Ferromagnetism induced by planar nanoscale CuO inclusions in Cu-doped ZnO thin films

Abstract

We report ferromagnetism above $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in $\mathrm{Zn}\mathrm{O}:x\mathrm{Cu}$ ($x$ in at. %) sputtered thin films. For $x<1$, a large magnetic moment of $1.6\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}∕\mathrm{Cu}$ was observed, which decreases monotonically with increasing $x$. We find evidence that the ferromagnetic moment is due to Cu-O planar nanophase inclusions in ZnO basal planes. The presence of CuO nanophase is confirmed by transmission electron microscopy, x-ray diffraction, and Raman spectroscopy studies. These inclusions are present even for $x<3$, where Cu-O structures of a few nanometers in size are observed. Field-cooled and zero-field-cooled magnetization measurements show a bifurcation for temperatures below $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.