Excitation of Electronic and Nuclear Spin Waves in a Flopped Antiferromagnet

Critical rf fields for parallel pumping in a flopped antiferromagnet are derived, and experimental results on excitation of electronic and nuclear spin waves in RbMnF₃ are reported. The theory includes expressions for the critical fields necessary for the excitation of two electronic spin waves, two nuclear spin waves, and one nuclear and one electronic spin wave. This involves computation of normal-mode frequencies and eigenvectors for the coupled nuclear and electronic spin systems. These normal modes for the flopped configuration show a high degree of ellipticity associated with the electronic motion, thus making the system ideal for spin-wave studies by parallel pumping. In particular, we predict that in the flopped state two nuclear spin waves can be excited in RbMnF₃ with critical rf fields of only about 0.1 Oe. Simultaneous excitation of one electronic and one nuclear spin wave by parallel pumping in RbMnF₃ has been performed in the temperature range from 2.4 to 13^∘{}K. Wave vectors up to 10⁵ cm^-1 are excited at a pumping frequency of 8.469 GHz. Using the experimental data to infer the product relaxation rates ηₖᵉηₖⁿ, we find order-of-magnitude agreement with Richards's theory for ηₖⁿ and with Cole and Courtney's experimental value for ηₖᵉ. In the product relaxation rates, there appears a minimum at a given k whose value increases linearly with temperature. There is no current theory which predicts such behavior. Excitation of two electronic spin waves could not be detected, since simultaneous electronic plus nuclear excitation was always the first process to become unstable.