Abstract
Abstract Co3O4 nanoparticles were prepared by mixing cobalt chloride (CoCl2 · 6H2O) aqueous solutions with sodium metasilicate nonahydrate (Na2SiO3 · 9H2O) aqueous solutions. The diameters of these particles were estimated from the X-ray diffraction patterns as ranging from 3 to 30 nm. The particle sizes were controlled through the annealing conditions and by the Si concentration of the solutions. Magnetization measurements were performed on the various sizes of Co3O4 nanoparticles. The blocking temperatures Tb were observed to be between 4 and 28 K, while the Neel temperature of Co3O4 bulk crystal is known to be TN = 33 K. At the blocking temperature, the superparamagnetic spins in the particle are supposed to be blocked against the thermal fluctuation energy. The saturation magnetization Ms increased dramatically for particle sizes below about 10 nm. The calculated effective magnetic moment μeff for a Co3O4 particle of 3 nm was twice as large as that of bulk crystal. These magnetic anomalies, which are dependent on particle size, could be due to the surface spins of the extremely small Co3O4 particles.
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