Co–CoO nanoparticles prepared by reactive gas-phase aggregation

Abstract

The technique of gas-phase aggregation has been used to prepare partially oxidized Co nanoparticles films by allowing a controlled flow of oxygen gas into the aggregation zone. This method differs from those previously reported, that is, the passivation of a beam of preformed particles in a secondary chamber and the conventional (low Ar pressure) reactive sputtering of Co to produce Co-CoO composite films. Transmission electron microscopy shows that the mean size of the particles is about 6 nm. For sufficiently high oxygen pressures, the nanoparticles films become super-paramagnetic at room temperature. X-ray diffraction patterns display reflections corresponding to fcc Co and fcc CoO phases, with an increasing dominance of the latter upon increasing the oxygen pressure in the aggrega- tion zone, which is consistent with the observed reduction in saturation magnetization. The cluster films assembled with particles grown under oxygen in the condensation zone exhibit exchange-bias fields (about 8 kOe at 20 K) systematically higher than those measured for Co-CoO core-shell nanoparticles prepared by oxidizing preformed particles in the deposition chamber, which we attribute, in the light of results from annealing experiments, to a higher ferromagnetic-antiferromagnetic (Co-CoO) interface density.