ELECTRONIC AND MAGNETIC PROPERTIES OF IMPURITIES AND CLUSTERS ON METAL SURFACES

VERONESE, MARCO

ELECTRONIC AND MAGNETIC PROPERTIES OF IMPURITIES AND CLUSTERS ON METAL SURFACES

VERONESE, MARCO

2004-04-06

http://thesis2.sba.units.it/store/handle/item/12556

http://hdl.handle.net/10077/11585

Doctoral Thesis

Contributor(s)

SENATORE, GAETANO

Abstract

The electronic structure and the magnetism of transition metal single atoms and magnetic impurities on metal surfaces have been studied by means of x-ray absorption dichroic techniques (XMCD, and magnetization measurements) in high magnetic fields and at low temperatures. The unique capabilities of XMCD to probe the both spin and orbital magnetic moments with element selectivity have been applied to diluted systems with low surface concentration (1012 atoms cm-2 ) down to the limit of isolated atoms. This kind of measurements have been applied to surface impurity systems only in the last few year thanks to the availability of high flux and brilliance synchrotron radiation sources. At the present these measurements represent the state of the art in the study of the magnetism of surface impurities. Results are reported for three different classes of systems: Mn and V impurities on the surface of alkali metals (Cs, K, N a); M n, V, Fe, Co single atoms on the Cu(1 00) surface an d Co single atoms on the Pt(111) surface. Furthermore results have been presented, regarding the magnetic moment distribution of a Mn monolayer coupled to the Fe(1 00) surface. Manganese on potassium has been shown to possess an atomic like magnetic d5 electronic configuration. This electronic configuration is particularly stable for small hybridization changes induced by alkali metals of increased electronic density. lf the hybridization is further increased as on Al(100) the spectroscopic features related to atomic multiplets are suppressed together with magnetism. Vanadium impurities on alkali metal surfaces also show atomic multiplet features. The electronic configuration is not an atomic-like configuration as d3 or d4 . The orbital moment is found to be small, less than 0.5 !-ls with respect to the purely atomic configurations and to be antiparallel to the spin. lt decreases along the alkali metals column (group 1A) of the periodic table, as the electronic density is increased, i.e. going from Cs to Li. This effect is related to an increased hybridization with the substrate due to the larger electronic density. The measured magnetic moments are of the order of 3)-ls, and cannot be explained with simple atomic parameters. Two possible mechanisms have been proposed to explain the partial quenching of the orbital moment, the effect of a weak crystal field and the effect of hybridization through the larger width of Friedel resonant bound state for the early elements of 3d transition metal series. Fe single atoms on Cu(100) surface have a large ot1f of plane magnetic anisotropy of 1.5 meV/atom and enhanced orbital magnetic moment with the respect to the bulk. The Cu substrate does not contribute considerably to the anisotropy as in the case of Co o n Pt. At the apposite Co single atoms on Cu(1 00) surface do not show any dichroism an d hence magnetism. This result ca n be explained on the basis of the Kondo effect, since Co single atoms have been found to be a Kondo system with T K=88 K. A comparison of the experimental data for V, Mn, Fe, Co impurities on Cu(100) with the calculated magnetic behavior of adatoms along the 3d series, gives an overall reasonable agreement, with important deviations for V and Co impurities. Co single atoms on the Pt(111) surface have an extraordinary large out of plane magnetic anisotropy of about 9.2 meV/atom due to the interplay between an enhanced orbital moment, consequence reduced coordination of the single atom, at the surface, and the effect of Pt hybridization, consequence of a d-d mixing between Co and Pt orbitals. This result is of particular relevance since this magnetic anisotropy is the highest measured, by now, for any system.