Effect of Gd magnetic impurities on the superconducting properties of highly disordered molybdenum nitride microstrips

Abstract

We investigated the impact of Gd magnetic impurities on the superconducting properties of disordered MoN x thin films grown by reactive co-sputtering at room temperature. The samples were fabricated in the two gun cluster using Mo and Gd targets in an Ar + N2 atmosphere with a total pressure of 0.67 Pa and a N2 concentration of 7%. The study focuses in thin films with thickness between 22 and 27 nm. We modified the chemical composition by varying the power of the Gd target from 0 to 24 W, fixing the Mo in 100 W. The chemical state of the samples was analysed using X-ray Photoelectron Spectroscopy. The samples primarily exhibit an amorphous metallic Mo and Gd nitride structure. With the addition of Gd, the superconducting critical temperature is suppressed, dropping from 7.4 K for pure MoN x to below 3 K for samples containing ≈ 4.5 at.% Gd. Furthermore, the temperature dependence of the upper critical field (H c2) and the vortex dissipation mechanism are also affected. Specifically, H c2(0) is reduced from 10 to 7.7 T for samples with 3.1 at.% Gd. Moreover, at a temperature of 3 K, there is an increase in the magnetic field dependence of the critical current density for low Gd doping. These alterations also impact the instability of the vortex lattice, limiting the maximum achievable vortex velocity during dissipation.