The size-dependent magnetic properties of Co3O4 nanoparticles

Co3O4 nanoparticles were prepared by mixing cobalt chloride (CoCl2 center dot 6H(2)O) aqueous solutions with sodium metasilicate nonahydrate (Na2SiO3 center dot 9H(2)O) 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 T-b were observed to be between 4 and 28 K, while the Neel temperature Of Co3O4 bulk crystal is known to be T-N = 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 M-s increased dramatically for particle sizes below about 10 nm. The calculated effective magnetic moment mu(eff) for a Co3O4 particle of 3 rim 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. (c) 2005 Elsevier Ltd. All rights reserved.