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Magnetic monopole predicted by Dirac nearly a century ago was found in spin ice as quasi-particle(2). My question is Why magnetic monopole found in spin ice don't modify the Maxwell's Equations? (I know they are not elementary particles but quasi-particles.)

(1) Dirac, P. A. M. Quantised singularities in the electromagnetic field. Proc. R. Soc. A 133, 60–72 (1931)

(2) Castelnovo, C., Moessner, R. & Sondhi, S. Magnetic monopoles in spin ice. Nature 451, 42–45 (2008).

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1 Answer 1

These are "fake monopoles", in the sense that the north and south poles are not actually separated. They are the ends of thin tubes which behave like Dirac strings - like long thin twisted magnets. The tubes are formed due to geometrical frustration, which forces the magnetic field to be orientated either toward the outside or toward the inside of the tetrahedral cells. The tetrahedral cells end up by having two faces with "spin in" orientation and two faces with "spin out" orientation. Two tetrahedral cells couple one another like tiny magnets, so that tiny magnetic tubes form. The ends seem to be separated north and south magnetic poles, while in fact they are connected through the tube.

Had they been genuine monopoles, they would have indeed modified Gauss's law for magnetism, and Faraday's law of induction, by adding terms corresponding to magnetic charge density and current.

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Thank you very much for the answer! –  Jeremy Sep 17 '12 at 1:54

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