Single crystal growth of Nd123 superconductive oxides: New process to control critical currents

To investigate whether or not the anomalous peak effect in the magnetization hysteresis (M-H) loops is an intrinsic property of NdBa₂Cu₃O_7-δ (Nd123), different series of oxygen heat treatments were performed in a pure oxygen gas flow using Nd123 single-crystals, and critical current density (J_c)-H curves at 77 K were determined from M-H loops. The J_c-H curve for H parallel to either the a-axis or the c-axis of a Nd123 crystal heat treated at 340°C for 200 h exhibited no anomalous peak effect. In contrast, the J_c-H curve with parallel to the c-axis of a Nd123 crystal subjected to a two-step heat treatment (500°C for 100 h and 340°C for 200 h) exhibited an anomalous peak effect. Further annealing (900°C for 100 h, quenched, 340°C for 200 h) almost resulted in the disappearance of the peak effect. We propose that a reversible solid-state reaction, such as spinodal decomposition, modifies the nanoseale structure of the bulk Nd123 crystal. A wave-like fluctuation (wave length ≈ 50 nm) of the Ba/Nd atomic ratio in the Nd123 crystal with the anomalous peak effect, which was observed by using transmission electron microscope (TEM) equipped with an energy dispersive X-ray spectroscopy (EDS) system, supports this proposed reaction.