Photon Factory Activity Report 2018 #36 (2019)

BL-7A/2017G060

Strain-Induced Reversible Manipulation of Orbital Magnetic Moments
in Ni/Cu Multilayers on Ferroelectric BaTiO3
Jun Okabayashi1*, Yoshio Miura2, and Tomoyasu Taniyama3
1Research Center for Spectrochemistry, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033
2Research Center for Magnetic and Spintronic Materials, National Institute for Materials Science (NIMS), Tsukuba 305-0047
3Department of Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602

The coupling between ferromagnetic andferroelectric properties has recently attracted
considerable attention toward the creation of novel
devices using multiferroic controlling of theirproperties. In particular, the hetero-interfaces in thinfilms comprising both ferromagnets and electricallypolarized materials produce a rich variety ofpossibilities for creating multifunctional properties.
Modulation of interfacial lattice constants by anelectric field (E) induces interfacial changes inmagnetism. The interfacial lattice distortion produces
variations in magnetic properties, which arerecognized as inverse magneto-striction effects.
Moreover, magnetic anisotropy is also tuned bylattice distortions. Recent attempts have been focusedon the modulation of the number of charge carriers atthe interface between an ultrathin ferromagnetic layer
and an oxide-barrier insulator in magnetic tunnel
junctions. Other approaches, which are our focus inthis study, are based on the interfacial mechanicalstraincoupling
betweenferromagneticandferroelectric layers using multiferroic hybrid
structures. In this study, we focus on thin Ni layers
sandwiched by Cu layers exhibit perpendicularmagnetic anisotropy (PMA) because of the interfacial
tensile strain in the Ni layers.
Recently, E-control of the magnetic properties ofNi/Cu multilayers on BaTiO3 was achieved; themagnetization was switched from the perpendicularaxis to the in-plane easy axis by tuning the lattice
distortion through the application of E [1]. To
understand this relationship and the elastic
phenomena from the view point of morb, wedeveloped a technique by applying an electric field in
XMCD measurements (EXMCD) to clarify themechanism of the electric-field-induced changes in
the magnetic anisotropy of Ni/Cu multilayers onBaTiO3 hetero-structures through lattice distortions.
The samples were grown by using ultra-highvacuum molecular beam epitaxy on [100]-oriented
0.5-mm-thick BaTiO3 single crystal substrates.
Therefore, the bias voltage of 400 V applied between
top and bottom electrodes means the electric field of
8 kV/cm. The stacked structures are shown in Fig. 1a.
The XAS and XMCD measurements for the Ni andCu L-edges were performed at the KEK-PF BL-7A
beamline, at room temperature. The EXMCD
measurements were performed using the partial
fluorescence yield (PFY) mode.
Figure 1b shows the Ni L-edge XAS and XMCD
spectra obtained in the PFY mode by applying anelectric field, E, of positive bias of 8 kV/cm. Sample
surfaces are connected to ground and E is applied tothe back side of the BaTiO3 substrates. The spectralline shapes of the XAS and XMCD are modulated by
E in normal incident (NI) case in spite of fixedsample measurement position. By comparing theresults with and without E, we observed a slightvariation in the peak asymmetries between the L3 andL2 edges. These results reveal that the orbital
moments are modulated by applying E, thereby
resulting in changes of magnetic anisotropy. The
values of spin and orbital moments are estimated tobe 0.56 and 0.055 µB, respectively, for an electricfield of 0 kV/cm, and 0.56 and 0.045 µB, respectively,
for an electric field of 8 kV/cm. The modulation of
the orbital magnetic moments by 0.01 µB upon
applying E is related to the induced lattice distortionof 2% from the BaTiO3 substrates. Moreover, after
releasing E to zero, spectral line shapes also revert tothe pristine state [2]. Therefore, we conclude thereversible changes of morb is induced by lattice
distortion, which is the demonstration of novel
orbital-elastic effect.

Fig. 1, (a) Schematic structure of the sample. The
electrodes for applying the electric field are mounted
at the top (Ni/Cu layer) and the bottom (BaTiO3substrate) of the films. (b) XAS and XMCD spectra
obtained at Ni L-edges in partial-fluorescence-yieldmode with and without an applied electric field.

References[1] Y. Shirahata et al. NPG Asia Materials 7, e198 (2015).
[2] J. Okabayashi et al., NPJ Quantum Mater. 4, 21 (2019).
Press release in Japanese*jun@chem.s.u-tokyo.ac.jp