On the multiferroic skyrmion-host GaV₄S₈

Abstract

The lacunar spinel GaV₄S₈ exhibits orbital ordering at 44 K and shows a complex magnetic phase diagram below 12.7 K, which includes ferromagnetic and cycloidal spin order. At low but finite external magnetic fields, Néel-type skyrmions are formed in this material. Skyrmions are whirl-like spin vortices that have received great theoretical interest because of their non-trivial spin topology and that are also considered as basic entities for new data-storage technologies. Interestingly, we found that the orbitally ordered phase shows sizable ferroelectric polarisation and that excess spin-driven polarisations appear in all magnetic phases, including the skyrmion-lattice phase. Hence, GaV₄S₈ shows simultaneous magnetic and polar order and belongs to the class of multiferroics materials that attracted enormous attention in recent years. Here, we summarise the existing experimental information on the magnetic, electronic and dielectric properties of GaV₄S₈. By performing detailed magnetic susceptibility, resistivity, specific heat and dielectric experiments, we complement the low-temperature phase diagram. Specifically, we show that the low-temperature and low-field ground state of GaV₄S₈ seems to have a more complex spin configuration than purely collinear ferromagnetic spin order. In addition, at the structural Jahn–Teller transition the magnetic exchange interaction changes from antiferromagnetic to ferromagnetic. We also provide experimental evidence that the vanadium V₄ clusters in GaV₄S₈ can be regarded as molecular units with spin 1/2. However, at high temperatures deviations in the susceptibility show up, indicating that either the magnetic moments of the vanadium atoms fluctuate independently or excited states of the V₄ molecule become relevant.

Keywords:

• Lacunar spinels
• phase diagrams
• orbital order
• multiferroicity
• skyrmions