  Using first-principles calculations, we explore the role of an
anti-ferromagnetic heavy-metal, L1$_0$-IrMn, as a capping layer in a
perpendicular magnetic tunnel junction (\emph{p}-MTJ). A comparative study is
conducted by employing conventional non-magnetic heavy-metals (Ta, W or Mo)
capping layers along with an anti-ferromagnetic IrMn in a symmetric-MTJ
X/FeCo/MgO/FeCo/X, where X=Ta, W, Mo or IrMn. Firstly, the calculations without
including spin-orbit coupling (SOC) are presented where the highest TMR is
achieved in IrMn-IrMn MTJ compared to that of Ta-Ta, W-W and Mo-Mo MTJs. The
origin of this large TMR is attributed to, both, the large spin-polarization
due to reduced lattice-mismatch and the non-identical signatures of both the
anti-parallel conduction-channels caused by the anti-ferromagnetic ordering of
IrMn that reflects the spins at IrMn/FeCo interface. Moreover, when SOC is
switched-on, the increase of TMR is observed in all the MTJs with a
particularly giant enhancement in IrMn-IrMn MTJ. This SOC-induced increase in
TMR is ascribed to the mixed contribution of $\Delta_1$ and $\Delta_5$ states
and the additional increase in the band levels of the out-of-plane orbitals,
\emph{p}$_z$, \emph{d}$_{z}^2$ and \emph{d}$_{xz}$ due to the lifting of
degeneracy. Furthermore, it was also observed that the lattice-mismatch-induced
strain might create an orbital reconstruction within the capping layer,
probably, due to the crystallographic deformation and, in turn, in the adjacent
FeCo-electrode. This microscopic mechanism also plays an additional decisive
role in enhancing the TMR. Finally, our results indicate that IrMn can offer
giant TMR in future spin-orbit toque (SOT) based MRAM devices with a
straightforward design strategy.
