Nature

Letter

Nature 461, 956-959 (15 October 2009) | doi:10.1038/nature08500; Received 18 June 2009; Accepted 14 September 2009

Measurement of the charge and current of magnetic monopoles in spin ice

S. T. Bramwell1,5, S. R. Giblin2,5, S. Calder1, R. Aldus1, D. Prabhakaran3 & T. Fennell4

  1. London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, 17–19 Gordon Street, London WC1H 0AH, UK
  2. ISIS Facility, Rutherford Appleton Laboratory, Chilton, Oxfordshire OX11 0QX, UK
  3. Department of Physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, UK
  4. Institut Laue-Langevin, 6 rue Jules Horowitz, 38042 Grenoble, France
  5. These authors contributed equally to this work.

Correspondence to: S. T. Bramwell1,5 Correspondence and requests for materials should be addressed to S.T.B. (Email: s.t.bramwell@ucl.ac.uk).

The transport of electrically charged quasiparticles (based on electrons or ions) plays a pivotal role in modern technology as well as in determining the essential functions of biological organisms. In contrast, the transport of magnetic charges has barely been explored experimentally, mainly because magnetic charges, in contrast to electric ones, are generally considered at best to be convenient macroscopic parameters1, 2, rather than well-defined quasiparticles. However, it was recently proposed that magnetic charges can exist in certain materials in the form of emergent excitations that manifest like point charges, or magnetic monopoles3. Here we address the question of whether such magnetic charges and their associated currents—'magnetricity'—can be measured directly in experiment, without recourse to any material-specific theory. By mapping the problem onto Onsager's theory of electrolytes4, we show that this is indeed possible, and devise an appropriate method for the measurement of magnetic charges and their dynamics. Using muon spin rotation as a suitable local probe, we apply the method to a real material, the 'spin ice' Dy2Ti2O7 (refs 5–8). Our experimental measurements prove that magnetic charges exist in this material, interact via a Coulomb potential, and have measurable currents. We further characterize deviations from Ohm's law, and determine the elementary unit of magnetic charge to be 5

mu

B Å-1, which is equal to that recently predicted using the microscopic theory of spin ice3. Our measurement of magnetic charge and magnetic current establishes an instance of a perfect symmetry between electricity and magnetism.

  1. London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, 17–19 Gordon Street, London WC1H 0AH, UK
  2. ISIS Facility, Rutherford Appleton Laboratory, Chilton, Oxfordshire OX11 0QX, UK
  3. Department of Physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, UK
  4. Institut Laue-Langevin, 6 rue Jules Horowitz, 38042 Grenoble, France
  5. These authors contributed equally to this work.

Correspondence to: S. T. Bramwell1,5 Correspondence and requests for materials should be addressed to S.T.B. (Email: s.t.bramwell@ucl.ac.uk).

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http://www.nature.com/nature/journal/v451/n7174/full/nature06433.html

Letter

Nature 451, 42-45 (3 January 2008) | doi:10.1038/nature06433; Received 27 June 2007; Accepted 29 October 2007

Magnetic monopoles in spin ice

C. Castelnovo1, R. Moessner1,2 & S. L. Sondhi3

  1. Rudolf Peierls Centre for Theoretical Physics, Oxford University, Oxford OX1 3NP, UK
  2. Max-Planck-Institut für Physik komplexer Systeme, 01187 Dresden, Germany
  3. PCTP and Department of Physics, Princeton University, Princeton, New Jersey 08544, USA

Correspondence to: C. Castelnovo1 Correspondence and requests for materials should be addressed to C.C. (Email: castel@physics.ox.ac.uk).

Electrically charged particles, such as the electron, are ubiquitous. In contrast, no elementary particles with a net magnetic charge have ever been observed, despite intensive and prolonged searches (see ref. 1 for example). We pursue an alternative strategy, namely that of realizing them not as elementary but rather as emergent particles—that is, as manifestations of the correlations present in a strongly interacting many-body system. The most prominent examples of emergent quasiparticles are the ones with fractional electric charge e/3 in quantum Hall physics2. Here we propose that magnetic monopoles emerge in a class of exotic magnets known collectively as spin ice3, 4, 5: the dipole moment of the underlying electronic degrees of freedom fractionalises into monopoles. This would account for a mysterious phase transition observed experimentally in spin ice in a magnetic field6, 7, which is a liquid–gas transition of the magnetic monopoles. These monopoles can also be detected by other means, for example, in an experiment modelled after the Stanford magnetic monopole search8.

  1. Rudolf Peierls Centre for Theoretical Physics, Oxford University, Oxford OX1 3NP, UK
  2. Max-Planck-Institut für Physik komplexer Systeme, 01187 Dresden, Germany
  3. PCTP and Department of Physics, Princeton University, Princeton, New Jersey 08544, USA

Correspondence to: C. Castelnovo1 Correspondence and requests for materials should be addressed to C.C. (Email: castel@physics.ox.ac.uk).

Abstract