Logarithmic to nonlogarithmic flux creep transition and magnetic flux hardening in BiSrCaCuO superconducting ceramics

A sharp temperature and field dependent transition from a logarithmic to a non-logarithmic flux creep behavior has been found in BiSrCaCuO superconductors. The transition is not followed by any noticeable change in the magnetization. However, vibrating reed measurements show that it coincides with a strong hardening of the magnetic structure. The transition occurs at fields and temperatures as well below the reversibility line. The possible relation to the recently proposed flux lattice melting is discussed