Sn(2+) Doping: A Strategy for Tuning of Fe(3)O(4) Nanoparticles Magnetization Dipping Temperature/Amplitude, Irreversibility, and Curie Point

Doped magnetite (Sn(x)Fe(3-2/3x)O(4)) nanoparticles (NPs) (12–50 nm) with different amount of Sn(2+) ions (x) were synthesized using co-precipitation method. Sn(2+) doping reduces the anticipated oxidation of Fe(3)O(4) NPs to maghemite (γ-Fe(2)O(3)), 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-Fe(2)O(3) at the NPs surfaces. Along with an increase in the dipping temperature, we found that doping with Sn(2+) 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(0)[1 − γ(T/T(C))](β)) and a modified Curie–Weiss law (M = − α[1/(T − T(C))(δ)]) is developed in order to explain the observed M-T behavior at different applied external magnetic fields and for different Sn(2+) 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.