Measurement of Tc suppression in tungsten using magnetic impurities

We have measured the effects of dilute magnetic-atom doping on the superconducting transition temperature of tungsten thin films. Our “$T_c$ tuning” technique is accurate, precise, and simple. Experiments were performed using dc-magnetron-sputtered tungsten films with undoped values of $T_c$ in the range of 70–150 mK. The magnetic-atom doping was achieved using ion implantation. Specific $T_c$ suppressions of between 5% and 65% were targeted and observed in this study. The transition width of each undoped sample was ≈1 mK and the transition widths remained sharp after implantation with ${}^{56}{\mathrm{Fe}}^{\mathrm{+}}$ ions. Our data are in good agreement with predictions of a linear dependence of $T_c$ suppression with increasing magnetic-atom concentration, in the small concentration limit. At higher concentrations, antiferromagnetic coupling between the magnetic dopant atoms becomes important and the $T_c$ -suppression effect is diminished. We use our $T_c$ data to calculate the Abrikosov–Gor’kov (AG) and Ruderman–Kittel–Kasuya–Yosida (RKKY) spin–flip relaxation parameters ${\tau }_{\mathrm{AG}}$ and ${\tau }_{\mathrm{RKKY}}.$ We conclude with a brief discussion of applications of the $T_c$ -tuning technique, and present our plans for future studies in this area.