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

Abstract

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 $6.1{\ensuremath{\mu}}_{B}∕\mathrm{Co}$ and a high Curie temperature ${T}_{C}$ of $790\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ are observed in $(4\phantom{\rule{0.3em}{0ex}}\mathrm{at}.\phantom{\rule{0.2em}{0ex}}%)$ 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 $(\ensuremath{\sim}200\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$, 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.