Superconducting tunneling as a probe of sputtered oxide barriers

The tunneling properties of sputtered oxide barriers were studied in Pb/oxi de/ferromagnet junctions. The initial oxide/ferromagnet bilayer was made wi thout breaking vacuum. The bilayer was exposed to atmosphere before the dep osition of a Pb counterelectrode in a separate vacuum chamber. I-V curves a nd conductance measurements at 1.5 K confirmed the presence of single-step, elastic tunneling in these structures. Separate experiments involving gold bottom electrodes, variation of exposure times in air between bilayer and Pb depositions, and plasma oxidation proved that the oxide layer is permeab le, allowing for oxidation of the ferromagnetic base electrode. This reveal ed that the tunneling channels were due to the contribution of small-area j unctions at thermally oxidized "pinhole" sites on the bottom electrode. (C) 1999 American Institute of Physics. [S0003-6951(99)03327-6].