二氧化铬基复合氧化物的高温高压合成及其输运性质研究
摘要
近年以来，高自旋极化磁性材料由于其重要的理论研究价值和在新型自旋电子器件中的广泛应用前景而成为凝聚态物理领域的研究热点。例如，具有高自旋极化率的二氧化铬（CrO_2），四氧化三铁（Fe3O4）和赫斯勒合金（Heusler alloys）等，都是重要的研究对象。其中，半金属磁体CrO_2引起了广泛的关注。能带结构计算表明CrO_2具有100%的自旋极化率。实验中已制备出自旋极化率高达98.4%的单晶CrO_2薄膜。因此，CrO_2是一种应用于磁隧道结和其它自旋电子器件的理想材料。此外，在冷压CrO_2粉末压结体中已经发现很高的隧穿磁电阻（TMR）效应。通过在CrO_2粉末压结体中引入第二相物质，构成CrO_2基复合氧化物，可以进一步提高磁电阻（MR）效应。目前，引入的第二相物质包括：Cr_2O_3、TiO_2和聚合物等。然而，由于CrO_2在常压下的亚稳态特征，致使CrO_2的合成具有很大的挑战性。因此，对冷压CrO_2粉末压结体及CrO_2基复合氧化物的磁电阻效应研究多采用商业CrO_2磁粉，所以关于它们的磁电阻效应研究结果具有一些共同的特征，如磁电阻都来源于晶界间的自旋相关隧穿效应；以及电导在低温区域与温度exp(-1/T1/2)呈线性关系，这说明低温时它们的电导主要来源于库仑阻塞效应影响的自旋相关隧穿。迄今为止，在冷压CrO_2粉末压结体及CrO_2基复合氧化物的磁电阻效应研究中，关于采用其它CrO_2源的研究报道并不多见，并且关于晶粒尺寸和第二相物质对CrO_2磁电阻效应的影响缺乏很详细的研究。本论文采用高温高压方法合成了高纯度的CrO_2-TiO_2复合氧化物和CrO_2-SnO_2复合氧化物，并且详细地研究了微观结构和引入物质对其磁输运性质的影响。
1.以CrO_3为原料，利用高温高压方法在压强为1GPa，400-600℃的温度范围内成功合成了高纯度的CrO_2压结块体，并研究了微观结构对CrO_2块体磁输运性质的影响。研究表明，在500℃、550℃和600℃下合成的CrO_2晶粒表面存在Cr_2O_3绝缘层，厚度大约为6nm，而在400℃和450℃下合成的CrO_2晶粒表面并没有观察到Cr_2O_3绝缘层。CrO_2块体在5K下的饱和磁化强度高达131.7emu/g，非常接近于CrO_2的理论值（Ms=133emu/g），这说明CrO_2块体的纯度非常高。在500℃、550℃和600℃下合成的CrO_2块体磁电阻明显高于在400℃和450℃下合成的CrO_2块体磁电阻，这是由于绝缘层Cr_2O_3势垒的存在，增强了磁电阻效应。所有CrO_2块体在低温下的磁电阻随磁场的变化关系由两个区域组成：在低场区域（H10kOe），磁电阻随磁场增加迅速地增大；而在高场区域(H10kOe)，磁电阻随磁场增加近似于线性地缓慢增大。低场磁电阻随M/Ms的变化关系曲线可以利用(M/Ms)2的高阶项很好地拟合，这说明低场磁电阻来源于CrO_2晶粒间的自旋相关隧穿效应，同时CrO_2晶粒间的磁耦合作用不可忽略。高场磁电阻可能是CrO_2的固有属性，或者来源于磁场诱导隧穿势垒的改变。CrO_2块体的磁电阻随温度上升而快速地下降，这可能是由于CrO_2块体的自旋极化率随温度上升而减小的缘故，或者是CrO_2块体中自旋无关隧穿随温度上升而占主导地位。在500℃、550℃和600℃下合成的CrO_2块体表现为绝缘性，电阻率-温度（-T）曲线可以用Fluctuation-Induced Tunneling（FIT）模型解释。但是在240K以上，-T曲线开始背离FIT模型，这说明电导还可能来源于高阶非弹性隧穿的贡献。在400℃和450℃下合成的CrO_2块体在低温区域的-T曲线可以用FIT模型很好地拟合，然而随温度上升，CrO_2块体出现绝缘体-金属体（I-M）转变，这是晶界电阻和晶粒内电阻竞争的结果。在转变（TIM）温度以上，晶粒内电阻开始占主导地位，电阻率随温度T2呈线性变化，这说明电阻主要归结于晶粒内电子-电子散射和电子-声子散射。
2.以CrO_3和H2TiO_3为原料，利用高温高压方法在压强为1GPa，温度分别为400℃和500℃下合成了一系列(CrO_2)x-(TiO_2)1-x（x=80%，70%，60%，50%，40%）复合氧化物。400℃下合成的CrO_2-TiO_2复合氧化物中，纳米TiO_2为锐钛矿型结构，粒径大约为40nm；500℃下合成的CrO_2-TiO_2复合氧化物中，棒状TiO_2为金红石型结构，晶粒长度达到300nm。CrO_2-TiO_2复合氧化物的饱和磁化强度随CrO_2含量减少而系统地下降，并且复合氧化物中CrO_2的饱和磁化强度非常接近于CrO_2的理论值，这说明复合氧化物中CrO_2和TiO_2纯度非常高。通过引入绝缘体TiO_2，增强了CrO_2晶粒间的势垒，从而使CrO_2-TiO_2复合氧化物的磁电阻高于CrO_2块体的磁电阻。CrO_2-TiO_2复合氧化物在低温下的磁电阻曲线同样由两个区域组成，即低场磁电阻和高场磁电阻。复合氧化物的低场磁电阻曲线可以用-(M/Ms)2曲线拟合，这说明低场磁电阻来源于CrO_2晶粒间的自旋相关隧穿效应。另外，绝缘体TiO_2的引入使CrO_2-TiO_2复合氧化物的电阻率比同样条件下合成的CrO_2块体电阻率至少大一个数量级。CrO_2-TiO_2复合氧化物的-T曲线可以用FIT模型解释，但是在240K以上时，-T曲线开始背离FIT模型，这说明电导还可能来源于高阶非弹性隧穿的贡献。
