Large-proximity-induced anomalous Hall effect in ${\mathrm{Bi}}_{2-x}{\mathrm{Sb}}_x{\mathrm{Te}}_{3-y}{\mathrm{Se}}_y/{\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ heterostructure prepared by film transfer method

The magnetic proximity effect is one of the powerful approaches to realize quantum anomalous Hall effect in topological insulators (TIs) targeting at high temperatures. Various TI/ferromagnetic-insulator (FMI) heterostructures have extensively been investigated by using a van der Waals epitaxial growth technique of TI films grown on FMI substrates. However, FMI materials which can be used as a substrate are strictly limited due to the lattice mismatching between TI and FMI, not succeeding to boost the quantization temperature to be higher. Here, we show that a large anomalous Hall effect, comparable to the best value so far reported for the heterostructure interfaces fabricated by epitaxial growth techniques, is realized for ${\mathrm{Bi}}_{1.5}{\mathrm{Sb}}_{0.5}{\mathrm{Te}}_{1.7}{\mathrm{Se}}_{1.3}$ (BSTS)/${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ (CGT) heterostructure by transferring an epitaxially grown BSTS thin film floating on a ultrapure water directly on a CGT substrate (wet transfer method). The fundamental discussions about the nature of magnetic proximity effect were given in the aspect of the quality of the CGT substrate and the atomic orientation of the TI/CGT interface. The large magnetic proximity effect shown in our present studies can be applicable for a variety of FMI substrates and therefore can pave a route for promoting magnetic TIs both in basic science and applications.

Figure 1

Schematic model of BSTS/CGT heterostructure and its preparation processes. (a) Schematic model of BSTS/CGT structure and optical image in $200\times 200\mu {\mathrm{m}}^2$. (b) Schematic model of film transferred BSTS/CGT preparation processes. (c) The detailed processes for each preparation step.

Figure 2

Raman spectra of pristine BSTS, CGT, and BSTS/CGT heterostructure. (a) The Raman spectra of the samples corresponding to each preparation process. (b) Comparison between the Raman spectra of BSTS/CGT heterostructure (black) and the sum of the pristine BSTS and CGT (red). The spectra in red were calculated by a linear combination of both BSTS and CGT spectra in (a) as $I_{\mathrm{tot}}=I_{\mathrm{BSTS}}+0.6I_{\mathrm{CGT}}$.

Figure 3

The electronic transport of BSTS/CGT heterostructure. (a) Sheet resistance of BSTS/CGT, pristine BSTS film, and CGT flake. The thickness of the BSTS film is 21 nm, which is thin enough to suppress the bulk contributions to realize the surface dominant transport. (b) Hall resistivity of BSTS/CGT and pristine BSTS film at 7 K. The fitting curve of the Hall resistivity of BSTS/CGT base on the parallel circuit model is shown as a white dashed line. The inset in (b) is a schematic image of the band picture of before and after transferring BSTS on CGT. Due to the p-n junction formed at the interface of CGT and the bottom surface of BSTS, the chemical potential of BSTS shifts to the lower energy level by ending up the bottom BSTS changes from $n$ to $p$ type.

Figure 4

AHE in BSTS/CGT heterostructure. (a) Hall resistivity at low $B$, (b) $R_{\mathrm{AH}}$ under low $B$. The $R_{\mathrm{AH}}$ was extracted by subtracting the positive slope contributed by hole carriers as the background.

Figure 5

Comparison between $R_{\mathrm{AH}}$ of BSTS/CGT and that of the pristine CGT at 7 K. (a) Temperature dependence of $R_{\mathrm{AH}}$ of BSTS/CGT and magnetization of CGT substrate. (b) $B$ dependence of $R_{\mathrm{AH}}$ of BSTS/CGT and magnetization of CGT. In the figures, the whole behaviors between $R_{\mathrm{AH}}$($T,B$) and $M$($T,B$) are perfectly matched with each other, which is firm evidence to confirm the successful observation of AHE in BSTS/CGT.

Figure 6

Summary of $R_{\mathrm{AH}}$ and ${\theta }_{\mathrm{AH}}$ among proximity-induced TI/FMI systems at around 2–7 K. The values of $R_{\mathrm{AH}}$ and ${\theta }_{\mathrm{AH}}$ of Epi-growth systems (triangles) and film transferred systems (circles). These data, taken from Refs. [15, 17, 18, 19, 20, 21], are plotted as a function of carrier density together with the present work.