Quantized electronic fine structure with large anisotropy in ferromagnetic Fe films

We report on the spectroscopic observation of a quantized electronic fine structure near the Fermi energy in thin Fe films grown on W(110). The quantum well states are detected down to binding energies of $\sim$10 meV by angle-resolved photoelectron spectroscopy. The band dispersion of these states is found to feature a pronounced anisotropy within the surface plane: It is free-electron-like along the $\overline{\Gamma H}$ direction while it becomes heavy along $\overline{\Gamma N}$. Density functional theory calculations identify the observed states to have both majority and minority spin character and indicate that the large anisotropy can be dependent on the number of Fe layers and coupling to the substrate.

Figure 1

First-order LEED spots for 20 ML bcc-Fe(110) on W(110) measured at electron energy of 81 eV and the surface Brillouin zone with high-symmetry points.

Figure 2

(a) Energy distribution curves (EDC) at the $\overline{\Gamma }$ point for Fe layers on W(110) with film thicknesses between 18 ML (bottom EDC) and 23 ML (top EDC). The spectra were obtained by integration over a $k_{\left|\right|}$ range of $\pm$0.05 Å${}^{-1}$. Peak positions corresponding to quantum well states are marked by vertical lines. (b) Binding energy evolution of the peak positions as a function of film thickness.

Figure 3

Raw ARPES data (left) and corresponding second derivatives (right) for 20 ML Fe/W(110) along $\overline{\Gamma H}$ in (a) and (b), for 22 ML along $\overline{\Gamma H}$ in (c) and (d), and for 22 ML along $\overline{\Gamma N}$ in (e) and (f). The second derivative images are divided into segments as indicated by horizontal lines. The maximum values of the grayscale were chosen individually for all of these segments in order to provide an optimal representation of all spectral features. The data evidence a free-electron-like dispersion along $\overline{\Gamma H}$ and a flat-band behavior along $\overline{\Gamma N}$.

Figure 4

(a) Fermi surface for 22 ML Fe/W(110) obtained by ARPES experiments. The spectrum shows the second derivative of the ARPES data set. The experimental result is compared to the calculated Fermi surface of a free-standing 20 ML Fe slab in (b). The calculated bands are exchange split into minority (black) and majority (blue) states.

Figure 5

GGA band structures for a free-standing 20 ML Fe(110) film along the $\overline{\Gamma H}$ direction in (a), for a 20 ML film along $\overline{\Gamma N}$ in (b), for a 22 ML film along $\overline{\Gamma N}$ in (c), and for a 20 ML film along $\overline{\Gamma N}$ on W in (d). The size of the circles indicates the amount of Fe surface and subsurface character, the horizontal axis is labeled in Å${}^{-1}$.