Infuence of pressure on magnetic phase transitions in the ferromagnetic superconductor UGe2_2 -- phenomenological approach

We propose a thermodynamic model of free energy expansion up to eighth order of magnetisation to describe the complex magnetic phase transitions in ferromagnetic superconductor UGe$_2$. The model successfully describes transitions between ordered phases which take place without changing of magnetic structure but only of magnetisation which is the case of UGe$_2$ where two magnetic phases with same structure but with different magnitude occur with decreasing of temperature. We consider the infuence of pressure on magnetic transitions in the simplest form of including it only in pressure dependence of Curie temperature for the transition between the paramagnetic and themagnetic phase with the lower magnetic moment FM1. Our results show that for pressures lower than some limiting pressure the transition betweenlow-and high-magnetisation phase remains of crossover type. Above this limiting pressure this transition changes to real thermodynamic transition of first order.Comment: 15 pages, 4 figures. arXiv admin note: text overlap with arXiv:2108.0951

The existence of a stable noncollinear phase in a Heisenberg model with a complex structure

We have analyzed the properties of a noncollinear magnetic phase obtained in the mean-field analysis of the model of two coupled Heisenberg subsystems. The domain of its existence and stability is narrow and depends on the ratio between the averaged over nearest neighbours microscopic exchange parameters.Comment: 7 pages, miktex, 3 figure

Phase transitions to spin-triplet ferromagnetic superconductivity in neutron stars

Effects of the anisotropy of Cooper pairs in spin-triplet ferromagnetic superconductors are investigated on the basis of the Ginzburg-Landau theory. A special attention is paid to the triggering of the superconducting state by the ferromagnetic order. The ground states of these superconductors are outlined and discussed. The idea about a possible coexistence of ferromagnetism and spin-triplet superconductivity in neutron stars is introduced.Comment: 10 pages Latex2e, 2 figs, Proc. of the Leiden Workshop on Realistic Models in Astrophysical Matter (AIP, N.Y., 2004

Phenomenological description of anisotropy effects in some ferromagnetic superconductors

We study phenomenologically by using the previously derived Landau free energy, the role of anisotropy in ferromagnetic superconductors UGe2, URhGe, and UCoGe. The three compounds are separately discussed with the special stress on UGe2. The main effect comes from the strong uniaxial anisotropy of magnetization while the anisotropy of Cooper pairs and crystal anisotropy only slightly change the phase diagram in the vicinity of Curie temperature. The limitations of this approach are also discussed.Comment: 12 pages, 4 figure