In the lacunar spinel ${\mathrm{GaV}}_4{\mathrm{S}}_8$, the interplay of spin, charge, and orbital degrees of freedom produces a rich phase diagram that includes an unusual Néel-type skyrmion phase composed of molecular spins. To provide insight into the interactions underlying this complex phase diagram, we study the spin excitations in ${\mathrm{GaV}}_4{\mathrm{S}}_8$ through inelastic neutron scattering measurements on polycrystalline and single crystal samples. Using linear spin wave theory, we describe the spin wave excitations using a model where ${\mathrm{V}}_4$ clusters decorate an fcc lattice. The effective cluster model includes a ferromagnetic interaction and a weaker antisymmetric Dzyaloshinskii-Moriya interaction between the neighboring molecular spins. Our work clarifies the spin interactions in ${\mathrm{GaV}}_4{\mathrm{S}}_8$ and supports the picture of interacting molecular clusters.

• Received 23 October 2021
• Accepted 9 December 2021

DOI:https://doi.org/10.1103/PhysRevB.104.224425