FeMn
thickness, so that the expected Fe XMLD signal would
be within the noise of the measurement of the spectra of
Ref. 21, which were taken for a 15 ML FeMn film.
Fig. 2 shows a series of magnetic domain images of the
Co layer from a sample in which a 0–25 ML wedge of
FeMn was deposited on top of a continuous film of 6 ML
Co/Cu(001). The left column shows the XMCD contrast
at the Co L3 absorption maximum. The right column
shows images of the same spot of the sample, acquired
with linear polarization of the x-rays. As in Fig. 1, the
FeMn thickness increases from the top to the bottom of
the images. Panels (a) through (g) show images obtained
for different azimuthal angles of incidence, indicated by
an arrow at the right hand side of each panel. Note that
the field of view slightly shifted due to readjustment of
the sample. The azimuth angle was read from a dial at
the sample holder with an accuracy of 1◦. 0◦ corresponds
to the nominal [010] direction of the Cu substrate; how-
ever, as will be outlined below, the angular dependence
of the magnetic contrast indicates that the real [010] di-
rection was at −2◦ azimuth angle. This deviation from
the nominal direction is within the accuracy with which
the substrate could be oriented upon mounting to the
sample holder.
The data have been taken in the sequence from (a) to
(g). Typical acquisition times were 4 minutes per he-
licity for the circular polarization, and 20 minutes per
polarization direction and photon energy for the linear
polarization. Including the necessary sample manipu-
lations, the time to obtain the data of Fig. 2 totalled
28 hours. The time evolution of the domain pattern is
clearly visible in the (a)–(g) series from the shift of the
transition line between paramagnetic and antiferromag-
netic FeMn towards higher thicknesses. We attribute this
to progressing contamination and possibly oxidation by
residual gas of the surface of the FeMn layer. The Co
layer, however, which is investigated here, is protected
against contamination by the FeMn overlayer.
The right column of Fig. 2 shows images taken with
linear x-ray polarization. The upper parts of the im-
ages clearly reveal identical domain pattern as seen with
XMCD. The behavior of the XMLD contrast follows the
geometric expectations including the reversal of contrast
between panels (a) and (c), and near vanishing of the
contrast in panel (b) for the azimuthal angle of incidence
close to 45◦ with respect to the magnetization. The most
prominent observation, and the main result of this work,
is the strong suppression of the XMLD contrast visible
in the bottom parts of the images which correspond to
domains with 〈100〉 direction of magnetization.
In the following we discuss the angular dependence of
XMCD and XMLD contrast in detail. In Fig. 3 we show
the Co XMCD contrast as a function of the azimuthal
FIG. 2: Magnetic domain images of a FeMn/Co bilayer on
Cu(001), acquired at the Co L3 edge with circular polarization
(left column) and linear polarization (right column). Rows
(a)–(g) correspond to different azimuthal angles of incidence,
as labeled at the right hand side and indicated by arrows.
Local magnetization directions are indicated by arrows. The
FeMn thickness increases from top to bottom in each image,
from 7.2 to 14.7 in (a), gradually shifting to from 9.4 to 16.9
ML in (g).
angle of incidence obtained from the data of Fig. 2. Fig.
3 (a) shows the contrast of the domains in the upper part
of the images, where the FeMn is paramagnetic and Co
domains are magnetized along 〈110〉 directions. Panel (b)
presents the contrast from the lower part of the images,
where the FeMn is antiferromagnetic and Co domains
 [110]
 [110]
 [110]
6040200-20
x-ray incidence azimuth ϕ  (deg)
(b)  [010]
 [100]
 [010]
FIG. 3: Angular dependence of the Co L3 XMCD contrast:
(a) the XMCD signal of three domains with magnetization
directions [11̄0], [1̄10], and [110] represented by solid trian-
gles, circles, and squares, respectively (the crosses at −20◦
are measurements from a Co/Cu(001) reference film without
FeMn layer), (b) the XMCD signal of three domains with
magnetization directions [01̄0], [1̄00], and [010] represented
by open triangles, circles, and squares, respectively. The solid
lines are the result of simultaneous sin(ϕ) fits to the data.
-0.01
6040200-20
x-ray incidence azimuth ϕ  (deg)
 <110>
 <100>
FIG. 4: Angular dependence of the Co L3 XMLD contrast.
The contrast between Co magnetic domains with mutually
perpendicular magnetization direction along 〈110〉 directions
is represented by solid symbols, the contrast between Co do-
mains with perpendicular magnetization direction along 〈100〉
directions is represented by open symbols. The solid lines are
the result of simultaneous sin(2ϕ) fits to the data.
align along 〈100〉 directions. The lines correspond to a
sine fit. The fit reveals a −2◦ phase shift, which can
be attributed to the inaccuracy of the sample mounting.
