Metamagnetic phase transition in ferromagnetic superconductor URhGe

Abstract

Ferromagnetic superconductor URhGe has orthorhombic structure and possesses spontaneous magnetisation along the c-axis. Magnetic field directed along the $b$-axis suppresses ferromagnetism in $c$-direction and leads to a metamagnetic transition into polarised paramagnetic state in the $b$-direction. The theory of these phenomena based on the specific magnetic anisotropy of this material in $(b,c)$ plane is given. Line of the first order metamagnetic transition ends at a critical point. The Van der Waals - type description of behaviour of physical properties near this point is developed. The triplet superconducting state destroyed by orbital effect is recreated in vicinity of the transition. It is shown that the reentrance of superconductivity is caused by the sharp increase of magnetic susceptibility in $b$ direction near the metamagnetic transition. The specific behaviour of the upper critical field in direction of spontaneous magnetisation in UCoGe and in UGe$_2$ related to the field dependence of magnetic susceptibility is discussed.