Prominent bulk pinning effect in theMgB2superconductor

We report the magnetic-field dependence of the irreversible magnetization of the recently discovered binary superconductor ${\mathrm{MgB}}_2.$ For the temperature region of $T<\mathrm{}{0.9T}_c,$ 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 $M\left(H\right)\mathrm{}$ were well described by the critical-state model without considering the reversible magnetization and the surface-pinning effect. It was also found that the $M\left(H\right)\mathrm{}$ curves at various temperatures scaled when the field and the magnetization were normalized by the characteristic scaling factors $H^{*}\mathrm{}\left(T\right)\mathrm{}$ and $M^{*}\mathrm{}\left(T\right),$ respectively. This feature suggests that the pinning mechanism determining the hysteresis in $M\left(H\right)\mathrm{}$ is unique below $T_c.$