  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 $\Delta_1$ electron at Al/Fe
interface and the full confinement at 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 $G_{\uparrow\uparrow}$ is 2.13 ML
Fe (9 ML Al), close to 2.25 ML Fe (8.33 ML Al) calculated by 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\times10$$^5$\%
is achieved. Our results provides a new path for designing and applying
quantum-well resonances in spintronics devices.
