Magnetite Fe₃O₄ is one of only half-metallic materials (conductor for one spin direction and insulating for the other one) to have a Curie temperature (Tc = 860 K) above the room temperature. For this reason, this ferrimagnetic oxide constitutes a material of choice for applications in spin electronics devices (magnetoresistive read heads, nonvolatile MRAM memories). These properties make it possible to predict significant tunnel magneto-resistance (TMR) effects at room temperature if one uses this oxide as electrode in a magnetic tunnel junction (JTM). Obtaining strong TMR effects passes nevertheless by the understanding of the electronic and magnetic properties of the Fe₃O₄ thin films which are very different from those of bulk material. In particular magnetic measurements show that the magnetization of thin films at high field is considerably reduced compared to that of a monocrystal and that it is impossible to obtain saturation even with strong magnetic field. The presence of antiphase boundaries (APBs), structural defect present only in thin films, could be at the origin of these behaviors. This is why we undertook to study the role of the antiphase boundaries on the magnetic properties of Fe₃O₄thin films. For this, thin films of Fe₃O₄ were worked out by molecular beam epitaxy (MBE) assisted by monoatomic oxygen plasma. The structural studies show the perfect epitaxy of the layers. Nevertheless the transmission electronic microscopy analyses reveal also the existence of antiphase booundaries, of which we could vary the density with the film thickness.