[Paper] In Situ Local Imaging of Ferromagnetism and Superconductivity in RbEuFe$_4$As$_4$

Our new study about in-situ imaging magnetic domains and Meissner screening in helical magnetic superconductor RbEuFe$_4$As$_4$ was published in Nano Letters on July 15, 2024.

Our research is motivated by the intriguing coexistence and interplay between superconductivity and ferromagnetism, particularly in unconventional superconductors. Despite theoretical predictions by Anderson and Suhl that non-uniform magnetic phases could coexist with $s$-wave superconductivity if the ordering wavelength is shorter than the superconducting coherence length, experimental investigations have been limited by the low transition temperatures in known materials. The discovery of iron-based superconductors, specifically the Eu-containing 1144-type superconductor family, has opened new avenues for exploring these phenomena. Our research investigates the intriguing coexistence and interplay between superconductivity and ferromagnetism in the helical magnetic superconductor RbEuFe$_4$As$_4$. Using scanning SQUID microscopy, we achieved a micrometer-scale spatial resolution to image this material's in-situ diamagnetic and ferromagnetic responses. 

We observed a significant suppression of superfluid density near the magnetic phase transition temperature ($T_m$), attributed to enhanced magnetic scatterings between Eu spins and Fe 3$d$ conduction electrons. This highlights the strong impact of magnetic fluctuations on superconductivity. Contrary to the expected ideal helical magnetic phase, we identified multiple ferromagnetic domains below $T_m$, suggesting a weak $c$-axis ferromagnetic component likely due to a Eu spin-canting effect. The formation of these domains indicates possible superconductivity-driven phenomena, such as domain Meissner and domain vortex-antivortex phases.

Movie: Susceptibility($\chi$) and magnetic flux($\Phi$) scan at small magnetic fields while warming up.

[1] In Situ Local Imaging of Ferromagnetism and Superconductivity in RbEuFe$_4$As$_4$
H. Man, Y. Iguchi, J.-K. Bao, D. Y. Chung, and M. G. Kanatzidis
Nano Letters, ASAP
https://doi.org/10.1021/acs.nanolett.4c02475