Surfactant effects in monodisperse magnetite nanoparticles of controlled size

TL;DR

This study synthesizes monodisperse magnetite nanoparticles with controlled sizes, coated with oleic acid, revealing surface effects that influence magnetic properties and have implications for biomedical uses.

Contribution

It demonstrates that oleic acid coating reduces surface spin disorder, resulting in magnetic properties similar to bulk magnetite in nanoparticles of 6-20 nm.

Findings

Saturation magnetization reaches bulk values

Oleic acid reduces surface spin disorder

High-field differential susceptibility is significantly larger

Abstract

Monodisperse magnetite Fe3O4 nanoparticles of controlled size within 6 and 20 nm in diameter were synthesized by thermal decomposition of an iron organic precursor in an organic medium. Particles were coated with oleic acid. For all samples studied, saturation magnetization Ms reaches the expected value for bulk magnetite, in contrast to results in small particle systems for which Ms is usually much smaller due to surface spin disorder. The coercive field for the 6 nm particles is also similar to that of bulk magnetite. Both results suggest that the oleic acid molecules covalently bonded to the nanoparticle surface yield a strong reduction in the surface spin disorder. However, although the saturated state may be similar, the approach to saturation is different and, in particular, the high-field differential susceptibility is one order of magnitude larger than in bulk materials. The relevance of these results in biomedical applications is discussed.