Local Structure And Magnetic Property Of Diluted Magnetic Oxides Doping With Cobalt

Diluted magnetic oxides (DMO) are promising spin sources, which are considered as important supporting materials for a new generation of spintronics. Preparing DMO with room temperature ferromagnetism (RTFM), exploring the origin of RTFM, and applicating DMO to spintronics are the hot topics in the field of spintronics. The effects of preparation parameters, such as doping concentration, substrate and its temperature, oxygen partial pressure and deposition rate, on the local structure and ferromagnetic ordering of DMO have been systematically studied. This dissertation discusses Co-doped ZnO and LiNbO₃ prepared by magnetron sputtering, as well as ion implantation and pulsed laser deposition, respectively. These researches concentrate on the correlations between magnetic properties and the local structure, the origin of RTFM in DMO. Employing the prepared DMO materials, magnetic tunnel junctions (MTJ) prototype spintronics are designed and fabricated, whose tunnel magnetoresistance (TMR) and the corresponding spin polarization, injection and trasnsport are explored.The results show that the Co-doped ZnO films deposited by magnetron sputtering possess a Curie temperature higher than 700K, and the magnetic moments of Co are intimately correlated to the doping concentration and the substrate. A giant magnetic moment of 6.1μB/Co is observed in (4 at.%) Co-doped ZnO insulating films. Converse magnetoelectric coupling between Zn0.96Co0.04O films and ferroelectric substrates can effectively enhance the ferromagnetic ordering. A super coupling mechanism based on bound magnetic polarons (BMP) is developed to understand the giant magnetic moment. Taking advantage of modulating the oxygen partial pressure, diluted magnetic insulators (DMI) and semiconductors (DMS) in single-crystal Zn0.96Co0.04O films can be controllably synthesized. Based on the insulating Co-doped ZnO films showing robust RTFM, as well as separating the effects of carrier concentrations and structural defects on magnetization, we unambiguously demonstrate that RTFM is profoundly correlated with structural defects, and the carriers involved in carrier-mediated exchange are by-products of defects created in ZnO. The origin of RTFM in DMO is then clearly clarified.Taking advantage of the prepared Co-doped ZnO films, fully epitaxial (Zn,Co)O/ZnO/(Zn,Co)O junctions are successfully fabricated. A positive TMR of 20.8% is obtained at 4 K and at 2 Tesla. Due to the improved crystallinity and compatibility of electrode/barrier interfaces, TMR can resist up to room temperature, indicating that spin injection and transport at room temperature are realized in the junctions. (Zn,Co)O-based MTJ with double-barriers show anomalous bias voltage dependent TMR, which are not only significant for deeply exploring spin phenomena, but also expand the range of bias voltage for using MTJ devices.The results also show that (5%) Co-doped LiNbO₃ is a real DMO with RTFM, demonstrating that the bound magnetic polarons based on defects are the origin of RTFM in DMO. Co-doped LiNbO₃ is a room temperature single-phase multiferroic, which possesses very similar ferromagnetic and ferroelectric Curie temperature, i.e., 540 K and 610 K, respectively.