Switching a Single Spin on Metal Surfaces by a STM Tip: Ab Initio Studies

The exchange coupling between single $3d$ magnetic adatoms (Cr, Mn, Fe, and Co) adsorbed on a Cu(001) surface and a Cr STM tip is studied with ab initio calculations. We demonstrate that the spin direction of single adatoms can be controlled by varying the tip-substrate distance, and the sign of the exchange energy is determined by the competition of the direct and the indirect interactions between the tip and the adatom. Based on the spin-dependent transport calculations, we find a magnetoresistance of about 70% at short tip-substrate distances.

Figure 1

Exchange energy as a function of the tip-adatom separation $D$. The inset is the setup for the calculations. In all calculations, the spin direction of the tip apex is fixed (spin up), while it can be reversed for adatoms (either to be spin up or spin down).

Figure 2

Spin-dependent charge density difference $\Delta {\rho }^{\sigma }$. The red and black arrows in the figure are the positions of the tip apex and the adatom, respectively. The units of the $x$ and $y$ axes are Å. (a) and (b) are the charge density difference for the spin-up and spin-down electrons at the tip-adatom separation of 4.0 Å (P configuration), respectively. (c) and (d) are the charge densities difference for the spin-up and spin-down electrons at the tip-adatom separation of 2.5 Å (AP configuration), respectively.

Figure 3

Projected density of states for the Cr adatom (left panels) and the Cr tip- apex (right panels) at 4.0 Å separation between them (P configuration); (a), (b) $4s$ states; (c), (d) $4p$ states; (e), (f) $3d$ states.

Figure 4

Zero bias transmission coefficient for the Cr tip and the Cr adatom at different tip-adatom separations, $D$, for P (a) and AP alignment (b). Positive values are for majority spins, negative values for minority spins.