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Chiral magnetic interactions stabilize modulated phases with a fixed sense of rotation in non-centrosymmetric helimagnets [1]. The inhomogeneous states theoretically arise as one-dimensional spirals or multidimensional -Y¡Skyrmion¡ textures. In magnetic nanostructures the chiral Dzyaloshinskii-Moriya exchange arises owing to broken inversion symmetry at surfaces and can stabilize vortex-like Skyrmion states [2]. In non-centrosymmetric cubic helimagnets, such as MnSi and FeGe, intrinsic Dzyaloshinskii-Moriya interactions are isotropic and cause a conical phase as the globally stable state in an external magnetic field, while skyrmion lattices and helicoidals transverse to the applied field are metastable solutions. Within a phenomenological theory [3] we show that small uniaxial distortions induced in cubic systems may stabilize Skyrmion lattice states in a wide range of magnetic field. Similarly, detailed investigations on modulated and localized states in ferromagnetic nanolayers under the combined influence of chiral exchange and a strong surface anisotropy show that Skyrmionic states can arise as metastable localized excitations in collinearly ordered states, or can form stable condensed phases in the form of Skyrmion lattices. These Skyrmions have convex shape. The solutions for these two-dimensional ´Baby-Skyrmions¡ are related to spherulitic domains in chiral nematic films with surface anchoring. As another mechanism leading to stabilization of Skyrmionic states, we investigate the modulation of the magnetic polarization that can arise near the ordering temperature due to longitudinal fluctuations [1]. While in zero and low fields, this mechanism leads to condensed pi/2 states in isotropic chiral magnets, it stabilizes pi-Skyrmion states in stronger applied fields. We consider the phase diagram for the transition between these states and the helicoidal and conical phases, which explains the major part of the so-called A-phase in the chiral cubic magnets with B20-structure as MnSi as a c ondensed phase of proper Skyrmions, which may form lattices or other forms of Skyrmionic textures. The results suggest the possibility of various transitions between different Skyrmionic states in the magnetic phase diagram of non-centrosymmetric magnets. [1] U.K. R\"oßler, A.N. Bogdanov, C. Pfleiderer, Nature (London) 442, 797 (2007); [2] A. N. Bogdanov, U.K. R\"oßler, Phys. Rev. Lett. 87, 037203 (2001); [3] A. N. Bogdanov, A. Hubert, J. Magn. Magn. Mater. 195, 185 (1999). |
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