Fe/MgO/Fe single-crystalline magnetotunneling structures were epitaxially grown on GaAs(001). An independent magnetization switching in the Fe electrodes was achieved by pinning the magnetization of the Fe-top electrode by antiferromagnetic coupling across a Cr spacer to another Fe film. Such Fe/Cr/Fe/MgO/Fe/GaAs(001) structures exhibit in-plane anisotropy, which ensures a discrete switching behavior of magnetization simplifying the shape of the hysteresis loop. The field dependence of magnetization was measured along the [110] and [100] crystallographic directions of GaAs(001) substrate and modeled by minimizing the total magnetic energy of the system. A peculiar magneto-optical behavior (similar to a reversed hysteresis loop) is explained by magneto-optical phase angles, which are determined mainly by the optical properties of the semiconducting GaAs substrate.