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Ferromagnet/insulator/superconductor (F/I/S) tunnel junctions consisting of Co and Bi 2 Sr 2 CaCu 2 O x (BSCCO) have been fabricated in order to study whether the superconducting properties are affected by the spin-polarized current. Co is a ferromagnetic metal with a spin polarization of approximately 35%, and BSCCO is a high-T c superconductor with critical temperature (T c ) of 80-90 K. The junctions with 100 × 100 µm 2 have been fabricated using photolithography on the cleaved BSCCO single crystal. As a reference, Au/I/BSCCO tunnel junctions have also been fabricated in the same manner. In both junctions, gap structures were clearly observed in dI /dV spectra. However, the gap energy of 28 mV obtained at 30 K for the Co/I/BSCCO junction was smaller than that obtained for the Au/I/BSCCO junction. The zero-bias conductance peak (ZBCP) was observed clearly for the Au/I/BSCCO junction. It is considered that these differences between the Co/I/BSCCO and the Au/I/BSCCO junctions are due to the spin-polarized current flowing through the junctions.

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