Article

Real-space observation of emergent magnetic monopoles and associated Dirac strings in artificial kagome spin ice

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Abstract

Magnetic monopoles have been predicted to occur as emergent fractional quasiparticles inside pyrochlore spin ice, a frustrated magnetic insulator. Experimental signatures of such emergent monopoles accompanied by Dirac strings have been detected by means of neutron scattering in reciprocal space in pyrochlore spin ice at sub-Kelvin temperatures, but their real-space observation has remained elusive. Here we report on direct, real-space observations of emergent monopoles and their associated Dirac strings in two-dimensional (2D) artificial kagome spin ice at room temperature using synchrotron X-ray photoemission electron microscopy. Magnetization reversal proceeds through the nucleation and avalanche-type dissociation of monopole–antimonopole pairs along 1D Dirac strings. This is in sharp contrast to conventional domain growth in 2D systems, providing a striking example of dimensional reduction due to frustration. The observed hysteresis, monopole densities and 1D Dirac-string avalanches are quantitatively explained by Monte Carlo simulations.

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Acknowledgements

The authors would like to thank: M. Horisberger, E. Deckardt, A. Weber, C. David, A. Steger, L. Le Guyader, A. Kleibert, J. Raabe, D. Eastwood and D. Atkinson for their help. We also acknowledge helpful discussions with V. Lobaskin, W. Nahm and M. Sigrist. This work was supported by the Swiss National Science Foundation and the Science Foundation of Ireland (08/RFP/PHY1532 and 05/IN1/I853), and part of this work was carried out at the Swiss Light Source, Paul Scherrer Institute, Villigen, Switzerland. We also acknowledge receipt of an HEA Ireland equipment fund.

Author information

Author notes

    • Arantxa Fraile Rodríguez

    Present address: Departament de Física Fonamental and Institut de Nanociència i Nanotecnologia (IN2UB), Universitat de Barcelona, 08028 Barcelona, Spain

Affiliations

  1. Paul Scherrer Institute, 5232 Villigen PSI, Switzerland

    • Elena Mengotti
    • , Laura J. Heyderman
    • , Arantxa Fraile Rodríguez
    •  & Frithjof Nolting
  2. School of Physics, University College Dublin, Dublin 4, Ireland

    • Remo V. Hügli
    •  & Hans-Benjamin Braun

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Contributions

Sample preparation: E.M.; measurements: E.M., A.F.R.; analysis and interpretation: E.M., L.J.H., H.B.B.; theory and simulations: R.V.H., H.B.B.; preparation of the manuscript: E.M., L.J.H., H.B.B.; supervision of the project: L.J.H., H.B.B., F.N. All authors discussed the results and implications, and commented on the manuscript at all stages.

Competing interests

The authors declare no competing financial interests.

Corresponding authors

Correspondence to Laura J. Heyderman or Hans-Benjamin Braun.

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