XMCD studies of Co and Co–Pt nanoparticles prepared by vapour deposition

Abstract Co–Pt and Co nanoparticles (∼2 nm diameter) of different composition and crystal structure have been studied by X-ray magnetic circular dichroism (XMCD) at the Co L 2,3 edges. The magnetic properties of the samples, covered with a thin protecting layer of Al 2 O 3 , depend upon the composition, order and presence of Co oxide contamination. Pure Co nanoparticles reveal an orbital-to-spin moment ratio, μ l / μ s =0.13 at room temperature (RT) and μ l / μ s =0.11 at 3.4 K, Co/Pt mixed nanoparticles show a temperature independent μ l / μ s ratio: 0.13 both at RT and at 3.4 K. In contrast, Pt-shell Co-core samples have a ratio μ l / μ s =0.14 at RT and a strongly enhanced ratio μ l / μ s =0.20 at 3.4 K. The same samples with a small CoO contamination show an enhanced μ l / μ ss =0.25 at RT and μ l / μ ss =0.28 at 3.4 K. In the reversed configuration (Pt-core and a Co-shell), where the CoO is probably not at the Co–Pt interface but at the Co surface, the enhancement of μ l / μ s caused by the simultaneous increases of the orbital moment and decreases of the spin moment reaches a record value of μ l / μ ss =0.33 at 3.4 K and μ l / μ ss =0.21 at RT.