The effect of nanosized CoFe₂O₄ addition on the magnetic properties of GdBa₂Cu₃O_7-δ using AC magnetic susceptibility measurements

Superconductor samples of type (CoFe₂O₄)_xGdBa₂Cu₃O_7-δ (0≤x≤0.1) were synthesized by the conventional solid-state reaction technique, whereas nanosized CoFe₂O₄ was prepared by co-precipitation method with grain size of about 8.5 nm. The elemental content of the prepared samples was determined using particle induced X-ray emission (PIXE). The temperature dependence of real ( ${\chi }^{\prime }$) and imaginary ( ${\chi }^{″}$) components of AC magnetic susceptibility (ACMS) at different magnetic field amplitude (3–15 Oe) was investigated. The analysis of the temperature dependence of ACMS was performed using Bean critical state model. The values of the critical current density $J_{\text{c}}$ at T < $T_{\text{p}}$ ( $T_{\text{p}}$ is the inter-granular loss peak temperature) were calculated as a function of magnetic field and nanosized CoFe₂O₄ content. It was found that the low nanosized CoFe₂O₄ addition content (x = 0.01) improves the critical current density $J_{\text{c}}$ of Gd-123 superconducting phase. The observed variation of $J_{\text{c}}$ with temperature indicated that the weak links are changed from superconductor– normal metal–superconductor (SNS) for free sample to superconductor–insulator–superconductor (SIS) type of junctions for samples added with nanosized CoFe₂O₄ of x < 0.01. We also discussed the experimental results in the framework of the critical state model to estimate the effective volume fraction of the grains $f_{\text{g}}$ using Cole–Cole plots.