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 – 2 E intra‐ionic Co 2+ 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 ( α – β )〉 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 | α – β | ≈ 0.8 eV, with a positive value of ( α – β ) 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. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)