Magneto-optical investigation on the multiphase and stability of Fe oxide nanoparticles

Spinel ferrites magnetic nanoparticles have promising potential in different fields of application as magnetic recording, spintronics, sensoristics and biomedicine. Unlike their bulk counterparts, where the composition can be considered mainly as homogeneous, in nanosized magnetic ferrites the increased surface/volume ratio can give rise to a faster oxidation process, leading to a not pure chemical structure. Traditional magnetic measurements do not allow for discriminating between the different magnetic phases, in particular between the magnetite and the maghemite. Their magnetic properties have been analyzed by several studies but the possible oxidation of Fe2+ ions of the metastable magnetite phase to Fe3+ of maghemite can be relevant for the physical properties of the nanosystem. Magnetic circular dichroism (MCD) spectroscopy, being sensitive to specific electronic transitions[1], represents a simple and valid alternative to other expensive investigation techniques for the detection of different cationic species. In this study spherical iron oxide nanoparticles with increasing size between 5 to 10 nnm are investigated by MCD in the nUVvis- nIR range [2]. Moreover the time evolution of different nanoparticles is monitored to investigate the progressive oxidation. The measurements show the presence of two definite spectral structures, whose intensity evolved in time, likely originated by different magnetic contributions. The analysis of the MO hysteresis loops allows the deconvolution of the two phases. The results support the hypothesis of two distinct magnetic phases, related to the presence magnetite and maghemite, which transformation is size dependent. This work has been financed by the EC through EU-FP7 NANOPYME Project (No. 310516) and by Fondazione Cariplo through Project No. 2010-0612.