Distinction of spin-glass freezing from superparamagnetic blocking

Abstract

Experimental results on Eu x Sr 1− x S provide clear evidence for a cooperative phenomenon at the spin-glass transition, as distinguished from ordinary thermal blocking of superparamagnetic clusters. Only below the percolation threshold x p = 0.13 can single-clusters aspects be separated clearly (superparamagnetic regime). In the spin-glass regime for x > x p, susceptibility and remanent magnetization are studied near the freezing temperature in dependence on temperature, magnetic field and observation time. The anomalous slow relaxations of the remanent magnetization, which follow a power-law, exhibit a strong variation just near T f0, the transition temperature deduced from static magnetization measurements. In addition, T f values derived from ac- χ measurements are distinctly frequency dependent; the frequency variation decreases towards low frequencies and seems to saturate near the T f0 value. The strong sensitivity of χ( T f) to even small applied fields can be represented by a universal function independent of concentration. All these results emphasize the importance of the interactions among the spin clusters of spin glasses which are partially frustrated.