Magnetic properties of mechanochemically synthesized γ-Fe2O3 nanoparticles

The synthesis of mono-dispersed γ-Fe 2 O 3 nanoparticles by mechanochemical processing was demonstrated for the first time, via the solid-state exchange reaction Fe 2 (SO 4 ) 3  + 3Na 2 CO 3  → Fe 2 (CO 3 ) 3  + 3Na 2 SO 4  → Fe 2 O 3  + 3Na 2 SO 4  + 3CO 2 (g) and subsequent heat treatment at 673 K. The nanoparticles had a volume-weighted mean diameter of 6 nm and a narrow size distribution with the standard deviation of 3 nm. The particles showed a superparamagnetic nature with the superparamagnetic blocking temperature of 56.6 K. The anisotropy constant was 6.0 × 10 6  erg/cm 3 , two orders of magnitude larger than the magnetocrystalline anisotropy constant of bulk γ-Fe 2 O 3 . The detailed analysis of the magnetic properties indicated that the γ-Fe 2 O 3 nanoparticles had a core–shell structure, consisting of a ferrimagnetic core of ∼4 nm in diameter having a collinear spin configuration and a magnetically disordered shell of ∼1.2 nm in thickness.