Abstract
AC susceptibility measurements were carried out on K3C60 crystals. The complex susceptibility χ=χ′+χ′′ shows dependence on dc magnetic-field amplitude H dc and ac amplitude h ac. The temperature where χ′′ is maximal, T p, exhibits a weak dependence on ac frequency f: T p rises with increasing f. Increasing H dc or h ac enhances the effect. The inverse temperature, T p-1, roughly obeys the linear relationship T p-1≈-5×10-5H dcln f+b(H dc), where b is a constant and SI units are used. This is in qualitative agreement with the predictions of the thermally activated flux creep model. This suggests that in the experiment flux lines are pinned at pinning sites and ac losses are mainly due to flux pinning losses. The temperature T p is found to obey an empirical scaling law (1-T p/T c)3/2≈4×10-4H dc[ln (f0/f)+25]+1.4×10-2. The scaling relationship is in good agreement with the predicted equation based on the model of thermally activated flux motion.
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