Magnetization curves of (La_1−x Sr _x )₂CuO_4−δ single crystals as functions ofx and δ

Abstract

The second peak effect in magnetization curves for overdoped (La_1−xSr_x)CuO_4−δ single crystals was examined by means of systematic variations of Sr content x and oxygen deficiency δ. Oxygen defect concentration was found to sensitively affect the critical temperature T_c and the macroscopic pinning force F_p, resulting in significant changes in magnetization hysteresis loops. Observations of δ dependence of ΔM and F_p at the same reduced temperature T/T_c yielded an insight into the role of oxygen defects: increasing δ results in increasing the pinning center density N. The second peak field B_2pk seemed to be determined by the development of a percolating network of magnetically reversible regions from the observation that the temperature dependence of B_2pk showed similar behavior to the irreversibility field B_irr.