LINEAR AND CUBIC DYNAMIC SUSCEPTIBILITIES OF SUPERPARAMAGNETIC FINE PARTICLES

A consistent theory of linear and nonlinear (cubic) initial susceptibi lities of an assembly of uniaxially anisotropic noninteracting fine ma gnetic particles is presented. The expressions for the static (equilib rium) susceptibilities are obtained directly from the pertinent statis tical thermodynamics. The contributions of anisotropy emerge yet in th e first order and are analyzed for random and axes-aligned distributio ns. The ac susceptibilities are studied on the basis of the micromagne tic Fokker-Planck equation. Both a numerically exact solution for arbi trary 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 resu lts are used for a quantitative interpretation of recently published s et of data on Co-Cu precipitating alloys. In this connection the choic e of the particle size-distribution function is discussed.