Distance- and spin-resolved spectroscopy of iridium atoms on an iron bilayer

The induced spin polarization of Ir atoms on a ferromagnetic Fe double layer on W(110) has been investigated with spin-polarized scanning tunneling microscopy. An unoccupied state is observed with a spin polarization exceeding $60\%$ that is inverted with respect to the Fe layer. This inversion is due to the tunneling gap acting as an orbital and spin filter. Distance dependent measurements show that the spin polarization remains approximately constant over the entire experimentally accessible range, from far in the tunneling regime to 1 Å from the point of contact formation. This is corroborated by density functional theory calculations which show that the inversion of spin polarization occurs within 0.5 Å of the adatom.

Figure 1

(a) Topograph ($700\text{mV}$, $32\times 35{\text{nm}}^2$) of Ir atoms adsorbed on magnetic Fe domains measured with a spin-polarized tip. Single Ir atoms are readily distinguishable as protrusions, which is further illustrated by the inset ($1\text{V}$) where the color scale has been adjusted. Defects of the Fe layer appear as depressions. (b) $dI/dV$ map recorded simultaneously with the topograph. At the applied sample voltage of $700\text{mV}$ the observed large scale contrast stems from differently polarized magnetic domains. Since the magnetic domains are polarized out of plane, this contrast directly reflects out-of-plane spin sensitivity of the tip. For further evaluation we refer to domains with lower (higher) $dI/dV$ signal at $700\text{mV}$ by using $\alpha \left(\beta \right)$ as index. Atoms are indexed according to the domain they are adsorbed on. The positions of two Ir atoms adsorbed on differently polarized Fe domains are indicated by circles in (a) and (b).

Figure 2

(a, d) Spin-polarized $dI/dV$ spectra of Fe domains and Ir atoms, respectively. (b, e) Majority ($n_{\text{maj}}$) and minority ($n_{\text{min}}$) vacuum densities of states $8.6\text{Å}$ above the hollow site of the Fe surface and $5\text{Å}$ above the Ir atom, respectively. (c) Experimental asymmetries of Fe domains and Ir atoms on Fe calculated via Eq. (2) from the data in (a) and (d), respectively. (f) Corresponding calculated spin polarizations [via Eq. (1) from the data in (b) and (e)]. Comparison of (c) and (f) in combination with Eq. (3) suggests that the spin polarization of the tip changes sign between regions I, II, and III. In regions I and III the tip is more sensitive to minority states while for region II majority electrons prevail. A one-to-one correspondence between the $dI/dV$ curves and $n_{\text{maj}}$ or $n_{\text{min}}$ may not be expected as the tip is only partially spin polarized.

Figure 3

(a) Total DOS of an adsorbed Ir atom and average total DOS of four nearest-neighbor Fe atoms calculated with vasp. Majority (minority) states are represented as positive (negative) values with solid (dashed) lines. (b) Orbital decomposition of the Ir DOS in (a) over a narrower energy region around the Fermi level. The $s{p}_z$ state was multiplied by 4. (c) Asymmetries of single Ir atoms measured with a spin-polarized tip at conductances from $0.4\text{nS}$ to $4\mu \text{S}$, which correspond to distances $\mathrm{\Delta }z$ of 5.4 to $1\text{Å}$ from the formation of the single-atom contact at $\approx 35\mu \text{S}$. (d) Spin polarization of single Ir atoms at different distances $\mathrm{\Delta }s$ into the vacuum calculated via DFT using vasp. $\mathrm{\Delta }s=0\text{Å}$ is at the Ir atom.

Figure 4

Distance dependence of the spin polarization of the Ir adatom (upper panels) and the bare second Fe layer on W(110) (lower panels) integrated over the energy intervals $[-50\text{meV},50\text{meV}]$ (a, c) and $[300\text{meV},400\text{meV}]$ (b, d).