Giant magnetic moment in an anomalous ferromagnetic insulator: Co-doped $\mathrmZnO$

Ferromagnetic insulators that exhibit strong ferromagnetism at the atomic level are believed to be suitable for magnetic dielectric barriers in spintronic devices and solid-state qubits in quantum computing. Here a giant magnetic moment of 6.1μB∕Co and a high Curie temperature TC of 790 K are observed in (4 at. %) Co-doped ZnO films, which is not carrier mediated, but co-exists with the dielectric state. Direct current reactive magnetron co-sputtering is used to grow Zn0.96Co0.04O dilute magnetic insulator on LiNbO3 (104) substrates at considerably low growth temperature (∼200 °C), which is significant for complementary metal oxide semiconductor technology. X-ray photoelectron spectroscopy and x-ray-absorption spectroscopy reveal a solid solution of cobalt in ZnO, where Co is in the 2+ state substituting for Zn. A supercoupling mechanism in terms of bound magnetic polarons is proposed to discuss the ferromagnetism in the dielectric ground state of Co:ZnO, which would lead to different consideration for the origin of giant magnetic moment and high-temperature ferromagnetism in transition-metal doped oxides.