Skyrmions represent topologically stable field configurations with particle-like properties. We used neutron scattering to observe the spontaneous formation of a two-dimensional lattice of skyrmion lines, a type of magnetic vortices, in the chiral itinerant-electron magnet MnSi. The skyrmion lattice stabilizes at the border between paramagnetism and long-range helimagnetic order perpendicular to a small applied magnetic field regardless of the direction of the magnetic field relative to the atomic lattice. Our study experimentally establishes magnetic materials lacking inversion symmetry as an arena for new forms of crystalline order composed of topologically stable spin states. 1 arXiv: 0902.1968v1 [cond-mat.str-el]
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]
Recent small angle neutron scattering suggests, that the spin structure in the A-phase of MnSi is a so-called triple-Q state, i.e., a superposition of three helices under 120 degrees. Model calculations suggest that this structure in fact is a lattice of so-called skyrmions, i.e., a lattice of topologically stable knots in the spin structure. We report a distinct additional contribution to the Hall effect in the temperature and magnetic field range of the proposed skyrmion lattice, where such a contribution is neither seen nor expected for a normal helical state. Our Hall effect measurements constitute a direct observation of a topologically quantized Berry phase that identifies the spin structure seen in neutron scattering as the proposed skyrmion lattice.PACS numbers: 72.80. Ga, Many years ago Skyrme showed that topologically stable objects of a nonlinear field theory for pions can be interpreted as protons or neutrons [1,2]. This seminal paper inspired the search for topological stable particlelike objects in a broad range of fields ranging from highenergy to many areas of condensed matter physics. For instance, twenty years ago it has been predicted that skyrmions exist in anisotropic spin systems with chiral spin-orbit interactions, where they are expected to form crystalline structures [3,4]. Lattices of skyrmions have also been suggested to occur in dense nuclear matter [5] or in quantum Hall systems near Landau level filling factor ν = 1 [6]. However, thus far the experimental evidence is only indirect [7,8].Recently we reported microscopic evidence of a skyrmion lattice in the A-phase of the transition metal compound MnSi using small angle neutron scattering (SANS) [9]. The SANS data shows magnetic Bragg peaks with a hexagonal symmetry consistent with the superposition of three helices under an angle of 120 degrees -a so-called triple-Q structure. The three helices are thereby confined to a plane strictly perpendicular to the applied magnetic field. A detailed theoretical analysis [9] of an appropriate Ginzburg-Landau model suggested that a lattice of anti-skyrmion lines forms in the A-phase of MnSi, similar to the vortex lattice in superconductors.However, whether the spin structure in the A-phase indeed represents a skyrmion lattice depends crucially on the phase relationship of the helices that are superimposed. This phase information could not be extracted from the SANS data. In contrast to neutron scattering the phase relationship of the helices, and thus existence of topologically nontrivial spin structures, may be established directly by means of the so-called topological Hall effect (THE) [10]. The perhaps most convincing example of a topological Hall effect has been reported for 3D pyrochlore lattices [11,12]. However, in these systems the non-coplanar spin structure is due to frustration on short length scales, i.e., the spin structure is not a continuous field for which topological properties may be defined in a strict sense. The topological Hall effect has also been considered, e.g., in La 1...
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