Superconducting anisotropy and evidence for intrinsic pinning in single   crystalline MgB$_2$
  We examine the superconducting anisotropy $\gamma_c = (m_c / m_{ab})^{1/2}$ of a metallic high-$T_c$ superconductor MgB$_2$ by measuring the magnetic torque of a single crystal. The anisotropy $\gamma_c$ does not depend sensitively on the applied magnetic field at 10 K. We obtain the anisotropy parameter $\gamma_c = 4.31 \pm 0.14$. The torque curve shows the sharp hysteresis peak when the field is applied parallel to the boron layers. This comes from the intrinsic pinning and is experimental evidence for the occurrence of superconductivity in the boron layers. 
Ken'ichi Takahashi, Toshiyuki Atsumi, Nariaki Yamamoto, Mingxiang Xu,   Hideaki Kitazawa, and Takekazu Ishida