Tartakovskaya E. Magnetic phase transitions and elementary excitations in nanosystems with long-range interactions

Thesis is aimed on study the effect of the dipole and elastic interactions, as well as various kinds of indirect exchange on the reorientational phase transitions and spin dynamics in nanosystems. Many of the models discussed here satisfy the definition of magnon crystals, which presents a new and technologically forward-looking industry of nanomagnetizm. The thesis focuses on theoretical studies and the connection between theory and experimental testing of the theoretical results by the available experimental techniques: Brillouin light scattering, ferromagnetic resonance, and the method of neutron scattering in the material. It is proved that for two-sublattice easy-plane two-dimensional antiferromagnets, and ferrites close to the point of compensation, a true long-range order can exist. The influence on this effect of long-range part of the dipole forces and the Dzyaloshinskii-Moriya interaction is investigated. The theory of the dynamics of magnetic rare-earth multilayer and the successful application of this theory in planning and explaining the experiments on inelastic neutron scattering is presented. A theory describing the spin-wave excitations of long cylindrical ferromagnetic nanowires is developed. Theoretical analysis of the phase transition, experimentally observed in the ordered hexagonal superlattice of permalloy wires proves that it occurs due to dipole interaction between the wires. It was also proved that it is a first-order transition. The dispersion laws for spin waves propagating due to dipolar interaction along different directions of ordered arrays of nanospheres are calculated.