Voytenko A. Influence of Relativistic Interactions on Concentration and Temperature Phase Transitions in Two-Dimensional and Three-Dimensional Magnets

The candidate's thesis of physical and mathematical sciences on speciality 01.04.11 - Magnetism. - Taurida National V.I. Vernadskiy University, Simferopol, 2011. Models of concentration phase transitions are built for Fe1-xCox alloy monolayers and films of several tens of atomic layers taking into account the magnetic dipole and the magnetoelastic interactions, and also the single-ion anisotropy depending on concentration of Сo atoms. In the systems considered, in addition to the experimentally observed phases - the "easy axis" and the "easy plane", the realization of angular and domain phases is predicted. In the systems where the influence of "planar" magnetoelastic interactions was taken into account, the realization of "in-plane easy axis phase" was also predicted. In non-Heisenberg ferromagnetic with compensated single-ion anisotropy, the accounting of giant magnetoelastic interaction leads to the realization of intermediate quadrupolar-ferromagnetic phase and changes the phase transition type. Besides, the temperature of the phase transition from the ferromagnetic phase is increased with increasing of the magnetoelastic interaction. The temperature of the phase transition from the quadrupolar phase practically does not depend on the magnetoelastic interaction parameter. The investigation of the temperature phase transitions in the monocrystal of U2Co15Si2 alloy with competing anisotropies has shown that two spin-flip phase temperature transitions of the first kind are possible in the three-dimensional sample: the easy axis - the angular phase and the easy plane - the angular phase. In a film of U2Co15Si2 alloy only one spin-flip phase transition of the first kind is possible from the easy plane phase into the easy axis phase through the domain phase.