【作者】 李国庆；

【导师】 鲜于泽；

【作者基本信息】 东北大学 ， 材料物理与化学， 2000， 博士

【摘要】 庞磁电阻效应（CMR）因其极其敏感的潜在应用价值和包含的极其复杂磁相互作用机理，而成为近几年材料物理学和凝聚态物理学中的一个重要课题。本文用固相反应法制备所有样品，系统地研究了A位三元掺杂的AMnO₃类钙钛矿结构Mn基庞磁电阻材料的磁性、导电性和磁电阻效应。实验中发现了若干新现象，提出了关于庞磁电阻效应机制的进一步见解，丰富了对庞磁电阻效应的认识。 首先用振动样品磁强计较系统地研究了Ca掺杂（La，Nd）_1-xCa_xMnO₃（x=0.21，0.33，0.41）系类钙钛矿型Mn氧化物的磁性、导电性和磁电阻效应及其与掺杂量及烧结温度、烧结时间、烧结次数等工艺条件的关系。结果表明烧结温度和烧结次数等材料制备工艺的变化都会改变表证材料性能的居里温度T_C、电阻率转变温度T_P和磁电阻效应。材料的磁性能及输运特性对固相反应工艺敏感。提高烧结温度可以明显地改善材料的磁电阻性能，但是会降低材料的电阻率转变温度，使出现庞磁电阻效应的温度向低温方向移动；延长烧结时间和增加研磨次数，也可以使材料性能稳定，提高材料的庞磁电阻效应。碱土元素掺杂比例的改变使Mn的平均价态产生变化，同时也改变了体系组成的离子平均半径，从而影响到钙钛矿结构的畸变程度，在掺杂比例为0.21～0.41之间，存在着具有磁电阻性能和导电性转变温度T_P峰值的碱土掺杂比例。 在研究Ca掺杂Mn氧化物性能的基础上，开辟了不同碱土元素掺杂的La_1／3Nd_1／3B_1／3MnO₃（B=Ca，Ba，Sr，Pb）类钙钛矿型Mn氧化物系，用振动样品磁强计对其磁性、导电性和磁电阻效应进行了研究。结果表明低温下都表现出软铁磁性的四种碱土金属离子掺杂类钙钛矿型Mn氧化物在室温下，除了加Sr的样品外，其铁磁性特征都消失，说明室温时除了加Sr的样品外，都发生了铁磁性向顺磁性的磁相变。但室温下已经进入顺磁状态的掺Ca，Ba，Pb样品中La_1／3Nd_1／3Ca_1／3MnO₃材料的磁化曲线表现为完全的顺磁性，而La_1／3Nd_1／3B_1／3MnO₃（B=Ba，Pb）样品都表现出寄生铁磁性的磁化曲线特征。同时La_1：3Nd_1／3Ba_1／3MnO₃样品的电阻率随温度的变化在高于T_P的230K附近又出现另外一个对磁场敏感的电阻率转变峰，外磁场强烈抑制这一额外的峰。室温寄生铁磁性现象和电阻率转变双峰现象的出现都归因为材料中存在铁磁—反铁磁磁双相的竞争。 进而，用超导量子干涉仪研究了La_1／3Nd_1／3Ba_1／3MnO₃样品的磁性和La_1／3Nd_1／3B_1／3MnO₃（B=Ca，Ba，Sr）类钙钛矿型Mn氧化物大磁场下的输运特性。结果表明La_1／3Nd_1／3Ba_1／3MnO₃样品在13（?）e微弱磁场下零场降温和带场降温测试的热磁曲线的磁性表现出截然不同的特性。零场降温后的热磁曲线表现出反铁磁磁性的特征，材料的磁化强度随温度的上升而增加，在230K出现最大峰值。而带场降温测试过程中，材料经过磁相变点建立磁有序后，随温度的进一步降低，磁化强度继续增加，表现出典型的铁磁性特征。带场降温和零场降温曲线在高温区重合，但在峰值以下，磁矩表现出明显的更多还原

【Abstract】 The colossal magnetoresistant （CMR） effect has become an important research subject in the fields of material science and condensed matter physics since the recent years due to its potential sensitive application value and extremely complex magnetic interaction mechanism involved. In this work, Mn-based perovskite-like oxide AMnO₃ where A is substituted by three elements were obtained by so-called Solid Phase Reaction Method, and their magnetism, electrical conductivity and magnetoresistant effect were investigated systematically. All the new phenomena observed experimentally and new opinions put forward subsequently enriched the knowledge about CMR.The relationship between the CMR effect as well as properties concerned and stoichiometry along with the preparation conditions of sample such as sintering temperature, sintering period and grinding times was researched on the Vibrating Sample Magnetometer （VSM） for the （La, Nd）_1-xCa_xMnO₃（x=0.21, 0.33, 0.41）. The results show that the magnetic and transporting properties is quite sensitive to solid phase reaction procedure concluded by the fact that the change of either sintering temperature or grinding times, which reflect the preparation procedure, will affect both the Curie temperature T_C, electrical resistivity transitional temperature T_P and CMR effect, witch characterize the properties of materials. With the increase of either sintering temperature or grinding times, the CMR effect increases generally and both the Curie temperature T_C and electrical resistivity transitional temperature T_P decrease monotonously. Alkali earth element doping will change the average electronic valence of Mn ion, the average ion diameter and accordingly the distortion degree of perovskite structure that will lead to varying the magnetoresistivity. There is an optimizing peak for CMR and T_P when the doping amount is between 0.21 and 0.41（near 0.33）.Then, the same work as done above for （La, Nd）_1-xCa_xMnO₃（x=0.21, 0.33, 0.41） is done for La_1/3Nd_1/3B_1/3MnO₃（B=Ca, Ba, Sr, Pb）. All samples show soft ferromagnetism at low temperature, and the fact that their ferromagnetism except Sr doped disappears at room temperature indicates that ferromagnetism-paramagnetism phase transition has happened. While La_1/3Nd_1/3B_1/3MnO₃（B=Ba, Pb） shows interesting so-called Parasitic Ferro- magnetism at room temperature, only La_1/3Nd_1/3Ca_1/3MnO₃ is complete paramagnetism. And the curve of temperature dependence of electrical resistivity for La_1/3Nd_1/3Ba_1/3MnO₃ shows another peek sensitive to magnetic field, near 230K above T_P. The extra peak is restrained strongly by applied field. This is very likely aroused by double magnetic phases, which cause two critical conductive transitions. The reason for parasitic ferromagnetism and double peak of electrical resistivity transition was reduced to ferromagnetism-antiferromagnetism conflict that exists更多还原