55Mn Nuclear Acoustic Resonance in Antiferromagnetic RbMnF3

Abstract

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> (2HeHa)1/2. For k ∥[100] and H0 in either the (001) or(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.