The angular dependence of the XMCD contrast of the
individual domains confirms very nicely the assignment
of magnetization directions in Fig. 2. A small vertical
offset may be attributed to instrumental asymmetries,
as for example different intensities of the two helicities.
Importantly, the amplitude of the XMCD contrast is only
about 5% lower in panel (b) than in panel (a). The latter
is equal to the contrast of Co domains in a Co/Cu(001)
reference sample without FeMn layer, which is indicated
in Fig. 3 (a) by crosses at −20◦ incidence azimuth.
Fig. 4 shows the angular dependence of the Co XMLD
contrast from data of Fig. 2. Solid and open symbols rep-
resent the contrast between domains with mutually per-
-0.03
-0.02
-0.01
784782780778776774772770
photon energy  (eV)
 Dhesi et al., Co/Cu(001)vic 4°
 Kuch et al., Co/FeMn/Cu(001)
Co L3
FIG. 5: Comparison of different experimental Co XMLD
data from literature. (a): Polarization-averaged absorption
at the Co L3 edge, (b): Difference between absorption for
parallel and perpendicular x-ray polarization. Markers and
thin lines: Fig. 1 of Ref. 30, for 6 ML Co on 4◦ miscut Cu(001),
magnetization along 〈110〉. Thick lines: Data from Fig. 4 of
Ref. 21, for 15 ML FeMn/6 ML Co/Cu(001), magnetization
along 〈100〉, scaled to the same absorption maximum. The
data of Ref. 30 have been shifted in energy by −1.34 eV for
overlap of the absorption curves in (a).
pendicular magnetization direction in the regions where
the FeMn layer is paramagnetic and antiferromagnetic,
respectively. The solid lines are the result of sine fits for
which the phases were fixed at +43◦ and −2◦, respec-
tively, using the result of the fits of Fig. 3. Again, the
data confirm the assignment of the magnetization axes in
Fig. 2. Based on the fits we are able to quantify the sup-
pression of the XMLD contrast in 〈100〉 domains as com-
pared to the 〈110〉 domains to be a factor of 0.28. Such
a large effect of magnetization direction on any physi-
cal quantity is rather unusual in 3d metals, which ex-
hibit only a weak spin–orbit coupling. In order to prove
that we see a genuine magneto-crystalline anisotropy we
next discuss the role of spin non-collinearity and make
comparison to other experimental data and theoretical
calculations.
Besides changing the easy axis direction in the Co
layer, magnetic ordering of the FeMn may also induce
a small non-collinearity of the Co moments. Such
a non-collinear fanning out of the FM moments in
Fe/MnF2 bilayers as a consequence of the AF–FM cou-
pling was recently suggested on the basis of Mössbauer
spectroscopy.31 We consider now if a similar scenario can
explain our XMLD data. A distribution of the Co spins
around a mean direction would lead to a reduction of
both the XMCD and XMLD signals compared to the
fully aligned case. While the reduction of the XMCD
signal is proportional to the reduction of the net mo-
ment, XMLD, due to its different angular dependence, is
more sensitive and depends also on the distribution of the
fanning angles. In the extreme case of moments oriented
-0.02
-0.01
2520151050-5
energy  (eV)
 [110]
 [100]
FIG. 6: Calculated XMLD in the fcc Co for magneti-
zation along two different crystallographic directions. (a):
Polarization-averaged absorption spectrum for [100] magneti-
zation. (b): XMLD difference for magnetization along [100]
(solid line) and along [110] (line and symbols). The data for
[100] magnetization are reproduced from Ref. 9. The energy
scale is relative to the absorption edge.
at 45◦ with respect to the net magnetization, the XMLD
would be reduced to zero, while the XMCD would still be
at 71% of its maximum value. Using the reduction factor
of 0.95 of the net magnetization, obtained from XMCD
contrast, and assuming both binary (moments point at a
fixed angle on either side of the net magnetization) and
normal distributions (Gaussian distribution of angles) of
the fanning angle we arrive at a reduction factor of about
0.81 for the XMLD contrast. Therefore fanning of the Co
moments due to the interaction with the FeMn layer, if
at all present, can explain only a small fraction of the
observed effect, which amounts to the reduction factor of
0.28.
Since the stability of the instrument is not sufficient to
acquire series of microspectroscopic images for different
photon energies, we use published data to compare exper-
imental Co XMLD spectra for magnetization along [100]
and [110] directions. The [100] data are taken from Ref.
21, in which a 15 ML FeMn/6 ML Co bilayer on Cu(001)
was measured. The spectra for Co magnetized along [110]
direction are taken from the work of Dhesi et al., in which
6 ML Co on Cu(001), miscut by 4◦, was measured.30 The
two spectra at the Co L3 edge, rescaled to the same ab-
sorption maximum and shifted to the common position
of the absorption edge, are compared in Fig. 5. Both
spectra had been measured under similar conditions.32
Note that the peak-to-peak ratio of the XMLD spectra
of Fig. 5 (b) cannot be compared directly to the ratio
of the XMLD asymmetry magnitudes of Fig. 4, which
corresponds to the contrast between XMLD signal at the
energies marked by arrows in Fig. 5.33 Although smaller
than the asymmetry anisotropy of 3.6 from Fig. 4, the ra-
tio of peak-to-peak XMLD of 2.3 between the two curves
obtained from Fig. 5 still indicates substantial magneto-
crystalline anisotropy of the XMLD signal.
