The role of structural defects in frustrated magnetism

Orateur : Jon GOFF (Royal Holloway, University of London)

Pyrochlore systems are ideally suited to the exploration of geometrical frustration in three dimensions, and their rich phenomenology encompasses topological order and fractional excitations. The sample dependence of their physical properties and the identification of different magnetic ground states for nominally stoichiometric samples clearly points towards the importance of low levels of structural disorder. Quantum spin liquids have been elusive experimentally, in part because disorder induces competing glassy states instead of entangled ones. However, recent theoretical predictions suggest the presence of structural disorder can be used to stabilise classical and quantum spin liquids, and it can lead to new magnetic degrees of freedom, the formation of topological spin glasses and the formation of entirely novel quantum spin liquids. Here I shall describe how diffuse neutron scattering studies have been used to determine the decisive role of structural defects in the magnetism of frustrated pyrochlores. In the case of the classical spin ice dysprosium titanate we have shown that oxygen vacancies are responsible for slowing down the long-time monopole dynamics at sub-Kelvin temperatures. For the quantum analogue ytterbium titanate, samples in which the oxygen vacancies have been eliminated by annealing in oxygen exhibit a transition to a ferromagnetic phase, and this is the true magnetic ground state. The presence of low levels of oxygen vacancies stabilizes a spin liquid phase.

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