Structure effects on the magnetic behavior in Fe oxide based nanoparticles

Abstract

A key question in magnetic nanoparticles (NP) is how the nanostructure modifies their magnetic and electronic properties. As the particle size is reduced, deviations from bulk behaviour have been widely reported and attributed to both the appearance of a magnetically disordered surface layer and finite-size effects, thus giving rise to a decrease in the particle magnetization, an exchange field acting on the core and an increase in the anisotropy. In order to elucidate those phenomena we have carried out STEM, XAS, XMCD and magnetization measurements in Fe oxide based NP models, aiming at the real-space characterization at the sub-nanometer scale of a single NP to characterize the particle structure, surface magnetization, and the dependence of the latter on the strength of the surface bond to a bio-molecule coating. We also aim at the quantification of the orbital and spin contributions to the NP magnetic moment. The key role of the crystal quality is thus suggested, because particlelike behaviour above about 5 nm in size is observed only when NP are structurally defective. These conclusions are supported by Monte Carlo simulations. It is also shown that thermal decomposition is a chemical route capable of producing high-crystal quality NP with bulklike properties