Strong flux pinning in YBa₂Cu₃O_7−δthin films due to nanometer-sized precipitates

Abstract

The magnetic field angle dependence of the critical current density Jc(H,θ) was measured in 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 showed Jc(θ) peaks around with the shape of a stratovolcano (i.e., like Mount Fuji) and (2) films that showed high, broad Jc(θ) peaks centered at in addition to less prominent peaks. Transmission electron microscope observations revealed that the films in category 1 contained a high density of very small precipitates, most of which were less than 7 nm, and that the films in category 2 contained a high density of precipitates whose typical diameters ranged from 5 to 25 nm. The Jc(H,θ) data were analyzed based on the angular-dependent coherence length ξ(θ) within an anisotropic Ginzburg–Landau approximation. The pinning of the films in category 1 can be described by a direct summation of the core pinning interaction that is due to small point defects whose diameters are less than 2ξ. 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 also be explained by a similar analysis for spherical pinning centers whose diameters are larger than 2ξ.