Influence of fluorine in magnetic properties of Hg-1223 high-temperature superconductors

Bulk pure-phase samples of HgBa₂Ca₂Cu₃O_y with high quality were prepared using a solid-state reaction with BaF₂ as a part of the starting material. X-ray diffraction data show that the crystal structure is nearly the same as for starting compounds without fluorine with a = 3.85 A ̊ and c = 15.85 A ̊. The effect of fluorine in the starting material on the magnetic properties was studied by DC susceptibility and hysteresis-loop measurements, and our results revealed a remarkable enhancement of hysteretic magnetization in samples prepared using BaO/BaF₂. Fluorine in Hg-1223 also shows an effect similar to that of oxygenation, which increases T_{c onset} of the as-synthesized samples to 133 K. The asymmetric magnetization-field data indicate the presence of a surface barrier, and the field dependence and time dependence of the magnetization data display an enhancement of hysteresis upon replacement of 10% of BaO by BaF₂ as a result of increased concentration of pinning centers without changing the nature of the individual pinning.