Coexistence of ferromagnetism and topology by charge carrier engineering in the intrinsic magnetic topological insulator $\mathrm{Mn}{\mathrm{Bi}}_4{\mathrm{Te}}_7$

Intrinsic magnetic topological insulators (MTIs) $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ and $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4/{\left({\mathrm{Bi}}_2{\mathrm{Te}}_3\right)}_n$ are expected to realize the high-temperature quantum anomalous Hall effect and dissipationless electrical transport. However, there is still a lack of ideal MTI candidates with magnetic ordering of the ferromagnetic (FM) ground state. Here, we show a MTI sample of $\mathrm{Mn}{\left({\mathrm{Bi}}_{0.7}{\mathrm{Sb}}_{0.3}\right)}_4{\mathrm{Te}}_7$ which holds the coexistence of a FM behavior state and topological nontriviality. The dramatic modulation of the magnetism is induced by a charge carrier engineering process via the Sb substitution in the $\mathrm{Mn}{\mathrm{Bi}}_4{\mathrm{Te}}_7$ matrix with antiferromagnetic ordering. The evolution of magnetism in $\mathrm{Mn}{\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_4{\mathrm{Te}}_7$ is systematically investigated by our magnetic measurements and theoretical calculations. The clear topological surface states of the FM sample of $\mathrm{Mn}{\left({\mathrm{Bi}}_{0.7}{\mathrm{Sb}}_{0.3}\right)}_4{\mathrm{Te}}_7$ are further verified by angle-resolved photoemission spectroscopy. The demonstration of the intrinsic FM-MTI of $\mathrm{Mn}{\left({\mathrm{Bi}}_{0.7}{\mathrm{Sb}}_{0.3}\right)}_4{\mathrm{Te}}_7$ in this paper sheds light on further material optimization of intrinsic MTIs and paves the way for further studies to clarify the relationships between topology, magnetism, and charge carriers in topological materials.

Figure 1

Characterization of $\mathrm{Mn}{\left({{\mathrm{Bi}}_1}_{-x}{\mathrm{Sb}}_x\right)}_4{\mathrm{Te}}_7$. (a) Crystal structure of $\mathrm{Mn}{\left({{\mathrm{Bi}}_1}_{-x}{\mathrm{Sb}}_x\right)}_4{\mathrm{Te}}_7$. (b) Single crystal x-ray diffraction patterns of the four samples. Inset: Optical images of the as-grown sample. (c) Energy dispersive spectra after normalized with the intensity of the Te element. The curves are offset for clarity.

Figure 2

Magnetism measurement of $\mathrm{Mn}{\left({{\mathrm{Bi}}_1}_{-x}{\mathrm{Sb}}_x\right)}_4{\mathrm{Te}}_7$ under $\mathrm{H}//c$ H // $c$. (a) Field cooling (FC)-zero field cooling (ZFC) curves of the six samples. The curves are offset for clarity. (b) The magnetic field dependence of the magnetization from 2 to 20 K.

Figure 3

Angle-resolved photoemission spectroscopy (ARPES) images of $\mathrm{Mn}{\left({{\mathrm{Bi}}_1}_{-x}{\mathrm{Sb}}_x\right)}_4{\mathrm{Te}}_7$ at $x=0.3$. (a)–(f) ARPES measurement at the photon energies from 7.2 to 23 eV. The topological surface states are marked by the red arrows.

Figure 4

Electrical transport and the evolution diagram of $\mathrm{Mn}{\left({{\mathrm{Bi}}_1}_{-x}{\mathrm{Sb}}_x\right)}_4{\mathrm{Te}}_7$ under $\mathrm{H}//c$. (a) Magneto resistivity and (b) the anomalous Hall after subtracting the background at the selected ratios: $x=0.3$, 0.36, and 0.4. The curves of $x=0.36$ and 0.4 are offset for clarity. (c) Carrier densities extracted from the Hall signals at 20 K. (d) Anomalous Hall effect (AHE) ratio at 2 and 5 K and the coercive field of the samples. The background colors represent the antiferromagnetic (AFM), ferromagnetic (FM), and ferrimagnetic region.

Figure 5

The energy differences and the electronic structures between the antiferromagnetic (AFM) and ferromagnetic (FM) states in $\mathrm{Mn}{\left({{\mathrm{Bi}}_1}_{-x}{\mathrm{Sb}}_x\right)}_4{\mathrm{Te}}_7$. (a) The energy of the AFM state after subtracting the linear background. Inset: The original data of the AFM energy. (b) The band structures and the related density of states (DOS) for the AFM state. (c) and (d) The energy, band structure, and the DOS for the FM state. (e) and (f) Energy differences of FM-AFM at the spin-orbit coupling $\left(\mathrm{SOC}\right)=1.0$ and 0.8, respectively. The black dashed lines show the exact position of the Van Hove singularity (VHS). The zero point of the $E-E_F$ is set at the Dirac point of the surface state.