Moodera-Research

In the past few years of MTJ development, the establishment of coherent tunneling has led to giant TMR of over 600% at room temperature. Thus MgO is a highly promising spin injection barrier for future semiconductor spintronics. Because of such technological importance, we study the epitaxial growth of MgO on (100)-Si by molecular beam epitaxy [1]. MgO matches Si with 4:3 cell ratio, which renders Fe to be 45° rotated relative to Si, in sharp contrast to the direct epitaxial growth of Fe on Si. It is observed that the compressive strain from Si leads to the formation of small angle grain boundaries in MgO below 5 nm, and also affects the transport characteristics of Fe/MgO/Fe magnetic tunnel junctions formed on top. In addition, we show that by introducing oxygen vacancies in the MgO barriers of epitaxial Fe/MgO/Fe MTJs, symmetry-breaking scatterings are introduced and hence open up channels for noncoherent tunneling processes that follow the normal WKB approximation [2]. The evanescent waves inside the MgO barrier thus experience two-step tunneling: the coherent process followed by the noncoherent process, and thus lead to lower tunnel magnetoresistance, higher junction resistance, as well as increased bias and temperature dependence. The characteristic length of the symmetry scattering process was determined to be about 1.6 nm.

[1] Miao, Chang, Veenhuizen, Thiel, Seibt, Eilers, Münzenberg, Moodera, APL 93, 142511 (2008).

[2] Miao, Park, Moodera, Seibt, Eilers, Münzenberg, PRL 100, 246803 (2008).