Angular dependence of the bulk nucleation field H_c2 of aligned MgB₂ crystallites

The angular dependence of the bulk nucleation field of a sample made of aligned MgB₂ crystallites was obtained using dc magnetization and ac susceptibility measurements. A good fitting of the data by the three-dimensional anisotropic Ginzburg-Landau theory attests to the bulk nature of the critical field H_c2. We found a mass anisotropy ratio ɛ²≈0.39 that implies an anisotropy of the Fermi velocity, with a ratio of 1.6 between the in-plane and perpendicular directions, if an isotropic gap energy is assumed. For an s-wave anisotropic gap this ratio could increase to 2.5. Besides the fundamental implications of this result, it also implies the use of texturization techniques to optimize the critical current in wires and other polycrystalline forms of MgB₂.