Photon Factory Activity Report 2018 #36 (2019)

BL-7A/2017G060

Perpendicular magnetic anisotropy in W-inserted Fe/MgO interfaces
studied by X-ray magnetic circular dichroism
Yuki Iida1,2,*, Qingyi Xiang2, Jun Okabayashi3, Thomas Scheike2, Xiandong Xu2, Kazuhiro Hono1,2,Hiroaki Sukegawa2 and Seiji Mitani1,2Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki Japan.
2
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.
1explained by orbital moment anisotropy as the Keffis proportional to ΔMorb [4,5]. Further, we foundthat Fe and W were completely mixed by thestructure analysis of Fe/W(0.05 nm)/MgO usingtransmission electron microscopy. It suggests thatW penetrates into Fe due to the large mass of W
atoms.
1.53Keff (MJ/m )Large perpendicular magnetic anisotropy(PMA) energy is strongly required to stabilize thedirection of magnetization in magnetic tunnel
junctions. The highest PMA energy (Keff) wasreported at single-crystalline Fe/MgO interfaces[1]. In order to improve Keff values, heavy metal
doping to the interfaces has been investigatedthoroughly [2], First-principles calculationsuggests that the large interface PMA energy canbe expected when the lattice constant of inserted W
is distorted and matched to that of Fe or MgAl2O4[3]. In this study, we aim to achieve the large PMA
energy by inserting W to epitaxial Fe/MgO
interface and investigate interface spin and orbitalstates.
The samples of MgO-seed(5 nm)/Cr-buffer(30 nm)/Fe(0.7 nm)/MgO(2 nm) weredeposited on the MgO substrate using molecularbeam epitaxy at 150˚C, while W was deposited toFe/MgO interfaces or into Fe using rf sputtering atroom temperature. An annealing process wascarried out at 800˚C for MgO substrate and Crbuffer, at 250˚C for Fe, 400˚C for top-MgO. X-ray
absorption spectroscopy (XAS) and magnetic
circular dichroism (XMCD) measurements werealso performed for Fe L2,3 edges to evaluate thespin and orbital magnetic moments Mspin, Morb forgrazing incidence (GI) and normal incidence (NI)geometries.
As shown in Fig. 1, in the case of Fe/W/MgO,
PMA energies decrease by the insertion of W,
while larger PMA energies can be obtained thanthat of normal Fe/MgO interfaces when 0.05nmthick W is inserted into Fe.
Mspin and Morb of various Fe(0.7 nm)/W(tw nm)/MgO(2 nm) heterostructures were deduced fromXMCD analyses. Figure 2 shows XAS and XMCD
taken at NI and GI. In the case of 0.15nm thick W
insertion, XMCD in both NI and GI keep resemble
line shapes as shown in Fig. 2(b), suggesting thatthe anisotropy of orbital moment ΔMorb decreases
with the insertion of W. Therefore, the PMA can be

Fe(5ML)/W/MgO
Fe(4ML)/W/Fe(1ML)/MgO
Fe(3ML)/W/Fe(2ML)/MgO

1.00.50.0
0.00 0.05 0.10 0.15 0.20 0.25
tW (nm)Fig. 1 Keff values depending on tW bymagnetization measurement. Each line
indicates the position of W in Fe/MgO
interfaces.

Fig. 2 (a) Fe-L2,3 edges absorption spectra in NIand (b) XMCD in Fe(0.7 nm)/W(0.15 nm)/MgO(2 nm).
References[1] J. W. Koo et al., Appl. Phys. Lett. 103, 192401 (2013).
[2] T. Nozaki et al., NPG Asia Mater. 9(12), e451 (2017).
T. Nozaki et al., APL MATERIALS 6, 026101 (2018).
Y. Matsumoto et al., IEEE Trans. Magn. 51, 1 (2015).
Y. Matsumoto et al., Appl. Phys. Express 10, 063005 (2017).
[3] K. Masuda and Y. Miura, Phys. Rev. B 98, 224421 (2018).
[4] Y. Iida et al. Appl. Phys. Lett. 113, 252401 (2018).
[5] J. Okabayashi et al., Appl. Phys. Lett. 105, 122408 (2014).
*IIDA.Yuki@nims.go.jp