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The compound NiCo 2 O 4 , with spinel-related structure, has been prepared by thermal decomposition of metal nitrates and its bulk structural properties examined by means of magnetic measurements, neutron diffraction, X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). The results suggest a delocalised electron distribution on the octahedral sites with average oxidation states of z3.5 and z2.5 for nickel and cobalt, respectively, and lead to a cation distribution for NiCo 2 O 4 of {Ni 3z 0.1 Co 2z 0.9 } tet [Ni 3.5z 0.9 Co 2.5z 1.1 ] oct O 4 . This electronic configuration is consistent with magnetisation measurements if applied magnetic fields cause a charge redistribution on the octahedral sites to favour Co 3z and Ni 3z . The surface of NiCo 2 O 4 was examined by X-ray photoelectron spectroscopy (XPS) and found to have a different composition containing Co 2z , Co 3z , Ni 2z , Ni 3z and, probably, Ni 4z .

Magnetite nanoparticles with two different sizes, 5 and 17 nm, have been prepared by the decomposition of organic precursors in an organic media in the presence of oleic acid. The particles were characterized by TEM, X-ray diffraction, and IR and Mo ¨ssbauer spectroscopy to clarify their structural and physicochemical properties. The samples consist of noninteracting magnetite nanoparticles in both cases with a more uniform size distribution and higher crystal order degree than particles of similar sizes prepared by coprecipitation. A controlled heat treatment of the samples in air leads to the transformation of magnetite to maghemite, which can be followed by the appearance of many IR bands forbidden for a spinel structure. In addition to that, an important reduction of saturation magnetization and coercivity at low temperature takes place whenever the oleic acid is preserved. Finally, Mo ¨ssbauer spectra at different temperatures clearly show the effect of the nature of the iron oxide phase, the particle size, particle interactions, and structural order at the surface in both magnetite and maghemite as a consequence of oleic coating.

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A considerable increase in the saturation magnetization, M s (40%), and initial susceptibility of ultrasmall (<5 nm) iron oxide nanoparticles prepared by laser pyrolysis was obtained through an optimized acid treatment. Moreover, a significant enhancement in the colloidal properties, such as smaller aggregate sizes in aqueous media and increased surface charge densities, was found after this chemical protocol. The results are consistent with a reduction in nanoparticle surface disorder induced by a dissolutionÀ recrystallization mechanism.

The oldest known magnetic material, magnetite, is of current interest for use in spintronics as a thin film. An open question is how thin can magnetite films be and still retain the robust ferrimagnetism required for many applications. We have grown 1-nm-thick magnetite crystals and characterized them in situ by electron and photoelectron microscopies including selected-area x-ray circular dichroism. Well-defined magnetic patterns are observed in individual nanocrystals up to at least 520 K, establishing the retention of ferrimagnetism in magnetite two unit cells thick.

A direct method for the preparation of uniform magnetite nanoparticles with sizes around 30 nm and stable in aqueous media at pH 7 has been developed. This method is based on the precipitation of an iron (II) salt (FeSO4) in the presence of a base (NaOH) and a mild oxidant (KNO3). Reaction rate seems to be controlled by the iron salt concentration and the presence of ethanol in the media. Thus lower iron concentration and a water/ethanol ratio equal to one lead to the formation of the smallest particles, 30 nm in diameter. Colloidal suspensions of these particles were directly obtained by simple ultrasonic treatment of the powders leading to very stable ferrofluids at pH 7. Sulphate anions present at the particle surface seem to be responsible for the colloidal stability, providing a biocompatible character to the suspensions. The structural, morphological and magnetic characterization of the nanoparticles is also described and suggests that the smallest particles have a diameter close to the limit between monodomain–multidomain magnetic structure, which could account for the high powder absorption of magnetic fields. According to this calorimetric experiments resulted in specific power absorption rates of ca 80–95 W g−1, which are among the highest values reported in the literature and make these nanoparticles very interesting for hyperthermia.

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