Magnetization and magneto-acoustics studies on UCo₂Si₂ single crystal

We report results of magnetization and magneto-acoustic studies on a UCo₂Si₂ single crystal in high magnetic fields. This compound has a tetragonal crystal structure and orders antiferromagnetically at T_N = 83 K. At low temperatures, it shows a first-order metamagnetic transition (MT) at 45 T with very small hysteresis (μ₀ΔH_cr = 0.16 T) to a ferrimagnetic state. This transition is seen as a sharp jump in the magnetization when the magnetic field is applied along the c direction. 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 only Δα shows a very sharp peak at the MT, Δv/v displays a more complicated behavior. In addition the sound-velocity anomaly exhibits a non-monotonous temperature evolution with maximum effects at 30 K, which can be due to the transition change from the first to the 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 magneto-elastic interaction in this material.