Low Temperature Studies of Novel Magnetic Materials

这个项目是一个研究项目，研究在低温下表现出不寻常物理性质的磁性材料。这项研究将集中在两类材料上：几何受挫磁体和钙钛矿型锰氧化物，尽管这项研究也将扩展到探索其他有趣的磁性材料的物理，如铁磁半导体。几何受阻磁体是一种材料，其中原子自旋之间的相互作用由于结构有序的磁子晶格的几何形状而相互竞争。这些材料已被证明在低温下表现出各种奇异的行为，包括在最小无序、自旋液体状态和“自旋冰”(与冻结水中发现的受挫质子态的磁性相似)存在下的玻璃性。这项研究将探索几何受挫磁铁的物理学，重点是研究大型稀土自旋系统中的玻璃态慢动力学和量子弛豫。钙钛矿型锰氧化物是以锰为基础的磁性氧化物。这些材料的磁自由度、电子自由度和晶格自由度之间具有很强的耦合性，并且它们表现出一系列不寻常的物理性质。最近的实验和理论工作表明，本征磁电相分离成电荷有序和导电的铁磁区是这些材料物理中的一个重要元素。这项研究将研究不同磁电子相的微观共存所产生的新的磁现象。一个研究领域将是与相分离相关的时间依赖行为的调查。这一行为类似于在自旋玻璃材料中观察到的行为，但它的起源却截然不同，因为它是由不同相的大规模共存引起的。第二个研究领域将是研究不寻常的低温超磁转变，这种转变非常尖锐，只有在远低于自旋-自旋相互作用能量标度的温度下才会出现。本课题研究的重点是磁性材料的物理性质。这些材料很重要，因为它们展示了在其他系统中无法访问的基本原理。它们在技术上也很重要，特别是在计算机内存应用中。这项工作的影响将是更好地理解这些令人着迷的材料系统。特别是，这项研究将阐明一类被称为几何受挫磁铁的材料的性质。在这些材料中，磁性原子的排列使得原子相互作用的局部能量不能在整个系统中同时最小化。了解磁体中的几何受挫对超导结阵列和神经网络等各种复杂系统都有影响，并有可能提供对系统的洞察，这些系统可能构成未来新的计算范例的基础。另一类将被研究的材料是“巨磁电阻”锰氧化物，这是一种当置于磁场中时其电阻发生剧烈变化的材料。这项研究将集中在它们在低温下的不同寻常的相分离特性，即样品自发地分成导电的微观区域和不导电的区域。这项研究的更广泛影响将是广泛的学生的教育体验。首席调查员在研究过程的每个阶段都有与研究生和本科生合作的良好记录，拟议的研究将支持这一教育

This project is a research program to study magnetic materials that display unusual physical properties at low temperatures. The research will focus on two classes of materials: geometrically frustrated magnets and perovskite manganites, although the research will also extend to explore the physics of other interesting magnetic materials such as ferromagnetic semiconductors. Geometrically frustrated magnets are materials in which the interactions between atomic spins compete with each other due to the geometry of a structurally well-ordered magnetic sublattice. These materials have been shown to display a variety of exotic behavior at low temperatures, including glassiness in the presence of minimal disorder, spin liquid states, and "spin ice" (a magnetic analog to the frustrated protonic state found in frozen water). The research will probe the physics of geometrically frustrated magnets with particular emphasis on investigations of glassy slow dynamics and quantum relaxation in systems of large rare earth spins. The perovskite manganites are magnetic oxides based on manganese. These materials have a strong coupling between the magnetic, electronic, and lattice degrees of freedom, and they show a wide range of unusual physical properties. Recent experimental and theoretical work has indicated that intrinsic magnetoelectronic phase separation into charge-ordered and conducting ferromagnetic regions is an important element of the physics of these materials. The research will investigate novel magnetic phenomena resulting from this microscopic coexistence of different magnetoelectronic phases. One area of research will be investigations of time-dependent behavior related to phase separation. This behavior is analogous to that observed in spin glass materials, yet it is quite different in origin, since it arises from the large scale coexistence of different phases. A second area of research will be an investigation of unusual low temperature metamagnetic transitions, which are extremely sharp and appear only at temperatures well below the energy scale of the spin-spin interactions. This research project focuses on the physical properties of magnetic materials. Such materials are important in that they demonstrate fundamental principles which cannot be accessed in other systems. They also are important technologically, particularly in computer memory applications. The impact of this work will be in the development of a better understanding of these fascinating material systems. In particular, the research will elucidate the nature of a class of materials known as geometrically frustrated magnets. In these materials, the magnetic atoms are arranged such that the local energy of the atomic interactions cannot be simultaneously minimized throughout the system. Understanding geometrical frustration in magnets has implications for complex systems as diverse as superconducting junction arrays and neural networks, and has the potential for providing insight into systems that may form the foundation for novel future computational paradigms. Another class of materials, which will be studied is the "colossal magnetoresistance" manganites, which are materials whose electrical resistance changes drastically when placed in a magnetic field. The research will focus on their unusual phase-separated properties at low temperatures, whereby the samples spontaneously separate into microscopic regions, which conduct electricity and regions which do not.The broader impact of the research will be in the enhanced educational experience of a broad range of students. The principal investigator has a strong record of working with both graduate and undergraduate students in every stage of the research process, and the proposed research would support this educational