Spin-polarized unoccupied quantum-well states in ultrathin Co films on Cu(100)

The unoccupied quantum-well states within ultrathin ferromagnetic Co overlayers on Cu(100) have been investigated by spin-resolved inverse photoemission. Two-dimensional discrete quantum-well states with exchange splittings ranging from 0.1 to 0.2 eV have been identified for film thicknesses up to ten atomic layers of Co. Their energy dependence as a function of the film thickness is described within the phase-accumulation model. For larger thicknesses, the $\mathrm{sp}$ band is formed and, as a consequence, the spectral emission features are reduced to one direct transition with a spin splitting of 0.4 eV, which is characterized by the three-dimensional band structure.