Study Of Spin-polarized Oxide Material And Device

In the age of information explosion, information technology is rapidly moving forward with Moore’s Law that the number of transistors on integrated circuits doubles approximately every18months.However, with the reduction of the device size and the increase of integration degree, the high-speed development will soon meet the insuperable obstacle because the energy consumption per unit area will rise rapidly and result in serious heat damage problems and quantum confinement effect. The challegenging situation prompts the advent of spintronics, which controls both charge and spin of electrons. Spintronics operates with different principle comparering with tradition microelectronic devices, making devices with faster speed, lower energy consumption and smaller size. It includes spin polarized current generation, spin injection, spin transport and so on. The foremost thing of spintronics is generate spin-polarized current. Therefore, spin-polarized materials are the the foundation of spintronics and lots of work focuses on exploring the spin-polarized material in recent years, such as magnetic semiconductors, halt metal, topological insulators and so on.The preparation of spin-polarized materials and design of spintronics devices are two main parts of spintronics. Based on the previous studies on spintronics, we have prepared some novely spin-polarized materials and devices and studied their properties.Magnetic semiconductorsare most important spin-polarized materials due to its high spin injection efficiency into semiconductors and have attracted considerable attention because of their physicsand potential applications during the past several years. Among various kinds of magnetic semiconductors wide band-gap semiconductorsare most potential.Room temperature ferromagnetism has been widely observed intransition metal dopedoxide semiconductors such as ZnO, ln2O3, TiO2and so on.Howeverfew studies on spin-polarization have been reportedcomparing with lots ofwork focusing on structure, magnetic properties of magnetic semiconductors. In this thesis we prepared magnetic semiconductors anddemonstrated the spin-polarization by magnetic tunnel junction, anomalous Hall effects and magnetoresistance.In order to satisfy the ever-increasing demand for faster, smaller, and non-volatile electronicsin information storage and processing technology, various novel electronic devices such as spin field-effecttransistors, multiferroic memories, racetrack memories, and memristors have been proposed and investigated.All these novle devices utilize new information storage and processing principleand will have great application in the future. We have designed and preparated a magnetic tunnel junction combining memristance and magnetoresistance which have advances such as high density, low power consumption and non-volatile information storage. We also observed spin dependent voltage shifts in magnetic tunneljunction leading to a large magnetoresistance, which have potential application as a magnetic sensor.The main results are as follows:(1) We growed Zn0.32Co0.68O1-v (v means oxygen vacancies) condensed magnetic semiconductorby alternately depositing0.5nm Co layers and0.5nmZnO layers at 20℃. The bottom and top ferromagneticelectrodes were patterned by shadow masks into a cross configurationwith a junction area of0.1mm X0.1mm.In the tunnel junction, room temperature magnetoresistance was observed, which gradually increased to19.7%with decreasing temperature to2K. According to the model of Julliere, a spin-polarization of25%in Zn0.32Co0.68O1-v layer can be deduced fromthe maximum TMR ratio of19.1%at2K.The temperature dependence of junction resistance indicates the existence of directed inelastic hopping of electrons because of the defects in the barrier layer.Several factors such as tunneling via impurity statesand inelastic scattering by magnon excitations can weaken theTMR, and the spin polarization derived from the TMR ratio isunderestimated. By contrast, the spin polarization of the Zn0.32Coo.6801-v CMS was estimated to be36.1%by spin-dependent variable range hopping. The spin polarization of carriers in ferromagnetic semiconductors was also revealed by anomalous Hall effects.(2) Epitaxial ln2O3films with embedded Fe3O4oriented nanocolumns were deposited by pulsed laserdeposition (PLD) on Y-stabilized ZrO2(111) substrates. By control the growth temperature and rate, magnetic ions aggregated in the form of Fe3O4nanocolumnsembedded in the epitaxial ln2O3With size of about several tens nanometers. The XRD patterns suggests a very strong preferred orientation along the (111) axis.Thenanocompositefilms show ferromagnetism atroom temperature with strong magnetic anisotropy, which can be attributed to the presence of Fe3O4nanocolumns. Magnetotransport measurements demonstrate a transition from negativemagnetoresistance to positive magnetoresistance as the measuring temperature increases. Quantitativecoincidence between the anomalous Hall resistivity as a function of the magnetic field and themagnetic hysteresis loop is observed at room temperature, indicating the spin polarization nature ofcarriers.The average transmittance in the visible range is higher than60%and a clear magneto-optic Kerr hysteresis loop is alsorecorded in these nanocomposites.All these results indicatethat these novel magnetic nanocomposite films may find wideapplications in future spintronic devices.(3) Using amagnetron sputtering machine and shadowmask, we designed and preparedCo/CoO-ZnO/Co MTJs which combine memristance and magnetoresistance. The CoO-ZnO nanon composite barrier layer was formed by deposited2nm ZnO on the Co layer. An antiferromagneticinsulating CoO thin layer was produced on the surfaceof the bottom Co layer during the deposition of ZnO, due to the oxidation of Co electrode. Magnetism and XPS results suggest that the thickness of CoO is about2nm.By CoO-ZnO nanon composite barrierlayers, we successfully simultaneously realize large memristance and tunnelingmagnetoresistance.TheThe bipolar resistance switching ratio is highup to90, and the TMR ratio of the high resistance state gets to8%at room temperature, which leads to threeresistance states. The HRS resistancedecreases with increasing temperature, which is a typical feature oftunneling transport through a continuous insulating barrier. On the contrary, the LRS resistance increases with increasingtemperature, showing metallic-like transport behavior. The bipolar resistance switching is explained by the metal-insulator transition of CoO1-v layer due to the migration of oxygen ions between CoO1-v and ZnO1-v.(4) We prepared magnetic tunneljunctions with asymmetry barrier CoO-ZnO using amagnetron sputtering machine and shadowmask. A spin dependent voltage shift of mVmagnitude was observed in the junction under AC voltage due to the current rectification effect. Due to the spin dependent voltage, a large magnetoresistanceof-95%was observed at1.2nA and themagnetoresistance reversed its sign with the current variation.A21%magnetoresistance was observed at-1nA. At the same time, the spin voltage varies and even reversed its sign with ZnO thickness. Based on the results we think that the the current rectification effect of AC voltage is the energy source of the spin motive fore in magnetic tunnel junctions.