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Flavien Museur presents

 Fragmentation in frustrated pyrochlore magnets: theory and experiment

Friday, May 26th 2023 at 10:00 am

Seminar room – Building A – CNRS

The defence will be in English. 



Frustrated magnets are a fascinating class of both theoretical models and materials. They have been a fruitful playground to test the limits of standard paradigms in statistical physics, as well as identify exotic phase transitions. Some of the most striking examples are the Holmium and Dysprosium titanates, which were proven to be realizations of so-called spin ices. In these materials, Ising-like spins occupy sites of a pyrochlore lattice, pointing in or out of the tetrahedra forming the lattice. They interact ferromagnetically so in the lowest energy configuration the spins satisfy a two in / two out ice-rule. No magnetic order is found down to 50 mK, but correlations develop in the structure factor in the form of characteristic pinch-point patterns which were observed in neutron scattering experiments. In addition, specific heat measurements show that there is an incomplete release of the entropy expected for Ising degrees of freedom. This points towards a description of spin ices using an emergent divergence-free field, which connects spin ices to a wider range of phenomena in condensed matter and statistical physics. Excitations off the low energy manifold of states form topological defects in the emergent field. As a consequence the emergent field is fragmented into two components: one providing the magnetic charge and the other one a fluctuating background. This innocuous mathematical property gains experimental relevance in Dysprosium and Holmium iridate pyrochlores, where the Iridium atoms order antiferromagnetically at about 130 K and provide a staggered local field to the rare-earth spins. The competition with the ferromagnetic exchange shifts the ground state towards a so-called monopole crystal. It can be described as a coexistence of an antiferromagnetic ordered phase with half of the ordered moment, and of a fluctuating phase with ferromagnetic correlations and a divergence-free constraint, associated with pinch-point scattering patterns and residual entropy. The physics of dipolar Ising pyrochlores can then be brought together in the form of a fragmentation phase diagram.
In this defense I will present some results of my PhD work, whose objective was to expand the knowledge of this phase diagram with an emphasis on a close collaboration between theory and experiment. First we studied theoretically the persistence of the fragmented phase at low temperatures where quantum fluctuations could become relevant. Then, we investigate the Holmium ruthenate pyrochlore in which Ho magnetic moments exhibit a transition at 1.5 K. We perform low temperature magnetic, specific heat and neutron scattering measurements and find that the Holmium ions have a partial ordered moment, a small residual entropy and peculiar diffuse scattering patterns. We interpret these results within the framework of fragmentation as a new fragmented structure, where the ordered fragment is this time of ferromagnetic nature