A low energy beam of spin polarized Li-8(+) has been employed to study the magnetic field distribution in an epitaxial thin film of 5.4% Mn doped GaAs(180 nm) on a (100) GaAs substrate via beta-detected NMR. The spectrum is a strong function of the implantation energy in the range 28-3 keV. In the magnetic layer, there is no indication of a missing fraction, and even more remarkable, there is a broad negatively shifted resonance. The spin lattice relaxation rate is, however, much faster in the Mn doped layer than in the substrate. A sharp peak characteristic of nonmagnetic GaAs is observed down to the lowest implantation energy, for which none of the Li should reach the substrate. This unexpected depth dependence is discussed. (C) 2008 Elsevier B.V. All rights reserved.