Photon Factory Activity Report 2018 #36 (2019)

BL-7A/2017G060

Perpendicular magnetic anisotropy in annealing-free La/CoFeB/MgO
studied by X-ray magnetic circular dichroism
Yuki Iida1,2, Jun Okabayashi3, Peng Sheng2, Masamitsu Hayashi2,4, and Seiji Mitani1,21Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki Japan.
Research Center for Magnetic and Spintronic Materials (CMSM), National Institute for Materials
Science (NIMS), Tsukuba, Ibaraki, Japan.
3
Research Center for Spectrochemistry, The University of Tokyo, Bunkyo, Tokyo, Japan.
4
Department of Physics, The University of Tokyo, Bunkyo, Tokyo, Japan.
2

Perpendicular magnetic anisotropy (PMA) is
one of the most important issues for perpendicularmagnetic tunnel junctions (p-MTJs). In p-MTJs,
CoFeB based alloys have been intensively studied,many efforts elucidated the mechanism of theoccurrence of PMA[1]. First principles
calculations show that the hybridization of Fe-3dz2and O-2pz states contributes to the interface PMA[2]. In the case of CoFeB/MgO, the easymagnetization axis is generally changed from inplane to perpendicular-to-plane direction by postannealing. However, interface PMA can also beobtained even for the annealing-free CoFeB/MgO
heterostructures, if specific materials, e.g., lowelectronegativity elements such as Zr, are used forbuffer layer underneth CoFeB [3,4]. In this study,
we investigated interface PMA in annealing-freeLa/CoFeB/MgO heterostructures, since La has
typically low electronegativity. Here, one of thepossible roles of low electronegativity is
modification of interface electronic structure, andthe other is a chemical role such as an oxygenabsorber.
CoFeB (1nm)/MgO (2nm) bilayer structureswith a 2-nm-thick La underlayer were prepared by
rf sputtering on a W buffered thermally oxidized Si
substrates. The whole stacking structures are
Si/SiO2-subs.//W-buffer(3nm)/La(2nm)/CoFeB(1nm)/MgO(2nm)/W-cap(1nm) which are grownat room temperature. We also prepared La(2nm)/CoFeB(1nm)/La(2nm) structure to investigatethat which interfaces contribute to PMA.
Figure 1 shows the M-H curves measured byusing a vibrating sample magnetometer for (a)La/CoFeB/MgO and (b) La/CoFeB/La. Blue andred lines indicate magnetization when magnetic
field was applied to perpendicular and in-plane
direction, respectively. Here, PMA was observedin La/CoFeB/MgO structure, but La/CoFeB/La
showed isotropic M-H curve. These results
indicate La/CoFeB interface does not havedominant influence for PMA.

In order to investigate the detailed mechanism of
the interface PMA, we performed X-ray circularmagnetic dichroism (XMCD) measurements.
Figure 2 shows (a) X-ray absorption spectra (XAS)
and (b) XMCD spectra of Fe-L2,3 edges in theLa/CoFeB/MgO structure. The normal componentof the orbital magnetic moment in Fe is slightly
larger than the in-plane one, and therefore it seems
to be qualitatively the same as the results in XMCD
study for Fe/MgO heterostructure [5]. From theseresults, the role of La buffer layer is likely
considered to absorb boron or oxygen of CoFeB,
and the improvement of CoFeB/MgO interface
brings about interface PMA.

Fig. 1. M-H curves for (a) Si/SiO2 sub.//W/La/CoFeB/MgO/W and (b) Si/SiO2 sub.//W/La/
CoFeB/La/W.

Fig. 2 (a) XAS and (b) XMCD spectra for Fe-L2,3edges in La/CoFeB/MgO heterostructures[1] S. Ikeda et al., Nat. Mater. 9, 721 (2010).
[2] H. X. Yang et al., Phys. Rev. B 84, 054401(2011).
[3] M. Yamanouchi et al., J. Appl. Phys. 109,07C712(2011).
[4] W. Skowronski et al., Phys. Rev. B 91,184410 (2015).
[5] J. Okabayashi et al., Appl. Phys. Lett. 105,122408 (2014).