3.向CrO_2中引入宽带隙半导体SnO_2，合成CrO_2-SnO_2复合氧化物。以CrO_3和SnO_2为原料，利用高温高压方法在压强为1GPa，温度分别为400℃和500℃下合成了一系列(CrO_2)1-x-(SnO_2)（xx=0%-80%）复合氧化物。研究表明，CrO_2-SnO_2复合氧化物中SnO_2纯度非常高，并且CrO_2的饱和磁化强度非常接近于理论值。SnO_2的引入增强了磁电阻效应，与相应的CrO_2块体磁电阻相比，400℃和500℃下合成的CrO_2-SnO_2复合氧化物磁电阻最大提高了-16.7%和-10.7%。CrO_2-SnO_2复合氧化物在低温下的磁电阻曲线同样由低场磁电阻和高场磁电阻两个区域组成。500℃下合成的CrO_2-SnO_2复合氧化物低场磁电阻曲线都可以用-(M/Ms)2曲线拟合。在400℃下合成的CrO_2-SnO_2复合氧化物中，当x60%时，低场磁电阻曲线可以用-(M/Ms)2曲线拟合；但是当x60%以后，低场磁电阻曲线开始背离-(M/Ms)2曲线，然而可以用(M/Ms)2的高阶项拟合，这表明x60%的复合氧化物中，CrO_2晶粒间磁耦合作用不可忽略。CrO_2-SnO_2复合氧化物的电阻率高于CrO_2块体的电阻率，并且随x增加而增大。在500℃下合成的CrO_2-SnO_2复合氧化物显示了绝缘性，-T曲线都可以用FIT模型解释，但是当T240K时，-T曲线开始背离FIT模型，这说明电导还可能来源于高阶非弹性隧穿的贡献。在400℃下合成的CrO_2-SnO_2复合氧化物在低温区域的-T曲线可以用FIT模型拟合，随温度升高，x60%的复合氧化物发生I-M转变，这是由于晶界电阻和晶粒内电阻竞争导致的结果。当T TIM时，晶粒内电阻开始起主导作用，-T2曲线呈线性关系，这说明电阻主要归结于晶粒内的电子-电子散射和电子-声子散射。
4.研究了CrO_3和金属Sn在高温高压下的反应，获得了高纯度的(CrO_2)_(1-x)-(SnO_2)_x（x=10%，20%，30%）复合氧化物。分析表明，随x增加，CrO_2-SnO_2复合氧化物的孔洞减少、结构致密。金属Sn吸收CrO_3释放的氧气，生成的SnO_2破坏了CrO_2晶粒表面。CrO_2-SnO_2复合氧化物在低温下的磁电阻曲线同样由两个区域组成，即低场磁电阻和高场磁电阻。低场磁电阻随M/Ms的变化关系曲线可以用(M/Ms)2的高阶项很好地拟合，这说明CrO_2晶粒间的磁耦合作用不可忽略。CrO_2-SnO_2复合氧化物的磁电阻高于CrO_2块体的磁电阻。但是，电阻率反而变小，这是因为Sn的引入，吸收了CrO_3释放的氧气，增加了结构致密性。CrO_2-SnO_2复合氧化物的-T曲线可以用FIT模型解释，但是当T240K时，-T曲线开始背离FIT模型，这说明电导还可能来源于高阶非弹性隧穿的贡献。
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 (CrO_2),magnetite and heusler alloys are all important research objects, as they have high spinpolarization. Among them, half-metallic magnet CrO_2gets the most widespreadattention. Because band structure calculations predicted100%spin polarization of theelectrons at the Fermi level of CrO_2. And Single-crystal CrO_2films have beenprepared with the spin polarization up to98.4%. So CrO_2is 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 CrO_2powder compacts andCrO_2-based composite oxides. In order to further enhance the MR effect, theCrO_2-based composite oxides are obtained by making a composite of CrO_2with asecondary phase. Now, the secondary phase materials include Cr_2O_3, TiO_2andpolymer etc. However, because CrO_2is metastable at ambient pressure, it is hard tosynthesize. Therefore, all these cold-pressed CrO_2powder compacts and CrO_2-basedcomposite oxides are made from commercial CrO_2powders. 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 CrO_2powdercompacts and CrO_2-based composite oxides made from different CrO_2sources otherthan commercial CrO_2powders. The MR of CrO_2particles with different sizes andshapes are also of great interest. Herein, in the thesis, high-purity CrO_2-TiO_2composite oxides and CrO_2-SnO_2composite oxides were synthesized using hightemperature and high pressure (HTHP) method. The influence of microstructure and secondary phase on magnetotransport property of the CrO_2-based composite oxideswas investigated detailedly.
