Shot Noise in Epitaxial Double-Barrier Magnetic Tunnel Junctions

We demonstrate that shot noise in Fe/MgO/Fe/MgO/Fe double-barrier magnetic tunnel junctions is determined by the magnetic configuration of the junction-the P-state with the magnetic moments of all three ferromagnetic layers aligned parallel, the AP1 state with the central electrode magnetized opposite to its neighbors, and two different AP2 states with the magnetic moment of the upper or bottom electrode aligned opposite to the other two. We also show that the asymmetry between both MgO barriers is another important factor which affects the shot noise. Some voltages present an enhancement in both conductance and shot noise, which indicates that resonant tunneling through quantum well states formed in the middle magnetic layer is taking place. When the junctions are heated to 60 K, the resonance tunneling anomalies in the shot noise smear out, but they survive in the differential conductance. On the other hand, at low voltages (below 200 mV) and low temperatures (below 4 K) the shot noise tends to decrease, probably due to multi-step tunneling via localized defect states in the tunnel MgO barrier. The theoretical model of sequential tunneling proposed for this system, which takes into account spin relaxation, successfully describes the experimental observations in the bias range between 200 mV and 500 mV, where the influence of tunneling through barrier defects and resonant states inside the central electrode is negligible.