Ground State Selection in the XY Pyrochlore Magnet Er2Ti2O7 and its Stability to Chemical Pressure and Quenched Impurities

Er₂Ti₂O₇ is a quantum S_eff=1/2 XY pyrochlore magnet that orders into a non-linear, non-coplanar Néel state below T=1.2 K via ground-state selection proposed to originate from an order-by-disorder mechanism [1,2]. As the ground state selection is relatively weak, it is interesting to examine its stability to perturbations such as magnetic dilution and chemical pressure. Indeed, recent theory strongly suggests rich phase diagrams resulting from such instabilities [3,4,5,6]. Magnetic dilution was achieved by substituting a small fraction of the S_eff=1/2 Er³⁺ ions for non-magnetic Y³⁺ ions; while chemical pressure resulted from the replacement of non-magnetic Ti⁴⁺ ions within the pyrochlore structure by other non-magnetic ions such as Pt⁴⁺. I will discuss our recent experimental results [7,8], with a focus on neutron scattering, where these chemical perturbations demonstrate the richness of the phase space in which XY pyrochlore magnets, such as Er₂Ti₂O₇, reside - a consequence of anisotropic exchange and order-by-disorder effects. [1] L. Savary et al., Phys. Rev. Lett. 109, 167201 (2012), [2] M.E. Zhitomirsky et al., Phys. Rev. Lett. 109, 077204 (2012) [3] Maryasin, V. S., and M.E. Zhitomirsky, Phys. Rev. B 90, 094412 (2014), [4] Andreanov, A., and P. A. McClarty, Phys. Rev. B 91, 064401 (2015), [5] H. Yan et al., Phys. Rev. B 95, 094422 (2017) [6] E.C. Andrade et al., arXiv:1710.06658 (2017), [7] J. Gaudet et al., Phys. Rev. B 94, 060407(R) (2016), [8] A. M. Hallas et al., Phys. Rev. Lett. 119, 187201 (2017)