  We demonstrate for the first time the magnetic field distribution of the pure
vortex state in lightly doped Mg$_{1-x}$Al$_x$B$_2$ ($x\leq 0.025$) powder
samples, by using $^{11}$B NMR in magnetic fields of 23.5 and 47 kOe. The
magnetic field distribution at T=5 K is Al-doping dependent, revealing a
considerable decrease of anisotropy in respect to pure MgB$_2$. This result
correlates nicely with magnetization measurements and is consistent with
$\sigma$-band hole driven superconductivity for MgB$_2$.
