Evidence of Current Stabilization after a Long-Time Decay in High-Tc Superconductors

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In this work, we studied the flux creep phenomenon over a long period [viz. about 1.5 10E7 s (i.e. for 6 months)] at liquid nitrogen temperature. For this purpose, four high-Tc ring-shaped samples were field-cooled (one sample) or ferromagnetic field-cooled (three samples) in order to induce a persistent current. The resulting current decay was measured using a Hall probe system and the results obtained revealed low relaxation rates in the ferromagnetic field-cooled samples. Also, the experimental data were found to depart from the prediction of the classical models after a long enough time. The slope of the logarithmic current decay plot exhibited an oscillatory phenomenon at ca. 2 10E5 s (about 55 hours). Oscillations vanished at ca. 4-5 10E6 s (46-55 days), after which the induced current remained stable throughout the remainder of the experiencing period.