Quantum-well resonances caused by partial confinement in MgO-based magnetic tunnel junctions

Quantum-well resonance is achieved through partial confinement in magnetic tunnel junctions (MTJs), which provides an additional operable degree of freedom to regulate quantum-well levels. Using Al/Fe/MgO/Fe/Al and Ag/Al/Fe/MgO/Fe/Al/Ag MTJs as examples, via first-principles calculations, we demonstrate that the partial confinement of ${\mathrm{\Delta }}_1$ electrons at the Al/Fe interface, and the full confinement at the Fe/MgO interface combine to produce quantum-well resonances in Fe. The quantum-well levels of Fe can be periodically adjusted by two degrees of freedom: Fe and Al thickness. The oscillation period obtained from conductance $G_{\uparrow \uparrow }$ of MTJs is 2.13 ML Fe (9 ML Al), close to 2.25 ML Fe (8.33 ML Al) calculated by Fermi wave vector in the bcc-Fe (fcc-Al) band. The combination of long and short periods enables quantum-well levels to be finely adjusted. An ultrahigh optimistic TMR effect of $3.05\times {10}^5$% is achieved. Our results provide a way for designing high-performance spintronics devices.