Abstract
Magnetization vs. temperature measurements were carried out on superconducting YBa2Cu3Ox (YBCO) single crystals. Samples with oxygen concentrations corresponding to x = 6.90, 6.85, 6.78 and 6.72 were studied. Zero field cooling (ZFC), field cooled cooling (FCC) and field cooled warming (FCW) prescriptions were employed. Measurements were performed in the temperature interval between 20 K and 120 K in constant magnetic fields ranged from 0.05 T to 5 T. A negative slope was observed in the FCC and FCW curves below a field-dependent minimum located in temperatures less than the irreversibility point (Tirr). This negative slope is identified as the high-field paramagnetic effect (HFPE). With a fixed magnetic field, the magnitude of the observed HFPE increases as the temperature decreases. The field dependence of the HFPE at fixed temperatures is not trivial. The overall magnitude of the measured HFPE depends on the oxygen content and becomes smaller when x decreases. The results are discussed on the basis of the effects of flux compression and spin moment polarization inside the vortex cores.
Highlights
Some specific values of magnetic field and temperature superconductors go into Meissner state, i.e. they expel the magnetic flux from the inside
It was found in textured YBa2 Cu3 Ox (YBCO) monoliths that above a “neutral zone” of fields, where the expected diamagnetic response was observed, the paramagnetic effect reappeared at relatively higher fields and became stronger as the field was increased
The critical temperatures of our samples were estimated from the intersection point of the extrapolated straight lines which fit the Zero field cooling (ZFC) measurements in the superconducting and normal states performed in a magnetic field of 1 mT
Summary
Some specific values of magnetic field and temperature superconductors go into Meissner state, i.e. they expel the magnetic flux from the inside. The effect is eventually suppressed when the field intensity surpasses a given value This phenomenon was called the Wohlleben effect or Paramagnetic Meissner Effect (PME). Later on, it was found in textured YBCO monoliths that above a “neutral zone” of fields, where the expected diamagnetic response was observed, the paramagnetic effect reappeared at relatively higher fields and became stronger as the field was increased. Published under licence by IOP Publishing Ltd doi:10.1088/1742-6596/568/2/022036 produced by the d-wave symmetry of the order parameter [5] and formation of spontaneous currents Those currents would be caused by the occurrence of Josephson π junctions between grains [6]. We present some magnetization measurements as an effort to elucidate some possible causes of the appearance of the HFPE in deoxygenated YBCO single crystals
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