Mode Crystallography Analysis through the Structural Phase Transition and Magnetic Critical Behavior of the Lacunar Spinel GaMo₄Se₈

Abstract

In the lacunar spinels, with the formula AB 4 X 8 , transition-metal ions form tightly bound B 4 clusters resulting in exotic physical properties such as the stabilization of Néel-type skyrmion lattices, which hold great promise for energy-efficient switching devices. These properties are governed by the symmetry of these compounds with distortion of the parent noncentrosymmetric F 4̅3 m space group to the polar R 3 m , with recent observation of a coexisting Imm 2 low-temperature phase. In this study, through powder neutron diffraction, we further confirm that a metastable Imm 2 coexists with the R 3 m phase in GaMo 4 Se 8 and we present its structure. By applying the mode crystallography approach to the distortions together with anisotropic microstrain broadening analysis, we postulate that the formation origin of the minority Imm 2 phase stems from the high compressive stress observed in the R 3 m phase. Bond valence sum analysis also suggests a change in electronic configuration in the transition to Imm 2 which could have implications on the electrical properties of the compound. We further establish the nature of the magnetic phase transition using critical exponent analysis obtained from single-crystal magnetization measurements which shows a mixture of tricritical mean-field and 3D Heisenberg behavior [β = 0.22(4), γ = 1.19(1), and δ = 6.42(1)]. Magnetoentropic mapping performed on a single crystal reveals the signature of a positive entropy region near the magnetic phase transition which corresponds to the skyrmion phase field observed in a polycrystalline sample.

Magnetism in the Néel skyrmion host GaV$_4$S$_8$ under pressure

“…To further understand the crystal structures, and the effect of pressure, we can consider related chemically substituted systems, the effect of substitution being similar to that of an external pressure. In GaMo 4 Se 8 , when the Jahn-Teller distortion is decomposed into different normal modes, it has been shown that [26] different amplitudes of these modes results in either the F 43m phase (stabilized in GaV 4 S 8 at low P , low T ), or Imm2 phase (stabilized in GaV 4 S 8 at high P , high T ). Further, Ref.…”

Magnetism in the Néel-skyrmion hostGaV4S8under pressure

“…To further understand the crystal structures, and the effect of pressure, we can consider related chemically substituted systems, where the effect of substitution is similar to that of an external pressure. In GaMo 4 Se 8 , when the Jahn-Teller distortion is decomposed into different normal modes, it has been shown that [27] different amplitudes of these modes results in either the F 43m phase (stabilized in GaV 4 S 8 at low P, low T ), or Imm2 phase (stabilized in GaV 4 S 8 at high P, high T ). Further, Ref.…”

Evidence of a cluster spin-glass phase in the skyrmion-hosting GaMo4S8 compound

“…polarised neutron-scattering measurements, as well as low-temperature, high-resolution, structural studies to clarify whether the JT distortion drives the compound into a single R 3 m phase or if any additional competing phases are present similarly to what was found in GaMo 4 Se 8 . 28,29…”

“…polarised neutron-scattering measurements, as well as low-temperature, high-resolution, structural studies to clarify whether the JT distortion drives the compound into a single R3m phase or if any additional competing phases are present similarly to what was found in GaMo 4 Se 8 . 28,29 Author contributions Jose ´F. Malta: synthesis, characterisation, physical properties measurements, analysis of the results, investigation and writing.…”

“…In the case of the isostructural Mo compounds, the existence of skyrmions was first hypothesised to occur in GaMo 4 S 8 through theoretical studies, namely Monte-Carlo simulations. 5 Only recent experimental studies disclosed the evidence of skyrmions in GaMo 4 Se 8 28,29 and in GaMo 4 S 8 . 30 Reports on GaMo 4 S 8 , both on its synthesis as well as on the characterisation of physical properties, remain scarce comparatively with the other skyrmion-hosting lacunar spinels.…”