Magnetic field dependence of pairing interaction in ferromagnetic superconductors with triplet pairing

A microscopic description is developed of superconductivity in ferromagnetic materials with triplet pairing triggered by the exchange of magnetic fluctuations. Instead of a widely used paramagnon model, we work with a phenomenological spectrum of fluctuations in the orthorhombic ferromagnet with strong magnetic anisotropy. An interplay between pressure dependence of the Curie temperature and the critical temperature of superconducting transition is established such that, for example, the increase of the Curie temperature as a function of pressure is accompanied by the decreasing critical superconducting temperature. Depending on an external field orientation parallel or perpendicular to the direction of spontaneous magnetization, the effective amplitude of pairing interaction proves to be a decreasing or increasing function of the magnetic field, which allows us to explain the drastic difference in magnitudes of the upper critical field in these directions.