Magnetic domain walls in strain-patterned ultrathin films

We present a comparison of the characteristics of magnetic domain walls in an atomic monolayer of Co on Pt(111) and a Ni/Fe atomic bilayer on Ir(111) based on spin-polarized scanning tunneling microscopy measurements. In both cases, the films exhibit a roughly triangular dislocation line pattern created by epitaxial strain relief, as well as out-of-plane ferromagnetic order. Domains with opposite magnetization are separated by domain walls with a unique rotational sense, demonstrating the important role of the Dzyaloshinskii-Moriya interaction induced by the Co/Pt and Fe/Ir interfaces. The domain walls in Co/Pt(111) are straight and usually found in geometrical constrictions of the film, where they can minimize their length. In contrast, the domain walls in Ni/Fe/Ir(111) follow complicated paths, which can be correlated to the structural triangular pattern. The comparison between the two systems shows that the structural patterns, despite their similarity, have a different impact on the domain walls. In the Co/Pt(111) case, the magnetic state is not influenced by the dislocation line network, in contrast to the Ni/Fe/Ir(111) system in which the formation of the walls is favored at specific positions of the structural pattern.

Figure 1

Structure of ultrathin films. (a) STM constant-current map of a Co/Pt(111) sample, with the inset showing the details of the dislocation lines network. The Co coverage is about one atomic layer. (b) STM constant-current map of a Ni/Fe/Ir(111) sample. The Fe coverage is about 0.7 atomic layer, and the Ni coverage is about 0.9 atomic layer. The yellow boxes in the inset mark long bridge lines. The inset shows in more detail the dislocation lines. (c) Sketch showing the arrangement of the atoms corresponding to the structural patterns. The overview images were partially differentiated to improve the visibility of the topographic features. Measurement parameters are 250 mV in the main plot in (a), $-1$ V in the inset in (a), 100 mV in the main plot of (b), and 1 nA, 4 K, and a Cr bulk tip in all plots.

Figure 2

Spin-polarized differential conductance maps of a Co/Pt(111) sample, measured with a tip sensitive to (a) the out-of-plane and (b) in-plane sample magnetization components, respectively. The domain walls, indicated by the green ellipses, are located at geometrical constrictions; a closer view of one wall is shown in the inset. The direction of the in-plane component of the tip magnetization is indicated by the black arrow. Measurement parameters are 250 mV, 1 nA, 4 K, 0 T, and a Cr bulk tip.

Figure 3

Spin-resolved differential conductance maps of a Ni/Fe/Ir(111) sample measured with (a) an out-of-plane sensitive tip and (b) a tip sensitive to both the out-of-plane and in-plane components of the magnetization. The blue box in (a) marks part of the area shown in (b). Two domain walls are shown in a closer view in the inset. The presence of ${360}^{\circ }$ domain walls in (b) is marked with white boxes. The direction of the tip's in-plane magnetization component is indicated by the black arrow. Measurement parameters are 100 mV, 1 nA, 4 K, 0 T [in (a)], 200 mT [in (b)], and a Cr bulk tip.

Figure 4

(a) STM constant-current map and (b) the simultaneously recorded spin-polarized differential conductance map of a Co monolayer on Pt(111). The signal in the white box is averaged in the short direction and plotted against the long direction in (c). The black dashed line is a fit to the data points using Eq. (3), which requires the assumption that the tip magnetization has the same direction as the magnetic domains, i.e., the out-of-plane direction. Measurement parameters are $-770$ mV, 1 nA, 4 K, 0 T, and a Cr bulk tip.

Figure 5

(a) STM constant-current map and (b) the simultaneously recorded spin-polarized differential conductance map of a Ni/Fe film on Ir(111). (c) The values of the differential conductance in (b) at points located on the bridge lines, as a function of the distance to the long bridge line marked in (a). Two domain wall profiles were fitted to the data (green dots) with the constraint that the wall width is either 1 or 4 nm (blue and orange lines, respectively). In the inset, the differential conductance profile [averaged over the area in the box indicated in (b)] is shown, as well as a fit (red line). The fit gives a wall width of 2.9 nm. Measurement parameters are 100 mV, 1 nA, 4 K, 0 T, and a Cr bulk tip.

Figure 6

(a) Constant-current map and (b)–(d) spin-resolved differential conductance maps showing the field dependence of the positioning of a domain wall in Ni/Fe/Ir(111). The constant-current map shows the details of the dislocation line pattern. When the field increases, the domain on the left grows, and the domain wall jumps from one bridge line to the next. The tip is sensitive to the out-of-plane component of the sample magnetization. Measurement parameters are 100 mV, 1 nA, 4 K, and a Cr bulk tip.