Highly enhanced orbital magnetism on cobalt cluster surfaces

Abstract

The ratio of the orbital to the spin magnetic moment has been measured for 2000-atom cobalt clusters in a copper matrix using magnetic x-ray dichroism. Averaging the result over all magnetic fields gives a value of 0.19(5) which is 2.2(6) times larger than the bulk value for face-centred-cubic cobalt. The orbital-to-spin ratio, at the cluster surface, is calculated from the measured high-field value of 0.16(2), using a simple surface model. The resulting value is almost twice that found for a plane surface and various possible reasons for this are given including changes in the nature of the cobalt-copper interface and an increase in the local density of states at the cluster surface (interface). The variation of the orbital-to-spin magnetization with the applied field provides the first evidence of changes in the surface magnetization of clusters in an external field.