Magnetic characterization of bulk C-added MgB₂

In order to find a cheap and efficient way to produce high performance bulk MgB₂ superconductors, a series of samples produced with different contents of carbon-coated, nanometer-sized amorphous boron was prepared to improve the magnetic properties. Therefore, a single-step solid-state reaction at the optimal reaction temperature of 805°C for 3 h in pure argon atmosphere was used. Small pieces cut from the bulk were characterized by magnetic hysteresis loop measurements in magnetic fields up to ± 7 T at temperatures ranging from 5 to 35 K using a physical property measurement system (PPMS). Critical current densities and flux pinning forces were calculated from the magnetization data. The pinning force data of all samples were compared using the pinning force scaling approach. The peak positions, h0, were found at h₀ ∼ 0.2, indicating a dominant flux pinning at the grain boundaries for samples with 50% and 30% carbon-coated boron, while the Eisterer scaling reveals a peak position of 0.44 for the 10% sample, which indicates pinning at point defects.