Superconductivity Reinforced by Magnetic Field and the Magnetic Instability in Uranium Ferromagnets

TL;DR

This paper reviews recent findings on ferromagnetic superconductors URhGe and UCoGe, highlighting how magnetic fields and instabilities enhance superconductivity through magnetic fluctuations and effective mass variations.

Contribution

It provides new insights into the field-induced reinforcement of superconductivity and the role of magnetic instability in uranium ferromagnets, supported by experimental data on specific heat and thermal expansion.

Findings

Superconducting transitions observed via specific heat in UCoGe.

Field-dependent effective mass explains re-entrant and S-shaped superconducting phases.

Magnetic fluctuations and anisotropy are highly sensitive to field orientation.

Abstract

We review our recent results on ferromagnetic superconductors, URhGe and UCoGe. High quality single crystals of both compounds were successfully grown. The specific heat shows a clear jump related to the superconducting transition in UCoGe. The finite values of C/T at 0K are discussed in terms of the self-induced vortex state and the value of the ordered moment. With increasing fields for H // b-axis in URhGe, the jump of thermal expansion increases and shifts to lower temperature. The re-entrant and S-shaped superconducting phases for URhGe and UCoGe respectively are explained by the unusual field dependence of the effective mass, which is induced by the ferromagnetic instability when the field is applied along the hard magnetization b-axis. The magnetic fluctuations are very sensitive to the field orientation. This is reflected in the Hc2 and the anisotropy of the effective mass.