Effect of particle size on the flux pinning properties of YBa2Cu3O7–δ thin films containing fine Y2O3 nanoprecipitates

Abstract

The magnetic-field angle dependence of the critical current density, Jc(H, θ), was measured at various temperatures in co-evaporated YBa2Cu3O7–δ (YBCO) thin films. The YBCO films showed volcano-shaped Jc(θ) peaks around H//ab, and Jc(θ) peaks around H//c were not observed. Film A, deposited at a lower temperature than the commercial standard film B, showed lower Jc values at high temperatures (T ≥ 60 K) compared with film B, although film A showed higher Jc at T = 20 K. Plan-view transmission electron microscope observations revealed that films A and B contained a high density of fine Y2O3 nanoprecipitates. The modes in the distribution of their cross-sectional areas are 10–20 nm2 in film A and 20–30 nm2 in film B. Because of the smaller particle size, film A showed lower Jc at high temperatures owing to the smaller elementary pinning force, fp, but showed higher Jc at 20 K where the temperature-dependent coherence length ξab(T) was short (∼2 nm) and comparable with the radius of Y2O3 nanoparticles. Film A showed anisotropic scaling behavior at T = 70–80 K, and the T dependence of Jc followed ∼(1 − T/Tc)m(1 + T/Tc)2 (m ≈ 2.5), which was expected from a simple flux-pinning model.