# Theory of the magnetic domains phases in ferromagnetic superconductors

## TL;DR

This paper develops a theoretical framework for understanding magnetic domain phases in ferromagnetic superconductors, explaining the transition from sinusoidal to soliton-like domains and the emergence of a ferromagnetic vortex state.

## Contribution

It introduces a model describing the evolution of magnetic structures and predicts a first-order transition to a vortex state in ferromagnetic superconductors.

## Key findings

- Magnetic structure transitions from sinusoidal to soliton-like domains.
- Identification of a first-order transition to a ferromagnetic vortex state.
- Domain walls generate vortices perpendicular to those in the domains.

## Abstract

Recently discovered superconducting P-doped EuFe$_2$As$_2$ compounds reveal the situation when the superconducting critical temperature substantially exceeds the ferromagnetic transition temperature. The main mechanism of the interplay between magnetism and superconductivity occurs to be an electromagnetic one and a short period magnetic domain structure was observed just below Curie temperature [Stolyarov et al., Sci. Adv. \textbf{4}, eaat1061 (2018)]. We elaborate a theory of such transition and demonstrate how the initial sinusoidal magnetic structure gradually transforms into a soliton-like domain one. Further cooling may trigger a first-order transition from the short-period domain Meissner phase to the self-induced ferromagnetic vortex state and we calculate the parameters of this transition. The size of the domains in the vortex state is basically the same as in the normal ferromagnet, but with the domain walls which should generate the set of vortices perpendicular to the vortices in the domains.