Magnetism of iron on tungsten (001) studied by spin-resolved scanning tunneling microscopy and spectroscopy

The system of $\mathrm{Fe}$ on $\mathrm{W}\left(001\right)$ has been investigated using spin-integrated as well as spin polarized scanning tunneling microscopy and spectroscopy. The electronic structure in the pseudomorphic growth regime and the beginning of strain relief has been studied and a pronounced layer dependence of the differential conductance has been observed. The spin-resolved measurements focus on the magnetic structure in the pseudomorphic regime. The domain structure is investigated with high spatial resolution and the fourfold anisotropy can directly be deduced from the maps of differential conductance. Quantitative analysis of the intensity of the signal reveals a layer dependent easy axis in the pseudomorphic regime. While the second and third monolayer are magnetized along $⟨110⟩$ directions of the surface the fourth monolayer has an easy axis along $⟨100⟩$. These high-symmetry directions and therefore also the layer-dependent easy axes enclose an angle of $45°$.

Figure 1

Sample of $5.1\mathrm{ML}$ of $\mathrm{Fe}$ on $\mathrm{W}\left(001\right)$ grown at $T\approx 550\mathrm{K}$. (a) Topography, the dashed rectangle marks the image area investigated more closely in Fig. 2; (b) $dI∕dU$ map corresponding to (a) at $U=+0.2\mathrm{V}$, the marks (a–e) indicate steps of the underlying $\mathrm{W}\left(001\right)$ substrate; (c) height profile along the line in (a) with a sketch of the local coverage; (d) corresponding $dI∕dU$ signal along the line in (b) with dotted lines indicating the signal strength for the different local coverage.

Figure 2

Sample of $5.1\mathrm{ML}$ of $\mathrm{Fe}$ on W(001) grown at $T\approx 500\mathrm{K}$. (a) Topography at $U=+1.0\mathrm{V}$, the dashed line indicates a step of the tungsten substrate, labels refer to local coverage; (b) $dI∕dU$ map corresponding to (a) at $U=+0.9\mathrm{V}$ taken from full spectroscopy (stabilization parameters: $U=+1.0\mathrm{V}$, $I=1.0\mathrm{nA}$); (c) model of the $\left(9\times 1\right)$ reconstruction cut along the [100] direction perpendicular to the dislocation lines; (d) histogram of left/upper and right/lower terrace in (a) shifted by $1.6\mathrm{\AA }$ with respect to each other; (e) layer-resolved $dI∕dU$ spectra shifted by $dI∕dU=2\mathrm{arb}$. units for clarity, labels refer to the local coverage [cf. (a)]; inset, $dI∕dU$ spectrum of the 1st ML from a different measurement; (f) $dI∕dU$ spectra taken on protrusions and depressions of the reconstructed areas; inset, topography of local coverage indicated with C.

Figure 3

Topography of $\mathrm{Fe}$ on $\mathrm{W}\left(001\right)$ in the pseudomorphic regime with different growth conditions. (a) 2.2 ML $\mathrm{Fe}$ on $\mathrm{W}\left(001\right)$ grown at $T\approx 525\mathrm{K}$; (b) 3.2 ML $\mathrm{Fe}$ on $\mathrm{W}\left(001\right)$ grown at $T\approx 525\mathrm{K}$; (c) 3.0 ML $\mathrm{Fe}$ on $\mathrm{W}\left(001\right)$ grown at $T\approx 550\mathrm{K}$.

Figure 4

(Color) Sample of $3.0\mathrm{ML}$ of $\mathrm{Fe}$ on $\mathrm{W}\left(001\right)$ grown at $T\approx 550\mathrm{K}$. (a) Topography, arrows indicate 3rd ML islands which will be referred to in Sec. 3B 3; (b) corresponding $dI∕dU$ map measured at $U=-0.8\mathrm{V}$, color: 2nd ML /gray: 3, $4\mathrm{ML}$; (c) $dI∕dU$ map corresponding to (a) at $U=-0.1\mathrm{V}$, color: 4th ML /gray: 2, 3 ML; (d) magnetization of 2nd ML with respect to tip magnetization; (e) magnetization of 4th ML with respect to tip magnetization; (f) sketch of the $45°$ rotation between 2nd and 4th ML magnetization.

Figure 5

(Color) Sample of $3.0\mathrm{ML}$ of $\mathrm{Fe}$ on $\mathrm{W}\left(001\right)$ grown at $T\approx 550\mathrm{K}$. (a) $dI∕dU$ map at $U=-0.1\mathrm{V}$; (b) sketch of the ${45}^{°}$ rotation between 2nd and 4th ML magnetization.

Figure 6

(Color) Sample of $3.0\mathrm{ML}$ of $\mathrm{Fe}$ on $\mathrm{W}\left(001\right)$ grown at $T\approx 550\mathrm{K}$. (a) Topography at $U=+0.05\mathrm{V}$; (b) $dI∕dU$ map corresponding to (a) at $U=+0.05\mathrm{V}$; (c) sketch of the ${45}^{°}$ rotation between 2nd and 4th ML magnetization; (d) $dI∕dU$ spectra of different magnetic domains (i, ii, iii) of the 2nd ML and asymmetry; (e) $dI∕dU$ spectra of different magnetic domains (1,2) of the 4th ML and asymmetry; (f) $dI∕dU$ spectra of the 3rd ML.

Figure 7

(Color) Sample of $3.2\mathrm{ML}$ of $\mathrm{Fe}$ on $\mathrm{W}\left(001\right)$ grown at $T\approx 525\mathrm{K}$. (a) Topography at $U=+0.1\mathrm{V}$; (b) $dI∕dU$ map corresponding to (a) at $U=+0.1\mathrm{V}$; (c) sketch of the $45°$ rotation between 3rd and 4th ML magnetization.

Figure 8

Sketch of the unit cell of pseudomorphic $\mathrm{Fe}∕\mathrm{W}\left(001\right)$ together with the coverage dependent easy magnetization axes.