Nonlinear dynamic susceptibilities and field-induced birefringence in magnetic particle assemblies

摘要: In a systematical order the superparamagnetic theory, dating back to the cornerstone works by L. N'eel and W.F. Brown (Jr.), is extended for the situation of nonlinear magnetization. Our basic formalism is the Fokker–Planck equation for the orientational distribution function, which accounts for the rotations of the magnetic moment of a single‐domain nanoparticle being in temperature equilibrium with a thermal bath. Starting with recalling the linear dynamic susceptibilities of solid and fluid superparamagnets, we then step over the limits of the linear response theory and analyze magnetization kinetics of a nanoparticle assembly in terms of the higher‐order susceptibilities. Equivalently, one may refer to this as to the the problem of higher harmonics generation in a nanoparticle assembly subjected to an AC magnetic field of finite amplitude. The theory given applies for the low‐frequency range, that is the time rate of …