Magnetization and 11 B NMR study of Mg 1-x Al x B 2 superconductors

We demonstrate the magnetic-field distribution of the pure vortex state in lightly doped ${\mathrm{Mg}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}{\mathrm{B}}_{2} (xl~0.025)$ powder samples, by using ${}^{11}\mathrm{B}$ nuclear magnetic resonance in magnetic fields of 23.5 and 47 kOe. The magnetic-field distribution at $T=5 \mathrm{K}$ is Al-doping dependent, revealing a considerable decrease of anisotropy in respect to pure ${\mathrm{MgB}}_{2}.$ This result correlates nicely with magnetization measurements and is consistent with $\ensuremath{\sigma}$-band hole driven superconductivity for ${\mathrm{MgB}}_{2}.$