1. With CrO_3as the precursor, pure CrO_2grains compacts were synthesizedunder400-600℃and1GPa conditions by HTHP method. The influence ofmicrostructure on magnetotransport property of the CrO_2compacts was studied. Theresults show that the surface of the CrO_2grains synthesized at500,550and600℃have a Cr_2O_3layer about6nm thick. However, the surface of the CrO_2grainssynthesized at400and450℃does not have Cr_2O_3layers. The saturationmagnetization (Ms) of the CrO_2compacts can reaches131.7emu/g at5K and is veryclose to the theoretical value (Ms=133emu/g, i.e.,2μBper formula unit). The MR ofthe CrO_2compacts synthesized at500,550and600℃is larger than that of the CrO_2compacts synthesized at400and500℃. This is because of the presence of theinsulating layer Cr_2O_3which improves the tunneling barrier and thus enhances theMR effect. The MR of all the CrO_2compacts 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 CrO_2compactscan 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 CrO_2crystals,and the magnetic coupling interaction between the CrO_2grains is not negligible. Thehigh-field MR of the CrO_2compacts might be intrinsic to CrO_2, 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 CrO_2compacts decreases quickly with increasingtemperature. This might be due to the decrease of the spin polarization of CrO_2withincreasing temperature. Spin-independent tunneling could also cause the decrease ofMR with increasing temperature, which becomes dominant at high temperature. TheCrO_2compacts synthesized at500,550and600℃display insulation property. Theresistivity-temperature (-T) curves of the CrO_2compacts can be fitted well by afluctuation-induced tunneling (FIT) model. But above240K, the resistivity of theCrO_2compacts 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 CrO_2compacts 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 (CrO_2)x-(TiO_2)1-x(x=80%,70%,60%,50%and40%) compositeoxides were prepared from CrO_3and H2TiO_3under1GPa,400and500℃conditionsby HTHP method. Nano TiO_2in the CrO_2-TiO_2composite oxides synthesized at400℃is anatase structure and the crystallite size of nano TiO_2is about40nm. TiO_2rodin the CrO_2-TiO_2composite oxides synthesized at500℃is rutile structure and itsgrain size reaches300nm in length. The saturation magnetization of the CrO_2-TiO_2composite oxides systematically decreases with the decrease in CrO_2concentration.And the saturation magnetization of CrO_2in the composite oxides is very close to thetheoretical value. This suggests that both CrO_2and TiO_2in the CrO_2-TiO_2compositeoxides are very pure, and defects such as Cr solution in TiO_2or Ti solution in CrO_2are neglectable. The introduction of TiO_2improves the tunneling barrier. So the MRof the CrO_2-TiO_2composite oxides is larger than that of pure CrO_2compacts. TheMR curves of the CrO_2-TiO_2composite 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 CrO_2-TiO_2composite 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 CrO_2grains. In