A Preisach model for systems with magnetic order

We present a generalized version of the Preisach model which includes a critical magnetic ordering temperature T-c. The spontaneous moment of the Preisach 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 landscape, which is modelled by incorporating the critical temperature dependence of the spontaneous moment into the Preisach distribution parameters (h) over 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 activation 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 of the hysteresis loop, and of the field cooled and zero field-cooled moment of a broad spectrum of magnetic systems, from superparamagnetic suspensions of fine particles to 'conventional' ferromagnets. (C) 2000 Elsevier Science B.V. All rights reserved.