High-pressure sintering of highly dense MgB₂ and its unique pinning properties

We report on the sintering of highly dense MgB₂ at high-pressure condition (p>3 GPa) and its transport and pinning properties. The grains for MgB₂ sintered at high pressure were compactly connected without discernible voids and the superconducting transition width was less than 1 K. The magnetization hysteresis M(H) curves at various temperatures were very symmetric with respect to H-axis and nearly overlapped with each other when the field and the magnetization were normalized by the characteristic value H^* and M^* at each temperature, respectively. This feature suggests that the pinning mechanism is unique below T_c. Compared to the recent report on the scaling of vortex pinning force (F_p(H)∝BJ_c~HJ_c) for T>0.5×T_c, our analysis gives a simple and direct evidence of the unique pinning in MgB₂ over broad temperature region, 0.12×T_c<T<0.9×T_c.