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Unwinding of a Skyrmion Lattice by Magnetic Monopoles

Science  31 May 2013:
Vol. 340, Issue 6136, pp. 1076-1080
DOI: 10.1126/science.1234657

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Hedgehogs, Whirls, and Zippers

Topologically ordered materials at certain ranges of temperature and magnetic field can form a regular lattice of magnetic whirls called skyrmions. Milde et al. (p. 1076) studied the destruction of a skyrmion lattice with varying magnetic fields by imaging the surface magnetic structure. Magnetic force microscopy revealed a pairwise merging of skyrmions on the surface. Furthermore, in the bulk, a hedgehog-like spin structure with the properties of the elusive magnetic monopole was needed to "zip" together the corresponding skyrmion lines.

Abstract

Skyrmion crystals are regular arrangements of magnetic whirls that exist in a wide range of chiral magnets. Because of their topology, they cannot be created or destroyed by smooth rearrangements of the direction of the local magnetization. Using magnetic force microscopy, we tracked the destruction of the skyrmion lattice on the surface of a bulk crystal of Fe1−xCoxSi (x = 0.5). Our study revealed that skyrmions vanish by a coalescence, forming elongated structures. Numerical simulations showed that changes of topology are controlled by singular magnetic point defects. They can be viewed as quantized magnetic monopoles and antimonopoles, which provide sources and sinks of one flux quantum of emergent magnetic flux, respectively.

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