Ultrasonic and magnetic studies of Nd_0.5Sr_0.5MnO₃

The results of systematic ultrasonic and magnetic studies of the magnetoresistive compound Nd_0.5Sr_0.5MnO₃ are reported. With decreasing temperature the pronounced acoustic-mode hardening was observed at about T_CO=145 K. The significant softening of the acoustic mode was observed in the vicinity of the insulator-to-metal transition induced by magnetic field. The T-B phase diagram was obtained from the ultrasonic measurements. The change of sound velocity around T_CO was explained by a strong coupling between the acoustic phonons and the CO states. The peculiarities of the sound propagation at the phase transition can be attributed to the phase segregation in the ground state in Nd_0.5Sr_0.5MnO₃. An enhancement in magnetic correlations at temperatures below 20 K, as seen by magnetization and ultrasound measurements, was observed in Nd_0.5Sr_0.5MnO₃. We discuss this effect in terms of possible short-range magnetic ordering on the rare-earth sublattice.