Enhanced critical current density in BaFe2(As0.66P0.33)2 nanocomposite superconducting films

The high upper critical field and low anisotropy of the 122-type iron-based superconductor BaFe₂As₂ makes it promising for use in superconducting high field magnets. However, its critical current density (J_c) in high magnetic fields needs to be further improved. Here we show that for the film prepared by pulsed laser deposition method by controlling the deposition parameters (higher substrate temperature and lower growth rate), the crystallinity of BaFe₂(As_0.66P_0.33)₂ (Ba122:P) matrix is improved while maintaining a high density of incoherent BaZrO₃ (BZO) nanoparticles (NPs) which together lead to significantly increased self field J_c. Our Ba122:P nanocomposite films also exhibit increased in-field J_c, reduced angular anisotropy of J_c and reduced detrimental effects of thermal fluctuations (creep rate) over a wide range of temperatures and magnetic field strength. The BZO NP doped Ba122:P films show high in-field J_c over 2.1 MA cm^-2 even at 4 K and 9 T (μ₀H∣∣c), which is significantly higher than that of standard Ba122:P films and conventional alloy superconducting wires. To understand the contribution of the various pinning centers, we applied a simple model, which was developed for cuprates, to Ba122:P film with all the parameters used derived by fitting to a limited set of experimental data (no free parameters) such that temperature, angle and field properties at other experimental conditions are then calculated. This simple model fits very well to the experimental results in these two very different material systems. We discuss the effectiveness of natural defect and BZO NPs on the ratio of J_c to the depairing current density. The superconducting properties for 122-type iron-based superconductors obtained through this work are considered promising for high-field applications.