Changes in the magnetism of small supported cobalt particles during the oxidation process observed by ferromagnetic resonance

Ferromagnetic resonance (FMR) under ultrahigh vacuum conditions was used to study the influence of oxygen on the magnetic properties of small cobalt particles deposited on the reconstructed surface of a sapphire single crystal [(root 31 x root 31)R +/- 9 degrees Al2O3(0001)]. The combination of FMR-results and thermal desorption spectroscopy shows that at intermediate oxygen dosages only the outer sphere of the particles is oxidized and a magnetic core still remains. These small cores tend to exhibit isotropic superparamagnetic behaviour with decreasing size of the core, i.e. increasing oxygen dosage. At higher dosages the resonance signal vanishes due to the complete formation of cobalt oxide. Additionally, we investigated annealed particles, which possess a higher degree of crystallinity and are larger due to coalescence. The oxidation behaviour of these annealed samples is similar to that of non-annealed ensembles except that a higher dosage of oxygen is necessary to oxidize the larger particles completely. (C) 1999 Elsevier Science B.V. All rights reserved.