Critical current densities and irreversibility fields of new high-T_c Ba₂CaCu₂O₄(O,F)₂ superconductor

Ba₂CaCu₂O₄(O_2-yF_y) (F-0212), a new high-T_c superconductor (HTSC) including fluorine at an apical site, is an attractive material for application because it shows a highest T_c of 108 K and has thin spacing between conduction layers that is favorable for high critical current density (J_c) and irreversibility field (B_irr). The polycrystalline samples of F-0212 were synthesized under high pressure. A doping state was controlled from under doping (T_c = 82 K) to slightly-over doping (T_c = 106 K) via optimally-doping (T_c = 108 K). From the DC magnetization hysteresis loops, we determined the intragrain J_c and B_irr using Bean's critical state model. The J_c and B_irr properties of F-0212 were found to improve rapidly as the doping state changes from under doping to slightly-over doping to via optimally doping state. The irreversibility lines for the optimally- and slightly-over doped F-0212 are higher than optimally-doped Bi-2212. However, they are much lower than that of Cu-1212 (Y-123) though the spacing of F-0212 is thinner than that of Cu-1212. This indicates that property of the spacing is important to enhance B_irr as well as its thickness. An anisotropy factor for under doped F-0212 was estimated to 118 from a 3D-2D crossover field of about 0.28 T