Correlation between size, shape and magnetic anisotropy of CoFe2O4 ferrite nanoparticles

The present work reports the effect of particle size and shape of CoFe2O4 (CFO) nanoparticles on magnetic properties and their use in device applications as permanent magnets at room temperature. A set of CFO samples with different particle sizes and shapes were synthesized via the polymeric method by sintering at temperatures ranging from 300 degrees C to 1200 degrees C. These materials were characterized structurally by x-ray diffraction, morphologically by scanning electron microscopy, and microstructurally by transmission electron microscopy. The morphology of these CFO samples shows size-dependent shapes like spherical, pyramidal, lamellar, octahedral and truncated octahedral shapes for the particle sizes ranging from 7 to 780 nm with increasing sintering temperature. The emergence of magnetic properties was investigated as a function of particle size and shape with a special emphasis on permanent magnet applications at low and room temperatures. The values of saturation and remanent magnetization were found to increase monotonously with a particle size up to 40 nm and from thereafter they were found to remain almost constant. The other magnetic parameters such as coercivity, squareness ratio, anisotropy constant and maximum energy product (BHmax) were observed to increase up to 40 nm and then decreased thereafter as a function of particle size. The underlying mechanism responsible for the observed behavior of the magnetic parameters as a function of particle size was discussed in the light of coherent rotation, domain wall motion and shape induced demagnetization effects. The significant values of BHmax - the figure of merit of permanent magnets - observed for single domain particles (particularly, 14 nm and 21 nm) were found to have suitability in permanent magnetic technology.