Fe on W(001) from continuous films to nanoparticles: Growth and magnetic domain structure

The evolution of the structural and magnetic properties of Fe films during growth on the W(001) surface have been studied with low energy electron diffraction, real-time low energy electron microscopy, and quasi-real-time, spin-polarized low energy electron microscopy in the absence of a magnetic field (virgin state). Depending on the growth temperature, different growth modes are observed: growth of atomically rough and highly strained (10.4% tensile) pseudomorphic films at room temperature, kinetically limited layer-by-layer growth (quasi–Frank-van der Merwe growth mode) of smooth pseudomorphic films up to 4 monolayers at around 500 K and growth of fully relaxed three-dimensional Fe islands on top of a thermodynamically stable 2-monolayer-thick wetting layer (Stranski-Krastanov growth mode) at and above 700 K. Around 500 K, layered growth is terminated by partial (2 monolayers) dewetting of the metastable Fe film and formation of thin, partially relaxed, elongated islands on a thermodynamically stable 2 monolayer film. Ferromagnetic order is first detected during growth at room temperature at 2.35 monolayer Fe film thickness. The magnetization is in-plane with a thickness-dependent direction, rotating from the substrate $\langle 110\rangle$ directions at 3 monolayers toward the $\langle 100\rangle$ directions at 4 monolayers and back again toward the $\langle 110\rangle$ directions at about 8 monolayers. The in-plane spin reorientation that occurs at room temperature is accompanied by significant changes of the magnetic domain structure. In the Frank-van der Merwe growth regime, large magnetic domains are observed in metastable 3 and 4 monolayer films. The isolated three-dimensional Fe islands that form in the Stranski-Krastanov regime have vortex, quasi-single domain ($C$ state), or single magnetic domain structures, depending on their size and shape. The detailed results that are obtained with high thickness, lateral and azimuthal angular resolution with spin-polarized low energy electron microscopy are compared with earlier laterally averaging and laterally resolving magnetic studies, and discrepancies are explained.

Figure 1

LEEM image intensity oscillations during deposition of Fe on the W(001) surface at several deposition temperatures. The thickness scale is in pseudomorphic MLs. The red vertical lines indicate 2.63 and 3.21 ML, respectively. See the text for the discussion about these thicknesses. The imaging electron energy is 9.0 eV.

Figure 2

LEEM video frames from the early stages of Fe growth on W(001) at 600 K. (a) Clean surface, (b) 1.5 ML, (c) 2 ML, (d) 2.2 ML. The imaging energy is 9.0 eV.

Figure 3

LEEM video frames of the film morphology after nucleation of 3D Fe crystals on W(001) at several deposition temperatures. (a) 700 K, 2.95 ML, (b) 600 K, 3.63 ML, (c) 500 K, 4.0 ML.

Figure 4

LEEM video frames showing (a) 3D crystals at 600 K, 6.0 ML (in LEEM) and (b) quasi-2D elongated crystals at 500 K, 5.0 ML (in SPLEEM). In (a) 3 ML (dark gray) and 2 ML (light gray) regions are present between crystals (black). Note that panel (a) was taken from a different experiment than that shown in Fig. 3. The imaging electron energy is (a) 9.0 eV and (b) 0.5 eV.

Figure 5

Selected SPLEEM images taken during growth of Fe on W(001) at room temperature: (a) 2.98, (b) 3.76, (c) 5.01, (d) 6.66, (e) 7.36, (f) 8.62 ML. The figure shows from left to right: image taken with polarization (1) parallel to [100] direction, (2) parallel to [010] direction, as arrows point, respectively; composite images that present (3) magnitude of the magnetic asymmetry and (4) magnetization direction; and (5) histogram of the angular distribution of the magnetization direction calculated pixel by pixel from (4). The imaging electron energy is 0.5 eV. See Supplemental Material for a full set of results [33].

Figure 6

(a) Magnitude of the magnetic asymmetry and (b) angular distribution of the magnetization direction averaged over the field of view of the SPLEEM images obtained during Fe film growth at room temperature (Fig. 5, and see Supplemental Material [33]), with $\varphi =0^{\circ },{90}^{\circ },{180}^{\circ }$, and ${270}^{\circ }$ corresponding to in-plane $\langle 100\rangle$ directions. The color code in panel (b) represents the pixel number at each angle relative to the maximum value (dark red) of the whole angular distribution.

Figure 7

Local (pixel-by-pixel) data derived from the SPLEEM images obtained during Fe film growth at room temperature (Fig. 5, and see Supplemental Material [33]). The properties of the three domain orientations (green, cyan, blue), which dominate at different coverages, are (a) fractional coverage, (b) magnetic asymmetry, (c) magnetization direction, (d) FWHM of the angular distribution of the magnetization direction, (e) fractional residue after domain fitting, and (f) total domain wall length per unit area.

Figure 8

SPLEEM images of films grown at 400 K, measured at room temperature: (a) 3 ML, (b) 4 ML, (c) 5 ML, and (d) 6 ML. The figure shows from left to right, image taken with polarization (1) parallel to [100], (2) parallel to [010], composite images that present (3) magnitude of the magnetic asymmetry and (4) magnetization direction and (5) the histogram of the angular distribution of the magnetization direction calculated pixel by pixel from (4). The arrows in the top row indicate the [100] and [010] directions, respectively, the arrows in the second row indicate the magnetization directions in the domains. The imaging electron energy is 0.5 eV.

Figure 9

(a) LEEM and (b, c) SPLEEM images with the polarization parallel to [100] and [010], respectively, of a 5.8-ML-thick Fe film grown on W(001) at 600 K, measured at room temperature. The (d) composite image of images (b) and (c) shows the magnitude of the magnetic asymmetry. The angular distribution of the magnetization direction in 3- and 4-ML-thick regions are shown in panels (e) and (f); that in the 3D crystals is shown in panel (g). (h) Pixel-by-pixel analysis of these images gives the angular magnetization distribution histograms. The imaging electron energy is 0.5 eV.

Figure 10

(a) LEEM and (b, c) SPLEEM images of a 5-ML-thick film grown at about 700 K, measured at room temperature. The polarization direction in panels (b) and (c) is parallel to [100] and [010], respectively. The following rows show the corresponding magnified images of individual crystals and their interpretation: (d–g) a large square crystal, (h–k) a large rectangular crystal, and (l–o) a small rectangular crystal with vortex, $C$-state, and single domain structure, respectively.

Figure 11

SPLEEM images of films grown at room temperature. (a) 8 ML and (b) 9.8 ML. The figure shows from left to right: image taken with polarization (1) parallel to [100] direction, (2) parallel to [010] direction as arrows point, respectively; composite images that present (3) magnitude of the magnetic asymmetry and (4) magnetization direction; and (5) histogram of the angular distribution of the magnetization direction calculated pixel by pixel from (4). The imaging electron energy is 0.5 eV.