Effects of Two-Band Superconductivity on the Flux-Line Lattice in Magnesium Diboride

We present neutron scattering from the flux line lattice (FLL) in MgB₂. Between $0.5$ and 0.9 T the FLL undergoes a 30^∘ reorientation, and simultaneously the scattered intensity falls sharply consistent with the weaker superconducting π band being suppressed with increasing field. We speculate that the π and σ bands favor different FLL orientations, and that the reorientation is driven by the suppression of the π band. When the c axis of the crystal is rotated 45^∘ to the applied field the penetration depth anisotropy could be measured, and rises both as a function of applied field and temperature.