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 (Jc(0 T,77 K) = 1 x 107 A cm-2) compared to the μ-films (Jc(0 T,77 K) = 3 x 106 A cm-2). The accommodation field B* that determines the end of the low field plateau of Jc is three times higher in the n-film than in the μ-film. In higher fields Jc(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