VORTEX PINNING AND FLUX CREEP IN BI2SR2(CA1-XYX)CU2O8+Y, BI2SR2(CA1-XGDX)CU2O8+Y

We present magnetisation measurements on powders of Bi2Sr2(Ca(1-x)R(x))Cu2O8+y (R = Y,Gd). The observed magnetic relaxations and the field dependence of the pinning force have been described in terms of a thermally activated flux-creep model without the need to consider phase transitions of the vortex system. We have found that the intra-grain J(c) decreases rapidly with rare earth or Y substitution. Even in the concentration range for which T(c) remains constant, J(c) decreases: J(c)(x = 0)/J(c)(x = .32) = 5.7.