Adatoms in graphene as a source of current polarization: Role of the local magnetic moment

We theoretically investigate spin-resolved currents flowing in large-area graphene, with and without defects, doped with single atoms of noble metals ($\mathrm{Cu}$, $\mathrm{Ag}$, and $\mathrm{Au}$) and $3d$-transition metals ($\mathrm{Mn}$, $\mathrm{Fe}$, $\mathrm{Co}$, and $\mathrm{Ni}$). We show that the presence of a local magnetic moment is a necessary but not sufficient condition to have a nonzero current polarization. An essential requirement is the presence of spin-split localized levels near the Fermi energy that strongly hybridize with the graphene $\pi$ bands. We also show that a gate potential can be used to tune the energy of these localized levels, leading to an external way to control the degree of spin-polarized current without the application of a magnetic field.

Figure 1

(a) Representation of devices considered in this work. (b) Possible adsorption sites for atoms on pristine graphene. (c) Geometry for $\mathrm{Co}@\mathit{MV}$, similar to all other atoms. (d) $\mathrm{Co}@585$, similar to all other atoms. (e) $\mathrm{Co}@555777$ (similar to $\mathrm{Mn}$:$\mathrm{Co}$:$\mathrm{Ni}@555777$). (f) $\mathrm{Au}@555777$. (g) $\mathrm{Cu}@555777$ (similar to $\mathrm{Ag}@555777$).

Figure 2

Schematic representation of the unitary cell in the real (a) and reciprocal (b) spaces, where we define the symmetry points used to plot the energy bands.

Figure 3

Current polarization with $V_{\mathrm{bias}}=50\mathrm{meV}$ (a) for all studied cases, (b) as a function of the local magnetic moment, and (c) as a function of the magnetic moment induced at the graphene carbon atoms. The solid lines guide the eye.

Figure 4

Band structure and spin-resolved transmittance for (a) and (b) $\mathrm{Au}@\mathit{MV}$, (c) and (d) $\mathrm{Co}$ over pristine graphene, and (e) and (f) Ni@555777. The solid (dashed) lines correspond to the majority (minority) spin channels. For the sake of comparison, we also show the transmittance for pristine graphene. The smooth behavior of all curves indicates the convergence, regarding the $k_{\perp }$ points, to the true 2D nature of graphene. In the energy bands, we also indicate the position of the Dirac cone (with a vertical dashed line), corresponding to the $K$ symmetry point in a hexagonal supercell.

Figure 5

Spin-resolved transmittance for $\mathrm{Mn}@\mathit{MV}$. The distance between $\mathrm{Mn}$ atoms are $13.0$, $17.3$, $21.6,$ and $26.0$ Å in (a)–(d), respectively.

Figure 6

(a) Spin-resolved current for the $\mathrm{Mn}@\mathit{MV}$ complex where the circles (squares) represent the majority (minority) spin. (b)–(f) Transmission probability for $V_{\mathrm{gate}}=3.0$, $1.0$, $0.0$, $-1.0,$ and $-3.0$ V, respectively. Vertical lines indicate the position of the valleys in the transmittance. The Fermi energy is fixed by the infinite graphene leads. (g) CP as a function of $V_{\mathrm{gate}}$.