Low Temperature Studies of Novel Magnetic Materials

Pennsylvania State Univ University Park, University Park PA

Investigators

Abstract

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