Effect of implanted metal impurities on superconducting tungsten films

The superconducting transition temperature of more than 30 thin-film tungsten samples was measured using a dilution refrigerator. The samples were fabricated using a 99.999% pure tungsten target and a dc magnetron sputtering system. Individual films were then doped with metal impurity ions using an accurate ion implantation technique. The effect of the metal-ion doping on the superconducting transition temperature was measured for samples with superconducting transitions in the range of 40-150 mK. Magnetic dopant species including Ni, Co, and Fe resulted in suppressed values of the tungsten T-c. The suppression was linear with increasing dopant concentration, for concentrations up to tens of ppm. For higher concentrations of magnetic atoms, the data are consistent with the Abrikosov-Gor'kov theory [Soviet Physics JETP 12, 1243 (1961)] modified by antiferromagnetic impurity-impurity interactions. By contrast, tungsten films implanted with Mg or Cr showed little change in T-c after doping. In this article, we present data from cryogenic experiments on these films. We also present x-ray diffraction (XRD) spectra for a subset of the films. Our XRD data confirm that the observed suppression in T-c for the magnetically doped samples is not due to any structural changes (e.g., lattice distortion or damage) induced by the implantation process. (C) 2002 American Institute of Physics.