UNOCCUPIED ELECTRONIC STATES OF ULTRATHIN FILMS OF CO/CU(111) AND FE/CU(111)

Angle-resolved inverse photoemission has been used to study changes in the unoccupied electronic states for ultrathin films of Co and Fe epitaxially deposited on Cu(111). Combining inverse photoemission and photoemission measurements of the occupied valence states reveals changes in the exchange splitting of the 3d band. The exchange splitting in a 15-angstrom-thick Co film is DELTAE(ex) = 1.05+/-0.1 eV, a value close to that measured for bulk Co(0001), DELTAE(ex) = 1.1+/-0.1 eV. Similar electronic structure is observed in thinner epitaxial Co films, 15 angstrom down to 2 angstrom thick, despite the fact that films cease to be continuous below 4 angstrom (i.e., 2 layers). The inverse photoemission spectra for the Fe/Cu(111) films show a more complicated evolution with increasing film thickness. Above 10 angstrom, the Fe films undergo a structural transition to the bcc(110) phase showing a 0.5-eV shift higher in energy of the uppermost minority-spin 3d band, indicative of a transition to the higher-spin-state bcc phase with an exchange splitting of 1.9+/-0.2 eV. The lower energy of the uppermost minority-spin 3d band for the thinner films possibly indicates reduced exchange splitting in the low-spin phase of fcc Fe(111).