Magnetic properties of Fe films on flat and vicinal Au(111): Consequences of different growth behavior

The epitaxial growth of Fe on flat Au(111) differs from Fe on vicinal Au(111). One consequence is a different critical Fe overlayer thickness at which a structural phase transition from fcc(111) to bcc(110) takes place. The critical thickness is higher for Fe on vicinal Au(111) than on flat Au(111). Accompanied with the structural phase transition is a spin-reorientation transition of the easy-magnetization direction from out of plane to in plane. In agreement with the impeded phase transition we observe an impeded spin-reorientation transition on vicinal Au(111). The spin-reorientation transition, however, proceeds in a narrow coverage range while the structural phase transition occurs gradually over a larger thickness range. A further consequence of the different growth behavior is a different topography for thicknesses beyond the phase transition. Fe on flat Au(111) exhibits a sixfold symmetry, Fe on vicinal Au(111) only a twofold symmetry. The different symmetries influence the magnetic properties: while for bcc Fe(110) on flat Au(111) no easy-magnetization direction could be determined, we identified a preferred direction for Fe on vicinal Au(111) which is perpendicular to the step edges. The twofold symmetry causes an uniaxial magnetic behavior. This knowledge of the magnetization behavior is essential for correctly analyzing spin-resolved measurements of the electronic structure as demonstrated by photoemission measurements.

Figure 1

Photoemission spectra from vicinal Au(111) measured for normal electron emission at RT (except for the coverage of 8.0 ML, which was measured at $T=145\text{K}$) as a function of Fe overlayer thickness. The excitation energy was $\hslash \omega =21.22\text{eV}$. In order to be able to compare the spectra with each other, they were normalized to equal background intensity.

Figure 2

Determination of the electron attenuation length $\lambda$ for Fe from the PE intensities of the $\text{Au}5d$ states at $E-E_{\text{F}}\approx -6\text{eV}$. The solid dots represent our measurements for the intensity of the $\text{Au}5d$ states with increasing Fe overlayer thickness. The solid line represents a linear fit through the data points from which $\lambda$ was determined. The empty dots are obtained from PE data taken by Fujisawa et al. (Ref. 37). The dotted line represents a linear fit through the data points. $\lambda$ is about an order of magnitude smaller in comparison with our result and other results in literature.

Figure 3

(Color online) (a) Polar MOKE measurements performed at $T=115\text{K}$ for various Fe overlayer thicknesses on Au(23 25 25). At an overlayer thickness of 1.1 ML we observed a paramagnetic behavior of the Fe structures. For a coverage up to 4.4 ML the easy-magnetization direction is out of plane. (b) Result of the longitudinal MOKE measurements of different Fe overlayer thicknesses on Au(23 25 25). The measurements were performed at $T=115\text{K}$ and at $T=295\text{K}$. Above 5 ML of Fe on Au(23 25 25) the easy-magnetization direction is in the film plane and stays in plane for the entire Fe overlayer thickness range studied. The magnetic field was applied perpendicular to the step edges. The sweeping speed of the field was about 5 Oe/s.

Figure 4

(Color online) Illustration of the matching between the bcc(110) Fe unit cells on (a) flat and (b) vicinal Au(111). For simplicity the same lattice constant for Fe and Au was chosen. (a) For bcc(110) Fe six orientations of the unit cells are possible in the KS orientation (black atoms) and three orientations for the NW-growth mode (white atoms). (b) For bcc(110) Fe on Au(23 25 25), only two of three growth directions are found (Ref. 17): four orientations of the bcc(110) unit cells are possible in the KS mode (black atoms) and two in the NW mode (white atoms). The growth of the crystallites along the ${\left[01\overline{1}\right]}_{\text{fcc}}$ direction is not realized in the case of Fe/Au(23 25 25). This excludes the bcc(110) unit cells with an orientation of the ${\left[001\right]}_{\text{bcc}}$ parallel (NW mode) and close to parallel (KS mode) to ${\left[01\overline{1}\right]}_{\text{fcc}}$. Please note that the terrace width in this sketch was chosen to illustrate the orientations of the bcc(110) unit cells and does not correspond to the actual terrace width of Au(23 25 25).

Figure 5

(Color online) MOKE measurements of 5 ML of Fe on flat Au(111) with the field applied along different directions as depicted in Fig. 4. All measurements were performed at $T=115\text{K}$. The hysteresis curves are very similar to each other and the shape of the curves is different from an easy-magnetization behavior. The sweeping speed of the field was about 5 Oe/s.

Figure 6

(Color online) MOKE measurements of 5 ML of Fe on vicinal Au(111) with the field applied along different directions. All measurements were performed at $T=115\text{K}$. The sweeping speed of the field was about 5 Oe/s.

Figure 7

(Color online) Spin-resolved PE measurements $\left(\hslash \omega =21.22\text{eV}\right)$ of 5 ML Fe/Au(111) for normal electron emission in dependence of the direction in which the sample was remanently magnetized prior to the measurement.

Figure 8

(Color online) Spin-resolved PE measurements $\left(\hslash \omega =21.22\text{eV}\right)$ of 5 ML Fe/Au(23 25 25) for normal electron emission in dependence of the direction in which the sample was remanently magnetized prior to the measurement.