Strong flux pinning in YBa2Cu3O7−δ thin films due to nanometer-sized precipitates

Abstract

The magnetic field angle dependence of the critical current densityJc(H,θ) was measuredin epitaxial YBa2Cu3O7−δ (YBCO) thin films with strong flux pinning(Jc>25 GA m−2 at 77 K). The YBCO films were classified into two categories: (1) films that showedJc(θ) peaks around with the shape of a stratovolcano (i.e., like Mount Fuji) and (2) films that showed high, broadJc(θ) peaks centered at in addition to less prominent peaks. Transmission electron microscope observations revealed that the filmsin category 1 contained a high density of very small precipitates, most ofwhich were less than 7 nm, and that the films in category 2 contained a highdensity of precipitates whose typical diameters ranged from 5 to 25 nm. TheJc(H,θ) data were analyzed based on the angular-dependent coherence lengthξ(θ) within an anisotropic Ginzburg–Landau approximation. The pinning of the filmsin category 1 can be described by a direct summation of the core pinninginteraction that is due to small point defects whose diameters are less than2ξ. The high,broad Jc(θ) peaks centered at in the films in category 2 were due to a high density of larger precipitates, and they can alsobe explained by a similar analysis for spherical pinning centers whose diameters are larger than2ξ.