Magneto-Optical Studies on Polycrystalline MgB2 Bulks Manufactured by Different Processes

Magneto optical imaging (MOI) was performed for polycrystalline MgB₂ bulks manufactured by various fabrication processes to investigate the relationship between critical current properties, microstructures and current obstacles. In dense samples prepared by the diffusion method, steep field gradients corresponding to high J_c were shown by MOI, even though some larger scale current obstacles such as cracks and local porous regions were found. For a crack-free, dense sample, high J_c over 0.4 MA/cm² at 20 K at 150 mT was found both by MOI and magnetization hysteresis measurements. On the other hand, relatively uniform field distribution without large-scale current obstacles was observed by MOI for a porous in-situ processed sample, in spite of its much lower Jc. This suggests that uniformly dispersed current obstacles on scales finer than the MOI resolution (~ few mum) exist in porous bulks. More homogenous microstructure without such fine-scale obstructions and elimination of voids and cracks are all important for improving J_c of MgB₂.