Electronic structure and magnetism of strained bcc phases across the fcc to bcc transition in ultrathin Fe films

We investigated the electronic structure of the bcc metastable phases involved in the fcc to bcc transition of Fe. Ultrathin Fe films were grown on a 2-monolayer (ML) Ni/W(110) substrate, where a fcc lattice is stabilized at low Fe coverages and the transition proceeds through the formation of bcc nuclei showing a specific “Kurdjumov-Sachs” orientation with the substrate. A comprehensive description of the electronic structure evolution is achieved by combining spin-resolved UV photoemission spectroscopy and ab initio calculations. According to our results, an exchange-split band structure is observed starting from 2 ML of Fe, concomitant with the formation of ferromagnetic bcc nuclei. Continuous modifications are observed in the spin-resolved photoemission spectra for increasing Fe coverage, especially for what concerns the minority states, possibly indicative of the progressive relaxation of the strained bcc phase starting from the bcc/fcc interface.

Figure 1

LEED patterns for increasing Fe coverages on 2-ML Ni/W(110). The electron beam energy is 163 eV. In the insets: closeup of the diffraction pattern observed near the (11) fcc spot. The beam energy selected for the insets is 125 eV. The schematic of panel (d) shows the LEED pattern expected from a bcc surface reconstructed according to the KS relation with white and gray dots, superimposed on the fcc pattern of Ni(111) (black dots). Gray dots single out only two of the six equivalent KS domains, thus reproducing the experimental pattern of panel (b). The red rectangle shows the bcc Fe(110) primitive cell, rotated by $\alpha =54.8^{\circ }$ with respect to the $\mathrm{W}\left[1\overline{1}0\right]$ direction.

Figure 2

Photoemission spectra for the 2-ML Ni/W(110) substrate (red line) and for selected Fe coverages (${\mathrm{\Theta }}_{\mathrm{Fe}}$, black lines). The spectra were acquired at normal photoelectron emission, with the substrate kept at 30 K. He i radiation was used for all but the top spectrum (open dots), acquired with He ii radiation. Source satellites were subtracted from the He i spectra. Capital letters mark the position of the relevant features discussed in the text: ${\mathrm{A}}^{\prime }$ (1-1.4 eV), ${\mathrm{B}}^{\prime }$ (0.6 eV), A (0.7 eV), B (0.3 eV), and C close to $E_{\mathrm{F}}$.

Figure 3

Spin-resolved photoemission spectra for selected Fe coverages on 2-ML Ni/W(110). Black dots: spin-integrated spectra. Blue and red triangles: photoemission signal $I^{\uparrow \downarrow }$ from majority and minority (with respect to the magnetization direction) electrons. Green dots: degree of the photoelectrons’ spin polarization $P=\left(I^{\uparrow }-I^{\downarrow }\right)/\left(I^{\uparrow }+I^{\downarrow }\right)$, retrieved by correcting the measured spin asymmetry by the Sherman function of the detector ($\mathrm{S}=0.14$, according to our previous calibration [30]). Two sets of spectra were acquired at normal photoelectron emission, with the substrate kept at 30 K and magnetized along two opposite directions, in order to cancel out any instrumental asymmetry [30, 46]. He i radiation was used and source satellites subtracted.

Figure 4

(a) Spin-resolved photoemission spectra (minority channel) for increasing Fe coverage (${\mathrm{\Theta }}_{\mathrm{Fe}}$), together with the spectra acquired on a reference bcc Fe(110) surface at RT. He i radiation was used and source satellites subtracted. (b) BE position of feature B as a function of the Fe thickness (displayed on a logarithmic scale).

Figure 5

Photoelectron polarization in the W(110) plane for selected Fe coverages on 2-ML Ni/W(110), computed according to the formula reported in Fig. 3, i.e., by taking the difference of the spin-up and spin-down intensities $I^{\uparrow \downarrow }$, normalized by their sum. Each data point represents the electron polarization measured, at a given binding energy, along the main axis of our apparatus $\left(P_{\parallel }\right)$ and perpendicular to it $\left(P_{\perp }\right)$. These two directions correspond to the $\mathrm{W}\left[001\right]$ and $\left[1\overline{1}0\right]$ directions at normal electron emission, respectively. Samples were magnetized along the $\mathrm{W}\left[001\right]$ direction and measured by using He i radiation in magnetic remanence. The electron polarization is evaluated from the spectra acquired in the energy range spanning from 3 eV to $E_{\mathrm{F}}$. Following Refs. [30, 46], the instrumental asymmetries have been eliminated from the data by repeating each spin-resolved measurement with the sample magnetized along the opposite direction.

Figure 6

(a) Sketch of the primitive orthorhombic cell used to simulate the deformation of the Fe lattice. A wireframe is added (thin blue lines), representative of the primitive bcc cell structure. (b) Atomic stacking in the direction perpendicular to the surface for the bcc lattice [letters A and B identify Fe atoms on different ${\left(110\right)}_{\mathrm{bcc}}$ planes]. A set of relations mapping the Cartesian coordinate system used in the simulation onto the crystallographic directions of the bcc lattice is also shown. (c) Spin-resolved band structure computed along the $\mathrm{\Gamma }\mathrm{N}$ direction of the bcc Brillouin zone. The bands are labeled according to the standard symmetry group notation for the ${\left(110\right)}_{\mathrm{bcc}}$ surface [53]. Dashed lines highlight ${\mathrm{\Sigma }}_2$ bands. The states with a $d_{x^2-y^2}$ or $d_{xy}$ character are drawn with thin lines. As explained in the main text, the simulated bands have been rigidly shifted upwards; the original position of $E_{\mathrm{F}}$ can be retrieved from the small arrows placed along the right axes. Blue lines: BE vs internal momentum $k_{\perp }$ for photoelectrons excited by He i radiation. Symbols: experimental position of features A (majority spins, up triangles) and B (minority spins, down triangles) for the 2- and 40-ML Fe films.