SQUID and magneto-optic investigations of flux turbulence

Instabilities of the critical state ("turbulence") are observed in various high-T-c, single crystals by means of magneto-optic (MO) imaging. Using the same samples as in the MO investigations, we observe for the first time ch aracteristic steps in the magnetic moment measured by means of a SQUID syst em, which allows a continuous temperature sweep. A direct correlation betwe en the steps and the MO images is established. Using this SQUID measurement s, a much wider range of field and temperature can be covered. As a result, we find turbulence not only in a narrow temperature window, but also in a field window ranging between 0 and 80 mT. Furthermore, we present tests for turbulence in several different high-T-c samples.