Photon Factory Activity Report 2022 #40 (2023)

BL-7A/2021G069
Intrinsic Magnetic proximity eﬀect at the Cox Fe3−x O4 /Pt interface
studied by XMCDJun Okabayashi1∗ , Shoto Nodo2 , and Taro Nagahama2
12

Research Center for Spectrochemistry, The University of Tokyo, Bunkyo-ku, Tokyo113-0033, Japan
Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-8628, JapanThe charge-to-spin current conversion phenomena
via spin-orbit interactions have been intensively investigated in the field of spintronics. In particular, thespin Hall eﬀect using heavy metal (HM) elements candirectly convert a charge current into a spin current,
and various useful phenomena have been reported inbilayers with ferromagnetic insulators (FMIs) throughangular momentum exchange at the HM/FMI interface. The magnetic proximity eﬀect (MPE), which induces ferromagnetism in a nonmagnetic HM throughinterfacial exchange interaction, also need to be investigated for interfacial materials designing [1]. The
induced ferromagnetism has been examined in termsof the anomalous Hall eﬀect and an anisotropic magnetoresistance eﬀect in the HM layer. One significant
issue with the MPE is the diﬃculty of experimentally
detecting induced magnetic moments of HMs.
X-ray magnetic circular dichroism (XMCD) is a
powerful tool for investigating this induced magnetism. Significant Pt magnetic moment signals cannot be detected by XMCD in the Y3 Fe5 O12 /Pt [2]and CoFe2 O4 /Pt systems [3]. Previous XMCD reports of CoFe2 O4 /Pt grown by pulsed laser deposition(PLD) found alloying at the interface in Pt deposited
at high temperatures (400◦ C). Since this magnetic signal originates not from the MPE but from the alloying at the interface, the appearance of induced magnetic moments in Pt largely depends on the interfacial structures. This suggests that the MPE depends
on the growth temperature and growth mode suchas molecular beam epitaxy (MBE), PLD, or sputtering deposition. This arises from the diﬀerence in kinetic energies, with atoms deposited by MBE having
lower kinetic energies than those deposited by sputtering, which suppresses the diﬀusion of atoms at theinterfaces. Therefore, in this study, we employ MBEfor the growth of CFO/Pt bilayers to understand theMPE.
Sample structures were MgO (001) substrate/ MgO(20 nm)/ NiO (5 nm)/ Cox Fe3−x O4 (CFO) (50 nm)/Pt (tPt nm). MgO substrates. For Co and Fe Ledges, XAS and XMCD were performed at BL-7A in
the Photon Factory. The photon helicity was fixed,
and a magnetic field of ±1.2 T was applied parallel tothe incident polarized soft x-ray beam to obtain theabsorption signals, defined as µ+ and µ- spectra. Total elections yield (TEY) mode was adopted, and allmeasurements were performed at room temperature.

Figure 1(a) shows the XAS and XMCD of CFO/Pt
after the annealing at 500◦ C for the Fe and Co Ledges. The XAS intensities of Co are suppressed owing to the composition ratio of 1:10, resulting in acomposition of Co0.27 Fe2.73 O4 with x=0.27. The CFO
with low Co composition exhibits perpendicular magnetic anisotropy (PMA) [4,5]. The XAS and XMCD
line shapes for Fe L edges show distinctive features dueto three kinds of Fe states (Fe3+ in Oh , Fe3+ in Td ,and Fe2+ in Oh ). For the Fe L edges, although the difference in XAS is small, clear diﬀerential XMCD lineshapes are detected. The Fe3+ state with Td symmetry exhibits an opposite sign, which is common forspinel-type ferrite compounds. The XMCD hysteresis
curves taken at the Fe3+ (Oh ) peak photon energy arealso displayed. The shapes of hysteresis curves are different between 500 and 600◦ , which is consistent with
the SQUID measurements. The XMCD at Pt L-edges
is also performed and interpreted consistently. Large
XMCD signals in the Co L edge correspond to contribution of the orbital moment through the distorted
Co2+ (3d7 ) Oh site, which might be also related to theMPE [6].

Fig. 1, XAS and XMCD of Co0.23 Fe2.77 O4 facing on Pt
layer after annealing at 600◦ C. Inset shows the hesteresis
curve at Fe3+ (Oh ) site.

References[1]P.K. Manna et al., Phys. Rep. 535, 61 (2014).
[2]T. Kikkawa et al,, Phys. Rev. B 95, 214416 (2017).
[3]H. B. Vasili et al,, ACS Appl. Mater. Interfaces 10, 12031(2018).
[4]M. Morishita et al., Phys. Rev. Mater. 7, 054402 (2023).
[5]J. Okabayashi et al., Phys. Rev. B 105, 134416 (2022).[6]S.
Noto et al., ACS Omega, in press (2023).
*e-mail: jun@chem.s.u-tokyo.ac.jp