Magneto-acoustic properties of UCuGe

The hexagonal intermetallic compound UCuGe is an antiferromagnet with TN = 48 K. In magnetic fields of 38 – 45 T, applied along the hexagonal c axis, a spin-flop phase transition in this compound has been observed at 4.2 K [1]. We report on sound-velocity and sound-attenuation results obtained on a UCuGe single crystal at different frequencies. Thereby, a longitudinal ultrasonic wave was propagated along the [001] direction with static (up to 18 T) and pulsed (up to 57 T) magnetic fields applied along the same direction. The temperature dependences of the sound velocity and of the attenuation display a pronounced anomaly at TN, which is evidence for a strong magneto-elastic interaction. The pulse-field measurements at 4.2 K show a minimum in the sound velocity followed by a jump-like anomaly at 37 T, and another kink-like anomaly at 45-46 T. These anomalies are clearly connected with known field-induced spin re-arrangements and can be attributed to the symmetry changes at the start and end of the spin-flop transition. In the paramagnetic state (T > TN), the temperature dependences for both acoustic characteristics show large frequency-dependent changes. The observed linear frequency dependence of these changes reveals the presence of unusual relaxation processes. The origin of these anomalies will be discussed.