Twisted, localized, and modulated states described in the phenomenological theory of chiral and nanoscale ferromagnets

The goal of the present thesis is to treat a wide diversity of problems in magnets by the tools of the modern computational micromagnetism. As an application of the self-elaborated numerical techniques this thesis includes a phenomenological description of magnetic phenomena in magnets with different geometry on nanoscale (part I) and in non-centrosymmetric magnets with chiral Dzyaloshinskii-Moriya interactions (part II). Part I. Nanomagnetic systems are well-prepared and controlled objects which induce growing interest for the understanding of magnetic behavior down to nanoscale. From one side, experiments on these carefully prepared and defect-free nanoobjects can serve as tests on classical results from micromagnetism. From the other side, nanosystems with multiformity of structural design provide an arena for the research of different aspects of surface-induced interactions. Surface-induced interactions in nanomagnets appear due to complex changes of the electrical and magnetic properties of surface layers and essentially modify their properties. The role of surfaceinduced contributions substantially increases with a perpetual miniaturization of magnetic nanostructures. Theoretical studies of the present thesis focus mainly on the effect of surface-induced uniaxial anisotropy in the stability and phase transitions of different homogeneous and inhomogeneous magnetization distributions in nanomagnets. Surface anisotropy competing with the intrinsic magnetocrystalline cubic anisotropy is found to influence the magnetization processes in various geometries of nanosystems, such as extended magnetic nanolayers, nanowires/nanotubes, and very small single-domain nanoparticles. In the present thesis, a micromagnetic model to describe magnetization processes in systems with competing magnetic anisotropies has been extended, generalized, and adapted to investigate nanomagnetic systems. Micromagnetic calculations have been implemented for detailed analysis of recent experimental results: (i) In layered magnetic nanostructures, e. g., in diluted magnetic semiconductors as (Ga,Mn)As and (In,Mn)As, competing anisotropies are known to result in the formation of specific multidomain states. A comprehensive analysis of the spin configurations in coexisting phases and parameters of domain structures versus the applied field is given for different values of the magnetic anisotropies and relative orientations of the anisotropy axes. Phase diagrams in components of applied and internal magnetic fields have been presented for such systems. These phase diagrams display regions of multi-phase domain structures. Lability lines of the phase diagrams span the regions with a remarkable transformation of the internal do-