Fishtail phenomenon in Ho-added PMP YBCO

The Y_{1- x} Ho_xBa₂Cu₃O_y superconductors were fabricated by the powder melting process (PMP) method and the magnetic hysteresis loops of these samples were measured at different temperatures by a SQUID magnetometer (up to 7 T). Critical current density was calculated by the Bean critical state model. The results indicate that the fishtail effect is observed in Y_0.6HO_0.4Ba₂Cu₃O_y below 60 K, while no fishtail can be found in Y_0.8Ho_0.2Ba₂Cu₃O_y until 30 K. The peak field at which J_c reaches its maximum decreases with the increasing magnetic field. In the Y_0.6Ho_0.4Ba₂Cu₃O_y sample, he fishtail effect disappears when the temperature is raised to 77 K. In addition, after the Y_0.6Ho_0.4Ba₂Cu₃O_y sample was re-sintered at high temperature for a long time, the fishtail can be found in two field directions (H//C and H⊥C). It is considered that the paramagnetism in the sample may be responsible for the fishtail effect in Ho-added YBCO. Also, the cation disorder created by Ho addition may be another reason for the fishtail.