High-field ultrasonics in the heavy fermion antiferromagnet URu2Si2

Abstract In the antiferromagnetic heavy fermion superconductor URu 2 Si 2 we observe the spin flop transition around 39 T below the Neel temperature T N using longitudinal sound. The c 33 -mode exhibits only one-step like anomaly at the spin flop transition in contrast to the three steps found in magnetization, magnetoresistance measurements and the longitudinal c 11 mode. This behaviour can be explained using the Gruneisen parameter coupling for the field dependence of the elastic constants. Above the Neel temperature of 17.5 K we find a softening of the c 33 elastic constants in high magnetic fields.