Magnetic relaxation and flux pinning in YBCO films prepared by PLD from a nanocrystalline target

The magnetic flux pinning properties of YBCO films prepared by pulsed laser deposition from nano- (n) and microcrystalline (μ) targets are studied and compared by magnetic relaxation and hysteresis measurements. The most striking difference between the films is the higher critical current density of the n-films ( J c (0 T,77 K) = 1 × 10 7 A cm −2 ) compared to the μ-films ( J c (0 T,77 K) = 3 × 10 6 A cm −2 ). The accommodation field B * that determines the end of the low field plateau of J c is three times higher in the n-film than in the μ-film. In higher fields J c ( B ) follows a power-law behaviour with exponents α = −0.6 in the intermediate field region and −2 in the high fields. The results are discussed on the basis of the recent strong pinning theories and it is concluded that the observed stronger pinning in the n-film is due to higher density of extended defects.