addition, the resistivity ofthe CrO_2-TiO_2composite oxides is at least one order of magnitude larger than that ofthe CrO_2compacts synthesized under the same conditions. The-T curves of all theCrO_2-TiO_2composite 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 (CrO_2)_(1-x)-(SnO_2)_x(x=0%-80%) composite oxides were preparedby using CrO_3and SnO_2as the source materials under high temperature (400and500℃) and high pressure (1GPa) conditions. The research shows that the CrO_2in theCrO_2-SnO_2composite oxides is very pure, and the saturation magnetization of CrO_2in the CrO_2-SnO_2composite oxides is very close to the theoretical value. The introduction of the SnO_2enhances MR effectively. The MR of the CrO_2-SnO_2composite oxides synthesized at400and500℃maximally increases-16.7%and-10.7%than that of pure CrO_2compacts synthesized under the same conditions,respectively. The MR curves of the CrO_2-SnO_2composite oxides also consist of thelow-field MR and the high-field MR at low temperatures. The low-field MR curves ofthe CrO_2-SnO_2composite oxides synthesized at500℃are fitted well by-(M/Ms)2curves. In the CrO_2-SnO_2composite 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 CrO_2grains is notnegligible. The resistivity of the CrO_2-SnO_2composite oxides is larger than that ofpure CrO_2compacts and increases with increasing x. The CrO_2-SnO_2compositeoxides 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 CrO_2-SnO_2composite oxides synthesized at400℃can be fitted well by FIT model at lowtemperature. With increasing temperature, the CrO_2-SnO_2composite 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 CrO_3and Sn was studied by HTHP method, and(CrO_2)_(1-x)-(SnO_2)_x(x=10%,20%and30%) composite oxides were prepared. Theresults show that the CrO_2-SnO_2composite oxides are dense and cavities decreasewith increase of x. The SnO_2was obtained by the reaction of Sn and oxygen from thedecomposition of the CrO_3, and it damaged the surface of CrO_2grains. The MRcurves of the CrO_2-SnO_2composite oxides also consist of the low-field MR and thehigh-field MR at low temperatures. The low-field MR of the CrO_2-SnO_2compositeoxides can be fitted well by higher-order terms of-(M/Ms)2, which implies that themagnetic coupling interaction between the CrO_2grains is not negligible. The MR of the CrO_2-SnO_2composite oxides is larger than that of pure CrO_2compacts. But theresistivity of the composite oxides is smaller. This is because of the introduction of Snwhich absorbs oxygen from the decomposition of the CrO_3and increases the structuredenseness. The-T curves of all the CrO_2-SnO_2composite 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.
