Linear and cubic dynamic susceptibilities of superparamagnetic fine particles

A consistent theory of linear and nonlinear (cubic) initial susceptibilities of an assembly of uniaxially anisotropic noninteracting fine magnetic particles is presented. The expressions for the static (equilibrium) susceptibilities are obtained directly from the pertinent statistical thermodynamics. The contributions of anisotropy emerge yet in the first order and are analyzed for random and axes-aligned distributions. The ac susceptibilities are studied on the basis of the micromagnetic Fokker-Planck equation. Both a numerically exact solution for arbitrary frequency and a reliable low-frequency approximation are given. The obtained description Droves to be more accurate as compared to the one based on the customary superparamagnetic blocking model. The results are used for a quantitative interpretation of recently published set of data on Co-Cu precipitating alloys. In this connection the choice of the particle size-distribution function is discussed.