Spin polarization measurements in Fe$_{4}$N/MgO/NbN tunnel junctions using quasiparticle tunneling spectroscopy

In spintronic applications, it is thought that Fe$_{4}$N is useful because it has been theoretically expected to have high spin polarization. Fe$_{4}$N/MgO/CoFeB magnetic tunnel junctions (MTJs) were fabricated, and an inverse tunnel magnetoresistance (TMR) effect was reported by our groups. However, spin polarization of Fe$_{4}$N is yet incompletely understood. We investigated spin polarization of Fe$_{4}$N, using quasiparticle tunneling spectroscopy (QTS), and the measured spin polarization of Fe$_{4}$N was compared with that in Fe$_{4}$N/MgO/CoFeB MTJs. Spin polarization of ferromagnetic materials can be directly measured by QTS. By using NbN and MgO as superconducting electrode and barrier layers, respectively, Fe$_{4}$N/MgO/NbN tunnel junctions were fabricated by magnetron sputtering. The thickness of MgO barrier layer was varied from 1.0 to 1.5 nm. The areal resistances of the Fe$_{4}$N/MgO/NbN tunnel junctions were close to those of Fe$_{4}$N/MgO/CoFeB MTJs, which had the same thickness of MgO barrier layer as those Fe$_{4}$N/MgO/NbN tunnel junctions. In QTS, spin polarization of Fe$_{4}$N was estimated to be $\sim -$0.68. This value is larger than spin polarization in MTJs.