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VOLUME 54 (1991) | ISSUE 6 | PAGE 335
Magnetic relaxation in Tl2Ba2CaCu2Oy single crystals
A qualitative change in the dynamics of the vortex structure in Tl-2212 single crystals is manifested at 7), =s 23 + 1 K. It is seen on the temperature dependence of the remanent magnetization, MR (T). It is also seen as an abrupt change, of nearly an order of magnitude, in the normalized logarithmic flux-creep rate S. The latter change is evidence of an increase in the effective height U0 of the barriers which determine the motion of the vortices. This increase is from ~ 19 meV at !T< 7} to U0 ~ 140 me V at 7} < Г< 60 К. No abrupt change in the relaxation rate is observed in Y!Ba2Cu207. TheSX T) curves for Tl-2212, Bi-2212, and 123-YBCO single crystals are qualitatively similar at Tj < T. They go through a maximum at 60-70K.