Photon Factory Activity Report 2017 #35 (2018)

BL-7A/2017G030

X-ray magnetic circular dichroism spectroscopy analysis ofgraphene/YIG heterostrcuture with Dirac cone spin polarization

Seiji SAKAI1, 2*, Takahiro WATANABE3, 1, Yoichi YAMADA3,111, 32
Songtian LI , Shiro ENTANI , Hiroshi NARAMOTO and Kenta AMEMIYA
1Quantum Beam Science Research Directorate / QST Future laboratory, National Institutes for
Quantum and Radiological Science and Technology QST, Takasaki 370-1292, Gunma, Japan
2Institute of Materials Structure Science, Photon Factory, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan3Institute pf Applied Physics, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8577, Japan
1 Introduction
Graphene and other two-dimensional materials arereceiving increasing attention as potential materials fornanospintronics devices due to their outstanding spintransport properties and low-dimensionality useful for
spin manipulation. The use of the proximity effect ingraphene/magnetic dielectrics heterostructures has been
proposed as an alternative to make up for the weakness ofthe spin-orbit interaction and gap-less nature of graphene
in developing graphene spin transistors [1,2]. Recently, it
has been revealed that a large degree of spin polarizationis generated in the Dirac cone electrons of single layer
graphene (SLG) proximity contacted with yttrium irongarnet (YIG, Y3Fe5O12) [3]. In this study, we investigatedthe electronic and magnetic states of the interface region
in the SLG/YIG heterostructure with the proximityinduced Dirac cone spin polarization by using X-ray
absorption (XAS) and magnetic dichroism (XMCD)
spectroscopy.
2 Experiment and Results
The SLG/YIG heterostructure was synthesized bytransferring a large SLG sheet, which was grown on a Cu
foil by chemical vapor deposition, on a YIG thin film
epitaxially deposited on a GGG(111) substrate. Inaccordance with the manifestation of the magnetic
proximity effect [3], the SLG/YIG sample was annealed
at 1000 K in ultra-high vacuum prior to the measurements,
in order to reduce residues and contamination at theSLG/YIG interface. The XAS and XMCD measurements
were performed at BL-7A of Photon Factory (KEK,Japan). The XAS and XMCD spectra were taken at the CK-edge, O K-edge and Fe L2,3-edge in the total electronyield (TEY) and partial electron yield (PEY) modes withlarger and smaller probing depths (~5 nm in the TEY
mode and < 2 nm in the PEY mode), respectively.
Figure 1 shows the C K-edge XAS spectra taken in thePEY mode with linearly polarized x-ray beams at grazing
incidence (GI) (θ = 30°) and normal incidence (NI) (θ =90°), respectively. A sharp peak A1 (~285 eV) and a broad
structure A2 (~292 eV) in the spectra are in agreementwith those of pristine graphite and attributed to thetransition from the 1s to unoccupied π and σ states ofgraphene, respectively. The remarkable change of the πpeak intensity between GL and NI confirms the adhesionof SLG on the surface of the YIG thin film. Since the πpeak shape is affected by the electronic structure aroundthe Κ and M points of graphene, the narrow shape of theπ peak indicates that there is no significant modificationof the π band of graphene at the SLG/YIG interface
different from the strong degradation of the graphitic
electronic structure caused by the chemical interfacialinteractions [4,5].

Fig. 1: C K-edge XAS spectra of the SLG/YIG
heterostructure taken at GI (θ = 30°) and NI (θ = 90°).
Figures 2a and 2b show (a) the XAS and XMCD
spectra at the Fe L2,3-edge taken at GI in the TEY and PEY
modes, and (b) the magnetic hysteresis loops obtained
from the asymmetry of the XAS intensity at the L3 and L2edges; I(L3) − I(L2) / I(L3)+I(L2), as a measure of themagnetization. In Fig. 2a, the spectra were measured at
100 K in remanence after magnetic saturation by applying
a pulsed magnetic field (300 Oe) along the incidence
direction of circularly-polarized x-ray beams (the circularpolarization factor PC = 0.8). In both the TEY and PEYmodes, the XAS spectra show a double-peak structure atboth the L3 and L2 edges consistent with the previous
report for YIG [6]. The shapes of the XAS and XMCD
spectra are almost similar regardless of the different
probing depth in the TEY and PEY modes. This indicatesthat the proximity contact with SLG has a minor impact
on the electronic structure and magnetism of YIG even in
the vicinity of the SLG/YIG interface, possibly related tothe physical nature of the interfacial interactions between
graphene and YIG which can be deduced from thegraphitic feature in the C K-edge XAS spectra (Fig. 1).
The weak XMCD intensity in remanence (Fig. 2a) is
ascribed to the small magnetization of YIG in remanencePhoton Factory Activity Report 2017 #35 (2018)as shown in Fig. 2b. The negligible remanent
magnetization of YIG at 300 K might be responsible forthe unsuccessful detection of spin polarization ofgraphene in the SLG/YIG heterostructure at 300 K incontrast to the remarkable spin polarization at 100 K [3].

suggested from the PEY spectrum with higher sensitivity
to the interface region located near the sample surface.
Fig. 3: O K-edge XAS spectra of the SLG/YIG
heterostructure taken at GI in the TEY and PEY modes.
Acknowledgement
We gratefully acknowledge helpful discussions with P. B.

Sorokin (MISiS), Z. I. Popov (MISiS), P. V. Avramov(Kyungpook Univ.), K. Ando (Keio Univ.) and Y.
Yamauchi (NIMS). This work was partly supported by
JSPS KAKENHI (Grant Nos. 16H03875 and 17H07376).

Fig. 2: a) Fe L2,3-edge XAS and XMCD spectra of theSLG/YIG heterostructure taken at GI and b) magnetic
hysteresis loops taken at GI with sweeping the magnetic
field along the x-ray incident direction. The measurement
temperatures are 100 K in Fig. 2a and 300 K (solidcircles) and 100 K (open circles) in Fig. 2b.
Figure 3 shows the TEY and PEY O K-edge XAS
spectra taken at GI with linearly polarized x-ray beams. Asharp peak B1 (~529 eV) and a broad structure B2 (~535eV) are attributed to the transitions to the unoccupiedoxygen 2p states (B1) hybridized with Fe 3d band and (B2)
weakly hybridized with Fe 4s and 4p states, respectively.
[7]. It is notable that the intensity of peak A with respect
to peak B is small in the PEY spectrum compared to thatin the TEY spectrum. The decrease of peak A can be
attributed to the decrease of the hole population in the O
2p–Fe 3d hybridized states. The redistribution of thecharge density between the less electronegative carbon tothe more electronegative oxygen at the SLG/YIG
interface might lead to the p-type doping of SLG as
observed in SPMDS [3] and also the decrease of the hole
population in the O 2p–Fe 3d hybridized states as

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* sakai.seiji@qst.go.jp