High-field magnetism and magneto-acoustics in UCo₂Si₂

We report results of magnetization and magneto-acoustic studies on a UCo₂Si₂ single crystal in high magnetic fields. This compound orders antiferromagnetically at the Néel temperature T_N = 83 K and shows at low temperatures a first-order metamagnetic transition (MT) at 45 T with very small hysteresis (µ0ΔH_cr = 0.16 T) to a ferrimagnetic state seen as a sharp jump in the magnetization when the magnetic field is applied along the c direction. The magnetization curve measured along the a axis shows no transition and is linear up to 60 T. The acoustic properties exhibit drastic anomalies in the vicinity of both magnetic phase transitions; the spontaneous and the field-induced one. At T_N, a pronounced change in the sound velocity Δv/v accompanied with a peak in the attenuation Δα has been observed. Whereas Δα only shows a very sharp peak at the MT, Δv/v displays a more complicated behavior; it has a non-monotonous temperature evolution with maximum effects at 30 K, which can be due to the transition changing from first to second order. Our results enabled us to map the phase diagram of UCo₂Si₂ in fields applied along the c axis. We discuss our observations in relation to the magnetism on the U site and the magnetoelastic interaction in this material. *Part of this work was supported by EuroMagNET under the EU contract No. 228043.