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_BTIn(t/τ₀), where τ₀ 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¯_c and dispersion σ_c, and a Gaussian interaction field distribution, with a self-consistent mean-field average h¯_int=km and dispersion σ_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