Analytic calculation of the longitudinal dynamic susceptibility of uniaxial superparamagnetic particles in a strong uniform DC magnetic field

A simple analytic equation which allows one to obtain a qualitative understanding of the physical processes underlying the behavior of the longitudinal complex susceptibility chi(\\)(omega) of an assembly of noninteracting uniaxial superparamagnetic particles subjected to a strong external uniform DC magnetic field is derived in the context of Brown's model of magnetic relaxation. It is shown that a knowledge of 3 time constants characterizing the magnetization relaxation, viz. the integral relaxation time tau, the effective relaxation time tau(ef), and the inverse of the smallest eigenvalue lambda(1) of the Fokker-Planck operator are sufficient to accurately predict the spectrum of chi(\\)(omega) in all frequency ranges of interest as well as the behavior of the equilibrium correlation function of the longitudinal component of the magnetization C-\\(t) in the time domain at all values of the barrier and external field parameters. (C) 2003 Elsevier Science B.V. All rights reserved.