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      Magnetic Monopoles in Spin Ice

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          Abstract

          Electrically charged particles, such as the electron, are ubiquitous. By contrast, 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. The most prominent examples of emergent quasiparticles are the ones with fractional electric charge e/3 in quantum Hall physics. Here 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 enables us to account for 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.

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          Most cited references15

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          Geometrical Frustration in the Ferromagnetic PyrochloreHo2Ti2O7

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            Spin Ice State in Frustrated Magnetic Pyrochlore Materials

            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 characterised, 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.
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              Current-induced resonance and mass determination of a single magnetic domain wall.

              A magnetic domain wall (DW) is a spatially localized change of magnetization configuration in a magnet. This topological object has been predicted to behave at low energy as a composite particle with finite mass. This particle will couple directly with electric currents as well as magnetic fields, and its manipulation using electric currents is of particular interest with regard to the development of high-density magnetic memories. The DW mass sets the ultimate operation speed of these devices, but has yet to be determined experimentally. Here we report the direct observation of the dynamics of a single DW in a ferromagnetic nanowire, which demonstrates that such a topological particle has a very small but finite mass of 6.6 x 10(-23) kg. This measurement was realized by preparing a tunable DW potential in the nanowire, and detecting the resonance motion of the DW induced by an oscillating current. The resonance also allows low-current operation, which is crucial in device applications; a DW displacement of 10 microm was induced by a current density of 10(10) A m(-2).
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                Author and article information

                Journal
                30 October 2007
                2007-10-31
                Article
                10.1038/nature06433
                18172493
                0710.5515
                f3685911-d823-49c1-8c82-266ea71f3243
                History
                Custom metadata
                Nature 451, 42-45 (2008)
                (6 pages, 6 figures) v2: fig 3 replaced with colour version. For the high-definition version of the paper click http://www-thphys.physics.ox.ac.uk/user/ClaudioCastelnovo/Publications/papersub.pdf
                cond-mat.str-el hep-th

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