Nonuniform magnetic states in ferromagnetic superconductors with low Curie temperature
We propose the theory of nonuniform magnetic states in the ferromagnetic superconductors (FS) with low Curie temperature θ and purely electromagnetic interaction. Due to θ in these compounds is smaller than the superconducting transition temperature the ferromagnetism nucleates in fully developed superconducting state while cooling the sample and the configuration of the emerging magnetic structure is significantly affected by the Meissner currents. We show that in the case of strong magnetic anisotropy of the FS, this leads to the appearance of a layered phase with a sine-like magnetization profile at the temperature slightly below θ instead of a homogeneous ferromagnetic state. With the decrease of temperature this phase smoothly transforms into domain-type structure with an unusual temperature dependence of the domain size. Also we calculate the critical temperatures corresponding to the transition to the phase with coexisting domains and vortices and suggest the explanation of the evolution of the period and hysteresis behavior of the magnetic domain structure recently observed in EuFe2 (As1-xPx)2.