ANISOTROPY EFFECTS ON THE NONLINEAR MAGNETIC-SUSCEPTIBILITIES OF SUPERPARAMAGNETIC PARTICLES

Equilibrium nonlinear susceptibilities of an overdamped three-dimensio nal rotator in a uniaxial anisotropy potential alpha cos(2) theta (the ta is the angle between the rotator axis and the potential symmetry ax is), which apply to independent magnetic particles and other rotationa lly bistable entities, are derived. In the crossover range from free-r otator to either two-state or plane-rotator regime induced by that pot ential, the dependences of the nonlinear susceptibilities on T can be steeper than those of the limit inverse-temperature power laws. The no nlinear susceptibilities can then resemble the high-temperature ranges of quantities diverging at low temperature, misleadingly suggesting i nterparticle interactions.