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Science 16 November 2001:
Vol. 294. no. 5546, pp. 1495 - 1501
DOI: 10.1126/science.1064761
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Review

Spin Ice State in Frustrated Magnetic Pyrochlore Materials

Steven T. Bramwell,1 Michel J. P. Gingras23*

A frustrated system is one whose symmetry precludes the possibility that every pairwise interaction ("bond") in the system can be satisfied at the same time. Such systems are common in all areas of physical and biological science. In the most extreme cases, they can have a disordered ground state with "macroscopic" degeneracy; that is, one that comprises a huge number of equivalent states of the same energy. Pauling's description of the low-temperature proton disorder in water ice was perhaps the first recognition of this phenomenon and remains the paradigm. In recent years, a new class of magnetic substance has been characterized, in which the disorder of the magnetic moments at low temperatures is precisely analogous to the proton disorder in water ice. These substances, known as spin ice materials, are perhaps the "cleanest" examples of such highly frustrated systems yet discovered. They offer an unparalleled opportunity for the study of frustration in magnetic systems at both an experimental and a theoretical level. This article describes the essential physics of spin ice, as it is currently understood, and identifies new avenues for future research on related materials and models.

1 Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK.
2 Department of Physics, University of Waterloo, Waterloo, Ontario, N2L-3G1, Canada.
3 Canadian Institute for Advanced Research, 180 Dundas Street, Toronto, Ontario M5G 1Z8, Canada.
*   To whom correspondence should be addressed. E-mail: gingras{at}gandalf.uwaterloo.ca.

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