Heat treatment and anomalous peak effect in J_c-H curve at 77 K for NdBa₂Cu₃O_7-δ single-crystal superconductor

To investigate whether for NdBa₂Cu₃O_7-δ (Nd 123) an anomalous peak effect in the magnetization hysteresis (M-H) loops at 77 K is an intrinsic property or not, different series of heat treatments were performed in a pure oxygen gas flow using Nd123 single crystal, and critical current density (J_C)-H curves at 77 K were determined from the M-H loops. The J_C-H curves with either H ∥ a-axis or H ∥ c-axis for a constant heat treatment at 340°C for 200 h showed no anomalous peak effect. On the other hand, when two step heat treatments, where the first step heat treatment was set to be at 500°C for 100 h and the second one was set to be at 340°C for 200 h, were carried out, the J_C-H curve at 77 K with only H ∥ c-axis exhibited an anomalous peak effect. These results indicated that for Nd123 the anomalous peak effect in the J_C-H curve at 77 K (in other words, the M-H loop) with H ∥ c-axis was considered to be not due to an intrinsic property. Further it was thought that the solid reaction occurred at 500°C and this solid reaction resulted in the formation of an anisotropic structure, causing an anomalous peak effect in the J_C-H curve at 77 K with only H ∥ c-axis. Further the anomalous peak effect could be eliminated by the heat treatment. Therefore this solid reaction was found to be a reversible reaction. Accordingly the existence of the anomalous peak in the J_c-H curve at 77 K could be considered to be controlled by the heat treatment. We propose that the phase separation which is derived from the spinodal decomposition of an unstable solid solution takes place and the nano-scale microstructure resulting from phase separation leads to the anomalous peak effect and anisotropic property in the J_C-H curve at 77 K.