Magnetoresistance study in Ni–Al–Ni and Al–Ni–Al tunneling junction systems

Abstract

Magnetoresistance (MR) in Ni–Al–Ni and Al–Ni–Al tunneling junction systems have been studied at temperatures between 40 mK and 4 K and in magnetic fields up to 3 T. In Ni–Al–Ni system, the resistance increases with increasing applied magnetic field at low fields. This is a typical anisotropy MR for ferromagnetic Ni wire electrodes. The resistance starts to decrease on increasing the magnetic field to roughly around 1.5 and 0.2 T for samples at 40 mK and 1.3 K, respectively. This does not happen for samples at 4 K. This phenomenon is attributed to the electron tunneling effect which is changed from cooper pair electron tunneling below the critical magnetic field H c of superconducting Al to quasiparticle tunneling above the H c. From the I– V characteristics, the superconductivity energy gap is roughly proportional to (1.2–0.2)/4 mV, and the tunneling junction barrier of the two Al 2O 3 tunneling junctions is roughly proportional to 0.2 mV. In Al–Ni–Al system, the variation of MR can be understood by the same H c mechanism. However, the anisotropy MR was not observed due to our experimental limitation of error. The anisotropy MR of the Ni electrode wires in Ni–Al–Ni case, and MR variation due to the H c of the Al island in Ni–Al–Ni case and Al electrodes in Al–Ni–Al case has been experimentally observed. Finally, this study provides us with a powerful method to study the critical magnetic field H c of various superconductors.