1.以CrO_3为原料，利用高温高压方法在压强为1GPa，400-600℃的温度范围内成功合成了高纯度的CrO_2压结块体，并研究了微观结构对CrO_2块体磁输运性质的影响。研究表明，在500℃、550℃和600℃下合成的CrO_2晶粒表面存在Cr_2O_3绝缘层，厚度大约为6nm，而在400℃和450℃下合成的CrO_2晶粒表面并没有观察到Cr_2O_3绝缘层。CrO_2块体在5K下的饱和磁化强度高达131.7emu/g，非常接近于CrO_2的理论值（Ms=133emu/g），这说明CrO_2块体的纯度非常高。在500℃、550℃和600℃下合成的CrO_2块体磁电阻明显高于在400℃和450℃下合成的CrO_2块体磁电阻，这是由于绝缘层Cr_2O_3势垒的存在，增强了磁电阻效应。所有CrO_2块体在低温下的磁电阻随磁场的变化关系由两个区域组成：在低场区域（H10kOe），磁电阻随磁场增加迅速地增大；而在高场区域(H10kOe)，磁电阻随磁场增加近似于线性地缓慢增大。低场磁电阻随M/Ms的变化关系曲线可以利用(M/Ms)2的高阶项很好地拟合，这说明低场磁电阻来源于CrO_2晶粒间的自旋相关隧穿效应，同时CrO_2晶粒间的磁耦合作用不可忽略。高场磁电阻可能是CrO_2的固有属性，或者来源于磁场诱导隧穿势垒的改变。CrO_2块体的磁电阻随温度上升而快速地下降，这可能是由于CrO_2块体的自旋极化率随温度上升而减小的缘故，或者是CrO_2块体中自旋无关隧穿随温度上升而占主导地位。在500℃、550℃和600℃下合成的CrO_2块体表现为绝缘性，电阻率-温度（-T）曲线可以用Fluctuation-Induced Tunneling（FIT）模型解释。但是在240K以上，-T曲线开始背离FIT模型，这说明电导还可能来源于高阶非弹性隧穿的贡献。在400℃和450℃下合成的CrO_2块体在低温区域的-T曲线可以用FIT模型很好地拟合，然而随温度上升，CrO_2块体出现绝缘体-金属体（I-M）转变，这是晶界电阻和晶粒内电阻竞争的结果。在转变（TIM）温度以上，晶粒内电阻开始占主导地位，电阻率随温度T2呈线性变化，这说明电阻主要归结于晶粒内电子-电子散射和电子-声子散射。
2.以CrO_3和H2TiO_3为原料，利用高温高压方法在压强为1GPa，温度分别为400℃和500℃下合成了一系列(CrO_2)x-(TiO_2)1-x（x=80%，70%，60%，50%，40%）复合氧化物。400℃下合成的CrO_2-TiO_2复合氧化物中，纳米TiO_2为锐钛矿型结构，粒径大约为40nm；500℃下合成的CrO_2-TiO_2复合氧化物中，棒状TiO_2为金红石型结构，晶粒长度达到300nm。CrO_2-TiO_2复合氧化物的饱和磁化强度随CrO_2含量减少而系统地下降，并且复合氧化物中CrO_2的饱和磁化强度非常接近于CrO_2的理论值，这说明复合氧化物中CrO_2和TiO_2纯度非常高。通过引入绝缘体TiO_2，增强了CrO_2晶粒间的势垒，从而使CrO_2-TiO_2复合氧化物的磁电阻高于CrO_2块体的磁电阻。CrO_2-TiO_2复合氧化物在低温下的磁电阻曲线同样由两个区域组成，即低场磁电阻和高场磁电阻。复合氧化物的低场磁电阻曲线可以用-(M/Ms)2曲线拟合，这说明低场磁电阻来源于CrO_2晶粒间的自旋相关隧穿效应。另外，绝缘体TiO_2的引入使CrO_2-TiO_2复合氧化物的电阻率比同样条件下合成的CrO_2块体电阻率至少大一个数量级。CrO_2-TiO_2复合氧化物的-T曲线可以用FIT模型解释，但是在240K以上时，-T曲线开始背离FIT模型，这说明电导还可能来源于高阶非弹性隧穿的贡献。
3.向CrO_2中引入宽带隙半导体SnO_2，合成CrO_2-SnO_2复合氧化物。以CrO_3和SnO_2为原料，利用高温高压方法在压强为1GPa，温度分别为400℃和500℃下合成了一系列(CrO_2)1-x-(SnO_2)（xx=0%-80%）复合氧化物。研究表明，CrO_2-SnO_2复合氧化物中SnO_2纯度非常高，并且CrO_2的饱和磁化强度非常接近于理论值。SnO_2的引入增强了磁电阻效应，与相应的CrO_2块体磁电阻相比，400℃和500℃下合成的CrO_2-SnO_2复合氧化物磁电阻最大提高了-16.7%和-10.7%。CrO_2-SnO_2复合氧化物在低温下的磁电阻曲线同样由低场磁电阻和高场磁电阻两个区域组成。500℃下合成的CrO_2-SnO_2复合氧化物低场磁电阻曲线都可以用-(M/Ms)2曲线拟合。在400℃下合成的CrO_2-SnO_2复合氧化物中，当x60%时，低场磁电阻曲线可以用-(M/Ms)2曲线拟合；但是当x60%以后，低场磁电阻曲线开始背离-(M/Ms)2曲线，然而可以用(M/Ms)2的高阶项拟合，这表明x60%的复合氧化物中，CrO_2晶粒间磁耦合作用不可忽略。CrO_2-SnO_2复合氧化物的电阻率高于CrO_2块体的电阻率，并且随x增加而增大。在500℃下合成的CrO_2-SnO_2复合氧化物显示了绝缘性，-T曲线都可以用FIT模型解释，但是当T240K时，-T曲线开始背离FIT模型，这说明电导还可能来源于高阶非弹性隧穿的贡献。在400℃下合成的CrO_2-SnO_2复合氧化物在低温区域的-T曲线可以用FIT模型拟合，随温度升高，x60%的复合氧化物发生I-M转变，这是由于晶界电阻和晶粒内电阻竞争导致的结果。当T TIM时，晶粒内电阻开始起主导作用，-T2曲线呈线性关系，这说明电阻主要归结于晶粒内的电子-电子散射和电子-声子散射。
