The low-field Hall effect and susceptibility of archetypal spin glasses-evidence for a second magnetic transition

Abstract

The authors have measured the very-low-field Hall coefficient and AC susceptibility of a number of Au-Mn, Au-Fe and Ag-Mn spin-glass alloys over the temperature range 4.2-300 K. The relation R11=R0+(Rs/ mu 0) chi m has been found to be inapplicable in all the alloys, but the modified relation RH=R0+(Rs/ mu 0) chi M0.75 fits all the alloys over the whole temperature range investigated. Changes of R0 and Rs occur across the freezing temperature and also at a temperature in the paramagnetic region which coincides with the temperature where deviations from Curie-Weiss behaviour take place. It is suggested that at this temperature, a subtle second magnetic transition occurs where individual moments first couple under the RKKY interaction. As a result, spin glasses are characterised by three regions; spin glass, intermediate paramagnetic and Curie-Weiss paramagnetic regions. The upper transition temperature appears to be closely associated with the temperature at which the magnetic component of the electrical resistivity reaches a maximum in Au-Mn alloys. The changes of R0 and Rs across the freezing temperature have been associated with changes in the anisotropy of ordinary and skew scattering resulting from the combination and blocking of clusters immediately above the freezing temperature.