Effect of size and shape dependent anisotropy on superparamagnetic property of CoFe2O4 nanoparticles and nanoplatelets

Superparamagnetic cobalt ferrite (CoFe2O4) spherical nanoparticles and rhomboidal nanoplatelets were synthesized by co-precipitation at 80 °C (S1) and hydrothermal route at 150 °C (S2). X-ray diffraction (XRD) pattern confirms formation of cubic inverse spinel structure of as prepared cobalt ferrite samples (S1 and S2) with average crystallite size of 13 nm and 18.7 nm for S1 and S2 respectively. Transmission electron microscopy (TEM) reveals spherical and rhomboidal shaped with average particle size 16.7 nm (S1) and 19.8 nm (S2). The zero field cooled magnetization MZFC vs. T exhibit a broad maxima at 400K and 510 K for S1 and S2 respectively. The blocking temperature TB is obtained as 310 K and 341 K for S1 and S2 respectively, by fitting coercive field at different temperatures to T1/2 law. The morphology of S1 and S2 corresponds to shape dependence of continuum approach. The effective demagnetization factors estimated as ΔN1=0 and ΔN2=0.749 for S1 and S2 samples respectively. The uniaxial anisotropy and shape anisotropy observed to be dominant in spherical shaped and rhomboidal shaped CoFe2O4 nanoparticles respectively. The uniaxial anisotropy constant of S1 sample is estimated as 56 (kJ/m3) at TB=310K whereas the effective anisotropy constant for S2 sample is 627 (kJ/m3) at TB=341K, in which shape anisotropy constant 605 (kJ/m3) dominates over contribution from uniaxial anisotropy constant 22 (kJ/m3) in S2 sample.