Spin polarization of Nb/Ni nanocontacts measured by Andreev reflection in planar thin-film geometry

Abstract Andreev reflection in point contacts between a ferromagnet and a superconductor has been developed in recent years as a method to measure the degree of spin polarization P of the ferromagnet. We have developed the method for planar thin-film edge contacts prepared by a seven-step process. Measurements of the conductance of an Nb/Cu edge junction showed the expected doubling of conductance for bias levels in the range ±1.1 mV, corresponding to the superconducting energy gap of the Nb film ( T SC = 6.5 K ) . For Ni, however, the conductance falls to 20% of its normal value within the energy gap. A fit of the data gives P = 70 % , which is much greater than the currently accepted value of 46%. Possible explanations for the elevated spin polarization in these planar junctions are d-band narrowing and electron transfer to incorporated oxygen.