Spin-Charge-Lattice Couplings in the Mn Oxide Systems

取代的La锰氧化物(准立方和层状化合物)提供了一个在局域结构水平上研究自旋-电荷-晶格关联的难得机会，并将这种关联与宏观现象如巨磁电阻(CMR)和电荷有序(CO)联系起来。局域探针研究(扩展X射线吸收精细结构(EXAFS)和中子对分布函数(PDF)分析)表明，随着零场铁磁转变温度T_c的升高，极化子诱导的结构发生了显著的变化。此外，还有一个与CMR行为有关的大而反常的磁致伸缩。这一个人研究人员奖支持一个项目，该项目首次确定作为磁场函数的场诱导的局部结构变化，并将EXAFS测量扩展到几种层状化合物。高场测量将探索磁化最初是如何在Tc以下发展的。对层状材料的测量将确定局部变形的各向异性。这些都是在这些系统中发展更好的铁磁性和电输运模型的重要结果。建议的项目将为研究生提供论文主题，并为本科生提供几个高级论文主题。学生们将在PI的指导下进行团队合作。他们将获得在国家机构进行研究的经验。随着样品在低温下磁化，一些被取代的La锰氧化物(部分La被钙或锶取代)显示出Mn-O键的局域原子扭曲的异常减少。在接近磁转变温度的温度下，样品的电阻率也有很大的变化，并且有很大的反常磁致伸缩(施加磁场时样品大小的变化)。这一个人研究人员奖支持一个项目，该项目旨在调查在较大的外加磁场中局部失真是如何变化的，并将这些结果与样品磁化、磁致伸缩和电阻率相关联。这些结果还应该确定这些材料中磁化过程的初始阶段是如何发生的。将对层状锰氧化物化合物进行类似的测量-但此处需要分别确定各层中和垂直于各层中的扭曲。建议的项目将为研究生提供论文主题，并为本科生提供几个高级论文主题。学生们将在PI的指导下进行团队合作。他们将获得在国家设施进行研究的经验。

The substituted Lanthanum manganites (quasicubic and layered compounds) provide an unusual opportunity to investigate spin-charge-lattice correlations at the local structure level and connect the correlations with macroscopic phenomena such as Colossal Magnetoresistance (CMR) and charge ordering (CO). Local probe studies (Extended X-ray Absorption Fine Structure (EXAFS) and neutron Pair Distribution Function (PDF) analysis) show that there is a significant, polaron-induced, structural change as the temperature is increased through the ferromagnetic transition temperature, Tc, in zero field. In addition, there is a large and anomalous magnetostriction that is connected with the CMR behavior. This individual investigator award supports a project to determine for the first time the field-induced changes in the local structure as a function of magnetic field and extend the EXAFS measurements to several of the layered compounds. The high field measurements will probe how the magnetization initially develops just below Tc. The measurements on the layered materials will determine the anisotropy of the local distortions. These are important results for developing better models of ferromagnetism and electrical transport in these systems. The proposed project will provide the thesis topic for a graduate student as well as several senior thesis topics for undergraduates. The students will work as a team under the PI. They will gain the experience of performing research at a national facility.Some of the substituted the Lanthanum manganites (with part of the La replaced with Ca or Sr) show an unusual decrease in the local atomic distortions of the Mn-O bond as the sample becomes magnetized at low temperatures. For temperatures near the magnetic transition temperature there is also a large change in the resistivity of the sample and a large anomalous magnetostriction (a change in sample size when a magnetic field is applied). This individual investigator award supports a project to investigate how the local distortion changes in a large applied magnetic field and to correlate these results with the sample magnetization, magnetostriction and electrical resistivity. These results should also determine how the initial stages of the magnetization process take place in these materials. Similar measurements will be carried out on a layered manganite compound - but here the distortions in the layers and perpendicular to the layers need to be determined separately. The proposed project will provide the thesis topic for a graduate student as well as several senior thesis topics for undergraduates. The students will work as a team under the PI. They will gain the experience of performing research at a national facility.