Magnetite nanoparticles with almost bulk magnetic properties: the role of the surfactant

Uniform magnetite nanoparticles of 6, 10 and 17 nm were synthesised by thermal decomposition of an iron precursor. Oleic acid was used as surfactant. Saturation magnetization M$_{s}$ reaches the expected value for bulk magnetite at low temperature, in contrast to results in small particle systems for which M$_{s}$ is usually much smaller due to surface spin disorder. The coercive field for the 6 nm particles is also in agreement with 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, such that the new O$^{2-}$ surface ligands partially reconstruct the crystal field of the surface Fe cations, as suggested by XPS. This may be of relevance in biomedical applications to reduce the strength of the magnetic field required to obtain a high M$_{s}$ and opens the question of whether M$_{s}$ above the bulk value may be obtained by taking advantage of the orbital contribution. Work funded by Spanish NAN2004-08805-CO4-02 and NAN2004-08805-CO4-01, and CONSOLIDER CSD2006-12