The theory for thermally assisted flux flow (TAFF) in the limit of small driving forces is used to derive exact expressions for the time-dependent behaviour of the magnetisation and permeability. The problem is especially relevant for the high-temperature superconductors where large variations of the transition temperature in small DC fields are observed as a function of the frequency of the probing AC field. The parameters of the theory are extensively discussed in relation to the present understanding of flux pinning.
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