【作者】 肖循；

【导师】 袁松柳；

【作者基本信息】 华中科技大学 ， 材料物理与化学， 2007， 博士

【摘要】 钙钛矿锰氧化物由于在居里温度Tc附近表现出庞磁电阻效应（CMR）,具有100 %自旋极化率及存在着复杂的电子、晶格、自旋等相互作用而成为凝聚态物理、材料物理等领域的主要研究对象之一。双交换作用是解释钙钛矿锰氧化物电输运行为和磁性质的重要理论,但是,掺杂怎样影响双交换作用以及异性离子间是否有铁磁作用一直是一个未解决的问题。本论文主要以钙钛矿锰氧化物La_2/3Ca_1/3MnO₃、La_2/3Sr_1/3MnO₃为研究对象,通过在其A位和B位进行特殊形式的掺杂,对掺杂体系的电输运和磁性质进行研究,从而为掺杂如何影响双交换作用、异性离子间是否有铁磁作用以及提高室温处的磁电阻效应提供实验和理论研究基础。主要研究内容包括如下几方面:1.介绍了磁电子学的相关内容,对最具应用前景的磁电子材料——钙钛矿锰氧化物的结构、电输运行为与磁性质进行了较全面的概括,综述了钙钛矿锰氧化物中掺杂对双交换和本征磁电阻的影响,在此基础上提出了论文的选题依据和研究意义。2.在A位掺杂研究中,通过改变A位平均离子半径<rA>以及固定<rA>改变失配因子σ2,研究了掺杂对双交换作用的影响以及样品的居里温度和CMR的变化规律。实验发现随着<rA>的减小,或者固定<rA>时随着σ2的增加,Mn-O-Mn键键角小于180o,双交换作用被削弱。因此,样品的Tc降低而CMR增大。3. Cr³⁺具有和Mn⁴⁺相同的电子组态,因此Cr³⁺与Mn³⁺间是否具有双交换作用是一个未解决的问题。通过保持[Mn³⁺]/([Mn⁴⁺]+[Cr³⁺])之比为择优比例2:1,采用特殊形式的B位Cr掺杂,制备了样品La_2/3+yCa_1/3-yMn_1-yCr_yO₃,并和采用通常的掺杂方式样品La_2/3Ca_1/3Mn_1-yCr_yO₃进行了电输运和磁性质的比较研究。实验发现Cr的掺杂对样品的Tc和绝缘体-金属转变温度Tp的下降影响较小;在低掺杂量时,La_2/3Ca_1/3Mn_1-yCr_yO₃的电阻-温度曲线有“双峰”现象,而La_2/3+yCa_1/3-yMn_1-yCr_yO₃的电阻-温度曲线没有“双峰”现象。同时,与La_2/3Ca_1/3Mn_1-yCr_yO₃比较,La_2/3+yCa_1/3-yMn_1-yCr_yO₃的磁化强度和导电性在低温区域增强。我们提出在Tc附近的高温区域,Mn³⁺与Cr³⁺之间存在着非常弱的双交换作用。但是当温度降低,这种铁磁作用导致Mn³⁺与Cr³⁺平行排列,因此Mn³⁺与Cr³⁺之间存在着的弱的双交换作用增强;所以在Mn³⁺与Cr³⁺间就有可能的电子传输,这将增强样品的电导。Mn³⁺-O-Cr³⁺间的作用类似于Mn³⁺-O-Mn⁴⁺间的双交换作用但比后者要弱一些。4.同样,当掺杂量y≤0.1时,La_2/3Sr_1/3Mn_1-yCr_yO₃的电阻-温度曲线有“双峰”现象,而La_2/3+ySr_1/3-yMn_1-yCr_yO₃的电阻-温度曲线没有双峰现象。与La_2/3Sr_1/3Mn_1-yCr_yO₃比较,La_2/3+ySr_1/3-yMn_1-yCr_yO₃的导电性在低温区域增强,而且其室温附近的CMR比采用通常掺杂方式的样品有较大幅度的提高。这可能是因为Mn³⁺与Cr³⁺之间存在着的弱的双交换作用随温度降低和磁场的作用增强引起的CMR的提高。5.保持Mn³⁺与Mn⁴⁺的比例为2:1,采用La_（2+4y）/3Ca_（1-4y）/3Mn_1-yCu_yO₃的形式掺杂,并和采用通常掺杂方式的样品La_2/3Ca_1/3Mn_1-yCu_yO₃进行了比较研究。实验发现Cu在La_2/3Ca_1/3Mn_1-yCu_yO₃中有助熔作用,但是在La_（2+4y）/3Ca_（1-4y）/3Mn_1-yCu_yO₃中则没有。由于助熔作用,La_2/3Ca_1/3Mn_1-yCu_yO₃的Tp几乎不下降,且磁电阻没有明显的增强;La_（2+4y）/3Ca_（1-4y）/3Mn_1-yCu_yO₃电阻率及其峰值随着掺杂量的增加而增加,而且Tp也随之下降,磁电阻在较宽的温度范围内均得到了增强。6. Cu对La_2/3Sr_1/3Mn_1-yCu_yO₃和La（2+4y）/3Sr（1-4y）/3Mn_1-yCu_yO₃都没有明显的助熔作用。随掺杂量y增加,样品Tp降低,电阻率升高。La（2+4y）/3Sr（1-4y）/3Mn_1-yCu_yO₃磁电阻值比La_2/3Sr_1/3Mn_1-yCu_yO₃有较大幅度的提高,在室温附近有较大的磁电阻效应。7. La_（2+4y）/3（Ca,Sr）_（1-4y）/3Mn_1-yCu_yO₃的Tc和Tp都低于样品La_2/3（Ca,Sr）_1/3Mn_1-yCu_yO₃的值,可见保持Mn³⁺与Mn⁴⁺的比例为2:1并没有使La_（2+4y）/3（Ca,Sr）_（1-4y）/3Mn_1-yCu_yO₃具有较好的铁磁性。这说明A位上两种元素的比例变化导致的容忍因子t、<rA>和σ²的改变也是影响双交换的重要因素。总之,以钙钛矿锰氧化物为基体,通过特殊设计的A位和B位掺杂方式,在一定程度上能阐明A位变化对双交换作用的影响及B位异性离子间是否有铁磁作用。这一方法为研究钙钛矿锰氧化物中的双交换理论提供了一个全新的思路,也能为寻求室温磁电阻材料提供参考。更多还原

