Magneto-optical spectroscopy of (Zn,Co)O epilayers

We present a magneto-optical study of (Zn,Co)O layers grown by molecular beam epitaxy. We observed sharp lines related to (4)A(2)-E-2 intra-ionic Co2+ transitions, and to the A, B and C excitons. Intra-ionic transitions observed by absorption and photoluminescence were used to determine the values of the parameters describing the isolated Co ions, such as the easy-plane magnetic anisotropy and the g-factor of the S = 3/2 Cobalt spin. Excitonic transitions observed in reflectivity were used to determine the giant Zeeman splitting and to estimate the effective coupling < N-0(alpha-beta)>(A,B) = 0.4 eV between excitons and Cobalt spins. Due to the electron-hole exchange within the exciton, this effective spin-exciton coupling is much weaker than the exchange integrals for free carriers, estimated to be N-0 vertical bar alpha-beta vertical bar approximate to 0.8 eV, with a positive value of (alpha-beta) if the normal ordering of the valence band of ZnO is assumed. The Zeeman splitting of diluted samples and the magnetic circular dichroism (for a higher Co content) are proportional to the magnetization of the paramagnetic, isolated Cobalt ions. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.