  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.
