Andreev reflection measurements in nanostructures with amorphous WC_x superconducting contacts

Point-contact Andreev reflection measurements of Co/ and Cu/tungsten-carbide (WC$_x$) contacts are presented. Metallic thin films were patterned by e-beam-lithography and lift-off; tungsten carbide superconducting tips were grown directly on the pre-patterned samples by decomposition of a metallo-organic vapour (tungsten hexacarbonyl) under a focused Ga$^+$-ion beam (FIB). Current-voltage measurements as a function of temperature and magnetic field clearly showed the signatures of Andreev reflection. The experimental conductance-voltage curves were analyzed within the Blonder-Tinkham-Klapwijk theory. The results highlight the possibilities, advantages and disadvantages of using FIB-produced amorphous WC$_x$ tips for point-contact spectroscopy in metallic nanostructures.