Elastic response of U₃Cu₄Ge₄ to spontaneous and field-induced phase transitions

U₃Cu₄Ge₄ is a uniaxial ferromagnet that displays a first-order magnetization process (FOMP) at 25 T for field applied along the hard b axis. Here, we report on ultrasound and magnetostriction measurements of U₃Cu₄Ge₄ in static and pulsed magnetic fields up to 40 T. The FOMP causes stepwise anomalies in the magnetoelastic properties. U₃Cu₄Ge₄ elongates along the a and c axes and shrinks along the b axis, leading to an almost zero volume effect. The sound velocities of the longitudinal and transverse acoustic waves decrease sharply at the FOMP, whereas the sound attenuation shows pronounced peaks. An analysis of the ultrasound data using a mean-field theory suggests the existence of quadrupolar interactions and crystal-electric-field effects. The 5 f electronic states are between itinerant and localized, typical of uranium-based intermetallic compounds.