Epitaxial growth and properties of cobalt-doped ZnO on α-Al2O3 single-crystal substrates -: art. no. 054424

Co-doped ZnO(CoxZn1-xO) is of potential interest for spintronics due to the prediction of room-temperature ferromagnetism. We have grown epitaxial CoxZn1-xO films on Al2O3(012) substrates by metalorganic chemical vapor deposition using a liquid precursor delivery system. High concentrations of Co(xless than or equal to0.35) can be uniformly incorporated into the film without phase segregation. Co is found to be in the +2 oxidation state, independent of x, by both surface-sensitive core-level x-ray photoemission and bulk-sensitive optical absorption spectroscopies. This material can be grown n-type by the deliberate incorporation of oxygen vacancies, but not by inclusion of similar to1 at. % Al. Semiconducting films remain ferromagnetic up to 350 K. In contrast films without oxygen vacancies are insulating and nonmagnetic, suggesting that exchange interaction is mediated by itinerant carriers. The saturation and remanent magnetization on a per Co basis was very small (<0.1 mu(B)/Co), even in the best films. The dependence of saturation magnetization, as measured by optical magnetic circular dichroism, on magnetic field and temperature, agrees with the theoretical Brillouin function, demonstrating that the majority of the Co(II) ions behave as magnetically isolated S=3/2 ions.