Ferromagnetism in Transparent ZnO:xCu Sputter Deposited Thin Films

Currently, detailed understanding on the intrinsic or extrinsic nature of the diluted magnetic oxides when doped with cations (magnetic or nonmagnetic) is an intricate issue. We report a systematic study of ferromagnetism (FM) above room temperature in transparent ZnO:xCu (x in at.{\%}) films deposited by reactive magnetron sputtering. Cu$^{2+}$ ions are found to be in wurtzite structure for x$<$3, whereas CuO phase is deciphered for x$>$3, and located within the ZnO lattice with the associated stacking faults or at grain boundary regions. An anomalous large magnetic moment (\textbf{\textit{M}}) of $\approx $1.76$\pm $0.2 $\mu _{B}$/Cu is observed for x$\approx $0.6. \textbf{\textit{M}} decreases drastically ($<<$ 0.4 $\mu _{B}$/Cu) for x$>$1 due to increased Cu-O-Cu anti-FM interactions. Micro-Raman spectral studies reveal plausible Cu-O clusters of few {\AA} in ZnO lattice giving rise to anomalous high \textbf{\textit{M}} even at low concentrations (x$<$1) in ZnO lattice. Detailed discussion will be presented on related results.