Interface-induced d⁰ ferromagnetism in undoped ZnO thin films grown on different oriented sapphire substrates

The origin of d⁰ ferromagnetism in ZnO material still remains an open question. Here, we report a systematic study of the structural, optical, Raman and magnetic properties of undoped ZnO films grown on a-, c-, m- and r-plane sapphire substrates by radio frequency magnetron sputtering at room temperature. It is found that the polarity of the substrate does not affect the preferential growth of undoped ZnO film along the c-axis, but it will obviously affect the species and concentration of intrinsic defects in films, thereby regulating the optical and Raman properties of undoped ZnO films. Magnetic measurement reveals that all undoped ZnO films exhibit clear hysteresis loops at room temperature, confirming the presence of room temperature ferromagnetism. Furthermore, the relationship between intrinsic defects and magnetic properties was discussed, which suggests that the observed ferromagnetic order has nothing to do with the internal intrinsic defects in undoped ZnO films, but is derived from the interface effects between the undoped ZnO films and the substrates.