Neutron Scattering study of Highly Frustrated Magnetic Materials

The behaviour of magnetic materials is primarily governed by the type and the strength of the exchange interactions within the system. These interactions force the magnetic moments of different atoms to be aligned either parallel or antiparallel to each other and in ordinary systems this results in the familiar cases of ferromagnetic and antiferromagnetic order. Nowadays there is a reasonable understanding of the physics of simple systems without competing interactions. Many systems, however, exhibit competing interactions, that is, interactions that do not all favour the same ordered state. In this case not all interactions can be minimised simultaneously - for example, the antiferromagnetic exchange interactions on a triangular lattice, and the system is said to be magnetically frustrated. Sufficiently strong competition (a high degree of frustration) leads to new physics that is manifested by the appearance of noncolinear ordering, novel critical exponents, rich phase diagrams, or an absence of long-range magnetic order at low temperatures, leading to magnetic analogues of liquids and ice.We propose to investigate the magnetic properties of a family of highly frustrated compounds by means of neutron scattering and bulk property measurements (magnetisation, ac and dc-susceptibility, specific heat). We propose to grow single crystals of these compounds, so that the all important questions of the influence of the single ion anisotropy and the interplay of the frustration and possible magnetic low-dimensionality can be addressed thoroughly. The scientific merit of the proposal, however, goes far beyond the field of solid-state magnetism. The effect of frustration on the statistical properties of many-body interacting systems (for example the associated glassiness and universal slowing down of their dynamics) has far reaching consequences in areas as diverse as protein folding, neural networks and the general theory of complexity.