In-situ local imaging of ferromagnetism and superconductivity in RbEuFe$_4$As$_4$

TL;DR

This study uses in-situ imaging to explore the coexistence and interaction of superconductivity and ferromagnetism in RbEuFe$_4$As$_4$, revealing complex magnetic domains and fluctuation effects near phase transitions.

Contribution

It provides the first high-resolution in-situ imaging of magnetic and superconducting responses in RbEuFe$_4$As$_4$, uncovering novel magnetic domain structures and fluctuation phenomena.

Findings

Suppression of superfluid density near magnetic transition

Multiple ferromagnetic domains observed

Weak c-axis ferromagnetic component from Eu spin-canting

Abstract

The coexistence of superconductivity and ferromagnetism is an intrinsically interesting research focus in condensed matter physics but the study is limited by low superconducting ($T_c$) and magnetic ($T_m$) transition temperatures in related materials. Here, we used a scanning superconducting quantum interference device to image the in-situ diamagnetic and ferromagnetic responses of RbEuFe$_4$As$_4$ with high $T_c$ and $T_m$. We observed significant suppression of superfluid density in vicinity of the magnetic phase transition, signifying fluctuation-enhanced magnetic scatterings between Eu spins and Fe 3$d$ conduction electrons. Intriguingly, we observed multiple ferromagnetic domains which should be absent in an ideal magnetic helical phase. The formation of these domains demonstrates a weak $c$-axis ferromagnetic component probably arising from Eu spin-canting effect, indicative of possible superconductivity-driven domain Meissner and domain vortex-antivortex phases as revealed in EuFe$_2$(As$_{0.79}$P$_{0.21}$)$_2$. Our observations highlight RbEuFe$_4$As$_4$ is a unique system which includes multiple interplay channels between superconductivity and ferromagnetism.