Properties of Some Magnetic Superconductors

Two solid solutions in the system GdRu₂-CeRu₂, in which both ferromagnetism and superconductivity have been observed, are studied by magnetic methods. The solid solution Gd0.082 Ce0.918 Ru₂, which has a Curie point θ above the critical temperature Tₛ for superconductivity, is both ferromagnetic and superconducting. In Gd0.04 Ce0.96 Ru₂, for which the expected θ<Tₛ, no ferromagnetic moment could be measured, although a small moment may be present and not detected by our methods. In solid solutions of increasing Gd content, when θ begins to exceed the expected Tₛ by a considerable margin, Tₛ suddenly drops toward zero; and when Tₛ>θ, θ approaches zero. Similar conclusions apply to the system GdOs₂-LaOs₂ when θ and Tₛ are related in the same ways. Both major and minor hysteresis loops have forms not previously observed and enable one to detect ferromagnetism and superconductivity when they exist. The molecular fields resulting from the interaction between Gd atoms, and the Curie points calculated therefrom by molecular field theory, increase with increasing temperature; this is in accordance with the theory of long-range exchange forces developed by Brout.