Effect of annealing on the superconducting properties of two amorphous alloys: Nb70Zr15Si15 and Zr85Si15

Abstract

The changes in the superconducting and electronic properties of amorphous Nb70Zr15Si15 and Zr85Si15 alloys with annealing were examined with an aim to evaluate the effect of structural relaxation on the superconductivity of metal-metalloid type amorphous alloys.Tc rises once from 3.99 to 4.42 K on annealing at temperatures below about 473 K for the Nb-Zr-Si alloy and from 2.71 to 2.75 K at temperatures below about 373 K for the Zr-Si alloy, and with further rising annealing temperature,td, lowers monotonically to a final relaxed value (≃3.15 K for Nb70Zr15Si15 and ≃2.49 K for Zr85Si15), which is independent of the previous thermal cycling. These results indicate that the thermal relaxation of an amorphous phase occurs through at least two stages. The lowering ofTc occurs exponentially withtd, and an activation energy for the relaxation process and the frequency of jump over the barrier were estimated to be about 2.03 eV and 2.4×1014 sec−1 for Nb70Zr15Si15 and about 1.28 eV and 1.2×1011 sec−1 for Zr85Si15, respectively. The high frequencies indicate that the relaxations occur more or less independently of each other in a non-co-operative manner. The dressed density of electronic states at the Fermi level,N(Ef) (1+λ), which was calculated from the measured values ofρn and (dHc2dT)Tc, exhibited a similar annealing temperature dependence to that ofTc. From this the change inTc on thermal relaxation was interpreted as due to the changes in λ and/orN(Ef). From the depressions ofJc(H) and fluxoid pinning force on annealing in a temperature range of 473 to 873 K, it was concluded that the structural relaxation from a less homogeneous quenched-in state to a homogeneous stable state occurred on the scale of coherence length (≃7.5 nm) during the annealing.