The effect of particle size on the magnetic and optical properties of cobalt oxide Co_3O_4 has been investigated. Single phase Co_3O_4 nanoparticles were synthesized by co-precipitation route and their structure was confirmed through X-ray diffraction analysis. The average crystallite sizes of the nanoparticles as determined from the X-ray diffraction were found to lie in the range of 13 to 59 nm. At room temperature these particles show paramagnetic response and the DC magnetic moment of the samples showed an increasing trend with increasing particle size. Both the antiferromagnetic Néel temperature T_{N} and the low temperature moment depict finite size effects in the AC susceptibility measurements. The optical absorption studies show that there are two direct band gaps in Co_3O_4, the main energy band gap and a sub-band located inside the main energy gap. The values of these energy band gaps for the 45 nm particles were determined to be 3.15 and 1.76 eV, respectively. The direct band gap in these particles clearly shows a blue shift with decreasing size attributable to the quantum confinement effect.
Hematite (α-Fe_{2}O_{3}) nanoparticles and hematite nanoparticles coated with polyvinylpyrrolidone (PVP) are synthesized chemically by co-precipitation. Prolysis and microemulsion methods respectively. An average size of nanoparticles (both coated and uncoated) was found by the broadening of the X-ray diffraction peaks using Scherrer's formula. It was found that coating reduced particle sizes. The attachment of the polymer on the surface of particles was confirmed by the Fourier transform infrared spectroscopy and thermogravimetric analysis. Optical ultraviolet/visible reflectance test also indicated the presence of PVP in coated nanohematites. In magnetic studies, it was observed that coating does not change main magnetic character of the α-Fe_{2}O_{3} nanoparticles however it reduces DC magnetization (as a function of applied field and temperature) to a considerable amount. Moreover it was observed that after coating Morin transition temperature T_{M} and its width ΔT_{M} shifts to lower values, which is another indication of size reduction.
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