  This work presents an ab-initio study of a few-layers hexagonal boron nitride
(hBN) and hBN-graphene heterostructure sandwiched between Ni(111) layers. The
aim of this study is to understand the electron transmission process through
the interface. Spin-polarized density functional theory calculations and
transmission probability calculations were conducted on Ni(111)/$n$hBN/Ni(111)
with $n$ = 2, 3, 4, and 5 as well as on Ni(111)/hBN-Gr-hBN/Ni(111). Slabs with
magnetic alignment in an anti-parallel configuration (APC) and parallel
configuration (PC) were considered. The pd-hybridizations at both the upper and
lower interfaces between the Ni slabs and hBN were found to stabilize the
system. The Ni/nhBN/Ni magnetic tunnel junction (MTJ) was found to exhibit a
high tunneling magnetoresistance (TMR) ratio at ~0.28 eV for $n$ = 2 and 0.34
eV for $n$ > 2, which are slightly higher than the Fermi energy. The observed
shifting of this high TMR ratio originates from the transmission of electrons
through the surface states of the $d_{z^2}$-orbital of Ni atoms at interfaces
which are hybridized with the $p_z$-orbital of N atoms. In the case of $n$ > 2,
the proximity effect causes an evanescent wave, contributing to decreasing
transmission probability but increasing the TMR ratio. However, TMR ratio, as
well as transmission probability, was found to be increased, by replacing the
unhybridized hBN layer of the Ni/3hBN/Ni MTJ with graphene, thus becoming
Ni/hBN-Gr-hBN/Ni. A TMR ratio as high as ~1200% was observed at an energy of
0.34 eV, which is higher than the Fermi energy. Furthermore, a design is
proposed for a device based on a new reading mechanism using the high TMR
observed just above the Fermi energy level.
