Magnetic and magnetoelastic properties of UCo₂Si₂ as studied by high-field magnetization and ultrasound measurements

We investigated an antiferromagnet UCo₂Si₂ by use of magnetization and ultrasound measurements in pulsed magnetic fields up to 60 T. It is found that the crystal UCo₂Si₂, which has the antiferromagnet type-I magnetic structure in zero field below T_N = 83 K, undergoes the metamagnetic phase transition to ferrimagnetic structure ++− type similarly to the UNi₂Si₂ with substitution of 10%–15% Ni by Pd or UPd₂Si₂. Therefore, similar phase transitions take place in the compounds with expected essentially different strength of the 5 f -d electron hybridization. In UCo₂Si₂, the transition occurs when the magnetic field is applied along the c axis at 45 T (at 1.5 K). The transition is extremely sharp and exhibits a small but non-negligible hysteresis. With increasing temperature, it becomes broader and vanishes at T_N. In our ultrasound measurements, the metamagnetic transition appears as anomalies in the sound velocity and attenuation. Our analysis suggests that the low-temperature changes in the sound velocity and attenuation predominantly are determined by an exchange renormalization caused by the sound waves.