Disturbance of Tunneling Coherence by Oxygen Vacancy in Epitaxial $\mathrm{Fe}/\mathrm{MgO}/\mathrm{Fe}$ Magnetic Tunnel Junctions

Oxygen vacancies in the MgO barriers of epitaxial $\mathrm{Fe}/\mathrm{MgO}/\mathrm{Fe}$ magnetic tunnel junctions are observed to introduce symmetry-breaking scatterings and hence open up channels for noncoherent tunneling processes that follow the normal WKB approximation. The evanescent waves inside the MgO barrier thus experience two-step tunneling, the coherent followed by the noncoherent process, and lead to lower tunnel magnetoresistance, higher junction resistance, as well as increased bias and temperature dependence. The characteristic length of the symmetry scattering process is determined to be about 1.6 nm.

Figure 1

Comparison of the magnetoresistance (MR) loops at RT and 1 K for MTJs of the same 2.5 nm MgO barrier thickness but deposited at $180°\mathrm{C}$ (a) and RT (b). Inset shows the corresponding TMR bias dependence at 1 K.

Figure 2

Comparison of the $I\mathrm{\text{−}}V$ and the $dI/dV\mathrm{\text{−}}V$ curves of the MTJs with 2.5 nm MgO barrier deposited at $180°\mathrm{C}$ (a),(c) and RT (b),(d), the curves of spin-P (blue) and spin-AP (red) configurations are marked. Inset shows the low bias ($<100\mathrm{mV}$) region of the MTJ with RT deposited barrier. Measurement is performed at 1 K.

Figure 3

Schematic illustration of the noncoherent tunneling process. The lateral angular momentum of an incoming electron is conserved until a scattering happens, and the electron loses its coherence and enters the noncoherent tunneling process, which has a much larger decay wave vector inside the barrier.