Direct observation of magnetization distribution
in a magnetic heterostructure of topological insulator
RIKEN Center for Emergent Matter Science, Taro Nakajima
1. Introduction
A topological insulator is a material processing a
conducting surface state while its bulk interior is an
insulator [1]. The presence of the surface state is
ascribed to the non-trivial topology of the bandstructure. The dispersion relation of the topologicalsurface state is gapless unless the time reversal
symmetry is broken. In other words, the opening of theenergy gap on the topological surface states can becontrolled by introducing ferromagnetic order. Thisunique situation leads to a variety of fascinating
physical phenomena, such as quantum anomalous Hall
effect (QAHE). Chang et al. demonstrated the QAHE
using Cr-doped topological insulator thin films of(Bi,Sb)2Te3 (BST). They observed the quantization ofHall resistivity to h/e2 at low temperatures [2], where hand e are the Planck’s constant and the charge of anFig. 1: A schematic of magnetically doped TI film andelectron, respectively. The square shaped hysteresis

typical field dependence of Hall resistivity of the film

loop of ρyx centered at H=0 (Fig. 1) indicates that the[Cited from Ref. 2]

magnetic moments of doped Cr ions achieved thelong-range ferromagnetic order, and that the quantized Hall resistivity was obtained owing to thespontaneous magnetization at zero magnetic field.
Another root to introduce the time reversal symmetry breaking is to exploit magnetic proximity
effect. By preparing a topological insulator (TI) / magnetic insulator (MI) heterostructure, thesurface electrons in TI can be magnetically polarized via magnetic exchange coupling from the MIlayer. Quite recently, our collaborators have grown a heterostructure of BST and a ferromagnetic
insulator Cr2Ge2Te6 (CGT), which has a relatively high ferromagnetic ordering temperature of 61 K.
Furthermore, CGT has a layered structure, which has a very flat surface and good structural
matching with BST. This is a favorable situation to realize a large proximity effect on the interfacewith TI. In the present study, we have thus investigated magnetization distribution and possible
magnetic proximity effect in a BST/CGT heterostructure by means of polarized neutron reflectivity(PNR) measurements at SHARAKU in MLF of J-PARC.

2. Experiment
The thin film sample of CGT/BST heterostructure was grown by molecular beam epitaxy (MBE) onthe InP(111) substrate. Nominal thicknesses of each layer are as follows: InP substrate/BST(1nm)/CGT(13 nm)/BST(9 nm)/CGT(13 nm)/Al2O3(3 nm). The surface area of the film was 7 x 15 mm2.
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In the present PNR measurements, we employed a closed-cycle He refrigerator and anelectromagnet. We measured PNR spectra in magnetic field of 1 T without analyzer, at 300 and 3 K.
We also performed x-ray reflectivity (XRR) measurement using the identical sample at room
temperature. The PNR and XRR spectra are simultaneously analyzed using the GenX software.
3. Results
Figures 2(a) and 2(b) show the observed XRR and PNR data. Because the difference in nuclear
scattering length density (SLD) between BST and CGT is relatively small, the PNR data at 300 K
does not exhibit clear fringe patterns. On the other hand, we observed distinct fringe pattern in theXRR data owing to the difference in charge density between CGT and BST. At 3 K, where themagnetic moments in CGT layers are supposed to achieve long-range ferromagnetic orders, weobserved clear splitting between R+ and R-,
which are neutron reflectivity measured when
the incident neutrons have up and down spins,
respectively. The XRR and PNR data havebeen analyzed using the GenX software, andwefinallyfoundastructuralmodel

simultaneously satisfying all the reflectivity
data. The nuclear and magnetic SLD profiles
are shown in Figs. 2(c) and 2(d). We foundthat the interfaces between CGT and BST are
relatively sharp (roughness is about 1 nm),and that the magnitude of magnetization inFig. 2: (a) XRR and (b) PNR data of the CGT/BST

the CGT layer is 2.4(3) µB/Cr3+.

heterostructure. SLD profiles obtained from (c) XRR and (d)PNR measurements.

4. Conclusion
The present results revealed the long-range ordered ferromagnetism in the CGT layers andpresence of the sharp interfaces between CGT and BST layers. We also expected that the proximity
effect induced magnetization in the BST layers. However, any additional magnetic SLD in BSTlayers do not improve the fitting, suggesting that magnetic moments are present only in the CGTlayers. This is a sharp contrast to a previous PNR study on a EuS/Bi2Se3 bilayer [3], in which theproximity-effect induced magnetization was found in the TI (Bi2Se3) layer. We are still discussing
on this issue, and planning further experiments to elucidate the coupling between the surface
electrons on the TI layers and the magnetic moments in the MI layer.
References:[1] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010).
[2] C. Z. Chang et al., Science 340, 167 (2013).
[3] F. Katmis et al. Nature 533, 513 (2016).

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