Tailoring magnetoresistance at the atomic level: An ab initio study

The possibility of manipulating the tunneling magnetoresistance (TMR) of antiferromagnetic nanostructures is predicted in the framework of ab initio calculations. By the example of a junction composed of an antiferromagnetic dimer and a spin-polarized scanning tunneling microscopy tip we show that the TMR can be tuned and even reversed in sign by lateral and vertical movements of the tip. Moreover, our finite-bias calculations demonstrate that the magnitude and the sign of the TMR can also be tuned by an external voltage.

Figure 1

Setup for our transport calculations. (I) The Co-Cr heteronuclear dimer on the Cu surface. (II) and (III): The full device setup including the Cr-terminated tip and the dimer. These are positioned, respectively, at a large (5 Å) and a short (3 Å) tip-atom distance. Here $H$ is the tip-atom distance, $L$ is the vertical distance between atoms of the dimer and the substrate atoms underneath atoms, and $D$ is the distortion of the substrate.

Figure 2

Transmission curves for the parallel and antiparallel configurations at zero bias: (a) the Cr tip is positioned above the Co atom; (b) the Cr tip is position above the Cr atom. The numbers in the figure indicate the tip-atom separation. The insets describe the geometry used in the calculation. Note that all energies are measured relatively to $E_{\mathrm{F}}$.

Figure 3

Spin-resolved transmission coefficient in the tunneling regime ($H_{\mathrm{TA}}$ $=$ 5 Å) for the AP (a) and P (b) configurations when the Cr tip is above the Co atom. In the lower panels we show the partial density of states (PDOS) projected over the $sp$ orbitals of the Cr tip and the Co atom: (c) AP and (d) P. In all the panels positive values correspond to $\uparrow$ spins and negative values to $\downarrow$.

Figure 4

Spin-resolved transmission coefficient in the contact regime ($H_{\mathrm{TA}}$ $=$ 3 Å) for the AP (a) and P (b) configurations when the Cr tip is above the Co atom. In the lower panels we show the partial density of states (PDOS) projected over the $d$ orbitals of the Cr tip and the Co atom: (c) AP and (d) P. In all the panels positive values correspond to $\uparrow$ spins and negative values to $\downarrow$.

Figure 5

Transmission coefficient in the contact regime for the AP (a) and P (b) configurations when the Cr tip is positioned above the Cr atom of the dimer. In the lower panels we show the partial density of states (PDOS) projected over the $d$ states of the Cr tip and the Cr atom for both the AP (c) and P (d) configurations. In all panels positive values correspond to $\uparrow$ spins and negative values to $\downarrow$.

Figure 6

(a) TMR ratio as a function of external biases at the tunneling regime. The evolution of the transmission coefficient at different biases is shown in (b) $+$0.3 eV, (c) 0 eV, (d) $-0.6$ eV. The vertical lines in the figure denote the bias window.