Changes in vortex pinning produced by Fe doping of polycrystalline YBCO

For polycrystalline samples of YBa2Cu3-xFexO7-delta of various x up to 0.15 , the saturation remanent magnetization (M-R) was measured as a detailed fu nction of temperature (T) between 4.2 K and the x-dependent T-c, after it w as first confirmed experimentally that M-R is directly related to the avera ge vortex pinning forces, as predicted by the critical-state model. For eac h Fe concentration (x), the M-R versus T data were closely fitted by the em pirical relationship, M-R = M(3)t(beta) + M(2)t(1/2) where t = 1 - T/T-c an d M-1, M-2, and T are adjustable. Since M-1 much greater than M-2 and beta much greater than 1 for all x, the t(beta) term is dominant at low rand the t(1/2) term is the residual component at higher T. For all x, M-R extrapol ates to nearly the same values at T = 0. However, as x is raised, the decre ase of M-R with increasing T becomes much more rapid. Thus, the Fe doping o f YBCO has essentially no effect on the vortex pinning at zero temperature. but it does cause the thermal weakening of the pinning process to be great ly enhanced. (C) 2001 Elsevier Science B.V. All rights reserved.