Inversion of spin polarization above individual magnetic adatoms

Using a cryogenic scanning tunneling microscope, the spin-resolved differential conductance through a magnetic tunnel junction consisting of a Cr tip, a vacuum barrier, and a single Co adatom on a Co monolayer is measured. It is proved, that the spin polarization at the Fermi level above this atomic protrusion is reversed with respect to the atomically flat Co layer, and thus dominated by majority electrons. Experiments with the same Cr-tip electrode and Co adatoms or dimers on Pt(111) as the second electrode suggest that the inversion might be unique for single atomic protrusions due to the reduced coordination.

Figure 1

(Color online) Overview of the sample electrode with platinum(111) terraces (A), Co monolayer (B), Co adatoms on Co monolayer (C), Co adatoms on Pt(111) (D), and Co dimer on Pt(111) (E). A Co dimer (E) is higher than single Co adatoms (D) on the platinum terrace. Co adatoms on Co monolayer (C) appear higher than those at the Co-monolayer edge (G) at this bias voltage. A defect (F) is also visible on the platinum terrace. (Tunneling parameters: $I_{stab}=0.8\text{nA}$ and $V_{stab}=0.3\text{V}$.)

Figure 2

(Color online) (a) Spin-resolved $dI/dV$ curves acquired over an hcp area of a monolayer as illustrated in the inset of (b). (b) Spin-resolved $dI/dV$ curves acquired over a single adatom on an fcc region of a monolayer. Inset: topograph of the area with the single adatom sitting on the monolayer. The dislocation network on the monolayer surface is visible (Ref. 10). (c) Spin-resolved $dI/dV$ curves acquired over single adatoms sitting on fcc and hcp lattice sites on a platinum terrace. (Curves from fcc adatoms are vertically offset by 0.5 nS for clarity.) Inset: topograph colored with the simultaneously recorded $dI/dV$ map at $I_{stab}=0.3\text{nA}$ and $V_{stab}=-0.1\text{V}$ of an area with several fcc and hcp adatoms (vertical scale from 0 to 0.17 nm). (d) Spin-resolved $dI/dV$ curves acquired over the center of a dimer with both adatoms sitting on nearest-neighboring fcc sites on a platinum terrace. Inset: topograph of the area with a single adatom and the compact dimer (arrow). Curves in (a)–(d) change with the relative orientation of sample and tip magnetizations as indicated. [Tunneling parameters: $I_{stab}=1.0\text{nA}$, $V_{stab}=0.6\text{V}$, and $V_{mod}=10\text{mV}$ (rms).]

Figure 3

[(a)–(d)] Magnetic asymmetry $A_{mag}$ calculated from the spin-resolved $dI/dV$ curves in Figs. 2a, 2b, 2c, 2d, respectively. Note that the tip spin-polarization is negative around $E_F$ and has a sign reversal around $-0.1\text{V}$.