Static magnetization of immobilized, weakly interacting, superparamagnetic nanoparticles

摘要

The magnetization curve and initial susceptibility of immobilized superparamagnetic nanoparticles are studied using statistical-mechanical theory and Monte Carlo computer simulations. The nanoparticles are considered to be distributed randomly within an implicit solid matrix, but with the easy axes distributed according to particular textures: these are aligned parallel or perpendicular to an external magnetic field, or randomly distributed. The magnetic properties are calculated as functions of the magnetic crystallographic anisotropy barrier (measured with respect to the thermal energy by a parameter sigma), and the Langevin susceptibility (related to the dipolar coupling constant and the volume fraction). It is shown that the initial susceptibility chi is independent of sigma in the random case, an increasing function of sigma in the parallel case, and a decreasing function of sigma in the perpendicular case. Including particle-particle interactions enhances chi, and especially so in the parallel case. A first-order modified mean-field (MMF1) theory is accurate as compared to the simulation results, except in the parallel case with a large value of sigma. These observations can be explained in terms of the range and strength of the (effective) interactions and correlations between particles, and the effects of the orientational degrees of freedom. The full magnetization curves show that a parallel texture enhances the magnetization, while a perpendicular texture suppresses it, with the effects growing with increasing sigma. In the random case, while the initial response is independent of sigma, the high-field magnetization decreases with increasing sigma. These trends can be explained by the energy required to rotate the magnetic moments with respect to the easy axes.