XMCD studies of Co_x Pt_100–x nanoparticles prepared by vapour deposition and chemical synthesis

Abstract An almost linear relation between the Pt amount and the orbital to spin moment ratio (μ_l/μ_s) has been found in Co_x Pt_100–x nanoparticles prepared by vapour deposition and possessing a well defined chemically disordered crystallographic structure. X‐ray magnetic circular dichroism (XMCD) measurements done at the L_2,3Co edges show that a 4.0 nm, x = 100 Co reference sample has a ratio μ_l/μ_s = 0.097, comparable to values found for Co thin layers. An increase of the amount of Pt increases the μ_l/μ_s value: μ_l/μ_s = 0.11 for x = 78, μ_l/μ_s = 0.12 when x = 74 and finally, for x = 38 it becomes μ_l/μ_s = 0.140. Nanoparticles prepared by chemical synthesis with size proportional to changes of the stoichiometric ratio, from Co₂₅Pt₇₅ to Co₅₀Pt₅₀, show values of the orbital to spin moment ratio compatible with the trend found for the former ones. The samples prepared by chemical synthesis in a diameter range from 4 nm to 8 nm are characterized by a sharp size distribution of 10%. In contrast to the in situ prepared nanoparticles that are oxide free, however, they show a varying degree of surface oxidation depending on the ambient conditions they are exposed to. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)