Size-dependent superparamagnetic properties of Mn spinel ferrite nanoparticles synthesized from reverse micelles

MnFe2O4 nanoparticles in a size range of 4-14 nm have been synthesized from water-in-toluene reverse micelles by using sodium dodecylbenzenesulfonate (NaDBS) as surfactant. The blocking temperature, saturation magnetization, and coercivity of the nanoparticles are clearly size-dependent. The blocking temperature increases from 20 to 250 K when the mean size of the nanoparticles increases from 4.4 to 13.5 nm. The coercivity at 20 K increases from 30 to 300 Oe with increasing nanoparticle size. The field-dependent magnetization hysteresis disappears above the blocking temperature. Due to high saturation field, the saturation magnetization of the nanoparticles has been obtained by the extrapolation of the magnetization vs 1/H plot to 1/H = 0. The saturation magnetization decreases with decreasing nanoparticle size. The high saturation held and the size-dependent saturation magnetization suggest the existence of a magnetically inactive layer on MnFe2O4 nanoparticles. The linear fitting of the saturation magnetization vs 1/d plot gives the thickness of this inactive layer as 0.45 nm.