Magnetic-field-induced orientation of superconducting MgB₂ crystallites determined by x-ray diffraction

X-ray diffraction studies of fine polycrystalline samples of MgB₂ in the superconducting state reveal that crystals orient with their c axis in a plane normal to the direction of the applied magnetic field. The MgB₂ samples were thoroughly ground to obtain average grain size 5–10 μm in order to increase the population of free single crystal grains in the powder. By monitoring Bragg reflections in a plane normal to an applied magnetic field we find that the powder is textured with significantly stronger (0,0,l) reflections in comparison to (h,k,0), which remain essentially unchanged. The orientation of the crystals with the ab plane parallel to the magnetic field at all temperatures below T_c demonstrates that the sign of the torque under magnetic field does not alter, in disagreement with current theoretical predictions.