A Preisach model for systems with magnetic order

We present a generalized version of the Preisach model which includes a cri tical magnetic ordering temperature T-c. The spontaneous moment of the Prei sach subsystems is allowed to vary as mu(T) = mu(o)(1 - T/T-c)(Gamma). This introduces an explicit temperature dependence into the free energy landsca pe, which is modelled by incorporating the critical temperature dependence of the spontaneous moment into the Preisach distribution parameters (h) ove r bar(c) = (h) over bar(eo)(1 - T/T-c)(Gamma), sigma(c) = sigma(co)(1 - T/T -c)(Gamma), sigma(i) = sigma(io)(1 - T/T-c)(Gamma). Thermal overbarrier act ivation is described by a thermal viscosity field h(T)* = [k(B) T/mu(T)]1n( t(exp)/tau(0)). The model is able to replicate the temperature dependence o f the hysteresis loop, and of the field cooled and zero field-cooled moment of a broad spectrum of magnetic systems, from superparamagnetic suspension s of fine particles to 'conventional' ferromagnets. (C) 2000 Elsevier Scien ce B.V. All rights reserved.