Ultrasonic investigation of GaV₄S₈

In recent years, transition metal-chalcogenides with the cubic GaMo₄S₈-type structure have attracted much interest because of their fascinating properties such as superconductivity under pressure, metal-insulator transition, 4d ferromagnetism and various structural and magnetic instabilities at low temperatures. These phenomena reflect the strong coupling of structural, electronic and magnetic degrees of freedom in this system. Here, we present results of ultrasound investigations performed on a high-quality single crystal of the tetrahedral magnetic-cluster material GaV₄S₈. The temperature dependence of the sound velocity and attenuation shows a huge anomaly at T_s = 44 K followed by a pronounce feature at T_c = 12.5 K related to a structural transition and ferromagnetic ordering, respectively, as known from previous magnetic-susceptibility and specific-heat results. A field-induced transition at temperatures below T_c reveals a complex magnetic structure of GaV₄S₈, suggesting a competition of several interactions. Based on these measurements, we mapped out the B-T phase diagram of GaV₄S₈ and discuss the role of the lattice degrees of freedom in this material.