Surfactant effects in magnetite nanoparticles of controlled size

Magnetite Fe3O4 nanoparticles of controlled size within 6 and 20nm in diameter were synthesised by thermal decomposition of an iron organic precursor in an organic medium. Particles were coated with oleic acid. For all samples studied, saturation magnetisation M-s is size-independent, and reaches a value close to that expected 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 in agreement with coherent rotation, taking the bulk magnetocrystalline anisotropy into account. 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. (c) 2007 Elsevier B.V. All rights reserved.