The role of Fe₃O₄ nanoparticles on superconductivity of MgB₂

We report the effect of electrohydraulic treated nano Fe₃O₄ doping on the superconducting properties of MgB₂. An important goal of substitution / doping is enhancement of pinning in MgB₂. In spite of impressive progress on improving the critical current density (J_c ) at temperatures above 20 K, which is considered to be the benchmark operating temperature for this material, J_c rapidly drops with increasing magnetic field (H) due to its poor pinning ability. In general, MgB₂ has poor grain connection and a lack of pinning centers, and hence often exhibits a rapid decrease in critical current density (J_c) in high magnetic fields. Fortunately, through the formation of nanoparticle structures in bulk MgB₂ and thin films, the problem of the poor grain connection can be solved, and the flux pinning force can also be significantly enhanced due to an increase of pinning centers served by grain boundaries. In order to improve the performance of MgB₂ further, it is necessary to introduce more pinning centers, especially those consisting of nano-sized second-phase inclusions, which often provide strong pinning forces. In this direction, several types of nanoparticles have also been substituted/doped in MgB₂. Nano-Fe₃O₄ (of various sizes) doped polycrystalline MgB₂ samples were synthesized by encapsulation of well mixed high quality Mg, B and nano-Fe₃O₄ powders. In order to study the effect of magnetic particles on superconductivity, the composites of (MgB₂)_0.98(Fe₃O₄)_0.02 was synthesized with former sintered at different temperatures. The superconducting properties were investigated in the case of samples sintering at high temperature (750 ℃) and at low temperature (350 ℃). We have investigated the effect of the addition of several concentrations of magnetite Fe₃O₄ nanoparticles on the microstructure, critical temperature T_c and the critical current density J_c in the composite MgB₂-Fe₃O₄ material. The study was conducted using X-ray Diffraction (XRD), Energy Dispersive X-Ray Spectroscopy (EDS) and Vibrating Sample Magnetometer (VSM) for the magnetization measurements. (author)