【作者】 张彩萍；

【导师】 温戈辉；

【作者基本信息】 吉林大学 ， 凝聚态物理， 2014， 博士

【摘要】 具有独特的能带结构和较高自旋极化率的半金属磁性材料由于其重要的理论研究意义以及在自旋电子学器件领域广泛的应用前景而成为凝聚态物理的研究热点。事实上，从上世纪九十年代开始，人们就尝试把半金属磁体应用到自旋电子学器件当中。根据Julliere模型，铁磁金属/绝缘层/铁磁金属隧道结的磁电阻主要取决于绝缘层两侧的铁磁层中传导电子自旋极化率，它随着传导电子的自旋极化率的增加而增大。这一结果引发了科研工作者极大地兴趣，随后，对具有高自旋极化率的半金属磁体，例如霍伊斯勒合金，钙钛矿锰氧化物以及CrO2的研究接踵而至。而CrO2作为一种非常重要的过渡金属二元氧化物铁磁材料具有接近100%的传导电子自旋极化率，这已被自旋分辨光电子谱以及Andreev反射法证实。同时，CrO2还具有优良的导电性和较高的居里温度，因此成为自旋电子学应用领域的理想材料。但是CrO2在常压下属于压稳态，很容易分解为稳定的Cr2O3相。这使得CrO2的合成工作具有很大的挑战性。到目前为止，人们对CrO2的研究主要集中在CrO2粉末压结体和CrO2复合材料，CrO2纳米结构以及CrO2薄膜。在CrO2薄膜方面，主要集中在利用化学气相沉积技术，在(100)取向的TiO2，(001)取向的Al2O3以及多晶TiO2基底上制备出CrO2薄膜，并对其输运性质进行了研究，而对其它基底上生长的CrO2薄膜的微观结构以及微观结构对其输运性质的影响很少涉及；在CrO2复合材料方面，主要集中在对CrO2复合压结体的合成及其输运性质的研究，而对CrO2复合薄膜的研究没有涉及；此外利用化学气相沉积法在不同基底上很难制备出较薄的CrO2薄膜，难以满足自旋电子学器件的要求。本文我们首先研究了CrO2薄膜的微观结构对其输运性质的影响；其次，在单晶Si基底上合成出CrO2-Al2O3复合薄膜，并对其输运性质进行了研究；此外研究了CrO2在TiO2纳米管基底上的生长及其输运性质；最后在高氧压环境下制备出超薄的CrO2和Fe3O4薄膜。1.分别在(001)和(012)取向的蓝宝石基底上制备出(100)和(101)取向的CrO2薄膜。(100)取向的CrO2薄膜表面光滑且平整，而(101)取向的CrO2薄膜表面粗糙且具有较高的晶界密度。不同取向的CrO2薄膜的电阻R都随着温度T的升高而增大，表现为金属性。当温度低于35K时，(100)和(101)取向CrO2薄膜的电阻与温度没有明显的相关性，我们可以认为在温度低于35K时，薄膜中没有显著地自旋反转散射。在130-250K之间，电阻R与温度T2呈线性变化关系。这主要归因于CrO2晶粒内的电子-电子散射和电子-声子散射。在T>250K时，电阻R与温度T2开始背离线性关系。此外，在5-300K的温度范围内，(101)取向的CrO2薄膜的电阻率远远大于(100)取向的CrO2薄膜，这主要归因于(101)取向的CrO2薄膜具有较高的晶界密度以及含有少量的反铁磁性绝缘体Cr2O3。当温度为5K，磁场H<16kOe时，(100)取向CrO2薄膜表现出隧道磁电阻效应，当磁场16kOe<H<50kOe时，洛伦兹磁电阻效应起主导作用；而对于(101)取向的CrO2薄膜在温度为5K时，完全表现隧道磁电阻效应。这表明洛伦兹磁电阻和隧道磁电阻，与薄膜的质量和磁场有关。随着温度的升高，隧道磁电阻逐渐消失。室温下，CrO2薄膜的磁电阻是由其本征磁电阻决定的，不受薄膜质量的影响，可以用双交换机制来解释。2.利用化学气相沉积技术，以CrO3和AlBr3作为蒸发源，在(100)取向的单晶Si基底上，首次制备出CrO2-Al2O3复合薄膜。复合薄膜由较大的CrO2微粒以及围绕在其周围的较小的绝缘体氧化物Al2O3纳米颗粒组成。CrO2-Al2O3复合薄膜中各成分的含量是通过改变蒸发源CrO3的温度来调整的。研究表明，随着蒸发源CrO3的温度的升高，复合薄膜中CrO2的分子的摩尔百分比逐渐增加。研究了Al2O3的摩尔分数为0.38的CrO2-Al2O3复合薄膜的输运性质，低温时，复合薄膜为绝缘体，电阻R随着温度T的增加而减小，当T<120K，其导电机制可以用FIT模型很好的解释。随着温度的升高，CrO2-Al2O3复合薄膜出现绝缘体-金属的转变，其转变温度为175K。此外，在低温时，其表现为隧道磁电阻，当温度为5K，磁场为50kOe时，增强的隧道磁电阻达-15.2%。随着温度的升高，隧道磁电阻几乎消失，负的线性磁电阻起主导作用，这种负的线性磁电阻可以用双交换机制解释。3.研究了在TiO2纳米管基底上CrO2多晶薄膜的生长及输运性质。在实验过程中，蒸发源CrO3的温度不变，通过控制反应时间来控制薄膜的厚度。研究表明，在5-300K的温度区域内，不同生长时间得到的CrO2多晶薄膜的电阻都随着温度的升高而迅速减小，表现为绝缘性。这说明CrO2晶粒之间存在间隙，且这些间隙起到了势垒作用。在低温区域，CrO2多晶薄膜的导电机制可以用FIT模型很好的解释。随着温度的升高，实验数据开始背离由FIT模型拟合的曲线。这说明CrO2多晶薄膜开始显现出其它导电机制。此外，在低温时，不同生长时间得到的CrO2多晶薄膜都表现隧道磁电阻效应，生长时间为0.5h,1h,2h,3h时合成的CrO2多晶薄膜，在温度为5K，磁场为50kOe时，磁电阻值MR分别为-20.8%,-18.4%,-16.6%,-13.8%。表明随生长时间的增加，间隙势垒降低。随着温度的升高，隧道磁电阻逐渐减小，当温度在200K以上时，隧道磁电阻几乎消失，负的线性磁电阻起主导作用，这种负的线性磁电阻可以用双交换机制解释。4.研究了高氧压环境下Cr膜和Fe膜的氧化和磁学性质。研究表明，对于Cr薄膜，高氧压氧化后，在薄膜中会生成CrO2相，但薄膜中仍存在Cr以及Cr的其它氧化物，且薄膜的矫顽力为120Oe。对于Fe薄膜，高氧压氧化后，形成Fe3O4和Fe2O3薄膜相，且薄膜在室温下的矫顽力为415Oe。更多还原

【Abstract】 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.更多还原