Superconducting and normal state tunnelling in NbN-GdN-NbN junctions

Abstract

Simultaneous occurrence of insulating and ferromagnetic behaviour in GdN makes it an ideal spin filtering material suitably compatible with other transition metal nitrides, such as NbN. Here we present the effect of spin-filtering on the normal state and superconducting state tunnelling current in lithographically patterned Josephson junctions. A large suppression of junction critical current is observed below Tc of NbN, from which we estimate a spin-filtering efficiency of ∼75% in these room temperature grown GdN layers. The temperature dependence ofsuper-current deviates from the usualAmbegaokar-Baratoff relation at low temperatures, roughly scaling with the magnetic coercivity of the GdN barrier. The critical current Fraunhofer pattern shows highly suppressed side lobes and possible signature of a second harmonic Josephson current. In the normal state conductance we observe possible signatures of spin accumulation effect, which leads to a decrease in conductance at low bias voltages.