Field-induced Phenomena in Ferromagnetic Superconductors UCoGe and URhGe

TL;DR

This paper reviews experimental studies on ferromagnetic superconductors UCoGe and URhGe, highlighting their unique coexistence of ferromagnetism and superconductivity, large upper critical fields, and magnetic domain effects.

Contribution

It provides new experimental insights into the behavior of UCoGe and URhGe, especially regarding their upper critical fields and magnetic domain effects under various field orientations.

Findings

Large upper critical field Hc2 along the hard-magnetization axis in UCoGe.

Magnetic domain effects influence the initial slope of Hc2 in both materials.

Bulk superconductivity persists under magnetic fields, confirmed by specific heat measurements.

Abstract

We review our recent studies on ferromagnetic superconductors, UGe2, URhGe and UCoGe, where the spin-triplet state with the so-called equal spin pairing is realized. We focus on experimental results of URhGe and UCoGe in which the superconductivity occurs already at ambient pressure. The huge upper critical field Hc2 on UCoGe for the field along the hard magnetization axis (b-axis) is confirmed by the AC susceptibility measurements by the fine tuning of field angle. Contrary to the huge Hc2 along the hard-magnetization axis, Hc2 along the easy-magnetization axis (c-axis) is relatively small in value. However, the initial slope of Hc2, namely dHc2/dT (H -> 0) both in UCoGe and in URhGe indicates the large value, which can be explained by the magnetic domain effect detected in the magnetization measurements. The specific heat measurements using a high quality single crystal of UCoGe demonstrate the bulk superconductivity, which is extended under magnetic field for the field along c-axis.