title: Vacancy defects and monopole dynamics in oxygen-deficient pyrochlores.
abstract: The idea of magnetic monopoles in spin ice has enjoyed much success at intermediate temperatures, but at low temperatures a description in terms of monopole dynamics alone is insufficient. Recently, numerical simulations were used to argue that magnetic impurities account for this discrepancy by introducing a magnetic equivalent of residual resistance in the system. Here we propose that oxygen deficiency is the leading cause of magnetic impurities in as-grown samples, and we determine the defect structure and magnetism in Y2Ti2O7-δ using diffuse neutron scattering and magnetization measurements. These defects are eliminated by oxygen annealing. The introduction of oxygen vacancies causes Ti(4+) to transform to magnetic Ti(3+) with quenched orbital magnetism, but the concentration is anomalously low. In the spin-ice material Dy2Ti2O7 we find that the same oxygen-vacancy defects suppress moments on neighbouring rare-earth sites, and that these magnetic distortions markedly slow down the long-time monopole dynamics at sub-Kelvin temperatures.
authors.affiliation: Department of Physics, Royal Holloway, University of London, Egham TW20 0EX, UK.
authors.forename: G
authors.initials: G
authors.lastname: Sala
authors.affiliation: ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, UK.
authors.forename: M J
authors.initials: MJ
authors.lastname: Gutmann
authors.affiliation: Department of Physics, University of Oxford, Oxford OX1 3PU, UK.
authors.forename: D
authors.initials: D
authors.lastname: Prabhakaran
authors.affiliation: Department of Physics and Astronomy and Guelph-Waterloo Physics Institute, University of Waterloo, Waterloo, Ontario N2L3G1, Canada.
authors.forename: D
authors.initials: D
authors.lastname: Pomaranski
authors.affiliation: Department of Physics and Astronomy and Guelph-Waterloo Physics Institute, University of Waterloo, Waterloo, Ontario N2L3G1, Canada.
authors.forename: C
authors.initials: C
authors.lastname: Mitchelitis
authors.affiliation: Department of Physics and Astronomy and Guelph-Waterloo Physics Institute, University of Waterloo, Waterloo, Ontario N2L3G1, Canada.
authors.forename: J B
authors.initials: JB
authors.lastname: Kycia
authors.affiliation: Department of Physics, Royal Holloway, University of London, Egham TW20 0EX, UK.
authors.forename: D G
authors.initials: DG
authors.lastname: Porter
authors.affiliation: Theory of Condensed Matter group, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, UK.
authors.forename: C
authors.initials: C
authors.lastname: Castelnovo
authors.affiliation: Department of Physics, Royal Holloway, University of London, Egham TW20 0EX, UK.
authors.forename: J P
authors.initials: JP
authors.lastname: Goff