Temperature dependent magnetic properties of superparamagnetic CoFe2O4 nanoparticles

Cobalt ferrite (CoFe2O4) nanoparticles were prepared by chemical co-precipitation route at low temperature (70 degrees C). X-ray diffraction pattern confirms the cubic inverse spinel structure with an average crystallite size of similar to 14 nm. Transmission electron microscopy exhibits a narrow distribution of particle size with an average particle size of 15 nm. The synthesized nanoparticles exhibited a saturation magnetization of 23.2 emu/g and 33.6 emu/g at 300 K and 20 K respectively. The randomly oriented spin layer thickness of 1.74 nm calculated at room temperature reduces the value of magnetization in as-synthesized nanoparticles. Exponential decay was observed in coercivity (H-c) and remanence magnetization (M-r) with the increase in temperature from 20 K to 300 K. Blocking temperature of 250 K is found from d(ZFC - FC)/dt versus T curve. The plot of H-HT versus T revealed the superparamagnetic behavior and exhibited largest superparamagnetic moment at 300 K. The effective anisotropy constant and surface anisotropy constant were calculated to be 6.00 x 10(4) J/m(3) and 1.08 x 10(-4) J/m(2) respectively at blocking temperature.