HTHP Synthesis And Transport Properties Of CrO₂-based Composite Oxides

Recently，high spin-polarized magnetic materials have become research hotspotsof condensed matter physics, due to its importance in theoretical research andextensive application prospects in new spintronics devices. Chromium dioxide （CrO₂）,magnetite and heusler alloys are all important research objects, as they have high spinpolarization. Among them, half-metallic magnet CrO₂gets the most widespreadattention. Because band structure calculations predicted100%spin polarization of theelectrons at the Fermi level of CrO₂. And Single-crystal CrO₂films have beenprepared with the spin polarization up to98.4%. So CrO₂is an ideal candidate formagnetic tunnel junctions and other spintronics devices, where a high tunnelingmagnetoresistance （TMR） effect is expected. In addition, large magnetoresistance（MR） effect has also been found in cold-pressed CrO₂powder compacts andCrO₂-based composite oxides. In order to further enhance the MR effect, theCrO₂-based composite oxides are obtained by making a composite of CrO₂with asecondary phase. Now, the secondary phase materials include Cr₂O₃, TiO₂andpolymer etc. However, because CrO₂is metastable at ambient pressure, it is hard tosynthesize. Therefore, all these cold-pressed CrO₂powder compacts and CrO₂-basedcomposite oxides are made from commercial CrO₂powders. Thus they share somecommon features, i.e., their magnetoresistance is all due to spin-dependent tunnelingthrough grain boundaries, and their conductance is proportional to exp（-1/T1/2） at lowtemperature. Until now, there have been very limited reports about CrO₂powdercompacts and CrO₂-based composite oxides made from different CrO₂sources otherthan commercial CrO₂powders. The MR of CrO₂particles with different sizes andshapes are also of great interest. Herein, in the thesis, high-purity CrO₂-TiO₂composite oxides and CrO₂-SnO₂composite oxides were synthesized using hightemperature and high pressure （HTHP） method. The influence of microstructure and secondary phase on magnetotransport property of the CrO₂-based composite oxideswas investigated detailedly.1. With CrO₃as the precursor, pure CrO₂grains compacts were synthesizedunder400-600℃and1GPa conditions by HTHP method. The influence ofmicrostructure on magnetotransport property of the CrO₂compacts was studied. Theresults show that the surface of the CrO₂grains synthesized at500,550and600℃have a Cr₂O₃layer about6nm thick. However, the surface of the CrO₂grainssynthesized at400and450℃does not have Cr₂O₃layers. The saturationmagnetization （Ms） of the CrO₂compacts can reaches131.7emu/g at5K and is veryclose to the theoretical value （Ms=133emu/g, i.e.,2μBper formula unit）. The MR ofthe CrO₂compacts synthesized at500,550and600℃is larger than that of the CrO₂compacts synthesized at400and500℃. This is because of the presence of theinsulating layer Cr₂O₃which improves the tunneling barrier and thus enhances theMR effect. The MR of all the CrO₂compacts shows different magnetic fielddependences at low field and high field. At low field （H <10kOe）, the MR changesfast with magnetic field. While at high field （H>10kOe）, the MR is nearly linearwith the magnetic field and changes slowly. The low-field MR of the CrO₂compactscan be fitted well by higher-order terms of-（M/Ms）2, which shows that the low-fieldMR originates from the spin-dependent tunneling between neighboring CrO₂crystals,and the magnetic coupling interaction between the CrO₂grains is not negligible. Thehigh-field MR of the CrO₂compacts might be intrinsic to CrO₂, which might originatefrom the tiny change in chemical potential caused by applied magnetic field. Or itmight originate from the field-induced reduction of the effective height or width ofthe tunnel barriers. The MR of the CrO₂compacts decreases quickly with increasingtemperature. This might be due to the decrease of the spin polarization of CrO₂withincreasing temperature. Spin-independent tunneling could also cause the decrease ofMR with increasing temperature, which becomes dominant at high temperature. TheCrO₂compacts synthesized at500,550and600℃display insulation property. Theresistivity-temperature （-T） curves of the CrO₂compacts can be fitted well by afluctuation-induced tunneling （FIT） model. But above240K, the resistivity of theCrO₂compacts begins to deviate from FIT model. This means that the conductancemight also be contributed by the higher-order inelastic hopping conductance, whichhas power-law temperature dependence and increases quickly at high temperature. The CrO₂compacts synthesized at400and450℃show insulator-metal （I-M）transition that arises from the competitive result between grain boundary resistanceand intragranular resistance. Above transition （TIM） temperature, intragranularresistance begins to become dominant, and the resistivity varies linearly with the T2,which can be mainly attributed to electron-electron scattering and electron-phononscattering in grains.2. A series of （CrO₂）x-（TiO₂）1-x（x=80%,70%,60%,50%and40%） compositeoxides