Theory of chiral modulations in confining structures: epitaxial films of cubic helimagnets

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Recently synthesized epitaxial thin films of cubic helimagnets with non-centrosymmetric B20 structure (MnSi, FeGe, and others) represent a novel class of nanomagnetic systems exhibiting many physical phenomena unknown in bulk helimagnets. In particular, nanolayers of cubic helimagnets enable the observation of specific particle-like structures (skyrmions) predicted theoretically more than twenty years ago and the field-induced unwinding of helicoids via quantized steps. Our recent studies show that in nanolayers of cubic helimagnets the interplay between intrinsic chirality, structural confinement, and surface/interface induced interactions leads to the formation of specific confined chiral modulations which fundamentally differ from those commonly observed in bulk helimagnets. The aim of the project is to give a comprehensive theoretical description of confined chiral modulations and, thus, to establish physical relations between specific effects arising in cubic helimagnet nanolayers and peculiarities of their structures. Within the standard micromagnetic theory of helimagnetism rigorous solutions for helical and skyrmionic modulations in thin layers of cubic helimagnets will be derived in broad ranges of the thermodynamic parameters. Our theoretical findings will be used to construct magnetic phase diagrams for nanolayers of cubic helimagnets with different types of induced magnetic anisotropy. These results will be applied for detailed analyses of recent experiments in epitaxial films of MnSi and FeGe and to elucidate complex magnetization processes observed in these systems. They also should provide the theoretical basis for the emerging nanophysics of confined chiral modulations and supply practical recommendations for future investigations.