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

Abstract

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