M. Opel, K. -W. Nielsen, S. Bauer, S. T. B. Goennenwein, J. C. Cezar, D. Schmeisser, J. Simon, W. Mader, R. Gross: Nanosized superparamagnetic precipitates in cobalt-doped ZnO. 2008European Physical Journal B: 63. ISBN 1434-6028 http://dx.doi.org/10.1140/epjb/e2008-00252-4