BL-7A, 16A/2019G028

Perpendicular orbital and quadrupole anisotropies at Fe/MgO interfaces
detected by x-ray magnetic circular and linear dichroisms
Jun Okabayashi,1 Yuki Iida,2,3 Qingyi Xiang,2 Hiroaki Sukegawa,2 and Seiji Mitani2,31
Research Center for Spectrochemistry, The University of Tokyo
2Research Center for Magnetic and Spintronic Materials, National Institute for Materials Science (NIMS)
3
Graduate School of Pure and Applied Sciences, University of Tsukuba
MgO-based magnetic tunnel junctions (MTJs) havebeen
exploited
towardanultrahightunnel
magnetoresistance (TMR) through the large interfacial
PMA between MgO and transition-metal (TM) alloys.
The PMA of 1.4 MJ/m3 at the interfaces between MgO
and Fe has been achieved [1]. As an understanding of
PMA induced at the interface between the ferromagnetic
layer and the MgO barrier layer, the electronic andmagnetic structures of the interfaces have to be clarified
explicitly. In order to evaluate the PMA energy, it isnecessary to elucidate the anisotropy of orbital magnetic
moments along parallel and perpendicular directions tothe surface by x-ray magnetic circular dichroism (XMCD)[2]. A recent study of the theoretical calculations suggests
the existence of the quadrupole tensor (Qij) at the Fe/MgO
interface through the interfacial strain due to the latticemismatching of 3.8% between Fe and MgO [3]. Therefore,
the understanding of interfacial orbital magnetic moments
and quadrupole moments at Fe/MgO is necessary forunveiling the Fe/MgO-based PMA and further functional
interfacial PMA designing. Here, we report the linear
dichroism (XMLD) technique adopting for PMA at single
crystal Fe/MgO(001) interfaces and discuss the effect of
Qij for the PMA beyond the anisotropic orbital moments.
Samples were grown by using ultrahigh vacuum
electron-beam evaporation on MgO(001) substrates. The
XMCD and XMCD were performed at BL-7A and 16A,
respectively, in the Photon Factory (KEK). The total
electron yield mode was adopted, and all measurements
were performed at room temperature. For the XMLD
measurements, the sample was magnetized along thesample surface normal easy-axis direction and reversed tozero to use the remanent states and was rotated to thegrazing incidence geometry.
Figure 1 shows the x-ray absorption spectra (XAS) ofa 0.7-nm-thick Fe/MgO interface by different circular[Fig. 1(a)] and linear [Fig. 1(b)] polarized beams. TheXAS line shapes show clear metallic states without
oxidation are detected. Using the XMCD sum rules, morb
and mseff are estimated to be 0.21 and 1.79 µB, respectively,
for the normal component. Although ms is smaller thanthat in bulk, morb is enhanced at the interface. The inset ofFig. 1 shows the M-H curve in Fe L3-edge XMCD for theeasy-axis direction. Clear square-shape hysteresis
coincides with the magnetization curves measured bySQUID.
The XMLD sum rule suggests that the integrals ofXMLD line shapes are proportional to Qzz. We confirmedin Fig. 1(b) that the integral of XMLD intensities
converges to positive, which is related to the sign of Qzz indichroism to be positive; that is, unoccupied 3dz2 orbitals

are slightly elongated to the out-of-plane direction. We
note that XMLD is defined as (M⊥E) – (M∥E), where
M and E is the magnetization and the electric filed vectorof incident synchrotron, respectively, resulting that thepositive XMLD means preferential coupling with theout-of-plane unoccupied orbitals. However, since thedenominator in the sum rule is two orders of magnitudelarger than that of the numerator, the absolute value of Qzz
is estimated to be less than 0.01, which is interpreted asless than 1% unoccupied orbital polarization of Fe 3dstates. On the other hand, electron occupancies areslightly shrinking through the relation Qxx+Qyy+Qzz=0.
The XMCD with the angular dependence also deduces thequite small magnetic dipole term mT using the spin sumrule. These results are consistent with the fact that thecontribution of mT is two orders of magnitude smaller
than that of ms (1.73 µB) [4]. We note that the notation ofQzz is independent of spin-orbit coupling of Fe, suggestingthat the origin of Qzz is derived from the interfacial
asymmetry and strain.

FIG. 1. (a) XAS and XMCD by circularly polarized beams(b) XAS and XMLD by linearly polarized beams for Fe L-edges
at the Fe /MgO interface. The inset is the XMCD hysteresiscurve at the Fe L3-edge for surface normal direction [4].

References[1] J.W. Koo et al., Appl. Phys. Lett. 103, 192401 (2013).
[2] J. Okabayashi et al., Appl. Phys. Lett. 105, 122408 (2014).
[3] K. Masuda and Y. Miura, Phys. Rev. B 98, 224421 (2018).
[4] J. Okabayashi et al., Appl. Phys. Lett. 115, 252402 (2019);*jun@chem.s.u-tokyo.ac.jp