  We investigate the magnetization reversal in Fe/MgO(001) films with fourfold
in-plane magnetic anisotropy and an additional uniaxial anisotropy whose
orientation and strength are tuned using different growth geometries and post
growth treatments. The previously adopted mechanism of 180^{o} domain wall
nucleation clearly fails to explain the observed 180^{o} magnetization
reversal. A new reversal mechanism with two successive domain wall nucleations
consistently predicts the switching fields for all field orientations. Our
results are relevant for a correct interpretation of magnetization reversal in
many other epitaxial metallic and semiconducting thin films.
