Exchange-split electronic states of ultrathin Co layers on Cu(111)

By spin- and angle-resolved photoemission we have studied the electronic structure of ultrathin Co overlayers on Cu(111). For coverages below 2 ML, we observe electronic states that are different from those of bulk Co but we find no in-plane ferromagnetic order. Above 2 ML the spectra show exchange-split and spin-polarized states, whose binding energy depends very little on the overlayer thickness. A previously unresolved separation of 0.3 eV between two occupied bands of opposite spin character could be observed. The experimentally determined exchange splitting for the two sets of d bands near {Γ} is thus found to be 1.4±{}0.15 and 1.15±{}0.15 eV, respectively. These values are larger than those derived from previous spin-integrated measurements and in closer agreement with band-structure calculations.