Unexpected Anisotropic Mn-Sb Anti-site Distribution and Van der Waals Epitaxy of MnSb2Te4

TL;DR

MnSb2Te4 is a magnetic topological insulator whose properties are strongly influenced by Mn-Sb site mixing. This work demonstrates that anti-site distribution can be anisotropic and inversion-symmetry breaking within septuple layers, revealed by atomic-scale STEM-EDS and corroborated by magnetometry indicating ferrimagnetic behavior. It also reports van der Waals epitaxy of MnSb2Te4 on Sb2Te3 seeds to grow thin films on amorphous SiOx, enabling potential gating of the Fermi level in Si-based devices. Together, these findings link defect chemistry, topology, and magnetism, and open avenues for integrating MST into multifunctional devices with potential piezoelectric and nonlinear optical functionalities.

Abstract

Mn-Sb site mixing directly impacts both the magnetic and topological properties of MnSb2Te4. This study reveals, unlike previously believed, that these anti-sites can be unevenly distributed within the crystal. To that end, a polycrystalline sample was created with a two-step synthesis using MnTe and Sb2Te3 as precursors. DC-SQUID magnetometry was used to confirm its magnetic properties. In addition, the use of High-Resolution Scanning Transmission Electron Microscopy combined with Energy-Dispersive X-ray Spectroscopy allowed us to identify the presence of an inversion-breaking asymmetry in the anti-site distribution. This reduced-symmetry structure bears resemblance to the recently proposed class of Janus materials and thus warrants further exploration due to its potential for combining topology and magnetism with other effects, such as non-linear optics and piezoelectricity. Finally, to further elucidate the interplay between site mixing, doping, topology, and magnetism, a method for growing MnSb2Te4 thin films over amorphous SiOx using Sb2Te3 seeds is introduced. The successful Van der Waals epitaxy of MnSb2Te4 over Sb2Te3 seeds using Pulsed Laser Deposition is confirmed using Scanning Transmission Electron Microscopy. This represents a crucial step in incorporating these materials into a Si-based architecture, which offers the possibility of controlling the Fermi lever via gating.

Figures (8)

• Figure 1: Characterization of polycrystalline MnSb2Te4 sample. a) Schematic representation of the ideal crystal structure. The a-vectors indicate the primitive rhombohedral cell, while the b-vectors represent the hexagonal supercell. b) CBS SEM micrograph of the ingot as-synthesized, the crystallites have a considerable size of a couple hundreds of microns exhibiting sharp angular features. No compositional contrast is observed indicative of the ingot's uniformity. c) HAADF STEM images of a single grain. The crystal consists of perfectly arranged SL separated by a Van der Waals-like gap. The inset shows a zoom in version of the SL captured using drift corrected frame integration [DCFI] and post-processed with a radial Wiener filter. The difference in contrast due to atomic number clearly highlights the location of Mn in the structure. d) XRD of the polycrystalline sample compared with the simulated pattern of MnSb2Te4 (as reported by orujlu2021phase) and its precursors (Sb2Te3anderson1974refinement and MnTe kruger1967lattice). The asterisk indicate background peaks. e) Inverse pole figure of the gridded ingot obtained using EBSD. The selected area indicates the approximate location where the lamella was taken.
• Figure 2: Structure and defects of MnSb2Te4. a) HAADF STEM images of the thin film. SL of MnSb2Te4 are epitaxially grown over three QLs of Sb2Te3 seeds over amorphous SiOx. b) Intensity profile of the area marked in (a), the contrast in the HAADF image is generated by difference in atomic number enabling the identification of Mn in the structure. The Sb peaks show slight contrast variation from the Te suggesting Mn-Sb site mixing. c) HAADF STEM image of a single MnSb2Te4 grain. The only defects found in the grain were 3D voids where occasional QL (in yellow) could be found. d) HAADF STEM image with a Radial Wiener filter of the MnSb2Te4 thin film. Abundant bilayer defects are present which mediate the position of QL and SL in the film. Occasionally they even combine to form nonuple layers [NL]. Its corresponding intensity profile is plotted below. e) iDPC STEM image of the MST film showing a twin boundary. Due to oxidation, only QL can be bound at the upper surface of the film. f) HAADF STEM image of a particulate in the surface of the film transported during pulsed laser deposition. The particulate shows the peritectic decomposition of MnSb2Te4 into MnTe and Sb2Te3.
• Figure 3: Stichometry and atomic distribution in MnSb2Te4. a) Atomically resolved elemental map of a single MnSb2Te4 grain measured with EDS in STEM mode (integration over 5581 frames). The Mn-Sb anti-sites are asymmetrically distributed. b) Intensity profiles corresponding to Fig. (a) with integration width of 16 nm. A significant Mn signal can be found in the Sb planes and even within the Van der Waals gap as interstitials. The orange arrows indicate the asymmetry in the Sb planes with one plane showing more Mn signal and correspondingly less Sb. The asymmetry occurs within a septuple layer and it is consistent in all septuple layers and multiple parts of the grain. No asymmetry in the Te planes is detected. c) Elemental distribution within the thin film. In the presence of oxygen, Mn diffuses from the center of the film (area #1) to the surface (area #2) creating a Mn-rich amorphous oxide. d) Stoichiometry of the ingot (as determined by EDS in STEM mode), the bulk of the film, and the amorphous oxide (areas #1 and #2 in Fig. (c) respectively). e) Line profile of Fig. (c). The HAADF signal is plotted in blue and corresponds to the right-side axis. The increase of Mn and O near the surfaces is clearly visible.
• Figure 4: Magnetic properties of MnSb2Te4. a) Magnetic moment vs applied field (m vs H) loops at different temperatures of MnSb2Te4 powder. A clear hysteresis is present at 5 K indicating ferri/ferromagnetic order. b) m vs H loops at 5 K of the MnSb2Te4 thin film for different direction. The signal is poor due to the small film thickness and the diamagnetic response of silicon. The data point with bad residual are plotted with crosses and asterisks. Anisotropy is clearly observed, but no hysteresis can be discerned. No background signal has been removed. c) H/m plot during a -5mT FC of the powder sample. The plot is proportional to the inverse susceptibility and exhibits a Curie-Weiss behavior (straight line fit) with critical temperature near 25 K. d) FC and RM at -5mT of the powder sample. During FC, the magnetic moment develops a maximum near 20 K reminiscent of antiferromagnetic order. The RM is always decreasing, but has two inflection point. e) High-temperature H/m plot during a 500mT FC of the polycrystalline sample. The Curie-Weiss behavior changes slope around 50-70 K. f) High-temperature RM after the 500mT FC.
• Figure 5: Table of content/Thumbnail figure