Magnetic relaxation and electronic properties of the Eu1-xGdxS spin glass system from151Eu Mossbauer spectroscopy

Abstract

The hyperfine data on the spin glass system Eu1-xGdxS are obtained by 151Eu Mossbauer spectroscopy in the temperature range from 1.5 to 130K. The work concerns the systematics of the electronic properties and the problem of magnetic ordering and of possible relaxation phenomena associated with spin glass freezing. The isomer shift dependence on concentration corresponds primarily to a chemical pressure effect while the hyperfine field dependence is determined by electronic transfer effects. AC susceptibility has suggested a spin freezing transition at x approximately=0.5, successive ferromagnetic-spin-glass transitions at x 0.57. Mossbauer spectroscopy reveals microscopic inhomogeneity at x=0.6 and x=0.7. On the other hand, for compositions x=0.3 and x=0.5 well defined onsets of magnetic short-range order are found at Tc=37K (x=0.3) and Tf=22K (x=0.5). At lower temperatures the data are unambiguously interpreted in terms of spherical relaxation of cluster magnetisation. The average relaxation rates are described by Arrhenius laws, with activation energies roughly scaling with Tc and Tf respectively. For x=0.3, no spectral change is detected on passing the spin freezing temperature (Tf approximately 14K) from the higher ferromagnetic phase. The sharp low-temperature spectra observed for all the concentrations are evidence for the predominance of long-range exchange interactions.