Giant magnetic moment in an anomalous ferromagnetic insulator: Co-dopedZnO

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 $6.1{\mu }_B∕\mathrm{Co}$ and a high Curie temperature $T_C$ of $790\mathrm{K}$ are observed in $(4\mathrm{at}.\%)$ Co-doped $\mathrm{ZnO}$ films, which is not carrier mediated, but co-exists with the dielectric state. Direct current reactive magnetron co-sputtering is used to grow ${\mathrm{Zn}}_{0.96}{\mathrm{Co}}_{0.04}\mathrm{O}$ dilute magnetic insulator on ${\mathrm{LiNbO}}_3$ (104) substrates at considerably low growth temperature $(\sim 200°\mathrm{C})$, 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 $\mathrm{ZnO}$, 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 $\mathrm{Co}:\mathrm{ZnO}$, which would lead to different consideration for the origin of giant magnetic moment and high-temperature ferromagnetism in transition-metal doped oxides.