Nuclear Acoustic Resonance ofMn55in Antiferromagnetic RbMnF3

An intense frequency-dependent absorption of ultrasound in antiferromagnetic RbMn${\mathrm{F}}_3$ has been observed and attributed to resonant phonon coupling to the manganese nuclear spins. At 4.2 K, the maximum resonant absorption using longitudinal waves at $\nu =650\mathrm{}$ MHz was much greater than 40 dB/cm and was accompanied by a dispersion (shift in acoustic velocity) of greater than 1%. Both the field-dependent and field-independent modes, with the electronic spins in a flopped configuration, have been studied. The observed nuclear acoustic resonance absorption and dispersion are well accounted for by the magneto-elastic theory of Fedders. A semiclassical model of the magneto-elastic theory, which agrees in most respects with that of Fedders, is described. It is shown further that this model can be extended to explain the coupling of ultrasound to the nuclear spins of magnetic atoms (e.g., ${\mathrm{Mn}}^{55}$ in MnTe) and nonmagnetic atoms (e.g., ${\mathrm{F}}^{19}$ in RbMn${\mathrm{F}}_3$) in other antiferromagnets.