Effect of Magnetic Field on Sound Propagation near Magnetic Phase Transition Temperatures

The anomalous ultrasonic attenuation and velocity variation caused by the critical fluctuation of spins near the Curie and Néel temperatures are theoretically investigated and found to be strongly affected by an application of magnetic field. In the random phase approximation, the attenuation coefficient is expressed in terms of a sum of the two terms; a cross term of the static spin polarization and the two-spin correlation function, and a product of the two-spin correlation functions. In the magnetic field, the former term has a positive contribution to the attenuation, since this term has a finite value only when the static spin polarization exists. The latter term decreases in the field, owing to the suppression of spin thermal fluctuations due to the magnetic field. The magnitude of the contributions from these terms depends upon temperature, the strength of magnetic field and the nature of the exchange interaction in magnetic materials. The theory explains various types of the field dependence of the attenuation observed in magnetic materials including MnP, Dy and MnF2. A new attenuation peak found recently by Hirahara et al. in the paramagnetic phase of MnP under a magnetic field is explained on the basis of the present theory.