Ren, Y.; Schmahl, Wolfgang W.; Brecht, E.; Fuess, H.
Intergrain flux-pinning in relation to structural phase transformation and tweed formation in..
In: Physica C: Superconductivity and its Applications, Vol. 199, Nr. 3-4: S. 414-424
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Title full: Intergrain flux-pinning in relation to structural phase transformation and tweed formation in YBa2(Cu1-xFex)3O7- y and NdBa2(Cu1-xFex)3O7-y. Fe-doped superconductors REBa2(Cu1-xFex)3O7-y (RE=Y, Nd; 0≤x≤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 xc≈0.030 for YBa2(Cu1-xFex) 3O7-y and at xc≈0.021 for NdBa2(Cu1-xFex)3O7-y. The spontaneous strain εlunate=2(b-a)/(a+b) shows tricritical-like scaling as εlunate∼|x-xc|1 4, and tweed formation is indicated by diffraction-line-broadening effects. For YBa2(Cu1-xFex)3O7-y the onset temperature Tc 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°C for 10 h show a significantly steeper linear decrease of Tc with x in the tweed regime. Contrary to the Y-compound, NdBa2(Cu1-x Fex)3O7-y shows no plateau of Tc in the orthorhombic regime, and Tc 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 αJ for the intergranular 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 dαJ/dx at the critical composition xc 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.