Reversibility and time dependence of the magnetization in Ag:Mn and Cu:Mn spin glasses

We report dc-magnetization measurements on Ag:Mn and Cu:Mn spin glass samples (1-8% Mn concentrations), at temperatures of 4-30 K, and in fields of 4-400 Oe. The temperature at which the magnetization of a particular sample becomes reversible (T_R) is found to depend on the homogeneity of the sample, and on the magnitude of the measurement field. Inhomogeneous samples showed irreversible behavior far above the glass temperature (T _R≃1.5 T_G). After annealing, these samples showed a sharper peak in the magnetization at T_G∼T_R. We feel this is strong evidence toward a unique glass temperature for homogeneous samples in low fields. In higher fields (400 Oe) we found reversibility to begin at a temperature less than the magnetization peak (T_R≃0.9 T_G for 4% Ag:Mn).