Magnetic Properties of Ferromagnetic Superconductors

Solid solutions of ${\mathrm{GdRu}}_2$ in ${\mathrm{CeRu}}_2$ are known to be superconducting and ferromagnetic. Magnetic measurements are reported for the 4 and 8% alloys; these show negative initial susceptibility (zero permeability) and hysteresis loops characteristic of superconductors. The 8% alloy shows typical ferromagnetic saturation and hysteresis at 5°K, and at 1.3° it appears to be both ferromagnetic and superconducting at the same time. Detection of ferromagnetism in the superconducting 4% alloy is difficult and could not be established.