MAGNON CONTRIBUTION TO THE ULTRASONIC-ATTENUATION IN PLANAR FERROMAGNETS

The propagation of acoustic waves in magnetic insulators is studied theoretically, first from a general point of view and then in the weak coupling limit. Both the modulation of the exchange integral and the single-ion magnetostriction are considered as coupling mechanisms between the spin and the phonon systems. At low temperatures, where the spin system can be described by non-interacting magnons, the attenuation coefficient is calculated for a planar ferromagnet. For the 1-D material CsNiF3 explicit expressions for the ultrasonic attenuation coefficient αph are computed in both zero and finite external field. αph is found to be linear in the temperature and quadratic in the phonon frequency for H=0. However, in a finite field the frequency dependence becomes linear and the temperature and field dependences are rather complicated.