Longitudinal dynamic susceptibility of superparamagnetic particles with cubic anisotropy

We study the linear response of a system of single-domain ferromagnetic par ticles with cubic magnetic anisotropy to a weak external a.c. magnetic fiel d. By averaging the Gilbert equation with a fluctuating field for the magne tization of an individual particle we derive a system of recurrence equatio ns for the spectra of equilibrium correlation functions describing the long itudinal relaxation of the system. We find the solution of this system by u sing matrix continued fractions. We also evaluate the longitudinal relaxati on time and the spectrum of the complex-valued magnetic susceptibility. Fin ally, we show that the nature of susceptibility dispersion is determined by the anisotropy and dissipation parameters. (C) 1999 American Institute of Physics. [S1063-7761(99)01001-X].