Site Mixing for Engineering Magnetic Topological Insulators

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Site Mixing for Engineering Magnetic Topological Insulators

Yaohua Liu, Lin-Lin Wang, Qiang Zheng, Zengle Huang, Xiaoping Wang, Miaofang Chi, Yan Wu, Bryan C. Chakoumakos, Michael A. McGuire, Brian C. Sales, Weida Wu, and Jiaqiang Yan

Phys. Rev. X 11, 021033 – Published 12 May 2021

Abstract

The van der Waals compound, ${MnBi}_{2} {Te}_{4}$ , is the first intrinsic magnetic topological insulator, providing a materials platform for exploring exotic quantum phenomena such as the axion insulator state and the quantum anomalous Hall effect. However, intrinsic structural imperfections lead to bulk conductivity, and the roles of magnetic defects are still unknown. With higher concentrations of the same types of magnetic defects, the isostructural compound ${MnSb}_{2} {Te}_{4}$ is a better model system for a systematic investigation of the connections among magnetism, topology, and lattice defects. In this work, the impact of antisite defects on the magnetism and electronic structure is studied in ${MnSb}_{2} {Te}_{4}$ . Mn-Sb site mixing leads to complex magnetic structures and tunes the interlayer magnetic coupling between antiferromagnetic and ferromagnetic. The detailed nonstoichiometry and site mixing of ${MnSb}_{2} {Te}_{4}$ crystals depend on the growth parameters, which can lead to $\approx 40 %$ of Mn sites occupied by Sb and $\approx 15 %$ of Sb sites by Mn in as-grown crystals. Single-crystal neutron diffraction and electron microscopy studies show nearly random distribution of the antisite defects. Band structure calculations suggest that the Mn-Sb site mixing favors a ferromagnetic interlayer coupling, consistent with experimental observation, but is detrimental to the band inversion required for a nontrivial topology. Our results suggest a long-range magnetic order of Mn ions sitting on Bi sites in ${MnBi}_{2} {Te}_{4}$ . The effects of site mixing should be considered in all layered heterostructures that consist of alternating magnetic and topological layers, including the entire family of $MnTe ({Bi}_{2} {Te}_{3})_{n}$ , its Sb analogs, and their solid solution.

Received 1 September 2020
Revised 9 December 2020
Accepted 15 March 2021

DOI:https://doi.org/10.1103/PhysRevX.11.021033

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Magnetic order Point defects

Ferrimagnets Magnetic semiconductors Topological insulators

Crystal growth Density functional theory Magnetization measurements Neutron diffraction Scanning tunneling microscopy Susceptibility measurements

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Yaohua Liu ^1,*, Lin-Lin Wang², Qiang Zheng³, Zengle Huang⁴, Xiaoping Wang ¹, Miaofang Chi⁵, Yan Wu ¹, Bryan C. Chakoumakos ¹, Michael A. McGuire ⁵, Brian C. Sales ⁵, Weida Wu ⁴, and Jiaqiang Yan ^5,3,†

¹Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
²Division of Materials Science and Engineering, Ames Laboratory, Ames, Iowa 50011, USA
³Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, USA
⁴Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA
⁵Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

^*liuyh@ornl.gov
^†yanj@ornl.gov

Popular Summary

Topological materials are remarkable systems with properties impervious to external perturbation. When combined with intrinsic magnetism, such materials offer an ideal playground for exploring many exotic quantum phenomena. However, these materials, like all crystalline solids, have defects in their crystal lattices that can affect both magnetic and topological properties. Identifying these defects and their influence is vital in understanding and controlling the exotic physics and ultimately optimizing quantum device performance. In this work, we study ${MnSb}_{2} {Te}_{4}$ —a candidate magnetic topological insulator—and find randomly distributed mixing of Mn and Sb atoms at specific lattice sites. These defects present a double-edged sword: They stabilize favorable ferromagnetic interactions but are likely detrimental to the topological electronic properties.

Using ${MnSb}_{2} {Te}_{4}$ as a model system, we perform a thorough investigation of the concentration, distribution, magnetic, and electronic effects of magnetic defects by using crystal growth, magnetic and transport measurements, neutron diffraction, microscopy probes, and theoretical calculations. Details of the elemental composition and site mixing of ${MnSb}_{2} {Te}_{4}$ crystals can be controlled by varying growth conditions. Randomly distributed Mn and Sb defects lead to complex magnetic structures and tune the interlayer magnetic coupling but are detrimental to realizing nontrivial topology.

