3d遷移金属置換Bi(Pb)2212単結晶の磁束ピンニング特性

Abstract

A large amount of lead doping to Bi2212 superconductors is effective in decreasing their high electromagnetic anisotropy and introduces effective pinning sites, such as lamella structure interfaces due to compositional fluctuation of lead ions, resulting in greatly enhanced flux pinning properties up to high temperatures. Particularly in the high-Tc superconducting materials with lowered anisotropy, further improvement of critical current properties can be expected by doping a small amount of impurity elements, which generate locally weak superconducting regions. Based on this background, we have attempted to enhance flux pinning strength of Bi(Pb)2212 single crystals by 3d metal doping in the present study. Crystal boules with starting compositions of Bi1.6Pb0.6Sr1.8Ca(Cu1-xMx)2Oy (M = Fe, Co, Ni : x = 0, 0.001, 0.002, 0.005, 0.02) were grown by the floating zone method. Tc's of the single crystals annealed to be the carrier overdoped state were dramatically decreased with an increase of metal doping levels. Critical current performance of the 3d metal-doped Bi(Pb)2212 crystals was quite poor at high temperatures near Tc, The crystal with x= 0.001, 0.002, 0.005, however, showed improved Jc properties accompanied by larger second peak effects in their magnetization hysteresis loops when compared with the 3d metal free Bi(Pb)2212 crystal below 50 K. On the other hand, magneto-optical measurements revealed that anisotropy in the in-plane Jc properties (Jca > Jcb ) was systematically suppressed with an increase of the doping level x. These results suggested that a very small amount of 3d metal doping is an essentially effective method to enhance pinning strength of Bi-based superconductors due to their generation of point-defect-like pinning centers. 3d doping, however, does decrease the pinning strength at the lamella structure interfaces.