【作者】 赵强；

【导师】 毛河光； 温戈辉；

【作者基本信息】 吉林大学 ， 凝聚态物理， 2010， 博士

【摘要】 CrO₂是一种重要的半金属磁体,是目前凝聚态物理和自旋电子学领域研究的热点之一。但是CrO₂在常压下处于亚稳态,这使得其合成工作异常艰难。所以CrO₂的合成,尤其是CrO₂纳米结构的合成,及其物理性质的研究具有很重要的科学意义。本论文对CrO₂纳米结构的合成进行了探索,获得了一系列重要的结果。并对CrO₂纳米结构的磁、电性质进行了研究。采用高温高压方法合成了高纯度的CrO₂,并且通过对温度和压强的控制实现了CrO₂颗粒由直径为400nm粒子到长5μm棒的调控。研究了不同形状CrO₂颗粒的磁性。通过常压化学气相沉积法合成了具有不同取向的CrO₂薄膜。对其磁性研究发现,不同取向的CrO₂薄膜磁性存在差异。磁电阻研究表明CrO₂薄膜在室温及低温下呈现出由不同的机制所导致的磁电阻效应。借助于氧化铝模板,通过化学气相沉积法,首次合成大面积有序排列的单晶CrO₂纳米棒、纳米线阵列。磁性研究表明,CrO₂纳米棒、纳米线阵列具有优良的磁各向异性,在高密度垂直磁记录领域具有潜在应用价值。更多还原

【Abstract】 In recent years, half-metallic ferromagnets such as Chromium dioxide, magnetite, manganites and heusler alloys have been of considerable interest due to their potential applications in the next generation of electronic devices. Among them, Chromium dioxide （CrO₂） is very attractive. Because it has been reported with the highest spin polarization （>98%） ever measured by spin-polarized photoemission and point-contact Andreev reflection. And CrO₂ has good electronic conductivity as well as adequate Curie temperature about 395K, so it appears to be an ideal candidate for magnetic tunnel junctions and other spintronic devices. In addition, CrO₂ has been widely used in the magnetic recording industry for a long time and presents a high performance in magnetic recording devices. However, CrO₂ is a metastable oxide at ambient condition, it would decompose to Cr2O3 easily and which is known as the most stable one in all chromium oxides. This means that it is difficult to synthesize of purity CrO₂ at ambient pressure, in particular, CrO₂ nanostructures. In this thesis, we introduce some methods to synthesize high-purity CrO₂ powders and CrO₂ nanostructures （films and nanowires arrays）. The magnetic and electronic properties of CrO₂ powders and nanostructures were investigated.High-purity CrO₂ powders were prepared under high temperature （400 oC-600 oC） and high pressure （1GPa-5GPa） conditions. The shape, size and the grain boundary density of the CrO₂ powders can be tuned via simply changing the experiment conditions, such as temperature and pressure. We can tune the size of the CrO₂ powders from 5?m to several hundred. Magnetic properties of the CrO₂ powders prepared under different conditions were investigated also. The saturation magnetizations of the samples can reach to 105 emu/g, which accords with the best value ever reported. And we found that CrO₂ powders with low aspect ratio have smaller coercivity, which was due to the shape anisotropy. The synthesis of high-purity CrO₂ powders with controlled morphology could be of interest in tunneling magnetoresistance research, this may facilitate the further fundamental research on the TMR in granular CrO₂.（100）-oriented and （101）-oriented CrO₂ films had been grown on Al2O3 （001） and （012） substrates by atmospheric pressure CVD. The （100）-oriented CrO₂ film exhibited better crystalline qualities than （101）-oriented CrO₂ film. At room temperature, the MR of the （100）-oriented and （101）-oriented CrO₂ films are nearly same, and not sensitive to the qualities of the films, both show a linear dependence on magnetic field up to 2.5T. This result can be interpreted in terms of double-exchange mechanism. There are strong coupling between conduction electrons and local magnetic moment in CrO₂ films. When applied magnetic field aligns adjacent moments, the mobility of conduction electrons will be increased. Thus the resistance decreases with increasing magnetic field. While at 80K, the MR of the films exhibit a significant difference. The MR of both films decrease quickly at low fields, and decrease relatively slowly at higher fields. These phenomena imply that the MR in the CrO₂ films at 80K should mainly arise from intergrain tunneling MR. The intergrain tunneling MR is sensitive to the grain boundary density in CrO₂ films.In previous, most of the studies about CrO₂ were mainly focused on CrO₂ powder compacts. Only a few reports have been made on CrO₂ thin films and nanostructures patterned on especial substrates, despite the CrO₂ nanostructures are fundamental importance in the applications for high-density magnetic recording devices and spintronics devices. Here we further tried to prepare CrO₂ nanorod and nanowire arrays using AAO template- assisted chemical vapor deposition. Large area, vertically aligned CrO₂ nanorod and nanowire arrays with high density were obtained via the atmospheric pressure CVD assisted by AAO templates. The dense CrO₂ nanorod and nanowires arrange in a hexagonal close packed pattern over a large area, and the areal density can reaches to 3.3×10¹⁰ cm^-2. And the CrO₂ nanorod and nanowires show a high crystalline quality and uniform size. In our experiment, we found that in all cases the pores were not completely filled with CrO₂ nanowires. And the length of the CrO₂ nanowires is closely related to the pore diameter, which decreases with increasing pore diameter. The capillary action is introduced to explain the formation of the CrO₂ nanowires. The CrO₂ nanowires arrays exhibits excellent magnetic property even the interwires distance decrease to 65 nm, thus appears a good application in high-density magnetic recording devices. In addition, CrO₂ nanowires prepared by this method can separate from AAO template to form individual elements simply, and these individual crystalline CrO₂ nanowires may be ideal candidate in low-dimensionality half-metallic fundamental research and application in spintronic devices as building blocks.更多还原