Superlattices and Microstructures, Vol. 22, No. 4, 1997 Flux pinning by a barrier on the surface of YBaCuO grains studied in dynamic regime M. Polichetti Department of Physics, University of Salerno, National Institute of Matter Physics (INFM), 84081 Baronissi (SA), Italy F. G¨ om¨ ory Institute of Electrical Engineering, Slovak Academy of Sciences, 842 39 Bratislava, Slovakia (Received 8 July 1996) The model of a surface barrier was found to explain well the observed behaviour in quasi- static magnetization experiments on high T c superconductors. We study this phenomenon in dynamic conditions in an AC magnetic field with frequencies ranging from 3 Hz up to 115 kHz. The experimental data recorded on a polycrystalline YBaCuO sample are explained by a model supposing the existence of an intergrain critical state and the barrier on the surface of the grains. The most striking feature we found is the decrease of the surface barrier with increasing frequency. c  1997 Academic Press Limited Key words: surface pinning, AC susceptibility, irreversible magnetic properties. 1. Introduction Interaction between the magnetic flux and the surface of a superconducting material can influence strongly the flux dynamics under certain conditions [1]. Experiments on YBCO single crystals revealed the surface barrier to be the mechanism enabling a consistent explanation of the observed behaviour [2]. In the theoretical models currently used it is supposed that pinning of magnetic flux on a sample surface is a quasistatic process, i.e. it is independent of the velocity of the flux-line motion. A suitable tool to analyse the surface pinning in dynamic conditions is the AC magnetization technique if one compares the behaviour observed in a wide range of frequencies. We performed such experiments on a YBaCuO polycrystalline sample with weak intergrain currents at frequencies ranging from 3 Hz to 115 kHz. 2. Experiments The sample used in our experiments is a rod-shaped (4 × 3 × 14 mm 3 )YBa 2 Cu 3 O x polycrystal prepared by the modified citrate pyrolysis [3]. The grains are oriented randomly, and the intergrain contacts are of poor quality allowing us to mostly observe the interaction of the magnetic flux with the grains at the temperatures within ∼10 K below T c . The quasistatic DC magnetization loop measured at 89.5 K on a SQUID magnetometer is presented in Fig. 1. At this temperature the intergrain critical currents ( j cm ) are very small, and as the applied field reaches 0749–6036/97/080607 + 06 $25.00/0 sm960479 c  1997 Academic Press Limited