Study of the critical current density of a hot pressed MgB$_2$

We report the result of the study of a hot-pressed MgB$_{2}$. We found that the hot-pressec(2.0GPa and 900\r{ } for 30 min.) MgB$_{2}$ has Tc $\cong $ 39K and $\Delta $T= 1.93 K. Its critical current density, J$_{c}$ increase about an order of magnitude than the untreated one. A significant improvement of the critical current density versus B was also found. The measurements of XRD and x-ray near-edge absorption spectra of B, O, and Mg edge show that the impurity of MgO originally existed in the MgB$_{2}$ powder before hot press disappears and instead B$_{2}$O$_{3}$ and other unknown impurities of compounds and oxides appear in the sample. Our result suggests that MgO is not the major impurity that cause the enhancement.