Low Temperature Studies of Novel Magnetic Materials

该项目是一项研究计划，旨在研究在低温下显示不寻常物理特性的磁性材料。 该研究将集中在两类材料上：几何受抑磁体和钙钛矿锰氧化物，尽管该研究还将扩展到探索其他有趣的磁性材料的物理学，如铁磁半导体。几何阻挫磁体是其中原子自旋之间的相互作用由于结构良好有序的磁子晶格的几何形状而彼此竞争的材料。 这些材料已经被证明在低温下表现出各种奇异的行为，包括在最小无序、自旋液态和“自旋冰”（在冷冻水中发现的受抑质子态的磁性类似物）存在下的玻璃状。该研究将探索几何受挫磁体的物理学，特别强调大型稀土自旋系统中的玻璃慢动力学和量子弛豫的研究。钙钛矿锰氧化物是基于锰的磁性氧化物。 这些材料在磁、电子和晶格自由度之间具有很强的耦合，并且它们显示出广泛的不寻常的物理特性。 最近的实验和理论工作表明，电荷有序和导电铁磁区域的内在磁电子相分离是这些材料的物理学的一个重要因素。 这项研究将调查新的磁现象所造成的这种微观共存的不同磁电子阶段。 研究的一个领域将是与相分离相关的时间依赖行为的调查。 这种行为类似于在自旋玻璃材料中观察到的行为，但它在起源上完全不同，因为它是由不同相的大规模共存引起的。 研究的第二个领域将是对不寻常的低温变磁转变的调查，这种转变非常尖锐，只在远低于自旋-自旋相互作用能量标度的温度下出现。本研究项目的重点是磁性材料的物理特性。 这些材料很重要，因为它们展示了在其他系统中无法获得的基本原则。 它们在技术上也很重要，特别是在计算机存储应用中。 这项工作的影响将是更好地理解这些迷人的材料系统的发展。特别是，这项研究将阐明一类被称为几何受挫磁体的材料的性质。 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。 理解磁体中的几何挫折对超导结阵列和神经网络等复杂系统具有影响，并有可能提供对可能形成未来新计算范式基础的系统的洞察。 另一类将被研究的材料是“巨磁阻”锰氧化物，这是一种当置于磁场中时电阻会发生剧烈变化的材料。 该研究将重点关注它们在低温下不寻常的相分离特性，即样品自发地分离成微观区域，这些区域导电，而区域不导电。该研究的更广泛影响将是增强广大学生的教育体验。 主要研究者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