Henkel plots in a temperature and time dependent Preisach model

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Abstract
The effect of finite temperature T and observation time t on the Henkel plots of ac and thermally demagnetized systems has been investigated within the framework of a generalized Preisach model, in which it is assumed that thermally activated hopping will occur over all energy barriers W<W* =k/sub B/TIn(t//spl tau//sub 0/), where /spl tau//sub 0/ is a microscopic time, and will systematically drive the Preisach plane towards equilibrium. The Preisach distribution function is assumed to be a factorized product of a Gaussian coercive field distribution, with mean value h~/sub c/ and dispersion /spl sigma//sub c/, and a Gaussian interaction field distribution, with a self-consistent mean-field average h~/sub int/=km and dispersion /spl sigma//sub s/. Increases in temperature or observation time cause a progressive collapse of the hysteresis cycle, as expected, and also enhance demagnetizing-like curvature in Henkel plots, at least for ac demagnetized systems. An exception is a thermally demagnetized system with k=0, which has a linear Henkel plot independent of W*. Varying the effective time for thermal relaxation of the magnetization from branch to branch of the hysteresis cycle can have the effect of imitating mean field interactions of both magnetizing-like and demagnetizing-like sign in systems with k=0, and can even lead to Henkel plots which violate the lower boundary i/sub d/=-i/sub r/.