Defect Induced Room-Temperature Ferromagnetism in ZnO and MgO Thin FIlms and Device Development

Abstract: This thesis presents the discovery of defect induced room-temperature ferromagnetism in industrially important ZnO and MgO thin films, and establishes from a systematic study, in both ZnO and MgO films, the unique phenomenon of the sequences of transitions from ferromagnetism to para-, and eventually the well known diamagnetism of the bulk as a function of film thickness.Highly oriented and high quality dense thin films of ZnO and MgO have been deposited by reactive (balanced) magnetron sputtering under different ambience conditions and deposition temperatures. The ZnO thin films were deposited from a Zn metal target whereas the MgO thin films were deposited from an MgO ceramic target. Their magnetic properties have been studied as a function of both film thickness and variation in oxygen deposition pressure (for a given thickness) using a SQUID magnetometer. The ferromagnetic ordering in these materials is shown to arise from lattice defects situated at the cation sites. We discuss in detail the observed variation in their saturation magnetization, MS, as a function of the various deposition conditions and film characteristics (i.e. film thickness), and relate these to the nature and role of the intrinsic defects in giving rise to the observed magnetism. The in-plane saturation magnetization obtained in these films is at least two orders of magnitude larger as compared to what is measured in nanoparticles of similar dimensions. Furthermore it is shown that the magnetic properties in these thin films is directional dependent and that along the diagonal of the wurtzite structure at 45 degrees to the c-axis the MS values are about 60% larger. This we correlate with a calculation based on the structure which shows that the cation- cation distances along the diagonal is the shortest by similar magnitude. A Zn57O57 super-cell has been modelled using the Inorganic Crystal Structure Database (ICSD Diamond 3.0), from which we have calculated the shortest distance between two adjacent cation sites (i.e. potential cation vacancy sites) along the c-axis as well as perpendicular and along the diagonal (i.e. 45°) to the c-axis (along which the films have grown). Such possibilities to tailor defect induced ferromagnetism resulting in saturation magnetization of ? 5 emu/g, is indeed highly important information in understanding and designing thin film devices. In order to further tailor the physical property of polycrystalline ZnO thin films, un-balanced magnetron sputtering was used to obtain porous microstructured ZnO thin films to induce significant UV photoconductivity and demonstrate plausible device application.The above studies have been made possible using extensive characterization of the high quality films, in the thickness range from a few nanometers to almost a micron, using XRD for structure, Dual beam HRSEM/FIB and AFM for accurate film cross-sectioning and surface morphology, EDXS for elemental analysis and electrical/photo- conductivity measurements over a wide range of incident radiation from UV to visible.The overall conclusion is that the room-temperature ferromagnetic ordering in the ZnO and MgO thin films originates from cation vacancies which couple ferromagnetically and establish long range magnetic order.