Coexistence of ferromagnetism and superconductivity in a superconducting alloy

Abstract

A model for a superconducting alloy doped with magnetic impurities is solved exactly. The model consists of a combination of the BCS model and the Zener s-d exchange-interaction model. Its behaviour appears to be strongly dependent on the impurity concentration and the coupling parameters belonging to the BCS model and the s-d exchange-interaction model. At very low temperatures the system is always a ferromagnet or a superconductor. Coexistence of superconductivity and ferromagnetism is only possible at finite temperatures for certain values of the coupling parameters and the concentration of magnetic impurities. The transitions from the mixed state to the superconducting and to the magnetic state do not need to be continuous, i.e., second order.