Many possible applications have arisen in the domain of spin electronics since the prediction and experimental observation of high tunneling magneto-resistance (TMR) values in MgO-based tunnel junctions. These high TMR values are due to the crystallinity of the stack, which allows the electronic transport to be described by Bloch wave functions and gives a symmetry dependence of the spin filtering in the tunnel barrier. An original experimental set-up has been developed during this PhD that is able to measure the electronic properties of nano-objects under an applied magnetic field. It combines fast electronic acquisition cards and an atomic force microscope (AFM) equipped with a full metallic tip. This set-up is highly versatile and allows nano-objects to be electrically contacted without difficult nano-fabrication steps. This set-up has been used in order to study the influence of the interfaces on the electronic properties of the fully epitaxial magnetic tunnel junction Fe/MgO/Fe (100). Two interface resonant states (IRS) have been observed for the first time in this system at 0.2 eV and 1.1 eV above the Fermi energy of the minority spin electrons. These IRS drastically modify the tunnel transport and even reverse the dynamic TMR. The transport symmetry of the IRS has been found from a study of samples with different MgO thicknesses.