Kondo resonance in magnetic double tunnel junctions

The magneto-transport properties of double tunnel junctions are reported as a function of temperature and bias. The double tunnel junctions were formed with MgO tunnel barriers and CoFe magnetic electrodes in which the middle electrode is comprised of ultra-thin CoFe layers. Double magnetic tunnel junctions, comprised of CoFe/MgO/CoFe/MgO/CoFe, were deposited on SiO₂ through a sequence of metal shadow masks using dc magnetron sputtering at room temperature. Tunneling magnetoresistance and AC conductance measurements show a peak in the conductance at low bias voltages and temperatures (below ∼50 K) and a strong suppression of tunneling magnetoresistance in the same bias voltage and temperature regime. These features are strongly reminiscent of Kondo resonance effects previously observed previously in lithographically defined quantum dots and C₆₀ molecules placed between ferromagnetic electrodes.