Instability of the Critical State in NdBa₂Cu₃O_7-δ Single Crystals

Instabilities of the critical state are observed in various NdBa₂Cu₃O_7-δ single crystals by means of direct magneto-optic (MO) imaging while warming up flux states containing vortices of opposite polarity. Using the same samples as in the MO investigations, we observe for the first time characteristic steps in the integral magnetization measured by means of a SQUID system, which allows a constant temperature sweep. The integral measurement technique enables the study of the instability phenomenon ("turbulence") in a much wider range of temperatures and fields than the MO investigations, and also in the orientations H_a ∥ c and H_a ⊥ c. As a result, we find turbulence not only in a narrow window of temperatures (65 ≤ T ≤ 80 K), but also in a window of applied negative fields ranging between 30 and 80 mT.