4.研究了CrO_3和金属Sn在高温高压下的反应，获得了高纯度的(CrO_2)_(1-x)-(SnO_2)_x（x=10%，20%，30%）复合氧化物。分析表明，随x增加，CrO_2-SnO_2复合氧化物的孔洞减少、结构致密。金属Sn吸收CrO_3释放的氧气，生成的SnO_2破坏了CrO_2晶粒表面。CrO_2-SnO_2复合氧化物在低温下的磁电阻曲线同样由两个区域组成，即低场磁电阻和高场磁电阻。低场磁电阻随M/Ms的变化关系曲线可以用(M/Ms)2的高阶项很好地拟合，这说明CrO_2晶粒间的磁耦合作用不可忽略。CrO_2-SnO_2复合氧化物的磁电阻高于CrO_2块体的磁电阻。但是，电阻率反而变小，这是因为Sn的引入，吸收了CrO_3释放的氧气，增加了结构致密性。CrO_2-SnO_2复合氧化物的-T曲线可以用FIT模型解释，但是当T240K时，-T曲线开始背离FIT模型，这说明电导还可能来源于高阶非弹性隧穿的贡献。
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 (CrO_2),magnetite and heusler alloys are all important research objects, as they have high spinpolarization. Among them, half-metallic magnet CrO_2gets the most widespreadattention. Because band structure calculations predicted100%spin polarization of theelectrons at the Fermi level of CrO_2. And Single-crystal CrO_2films have beenprepared with the spin polarization up to98.4%. So CrO_2is 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 CrO_2powder compacts andCrO_2-based composite oxides. In order to further enhance the MR effect, theCrO_2-based composite oxides are obtained by making a composite of CrO_2with asecondary phase. Now, the secondary phase materials include Cr_2O_3, TiO_2andpolymer etc. However, because CrO_2is metastable at ambient pressure, it is hard tosynthesize. Therefore, all these cold-pressed CrO_2powder compacts and CrO_2-basedcomposite oxides are made from commercial CrO_2powders. 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 CrO_2powdercompacts and CrO_2-based composite oxides made from different CrO_2sources otherthan commercial CrO_2powders. The MR of CrO_2particles with different sizes andshapes are also of great interest. Herein, in the thesis, high-purity CrO_2-TiO_2composite oxides and CrO_2-SnO_2composite oxides were synthesized using hightemperature and high pressure (HTHP) method. The influence of microstructure and secondary phase on magnetotransport property of the CrO_2-based composite oxideswas investigated detailedly.
