We used Brillouin light scattering to probe and quantify the effect Al overlayers have on the magnetic properties of epitaxial Fe layers grown on GaAs(001). In addition, we correlate the magnetic properties with structural properties obtained using reflection high-energy electron diffraction. We unexpectedly find that an epitaxial Al overlayer induces a significant volume component to the uniaxial magnetic anisotropy energy in 1.1-3.0 nm Fe layers of magnitude (2.5 +/- 0.2)x10(5) erg/cm(3). Our data indicate that the origin of this volume component resides in the presence of an anisotropic strain and relaxation induced in the Fe layer. However, for thinner Fe layers, the overlayer suppresses the uniaxial magnetic anisotropy of Fe layers. We also find that the Al overlayer has no effect on the cubic magnetic anisotropy energy and effective magnetization as our measured values of these constants are consistent and in good agreement with previous reports of Fe layers with Au and Cu overlayers and those without an overlayer.