Sn²⁺ Doping: A Strategy for Tuning of Fe₃O₄ Nanoparticles Magnetization Dipping Temperature/Amplitude, Irreversibility, and Curie Point

Doped magnetite (Sn_xFe_3-2/3xO₄) nanoparticles (NPs) (12–50 nm) with different amount of Sn²⁺ ions (x) were synthesized using co-precipitation method. Sn²⁺ doping reduces the anticipated oxidation of Fe₃O₄ NPs to maghemite (γ-Fe₂O₃), 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-Fe₂O₃ at the NPs surfaces. Along with an increase in the dipping temperature, we found that doping with Sn²⁺ 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 = M₀[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 Sn²⁺ 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.