Guiding Spin Spirals by Local Uniaxial Strain Relief

We report on the influence of uniaxial strain relief on the spin spiral state in the Fe double layer grown on Ir(111). Scanning tunneling microscopy (STM) measurements reveal areas with reconstruction lines resulting from uniaxial strain relief due to the lattice mismatch of Fe and Ir atoms, as well as pseudomorphic strained areas. Magnetic field-dependent spin-polarized STM measurements of the reconstructed Fe double layer reveal cycloidal spin spirals with a period on the nm scale. Globally, the spin spiral wave fronts are guided along symmetry-equivalent [$11\overline{2}$] crystallographic directions of the fcc(111) substrate. On an atomic scale the spin spiral propagation direction is linked to the [001] direction of the bcc(110)-like Fe, leading to a zigzag shaped wave front. The isotropically strained pseudomorphic areas also exhibit a preferred magnetic periodicity on the nm scale but no long-range order. We find that already for local strain relief with a single set of reconstruction lines a strict guiding of the spin spiral is realized.

Figure 1

(a) STM topography of about 1.6 atomic layers of Fe on an Ir(111) surface (measurement parameters: $U=+0.2\mathrm{V}$, $I=1\mathrm{nA}$, $T=4.8\mathrm{K}$). The Fe ML grows pseudomorphically. The Fe DL consists of reconstructed ($R$) and strained ($S$) areas. (b),(c) Magnified topography images [see box in (a)] of a reconstructed area with periodic reconstruction lines due to uniaxial strain relief taken at $U=+0.2$ and $U=-0.2\mathrm{V}$, respectively. (d) Atomic structure model of the reconstructed Fe DL with a 5% horizontally compressed Fe top layer on a pseudomorphic hexagonal Fe bottom layer; this locally leads to bcc(110)-like areas (blue rectangles) separated by hollow site reconstruction lines (yellow, pink).

Figure 2

(a),(b) Spin-averaged topography images of the reconstructed Fe DL with three rotational domains taken at $U=+1.0$ and $U=+0.2\mathrm{V}$, respectively. The typical period of the reconstruction is 4.7 nm, compatible with about 6% strain relief, or $18/17$ Fe atoms in the two Fe layers. (c),(d) SP-STM topography images performed at the same location in an out-of-plane magnetic field of 4 T [14]. (e),(f) SP-STM topography images taken in orthogonal in-plane magnetic fields, exhibiting the respective in-plane magnetization component resulting from an alignment of the tip magnetization along the fields. The green and blue empty boxes indicate the maximum magnetic corrugation amplitudes of the different $\overrightarrow{q}$ vectors (measurement parameters: $I=1\mathrm{nA}$, $T=4.8\mathrm{K}$).

Figure 3

(a) Overview SP-STM topography image of the spin spiral in the Fe DL and the nanoskyrmion lattice in the Fe ML; the zigzag shape of the spin spiral wave fronts with an angle of $\theta \sim 154°$ is indicated (measurement parameters: $U=+1.0\mathrm{V}$, $I=1\mathrm{nA}$, $T=4.8\mathrm{K}$). (b) Sketch of the magnetic state of the reconstructed Fe DL, as deduced from the SP-STM measurements. (c),(d) SP-STM topography images of the zigzag spin spiral measured with an out-of-plane and in-plane spin-sensitive tip, respectively. The corresponding SP-STM simulations are in the insets (measurement parameters: $U=+0.5\mathrm{V}$, $I=1\mathrm{nA}$, $T=7.8\mathrm{K}$, $B_z=2.5\mathrm{T}$).

Figure 4

(a) Spin-averaged topography of the Fe DL on Ir(111) and (b) SP-STM measurement of the same area [14]. Comparison shows the magnetic contribution to the image in (b) (measurement parameters for both: $U=+0.2\mathrm{V}$, $I=1\mathrm{nA}$, $T=7.8\mathrm{K}$, $B_z=-1.55\mathrm{T}$). (c) Perspective SP-STM overview topography image. The FFT shown in the inset is taken from the dashed box (measurement parameters: $U=+0.2\mathrm{V}$, $I=1\mathrm{nA}$, $T=7.8\mathrm{K}$, $B_z=0\mathrm{T}$).