High-Frequency Sound Propagation near Magnetic Phase Transitions

The results of ultrasonic attenuation and velocity experiments near the magnetic phase transitions in RbMnF₃ and MnF₂ are presented. For T>TN, the data give additional evidence that the sound couples to the spin-energy-density fluctuations which decay via spin-lattice relaxation. The attenuation results for RbMnF₃ could be fitted with a single relaxation formula for the frequency range 90-930 MHz and for the temperature range of ε[=(T-TN)TN] from 10^-4 to 10^-2. The spin-lattice relaxation time τL was found to vary from 4×{}10^-10 to 2×{}10^-10 sec in the same ε range. Furthermore, both the critical attenuation and velocity changes are independent of sound-wave propagation direction. In MnF₂, we found from the critical velocity dispersion a constant relaxation time τL=(2.7±0.2)×10^-9 sec for ε≥10^-3, in good agreement with Kawasaki and Ikushima's quoted value. A small divergence was found for smaller ε. For T<TN, it was found that the maxima of the attenuation and the minima of the velocity shifted to lower temperatures with increasing frequency in RbMnF₃. In addition, in RbMnF₃, a second attenuation peak was found below TN which shifted to lower temperatures with increasing frequency. This subsidiary attenuation peak was found to be strongly sample dependent. In MnF₂, broad attenuation maxima and velocity minima are observed in the ordered region. Possible mechanisms for these effects are discussed.