Unexpected magnetic properties explained by the homogeneity of mixed ferrites
Résumé
In this work, we report on magnetic nanoparticles of CoxFe3-XO4 (x <= 1, step 0.2) ferrites synthesized by a solvothermal chemical route based on theoretical study on Fe-Co-O alloys. Both, X-Ray Diffraction (XRD) and Selected Area Electron Diffraction (SAED) indicate the presence of both the hematite and the magnetite in the sample before any substitution (x = 0). The substitution of iron by cobalt (x > 0) gives spinel structure of space group Fd-3m for all samples. Magnetization curves, recorded by a Vibrating Sample Magnetometer (VSM), are the sign of two exchange-coupled magnetic phases (x > 0). This was in disagreement with XRD and SAED analyses which showed only one structural phase. Large (similar to 250 nm and less) rectangular and small spheroidal nanoparticles have been found by Transmission Electron Microscopy (TEM). The increase of cobalt content favors the formation of monodisperse nanoparticles. Following a subsequent heat treatment, a fraction of the as prepared samples (depending on x) has been transformed into hematite phase with a disappearance of the exchange-coupling behavior. In situ Micro-Raman analyses were necessary to investigate on the growing of the nanoparticles and the ferrite-hematite transformation. We have also deduced, that the synthesized phase with the cobalt ferrite one (till x < 1) was the magnetite Fe3O4. Then, we have pointed out the effect of performing local spectroscopy and the effect of having mixed ferrites on the structural and the physical properties.