Contrasting magnetic behavior in MnSc_2X_4 (X = S, Se) spinel compounds investigated by magnetoelastic studies

TL;DR

This work investigates how magnetoelastic coupling shapes frustration-driven magnetism in MnSc2X4 spinels. Using ultrasound, dilatometry, ac-susceptibility, and specific heat, the authors map (H,T) phase diagrams and reveal a robust antiferromagnetic skyrmion 3q phase in MnSc2S4, contrasted by the absence of sharp transitions in MnSc2Se4. A mean-field exchange-striction model with an elastic-energy term reproduces the MnSc2S4 magnetostriction and phase sequence, while Monte Carlo simulations with bond randomness explain the broad specific-heat feature and lack of long-range order in MnSc2Se4. The results highlight how interionic distance and disorder govern frustration-driven textures in spinel magnets, informing strategies to engineer skyrmion-hosting materials via lattice and chemical tuning.

Abstract

The spinel compounds MnSc_2X_4 are highly frustrated and candidate materials for vortex-like 3q magnetic states, such as skyrmions, with propagation vectors in the [111] plane. Because of the strong magnetoelastic coupling, we could extract a refined magnetic (H, T) phase diagram for MnSc_2S_4 from ultrasound and dilatometry measurements. We found a variety of magnetic phases, including the skyrmion phase, which is stable down to lowest temperatures. In comparison, we investigated MnSc_2Se_4 , having a larger distance between the magnetic Mn^3+ ions using the same methods. Unlike in MnSc_2S_4 , we found no skyrmion phase and overall a lack of sharp anomalies indicative of phase transitions, neither in dilatometry nor ultrasound nor in specific heat and ac-susceptibility data. Motivated by our findings, we performed model calculations, which reproduced the experimentally observed magnetostriction and specific-heat results reasonably well.

Figures (11)

• Figure 1: Crystal structure of MnSc$_2X_4$ with cubic unit cell. Only the magnetic Mn sites are shown. These are located so that next-nearest neighbors are arranged in triangular planes perpendicular to the [111] direction. $J_1$, $J_2$, and $J_3$ represent the magnetic exchange interactions between nearest, next-nearest, and third-nearest neighbors, respectively. The visualization was done using VESTAvesta3.
• Figure 2: (a) Thermal-expansion data of MnSc$_2$S$_4$ up to 100 K. (b) Low-temperature thermal-expansion coefficient $\alpha$ in 0 and 1 T.
• Figure 3: (a), (b) Thermal expansion and (c), (d) magnetostriction of MnSc$_2$S$_4$ single crystals for $H\parallel \Delta L/L \parallel$[111] after zero-field cooling (ZFC) from the paramagnetic state. The curves are shifted for better visibility. The black dashed lines indicate the phase transitions and are guides to the eye.
• Figure 4: Relative sound velocity of the longitudinal acoustic mode ($\textbf{k}\parallel$u$\parallel\textbf{H}\parallel$[111], f = 81 MHz) in MnSc$_2$S$_4$ versus (a), (b) temperature and (c), (d) magnetic field, measured at selected temperatures and magnetic fields, respectively. The curves are shifted for better visibility. The black dashed lines indicate the phase transitions and are guides to the eye.
• Figure 5: Phase diagram of MnSc$_2$S$_4$. The numbers correspond to the labeling of the anomalies used in the text. The abreviations indicate the magnetic structure of the phases (see also schematic pictures in gao_2017). The shape of the markers indicates the method used to extract the transitions.