Spherical Neutron Polarimetry and the Phase Diagram of Magnetically Dilute Er₂Ti₂O₇

The ground state of XY pyrochlore antiferromagnet Er₂Ti₂O₇ has been a rich topic of research due to the selection of a non-coplanar k=0 magnetically ordered state ψ₂ through order by disorder. Several proposed mechanisms account for this selection of ψ₂ over the coplanar ψ₃, the alternate basis state of the Γ₅ irreducible representation. Recent theory work has proposed that magnetic dilution that will lead to an instability in the ground state selection, resulting in a ψ₃ ground state for a dilute Er₂Ti₂O₇. Through systematic substitution of non-magnetic Y³⁺ on the Er³⁺ site, we have probed the effect of such dilution with samples of 5%, 10%, and 20% Y³⁺. While the magnetic structures of ψ₂ and ψ₃ are indistinguishable in an unpolarized neutron scattering measurement, the contrast is clear using spherical neutron polarimetry(SNP). In this talk, I will present the results of our SNP study of these 3 diluted samples, following our previous work using inelastic neutron scattering.