Spin-Polarized Resonant Tunneling in Antiferromagnetic Heterojunctions of Graphene Nanoribbons with 3d Adatoms

Abstract

Binding, magnetic, and electron transport properties of $3d$ transition-metal (TM) adatoms ($\mathrm{Cr}$-$\mathrm{Cu}$) on an antiferromagnetic armchair graphene nanoribbon heterojunction are studied by means of density-functional-theory (DFT) calculations and nonequilibrium Green function (NEGF) formalism. The heterojunction emulates a system of two symmetric potential barriers and one well with resonances (quasibound states). Binding energies show a strong interaction between $3d$ TM adatoms and heterojunction hollow sites, where $3d$ TM adatoms prefer the edges rather than the middle of the graphene heterojunction. As for magnetic properties, heterojunctions with Cr, $\mathrm{Mn}$, and $\mathrm{Fe}$ have a ferromagnetic behavior with a total magnetic moment $\ensuremath{\ge}4{\ensuremath{\mu}}_{B}$, while $\mathrm{Co}$, $\mathrm{Ni}$, and $\mathrm{Cu}$ adatoms induce antiferromagnetism in the full system with total magnetizations $<3{\ensuremath{\mu}}_{B}$. Transmission curves show that electronic resonances undergo a spin-splitting effect induced by $3d$ TM adatoms, and current versus bias-voltage curves show spin currents from which spin-polarized ratios (SPRs) $>75\mathrm{%}$ for $\mathrm{Co}$ and approximately $100\mathrm{%}$ for $\mathrm{Ni}$ adatoms are calculated. Finally, an oscillatory behavior is observed in SPR curves at low bias due to a combined effect of resonant tunneling and spin filtering. Therefore, these results suggest that the present antiferromagnetic graphene heterojunction with $\mathrm{Co}$ and $\mathrm{Ni}$ adatoms is suitable for future spin and low-power nanoelectronics as a spin-dependent resonant tunneling device.