The preparation of Fe/MgO/Fe nanoparticles/MgO/Co double tunnel junctions by molecular beam epitaxy, for current-perpendicular-to-plane transport measurements on submicrometer-sized pillars, was discussed. The microstructural observations indicates that the samples exhibit a fully epitaxial layered structure with sharp and flat interfaces including separated Fe nanoparticles between the barriers. It was shown that the introduction of asymmetric MgO tunnel barrier, with different thickness, in the double junction leads to a clear observation of Coulomb staircase and associated tunnel magnetoresistance oscillations. Estimation of the capacitance of the system indicates that these transport phenomena were due to charging effects of the magnetic particles.
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