Particle size, spin wave and surface effects on magnetic properties of MgFe2O4 nanoparticles

Magnesium ferrite, MgFe2O4, nanoparticles with a mean diameter varying from ~6 to ~17 nm were successfully synthesized using a simple thermal decomposition method at different annealing temperatures ranging in between 400 and 600 °C. Pure spinel ferrite nanoparticles were obtained at temperatures lower than 500 °C, while the presence of hematite (?-Fe2O3) impurities was observed at higher temperatures. Single-phase samples show a superparamagnetic behavior at 300 K, the saturation magnetization (Ms) becoming larger with the increase of particles size. The temperature dependence of Ms was explained in terms of surface spin-canting as well as spin wave excitations in the core. Using a modified Bloch law, [Ms(T)=Ms(0)(1-?T?)], we observed a size dependent behavior of the Bloch constant ? and the exponent ?, whose values increase and decrease, respectively, as the particle size reduces.