Magnetoentropic mapping and computational modeling of cycloids and skyrmions in the lacunar spinels GaV$_4$S$_8$ and GaV$_4$Se$_8$

TL;DR

This study demonstrates that magnetoentropic mapping combined with numerical modeling effectively identifies magnetic phases like cycloids and skyrmions in lacunar spinels, revealing phase boundaries and characteristic behaviors.

Contribution

The paper introduces a rapid magnetoentropic mapping method, guided by a simple spin Hamiltonian model, to accurately characterize magnetic phases in GaV4S8 and GaV4Se8.

Findings

Excellent agreement between measurements and the minimal spin Hamiltonian.

Identification of high-entropy behavior as a precursor to skyrmion formation.

Observation of an anomalous high-entropy state in GaV4Se8 not explained by the model.

Abstract

We report the feasibility of using magnetoentropic mapping for the rapid identification of magnetic cycloid and skyrmion phases in uniaxial systems, based on the GaV4S8 and GaV4Se8 model skyrmion hosts with easy-axis and easy-plane anisotropies respectively. We show that these measurements can be interpreted with the help of a simple numerical model for the spin Hamiltonian to yield unambiguous assignments for both single phase regions and phase boundaries. In the two lacunar spinel chemistries, we obtain excellent agreement between the measured magnetoentropic features and a minimal spin Hamiltonian built on Heisenberg exchange, single-ion anisotropy, and anisotropic Dzyaloshinskii-Moriya interactions. In particular, we identify characteristic high-entropy behavior in the cycloid phase that serves as a precursor to the formation of skyrmions at elevated temperatures and is a readily-measurable signature of this phase transition. Our results demonstrate that rapid magnetoentropic mapping guided by numerical modeling is an effective means of understanding the complex magnetic phase diagrams innate to skyrmion hosts. One notable exception is the observation of an anomalous, low-temperature high-entropy state in the easy-plane system GaV$_4$Se$_8$, which is not captured in the numerical model. Possible origins of this state are discussed.