Exchange bias in the van der Waals heterostructure $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4/{\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$

The exchange bias effect, namely the horizontal shift in the magnetic hysteretic loop, is known as a fundamentally and technologically important property of magnetic systems. Though the exchange bias effect has been widely observed in normal magnetic heterostructure, it is desirable to raise such pinning coupling in topology-based multilayer structure. Furthermore, the exchange bias effect was theoretically proposed to be able to further open the surface magnetization gap in the recently discovered intrinsic magnetic topological insulator $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$. Such an exchange interaction can be ensured and programmed in the heterojunction, or applied to spintronics. Here we report the electrically tunable exchange bias in the van der Waals $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4/{\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ heterostructure. The exchange bias emerges over a critical magnetic field and reaches the maximum value near the band gap. Moreover, the exchange bias is experienced by net ferromagnetic (FM) odd-layers $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ rather than the pure FM insulator ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$. Accompanied by nonlocal signal, an unfamiliar antisymmetric peak endows a domain-related structure within interface of the heterostructure. Such van der Waals heterostructure provides a promising platform to study the novel exchange bias effect and explore the possible application of spintronics or $\mathrm{high}-{\mathrm{T}}_{\mathrm{c}}$ quantum anomalous Hall effect.

Figure 1

The image of the device and the observed exchange bias. (a) The sketch of $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4/{\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ Hall-bar device. Six terminals are marked, respectively, from ① to ⑥. (b) Typical negative exchange bias after positive magnetism polarization (2 T) and negative magnetism polarization $\left(-2\mathrm{T}\right)$ process in 5 K. Both protocols show symmetric $B_{\mathrm{EB}}=0.1\mathrm{T}$. $B=0\mathrm{T}$ is marked by green dashed line. (c) Increasing maximum negative polarization fields from -1 T to -2 T in 5 K. Minor (major) coercive field $B_{\mathrm{c}1} \left(B_{\mathrm{c}2}\right)$ is marked. (d) is the same as c except for positive polarization fields from 1 T to 2 T. All the above measurement is at $V_{\mathrm{g}}=60\mathrm{V}$.

Figure 2

The gate voltage dependence of exchange bias. a The gate voltage dependence of exchange bias with NMP process to -3 T from -40 V to 60 V in 5 K. The dashed vertical line marks the position of zero point. (b), (c) The resistance maps as a function of the magnetic field and gate voltage when the magnetic field is swept forward and backward respectively. The transition line corresponds to the coercive fields.

Figure 3

The temperature dependence of exchange bias. (a) The temperature evolution of exchange bias from 5 K to 20 K at $V_{\mathrm{g}}=60\mathrm{V}$. The dashed vertical line marks the zero field. (b), (c) The resistance maps as a function of the magnetic field and temperature when the magnetic field is swept forward and backward, respectively.

Figure 4

Special configuration of measurement to verify exchange bias. (a) Three Hall channels measured concurrently in ranges of $\pm 1$ T and $\pm 3$ T. (b) Three-terminals–grounded (ttg) signals measured with port $②, ④, ⑥$ grounding. (c) Schematic of current distribution in (b). Side and edge current is separated due to terminals grounding. Top/bottom surface and bulk current contributes to subsequent Hall signals. All the measurements are in 5 K and $V_{\mathrm{g}}=60\mathrm{V}$. The double coercive states $B_{\mathrm{c}1}$ and $B_{\mathrm{c}2}$ are marked by orange and green dashed lines, respectively.

Figure 5

Spin-flip process and the nonlocal measurement of the $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4/{\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ device. (a) Specific spin-flip process of $B_{\mathrm{c}1}$ and $B_{\mathrm{c}2}$ hysteresis loop. The colorful dashed frame represents that the bonding/pinning part exists in the interface of MBT/CGT. The blue arrow stands for the free part of MBT moments. (b)–(d) The nonlocal transport measurement with two nonlocal channels and one Hall channel. The sweeping range is gradually increased to reveal the variation of $B_{\mathrm{c}1}/B_{\mathrm{c}2}$ by orange/green marks.