Effect of magnetic iron impurity on the superconducting properties of an amorphous Nb₅₀Zr₃₅Si₁₅ alloy

The effect of magnetic iron impurity on the superconducting properties of amorphous Nb₅₀Zr_35-xSi₁₅Fe_x (x≤4 at %) alloys was examined. Doping with an iron impurity resulted in a linear depression of T_c and H_c2(T) and a decrease in {Mathematical expression} and ρ_n after reaching a maximum value at 0.5 to 1.0 at % iron. The observed decrease was about 35% for T_c, 85% for H_c2 at 2.0 K, 16% for {Mathematical expression} and 21% for ρ_n. Although the decrease in {Mathematical expression} occurs through the decrease in ρ_n as expected from the GLAG theory, the depression in T_c caused by magnetic impurity could not be explained in terms of the GLAG theory which is applicable to Nb-Zr-Si amorphous alloys without magnetic impurity, but was interpreted as arising from the pair-breaking effect in the superconducting nature due to magnetic scattering. However, the pair-breaking effect was found to be smaller by about one-tenth for the present amorphous superconductors than for crystalline superconductors, indicating the high stability of the superconductivity of the Nb-Zr-Si-Fe alloys against the magnetic scattering arising from the magnetic impurity. The reduced magnetic field at which the reduced fluxoid pinning force exhibits a maximum value increased with iron concentration, indicative of an enhancement of fluxoid pinning force. The enhancement in fluxoid pinning force was interpreted as arising from the increase in compositional, electronic and/or magnetic fluctuations by the dope of iron impurity.