Crossover from diffusive to tunneling regime in NbN/DyN/NbN ferromagnetic semiconductor tunnel junctions

Abstract

We have investigated NbN-DyN-NbN junctions with 1 to 10 nm thick DyN barriers. A crossover from diffusive (hopping) to tunneling-type transport was found in these junctions as the DyN thickness is reduced below $\ensuremath{\sim}$4 nm. We have also made a detailed study of magnetic and electrical properties of thicker DyN thin films deposited under similar conditions; DyN films were found to be ferromagnetic with ${T}_{\text{Curie}}\ensuremath{\sim}35\ifmmode\pm\else\textpm\fi{}5$ K. Electrical transport of the junctions with $\ensuremath{\sim}$10 nm DyN was understood in terms of Shklovskii-Efros (SE)-type variable range hopping (VRH) at low temperature between 90 and 35 K. We estimated localization length $\ensuremath{\xi}=5.6$ nm in this temperature range. Temperature dependence of resistance was found to deviate from SE-VRH below 35 K along with large suppression of resistance with magnetic field. This is correlated with onset of magnetism below 35 K. Large butterfly-shaped MR up to $\ensuremath{\sim}$40$%$ was found for the $\ensuremath{\sim}$10 nm thick DyN junction at 2 K. In the tunneling regime, barrier height of the tunnel junction was estimated $\ensuremath{\sim}$50 meV from the Simmons model. Signatures of spin filtering was found in temperature dependence of resistance in tunnel junction with $\ensuremath{\sim}$3 nm thick DyN. Cooper pair tunneling in these junctions below ${T}_{C}$ ($\ensuremath{\sim}$10.8 K) of NbN was understood according to S-I-S tunneling current model. We found coherent tunneling of Cooper pairs through a $\ensuremath{\sim}$1 nm thick DyN tunnel barrier with critical current ${I}_{C}\ensuremath{\sim}12$ $\ensuremath{\mu}$A. The critical current also showed modulation with magnetic field.