Magnetic dilution and domain selection in the XY pyrochlore antiferromagnet Er2Ti2O7

Below TN=1.1 K, the XY pyrochlore Er2Ti2O7 orders into a k=0 noncollinear, antiferromagnetic structure referred to as the ψ2 state. The magnetic order in Er2Ti2O7 is known to obey conventional three-dimensional (3D) percolation in the presence of magnetic dilution, and in that sense is robust to disorder. Recently, however, two theoretical studies have predicted that the ψ2 structure should be unstable to the formation of a related ψ3 magnetic structure in the presence of magnetic vacancies. To investigate these theories, we have carried out systematic elastic and inelastic neutron scattering studies of three single crystals of Er2−xYxTi2O7 with x=0 (pure), 0.2 (10%Y) and 0.4 (20% Y), where magnetic Er3+ is substituted by nonmagnetic Y3+. We find that the ψ2 ground state of pure Er2Ti2O7 is significantly affected by magnetic dilution. The characteristic domain selection associated with the ψ2 state, and the corresponding energy gap separating ψ2 from ψ3, vanish for Y3+ substitutions between 10% Y and 20% Y, far removed from the three-dimensional percolation threshold of ∼60% Y. The resulting ground state for Er2Ti2O7 with magnetic dilutions from 20% Y up to the percolation threshold is naturally interpreted as a frozen mosaic of ψ2 and ψ3 domains.