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

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 Hc of superconducting Al to quasiparticle tunneling above the Hc. 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 Al2O3 tunneling junctions is roughly proportional to 0.2 mV. In Al-Ni-Al system, the variation of MR can be understood by the same Hc 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 Hc 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 Hc of various superconductors