Appearance of the minority $d_{z^2}$ surface state and disappearance of the image-potential state: Criteria for clean Fe(001)

The unoccupied surface electronic structure of clean and oxidized Fe(001) was studied with spin-resolved inverse photoemission and target current spectroscopy. For the clean surface, we detected a $d_{z^2}$ surface state with minority spin character just above the Fermi level, while the image-potential surface state disappears. The opposite is observed for the ordered $p$($1\times 1$)O/Fe(001) surface: the $d_{z^2}$-type surface state is quenched, while the image-potential state shows up as a pronounced feature. This behavior indicates enhanced surface reflectivity at the oxidized surface. The appearance and disappearance of specific unoccupied surface states prove to be decisive criteria for a clean Fe(001) surface. In addition, enhanced spin asymmetry in the unoccupied states is observed for the oxidized surface. Our results have implications for the use of clean and oxidized Fe(001) films as spin-polarization detectors.

Figure 1

Spin-resolved IPE data for Fe(001). (a) Spectra for the clean surface taken with counters C1 and C2. The spectra of C2 reveal an extremely surface-sensitive minority feature close to $E_{\mathrm{F}}$. (b) Spectra of C2 in a limited energy range and with enhanced energy resolution ($\Delta E=390$ meV) for the clean surface and the surface exposed to 0.2 L of oxygen, showing high surface sensitivity of the minority feature just above $E_{\mathrm{F}}$.

Figure 2

Spin-resolved IPE spectra, obtained with C2, of clean Fe(001) (top) compared with $p$(1$\times 1$)O/Fe(001) (middle) and the oxygen-rich phase of Fe(001) (bottom). For the oxygen-rich phase, the LEED pattern with the extra spots (encircled) is presented.

Figure 3

TCS spectra from clean Fe(001) (top), $p$($1\times 1$)O/Fe(001) (middle), and the oxygen-rich structure (bottom). Inset: Corresponding spin asymmetries.