Photon Factory Activity Report 2017 #35 (2018)

BL-16A/2016S2-005

Magnetocrystalline anisotropy of L10-ordered FePt thin films studied byFe L-edge magnetic field angle-dependent x-ray magnetic circular dichroicsm
Keisuke IKEDA1* , Takeshi SEKI2 , Goro SHIBATA1 , Shoya SAKAMOTO1 , Yosuke NONAKA1 ,Zhendong CHI1 , Yuxuan WAN1 , Masahiro SUZUKI1 , Masako SAKAMAKI3 , Kenta AMEMIYA3 ,Koki TAKANASHI2 and Atsushi FUJIMORI112University of Tokyo, 7-3-1 Hongo, Bunkyo, 113-0033, JapanTohoku University, 2-1-1 Katahira, Sendai, 980-8577, Japan
3Photon Factory, 1-1 Oho, Tsukuba, 305-0801, Japan

1 Introduction
Recently, magnetic recording media with higher
recording density have been required, and materials withstrong perpendicular magnetic anisotropy (PMA) havebeen studied intensively. L10-ordered FePt is well known
as a candidate material because of its large magnetic
anisotropy energy (MAE) of about to 7×107 erg/cc. For
L10-ordered FePt, the MAE can be modified by changing
the degree of long-range chemical order S. Seki et al.
have shown that this S can be controlled by deposition
and annealing temperatures [1]. Despite its importance,
the origin of the PMA of L10-ordered FePt is still notunderstood.
Bruno [2] has derived a simple relationship that theMAE is proportional to the difference of the orbital
magnetic moments between the magnetic hard and easy
axis by perturbation theory with respect to spin-orbit
coupling within the tight-binding approximation. Beside
this calculation, van der Laan [3] has derived anotherterm that has been ignored by Bruno. This term is
proportional to the anisotropy of spin-density distribution,
namely, the magnetic dipole term. In the following, we
refer to the first term, which has been discovered byBruno, the spin-conservation term, and the second term,
which has been pointed out by van der Laan, the spin-flip
term after the spin direction of virtual states inperturbation theory. From the first-principles calculation
study of the MAE of L10-ordered FePt, Solovyev et al.
have indicated that not only the Fe 3d orbitals but also thePt 5d orbitals contribute to the MAE [4]. Theircalculation has shown that the relationship between theorbital magnetic moment anisotropy and the MAE
contradicts the Bruno’s relationship both at the Fe and Pt
sites. To resolve this issue, we have investigated the spinconservation term at the Fe and Pt sites, and concluded
that the spin-conservation term cannot be the origin of its
large PMA [5], consistent with the Solovyev’s study.
Then, our next purpose is to clarify the role of the spinflip term and attempt to understand the origin of the MAEof L10-ordered FePt.
In this work, we have performed magnetic field angledependent x-ray magnetic circular dichroism (XMCD)
measurements of FePt thin films at the Fe L2,3-edge toobtain the magnetic dipole term experimentally and toevaluate the contribution of the spin-flip term to the MAE.

2 Experiment
FePt thin films were grown on MgO (100) substrates
by the ultrahigh vacuum magnetron sputtering method.
The structure of the sample was MgO subs./Fe (1nm)/Au(30nm)/FePt (20nm)/Au (2nm). The deposition
temperature (TS) and annealing temperature (TA) ranged
from room temperature (R.T.) to 500 oC (see Table 1).
Magnetic field angle-dependent XMCD measurement
is an XMCD measurement with changing the magnetic
field direction while fixing the x-ray incident direction.
Frommagnetic
fieldangle-dependent
XMCD
measurement, one can obtain the uniaxial MAE Ku as
well as the magnetic dipole term (Qzz) [6]. In this study,
we have performed this measurement in order to deducethe magnetic dipole term of the Fe sites in L10-ordered
FePt samples with various degrees of long-range
chemical order S. For this measurement, we have
employed a vector XMCD apparatus at KEK-PF BL16A2.
Table 1: Sample preparation conditions and thedegree of long-range chemical order S of eachsample.
TS (oC)TA (oC)
SR.T.
0300
350
0.3
300
500
0.4
3 Results and Discussion
Figure 1 shows that the magnetic field angle-dependent
XMCD spectra of the sample with S=0.4. Here, wedefined θH as the angle measured from the sample normal
to the magnetic-field angle and the magnitude ofmagnetic field were below 1T. The incident x-ray
direction was 30 degree. One can observe that the XMCD
spectrum changes its sign with the magnetic-field angle.
Figure 2 shows that the magnetic field angle dependenceof the effective spin magnetic moments at the Fe site forvarious S. Dashed line is the fitting of the magnetic field
angle dependence of the effective spin magnetic moment
assuming the Stoner-Wohlfarth model. From this fit, one
can deduce the magnetic dipole term of each sample.
Figure 3 shows the deduced quadrupole Qzz comprisingPhoton Factory Activity Report 2017 #35 (2018)the magnetic dipole term plotted against S. The negativevalue of Qzz indicates that the orbitals having spin
magnetic moment are elongated along the z direction, andthis tendency was enhanced with increasing S. We
deduced the contributions of the spin-conservation term
and the spin-flip term of the Fe site to the MAE and thesummation of these two terms, as plotted against S inFig.4. The contribution of the spin-flip term shows anopposite contribution to the PMA, and its magnitude islarger than spin-conservation term which favors the PMA.
Totally, the contribution to the MAE from the Fe site is
opposite to the PMA. This tendency is consistent with thefirst-principles calculation done by Solovyev et al.[4]. We,
therefore, speculate that the the spin-flit term at the Pt site
makes the dominant contribution to the PMA of L10ordered FePt.

Fig. 1: Magnetic field-angle dependent x-ray magnetic
circular dichroism spectra of L10-odered FePt at the Fe
L2,3 edge.

Fig. 3: Quadrupole Qzz at the Fe site, which isproportional to the magnetic dipole, plotted against S. TheQzz shows a negative value and its magnitude increases
with the S.

Fig. 4: Magnetic anisotropy energy of the Fe site,
decomposed into the spin-conservation and spin–flipterms, and their summation, plotted against S. The total
contribution shows a negative value mainly originatingfrom the spin-flip term, indicating that the contribution ofthe Fe site is opposite to the perpendicular magnetic
anisotropy.

References[1] T. Seki et al., J. Phys. D: Appl. Phys. 44 335001(2011).
[2] P. Bruno, Phys. Rev. B 39, 865 (1989).
[3] G. van der Laan, J. Phys.: Condens. Matter 10, 3239(1998).
[4] I. V. Solovyev et al., Phys. Rev. B 52, 13419 (1995).
[5] K. Ikeda et al., Appl. Phys. Lett. 111, 142402 (2017).
[6] G. Shibata et al., npj Quantum Mater. 3 (2018).
Research Achievements
1. JSR2018 students awards
Fig. 2: Effective spin magnetic moment of Fe against
the magnetic field angle θH. The samples with S=0.4 and0.3 show the perpendicular magnetic anisotropy and S=0
shows in-plane magnetic easy axis originating from shapeanisotropy.

* k-ikeda@wyvern.phys.s.u-tokyo.ac.jp