Fixed magnetic nanoparticles: Obtaining anisotropy energy density from high field magnetization

Abstract

A simple method is proposed to obtain the effective anisotropy energy density Keff of an assembly of randomly oriented magnetic nanoparticles, from their hysteresis loops. It involves the fitting of a high field asymptotic expression of the magnetization in inverse powers of the applied field H, up to H−3. This is derived from the partition function formalism and the Stoner–Wohlfarth model for single domain nanoparticles. This method can be applied to ferrogels, frozen ferrofluids or magnetic nanoparticles powder (or any system where the nanoparticles are fixed in random directions, and not allowed to rotate), when dipolar interactions can be neglected. As a proof of concept, it is applied to a suspension of iron oxide nanoparticles in hexane, at different temperatures, obtaining the anisotropy energy density Keff as a function of temperature below the fusion point.