Scattering effect of the well-ordered MgB_4 impurity phase in two-step sintered polycrystalline MgB_2 with glycine addition

摘要

Glycine-doped MgB_2 bulk was prepared by two-step sintering in this study,first at 750°C and then 900°C.The MgB_4 particles are induced to precipitate where the dislocations concentrated after C substitution or along the steps of screw dislocation during crystal growth,forming ordered MgB_4 arrays throughout the MgB_2 grain.By means of atomic force microscope,the detected magnetic domains are arranged in agreement with the ordered MgB_4 particles after the measurement of magnetic hysteresis loop,which supported that the nano-scale MgB_4 domain structure brought strong scattering effects and indicated that atomic force microscopy could test the role of the impurities.As a result,the extrapolating upper critical field H_(c2)(0 K) is enhanced to 22.8 T for the sample with ordered MgB_4,while only 18.1 T for the un-doped sample underwent the same sintering program.Besides,carbon substitution contributed to the enhancement of H_(c2) as well.