Observing Monopoles in a Magnetic Analog of Ice

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Science  16 Oct 2009:
Vol. 326, Issue 5951, pp. 375-376
DOI: 10.1126/science.1181510

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A bar magnet has a north and south pole, and cutting it in half just creates two new poles, not two separated monopoles. However, a recent theoretical proposal suggested that defects in the spin alignment of certain oxide magnets can create separated effective magnetic monopoles (1). These materials are called spin ices because the lowest-energy orientation of the magnetic spins closely mimics the most stable arrangement of protons in water ice (2). On pages 415 and 411 of this issue, Fennell et al. (3) and Morris et al. (4) report measurements from neutron-scattering experiments showing that the low-energy excitations in spin ices are reminiscent of Dirac's elementary magnetic monopoles (5) that have so far eluded the searches of high-energy physicists. These dissociated north and south poles diffuse away from each other (6) in these oxides and leave behind a “Dirac string” of reversed spins that can be seen as patterns in the intensity of scattered neutrons.