1. With CrO_3as the precursor, pure CrO_2grains compacts were synthesizedunder400-600℃and1GPa conditions by HTHP method. The influence ofmicrostructure on magnetotransport property of the CrO_2compacts was studied. Theresults show that the surface of the CrO_2grains synthesized at500,550and600℃have a Cr_2O_3layer about6nm thick. However, the surface of the CrO_2grainssynthesized at400and450℃does not have Cr_2O_3layers. The saturationmagnetization (Ms) of the CrO_2compacts can reaches131.7emu/g at5K and is veryclose to the theoretical value (Ms=133emu/g, i.e.,2μBper formula unit). The MR ofthe CrO_2compacts synthesized at500,550and600℃is larger than that of the CrO_2compacts synthesized at400and500℃. This is because of the presence of theinsulating layer Cr_2O_3which improves the tunneling barrier and thus enhances theMR effect. The MR of all the CrO_2compacts 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 CrO_2compactscan 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 CrO_2crystals,and the magnetic coupling interaction between the CrO_2grains is not negligible. Thehigh-field MR of the CrO_2compacts might be intrinsic to CrO_2, 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 CrO_2compacts decreases quickly with increasingtemperature. This might be due to the decrease of the spin polarization of CrO_2withincreasing temperature. Spin-independent tunneling could also cause the decrease ofMR with increasing temperature, which becomes dominant at high temperature. TheCrO_2compacts synthesized at500,550and600℃display insulation property. Theresistivity-temperature (-T) curves of the CrO_2compacts can be fitted well by afluctuation-induced tunneling (FIT) model. But above240K, the resistivity of theCrO_2compacts 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 CrO_2compacts 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 (CrO_2)x-(TiO_2)1-x(x=80%,70%,60%,50%and40%) compositeoxides were prepared from CrO_3and H2TiO_3under1GPa,400and500℃conditionsby HTHP method. Nano TiO_2in the CrO_2-TiO_2composite oxides synthesized at400℃is anatase structure and the crystallite size of nano TiO_2is about40nm. TiO_2rodin the CrO_2-TiO_2composite oxides synthesized at500℃is rutile structure and itsgrain size reaches300nm in length. The saturation magnetization of the CrO_2-TiO_2composite oxides systematically decreases with the decrease in CrO_2concentration.And the saturation magnetization of CrO_2in the composite oxides is very close to thetheoretical value. This suggests that both CrO_2and TiO_2in the CrO_2-TiO_2compositeoxides are very pure, and defects such as Cr solution in TiO_2or Ti solution in CrO_2are neglectable. The introduction of TiO_2improves the tunneling barrier. So the MRof the CrO_2-TiO_2composite oxides is larger than that of pure CrO_2compacts. TheMR curves of the CrO_2-TiO_2composite 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 CrO_2-TiO_2composite 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 CrO_2grains. In addition, the resistivity ofthe CrO_2-TiO_2composite oxides is at least one order of magnitude larger than that ofthe CrO_2compacts synthesized under the same conditions. The-T curves of all theCrO_2-TiO_2composite 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 (CrO_2)_(1-x)-(SnO_2)_x(x=0%-80%) composite oxides were preparedby using CrO_3and SnO_2as the source materials under high temperature (400and500℃) and high pressure (1GPa) conditions. The research shows that the CrO_2in theCrO_2-SnO_2composite oxides is very pure, and the saturation magnetization of CrO_2in the CrO_2-SnO_2composite oxides is very close to the theoretical value. The introduction of the SnO_2enhances MR effectively. The MR of the CrO_2-SnO_2composite oxides synthesized at400and500℃maximally increases-16.7%and-10.7%than that of pure CrO_2compacts synthesized under the same conditions,respectively. The MR curves of the CrO_2-SnO_2composite oxides also consist of thelow-field MR and the high-field MR at low temperatures. The low-field MR curves ofthe CrO_2-SnO_2composite oxides synthesized at500℃are fitted well by-(M/Ms)2curves. In the CrO_2-SnO_2composite 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 CrO_2grains is notnegligible. The resistivity of the CrO_2-SnO_2composite oxides is larger than that ofpure CrO_2compacts and increases with increasing x. The CrO_2-SnO_2compositeoxides 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 CrO_2-SnO_2composite oxides synthesized at400℃can be fitted well by FIT model at lowtemperature. With increasing temperature, the CrO_2-SnO_2composite 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 CrO_3and Sn was studied by HTHP method, and(CrO_2)_(1-x)-(SnO_2)_x(x=10%,20%and30%) composite oxides were prepared. Theresults show that the CrO_2-SnO_2composite oxides are dense and cavities decreasewith increase of x. The SnO_2was obtained by the reaction of Sn and oxygen from thedecomposition of the CrO_3, and it damaged the surface of CrO_2grains. The MRcurves of the CrO_2-SnO_2composite oxides also consist of the low-field MR and thehigh-field MR at low temperatures. The low-field MR of the CrO_2-SnO_2compositeoxides can be fitted well by higher-order terms of-(M/Ms)2, which implies that themagnetic coupling interaction between the CrO_2grains is not negligible. The MR of the CrO_2-SnO_2composite oxides is larger than that of pure CrO_2compacts. But theresistivity of the composite oxides is smaller. This is because of the introduction of Snwhich absorbs oxygen from the decomposition of the CrO_3and increases the structuredenseness. The-T curves of all the CrO_2-SnO_2composite 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.
引文
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