SPIN DYNAMICS IN SOLID DILUTE PARAMAGNETS

The free-induction decay signals for the two-spin clusters, the ''matr ix,'' and the entire spin system as a whole are calculated for a real dipole-dipole interaction on the basis of a method for the cluster exp ansion of correlation functions previously developed by the authors. T he intracluster interactions, which generate a discrete spectrum, are taken into account exactly, and the interactions outside the clusters are taken into account on the basis of the Anderson-Weiss-Kubo (AWK) t heory of phase relaxation. The corresponding shape functions of the re sonance line are calculated. Before the clusters are separated, an AWK analysis in the limit of slow fluctuations of the local fields is in satisfactory agreement with the exact results obtained in the Anderson model of dipolar interactions, but a similar analysis for realistic f luctuation rates and Dt>1 leads to a significant qualitative differenc e: exp(-Dt) transforms into exp(-root D(1)t). After the clusters are s eparated, this difference is obliterated. The free-induction decay is almost exponential up to Dt similar to 10, and nonexponential behavior of the long-time asymptote appears sooner. This finding is not altere d significantly when the analysis is complicated by raising the maximu m rank of the clusters to 3 or more. (C) 1997 American Institute of Ph ysics.