Energy- and $k$-resolved mapping of the magnetic circular dichroism in threshold photoemission from Co films on Pt(111)

The magnetic circular dichroism in threshold photoemission (TPMCD) for perpendicularly magnetized fcc Co films on Pt(111) has been revisited. A complete mapping of the spectral function $I(E_B,k_x,k_y)$ (binding energy $E_B$, momentum parallel to surface $k_x$, $k_y$) and the corresponding TPMCD asymmetry distribution $A_{\mathrm{MCD}}(E_B,k_x,k_y)$ has been performed for one-photon and two-photon photoemission using time-of-flight momentum microscopy. The experimental results allow distinguishing direct from indirect transitions. The measurements reveal clear band features of direct transitions from bulk bands that show a nontrivial asymmetry pattern. A significant homogeneous background with substantial asymmetry stemming from indirect transitions superposes direct transitions. Two-photon photoemission reveals enhanced emission intensity via an image potential state, acting as intermediate state. The image potential state enhances not only intensity but also asymmetry. The present results demonstrate that two-photon photoemission is a powerful method for mapping the spin-polarized unoccupied band structures and points out pathways for applying TPMCD as a contrast mechanism for various classes of magnetic materials.

Figure 1

(a) Geometry of the momentum microscopy measurement yielding the magnetic circular dichroism texture of the full half space. (b) 3D spectral function $I$($E_{\mathrm{kin}},k_x,k_y$) is limited by the paraboloidic photoemission horizon and by the Fermi level. Adapted from Ref. [26].

Figure 2

$k$-microscopy on clean fcc Co(4.5 ML)/Pt(111) for 2PPE at $2h\nu$ = 6.2 eV. The in-plane component of the photon helicity vector $S_{\gamma }$ is denoted by arrows in (a, b). Intensity (a) and MCD asymmetry (b) $k_x\text{−}{k}_y$ sections for $E_B$ = 0.2 eV. Intensity (c) and MCD asymmetry (d) $k_y$-E sections for $k_x$ = 0. Corresponding intensity and MCD asymmetry profiles close to the $\overline{\mathrm{\Gamma }}$ point (e) and averaged over the whole paraboloid (f). The gray scale in (c) and the red-gray-blue scale in (d) quantify intensity and MCD asymmetry. ${\tilde{A}}_{\mathrm{MCD}}$ has been determined using Eq. (1).

Figure 3

$k$-microscopy on cesiated fcc Co(4.5 ML)/Pt(111) ($\mathrm{\Phi }=4.1$ eV) for 2PPE at $2h\nu$ = 6.6 eV. Intensity (a) and MCD asymmetry (b) $k_x\text{−}{k}_y$ sections for $E_B$ = 0.2 eV. Intensity (c) and MCD asymmetry (d) $k_y$-E sections for $k_x$ = 0. Dashed curves serve to guide the eye. Corresponding intensity and MCD asymmetry curves close to the $\overline{\mathrm{\Gamma }}$ point (e) and averaged over the whole paraboloid (f). Color scales, see Fig. 2.

Figure 4

$k$-microscopy on cesiated fcc Co(4.5 ML)/Pt(111) for 2PPE at $2h\nu$ = 3.3 eV and for $\mathrm{\Phi }$ = 2.3 eV (a–d) and 2.0 eV (e–h). Intensity (a, e) and MCD asymmetry (b, f) $k_y$-E sections for $k_x$ = 0. Corresponding intensity and MCD asymmetry curves close to the $\overline{\mathrm{\Gamma }}$ point (c, g) and averaged over the whole paraboloid (d, h). $A_{\mathrm{MCD}}$ has been determined using Eq. (1); color scales, see (b).

Figure 5

Energy scheme of a resonant 2PPE transition through an IPS. The bottom of the parabolic band state IS ($n=1$) is energetically separated from the vacuum level $E_{\mathrm{Vac}}$ by $E_1$. The bottom of the IPS appears in the photoelectron spectra at $\mathrm{\Delta }{E}_{\text{IS}}$ below the Fermi level $E_F$. $E_1$ and $\mathrm{\Delta }{E}_{\text{IS}}$ are related by the photon energy $h\nu$ and the work function $\mathrm{\Phi }$ (see text).

Figure 6

Calculated band structure of fcc Co taken from Refs. [24, 25]. Bands are numbered starting from the lowest energy. Dashed arrows indicate direct transitions into final bulk states. The vertical dashed line denotes the $k_z$ value of $\frac{1}{3}\mathrm{\Gamma }\mathrm{L}$.

Figure 7

Projection of a section of $\mathrm{\Gamma }$ to $\frac{1}{3}\mathrm{\Gamma }L$ of the fcc Co minority Fermi surface. This representation contains the symmetry points L, K, X. The turquoise feature originates from band 11 and shows a threefold symmetry. From [33].

Figure 8

The IPS selects particular initial bands for the photoemission process. The energetic position of the band $E_{B_{1,2}}$ is defined by the work function ${\mathrm{\Phi }}_{1,2}$ (as shown in a and b) and the photon energy $h\nu$.