Magneto-acoustic study of UCuGe

We present results of a magneto-acoustic study of single crystalline UCu_0.95Ge in static (up to 18 T) and pulsed (up to 60 T) magnetic fields. This intermetallic compound exhibits antiferromagnetic (AF) ordering at T_N = 48 K. The magnetic structure of UCuGe, characterized by the AF arrangement of the magnetic moments in the basal plane of the hexagonal lattice is determined by competing intra- and inter-layer exchange interactions. In magnetic fields applied along the c direction this material shows a phase transition at 38 T to a spin-polarized state with an U magnetic moment of 1.3µ_B. This phase transition is seen as a jump in the magnetization, accompanied by a hysteresis. The sound velocity and sound attenuation demonstrate pronounced anomalies in the vicinity of both the antiferromagnetic ordering and the field-induced phase transition proving the important role of magneto-elastic interactions in the physics of this actinide compound. We discuss our results in frame of a phenomenological model, which describes qualitatively the main experimental observations. In addition to the features accompanying the magnetic phase transitions, a broad and frequency-dependent step-like change of the sound velocity together with a broad maximum of the sound attenuation have been observed for the ultrasound wave propagating along the c axis at temperatures above T_N. We discuss these anomalies in relation to vacancy dynamics in the nonstoichiometric UCu_0.95Ge.