Magnetic anisotropy energy and the anisotropy of the orbital moment of Ni in NiÕPt multilayers
F. Wilhelm, P. Poulopoulos,* P. Srivastava,
†
H. Wende, M. Farle,
‡
and K. Baberschke
Institut fu ¨r Experimentalphysik, Freie Universita ¨t Berlin, Arnimallee 14, D-14195 Berlin-Dahlem, Germany
M. Angelakeris and N. K. Flevaris
Department of Physics, Aristotle University of Thessaloniki, 54006 Thessaloniki, Greece
W. Grange and J.-P. Kappler
Institut de Physique et Chimie des Mate ´riaux de Strasbourg (IPCMS), 23 rue du Loess, 67037 Strasbourg, France
G. Ghiringhelli and N. B. Brookes
European Synchrotron Radiation Facility (ESRF), Boı ˆte Postale 220, 38043 Grenoble, France
Received 21 September 1999; revised manuscript received 21 December 1999
Angular-dependent x-ray magnetic circular dichroism experiments are performed on Ni/Pt multilayers at the
Ni L
2,3
edges under magnetic fields up to 50 kG at a temperature of 10 K. By rotating the applied field away
from the easy axis a decrease of the ratio of the orbital-to-spin magnetic moment
L
/
S
of Ni is observed for
perpendicularly magnetized Ni/Pt layers which show considerable magnetic anisotropy energy 20 eV/
atom. Saturation effects in the recorded spectra, depending on both the thicknesses of Ni and Pt, are shown to
lead into erroneous conclusions in the determination of
L
/
S
and they have to be considered in the analysis.
An anisotropy of the Ni-orbital moment
L
as large as 22% of
L
is determined.
L
is demonstrated to be
the origin of the magnetic anisotropy energy of 3 d ferromagnetic elements.
Orbital magnetism of 3 d transition metals has become a
topic of major interest. The spin magnetic moment
S
is a
priori isotropic. In first approximation, by treating the spin-
orbit coupling in a second-order perturbation theory,
1
simi-
larly to the well-known picture of localized magnetism, a
simple relation may be derived linking the magnetic anisot-
ropy energy MAE to the anisotropy of the orbital magnetic
moment
L
, i.e., the difference
L
between an easy- and a
hard-magnetization axis:
2
MAE=
4
B
L
, 1
where is the spin-orbit coupling constant. The prefactor
depends on the electronic structure and, consequently, on the
thickness of ultrathin magnetic layers. It is a function of
L
and not a simple numeric constant as was shown by recent
first-principles calculations.
3–5
Few experimental data for
are available for Co Ref. 6 and Fe Ref. 7 layers only. In a
bulk material with cubic symmetry,
L
is almost quenched
and
L
is very small 10
-4
B
/atom.
8
In ultrathin mag-
netic layers
L
might be increased due to i reduced coor-
dination numbers at surfaces, interfaces, and atomic steps or
ii distortions of the cubic symmetry due to strain. In agree-
ment with Eq. 1, enhanced values of
L
have been re-
corded via x-ray magnetic circular dichroism XMCD in
thin Co Refs. 6,9–11 and Ni Ref. 12 films possessing
large MAE.
Ni allows us to study the largest variety of phenomena in
thin film magnetism.
8
Moreover, Ni-based films show in-
creasing technological interest. Earlier, perpendicularly mag-
netized Co layers had been considered as important materials
for magneto-optic MO recording.
13
However, the undesir-
ably high Curie temperature T
C
of Co has prompted a search
for alternative solutions. The alloying of Co layers with Ni,
for example, was shown to reduce T
C
while the samples
retained good features for storage applications.
14
Recently,
perpendicularly magnetized Ni/Pt multilayers at room tem-
perature have been also recognized as candidates for MO
recording.
15
Ni/Pt multilayers with very thin Ni layers 2
monolayers ML in the multilayer period are the only Ni-
based perpendicularly magnetized multilayers with relatively
large values of MAE.
16,17
For these reasons, we have selected the Ni/Pt multilayers
as a prototype to measure via XMCD the
L
of Ni and to
relate it to the estimated from magnetization-hysteresis-loops
MAE via Eq. 1. Angular-dependent XMCD allows the de-
termination of the ratio
L
/
S
and the separation of
L
and
S
via the application of sum rules.
18
However, it is shown
that the XMCD spectra are strongly influenced by saturation
effects in total electron yield TEY measurements as the
direction of incidence of the x rays approaches the film
plane.
19,20
Thus, erroneously large angular dependence of an
apparent
L
/
S
may be obtained even for bulklike, i.e., iso-
tropic samples. After correcting for the saturation effects we
are able to measure properly the angular dependence of the
true
L
/
S
and then to determine
L
. We show that MAE
scales with
L
and the largest value of
L
is observed
along the easy-magnetization axis as theory predicts.
1
A
prefactor 0.1, determined from Eq. 1, is compared to
previous experimental values for Co Ref. 6 and Fe Ref. 7
layers. It is only now, since the time that the link between
MAE and
L
in the itinerant ferromagnetism was derived
by theory,
1
that we may provide a set of experimental values
for MAE,
L
and for thin layers of all the 3 d ferromag-
netic elements.
PHYSICAL REVIEW B 1 APRIL 2000-I VOLUME 61, NUMBER 13
PRB 61 0163-1829/2000/6113/86474/$15.00 8647 ©2000 The American Physical Society