Anomalous zero-field muon spin relaxation in highly disordered magnets

Low-temperature zero-field muon spin relaxation spectra from several disordered magnetic materials show static-field relaxation behavior that cannot be properly represented by the Gaussian or Lorentzian distributions commonly used in the analysis of such data (Kubo-Toyabe relaxation). In the case of ${\mathrm{CeCu}}_{0.2}{\mathrm{Ni}}_{0.8}\mathrm{Sn}$, a Gaussian distribution of second moments, convoluted with a Gaussian form of field distribution, produces a closed-form modified Kubo-Toyabe relaxation function that fits the data well. Its characteristic feature is a shallow polarization minimum before recovery to the 1/3 asymptote. Any microscopic model that reproduces such shallow static Kubo-Toyabe relaxation must do so by generating an excess of low-field sites.