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]

1Since the 1950s Heisenberg and others have attempted to explain the appearance of countable particles in quantum field theory in terms of stable localized field configurations [1]. As an exception Skyrme's model succeeded to describe nuclear particles as localized states, so-called 'skyrmions', within a non-linear field theory [2]. Skyrmions are a characteristic of non-linear continuum models ranging from microscopic to cosmological scales [3,4,5,6]. Skyrmionic states have been found under non-equilibrium conditions, or when stabilised by external fields or the proliferation of topological defects. Examples are Turing patterns in classical liquids [7], spin textures in quantum Hall magnets [8], or the blue phases in liquid crystals [9], respectively. However, it is believed that skyrmions cannot form spontaneous ground states like ferromagnetic or antiferromagnetic order in magnetic materials. Here, we show theoretically that this assumption is wrong and that skyrmion textures may form spontaneously in condensed matter systems with chiral interactions without the assistance of external fields or the proliferation of defects. We show this within a phenomenological continuum model, that is based on a few material-specific parameters that may be determined from experiment. As a new condition not considered before, we allow for softened amplitude variations of the magnetisation -a key property of, for instance, metallic magnets. Our model implies that spontaneous skyrmion lattice ground states may exist quite generally in a large number of materials, notably at surfaces and in thin films as well as in bulk compounds, where a lack of space inversion symmetry leads to chiral interactions.The possibility that particle-like states may form spontaneously in continuous fields has motivated intense theoretical efforts in the past. Derrick and Hobart established by rather general arguments that particle-like configurations are not stable in the majority of nonlinear field models [10,11]. However, a few exceptions have been found. Skyrme showed that particle-like excitations of continuous fields exist in the presence of certain non-linear interactions [2]. As a drawback, the interactions considered by Skyrme are physically not transparent, because they involve higher order derivative terms that are technically intractable. Therefore, Skyrme's approach is not viable in the context of ordered states in condensed matter that are ruled by short range interactions. In contrast, a physically transparent exception to the Derrick-Hobart theorem has been recognized in systems with bro-2 ken inversion symmetry, where chiral interactions lead to skyrmion excitations in condensed matter systems [12,14,16]. Chiral interactions exist in many different systems, e.g., (i) spin-orbit interactions in non-centrosymmetric materials, also referred to as DzyaloshinskyMoriya (DM) interactions [13], (ii) in non-centrosymmetric ferroelectrics, (iii) for certain structural phase transitions, (iv) in chiral liquid crystals, and (v) in the form of Che...

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]