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

Magnesium ferrite, MgFe2O4, nanoparticles with a mean diameter varying from similar to 6 to similar to 17 nm were successfully synthesized using a simple thermal decomposition method at different annealing temperatures ranging in between 400 and 600 degrees C. Pure spinel ferrite nanoparticles were obtained at temperatures lower than 500 degrees C, while the presence of hematite (alpha-Fe2O3) impurities was observed at higher temperatures. Single-phase samples show a superparamagnetic behavior at 300 K, the saturation magnetization (M-s) becoming larger with the increase of particles size. The temperature dependence of M-s was explained in terms of surface spin-canting as well as spin wave excitations in the core. Using a modified Bloch law, [M-s(T)=M-s(0)(1-beta T)(alpha)], we observed a size dependent behavior of the Bloch constant beta and the exponent alpha, whose values increase and decrease, respectively, as the particle size reduces. (C) 2016 Elsevier B.V. All rights reserved.