Superconducting aluminum electrodes have typically been used to measure the spin polarization of tunneling current using superconducting tunneling spectroscopy but at temperatures well below . Using NbN superconducting electrodes we show that measurements at elevated temperatures are possible, thereby eliminating the need for the dilution refrigerators. Tunneling spin polarization values can be extracted from the experimental data by fits using a strong coupling theory of the quasiparticle density of states of NbN. Nb and Ta electrodes were also explored but NbN, with its much lower spin-orbit scattering rate, yet higher superconducting transition temperature, is the most attractive.
We note that the resistivity of our NbN films varies little with temperature even for thick NbN films. For example, the resistance of a NbN film thick is 1.2 times higher at than at .