Sn Doping: A Strategy for Tuning of FeO Nanoparticles Magnetization Dipping Temperature/Amplitude, Irreversibility, and Curie Point.

Doped magnetite (SnFeO) nanoparticles (NPs) (12-50 nm) with different amount of Sn ions (x) were synthesized using co-precipitation method. Sn doping reduces the anticipated oxidation of FeO NPs to maghemite (γ-FeO), making them attractive in several magnetic applications. Detailed characterizations during heating-cooling cycles revealed the possibility of tuning the unusual observed magnetization dipping temperature/amplitude, irreversibility, and Curie point of these NPs. We attribute this dip to the chemical reduction of γ-FeO at the NPs surfaces. Along with an increase in the dipping temperature, we found that doping with Sn reduces the dipping amplitude, until it approximately disappears when x = 0.150. Based on the core-shell structure of these NPs, a phenomenological expression that combines both modified Bloch law (M = M[1 - γ(T/T)]) and a modified Curie-Weiss law (M = - α[1/(T - T)]) is developed in order to explain the observed M-T behavior at different applied external magnetic fields and for different Sn concentrations. By applying high enough magnetic field, the value of the parameters γ and δ ≈ 1 which are the same in modified Bloch and Curie-Weiss laws. They do not change with the magnetic field and depend only on the material structure and size. The power β for high magnetic field was 2.6 which is as expected for this size of nanoparticles with the core dominated magnetization. However, the β value fluctuates between 3 and 10 for small magnetic fields indicating an extra magnetic contribution from the shell structure presented by Curie-Weiss term. The parameter (α) has a very small value and it turns to negative values for high magnetic fields.