Instability of the critical state in NdBa2Cu3O7-delta single crystals

Instabilities of the critical state are observed in various NdBa2Cu3O7-delt a single crystals by means of direct magneto-optic (MO) imaging while warmi ng up flux states containing vortices of opposite polarity. Using the same samples as in the MO investigations, we observe for the first time characte ristic steps in the integral magnetization measured by means of a SQUID sys tem, which allows a constant temperature sweep. The integral measurement te chnique enables the study of the instability phenomenon ("turbulence") in a much wider range of temperatures and fields than the MO investigations, an d also in the orientations H-a parallel to c and H-a perpendicular to c. As a result, we find turbulence not only in a narrow window of temperatures ( 65 less than or equal to T less than or equal to 80 K), but also in a windo w of applied negative fields ranging between 30 and 80 mT.