Correlation between anomalous peak effect in magnetic hysteresis loop and nanoscale structure for NdBa₂Cu₃O_7-δ single-crystal superconductor

To investigate the correlation between an anomalous peak effect in the magnetic hysteresis (M-H) loop and microstructure for the NdBa₂Cu₃O_7-δ (Nd123) single-crystal superconductor, M-H loops were measured with a superconducting quantum interference device (SQUID) magnetometer, and microstructures were observed by transmission electron microscopy (TEM). A TEM image (observed from [001] direction) of the Nd123 single crystal, which has an anomalous peak in the M-H loop, shows a nanoscale modulated structure (tweedlike image). This crystal also has the twin structure, as observed by polarized optical microscopy. In contrast, the Nd123 crystal, which has shown no anomalous peak effect, was found to possess no modulated structure, but twin structure was observed. Therefore the nanoscale structure (the modulated structure) is considered to be responsible for the anomalous peak effect in the M-H loop. Moreover temperature independence of the anomalous peak field in M-H loops was confirmed. We conclude that the nanoscale modulated structure may act as effective pinning center when its length matches that of the vortex lattice spacing.