Kondo effect and STM spectra through ferromagnetic nanoclusters

摘要

Motivated by recent scanning tunneling microscope (STM) experiments on cobaltclusters adsorbed on single wall metallic nanotubes [Odom {\em et al.}, Science{\bf 290}, 1549 (2000)], we study theoretically the size dependence of STMspectra and spin-flip scattering of electrons from finite size ferromagneticclusters adsorbed on metallic surfaces. We study two models of nanometer sizeferromagnets: (i) An itinerant model with delocalized s, p and d electrons and(ii) a local moment model with both localized d-level spins and delocalizedcluster electrons. The effective exchange coupling between the spin of thecluster and the conduction electrons of the metallic substrate depends on thespecific details of the single particle density of states on the cluster. Thecalculated Kondo coupling is inversely proportional to the total spin of theferromagnetic cluster in both models and thus the Kondo temperature is rapidlysuppressed as the size of the cluster increases. Mesoscopic fluctuations in thecharging energies and magnetization of nanoclusters can lead to largefluctuations in the Kondo temperatures and a very asymmetric voltage dependenceof the STM spectra. We compare our results to the experiments.