In this work we carefully analyze the role of the microstructure on the magnetic properties of Co-doped ZnO nanoparticles prepared by the vaporization-condensation method in a solar reactor. We show that a close correlation exists between microstructural features and the appearance of ferromagnetism. Both shape and size of the particles, as well as the microstructure, can be controlled by changing the pressure inside the evaporation chamber, as evidenced by transmission electron microscopy micrographs and high resolution electron microscopy ͑HREM͒. X-ray diffraction patterns and HREM make evident the absence of any significant Co segregation or any other phase different from würtzite type ZnO. On the other hand, electron energy loss spectroscopy analyses performed on several particles of würtzite type ZnO yielded an average Co concentration in good agreement with the nominal composition. Samples prepared in low pressure ͑Ϸ10 Torr͒ exhibit a very homogeneous microstructure and are ferromagnetic at low temperature but they have a very small saturation moment, well below that expected for a Co 2+ ion. Conversely, samples prepared at higher pressure conditions ͑Ϸ70-100 Torr͒ show a defective microstructure and are paramagnetic and increasing the Co content does not induce ferromagnetism.

Secondary ion mass spectrometry was used to profile the diffusion of oxygen in polycrystalline β-cristobalite and vitreous SiO2. The tracer concentration profiles of cristobalite are consistent with a model based on two mechanisms: bulk and short-circuit diffusion. The profiles of partially crystallized samples containing vitreous SiO2 and β-cristobalite were fitted using the sum of two complementary error functions and taking account of some interstitial-network exchange. The bulk oxygen diffusivity, in the temperature range 1240–1500 °C, is about five times greater for vitreous silica than for β-cristobalite.

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