Hydrothermal Synthesis Of CrO₂and HTHP Synthesis Of CrO₂-MgCO₃Composite Oxides

In recent years, with the development of spintronics, high spin-polarizedmagnetic materials have become the hotspot of condensed matter physics. The majorhigh spin-polarized magnetic materials include Chromium dioxide (CrO2), Ferroferricoxide (Fe3O4) and Heusler alloys, etc. By means of band structure calculations, it isfound that CrO2has100%spin polarization. Curie temperature of CrO2has reached394K, therefore, it is identified that CrO2is an ideal material for magnetic tunneljunctions and spintronics devices. However, due to the fact that CrO2is metastable atambient pressure, it is a great challenge to synthesize high-quality CrO2powdercompacts or CrO2-based composite oxides.In the present study, CrO2were synthesized using high temperature and highpressure (HTHP) method. Through characterization methods such as XRD, SEM andVSM, the influence of reaction time and concentration of CrO3on structure,morphology and magnetic properties of CrO2was investigated systematically. Theresults show that CrO2tends to be synthesized by shortening the reaction time andincreasing the concentration of CrO3. Besides, in this research H2O2substituted forH2O in order to increase the oxygen pressure during the process of reaction. Theinfluence of oxygen pressure on structure, morphology and magnetic properties ofhydrothermally synthesized CrO2has also been studied. It is identified that increasingthe oxygen pressure favor the synthesis of CrO2.Moreover, CrO2-MgCO3composite oxides were first synthesized using hightemperature and high pressure (HTHP) method. The structure and magnetotransportproperties of the CrO2-MgCO3composite oxides was explored in detail. In this research, with CrO3and MgC2O4as the raw materials, a series of(CrO2)x-(MgCO3)1-x(x=60%-80%) composite oxides were synthesized under500oC and1GPa conditions by HTHP method. The results indicate that high-purityCrO2-MgCO3composite oxides can only be synthesized when mole percent of CrO3is increased to60%. The study has also explored the magnetic properties ofCrO2-MgCO3(x=60%). Its Curie temperature has reached394K which is roughly inline with the Curie temperature of CrO2compacts. This indicates that MgCO3makesno difference to the Curie temperature of CrO2. Magnetoresistance effect ofCrO2-MgCO3(x=60%) has been studied as well. It showed that magnetoresistanceeffect of CrO2-MgCO3composite oxides is clearly higher than the correspondingCrO2compacts. This is because of the presence of MgCO3which improves thetunneling barrier between CrO2particles and thus enhances the magnetoresistance(MR) effect. The MR curves of CrO2-MgCO3composite oxide consist of two regionsat low temperature, namely, the low-field MR and the high-field MR. The low-fieldMR originates from tunneling magnetoresistance, while the high-field MR mightoriginates from the field-induced reduction of the tunnel barriers. The MR ofCrO2-MgCO3(x=60%) composite oxides decreases quickly with increasingtemperature. This might be due to that spin-independent tunneling becomes dominantat high temperature. By analyzing the conductive mechanism of CrO2-MgCO3composite oxide, it shown that CrO2-MgCO3composite oxide displays insulationproperty. The resistivity-temperature (ρ-T) curves of the CrO2-MgCO3(x=60%)composite oxide can be fitted well by a fluctuation-induced tunneling (FIT) model.This thesis studies the magnetic and magnetotransport properties of synthesizedCrO2-MgCO3(x=60%) composite oxide synthesized under different temperaturesand it is found that synthesis temperature has small influence on the properties ofCrO2-MgCO3(x=60%) composite oxide.