Growth, magnetism and ferromagnetic thickness gap in Fe films on the W(111) surface

The growth, structure, and magnetism of Fe films on the W(111) surface were investigated using low energy electron microscopy (LEEM) and diffraction (LEED), spin polarized LEEM (SPLEEM), and work function measurements. In contrast to an earlier report that Fe grows with fcc structure following initial pseudomorpic layer growth, we observe no evidence of the formation of fcc Fe over the entire thickness range studied, up to 18 monolayers (ML). Observations are instead consistent with the formation of a well-ordered, laterally (tensile) strained bcc Fe layer that gradually relaxes vertically and develops increasing disorder with increasing thickness. Ferromagnetic order appears at 6 ML, but surprisingly vanishes at 8 ML, and reappears just as suddenly at 9 ML during Fe deposition at room temperature. Ferromagnetism between 6 and 8 ML also vanishes at only 5 deg above room temperature. The magnetization direction of a monodomain structure remains constant before and after the ferromagnetic thickness gap at 8–9 ML until the formation of a multidomain structure at about 12 ML. Variations of exchange asymmetry in spin-polarized elastic electron scattering are also observed with increasing film thickness, particularly above 12 ML, that indicate changes in the spin-polarized electron band structure above the vacuum level. The evolution of magnetism and exchange asymmetry with increasing thickness and the appearance of the ferromagnetic gap are attributed to structural and morphological changes in the strained Fe layer, which eventually lead to a relaxed although highly disordered bcc Fe layer.

Figure 1

(Top) Normalized LEEM image intensity at 16 eV (◯) and 18 eV (□) and (bottom) work function change measured by mode (i) (□) and mode (ii) (◯) (see text) vs film thickness during Fe deposition on W(111) at room temperature. Mode (ii) results are shifted by $+$0.05 eV for clarity.

Figure 2

(a) LEEM $I$($V$) curves for the clean and Fe covered W(111) surface, measured during continuous Fe deposition at room temperature, are shown for film thickness from 0 to 18 ML at roughly 0.21 ML intervals. The data at 3 ML intervals are indicated by thick (red) curves. The curves are shifted vertically and intensity is scaled at larger thickness for clarity. (b) The position of the dominant intensity peaks are indicated by solid lines and are plotted vs energy. The energy scale is referenced to the instrument power supply (see text).

Figure 3

(a) As-measured LEEM $I$($V$) curves for clean W(111) and indicated Fe thicknesses. (b) Ratio of elastically scattered intensity in the diffraction spots to the background intensity integrated over all reciprocal space at incident energy 22 eV.

Figure 4

SPLEEM images obtained at incident energies (left) 1.4 eV and (right) 6.8 eV during Fe deposition at room temperature are shown for selected Fe film thicknesses. The incident spin polarization direction probes in-plane sample magnetization along the easy axis corresponding to the azimuthal direction ϕ $=$ 80° with respect to the instrument reference direction. Images on the left at 14 ML and above have been processed to enhance the weak domain contrast at 1.4 eV.

Figure 5

SPLEEM magnetic asymmetry as a function of Fe film thickness at the indicated incident energies, measured at 298.8 K for the in-plane direction at ϕ $=$ 80°. A ferromagnetic thickness gap is observed at all energies between 8 and 9 ML. The multidomain structure appears in the thickness range above 12 ML.

Figure 6

SPLEEM magnetic asymmetry as a function of (a) incident electron energy relative to the vacuum level and (b) in-plane azimuthal spin polarization direction (ϕ) are shown for incrementally thicker Fe films: (1) 6.29, (2) 6.45, (3) 6.99, (4) 7.49, (5) 8.50, (6) 9.20, (7) 10.0, (8) 10.69, (9) 10.85, (10) 11.07, (2) 11.36, (12) 12.18, (13) 12.71, (14) 13.23, (15) 18.07 ML. The measurements in (a) probed in-plane magnetization along the easy axis (ϕ $=$ 80°) for all thicknesses. Null results for the orthogonal in-plane (ϕ $=$ 170°) and out-of-plane directions are shown for 8.5 ML as well. The vertical dashed lines in (a) indicate the energies of the measurements shown in Fig. 5. In (b), measurements were made at incident energy $E$ $=$ 7.0 eV. The curves are offset for clarity.

Figure 7

(a) SPLEEM magnetic asymmetry at $E$ $=$ 7.0 eV is shown as a function of thickness during growth at several temperatures, 298.8 K (◯), 302.3 K (□), 303.9 K (△), 304.4 K (▽), 305.1 K ($♦$). (b) The width of the thickness range centered at 7 ML in which asymmetry is observed during deposition (◯) and the magnitude of asymmetry at 7 ML ($♦$) are shown for different deposition temperatures.

Figure 8

(a) SPLEEM magnetic asymmetry at $E$ $=$ 7.0 eV vs temperature is shown for the film thicknesses, 6.44 ML (◯), 7.04 ML (□), 7.47 ML (△). (b) The temperature $Tc$, defined at the point that asymmetry vanishes during heating, is shown as a function of thickness. The horizontal dashed line indicates room temperature.