Magnetic properties of mechanochemically synthesized γ-Fe2O3 nanoparticles

The synthesis of mono-dispersed γ-Fe2O3 nanoparticles by mechanochemical processing was demonstrated for the first time, via the solid-state exchange reaction Fe2(SO4)3 + 3Na2CO3 → Fe2(CO3)3 + 3Na2SO4 → Fe2O3 + 3Na2SO4 + 3CO2(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 × 106 erg/cm3, two orders of magnitude larger than the magnetocrystalline anisotropy constant of bulk γ-Fe2O3. The detailed analysis of the magnetic properties indicated that the γ-Fe2O3 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.

Research highlights
► Mechanochemical synthesis of maghemite nanoparticles using a new reaction route.
► The particles had a mean diameter of ∼6 nm and a narrow size distribution.
► Magnetocrystalline anisotropy constant was 100 times larger than bulk maghemite.
► Superparamagnetic blocking temperature was 56.6 K.
► Those magnetic properties were explained by using a core–shell structure model.