Substrate-dependent magnetization in Co-doped ZnO insulating films

Magnetization and local Co structure of 4 at. % Co-doped ZnO insulating films, prepared by direct current reactive magnetron cosputtering, have been studied as a function of substrates. Although all the Co:ZnO films possess a common feature that Co2+ replaces Zn2+, room-temperature ferromagnetism of Co:ZnO films is strongly dependent on the substrates. The reorganization of defects due to magnetoelectric coupling and a slight dissimilarity among Co K-edge x-ray-absorption near-edge structure spectra on different substrates reveal that the combination of Co:ZnO film grown on ferroelectric substrates and a suitable Co-O bond length likely serves as a key in enhancing the magnetic ordering. Furthermore, electric property measurements demonstrate trapped electrons at defect sites, which is of help for a better understanding of the bound magnetic polaron mechanism. These observations and the corresponding mechanisms responsible for the ferromagnetism enhancement would advance the insight into the controversial magnetic behaviors in Co:ZnO system.