Spin Ices and Magnetic Monopoles
Spin ices are geometrically frustrated magnetic systems. While frustration is usually associated with triangular or tetrahedral arrangements of magnetic moments coupled via antiferromagnetic exchange interactions, spin ices are frustrated ferromagnets. It is the local nature of the strong crystal field forcing the magnetic moments to point either in or out of a tetrahedron that renders ferromagnetic interactions frustrated in spin ices. Interestingly, it is the long range magnetic dipolar interaction and not the nearest-neighbor exchange coupling that causes the frustration and the consequential "two-in two-out" spin orientations and which leads to the spin ice phenomenology.
In a paper published in Science in September 2009, researchers Jonathan Morris and Alan Tennant from the Helmholtz-Zentrum Berlin für Materialien und Energie (HZB) along with Santiago Grigera from Instituto de Física de Líquidos y Sistemas Biológicos (IFLYSIB, CONICET) and other colleagues from Dresden University of Technology, University of St. Andrews and Oxford University described the observation of quasiparticles resembling monopoles. A single crystal of dysprosium titanate in a highly frustrated pyrochlore lattice (Fd3m) was cooled to 0.6 to 2 K. Using neutron scattering, the magnetic moments were shown to align in the spin ice into interwoven tube-like bundles resembling Dirac strings. At the defect formed by the end of each tube, the magnetic field looks like that of a monopole. Using an applied magnetic field to break the symmetry of the system, the researchers were able to control the density and orientation of these strings. A contribution to the heat capacity of the system from an effective gas of these quasiparticles is also described.
The effective charge of a magnetic monopole in a spin ice has been measured as 5 μB·Å−1 (Bohr magnetons per angstrom). The elementary constituents of spin ice are magnetic dipoles, so the emergence of monopoles is an example of the phenomenon of fractionalization.
Read more about this topic: Spin Ice
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