THERMAL FLUCTUATIONS OF FINE MAGNETIC PARTICLES

We study the influence of thermal fluctuations on the dynamics of a fi ne magnetic particle in a uniaxial potential. At low temperature, the transverse component of the magnetization precesses coherently about t he easy axis. With rising T, thermal fluctuations reduce the precessio n frequency and, as the thermal energy approaches the anisotropy poten tial, they drive a crossover to incoherent motion. Our findings agree with recent neutron scattering data.