Microstructure and magnetic properties of Co/Al2O3 nanocomposite powders

Nanocomposite powders of magnetic cobalt nanoparticles dispersed by nonmagnetic Al2O3 particles have been prepared by planetary ball milling. Ball milling of the CoO and Al mixture powder after a certain milling duration reduces CoO to (fcc and hcp) Co completely and oxidizes Al to α-Al2O3 simultaneously. The average grain sizes of the nanocomposite powders are 19 nm for Co and 28 nm for α-Al2O3 after the completion of the reduction reaction. By direct ball milling of the mixture of Co and Al2O3, the allotropic phase transformation of Co was observed and the average grain size of Co is reduced to 5 nm. For both the samples of the mechanochemical series and the direct milling series, the saturation magnetizations of the nanocomposite powders decrease with decreasing average grain size of Co. This may be due to the enhancement of the interface effects and the increase of the superparamagnetic particles with decreasing Co grain size. The coercivities of the Co/Al2O3 nanocomposite powders increase up to 380 Oe. The increasing grain boundaries with decreasing Co grain size result in the domain wall pinning which predicts the coercivity enhancement. In addition to the grain size effects, the reduction of the particle size toward the size region of single domain also contributes to the increase of coercivity.