Anisotropy and internal-field distribution of MgB2 in the mixed state at low temperatures

Magnetization and muon spin relaxation on $\mathrm{Mg}{\mathrm{B}}_{2}$ were measured as a function of the applied magnetic field at $2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Both indicate an inverse-squared penetration depth strongly decreasing with increasing field $H$ below about $1\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. Magnetization also suggests the anisotropy of the penetration depth increases with increasing $H$, interpolating between a low ${H}_{c1}$ and a high ${H}_{c2}$ anisotropy. Measurements of the torque as a function of the angle between the field and the $c$ axis of the crystal are in agreement with this finding, while also ruling out drastic differences between the mixed state anisotropies of the two basic length scales penetration depth and coherence length.