Magnetovoltage Measurements and Hysteresis Effects in Polycrystalline Superconducting Y₁Ba₂Cu₃O_7−x /Ag in Weak Magnetic Fields

Abstract

The magnetovoltage measurements (V–H curves) with different sweeping rates (dH/dt) of the external magnetic field in Ag-added polycrystalline YBa₂Cu₃O_7−x sample (YBCO/Ag) were investigated. The measurements of V–H curves were carried out as functions of the transport current (I) and temperature (T). Upon cycling H, all V–H curves measured for different values of I exhibit a clockwise hysteresis effects in forward region. The hysteresis effects in the V–H curve were interpreted in terms of two-level magnetic system, which considers the superposition of the external magnetic field and the local magnetic fields in the intergrain boundaries induced by magnetic dipole moment of neighbor superconducting grains. The analysis of magnetovoltage data showed that the flux trapping in the junction network has a negligible effect on the evolution of the V–H curves and the irreversibilities arise mainly from the flux trapping inside the grains. It is shown that the width of V–H curves shows a universal scaling behavior with respect to the applied current below the critical temperature T _c . The comparison of V–H curves of the YBCO/Ag sample with those of YBCO shows that adding Ag to the superconducting structure weakens the pinning properties of Josephson medium and provokes the instabilities in measured dissipation. The presence of Ag in the superconducting matrix causes marked decrease in hysteresis effects and makes the V–H curves dH/dt dependent. At high values of dH/dt, the instabilities and plateau regions in V–H curves increase significantly as compared to those of observed in YBCO. In addition, the interrelation between the evolution of V–H curves obtained for different values of I and the critical current I _c was demonstrated experimentally.