ACOUSTIC EXCITATION OF NUCLEAR-SPIN WAVES IN THE EASY-PLANE ANTIFERROMAGNETIC MATERIAL KMNF3

Ultrasound damping at T= 4.2 K in single crystal easy-plane antiferrom agnetic KMnF3 is studied experimentally as a function of the magnitude and direction of a constant magnetic field H at frequencies of 640-67 0 MHz, corresponding to the frequencies of nuclear spin waves. Two exp erimental situations are examined: in the first, the vector H lies in the easy magnetization plane (001), and in the second, H forms an angl e with (001). For longitudinal ultrasound waves propagating along the hard magnetization axis [001], it is found that the damping depends re sonantly on the magnitude of the field H. In the first case a single d amping maximum is observed, and in the second, two damping peaks that are well resolved with respect to the field. The angular dependence of the resonance damping signals on the direction of the constant magnet ic field is found to have a 90 degrees periodicity in all cases. The o bserved effects are explained by resonant ultrasonic excitation of nuc lear spin waves. On the basis of an analysis of the magnetoacoustic in teraction energy, it is shown that in the first case, nonzero oscillat ions of the antiferromagnetism vector L occur only in the basal plane, while in the second, oscillations of L occur both in the basal and a vertical plane, which are associated, respectively, with two branches of the nuclear spin waves. It is also shown that the 90 degrees period icity in the angular dependence of the damping signals is associated w ith a fourth order [001] axis. (C) 1997 American Institute of Physics.