Spin freezing and the ferromagnetic and reentrant spin-glass phases in a reentrant ferromagnet

The reentrant spin-glass (RSG) transition and the magnetic nature of the RSG and ferromagnetic (FM) phases of a standard reentrant ferromagnet (Fe-57-doped) NiMn were investigated using ac susceptibility and Mossbauer measurements. The spin-freezing temperature, at the observation time of the ac magnetic method, was determined by a peak in the in-phase component of nonlinear susceptibility. The distribution of the hyperfine field in the zero-field Mossbauer data consisted of two peaks in the FM phase but one antisymmetric peak in the RSG phase. The average hyperfine field rapidly increases so as to deviate from the Brillouin function below a certain temperature. The onset of deviation corresponds to spin freezing on the Mossbauer time scale. The two peaks in the hyperfine field in the FM phase were assigned to two kinds of spin groups having different relaxation times. The spin-freezing process can be explained based on the concept of melting of frustrated spins introduced by Saslow and Parker. The local magnetization, deduced based on the in-field Mossbauer data, was dependent on the applied field in the same manner as the magnetization in both the RSG and FM phases. This indicates that a spin-glass correlation coexists with ferromagnetic order in the RSG phase in a different way from the mean-field model for vector spin glasses. The application of a magnetic field induced one additional peak in the hyperfine field in the RSG phase. This implies that the long-range spin-glass order becomes unstable by applying a magnetic field in the RSG phase. Based on all the experimental results, we construct a comprehensive picture of a reentrant ferromagnet.