Direct Evidence of Spontaneous Abrikosov Vortex State in Ferromagnetic Superconductor EuFe$_2$(As$_{1-x}$P$_x$)$_2$ with $x=0.21$

TL;DR

This study provides direct experimental evidence of spontaneous vortex formation in a ferromagnetic superconductor using Magnetic Force Microscopy, revealing a transition in domain structures near the ferromagnetic transition temperature.

Contribution

First direct imaging of spontaneous vortex-antivortex pairs and domain structure transition in EuFe$_2$(As$_{0.79}$P$_{0.21}$)$_2$ ferromagnetic superconductor.

Findings

Observation of spontaneous vortex-antivortex pairs near $T_{FM}$

Transition from short to long period domain structures

Large domains and striped surface structures at low temperature

Abstract

Using low-temperature Magnetic Force Microscopy (MFM) we provide direct experimental evidence for spontaneous vortex phase (SVP) formation in EuFe$_2$(As$_{0.79}$P$_{0.21}$)$_2$ single crystal with the superconducting $T^{\rm 0}_{\rm SC}=23.6$~K and ferromagnetic $T_{\rm FM}\sim17.7$~K transition temperatures. Spontaneous vortex-antivortex (V-AV) pairs are imaged in the vicinity of $T_{\rm FM}$. Also, upon cooling cycle near $T_{\rm FM}$ we observe the first-order transition from the short period domain structure, which appears in the Meissner state, into the long period domain structure with spontaneous vortices. It is the first experimental observation of this scenario in the ferromagnetic superconductors. Low-temperature phase is characterized by much larger domains in V-AV state and peculiar branched striped structures at the surface, which are typical for uniaxial ferromagnets with perpendicular magnetic anisotropy (PMA). The domain wall parameters at various temperatures are estimated.