Intergrain flux-pinning in relation to structural phase transformation and tweed formation in YBa 2(Cu 1- xFe x) 3O 7- y and NdBa 2(Cu 1- xFe x) 3O 7- y

Abstract

Fe-doped superconductors REBa 2(Cu 1- x Fe x ) 3O 7- 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 x c≈0.030 for YBa 2(Cu 1- x Fe x ) 3O 7- y and at x c≈0.021 for NdBa 2(Cu 1- x Fe x ) 3O 7- y . The spontaneous strain ϵ=2( b- a)/( a+ b) shows tricritical-like scaling as ϵ∼| x- x c| 1 4 , and tweed formation is indicated by diffraction-line-broadening effects. For YBa 2(Cu 1- x Fe x ) 3O 7- 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 O 2 at 400°C for 10 h show a significantly steeper linear decrease of T c with x in the tweed regime. Contrary to the Y-compound, NdBa 2(Cu 1- x Fe x ) 3O 7- 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, J 0, and pinning force densities α J 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 dα J /d x 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.