【作者】 唐洁；

【导师】 袁松柳； 夏正才；

【作者基本信息】 华中科技大学 ， 材料物理与化学， 2004， 硕士

【摘要】 钙钛矿锰基氧化物中由于存在复杂的电子、晶格、自旋等相互作用而成为凝聚态物理、材料物理等的主要研究对象之一。特别是在其中发现了具有潜在应用前景的超大磁电阻效应(CMR),更加激发了人们对它的研究热情。但是,对于纯钙钛矿锰基氧化物单晶材料,其本征磁电阻效应只有在较高的外加磁场(约几特斯拉)和较窄的温度范围内才能达到实际应用所需的大小。对于多晶材料,虽然可以在较低的外磁场下获得明显的磁电阻效应,但这类磁电阻效应主要出现在低温范围(远低于居里温度),且它的大小随温度升高而迅速减小,对温度有很强的敏感性,因而直接将钙钛矿锰基氧化物应用于实际器件中还存在一些技术问题。对于基本结构为“铁磁金属-中间层-铁磁金属”的材料来说,在低场下均表现出较大的磁电阻效应,而且铁磁金属的自旋极化度越高,磁电阻就越大。由于锰基氧化物的自旋极化率很高(可以达到100%),因此在本论文中我们选择以La2/3Ca1/3MnO3为铁磁金属,通过与Cu的复合,对它的颗粒边界进行异质改性,来改变La2/3Ca1/3MnO3基体边界的电、磁性质,从而达到改善磁电阻效应的目的我们考虑是这种结构本身起了很重要的作用。论文主要包括以下几个方面:对锰基钙钛矿氧化物的电子结构、本征CMR效应、非本征磁电阻效应及其机理做了简要的概述。并介绍了截止目前人们针对两相复合体系,改变铁磁金属及其之间的中间层以改善低场磁电阻效应方面所取得的一些进展。并在此基础上提出本论文所研究的内容和方向。介绍钙钛矿锰基氧化物的制备工艺,着重介绍了溶胶-凝胶法。经过对比 发现溶胶-凝胶法制备材料更方便,简单,制备的材料性质更好。研究了La2/3Ca1/3MnO3的电、磁性质和磁电阻效应,指出烧结温度的高低影响颗粒尺寸大小,导致表相和体相在结构中所占的比重不同,很大程度上影响了材料的电、磁和磁电阻效应。采用两种方法制备Cu包覆LCMO颗粒体系,分别是CuO机械混合法和硝酸铜<WP=5>化学法。通过比较发现,化学法制备的样品,均匀性程度更高,重复性更好。同时我们发现样品烧结温度对样品性质的影响很大,因为它直接决定颗粒尺寸,尺寸太大和太小都不利于Cu对La2/3Ca1/3MnO3的包覆。最终我们确定了制备复合样品的最佳制备工艺。对我们制备的(1-x)La2/3Ca1/3MnO3/xCu复合体系的电、磁性质和磁电阻效应作了研究。发现在低场(0.3T)下复合样品在低于T<Tp出现高的磁电阻峰,但它不是La2/3Ca1/3MnO3的本征CMR效应。我们认为这种非本征的磁电阻峰的产生跟Cu的引入有关。Cu离子本身是一种磁性离子,在La2/3Ca1/3MnO3的颗粒边界引入一层Cu相关层,额外的磁性自旋也被引入到颗粒边界,增强了传导电子在颗粒边界的自旋相关散射。在外加磁场下,自旋磁矩有序排列,降低了电子自旋散射,电阻减小,从而产生大的磁电阻,达到55%,比几个特斯拉磁场下在纯LCMO中得到的最大磁电阻还要大。对于x=8.5%的复合样品在高场(5T)下磁电阻峰变宽,我们将其解释为本征CMR效应和非本征磁电阻峰同时在复合体系中同时存在的结果,是两种磁电阻效应的叠加。更多还原

【Abstract】 Manganese oxides with peroveskite structure have been the subject of intense study in recent years due to the complex interaction between electrons,lattices and spins. Specially, the potential technological application related to the colossal manganetoresistance (CMR) effect has activated the interesting on the materials. However, for a single crystal of a pure manganese oxide, the intrinsic CMR can be observed only at a high magnetic field (~ several Teslas) and in a narrow temperature window near Curie temperature. For a crystalline, although a higher magnetoresistance was observed at a lower magnetic field in the temperature much lower than the Curie temperature, the low field magnetoresistance decreased rapidly with increasing in the temperature. Thus, there are many technological problems as the manganese oxides were used to the electronic devices. The material which has the structure “FM/ML/FM” shows the low field MR(LFMR),where FM and ML represents the ferromagnetic metal and middle layer,respectively.So,we try to optimize the structure to enhance the LFMR and the sensitivity to the magnetic field.For the ferromagnetic metals,MR is higher with the higher spin polarization,so we try to bring Cu into the grain- boundaries of La2/3Ca1/3MnO3,changing the its electric and magnetic properties,to enhance the MR.In this thesis, our main investigation including:(1)A summary of the formation and its physical problems of the structure, properties and magnetoresistance in the manganese oxides has been given. We summarize the research work related to the FM/ML/FM material,and the fundamental of theory and experiment of the thesis has put forward on the basis of it. (2) The influences of different preparation procedures are compared and optimal one is chosen.We use the sol-gel to produce the manganese oxides. The electric ,magnetic properties and magnetoresistance of La2/3Ca1/3MnO3 are investigated.(3) For the different preparation procedures of composite La2/3Ca1/3MnO3/Cu,we observed <WP=7>that sintering temperature evidently affectes the results by changing the grain size of La2/3Ca1/3MnO3 ,the larger and smaller size both go against the effect.(4)A composite containing La2/3Ca1/3MnO3(LCMO) and Cu dependent material is investigated.Compared to LCMO,the composite has a lower insulator-metal transition temperature(TP) and a substantial reduction in the resistivity over the whole temperature range studied.For a low applied magnetic field(0.3T),the composite displays a high magnetoresistance (MR) peak near (TP) and a temperature-independent MR at low temperatures, a behavior diffenrent form LCMO that shows a monotonical increase in MR with cooling. The MR peak is different from the intrinsic CMR of LCMO,Our observation may reveal the special features of MR due to the introduction of Cu dependent material into the grain-boundary of LCMO,which is the extrinsic CMR. It is also found that the behavior of resistivity minimum appears at low temperature in the zero-field, but suppressed gradually in the applied field.We consider it from the spin-polarized scattering.更多还原