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]

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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MnSi is an itinerant magnet which at low temperatures develops a helical spin-density wave. Under pressure it undergoes a transition into an unusual partially ordered state whose nature is debated. Here we propose that the helical spin crystal (the magnetic analog of a solid) is a useful starting point to understand partial order in MnSi. We consider different helical spin crystals and determine conditions under which they may be energetically favored. The most promising candidate has bcc structure and is reminiscent of the blue phase of liquid crystals in that it has line nodes of magnetization protected by symmetry. We introduce a Landau theory to study the properties of these states, in particular, the effect of crystal anisotropy, magnetic field, and disorder. These results compare favorably with existing data on MnSi from neutron scattering and magnetic field studies. Future experiments to test this scenario are also proposed.

Chiral magnets like MnSi form lattices of skyrmions, i.e. magnetic whirls, which react sensitively to small electric currents j above a critical current density jc. The interplay of these currents with tiny gradients of either the magnetic field or the temperature can induce a rotation of the magnetic pattern for j > jc. Either a rotation by a finite angle of up to 15 • or -for larger gradientsa continuous rotation with a finite angular velocity is induced. We use Landau-Lifshitz-Gilbert equations extended by extra damping terms in combination with a phenomenological treatment of pinning forces to develop a theory of the relevant rotational torques. Experimental neutron scattering data on the angular distribution of skyrmion lattices suggests that continuously rotating domains are easy to obtain in the presence of remarkably small currents and temperature gradients.