FLUX-STRUCTURE RELAXATION IN A BI2SR2CA1CU2OX MONOCRYSTAL

Recently, we have carried out extensive electronic and magnetic property studies on a rather good monocrystal of Bi2Sr2Ca 1Cu2Ox in which we find transport critical currents of 105 A/cm2 along the a-b plane at 4.2 K and at fields as large as 3 T. These crystals, grown by the floating-zone technique by Balbashov (Moscow Power Institute), exhibit some unusual flux-flow, relaxation, and flux-melting properties. In our magnetic-relaxation measurements at any temperature, we go through the entire hysteretic cycle at least twice and stop at predetermined fields in both segments of the loop. A part of the typical relaxation data at 4.2 K in a field of 1 T is shown in the figure as a function of ln t. The data over 3 decades can be fitted with no measurable deviation to the form M(t)=A+Bln t. Both parameters A and B decrease, while the ratio B/A increases with increasing field, indicating that the flux structure becomes unstable at higher fields. At higher temperatures Jc (transport) decreases rather quickly (an order of magnitude less at 20 K than at 4.2 K) and the hysteretic loop collapses even at low fields (as low as 2 T at 20 K). Furthermore, the magnetization relaxes almost to a zero value at much shorter times (tens of seconds at 20 K, 1 T; see figure). Data on the effect of neutron-irradiation and flux-melting phenomena in these monocrystals will also be presented.