Low-field magnetization processes of hexagonal easy-plane altermagnet $α$-MnTe

TL;DR

This work investigates low-field magnetization processes in the hexagonal easy-plane altermagnet α-MnTe. The authors grow high-purity crystals by iodine-assisted chemical vapor transport and characterize their structure and magnetic properties using XRD, EDXS, BSE, magnetization, resistivity, and specific heat measurements, identifying a Néel transition at $T_N \approx 307$ K and a Mn$^{2+}$ spin state $S=5/2$. They observe complex, domain-driven magnetization with a weak ferromagnetic component along $c$ and a low-field anomaly near $\mu_0 H \approx 1$ T, suggesting metastable domain dynamics rather than conventional spin-flop. A phenomenological micromagnetic model incorporating higher-order anisotropic exchange and Dzyaloshinskii-Moriya-like invariants coupling the weak ferromagnetic moment to the Néel vector explains the absence of spin-flop transitions and attributes the irregular magnetization to redistribution of metastable domains, with implications for altermagnet spintronics.

Abstract

Single crystals of $α$-MnTe were synthesized by chemical vapor transport using iodine as the transport reagent. Structural characterization by powder x-ray diffraction confirmed the hexagonal structure (space group P6$_{3}$/mmc). Magnetization $M(T)$ and specific heat $C_p(T)$ measurements revealed an antiferromagnetic phase transition at $T_N \approx307$ K. The magnetic entropy derived from the $C_p(T)$ data is consistent with the $S = 5/2$ spin state of Mn$^{2+}$ ions. Angle- and field-dependent magnetization measurements indicate complex magnetic responses associated with domains, and show an anomaly around 1 T. These features are analyzed using a phenomenological micromagnetic model that includes higher-order anisotropic exchange interactions coupling the weak ferromagnetic component and the antiferromagnetic order parameter. The model captures the generic behavior of magnetic states and demonstrates that the observed uniaxial and unidirectional anisotropies arise from metastable domain configurations and irreversible magnetization processes.

Figures (9)

• Figure 1: (a) An image of single crystals grown by chemical vapor transport. (b) Back-scatter electron (BSE) map of a $\alpha$-MnTe single crystal, individually color-coded for Mn and Te ions. (c) BSE map of $\alpha$-MnTe. The circles represent the area in which the composition was analyzed using EDXS. At positions marked 1,2, and 3, the compositions were consistently found to be Mn$_{1.02(1)}$Te$_{0.98(1)}$, Mn$_{1.02(1)}$Te$_{0.98(1)}$, and Mn$_{1.02(1)}$Te$_{0.98(1)}$, respectively.
• Figure 2: Results of the profile matching by Rietveld refinement of $\alpha$-MnTe from the powder x-ray diffraction data taken at 298 K.
• Figure 3: DC magnetic susceptibility $\chi(T)$ measured under a zero-field-cooled (ZFC) protocol with an applied magnetic field of (a) 0.1 T and (b) 2 T, for fields oriented parallel to the crystallographic $\braket{2\overline{1}\overline{1}0}$, $\braket{1\overline{1}00}$, and $\braket{0001}$ axes. Inset of panel (a) illustrates the orientation of the $\braket{2\overline{1}\overline{1}0}$ and $\braket{1\overline{1}00}$-axes within the hexagonal basal plane; the $\braket{0001}$ axis is oriented perpendicular to this plane.
• Figure 4: Magnetization $M(H)$ measured at 2 K with the magnetic field applied parallel to the crystallographic $a$-, $m$-, and $c$- axes. The inset shows the field derivative $dM/dH$ as a function of $H$. Vertical dashed lines mark the critical fields for both positive and negative fields applied along the $\braket{2\overline{1}\overline{1}0}$- and $\braket{1\overline{1}00}$-axes. A sharp peak in the $dM/dH$ curve for $H\parallel c$ is attributed to weak ferromagnetism along the $c$-axis.
• Figure 5: Magnetic field derivative $dM/dH$ of the magnetization $M(H)$ curves measured at various temperatures.