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 Δω212, 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 Δω212. 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. © 1968 The American Physical Society.