Nonlinear response of superparamagnetic particles to a sudden change of a high constant magnetic field

For a system of superparamagnetic particles in a high external constant mag netic field, a technique for calculating the nonlinear response to a sudden change in the field direction and magnitude is proposed. A set of momentar y equations for the averaged spherical harmonics, which is derived from the Fokker-Planck equation for the magnetization-orientation distribution func tion is the basis of this technique. As an example, the nonlinear response of a system of particles with anisotropy of the easy-axis type is examined. For this case, a solution to the momentary equations is obtained by using matrix continued fractions. The magnetization relaxation time and the spect rum of the relaxation function are calculated for typical values of anisotr opy, dissipation, and nonlinearity parameters. It is shown that the magneti zation kinetics is essentially dependent on these parameters. (C) 2000 MAIK "Nauka/ Interperiodica".