Magnetic‐field dependence of T_g in bulk Cu:Mn and Cu:Mn/Cu multilayer systems

In order to ascertain the universality class of the long‐range metallic spin glass Cu:Mn, extensive measurements of the longitudinal component of the zero‐field‐cooled (M_zfc) and field‐cooled (M_fc) magnetization have been made. Measurements were made in both bulk Cu:Mn and Cu:Mn/Cu multilayer films in magnetic fields between 2 and 15 000 G. Mean field calculations of a Heisenberg spin glass with weak Dyzaloshinskii–Moriya‐type anisotropy predict Ising‐like behavior at low fields crossing over to Heisenberg behavior in high fields. Evidence consistent with crossover behavior in the transition temperature T_g (where T_g is defined by the onset of strong irreversibility in the magnetization, M_irr = M_fc − M_zfc) was observed at fields similar to previous magnetic‐field‐dependent studies. From the low‐field extrapolation of T_g to H=0, the zero‐field Ising‐like transition temperature was obtained while from the high‐field extrapolation to H=0 the zero‐field Heisenberg transition temperature was obtained, the latter significantly lower than the former. This analysis, applied to the multilayer systems, suggests that the lower critical dimension of both Ising and Heisenberg spin glasses lies between two and three.