Photon Factory Activity Report 2017 #35 (2018)

BL-28A, BL-16A/2015S2-003, 2016G096, 2016S2-005

ARPES and XMCD studies of the van der Waals ferromagnet Cr2Ge2Te6
Masahiro SUZUKI1,∗, Bin GAO2, Goro SHIBATA1, Shoya SAKAMOTO1, Keisuke KOSHIISHI1,Yosuke NONAKA1, Keisuke IKEDA1, Zhendong CHI1, Suguru NAKATA1, Kenta HAGIWARA1,Chun LIN1, Yuxuan WAN1, Koji HORIBA3, Hiroshi KUMIGASHIRA3, Kanta ONO3,Sungmo KANG5, Seungjin KANG5, Jaejun YU5, Masaki KOBAYASHI4, Sang-Wook CHEONG2,and Atsushi FUJIMORI1
1Department of Physics, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
2Rutgers Center for Emergent Materials, Rutgers University, Piscataway, New Jersey 08854, USA
3
Photon Factory, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 3050801, Japan
4
Center for Spintronics Research Network, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033,
Japan
5
Center for Novel States of Complex Materials Research, Department of Physics and Astronomy,Seoul National University, Seoul 151-747, Republic of Korea
1 Introduction
In recent years, van der Waals (vdW) ferromagnets,
which have attractive physical properties, have been
intensively studied in order to investigate the origin of theirferromagnetism in 2D crystals [1]. A representative vdW
ferromagnetic insulator Cr2Ge2Te6 (CGT) exhibits
ferromagnetism below ~ 61 K [2,3,4] and the magnetic
moment of 2.3 µB/Cr atom with the easy magnetization
axis along the c-direction [2]. As for the electric properties,
resistivity measurements on CGT have shown
semiconducting behaviors and its resistivity prominently
increases below ~100 K [2,5]. It has also been reported thatthe magnetic behavior of CGT atomic layer is welldescribed by the 2D Heisenberg model with single-ion
anisotropy [6].
To unveil the origin of the ferromagnetism in 2D
materials, understanding the physical properties from boththe electronic and magnetic structure points of view ishighly important.
2 ExperimentSingle crystals of CGT were synthesized using the flux
method. ARPES measurements were performed at beamline BL-28A of Photon Factory. The incident light wascircularly polarized in the energy range of 34-80 eV. ASCIENTA SES 2002 photoelectron analyzer (ScientaOmicron Co., Ltd.) was used. Measurements wereperformed under the vacuum of 2.0 ×10−8 Pa and thesample was cleaved in the analyzer chamber prior tomeasurements to obtain clean surfaces. The sample
temperature during the measurements was set to 150 K,
above the transition temperature, in order to avoid charging
effects. The binding energy (EB) was corrected bymeasuring the Fermi level (EF) of an Au foil that is inelectrical contact with the sample. The energy resolution
was set to ~50 meV.
X-ray magnetic circular dichroism (XMCD)
measurement was conducted at BL-16 of Photon Factoryusing a vector-type magnet. The sample temperature wasset to 50 K, below the transition temperature. Magnetic
field of 1 T was applied both parallel and perpendicular tothe ab-plane of the sample.
3 Results and Discussion
Fig. 1 shows ARPES results. A constant energy
mapping at E - EF = -0.35 eV is shown in Fig.1(a). Strong
intensity at the Γ point is observed, and the ARPES
spectra taken along the red line in Fig.1(a) is shown in Fig.
1(b). Hole-like band around the Γ point below Fermi levelis consistent with its insulating property.

Fig. 1 ARPES results of CGT. (a) Constant energy mappingat E - EF = -0.35 eV. (b) Hole-like band around the Γ point.

Fig. 2 k-integrated spectrum compared with spectra
simulated by DFT calculation taking into account the photoionization cross-section of each atomic orbital.

Photon Factory Activity Report 2017 #35 (2018)

We have also performed DFT calculations. The kintegrated spectra obtained by experiment and thecalculated spectra are shown in Fig. 2. The peak at thebinding energy of ~1.9 eV is not reproduced by calculation
without Coulomb interaction. On the other hand,calculation including Coulomb interaction Ueff = 1.1 eV
within the DFT+U method reproduces the peak position.
Indeed, the value Ueff = 1.1 eV can reproduce theferromagnetism in the DFT calculation, indicating that
Coulomb interaction between the Cr 3d electrons plays animportant role in determining not only the electronic
structure but also the magnetic ordering.
0.05
0.00

(a)

0.1
0.0

-0.05

Experimentin planeout of plane

3+Cr-0.1

Intensity (arb. unit)Intensity (arb. unit)

-0.10
-0.15
-0.20
0.1(c)(b)0.0

in planeout of plane
Dt = 0 eV-0.2
0.1(d)

0.02+-0.1

Cr-0.2in planeout of plane
Dt = 0 eV

-0.3
575

580

585

Photon Energy (eV)

590

3+

-0.1

Cr-0.2in planeout of plane
Dt = 0.10 eV
575

580

585

590

Photon Energy (eV)Fig. 3 XMCD spectra on CGT. (a) XMCD spectra ofdifferent direction of magnetic field. Cluster model
calculation estimating the valence of Cr is 2+ (b) and 3+(c), and 3+ with trigonal crystal field (d).

XMCD spectra of CGT are shown in Fig. 3. Differencesin the spectra were observed between the different
directions of magnetic field. In order to identify the originof the spectral differences, we have conducted clustermodel calculation as shown in Figs. 3(b) – (d). The spectralline shape can be well reproduced by assuming Cr3+, andnot Cr2+. Here, we include the trigonal crystal field (Dt)because CrTe6 cluster is elongated along the c-axis, whichmakes the spectral line shape different with respect to thedirections of magnetic field. This suggests that thedistortion of CrTe6 cluster is the origin of the difference.
For the future prospect, we would like to perform
ARPES measurements for other vdW compounds such as
Cr2Si2Te6 in order to investigate the effect of the different
ligand atoms, and also to perform a quantitative analysis ofthe XMCD spectra.
References[1] N. Smith, Nature 546 (2017) 216.
[2] X. Zhang et al., Jpn. J. Appl. Phys 55 (2016) 033001.
[3] V. Carteaux, et al., J. Phys.: Condens. Matter 7, 69(1995).
[4] G. T. Lin, et al, arXiv:1706.03239.
[5] J. Huiwen, et al., J. Appl. Phys. 114, 114907 (2013).
[6] C. Gong et al., Nature 546 (2017) 265.
*masahiro@wyvern.phys.s.u-tokyo.ac.jp