Unexpected magnetic properties explained by the homogeneity of mixed ferrites

Abstract In this work, we report on magnetic nanoparticles of Co x Fe 3−X O 4 (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 (∼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  3 O 4 . 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.