Intergrain Flux-Pinning in Relation to Structural Phase-Transformation and Tweed Formation in Yba2(Cu1-Xfex)3o7-Y and Ndba2(Cu1-Xfex)3o7-Y
Physica C 199(3-4): 414-424
Fe-doped superconductors REBa2(Cu1-xFex)3O7-y (RE = Y, Nd; 0 less-than-or-equal-to x less-than-or-equal-to 0.1) were investigated by conventional and synchrotron X-ray diffraction, thermogravimetric analysis and AC susceptibility measurements. The transition from orthorhombic to macroscopically tetragonal symmetry is analogous for both systems and occurs at x(c) almost-equal-to 0.030 for YBa2(Cu1-xFex)3O7-y and at x(c) almost-equal-to 0.021 for NdBa2(Cu1-xFex)3O7-y. The spontaneous strain epsilon = 2(b-a)/(a+b) shows tricritical-like scaling as epsilon approximately \x-x(c)\1/4, and tweed formation is indicated by diffraction-line-broadening effects. For YBa2(Cu1-xFex)3O7-y the onset temperature T(c) of superconductivity is almost constant below x = 0.03 (i.e., in the orthorhombic regime) and falls linearly for x> 0.03 in the tweed regime, extrapolating to zero near x = 0.2. Samples which were treated twice in flowing O2 at 400-degrees-C for 10 h show a significantly steeper linear decrease of T(c) with x in the tweed regime. Contrary to the Y-compound, NdBa2(Cu1-xFex)3O7-y shows no plateau of T(c) in the orthorhombic regime, and T(c) drops rapidly with dopant concentration x. The low-field AC susceptibility of each sintered sample is strongly dependent upon applied AC RMS and superimposed DC bias fields, owing to the weak-link network of the superconducting grains. Critical transport current densities, J0, and pinning force densities alpha(J) for the inter-granular vortices were estimated from AC susceptibility measurements. Both quantities are strongly suppressed by Fe-doping and the substitution of Nd ions for Y ions. A distinct change of the slope d-alpha(J)/dx at the critical composition x(c) for both the YBCO and the NdBCO series indicates a softening of the free energy gradient between superconducting grains and non-superconducting intergrain material due to the formation of twin-domain boundaries in the grains.