Anisotropy and internal field distribution of MgB2 in the mixed state at   low temperatures
  Magnetization and muon spin relaxation on MgB2 were measured as a function of field at 2 K. Both indicate an inverse-squared penetration depth strongly decreasing with increasing field H below about 1 T. Magnetization also suggests the anisotropy of the penetration depth to increase with increasing H, interpolating between a low Hc1 and a high Hc2 anisotropy. Torque vs angle measurements 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. 
M. Angst, D. Di Castro, D. G. Eshchenko, R. Khasanov, S. Kohout, I. M.   Savic, A. Shengelaya, S. L. Bud'ko, P. C. Canfield,...