Anomalous field dependence of anisotropy observed in MgB2 single crystals with a torque method

The field dependence of anisotropy in the mixed state of MgB2 single crystals is studied using torque magnetometry. At all temperatures measured, the anisotropy parameter γ, derived from the angular dependence of magnetic torque τ, increases with increasing field H and shows a plateau with a almost constant value of ∼4.5, indicating that the isotropic π band contribution to the superconductivity weakens with H and only the anisotropic σ band contribution remains in the plateau region. In the low field region below the plateau, we found a change in the functional form of γ(H) at H = H*. In addition, the step-like decrease in the H dependence of d(τ/H)/dH is observed at H*. These anomalies can be related to the change in the vortex lattice structure accompanied by the decrease of the π band contribution.