Ultrasonic Propagation in RbMnF3. III.F19Nuclear Acoustic Resonance

The resonant coupling of coherent radio-frequency phonons to the ${\mathrm{F}}^{19}$ nuclear-spin system in antiferro-magnetically ordered RbMn${\mathrm{F}}_3$ has been studied using a continuous-wave transmission spectrometer. With this technique, the resonant increase in the ultrasonic attenuation and/or the resonant dispersion in the phase velocity due to coupling with the spin system is detected. A phenomenological model of the spin-phonon interaction is developed which quite adequately explains the observed dependence of the nuclearacoustic-resonance (NAR) signal strength on the orientation of the externally applied magnetic field. The two phenomenological nuclear-spin-phonon coupling constants $g_1$ and $g_2$ have the experimentally determined ratio $\frac{g_1}{g_2}=18.5\mathrm{}\pm 3.0$. The similarity of this ratio to the ratio of the magneto-elastic coupling constants $\frac{b_1}{b_2}\gtrsim 10$ in this material is evidence that the observed spin-phonon interaction results from a magneto-elastically created virtual magnon which couples through the transferred hyperfine interaction to the ${\mathrm{F}}^{19}$ spin system to produce a spin transition. The temperature dependence of the signal strength is in good agreement with that predicted on the basis of the above coupling mechanism. The observed NAR line shape is quite complex and is found to depend on the temperature, power, and orientation.