Transition from granular to dilute magnetic semiconducting multilayers in ion-beam-deposited ZnO/Co

Multilayers of Co/ZnO, with varying nominal thickness of metal (2–10 A) and semiconductor (2–20 A), were prepared by ion-beam sputtering. Magnetic, transport, and magnetotransport measurements were carried out over a temperature range of 2.5 to 300 K. Upon decreasing Co thickness and increasing ZnO thickness in the multilayer stack, the properties of the samples undergo a crossover from those of granular metallic Co/semiconductor multilayers to a dilute magnetic semiconductor superlattice. We interpret ferromagnetism in the latter case as due to ordering in the Co-rich layers, mediated by carriers from lightly doped, high carrier concentration layers.