Ordering Phenomena in Transition Metal Oxides

Magnetic Monopoles in Spin Ice   

Electrically charged particles, such as the electron, are ubiquitous. Bycontrast, no elementary particles with a net magnetic charge have ever been observed, despite intensive and prolonged searches. We pursue an alternative strategy, namely that of realising them not as elementary but rather as emergent particles, i.e., as manifestations of the correlations present in a strongly interacting many-body system. We show that magnetic monopoles do emerge in a class of exotic magnets known collectively as spin ice: the dipole moment of the underlying electronic degrees of freedom fractionalises into monopoles! This explains a mysterious phase transition observed experimentally in spin ice in a magnetic field, which is a liquid-gas transition of the magnetic monopoles. These monopoles can also be detected by other means, e.g., in an experiment modelled after the celebrated Stanford magnetic monopole search.

Reference: C. Castelnovo, R. Moessner, and S. L. Sondhi, Nature 451, 42 (2008).