Field-Induced Phenomena in Ferromagnetic Superconductors UCoGe and URhGe

We review our recent studies on ferromagnetic superconductors, UGe 2 , 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 H c2 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 H c2 along the hard-magnetization axis, H c2 along the easy-magnetization axis ( c -axis) is relatively small in value. However, the initial slope of H c2 , namely d H c2 /d T ( 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.