Spin-injection properties and conductance spectra of Co/Au/YBaCuO and Co/Au/BiSrCaCuO tunnel junctions

Tunnelling spectra of Co/Au/YBa₂Cu₃O_y (YBaCuO) and Co/Au/Bi₂Sr₂CaCu₂O_x (BiSrCaCuO) tunnel junctions have been studied in order to understand the electrical properties of ferromagnet/high-T_c superconductor junctions, and to study the difference between YBaCuO and BiSrCaCuO. The energy gap structures and a zero-bias conductance peak (ZBCP) are clearly observed in conductance spectra for both Co/Au/YBaCuO and Co/Au/BiSrCaCuO junctions. This result can be compared with that of Au/YBaCO and Au/BiSrCaCuO junctions, a normal metal/superconductor (S/N) junction. In the case of Co/Au/YBaCuO and Co/Au/BiSrCaCuO junctions, the amplitude of the ZBCP is smaller than that of the S/N junctions. This result indicates that the tunnelling properties are strongly affected by a spin polarization of electrons flowing through the junctions. Furthermore, the amplitude of the ZBCP observed in the Co/Au/YBaCuO junction is smaller than that of the Co/Au/BiSrCaCuO junction. The difference is attributed to the difference in anisotropy of the two superconductors.