Nuclear Spin-Wave Relaxation and Narrowing of NMR Lines in Ferro- and Antiferromagnets

It is shown that if a well-defined nuclear spin-wave spectrum exists in a ferro- or antiferromagnet, the NMR linewidth is considerably narrower than would be calculated from the second moment 〈Δω²〉 due to the Suhl-Nakamura interaction. We argue that if the frequency pulling ωc is large compared to 〈Δω²〉1/2, only a small band of nearly degenerate modes can interact; so the situation is analogous to standard transition-probability calculations. This explains why the observed NMR linewidth in RbMnF₃ is an order of magnitude smaller than 〈Δω²〉1/2. Nuclear spin-wave relaxation rates are calculated for RbMnF₃. For zero wave vector the rate is 1.5 ×{} 10³ sec^-1 at 4.2^∘{}K in a field of 3000 Oe. This figure is much less than the NMR width, but is in close agreement with parallel-pumping data. It is suggested that the NMR width may be due largely to random inhomogeneities.