Prominent bulk pinning effect in newly discovered MgB_2 superconductor

We report the magnetic field dependence of irreversible magnetization of the recently discovered binary superconductor MgB$_{2}$. For the temperature region of $T< 0.9T_c$, the contribution of the bulk pinning to the magnetization overwhelms that of the surface pinning. This was evident from the fact that the magnetization curves $M(H)$ was well described by the critical-state model without considering the surface pinning effect. It was also found that the $M(H)$ curves at various temperatures scaled, when the field and magnetization were normalized by the characteristic scaling factors $H^\ast(T)$ and $M^\ast(T)$, respectively. This feature suggests that the pinning mechanism determining the hysteresis in $M(H)$ is unique below $T=T_c$.