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Doped magnetite (Sn  x  Fe 3-2/3 x  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.

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