  When an electron moves in a smoothly varying non-collinear magnetic
structure, its spin-orientation adapts constantly, thereby inducing forces that
act on both the magnetic structure and the electron. These forces may be
described by electric and magnetic fields of an emergent electrodynamics. The
topologically quantized winding number of so-called skyrmions, i.e., certain
magnetic whirls, discovered recently in chiral magnets are theoretically
predicted to induce exactly one quantum of emergent magnetic flux per skyrmion.
A moving skyrmion is therefore expected to induce an emergent electric field
following Faraday's law of induction, which inherits this topological
quantization. Here we report Hall effect measurements, which establish
quantitatively the predicted emergent electrodynamics. This allows to obtain
quantitative evidence of the depinning of skyrmions from impurities at
ultra-low current densities of only 10^6 A/m^2 and their subsequent motion. The
combination of exceptionally small current densities and simple transport
measurements offers fundamental insights into the connection between emergent
and real electrodynamics of skyrmions in chiral magnets, and promises to be
important for applications in the long-term.
