Inelastic tunneling spectroscopy of magnetic tunnel junctions based on CoFeB∕MgO∕CoFeB with Mg insertion layer

Abstract

Magnetic tunnel junctions (MTJs) based on textured MgO barriers have thus far shown the highest tunneling magnetoresistance (TMR) at room temperature. In contrast to traditional magnetic tunnel junctions, it appears that the large TMR observed in these systems arises from a type of coherent tunneling in which the symmetry of the Bloch state wave functions plays a critical role. We have fabricated MTJs with artificial asymmetric barriers by depositing a thin layer of Mg of varying thickness (0–10 Å) prior to the growth of the MgO barrier into otherwise identical CoFeB∕MgO∕CoFeB MTJs. The inelastic tunnel spectrum shows magnon and phonon excitation peaks similar to traditional Al2O3 barriers, and an additional peak at about 300 meV. The conventional interpretation that this peak corresponds to density of states of the s electrons in the ferromagnetic electrodes, however, does not apply in the MgO system.