The influence of magnetic impurities on superconductivity in the high entropy alloys [TaNb]_1-x(TiZrHf)_x

High entropy alloys are an unusual class of materials where random mixtures of selected elements are distributed on a simple crystalline lattice (e.g., BCC). The alloys [TaNb]_1-x(TiZrHf)_x are damage resistant refractory metals that also exhibit superconductivity with T_c<8K. Here we report results for examples where Gd is substituted into the lattice in small amounts. X-ray diffraction measurements show that, like the parent compounds, the substituted specimens form in the BCC structure. Chemical analysis measurements also confirm uniform mixing of the components. Magnetization measurements reveal ferromagnetic ordering near room temperature, likely due to localized Gd f-moments. At low temperatures, electrical resistivity, magnetization, and heat capacity measurements show that T_c is modestly suppressed. These results will be compared to earlier studies of matrix-impurity superconductors, where the evolution of T_c and the specific heat jump ΔC at T_c provide insights about the solute spin and the nature of the superconducting pairing.