The magnetic properties of epitaxial Fe,O-3(4), films on MgO(100) substrate have been investigated. Spin wave resonance (SWR) technique has been used to study the magnetic anisotropy as a function of temperature between 4-300 K. The ferromagnetic resonance (FMR) spectra exhibit unusual unidirectional character upon cooling the sample in an external field H-c,. At higher temperatures, generally both the bulk and the surface modes appear as long as the external field is applied within a small angular interval with respect to the film normal. The bulk and the surface FMR modes are well separated from each other when the field, H-ap,, is parallel to the cooling field, H-c,, which is normal to the film plane. However these FMR modes surprisingly overlap and become only a single and broader absorption line when the applied field is antiparallel to the cooling field. Using a field-induced unidirectional exchange surface and bulk anisotropy, with the usual magneto-crystalline anisotropy energy, we have simulated the experimental spectra and deduced anisotropy and magnetic stiffness parameters as a function of temperature.