Heat treatment and anomalous peak effect in J(c)-H curve at 77 K for NdBa2Cu3O7-delta single-crystal superconductor

To investigate whether for NdBa2Cu3O7-delta (Nd123) 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 parallel to a-axis or H parallel to c-axis for a constant heat treatment at 340 degrees C for 200 h showed no anomalous peak effect. On the other hand, when two step heat treatments, where the first step heal treatment was set to be at 500 degrees C for 100 h and the second one was set to be at 340 degrees C for 200 h, were carried out, the J(c)-H curve at 77 K with only H parallel to 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 parallel to c-axis was considered to be not due to an intrinsic property. Further it was thought that the solid reaction occurred at 500 degrees 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 parallel to 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.