Shot Noise in Magnetic Tunnel Junctions: Evidence for Sequential   Tunneling
  We report the experimental observation of sub-Poissonian shot noise in single magnetic tunnel junctions, indicating the importance of tunneling via impurity levels inside the tunnel barrier. For junctions with weak zero-bias anomaly in conductance, the Fano factor (normalized shot noise) depends on the magnetic configuration being enhanced for antiparallel alignment of the ferromagnetic electrodes. We propose a model of sequential tunneling through nonmagnetic and paramagnetic impurity levels inside the tunnel barrier to qualitatively explain the observations. 
R. Guerrero, F. G. Aliev, Y. Tserkovnyak, T. S. Santos, and J. S.   Moodera