Absence of Meissner State and Robust Ferromagnetism in the Superconducting State of UCoGe: Possible Evidence of Spontaneous Vortex State

TL;DR

This study investigates UCoGe, revealing that ferromagnetism persists within its superconducting state without exhibiting a Meissner effect, suggesting the presence of a spontaneous vortex state.

Contribution

It provides experimental evidence that UCoGe maintains ferromagnetism in its superconducting phase without a Meissner state, indicating a spontaneous vortex state.

Findings

Ferromagnetism persists below the superconducting transition.

No Meissner state observed in the superconducting phase.

Evidence of a spontaneous vortex state in UCoGe.

Abstract

We report ac magnetic susceptibility and dc magnetization measurements on the superconducting ferromagnet UCoGe (with superconducting and Curie temperatures of $T_{{\rm SC}} \sim 0.5$~K and $T_{{\rm Curie}} \sim 2.5$~K, respectively). In the normal, ferromagnetic state ($T_{{\rm SC}} < T < T_{{\rm Curie}}$), the magnetization curve exhibits a hysteresis loop similar to that of a regular itinerant ferromagnet. Upon lowering the temperature below $T_{{\rm SC}}$, the spontaneous magnetization is unchanged, but the hysteresis is markedly enhanced. Even deeply inside the superconducting state, ferromagnetism is not completely shielded, and there is no Meissner region, a magnetic field region of $H < H_{\rm c1}$ (a lower critical field). From these results, we suggest that UCoGe is the first material in which ferromagnetism robustly survives in the superconducting state and a spontaneous vortex state without the Meissner state is realized.