【Abstract】 Peroveskite Manganites are the subject of intense study for the fields of condensed physics and materials physics in recent years due to the discovery of colossal Magnetoresistance （CMR） effect near Curie Temperature Tc and 100% spin polarization, as well as the complex interaction between electrons,lattices and spins in these compounds. Double Exchange （DE） is an important theory in interpretation of the electrical transport and magnetic properties of manganites. However, the effect of doping on DE and the possibility of DE hetero-ionic coupling between Mn and other ions has not yet been conclusive, and remains an open question. In this thesis, we base on the polycrystalline manganese oxides of La_2/3Ca_1/3MnO₃ and La_2/3Sr_1/3MnO₃, introduce the foreign elements to A- and B-sites by a special method. The electrical transport and magnetic properties of the doped manganites have been investigated, which provides experimental and theoretic basis for the enhancement of CMR at room temperature and whether or not DE coupling between hetero-ionic. The main investigations are shown as followed:1. An introduction of spintronics has been presented. More attentions have been paid on the structure, electrical transport behavior, and magnetic properties of perovskite manganites, which are considered as the most promising magneto electronics materials. On the basis of the effect to DE and CMR by substituting in perovskite manganites, the basic thinking of subject selection and investigation significance is put forward.2. A series of samples have been prepared by decreasing the value of <rA> and by fixing <rA> and increasingσ², in order to study the effect to DE and CMR by substituting at A-sites. It is found that the angle of Mn-O-Mn bond deviates 180o and the DE is weaken with the decreasing <rA> and increasingσ², which results in the decrease of T_c and an enhancement of CMR.3. Cr³⁺ has the same electronic configuration as Mn⁴⁺, so whether or not DE coupling between them remains an open question. A special formula of La_2/3+yCa_1/3-yMn_1-yCr_yO₃ with [Mn³⁺]/([Mn⁴⁺]+[Cr³⁺]) ratio fixed at optimal proportion 2:1 is designed and a comparison of the magnetic and electrical transport properties with La_2/3Ca_1/3Mn_1-yCr_yO₃. It is found that the substituting Cr for Mn in manganites shows inefficiency in lowering Tc and Tp. At lower doping level, there are two peaks in the resistivity versus temperature curve for La_2/3Ca_1/3Mn_1-yCr_yO₃ and the additional peak disappears for La_2/3+yCa_1/3-yMn_1-yCr_yO₃. Compared with La_2/3Ca_1/3Mn_1-yCr_yO₃, the magnetization and conductivity are enhanced in La_2/3+yCa_1/3-yMn_1-yCr_yO₃ in the low temperature range. we suggest that there exists the poor DE between Mn³⁺ and Cr³⁺ coupling at high temperature so that Cr³⁺ cannot play the role of Mn⁴⁺ in the Mn³⁺-O-Cr³⁺ interaction. With the decrease in temperature, Cr³⁺ partially plays the role of Mn⁴⁺ in the Mn³⁺-O-Cr³⁺ interaction, Mn³⁺ and Cr³⁺ ions turn to be favor of ferromagnetically ordering, probability of the electron transfer between Mn³⁺ and Cr³⁺ ions increases, which would promote the conductivity. The DE interaction between Mn³⁺ and Cr³⁺ is similar to the DE interaction between Mn³⁺ and Mn⁴⁺ but the former is weaker than that for the latter.4. Similarly, at y≤0.1, the resistivity versus temperature curve for La_2/3Sr_1/3Mn_1-yCr_yO₃ displays double peaks, however, there has an intrinsic characteristic peak in the curve of La_2/3+ySr_1/3-yMn_1-yCr_yO₃. Compared with La_2/3Sr_1/3Mn_1-yCr_yO₃, the conductivity is enhanced in La_2/3+yCa_1/3-yMn_1-yCr_yO₃ in the low temperature range. Furthermore, there is a visible enhancement of CMR around room temperature, which corresponds with the enhancement of the DE interaction between Mn³⁺ and Cr³⁺ with a decreasing in temperature.5. A series of double-doping samples of La_（2+4y）/3Ca_（1-4y）/3Mn_1-yCu_yO₃ with the Mn³⁺/Mn⁴⁺ ratio fixed at 2:1 have been prepared and compared to La_2/3Ca_1/3Mn_1-yCu_yO₃ by the measurement of the structural, magnetic and electrical transport properties. It is found that Cu doping acts the role of fluxing action in La_2/3Ca_1/3Mn_1-yCu_yO₃ but cannot act it in La_（2+4y）/3Ca_（1-4y）/3Mn_1-yCu_yO₃. The Tp of La_2/3Ca_1/3Mn_1-yCu_yO₃ maintains around the Tp of La_2/3Ca_1/3MnO₃, but the Tp of La_（2+4y）/3Ca_（1-4y）/3Mn_1-yCu_yO₃ shifts towards lower temperature with the increasing of the doping content, meanwhile, the magnetoresistance of La_（2+4y）/3Ca _（1-4y）/3Mn_1-yCu_yO₃ is enhanced at a wide temperature window.6. In the samples La_2/3Sr_1/3Mn_1-yCu_yO₃ and La（2+4y）/3Sr（1-4y）/3Mn_1-yCu_yO₃, Cu doping cannot act the role of fluxing action in those manganites. The Tp of all samples shift towards lower temperature region with the increasing of doping content and the resistivity increase. Compared to La_2/3Sr_1/3Mn_1-yCu_yO₃, the CMR effect is enhanced in La（2+4y）/3Sr（1-4y）/3Mn_1-yCu_yO₃ especially at the room temperature region.7. The Tc and Tp of La_（2+4y）/3（Ca,Sr）_（1-4y）/3Mn_1-yCu_yO₃ are lower than that of La_2/3（Ca,Sr）_1/3Mn_1-yCu_yO₃, which implies that the ratio of Mn³⁺/Mn⁴⁺ maintained at 2:1 except for Cu2+ cannot make samples a good ferromagnetism. The influence of tolerance factor, <rA> andσ2 by altering the ratio of A-sites elements on magnetic properties and DE may be the main factor for the double-doping manganites.As a conclusion, the special doping method on A- and B-sites in manganites is a distinct technique to clarify the influence on DE and experimentally test the possibility of DE hetero-ionic coupling between Mn and other ions in a certain extent. Such a process presents a new way to study DE in manganites, which would be beneficial to search out the room temperature magnetoresistance materials also.更多还原