Phenomenological model for the frequency dependence of the susceptibility of spin glasses and related compounds

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Abstract
A phenomenological model is introduced to describe the frequency dependence of the AC susceptibility, chi ( omega ), of random magnetic materials. The model is applied to CuMn, (Eu, Sr)S, holmium borate glass and aluminosilicate glass. On the basis of the model criterion the various systems are classified, and it is possible to distinguish between spin glasses and superparamagnets. The authors' approach is to divide the spin system into two temperature-dependent parts: one with correlated spins (clusters) and one with non-correlated (free) spins. Thereby they obtained meaningful values for the activation energy Delta Ea and the intrinsic time constant tau 0 which follows from an Arrhenius law. In those cases where the authors were able to combine susceptibilities over twelve decades in frequency their analysis showed an enormous evolution towards long times for the cluster relaxation times near and below Tf.