尖晶石和钙钛矿体系中巨磁电阻效应及相关物性研究

【作者】 屈哲；

【导师】 张裕恒；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2006， 博士

【摘要】 强关联电子体系表现出很多吸引人的物理现象。特别是钙钛矿结构和尖晶石结构化合物中，由于它们所表现出的超大磁电阻效应（Colossal Magnetosistance，CMR）在提高磁存储密度以及磁敏感探测元件上具有十分广阔的应用前景，因而受到人们的广泛关注。同时，在这些强关联电子体系中还表现出诸如磁场或光诱导的绝缘体-金属转变、电荷有序、轨道有序、相分离等十分丰富的物理内容。这些物理问题涉及凝聚态物理中的许多基本问题，吸引了物理工作者广泛的关注，成为了当前物理研究中的热点之一。对超大磁电阻效应微观机制的研究，将会对凝聚态物理的许多领域的发展和完善起到重要的推动作用。 在本论文中，我们通过对铬基硫族尖晶石和钙钛矿锰氧化物进行掺杂研究，对CMR材料的物性以及磁阻机制进行了一些探索。部分内容已在国际科技期刊上发表。 本论文分为六章。 第一章简要介绍了凝聚态物理的强关联体系的基本概念，使我们对强关联体系物理有一个大致的了解。 第二章介绍了铬基硫族尖晶石和钙钛矿锰氧化物超大磁电阻材料丰富的物理内容，包括晶体结构，电子结构，磁性质，输运性质，掺杂效应，有序相和相分离等奇特的物理现象。通过本章，我们将了解到铬基硫族尖晶石和钙钛矿锰氧化物的基本物理性质，同时对诸如双交换作用，Jahn-Teller效应等物理概念有所认识，为进入该研究领域作好准备。 第三章研究了Fe位掺In对铬基硫族尖晶石化合物FeCr₂S₄的物性和磁电阻效应的影响。实验证明In离子掺入Fe位且以二价的形式存在。由于In离子的掺入导致在高于Tc时在顺磁背底里出现越来越明显的磁涨落。磁涨落引起了磁随机势的出现，并导致载流子被磁随机势局域。从而高温区的输运机制从未掺杂样品中的小极化子导电过渡到掺杂样品中的可变程机制。同时，通过研究FeCr₂S₄单晶样品电子顺磁共振谱的角度依赖，我们成功的解释了多晶样品中复杂的电子顺磁共振行为。 第四章研究了Cr位掺Mn对铬基硫族尖晶石化合物FeCr₂S₄磁性和磁输运性质的影响。实验证明Mn的磁矩与Cr的磁矩平行。同时，电阻随着掺杂增多降低，而磁电阻效应和居里温度随着掺杂增大，磁电阻效应峰也朝高温移动。这些结果提供了一种可行的既增大尖晶石材料磁电阻效应大小又使其磁电阻峰值温度升高的方法，这对于未来巨磁电阻效应的实际应用是很有帮助的。 第五章通过比较Mn位掺Cr和掺Ga对La_1-xSr_xMnO₃系列样品磁性的影响，证明Cr³⁺和Mn³⁺离子之间的超交换作用是铁磁的。这一结果对解决一直以来关于Cr³⁺和Mn³⁺之间相互作用的争论是有帮助的。更多还原

【Abstract】 Rich physics has been found in strongly correlated electron systems. Especially, chalco-genide spinels and perovskite mangnites have attracted considerable interests with an eye towards both an understanding of the colossal magnetoresistance （CMR） mechanism and potential applications in magnetic information store and low-field magnetic sensors. Beside the CMR effect, these materials also exhibit intriguing physical properties such as insulator-metal and/or structure transition induced by applied field or photo radiation, charge ordering, orbital ordering and phase separation etc. Once the mechanism of the CMR effect is fully elucidated, the progress in many fields of condensed matter physics will be definitely stimulated.In this thesis, the author devoted his effort to the study of the fundamental physics in Cr-based chalcogenide spinels and perovskite mangnites, by both experimental and theoretical methods.The whole thesis consists of six chapters.1. A brief introduction to the condensed matter physics and strongly correlated electron systems.In this chapter, some basic concepts, such as paradigm, symmetry broken, Fermi liquid theory and Mott insulator, are introduced. By these illustration, we may acquire a basic sight of the correlated electron physics.2. A brief overview of the progress of the CMR effect and related properties.This chapter devotes to to a review of the magnetoresistance phenomena and related physical properties of chalcogenide spinels and perovskite mangnites, such as the structural, magnetic, electronic transport, charge/orbital ordering and phase separation, etc. Some physics concept, such as double-exchange, Jahn-Teller effect, etc. are interpreted. This part is helping to build up a background for the research of the CMR effect and related properties of chalcogenide spinels and perovskite mangnites.3. The effect of Fe site In doping in Fe_1-xIn_xCr₂S₄.The Fe site doping is an effective method to explore the CMR effect in chalcogenide spinels. In this chapter, the effect of Fe site In doping in Fe_1-xIn_xCr₂S₄ is investigated. The results prove that In ions occupy Fe sites in the +2 valence state. The magnetic fluctuation is induced in the paramagnetic matrix with the In doping. Thus, the magnetic random potential appears and trapped the carrier. As a result, the transport mechanism in the high temperature region changes from small polaron hopping to variable-range hopping （VRH） with the introduce of In. By the aids of the angular dependent of the electron-更多还原