Transport properties of thin flakes of the antiferromagnetic topological insulator $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$

$\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$ was recently suggested as the first example of an antiferromagnetic topological insulator. However, lacking good quality of single crystals hindered its further investigation. Here, we report the detailed transport properties of several $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$ thin flakes in which samples are more homogeneous as compared to the bulk single crystals. We found all the samples exhibit antiferromagnetic transition around 25 K and the same field-driven magnetic transitions; however, temperature dependence of resistivities shows either insulating or metallic behaviors. Such behavior is in contrast with the as-grown thick single crystals in which only metallic behavior was observed. The Hall coefficients indicate the gradual decrease of the carrier density with decreasing the temperature for the sample with insulating behavior. Such difference may relate to different impurity content (antisite defects and/or Mn vacancies). Our findings indicate the bulk carrier can be localized at low temperature by introducing disorder while the magnetic ordering keeps invariant, which is quite crucial for realizing the theoretical proposed quantum anomalous Hall effect and axion insulators in this material.

Figure 1

Structural characterizations and transport properties of as-grown $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$ single crystals. (a) XRD pattern of as-grown $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$ single crystals. Note strong preferred orientation along [001]. (b) In-plane electrical resistivity of a single crystal measured from room temperature down to 2 K. All the as-grown crystals show metallic behaviors and antiferromagnetic transitions with $T_N=25\mathrm{K}$. (c) Atomic structure of the $R-3m$-group bulk $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$ with purple, brown, and blue balls showing Bi, Te, and Mn atoms, respectively.

Figure 2

(a) The optical image of a typical exfoliated $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$ thin flake with a standard Hall bar configuration. The device is made by lithography and liftoff techniques. The scale bar equals to 10 µm. (b) Atomic force microscopy image of the square marked in (a). The sample surface is quite smooth. (c) Line profile along the dashed line shown in (b), indicating the sample thickness is around 180 nm. (d, e) Elemental maps of Bi (red) and Mn (yellow) by scanning EDX. The scanning maps indicate homogeneous distribution of Bi and Mn in the sample.

Figure 3

Temperature dependences of the in-plane resistivity ${\rho }_{xx}\left(T\right)$ for two different thin flake samples. One shows metallic behavior, while the other one shows insulating behavior. Both samples show an antiferromagnetic transition with $T_N=25\mathrm{K}$.

Figure 4

Temperature dependences of the in-plane resistivity ${\rho }_{xx}\left(T\right)$ under various magnetic fields for two different thin flake samples. (a, b) ${\rho }_{xx}\left(T\right)$ under various magnetic fields along the $c$ axis for the metallic sample (a) and insulating sample (b). (c, d) ${\rho }_{xx}\left(T\right)$ under various magnetic fields perpendicular to the $c$ axis for the metallic sample (c) and insulating sample (d). Peaks near $T_N$ in ${\rho }_{xx}\left(T\right)$ for both samples are suppressed by increasing the magnetic fields. The ${\rho }_{xx}\left(T\right)$ of two samples show similar behaviors.

Figure 5

Magnetotransport properties of $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$ for the two samples. (a–d) Field dependence of in-plane resistivity ${\rho }_{xx}\left(H\right)$ with applied magnetic field parallel to the $c$ axis [metallic sample (a) and insulating sample (b)] and perpendicular to the $c$ axis [metallic sample (c) and insulating sample (d)] at various temperatures, respectively. (a, b) For $H//c,{\rho }_{xx}\left(H\right)$ show steep decrease in response to the AFM-to-CAFM transition at about 3 T. The kinks at higher magnetic field indicate the CAFM-to-FM transition. (c, d) ${\rho }_{xx}\left(H\right)$ show gradual decrease with increasing field for $H\perp c$ indicating the spin gradually transits from the AFM to FM. (e, f) Angular dependence of the resistivity ${\rho }_{xx}\left(H\right)$ (shifted vertically for clarity) at 2 K with $H$ rotated from the out-of-plane to in-plane direction for the metallic sample (e) and insulating sample (f). The tilt angle θ is defined as the angle between $H$ and the $c$-axis direction. The schematics in (a), (c), and (e) illustrate the setup of the magnetotransport experiments. The arrows indicate the kinks in ${\rho }_{xx}\left(H\right)$ due to the CAFM-to-FM or AFM-to-FM transitions.

Figure 6

The phase diagram of magnetic states in $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$ flakes. The circles and squares represent the critical magnetic field as a function of temperature for the metallic and insulating samples, respectively. There is no obvious disparity between two kinds of samples. The corresponding arrows represent the magnetic order formed by the Mn FM layers, showing AFM, canted AFM, and FM orders, respectively.

Figure 7

Hall effect of $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$. (a, b) Hall resistivity ${\rho }_{xy}\left(H\right)$ as a function of magnetic field at various temperatures for the metallic sample (a) and insulating sample (b). There is a sharp decrease of ${\rho }_{xy}\left(H\right)$ at around 3T in response to the AFM-to-CAFM transition for both samples. (c, d) Temperature dependence of Hall coefficient $R_H$ and carrier density $n_H$. For the insulating sample, $n_H$ obviously decreases with decreasing the temperature. For both samples, $n_H$ shows anomaly and reaches the minimum value around $T_N$.