Strongly Geometrically Frustrated Magnets

Abstract

The study of cooperative phenomena in magnetism has provided fertile ground for testing theories of interacting systems that possess different spatial dimensions, ranges, and sign of interactions, and that exhibit local anisotropy of the basic interacting unit, the magnetic spin. This study has also motivated the development of new classes of materials, from the oldest known type of magnets, namely ferromagnets, to modern substances embodied in the unusual random field and spin glass compounds. In this context, we use the term material class to mean a set of compounds that share both microscopic, as well as macroscopic, or bulk, properties. Thus for example, ferro magnets possess the microscopic uniform ferromagnetic­ type exchange or dipolar interaction between spins, in addition to a bulk low temperature magnetization approaching the theoretical saturated moment value, and characteristic critical behavior at the Curie, or order­ ing, temperature. Among the known classes of magnets, spin glasses are among the most fascinating, displaying in their bulk properties simul­ taneous sharp ordering features in their magnetic response while exhibiting no such anomalies in their thermal response (1). These properties are thought to arise from a ground state characterized not by a single potential well representing the uniform arrangement of perfectly ordered spins, as in a ferromagnet, but rather by an energy landscape with many nearly degenerate ground state configurations separated by barriers of random height ( 1 ). The microscopic parameters empirically associated with spin glass