Spin manipulation using electric currents is one of the most promising directions in the field of spintronics. We used neutron scattering to observe the influence of an electric current on the magnetic structure in a bulk material.In the skyrmion lattice of MnSi, where the spins form a lattice of magnetic vortices similar to the vortex lattice in type II superconductors, we observe the rotation of the diffraction pattern in response to currents which are over five orders of magnitude smaller than those typically applied in experimental studies on current-driven magnetization dynamics in nanostructures. We attribute our observations to an extremely efficient coupling of inhomogeneous spin currents to topologically stable knots in spin structures. 1 arXiv:1012.3496v1 [cond-mat.str-el]
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.Comment: 24 pages, supplementary information file include
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 Fe(1-x)Co(x)Si (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.
Time-resolved x-ray imaging shows that the magnetization dynamics of a micron-sized pattern containing a ferromagnetic vortex is determined by its handedness, or chirality. The out-of-plane magnetization in the nanometer-scale vortex core induces a three-dimensional handedness in the planar magnetic structure, leading to a precessional motion of the core parallel to a subnanosecond field pulse. The core velocity was an order of magnitude higher than expected from the static susceptibility. These results demonstrate that handedness, already well known to be important in biological systems, plays an important role in the dynamics of microscopic magnets.
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