  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, N\'eel-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 polarization and that excess spin-driven polarizations
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 summarize 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.
