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Measurements of the field and temperature dependence of the frequency-dependent susceptibility X(ω) the thermoremanent magnetization (TRM) and the isothermal remanent magnetization (IRM) have been performed on a series of mixed ruthenates with chemical formula Ca x Sr 1-x RuO 3 . The susceptibility data are analysed within the framework of a static scaling theory. Estimates for the critical exponents γ, β, and δ are consistent with a second-order transition from a paramagnetic to a ferromagnetic state, with exchange bond disorder. The remanence data are analysed within the framework of a Preisach model, which incorporates an explicit reference to the critical temperature T C . The analysis shows that, under certain conditions, the TRM and the IRM yield independent scans of the Preisach distribution along two orthogonal directions in the Preisach plane, the TRM along the bias field axis H, and the IRM along the dissipation field axis H d .

The irreversible response of a ferromagnetic pervoskite SrRuO3 with a Curie temperature TC=160 K has been characterized experimentally by measuring the temperature dependence of the field cooled and zero field cooled moments over the temperature interval 5 K⩽ T ⩽180 K , in a sequence of applied fields up to the coercive field, and also the major hysteresis loop over the field range ∣ha∣⩽40 kOe at a sequence of temperatures which span the ordered phase. The data have been analysed within the framework of a generalized version of the Preisach model of hysteresis which combines thermal fluctuations with the growth of the free energy barriers below TC. The numerical simulations replicate all of the principal structural systematics of the data, and yield details of the spectrum of Barkhausen instabilities in this material.

We present a generalized version of the Preisach model which includes a critical magnetic ordering temperature Tc. The spontaneous moment of the Preisach subsystems is allowed to vary as μ(T)=μ0(1−T/Tc)Γ. 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 c = h co (1−T/T c ) Γ , σ c =σ co (1−T/T c ) Γ , σ i =σ io (1−T/T c ) Γ . Thermal overbarrier activation is described by a thermal viscosity field h ∗ T =[k B T/μ(T)] ln (t exp /τ 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.