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 blockin
g temperature increases from 20 to 250 K when the mean size of the nanopart
icles 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 magneti
zation hysteresis disappears above the blocking temperature. Due to high sa
turation 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. T
he high saturation held and the size-dependent saturation magnetization sug
gest the existence of a magnetically inactive layer on MnFe2O4 nanoparticle
s. The linear fitting of the saturation magnetization vs 1/d plot gives the
thickness of this inactive layer as 0.45 nm.