Large orbital magnetic moments of small, free cobalt cluster ions Co${_n^+}$ with n ${ \leqslant 9}$

The size dependent electronic structure and separate spin and orbital magnetic moments of free eo;t (n = 4 9) cluster ions have been invest igated by x-ray ab orption and x-ray magnetic circular dichroism spectroscopy in a cryogenic ion trap. A very large orbital magnetic moment of 1.4 ± 0.1 /.LB per atom was determined for Cot , which is one order of magnitude larger than in the bulk metal. Large orbital magnetic moments per atom of~ 1 /.LB were also determined for CoJ , Cot , and Cot. The orbital contribution to the total magnetic moment shows a non-monotonic cluster sizo dependence: T he orbital contribution increases from a local minimum at n = 2 to a local maximum at n = 5 and then decreases with increasing cluster size. The 3d spin magnetic moment per atom is nearly constant and is solely defined by the number of 3d holes which shows that t he 3d majority ~pin states arc fully occupied , that is, 3d hole spin polarization is 100%. K eywords: orbital magnetic moment, XMCD, cobalt clusters, spin magnetic moment, magnetic anisotropy energy, spin polarization