Calculating coefficients for a system of moment equations used to describethe magnetization kinetics of a superparamagnetic particle in a fluctuating field

A system of equations is derived for moments [averages of spherical harmoni cs < Y-l,Y-m>(t)] that determine the dynamics of the magnetization M of a s uperparamagnetic particle in a fluctuating field. The system is derived by representing the Gilbert equation in a fluctuating field, and the correspon ding Fokker-Planck equation for the distribution function of M, in terms of angular momentum operators, which in turn makes it possible to express the coefficients of the system of moment equations in terms of Clebsch-Gordan coefficients. (C) 1999 American Institute of Physics. [S1063- 7834(99)02311 -4].