Ferromagnetic fragmented ground state in the Ho2Ru2O7 pyrochlore

by Dr Flavien Muser (Institut Néel, CNRS and Université Grenoble Alpes)

Wednesday 5 Jul 2023, 15:00 → 16:00 Europe/Zurich

SZ-WBGB/019

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 neutrons scattering experiments [1]. In addition, specific heat measurements show that there is an incomplete release of the entropy expected for Ising degrees of freedom [2]. This points towards a description of spin ices using an emergent field, which can be fragmented into two components using the Helmholtz decomposition: one providing the magnetic monopole charge and the other one a fluctuating background [3]. The titanates pyrochlore have no magnetic monopoles at low temperature; but in the dysprosium and holmium iridate pyrochlores, the Iridium atoms order antiferromagnetically at about 130 K and provide a staggered local field to the rare-earth spins, promoting monopoles in the ground state. The physics of dipolar Ising pyrochlores can then be brought together in the form of a fragmentation phase diagram [4].
In this talk 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. From the theoretical point of view I investigated the effect of quantum fluctuations on several fragmented phase [5]. My main experimental project was a detailed study of 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. 
The other experimental part of my PhD was centered around the exploration of the fragmentation phase diagram through the application of hydrostatic pressure. I performed neutron diffraction and specific heat measurements under pressure on the Ho₂Ir₂O₇ and Dy₂Ir₂O₇ respectively. These experiments proved challenging to interpret, but seemed to show a reduction in ordered moment and a decrease in specific heat consistent with a change towards a standard spin ice phase. 
 

References:
[1] T. Fennell et al., Science 326, 415 (2009).
[2] A.P. Ramirez et al.,  Nature 399, 333 (1999).
[3] M.E. Brooks-Bartlett et al., Physical Review X 4, 011007 (2014)
[4] E. Lhotel, L.D.C. Jaubert, P.C.W. Holdsworth, Journal of Low Temperature Physics 201, 710-737 (2020).
[5] F. Museur, E. Lhotel, and P. C. W. Holdsworth, Phys. Rev. B 107, 214425 (2023).