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

Abstract

The magnetic flux pinning properties of YBCO films preparedby pulsed laser deposition from nano- (n) and microcrystalline(μ) targets are studied and compared by magnetic relaxation and hysteresis measurements. Themost striking difference between the films is the higher critical current density of the n-films(Jc(0 T,77 K) = 1 × 107 A cm−2) comparedto the μ-films (Jc(0 T,77 K) = 3 × 106 A cm−2). Theaccommodation field B* that determines the end of the low field plateau ofJc is three times higher in the n-film than in theμ-film. Inhigher fields Jc(B) follows a power-law behaviour with exponentsα = −0.6 in the intermediatefield region and −2 in the high fields. The results are discussed on the basis of the recent strong pinningtheories and it is concluded that the observed stronger pinning in the n-film is due tohigher density of extended defects.