Mapping skyrmion stability in uniaxial lacunar spinel magnets from first principles

Abstract

The identification of general principles for stabilizing magnetic skyrmion phases in bulk materials over wide ranges of temperatures is a prerequisite to the development of skyrmion-based spintronic devices. Lacunar spinels with the formula GaM4X8 with M=V, Mo; X=S, Se are a convenient case study towards this goal as they are some of the first bulk systems suggested to host equilibrium chiral skyrmions far from the paramagnetic transition. We derive the magnetic phase diagrams likely to be observed in these materials, accounting for all possible magnetic interactions, and prove that skyrmion stability in the lacunar spinels is a general consequence of their crystal symmetry rather than the details of the material chemistry. Our results are consistent with all experimental reports in this space and demonstrate that the differences in the phase diagrams of particular spinel chemistries are determined by magnetocrystalline anisotropy, up to a normalization factor. We conclude that skyrmion formation over wide ranges of temperatures can be expected in all lacunar spinels, as well as in a wide range of uniaxial systems with low magnetocrystalline anisotropy. *

Magnetoentropic mapping and computational modeling of cycloids and skyrmions in the lacunar spinels GaV4S8 and GaV4Se8Zuo1, Kitchaev2, Schueller3 et al. 2021Phys. Rev. MaterialsSelf Cite5010View full textAdd to dashboardCite

“…Monte Carlo sampling was performed using an in-house implementation of magnetic Hamiltonian Monte Carlo [37], based on a generalized formulation of spin-spin interactions described in previous works [18,31]. Briefly, each tetrahedral V 4 cluster is assumed to have a collective moment equal to 1 µ B , [28,38] which is allowed to interact quasiclassically with nearest-neighbor tetrahedra through a combination of Heisenberg exchange and symmetry-restricted anisotropic Dzyaloshinskii-Moriya interactions [39,40].…”

“…However, the applicability of this technique to more general types of magnetic phase diagrams remains undemonstrated [17]. In particular, the magnetoentropic behavior of uniaxial systems remains unresolved, which includes both materials with bulk uniaxial symmetry and thin film systems where skyrmions may persist over wide temperature windows far below T c [18][19][20].…”

“…These interactions perturb the nominal ferromagnetic order by stabilizing long-wavelength modulations that lead to cycloid and skyrmion states. Depending on the strength of the uniaxial anisotropy, these non-collinear states may appear across an extended range of temperatures or only near T c [18].…”

“…This ferroelectic transition allows cycloidal modulations with propogation vectors in the plane perpendicular to 111 . GaMo 4 S 8 [18,29,30], and GaMo 4 Se 8 [31]. The magnetic phase boundaries of these materials have been mapped out by conventional techniques, including SANS, field-dependent magnetization, probe microscopy, and AC susceptibility [10,11,[31][32][33].…”

“…Furthermore, lacunar spinels are ferroelectric [10,11,34] and susceptible to structural phase transitions [31], meaning that skyrmion stability can be probed using electric fields and mechanical strain [32]. Finally, while the electronic structure of these materials is highly correlated and requires advanced computational methods to resolve [28,35], their microscopic magnetic behavior is well-captured using semi-classical spin models [18,30,36]. This combination of rich physics, abundant experimental data and quantitative theory establish the lacunar spinels as an ideal system for testing new methods for magnetic phase determination.…”