ACOUSTIC PROPERTIES OF Nd₂CuO₄ AT LOW TEMPERATURES

Abstract

Acoustic properties anomalies near the points of phase transition in the magnetic subsystem of Nd 2 CuO 4 have been studied. A spontaneous ferroelastic phase transition at T = 5 K and magnetic field induced ferroelastic phase transition (PT) associated with continuous spin reorientations at T < 0.5 K have been found. The exchange nature of magnetoelastic coupling is suggested.

Low-temperature magnetism ofNd2CuO4: An ultrasonic investigation

“…The dip intensities are enhanced as the temperature is decreased similarly to the Nd 3+ magnetic susceptibility, 14,25 suggesting a magnetoelastic coupling involving directly the Nd 3+ magnetic moments rather than the Cu 2+ ones. Furthermore, the Cu 2+ Néel phase transition has been detected neither in our measurements nor in previous ultrasonic studies, 16,17 indicating a weak coupling of the Cu 2+ sublattice with the acoustic phonons. The first dip observed at 67 K, which is revealed using the ultrasonic technique, is associated with phase I→phase II transition.…”

“…We focused our study on the C 66 elastic constant, which exhibits the strongest anomalies at low temperatures, and on the corresponding attenuation variations. Neither a spontaneous ferrroelastic transition 16,17 nor a paraelastic contribution 19 can explain our results; we rather propose to relate these anomalies to growing magnetic domains due to the frustration of the Nd 3+ magnetic subsystem arising from the competition between Nd 3+ -Cu 2+ and Nd 3+ -Nd 3+ interactions.…”

“…Hence, in a previous ultrasonic study of Nd 2 CuO 4 at a frequency of 54.3 MHz, anomalies found around 33.5 K on the C 66 and the C 11 -C 12 elastic moduli have been associated with the phase II→phase III magnetic transition. 16 Moreover, this study has revealed a strong anomaly ͑ϳ5%͒ around 5 K in the C 66 elastic constant, accompanied by a peak in the attenuation. While no frequency dependence of this anomaly has been reported, a shift towards the low temperatures, as well as a decrease in its amplitude up to the maximum magnetic field used ͑3 T͒, have been observed under a magnetic field applied along the ͓100͔ direction.…”

“…While no frequency dependence of this anomaly has been reported, a shift towards the low temperatures, as well as a decrease in its amplitude up to the maximum magnetic field used ͑3 T͒, have been observed under a magnetic field applied along the ͓100͔ direction. 16 Weaker anomalies ͑Ͻ0.1% ͒ have also been measured at the same temperature in the C 11 and C 44 elastic moduli. These anomalies around 5 K were attributed to a spontaneous ferroelastic phase transition, whose origin was later associated with a loss of spin orthogonality in the magnetic structure.…”

“…18 More recently, acoustic measurements at lower frequencies performed on polycrystalline NCCO bars revealed a softening of the Young's modulus below 20 K. 19 Because Ce doping shifts this anomaly to lower tempera-tures, the authors concluded that it could not be related to the minimum observed in C 66 ͑T͒ in the ultrasonic experiments performed on single crystals. 16,17 The authors interpreted rather this anomaly in terms of a paraelastic contribution from the relaxation of Nd 3+ 4f electrons between the levels of the ground state doublet split by the interaction with the antiferromagnetically ordered Cu 2+ lattice; the value of the splitting was then found to be in agreement with inelastic neutron scattering, 20 infrared transmission, 21 and specific heat experiments. 22 In this paper, we further investigate the magnetic properties of Nd 2 CuO 4 at low temperature with the ultrasonic technique.…”