Magnetic dilution and domain selection in the X Y pyrochlore antiferromagnet Er 2 Ti 2 O 7
Résumé
Below TN = 1.1 K, the XY pyrochlore Er2Ti2O7 orders into a k = 0 non-collinear, 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 Er 3+ is substituted by non-magnetic Y 3+. 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 Y 3+ substitutions between 10%-Y and 20%-Y, far removed from the 3D 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.
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