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Vacancy defects and monopole dynamics in oxygen-deficient pyrochlores

Nature Materials volume 13, pages 488–493 (2014)Cite this article

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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 Ti4+ to transform to magnetic Ti3+ 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.

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Figure 1: Oxygen-vacancy defect structure in Y2Ti2O7−δ.
Figure 2: Composition dependence of the diffuse scattering.
Figure 3: Magnetism on the Ti ions in Y2Ti2O7−δ.
Figure 4: Defect structure and magnetism on the Dy ions in Dy2Ti2O7−δ.
Figure 5: Monopole trapping by oxygen vacancies in Dy2Ti2O7.

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Acknowledgements

We thank M. Jura and T. Lehner for their help, and B. Gaulin, M. T. Hutchings, K. Refson, R. Moessner, S. L. Sondi, S. Dutton and U. Karahasanovic for helpful discussions. We acknowledge support from the South East Physics Network and the Hubbard Theory Consortium, and we are grateful for the financial support and hospitality of ISIS. This work was supported in part by EPSRC grants EP/G049394/1, EP/H033939/1 and EP/K028960/1, NSERC, the Helmholtz Virtual Institute ‘New States of Matter and Their Excitations’, and the EPSRC NetworkPlus on ‘Emergence and Physics far from Equilibrium’.

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Authors and Affiliations

  1. Department of Physics, Royal Holloway, University of London, Egham TW20 0EX, UK

    G. Sala, D. G. Porter & J. P. Goff

  2. ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, UK

    M. J. Gutmann

  3. Department of Physics, University of Oxford, Oxford OX1 3PU, UK

    D. Prabhakaran

  4. Department of Physics and Astronomy and Guelph-Waterloo Physics Institute, University of Waterloo, Waterloo, Ontario N2L3G1, Canada

    D. Pomaranski, C. Mitchelitis & J. B. Kycia

  5. Theory of Condensed Matter group, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, UK

    C. Castelnovo

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  1. G. Sala
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Contributions

J.P.G., C.C. and D. Prabhakaran designed the research. The neutron measurements were performed by G.S., M.J.G. and J.P.G.; the X-ray diffraction was performed by D.G.P. The crystals were grown by D. Prabhakaran The d.c. magnetization was measured by D. Prabhakaran and G.S. and the magnetic relaxation measurements were performed by D. Pomaranski, C.M. and J.B.K. Theoretical modelling and simulation was by G.S., M.J.G. and C.C. The manuscript was drafted by J.P.G., C.C. and G.S. and all authors participated in the writing and review of the final draft.

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Correspondence to J. P. Goff.

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Sala, G., Gutmann, M., Prabhakaran, D. et al. Vacancy defects and monopole dynamics in oxygen-deficient pyrochlores. Nature Mater 13, 488–493 (2014). https://doi.org/10.1038/nmat3924

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