Henkel plots in a temperature and time dependent Preisach model

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(B)T In(t/tau(0)), where tau(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)over bar (c)> and dispersion sigma(c), and a Gaussian interaction field distribution, with a self-consistent mean-field average <(h)over bar (int)> = km and dispersion sigma(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(d) = -i(r).