Synthesis And Magnetotransport Studies Of Chromium Dioxide Films

Half-metallic ferromagnets, which have unique band structure and high spinpolarization, have become research hotspots of magnetoelectronic devices andcondensed matter physics, owing to their importance in theoretical research andextensive application prospects in spintronic devices. Since last nineteen nineties, thehalf-metallic ferromagnets have been applied to spintronics devices. According toJullière’s model the magnetoresistance (MR) of ferromagnet/insulator/ferromagnettunnel junctions depends on the spin polarization of the ferromagnetic electrodes used.The MR increases with increasing spin polarization of the electrode materialsinvolved. This resulted in revived research interest in the class of half-metallicferromagnets such as Heusler alloys, manganites and CrO2. CrO2is an importantferromagnetic metallic binary oxide, with nearly100%spin polarization at the Fermilevel, which has been proved by spin-polarized photoemission and point-contactAndreev reflection. And CrO2has good electronic conductivity as well as adequateCurie temperature about395K, so it appears to be an ideal candidate for spintronicdevices. However, CrO2is a metastable oxide at ambient condition, it woulddecompose to Cr2O3easily and which is known as the most stable one in allchromium oxides. This means that it is difficult to synthesize purity CrO2at ambientpressure. Previous studies on CrO2have been devoted to CrO2powder compacts,CrO2composites, CrO2nano-structure and CrO2films. The CrO2films were usuallydeposited on Al2O3(001), isostructural TiO2(100) and on polycrystalline TiO2substrates by the atmospheric pressure chemical vapor deposition (CVD) method. Thetransport properties of epitaxial and polycrystalline CrO2thin films have beeninvestigated. However, little research on the effect of microstructure on themagnetotransport properties of CrO2films deposited on other substrates has been done. In the field of CrO2composites, synthesis and magnetotransport studies havebeen devoted to CrO2powder compacts. However, up to now, little research on thesynthesis and magnetotransport properties of CrO2composite films has been done. Inaddition, it is difficult to synthesize very thin CrO2films on different substrates by theatmospheric pressure CVD method. In this thesis, the influence of the microstructureof CrO2films on its magnetotransport properties has been investigated. Then,CrO2-Al2O3composite films were deposited on Si (100) substrates and itsmagnetotransport properties was studied. In addition, the growth andmagnetotransport properties of CrO2films deposited on TiO2nanotubes substrateswere investigated. At last, the CrO2and Fe3O4ultra-thin films have been preparedunder high pressure of oxygen.1. CrO2films have been deposited on sapphire substrates by atmospheric pressureCVD method. The (101)-oriented CrO2film deposited on (012) sapphire substrate hasa rougher surface than the (100)-oriented CrO2film on (001) sapphire substrate. Andsmaller CrO2grains and more grain boundaries are found in the (101)-oriented CrO2film. In the low temperature region, as no significant temperature dependence ofresistance in the films were observed below~35K (and hence no T2dependence), wecan say that there is no significant spin-flip scattering in the films below35K. Wefound that the resistance varies linearly with the T2about130K to250K, which canbe mainly attributed to electron-electron scattering and electron-phonon scattering ingrains. And R-T2curve also shows a change in slope at about250K. Moreover, weobserved that (101)-oriented CrO2films’ resistivity is higher than that of(100)-oriented CrO2films in full temperature range. This can be attributed to thepresence of Cr2O3in the (101)-oriented CrO2films and much more grain boundariesbetween CrO2crystallites. Below16kOe, the resistance of (100)-oriented CrO2filmsdecreases slowly with increasing magnetic field at5K. The negative MR can beinterpreted in terms of intergrain tunneling magnetoresistance (ITMR) effect. Above16kOe, the resistivity of (100)-oriented CrO2films increases with increasingmagnetic field, which is due to the ordinary Lorentz MR contribution. In contrast,(101)-oriented CrO2films show only ITMR effect at5K. It means that ITMR effectand Lorentz MR effect can directly be related to the crystalline quality of the films.The ITMR effect almost disappears at200K and300K. The MR of the film is linearwith magnetic field at200K and300K. This can be explained by the presence of double-exchange mechanism in CrO2.2. CrO2-Al2O3composite films were deposited on Si (100) substrates byatmospheric pressure CVD method using CrO3and AlBr3as precursors. TheCrO2-Al2O3composite films consist of large CrO2grains and Al2O3nanoparticles.The composition of the films was controlled by adjusting the temperature of CrO3during deposition. The magnetotransport properties of the CrO2-Al2O3composite filmwith Al2O3content of0.38(molar fraction) has been investigated. In the lowtemperature region, it is insulator and its temperature dependence of resistance can bewell explained by fluctuation-induced tunneling (FIT) model; above175K, it ismetallic. It shows large ITMR effect at low temperature, which reaches-15.2%at5K and50kOe. While at high temperature the ITMR effect in the film nearlydisappears, and the film shows a small linear MR, which should be due to thedouble-exchange mechanism in CrO2.3. The growth and magnetotransport properties of polycrystalline CrO2films onTiO2nanotubes were investigated. During the deposition, the thickness of the filmswas controlled by adjusting the reaction time. The resistance of polycrystalline CrO2films grown on TiO2nanotubes decreases rapidly with increasing temperature in fulltemperature range. It suggests that it is insulator and there are gaps between the CrO2grains. The gaps in polycrystalline CrO2films act as tunneling barrier. In the lowtemperature region, the resistance-temperature curves of the films can be fit well bythe FIT model. But the resistance of polycrystalline CrO2films begins to deviate fromFIT model with the increasing temperature. This means that the conductance mightalso be contributed by the higher-order inelastic hopping conductance. At lowtemperature, all of the films show large ITMR effect. When the reaction time is0.5h,1h,2h and3h, the MR of the films reaches-20.8%,-18.4%,-16.6%and-13.8%,respectively, at5K and50kOe. At high temperature, the films show small linear MR,which can be interpreted in terms of double-exchange mechanism.4. Chromium and iron films were treated under high-pressure oxygen atmosphereand elevated temperature. Structure and magnetic properties of chromium and ironfilms after treated were studied. According to studies, CrO2, Cr and other chromiumoxides film can be obtained by oxygenating pure chromium film under high pressureof oxygen, and the film has a large coercive force of120Oe. Fe3O4and Fe2O3filmcan be obtained by oxygenating pure iron film under high pressure of oxygen, and the film has a large coercive force of415Oe.