Ultrasonic Attenuation near and above the Spin-Flop Transition of MnF2

Abstract

The ultrasonic attenuation in Mn${\mathrm{F}}_2$ was measured at 4.2°K in steady magnetic fields up to 150 kG. These measurements were performed with 8 to 310-Mc/sec longitudinal and shear sound waves propagating along the [100], [110], and [001] crystallographic axes. The most prominent features of the data are: (1) For several acoustical modes, a sharp peak in the attenuation occurs near the spin-flop transition at 93 kG. This peak is observed only if H is within a few tenths of a degree from the [001] axis. (2) For two acoustical modes, a large absorption edge, as a function of magnetic field, exists near the spin-flop transition. The edge is observed only if H is within ∼2° from the [001] axis. Supplementary antiferromagnetic-resonance experiments, at $X$ band, were performed on the same single crystal of Mn${\mathrm{F}}_2$. Several mechanisms which may be responsible for the observed magnetic field variation of the ultrasonic attenuation are discussed.