High magnetoresistance of hexagonal boron nitride-graphene heterostructure-based MTJ through excited-electron transmission

TL;DR

This study uses ab-initio calculations to analyze electron transmission and high magnetoresistance in hBN-graphene heterostructure-based magnetic tunnel junctions, revealing enhanced TMR ratios near the Fermi energy.

Contribution

It demonstrates significantly increased TMR ratios in Ni/hBN-graphene heterostructures compared to pure hBN, highlighting the role of surface states and hybridization effects.

Findings

High TMR ratios (~1200%) observed near 0.34 eV energy level.

Transmission influenced by surface state hybridization and evanescent waves.

Replacing hBN with graphene enhances TMR and transmission probabilities.

Abstract

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.