Effect of $\varepsilon$-Fe$_2$O$_3$ on the superconducting properties of YBCO

The additives of ultra-small $\varepsilon$-Fe$_2$O$_3$ nanoparticles affect on the superconducting properties of the high-temperature superconductor YBa$_2$Cu$_3$O$_{7-d}$ (YBCO). It has been found that such additives can lead to an increase in the critical current density only without annealing or with short-term annealing during the preparation of YBCO + $\varepsilon$-Fe$_2$O$_3$ samples. Prolonged annealing at a temperature of 930$^\circ$C, which is common for solid-phase synthesis, leads to a significant suppression of the superconducting properties on the surface of the granules.