Photon Factory Activity Report 2019 #37 (2020)

BL-7A/2019G028

Investigation of perpendicular magnetic anisotropy
in sputter-deposited Fe/MgO heterostructures by X-ray absorption spectroscopy
Yuki Iida1, 2, *, Qingyi Xiang2, Jun Okabayashi3, Hiroaki Sukegawa2 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.
21 Introduction
Ultrathin Fe/MgO heterostructures are known to show
large interface perpendicular magnetic anisotropy (PMA)[1], which is an important factor for consisting of
perpendicular magnetic tunnel junctions (p-MTJs). It is
usually prepared by electron beam evaporation (EB) or
molecular beam epitaxy (MBE) while there are few reports
about sputter-deposition [1-4]. Considering the recent
developments of wafer bonding techniques for MTJs,
needs for sputter-deposition of Fe/MgO is increasing [5].
In this study, we prepared Fe/MgO heterostructureswith PMA by rf-sputtering and revealed the differences ofinterface states with EB-grown samples by the X-ray
absorption spectroscopy.
2 Experiment
The multilayered stack of MgO-seed(5 nm)/Crbuffer(30 nm)/Fe(0.7 nm)/MgO(2 nm) was deposited on
MgO(001) substrate by rf-sputtering. The sputterdeposition was performed at room temperature. MgOsubstrate, Cr-buffer and top-MgO were post-annealed at
500 ˚C. As the reference, we also prepared EB-grown
Fe/MgO, which stack is same with sputter-deposited
samples. The EB-growth was performed at 150 ˚C. MgOsubstrate, Cr-buffer, Fe and top-MgO were post-annealed
at 800 ˚C, 800 ˚C, 250 ˚C and 400 ˚C, respectively.
Magnetic properties were characterized by the vibrating
samples magnetometer (VSM). In order to investigate
more detailed interface states, the X-ray absorption
spectroscopy was performed at KEK-PF BL-7A using thetotal electron yield mode. Circularly polarized soft x-ray isincident in the direction perpendicular to the sample’ssurface. At the same time, the magnetic fields of ±1.0 T
were applied parallel to incident X- ray.
3 Results and DiscussionIn sputter-deposited samples, the saturation
magnetization Ms and the effective PMA energies Keff wasdeduced to be 1.78 T and 0.77 MJ/ m3, respectively, fromthe results of the VSM. On the other hand, Ms and Keff ofEB-grown samples was 1.90 T and 1.2 MJ/m3, respectively.
Although the Keff obtained in sputter-deposited samples
was larger than that in other ferromagnetic/oxide interfaces[6], it was not as high as that in EB-grown ones.
Figure 1 shows the X-ray absorption spectroscopy of Fe
L2, 3-edges for samples made by (a) sputtering and (b) EB.
In Fig.1 (a), the satellites were observed at 708, 722 eV,

while they were not confirmed in Fig.1 (b). These satellites
indicate the Fe surface is oxidized in sputter-deposited
samples with PMA. These results are different from thetheoretical calculation, where the PMA energies
significantly decrease at Fe-O/MgO interfaces [7].
Therefore, the observed PMA might be derived fromanother mechanism enhancing PMA at the interface
between Fe and Fe compounds. Further investigation
including structural analysis would be necessary.

Figure 1 Normalized XAS of Fe L2, 3-edges for samples
made by (a) sputtering and (b) EB. μ+, - indicates the rightand the left-hand side circularly polarized X-ray,respectively. In Fig.1 (a), black arrows indicate thesatellites.
References[1] J. W. Koo et al., Appl. Phys. Lett. 103, 192401 (2013).
[2] S. Yuasa et al., Nat. Mater. 3, 868 (2004).
[3] J. Okabayashi et al., Appl. Phys. Lett. 105, 122408 (2014).
[4] T. Nozaki et al., Phys. Rev. Appl. 5, 044006 (2016).
[5] K. Yakushiji et al., Appl. Phys. Express 10, 063002 (2017).
[6] S. Ikeda et al., Nat. Mater. 9, 721 (2010).
[7] H. X. Yang et al., Phys. Rev. B 84, 054401 (2011).

* IIDA.Yuki@nims.go.jp