Exotic Ground States: A Study in the Structural Effects on Frustration and Dimensionality

Abstract

Structural disorder and dimensionality play central roles in the characterization of structure and properties of crystalline materials. Although structural disorder is commonly considered an undesirable quality, structural disorder may be desirable when searching for new materials with exotic properties. Disorder can be used as a tunable parameter when considering atomic sizes, coordination preferences, and electronegativity differences can be varied by substituting elements into a given structure. Dimensionality can be viewed as another adjustable parameter when searching for new materials presenting unique challenges when characterizing new materials. One way to tune dimensionality is by chemical doping to affect coupling between layers/chains. The growth of phases with inherent structural disorder often involves tuning at the edge of structural stability which necessitates careful adjustment of synthetic parameters and presents unique characterization challenges. Understanding the effects of disorder and dimensionality could lead to better understanding of complex behavior. Many materials with exotic magnetic ground states exhibit magnetic frustration in one form or another. Magnetic frustration is a term which loosely describes the suppression of long-range magnetic ordering to temperatures much lower than the Weiss temperature in phases with magnetic correlations. Three parameters linked to magnetic frustration include: geometry of the magnetic sublattice, site disorder, and the effective dimensionality of the magnetic sublattice. Often these three parameters are inseparable in real systems, and investigating the fundamental differences between these geometrical frustration, structural disorder, and dimensionality would be of significant interest. This dissertation presents the growth, structure, and properties of single crystals of 3-d structurally disordered intermetallic phases Ln2Ag1-xGa10-y (Ln = La, Ce) and â-LnNiGa4 (Ln = Tb-Er), 2-d frustrated spin glasses MAl2S4 (M = Mn, Fe, Co, Ni), and quasi-1-d quantum, antiferromagnet K4Cu(MoO4)3 to highlight the effects of structural disorder and dimensionality on magnetic ground states.