Possible emergence of a skyrmion phase in ferroelectric GaMo$_4$S$_8$

TL;DR

This study predicts that GaMo4S8, a lacunar spinel, could host Neel-type skyrmion phases due to its unique electronic and magnetic properties, with potential for very small skyrmions and ferroelectric-ferromagnetic states.

Contribution

It introduces GaMo4S8 as a new candidate for skyrmion phases, highlighting the role of strong spin-orbit coupling and exchange anisotropy in stabilizing complex magnetic textures.

Findings

GaMo4S8 exhibits electric polarization driven by Jahn-Teller distortion.

Stronger spin-orbit coupling in 4d orbitals favors skyrmion stabilization.

Potential for ferroelectric-ferromagnetic ground state due to exchange anisotropy.

Abstract

Polar lacunar spinels, such as GaV$_4$S$_8$ and GaV$_4$Se$_8$, were proposed to host skyrmion phases under magnetic field. In this work, we put forward, as a candidate for N\'eel-type skyrmion lattice, the isostructural GaMo$_4$S$_8$, here systematically studied via both first-principles calculations and Monte Carlo simulations of model Hamiltonian. Electric polarization, driven by Jahn-Teller distortion, is predicted to arise in GaMo$_4$S$_8$, showing a comparable size but an opposite sign with respect to that evaluated in V-based counterparts and explained in terms of different electron counting arguments and resulting distortions. Interestingly, a larger spin-orbit coupling of 4d orbitals with respect to 3d orbitals in vanadium-spinels leads to stronger Dzyaloshinskii-Moriya interactions, which are beneficial to stabilize a cycloidal spin texture, as well as smaller-sized skyrmions (radius<10 nm). Furthermore, the possibly large exchange anisotropy of GaMo4S8 may lead to a ferroelectric-ferromagnetic ground state, as an alternative to the ferroelectric-skyrmionic one, calling for further experimental verification.