In this paper, based on three-dimensional micromagnetic numerical simulation, the influences of the interface layer formed by the atomic diffusion at the interface on magnetic properties in parallel SmCo/Fe bilayer and perpendicular SmCo/Fe bilayer are investigated. For the parallel system, whose nucleation occurs in the second quadrant, as the interface layer thickness increases, the nucleation field and the pinning field increase gradually though the remanence decreases gradually, hence the maximum energy product first goes up and then comes down. As a result, in the system there occurs the transition from the exchange-spring to the rigid magnet. For the perpendicular system, with the increase of the interface layer thickness, a gradual transition from the first quadrant to the second quadrant happens to its nucleation. Although the pinning field experiences the changes from decreasing to unchanging and to increasing, the nucleation field and remanence both rise gradually. Therefore, the energy product is enhanced gradually. During the demagnetization, there appears a spin deviation within the film plane: the parallel system shows a progress of generation and disappearance of the <i>flower</i> and <i>C</i> states; however, the perpendicular system shows a progress of generation and disappearance of the <i>vortex</i> state. With the increase of the ratio of the SmCo atomic diffusion in the interface layer of parallel SmCo/Fe bilayers, the nucleation and pinning field go up, but the remanence decreases, and hence the maximum energy product first rises and then drops. For the two easy axis orientations and any interface layer thickness, the nucleation field rises with the increase of interface exchange energy constant, indicating that the existence of an interface layer between the soft layer and hard layer enhances the exchange coupling interaction between them. The model in this paper well simulates the relevant experimental results [<ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://doi.org/10.1063/1.2769755 "> 2007 <i>Appl</i>. <i>Phys</i>. <i>Lett</i>. <b>91</b> 072509</ext-link>].
