Optimization of minute doping of Y_1−xRE_xBa₂Cu₃O_7−δ thin films with RE = Tb and Nd

Abstract

Doping of YBa₂Cu₃O_7-δ (YBCO) has become an effective means of increasing the flux pinning and critical current densities (J_c) in thin film superconductors, while maintaining the transition temperature (T_c). In previous research efforts, our group showed that doping (Y_1-xRE_x)BCO with typically deleterious rare earth (RE) elements can be used to improve the film’s current density via flux pinning when the x molar additions are less than 1%. However, data was only presented for different orders of magnitude (x = 0.1%, 1.0%, 10%) without consideration of optimization. The research presented here demonstrates that the deleterious RE elements can differ greatly in how broad the range of optimal doping concentration is, in addition to the relative doping concentration. Rare-earth elements Nd and Tb were compared due to the difference in degradation mechanisms: Nd additions results in Ba site substitution and Tb123 exhibits poor phase formation. Thin films of Nd and Tb doped YBCO films were grown by pulsed laser deposition (PLD) using standard deposition parameters for plain YBCO. The compositions studied were (Y_1-xRE_x)BCO where x was varied from 0.0001 to 0.025 for Nd and 0.005 to 0.015 for Tb. Targets for PLD were prepared using solid state reaction and sintering procedures. All films were characterized for J_c and T_c by vibrating sample magnetometry. Data for J_c(H,T) and T_c were compared to undoped YBCO films processed under the same conditions. The results show a measurable increase in flux pinning for both different concentrations and range of Nd and Tb doping, with little decrease in T_c.