MFe2O4 (M: Co2+, Ni2+) Nanoparticles: Mossbauer and X-ray Absorption Spectroscopies Studies and High-Temperature Superparamagnetic Behavior

MFe2O4 (M: Co2+, Ni2+) nanoparticles with different sizes and crystal-chemistry have been synthesized under solvothermal conditions. The inversion degree (x) of the mixed spinel structure (Fe1-xMx)[Fe1+xM1-x]O-4 has been investigated by XAS and Mossbauer spectroscopies obtaining values between 0.20 and 0.30 in the case of Co-ferrite samples and between 0.00 and 0.16 for the Ni-ferrite ones. In order to have a better understanding of the superparamagnetic behavior of the nanosized samples, it was necessary to do high. temperature magnetic measurements. Although Co-ferrite samples present higher magnetization values, their effective superparamagnetic moment is similar to those found for the Ni-ferrite ones. This suggests that the dipolar interactions may be stronger in the Ni-ferrite system, probably due to a less effective anisotropy that can be considered an impediment to the particle interactions. This fact is supported by the effective volume/single-particle volume ratio that presents similar values for both Co-ferrite and Ni-ferrite.