Magnetism of Co-doped ZnO epitaxially grown on a ZnO substrate by Li, L, Guo, Y, Cui, X, Zheng, R, Ohtani, K, Kong, C, Ceguerra, A, Moody, M, Ye, J, Tan, H, Jagadish, C, Liu, H, Stampfl, C, Ohno, H, Ringer, S, Matsukura, F