  A new spintronic theory has been developed for the magnetic tunnel junction
(MTJ) with single-crystal barrier. The barrier will be treated as a diffraction
grating with intralayer periodicity, the diffracted waves of tunneling
electrons thus contain strong coherence, both in charge and especially in spin.
The theory can answer the two basic problems present in MgO-based MTJs: (1) Why
does the tunneling magnetoresistance (TMR) oscillate with the barrier
thickness? (2) Why is the TMR still far away from infinity when the two
electrodes are both half-metallic? Other principal features of TMR can also be
explained and reproduced by the present work. It also provides possible ways to
modulate the oscillation of TMR, and to enhance TMR so that it can tend to
infinity. Within the theory, the barrier, as a periodic diffraction grating,
can get rid of the confinement in width, it can vary from nanoscale to
microscale. Based on those results, a future-generation MTJ is proposed where
the three pieces can be fabricated separately and then assembled together, it
is especially appropriate for the layered materials, e.g., MoS2 and graphite,
and most feasible for industries.