Our results apply to other magnetic topological materials because all crystalline compounds are susceptible to lattice defects. This work paves the way for fine-tuning of the magnetic and topological properties of these types of materials via defect engineering.

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Vol. 11, Iss. 2 — April - June 2021

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Images

Figure 1
Different growth temperatures result in distinct magnetic properties of ${MnSb}_{2} {Te}_{4}$ single crystals. (a) Ideal crystal structure of ${MnSb}_{2} {Te}_{4}$ , isostructural to ${MnBi}_{2} {Te}_{4}$ . (b),(c) Temperature dependence of magnetic susceptibility measured in a field-cooled mode with an applied magnetic field of 1 kOe. (d),(e) Field dependence of magnetization at 2 K. The applied magnetic field is either along ( $H ∥ c$ ) or perpendicular to ( $H ∥ a b$ ) the crystallographic $c$ axis. The crystal with $T_{N} = 19 K$ was grown at $620 ° C$ , while the one with $T_{C} = 34 K$ was grown at $640 ° C$ .
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Figure 2
Distinct interlayer coupling in different ${MnSb}_{2} {Te}_{4}$ single crystals resolved by single-crystal neutron diffraction. (a),(b) Neutron scattering pattern in the $[H 1 L]$ plane of the $T_{C} 24 K$ and $T_{N} 19 K$ , respectively. The data were collected at 6.5 K, where the sample $T_{N} 19 K$ shows Bragg peaks at half- $L$ positions but $T_{C} 24 K$ does not. (c) Temperature dependence of the normalized intensities of selected magnetic Bragg reflections for $T_{N} 19 K$ and $T_{C} 24 K$ . The solid lines are guides to the eye. (d),(e) The schematic diagram of the magnetic structures of $T_{C} 24 K$ and $T_{N} 19 K$ , respectively. Both have the same intraseptuple-layer ferrimagnetic order, but the FM interseptuple-layer coupling for $T_{C} 24 K$ is in contrast to the AFM one for $T_{N} 19 K$ . The magnetic space groups for them are $R \bar{3} m^{'}$ (no. 166.101) and $R_{I} − 3 c$ (no. 167.108), respectively. The sites connected by the purple triangular lattice represent the MnTe sheets while the other sites represent Mn mixed into the ${Sb}_{2} {Te}_{3}$ sheets.
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Figure 3
Presence and random distribution of Mn-Sb antisite defects in both ${MnSb}_{2} {Te}_{4}$ crystals from STEM and STM. (a),(b) HAADF STEM images along [310] for $T_{N} 19 K$ (a) and along [210] for $T_{C} 24 K$ (b). No stacking fault was observed. (c),(d) Sum intensity profile perpendicular to the septuple layers. (e) STM topographic image of $T_{C} 24 K$ (tunneling condition $- 0.4 V$ , 100 pA). (f) STM topographic image of $T_{N} 19 K$ (tunneling condition $- 0.4 V$ , 60 pA). The insets in (e) and (f) are the fast Fourier transform of the topographic images. (g),(h) Radial pair distribution function of ${Mn}_{Sb}^{'}$ antisites. Radial pair distribution function $g (r)$ measures the normalized density of ${Mn}_{Sb}^{'}$ in the distance of $r$ away from a reference ${Mn}_{Sb}^{'}$ . Two bin sizes, 0.5 and 1 nm, are used to calculate $g (r)$ .
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Figure 4
Mn-Sb site mixing favors FM interlayer coupling but is detrimental to the band inversion from DFT. (a) $(2 \times 2 \times 2)$ supercells of rhombohedral ${MnSb}_{2} {Te}_{4}$ in the most stable configuration with the Mn-Sb site-mixing ratio of $50 ∶ 50$ in the central layer and $25 ∶ 75$ in the Sb layer, showing AFM or FM interlayer couplings while keeping the inversion symmetry. This configuration (labeled as AS3) and the other inequivalent ones are listed in Table S7 and shown in Fig. S8 of the Supplemental Material [35]. $PBE + U + SOC$ band structures of (b) the ideal FM without site mixing, where the black arrow indicates the Weyl point in ideal FM along $Γ − Z$ due to band inversion. (c),(d) AFM-like and FM-like configurations with Mn-Sb site mixing. The top valence band is in blue and the green shadow indicates the projection on the $p$ orbital of Sb at Mn site.
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