Superparamagnetic Nanoparticles

The thermal fluctuations and relaxation of the magnetization of single domain ferromagnetic particles currently merit attention in view of their importance in the context of magnetic recording of media and rock magnetism, as well as in connection with the observation of magnetization reversal in isolated ferromagnetic nanoparticles and nanowires. The instability of the magnetization due to thermal agitation results in superparamagnetism because each fine particle behaves like an enormous paramagnetic atom having a magnetic moment ~104 - 105 Bohr magnetons. The pioneering theory of thermal fluctuations of the magnetization M of a single domain ferromagnetic particle due to Néel was further developed by Brown using the theory of the Brownian motion. In the context of the Brown continuous diffusion model,4,5 the magnetization dynamics of magnetic nanoparticles is similar to the rotation of a Brownian particle in a liquid and is governed by a Fokker-Planck equation for the probability density function W of M.