Research Paper

Journal of Nanoparticle Research

, Volume 11, Issue 8, pp 2105-2111

Co–CoO nanoparticles prepared by reactive gas-phase aggregation

  • J. A. GonzálezAffiliated withDepartamento de Física Aplicada, Universidad de Castilla-La Mancha
  • , J. P. AndrésAffiliated withDepartamento de Física Aplicada, Universidad de Castilla-La Mancha
  • , J. A. De ToroAffiliated withDepartamento de Física Aplicada, Universidad de Castilla-La Mancha Email author 
  • , P. MuñizAffiliated withDepartamento de Física Aplicada, Universidad de Castilla-La Mancha
  • , T. MuñozAffiliated withDepartamento de Física Aplicada, Universidad de Castilla-La Mancha
  • , O. CrisanAffiliated withDepartment of Physics and Astronomy, University of Leicester
  • , C. BinnsAffiliated withDepartment of Physics and Astronomy, University of Leicester
  • , J. M. RiveiroAffiliated withDepartamento de Física Aplicada, Universidad de Castilla-La Mancha

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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 aggregation 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.

Keywords

Co nanoparticles Gas-phase aggregation Exchange-bias Core-shell particles Aerosols Nanocomposites