The Preparation And Magnetoelectric Properties Of Epitaxial Chromium Oxide Films

Spintronic devices such as giant magnetoresistant ?GMR? devices and tunneling magnetoresistant ?TMR? devices have various applications, especially in the area of magnetic data storage. To apply magnetic materials with high spin polarization such as magnetic half metals in devices is believed to be an efficient way to obtain high MR ratio. Among all the theoretically predicted half metals that have 100% spin polarization at Fermi level, CrO₂ is one of the few experimentally proved ones and possesses the highest spin polarization so far reported.To practically apply CrO₂ in MR devices, it is crucial to find ways to get over some difficulties. In this work, we study the physical behavior of CrO₂ and make it adaptive to be applied in MR devices.The thesis mainly includes the following four aspects:1.High quality CrO₂ thin films are grown on single crystalline ?100? ?110? ?001? TiO₂ substrates using CVD. The crystalline structure and Surface morphology of films are characterized by XRD?SEM?AFM?HRTEM. Magnetization measurementswere carried out using VSM at room temperature to find that the easy axes of the films are along [001]. The coercive force He decreases with the thickness increases due to the impact of the stress caused by the lattice mismatch between substrate and sample interface. For the films grown on the ?100? ?100? ?001? three different TiO₂ substrates, they have different growthmodes leading to the introduction of defects, which affect the movement of magnetic domain.2. CrO₂ is a metastable compound and can be irreversibly decomposed to insuIatingantiferromagneticCr₂O₃at the surface. Cr1-xSnxO2 thin films are grownon single crystalline TiO₂ ?100? substrates using CVD.The doping concentration is 3% and Sn doping does not change the films'growth mode.With the introduction of defects, He of the easy axis increases and the magnetocrystallineanisotropy decreases. Our study also indicates that Sn-doping may not change the essential halfmetallic properties of CrO₂ by the first principles calculation. Sn-doped films can withstand a temperature up to 510?, significantly higher than whatun-doped films can do ?lower than 435??, which suggests that Sn-doping indeed enhances the thermalstability of CrO₂ films.Sn-doped CrO₂ is expected to be advantageousfor applications in spintronic devices.3. The study of damping coefficient is extremely important in that the devices get magnetic resistance effect through the magnetization reversal. The ferromagnetic resonance ?FMR? is conducted on the films of different Sn doping concentration, different thickness and the films grown on ?100? ?100? ?001? three different TiO₂ substrates. The changing rule of the resonance field is related to their saturation magnetization Ms and saturated magnetic field Hk. More important, as the increase of the Sn doping concentration the damping coefficient a is increasing. According to the theory put forward by the Kambersky, due to Sn doping changed the lattice constant, the splitting energy of eg-t2g caused by oxygen octahedral crystal field changes, leading to the change of a. The in-plane direction [010] damping coefficients a of the films grown on ?100? and ?100? TiO₂ substrates are quite different, due to the different growth mode.4. RT, magnetoresistance ?MR?, angular magnetoresistance ?AMR? and Hall effect are tested on the CrO₂ film grown on the ?100? TiO₂ substrate. RT tests find that resistance presents strong anisotropy, and it is much higher along [001] than that along [010]. This is because the lattice mismatch of the substrate and CrO₂ film are different alone b axis and c axis, causing the difference of the carrier mobility. MR and AMR have shown that the lorentz MR dominates at low temperature, while the spin related MR dominates at high temperature. The anomalous Hall effect is observed, and the Hall coefficient Ro and the anomalous Hall coefficient Rs are both isotropic. Ro decreases with increasing temperature. The changing rule of Rs meets Yanagihara's result by fitting. With the temperature increasing, the carrier concentration increases and the carrier mobility decreases. The carrier mobility along [010] and [001] are quite different when the temperature is between 140K and 200K, confirming the result above.