Investigations on the flux pinning and an irreversibility line in different YBCO modifications by ac susceptibility measurements

On polycrystalline specimens, melt grown thick films and sputtered thin films of YBa2Cu3O7-x we have investigated the imaginary part (χ'') of the ac susceptibility in dependence of frequency (1-50) kHz and ac amplitude (1 mOe-25 Oe). For the melt grown thick films and the sputtered thin films the observed loss-peak temperature (Tp) shifts only slightly to lower temperatures with increasing ac amplitude. This behavior is due to a strong flux-pinning mechanism. For the polycrystalline specimen the intergranular Tp decreases strongly with increasing ac amplitude above 1.5 Oe, resulting in a strong temperature dependence of the intergranular critical current density Jcj. With increasing frequency we observe an enhancement of Tp for all investigated YBCO modifications. From the frequency dependence of Tp we determine the flux pinning potential U0(T=0, HAC=0) to some eV. A irreversibility line was derived from the Tp dependence of the ac amplitude and will be discussed in terms of a phase transition from flux glass to flux flow. (orig.)