Trapping magnetic monopoles in spin ice

The characteristic diffuse neutron scattering measured using SXD

The characteristic diffuse neutron scattering measured using SXD at ISIS from an oxygen-depleted pyrochlore
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Electrically isolated charges are common in nature, most notably in the form of individual electrons. However the equivalent in magnetism, the magnetic monopole, has proved far more elusive. Magnetic monopoles have been observed as so-called quasi-particles in spin-ice - a type of exotic magnet - but the model only works at certain temperatures. New research published in the journal Nature Materials used the SXD instrument at ISIS to show that oxygen deficiencies could explain why at sub-Kelvin temperatures experimental results don’t match up to predictions. This understanding is crucial in the hunt for the magnetic monopole, detecting magnetic currents and the design of future spin-ice devices.

Magnetic monopoles were predicted back in 1931 by Paul Dirac. But, unlike electrons, we just don’t seem to be able to find them. Magnetic particles are usually observed as dipoles – a north and south combined. The existence of magnetic monopoles has important implications – it would explain the quantisation of electric charge, make Maxwell’s unification of electricity and magnetism more elegant, and support a number of grand unification theories. However, despite exhaustive searches by condensed matter scientists, particle physicists and astrophysicists, so far the hunt has failed.

Emergent magnetic monopoles were predicted in spin ice1, and in 2009 it was reported in the journal Science that neutron scattering experiments provided compelling evidence in support of this picture2,3.  Since then the idea has been successful at intermediate temperatures, but in some experiments at temperatures below 1K, just using monopole dynamics to describe the system wasn’t enough.

In particular, magnetisation dynamics in spin-ices happens on far longer timescales than predicted. Numerical simulations suggest that this could be down to defects in the crystal structure of the spin ice, causing the magnetic equivalent of residual resistance. A group of researchers led by Jon Goff at Royal Holloway, University of London, have proposed that oxygen deficiency is the cause.

The group used the SXD instrument at ISIS to study oxygen-deficient Y2Ti2O7-δ and Dy2Ti2O7- δ. Diffuse neutron scattering showed that oxygen vacancies were the most common form of defect in as-grown samples. This overturns previous assumptions that magnetic impurities were due to “stuffing” of the Ti lattice with rare earth ions. The group ran Monte Carlo simulations to reproduce the key features in the diffuse neutron scattering .

The group then looked at  the magnetism of Dy2Ti2O7 as-grown and after annealing in oxygen to eliminate oxygen vacancies. Susceptibility measurements showed that in the annealed samples magnetic impurities were almost eliminated, suggesting that oxygen vacancies were the cause.

Professor Jon Goff said, “We have clearly demonstrated that oxygen vacancies play a key role in monopole dynamics at low temperatures. Understanding these defects is crucial for experiments directed at single-monopole detection, the observation of monopole currents, and the design of potential spin-ice devices.”

Devices that use electrical current are everywhere - if we can harness magnetic currents in a similar way there is potential for huge industrial impact.

Professor Goff adds, “Our results help us to understand the resistance to the flow of magnetic charges, a phenomenon that is likely to be as important to the behaviour of magnetic devices as electrical resistance is to electrical devices. The next big milestone is to detect single monopoles; any attempt to do this will require very low densities of monopoles, and understanding the contribution from magnetic defects will be crucial in studying this.”

Sara Fletcher

Research date: April 2014

Further Information

See Vacancy defects and monopole dynamics in oxygen-deficient pyrochlores, Nature Materials 13, 488-493 (2014)

References

1. Castelnovo, C. Moessner, R., Sondhi, S. L. Magnetic monopoles in spin ice. Nature 451, 42-45 (2008).

2. Morris, D. J. P., et al. Dirac strings and magnetic monopoles in the spin ice Dy2Ti2O7. Science 326, 411-414 (2009).

3. Fennel, T. et al. Magnetic Coulomb phase in the spin ice Ho2Ti2O7. Science 326, 415-417 (2009).

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