Muon spin relaxation in a random ferromagnet: PdMn

Muon spin relaxation (ySR) experiments have been performed in transverse, longitudinal and zero applied field on the disordered ferromagnet PdMn (2.3 at.%). Muon relaxation rates in the paramagnetic phase are in good agreement with predictions of the Walstedt-Walker formula, and appear to have no contribution from muon coupling to the giant moment polarization cloud. The transverse field relaxation rate (yT) and the width of x the zero field quasistatic field distribution (a(T)) in the ferromagnetic phase are less than calculated from a uniform mean field model. This "shortfall" is found to be qualitatively consistent with the Sherrington-Kirkpatrick disordered mean field theory. Spin-lattice relaxation rates in zero applied field give clear evidence for critical slowing down of Mn spin fluctuations over a wide temperature range, and are consistent at low temperatures with a two magnon Raman scattering mechanism