were prepared from CrO₃and H2TiO₃under1GPa,400and500℃conditionsby HTHP method. Nano TiO₂in the CrO₂-TiO₂composite oxides synthesized at400℃is anatase structure and the crystallite size of nano TiO₂is about40nm. TiO₂rodin the CrO₂-TiO₂composite oxides synthesized at500℃is rutile structure and itsgrain size reaches300nm in length. The saturation magnetization of the CrO₂-TiO₂composite oxides systematically decreases with the decrease in CrO₂concentration.And the saturation magnetization of CrO₂in the composite oxides is very close to thetheoretical value. This suggests that both CrO₂and TiO₂in the CrO₂-TiO₂compositeoxides are very pure, and defects such as Cr solution in TiO₂or Ti solution in CrO₂are neglectable. The introduction of TiO₂improves the tunneling barrier. So the MRof the CrO₂-TiO₂composite oxides is larger than that of pure CrO₂compacts. TheMR curves of the CrO₂-TiO₂composite oxides also consist of two regions at lowtemperature, namely, the low-field MR and the high-field MR. The low-field MRcurves of all the CrO₂-TiO₂composite oxides are fitted well by a-（M/Ms）2curves,which indicates that the low-field MR originates from the spin-dependent tunnelingthrough grain boundaries between adjacent CrO₂grains. In addition, the resistivity ofthe CrO₂-TiO₂composite oxides is at least one order of magnitude larger than that ofthe CrO₂compacts synthesized under the same conditions. The-T curves of all theCrO₂-TiO₂composite oxides can be explained by FIT model. But above240K, the-T curves begin to deviate from the FIT model. This means that the conductance alsomight to originate from the higher-order inelastic hopping conductance.3. A series of （CrO₂）_1-x-（SnO₂）_x（x=0%-80%） composite oxides were preparedby using CrO₃and SnO₂as the source materials under high temperature （400and500℃） and high pressure （1GPa） conditions. The research shows that the CrO₂in theCrO₂-SnO₂composite oxides is very pure, and the saturation magnetization of CrO₂in the CrO₂-SnO₂composite oxides is very close to the theoretical value. The introduction of the SnO₂enhances MR effectively. The MR of the CrO₂-SnO₂composite oxides synthesized at400and500℃maximally increases-16.7%and-10.7%than that of pure CrO₂compacts synthesized under the same conditions,respectively. The MR curves of the CrO₂-SnO₂composite oxides also consist of thelow-field MR and the high-field MR at low temperatures. The low-field MR curves ofthe CrO₂-SnO₂composite oxides synthesized at500℃are fitted well by-（M/Ms）2curves. In the CrO₂-SnO₂composite oxides synthesized at400℃, when x60%, thelow-field MR of the composite oxides is fitted well by-（M/Ms）2curve. But when x <60%, the low-field MR of the composite oxides begins to deviate from-（M/Ms）2curve.This deviation could be corrected by adding higher-order terms of-（M/Ms）2, whichimplies that the magnetic coupling interaction between the CrO₂grains is notnegligible. The resistivity of the CrO₂-SnO₂composite oxides is larger than that ofpure CrO₂compacts and increases with increasing x. The CrO₂-SnO₂compositeoxides synthesized at500℃show insulation property. And their-T curves can befitted well by FIT model. But when T>240K, the-T curves begin to deviate fromFIT model, which means that the conductance also might to originate from thehigher-order inelastic hopping conductance. The-T curves of the CrO₂-SnO₂composite oxides synthesized at400℃can be fitted well by FIT model at lowtemperature. With increasing temperature, the CrO₂-SnO₂composite oxides with x60%show I-M transition, which is the competitive result between grain boundaryresistance and intragranular resistance. When T> TIM, intragranular resistance beginsto become dominant. And the-T2curves have a linear relation, which can bemainly attributed to electron-electron scattering and electron-phonon scattering ingrains.4. Reaction between CrO₃and Sn was studied by HTHP method, and（CrO₂）_1-x-（SnO₂）_x（x=10%,20%and30%） composite oxides were prepared. Theresults show that the CrO₂-SnO₂composite oxides are dense and cavities decreasewith increase of x. The SnO₂was obtained by the reaction of Sn and oxygen from thedecomposition of the CrO₃, and it damaged the surface of CrO₂grains. The MRcurves of the CrO₂-SnO₂composite oxides also consist of the low-field MR and thehigh-field MR at low temperatures. The low-field MR of the CrO₂-SnO₂compositeoxides can be fitted well by higher-order terms of-（M/Ms）2, which implies that themagnetic coupling interaction between the CrO₂grains is not negligible. The MR of the CrO₂-SnO₂composite oxides is larger than that of pure CrO₂compacts. But theresistivity of the composite oxides is smaller. This is because of the introduction of Snwhich absorbs oxygen from the decomposition of the CrO₃and increases the structuredenseness. The-T curves of all the CrO₂-SnO₂composite oxides can be explainedby the FIT model. But when T>240K, the-T curves begin to deviate from the FITmodel, which implies that the conductance also might to originate from thehigher-order inelastic hopping conductance.