Structural and magnetic properties of Fe oxide nanoparticles prepared by laser pyrolysis and annealed in high pressure hydrogen atmosphere were investigated. The annealing treatments were performed at 200 °C (sample A200C) and 300 °C (sample A300C). The as prepared sample, A, consists of nanoparticles with ~ 4 nm mean particle size and contains C (~ 11 at.%), Fe and O. The Fe/O ratio is between γ-Fe2O3 and Fe3O4 stoichiometric ratios. A change in the oxidation state, crystallinity and particle size is evidenced for the nanoparticles in sample A200C. The Fe oxide nanoparticles are completely reduced in sample A300C to α-Fe single phase. The blocking temperature increases from 106 K in A to 110 K in A200C and above room temperature in A300C, where strong inter-particle interactions are evidenced. Magnetic parameters, of interest for applications, have been considerably varied by the specific hydrogenation treatments, in direct connection to the induced specific changes of particle size, crystallinity and phase composition. For the A and A200C samples, a field cooling dependent unidirectional anisotropy was observed especially at low temperatures, supporting the presence of nanoparticles with core–shell-like structures. Surprisingly high MS values, almost 50% higher than for bulk metallic Fe, were evidenced in sample A300C.

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The laser pyrolysis technique was employed in the production of magnetic iron oxide nanometric powders due to its capability of producing highly homogeneous nanoparticles in continuous form. This technique consists of the laser-driven rapid heating of an iron precursor in vapor phase in presence of oxygen. Different samples were prepared by changing the experimental conditions of synthesis. We found that high crystallinities and good magnetic properties are attained at high density of the laser power and strong oxidation. By the contrary, softer conditions using low laser densities and soft oxidation conditions give in general smaller and poorly ordered nanoparticles. The particles obtained were in the range of 2 to 9 nm in diameter (TEM). All of them were superparamagnetic at room temperature with saturation magnetization values in the interval of 4-38 emu/g-sample. The samples consist in Fe 2 O 3 maghemite with carbon as the main impurity present on the surface in the form of C=O bonds.

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