To create artificial magnetic monopoles, the scientists merged tiny magnetic whirls, skyrmions, and at the point of merging, the physicists were able to create a monopole, which has similar characteristics to a fundamental particle postulated by Paul Dirac in 1931. Dirac postulated the existence of a fundamental particle to explain why electrons and protons carry electrical charges of the same size, a puzzle because the elements of the protons and electrons are completely different fundamental particles. Dirac, however, argued that the existence of a single magnetic monopole would be enough to explain that the charges of all fundamental particles have to be quantized, i.e. exactly an integer multiple of an elementary charge.
The newly discovered artificial monopoles fulfill the quantization requirement but miss solving Dirac's problem, as you will see.
Skyrmions, magnetic whirls, have been examined intensively. These whirls influence the movements of the electrons in exactly the same manner as magnetic fields. For this reason, artificial magnet fields are used to describe these whirls as well as their influence on the electrons. Even if these are not “real” magnetic fields, it is possible to measure them experimentally in the same manner as normal magnet fields as they deflect electrons.
The researchers asked questions as to the consequences of attempting to destroy the magnetic whirls. The group working under the direction of Prof. Eng from the Technischen Universität Dresden observed magnetic whirls with a magnetic force microscope: a tiny magnetic tip samples the surface of the magnets and measures the direction of the magnetization thus making the ca. 50 nanometer sized whirl visible.
They were able to observe on the surface that the magnetic whirls apparently coalesce when the skyrmion phase is destroyed.
What happens within the materials? Measurements taken by the group working under the direction of Prof. Pfleiderer in Munich using neutron scattering suggest that similar processes occur there, but individual whirls were not observed in this manner. For this reason, Stefan Buhrandt and Christoph Schütte working in Prof. Rosch’s group at the University of Cologne conducted computer simulations. These showed that the whirls neighboring the merging process observed on the surface in the experiment also occur within the materials.
The image schematically shows how two magnetic whirls merge into one.
Due to the fact that every whirl carries an artificial magnetic field, their creation or destruction occurs at the point of merging. “This means that an artificial magnetic monopole has to sit on this point,” says Rosch, “whenever two magnetic whirls merge in the experiment, an artificial magnetic monopole has flown through surface.”
“It is fascinating that something as fundamental as a magnetic monopole can be realized in a piece of material,” says Buhrandt.
Despite this, artificial monopoles cannot solve Dirac’s problem: only electrons in solid state, but not protons, feel the artificial magnet fields.
Reference: P. Milde, D. Köhler, J. Seidel, L. M. Eng, A. Bauer, A. Chacon, J. Kindervater, S. Mühlbauer, C. Pfleiderer, S. Buhrandt, C. Schütte, and A. Rosch, 'Unwinding of a Skyrmion Lattice by Magnetic Monopoles', Science 31 May 2013: 1076-1080. DOI:10.1126/science.1234657