Muon spin relaxation in a superparamagnet: Field dynamics in Cu98Co2

Zero-field muon-spin-relaxation techniques have been used in conjunction with magnetization measurements and numerical simulations to characterize the internal-field distributions and dynamics in a model superparamagnetic system, namely, dilute heterogeneous Cu₉₈Co₂. The internal dipolar field distribution in Cu₉₈Co₂ arising from the polydispersed, noninteracting magnetic Co clusters, is found to be Lorentzian in form, with a half width at half maximum which, for a given Co concentration, is independent of the details of distribution of cluster moments. Superparamagnetic relaxation of the clusters is shown to result in a root-exponential muon depolarization function with an associated relaxation rate that can be related directly to the mean fluctuation rate of the dipolar fields, but which is entirely independent of the width of the distribution of fluctuation rates. Analysis of the observed muon-spin-relaxation spectra of Cu₉₈Co₂ within the framework of the proposed model enables both the activation energies and intrinsic relaxation rates of the superparamagnetic Co clusters to be extracted.