Scattering effect of the well-ordered MgB₄ impurity phase in two-step sintered polycrystalline MgB₂ with glycine addition

Glycine-doped MgB₂ bulk was prepared by two-step sintering in this study, first at 750 C and then 900 C. The MgB₄ particles are induced to precipitate where the dislocations concentrated after C substitution or along the steps of screw dislocation during crystal growth, forming ordered MgB₄ arrays throughout the MgB₂ grain. By means of atomic force microscope, the detected magnetic domains are arranged in agreement with the ordered MgB₄ particles after the measurement of magnetic hysteresis loop, which supported that the nano-scale MgB₄ 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 MgB₄, 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. (orig.)