Intrinsic pinning effect and its enhancement by Al substitution in MgB ₂ single crystals

Anisotropic behavior of pinning properties in MgB₂ and Mg _0.88Al_0.12B₂ single crystals has been studied using torque τ (=M x H) magnetometry as a function of magnetic field H and angle θ between the H direction and the c axis. When the direction of H is apart from the ab plane, we do not observe remarkable pinning effect. On the other hand, as θ approaches 90°, the hysteresis in τ(H) and τ(θ) curve suddenly increases and shows the sharp maximum at θ 90°, which remind us of the so-called intrinsic pinning. However, it is found that this pinning effect is irrelevant to the change of anisotropy γ with temperature in MgB₂, and is strongly enhanced in Mg _0.88Al_0.12B₂ in spite of the decrease of γ with Al substitution. These results may indicate another plane-like pinning mechanism, which is extrinsic and become effective by Al substitution.