As mentioned in the Introduction, such a large
anisotropy is unusual in 3d metals since the spin–orbit
coupling is rather weak, e.g. the calculated magneto-
crystalline anisotropy energy in bulk fcc Co is only 2 µeV
per atom.34 Also the XMCD spectrum, which depends
essentially only on integral quantities, namely spin and
orbital moments, exhibits a very small anisotropy.9 As
pointed out by one of us and P. M. Oppeneer, the XMLD
signal in metallic Co depends only weakly on the small
valence band spin-orbit coupling. The major contribu-
tion to XMLD comes from the exchange splitting of the
2p levels (≈ 1 eV).9 The magneto-crystalline anisotropy
then arises from the fact that different final 3d states are
probed for different orientations of the sample magneti-
zation.
To assess the feasibility of our experimental data, we
used the calculated XMLD spectrum of Ref. 9 for the
[100] direction (what is referred to in Ref. 9 as “full cal-
culation”) and augmented these with equal calculations
for the [110] magnetization on the same system (see Fig.
6). In the calculations performed on bulk fcc Co a siz-
able anisotropy of XMLD is found, however, the [100]
exhibits larger XMLD magnitude contrary to the exper-
iment. Before dismissing these results as a disagreement
a few remarks are in order. First, the calculations were
done on bulk material while the experiment is performed
on a thin layer sandwiched by other materials, therefore a
good quantitative agreement is unlikely. Second, we can-
not judge the calculated anisotropy based on the present
data only. Note that due to a slight mutual shift of the
calculated spectra, the [100] contrast at the maximum
amplitude of the [110] XMLD would be rather small.
Such a shift is not present in Fig. 5, where [100] and
[110] spectra obtained on slightly different samples are
compared. Third, a possible non-collinearity of Co spins
due to the presence of the AF FeMn layer would lead to
local moments pointing neither along [110] nor fully along
[100]. Taking these uncertainties into account we draw a
modest, nevertheless non-trivial, conclusion that the the-
ory does not prohibit a magneto-crystalline anisotropy of
XMLD as large as observed in our experiment.
IV. CONCLUSIONS
We have presented a spectromicroscopic PEEM inves-
tigation of the magnetic domain pattern on Co/FeMn
bilayers using XMCD and XMLD as the contrast mech-
anism. The sensitivity of the method allows to visualize
even the tiny XMLD signal of the induced ferromagnetic
moments in the FeMn layer. We have found a factor
of 3.6 difference in the XMLD contrast between the Co
L3 signal from 〈110〉 and 〈100〉 domains in a single sam-
ple. We argue that this huge difference is mainly due to
an intrinsic magneto-crystalline anisotropy of XMLD of
the Co layer. Comparison of experimental XMLD spec-
tra obtained from different samples published previously
and ab initio calculations on bulk fcc Co suggest that
such an anisotropy is indeed possible.
Acknowledgments
We thank B. Zada and W. Mahler for technical assis-
tance, and S. S. Dhesi for providing the data from Ref. 30.
Financial support by the German Minister for Education
and Research (BMBF) under grant No. 05 SL8EF19 is
gratefully acknowledged. J. K. acknowledges the support
by an Alexander von Humboldt Research Fellowship.
∗ Electronic address: kuch@physik.fu-berlin.de; URL: http:
//www.physik.fu-berlin.de/~ag-kuch
† Present address: CNISM and Dipartimento di Fisica, Uni-
versità Roma Tre, Via della Vasca Navale 84, I-00146
Roma, Italy.
‡ Present address: Universität Duisburg–Essen, Institut für
Experimentelle Physik, Lotharstraße 1, D-47057 Duisburg,
Germany.
§ Present address: Department of Physics and HKU-CAS
Joint Lab on New Materials, The University of Hong Kong,
Hong Kong, China.
¶ Present address: SPring-8, 1–1–1 Kouto, Sayo-cho, Sayo-
gun, Hyogo 679-5198, Japan.
∗∗ Present address: Hiroshima Synchrotron Radiation Cen-
ter, 2–313 Kagamiyama, Higashi-Hiroshima, 739-8526 Hi-
roshima, Japan.
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mailto:kuch@physik.fu-berlin.de
http://www.physik.fu-berlin.de/~ag-kuch
http://www.physik.fu-berlin.de/~ag-kuch
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	Introduction
	Experiment
	Results and discussion
	 Conclusions
	Acknowledgments
	References
