55Mn Nuclear Acoustic Resonance in Antiferromagnetic RbMnF3

An intense, frequency‐dependent absorption of ultrasound in antiferromagnetic RbMnF₃ has been observed and attributed to resonant phonon coupling to the ⁵⁵Mn nuclei. At 4.2 K, the maximum resonant absorption using longitudinal waves at ν₀=650 MHz was >40 dB/cm and was accompanied by a dispersion (shift of the acoustic phase velocity) of greater than 1%. Both the field dependent and field independent modes were seen. The angular dependence of nuclear acoustic resonance (NAR) absorption has been studied at 4.2 K for H₀> (2H_eH_a)^1/2. For k ∥[100] and H₀ in either the (001) or$\text{(011̄)}$ plane, the NAR absorption for the field dependent mode vanished for φ=nπ/2 where φ=(k, H₀). For φ≅π/4 the absorption was a maximum. Qualitatively, the same angular dependence was observed in a second crystal of RbMnF₃, for k ∥[110] and H₀ in the (100) plane. NAR absorption for the field independent mode showed a different angular dependence. For k ∥[100] and H₀ in the (001) plane, NAR absorption was a maximum for φ=π/2 and vanished for φ=0. The observed nuclear acoustic interaction is well described by a theory due to Fedders.¹ In this theory, phonons are coupled to the electronic spins by a magnetoelastic interaction. The electronic susceptibility is calculated by a random phase approximation for the coupled electronic and nuclear spins. The NAR dispersion calculated by Fedders agrees well with experiment.