Revealing the correlation between real-space structure and chiral magnetic order at the atomic scale

We image simultaneously the geometric, the electronic, and the magnetic structures of a buckled iron bilayer film that exhibits chiral magnetic order. We achieve this by combining spin-polarized scanning tunneling microscopy and magnetic exchange force microscopy (SPEX) to independently characterize the geometric as well as the electronic and magnetic structures of nonflat surfaces. This new SPEX imaging technique reveals the geometric height corrugation of the reconstruction lines resulting from strong strain relaxation in the bilayer, enabling the decomposition of the real-space from the electronic structure at the atomic level and the correlation with the resultant spin-spiral ground state. By additionally utilizing adatom manipulation, we reveal the chiral magnetic ground state of portions of the unit cell that were not previously imaged with spin-polarized scanning tunneling microscopy alone. Using density functional theory, we investigate the structural and electronic properties of the reconstructed bilayer and identify the favorable stoichiometry regime in agreement with our experimental result.

Figure 1

(a) Large-scale constant current SP-STM image ($V_{\mathrm{s}}=50\mathrm{mV}, I_{\mathrm{T}}=100\mathrm{pA}$, Fe tip) and constant frequency shift MExFM image ($\mathrm{\Delta }{f}_{\mathrm{set}}=-12\mathrm{Hz}, z_{\mathrm{mod}}=100\mathrm{pm}$, and $V_{\mathrm{s}}=0.1\mathrm{mV}$, Fe tip). Both images: $43.5\times 43.5\mathrm{n}{\mathrm{m}}^2$. Scale bars are 10 nm. Color scales are (a) 0.4–500 pm and (b) 220–240 pm.

Figure 2

(a) Constant-current STM ($V_{\mathrm{s}}=50\mathrm{mV}, I_{\mathrm{T}}=100\mathrm{pA}$, PtIr tip) and constant frequency shift NC-AFM images (flattened by an offset, $\mathrm{\Delta }{f}_{\mathrm{set}}=-40\mathrm{Hz}, z_{\mathrm{mod}}=100\mathrm{pm}$, and $V_{\mathrm{s}}=0.1\mathrm{mV}$). (c) Line profiles of the STM and NC-AFM images along the horizontal lines as indicated in (a) and (b). The NC-AFM has been smoothed prior to taking the line profile by a Gaussian filter with two points. (d) Atomic resolution STM image (flattened by an offset, Gaussian smoothed by two points, $V_{\mathrm{s}}=1\mathrm{mV}, I_{\mathrm{T}}=48\mathrm{nA}$, PtIr tip) and the inset shows the fast Fourier transform (FFT) of (d). Sizes: (a) $8.5\times 7.3\mathrm{n}{\mathrm{m}}^2$, (b) $8.5\times 7.3\mathrm{n}{\mathrm{m}}^2$, and (d) $6.8\times 3.3\mathrm{n}{\mathrm{m}}^2$. Scale bars are 1 nm for all images. Color scales are (a) 0–54 pm, (b) 1–11 pm, and (d) 0–28 pm. The arrows indicate equivalent positions in all images.

Figure 3

Characterization of the reconstructed overstoichiometric (10:9) surface from ab initio supercell calulations. In (a), a simulated STM image is presented along with the atomic structure of the reconstructed surface. In (b), a more detailed analysis of the atomic and electronic structures along the two atomic rows indicated in (a) is given.

Figure 4

(a) Constant-current SP-STM ($V_{\mathrm{s}}=50\mathrm{mV}, I_{\mathrm{T}}=100\mathrm{pA}$, Fe tip) and constant frequency shift MExFM image (flattened by an offset, smoothed by a Gaussian filter with two points, $\mathrm{\Delta }{f}_{\mathrm{set}}=-19\mathrm{Hz}, z_{\mathrm{mod}}=100\mathrm{pm}$, and $V_{\mathrm{s}}=0.0\mathrm{mV}$, Fe tip). Scale bars are 1 nm. The tip magnetization is out of plane as indicated by the symbol. (c) and (d) Constant-current SP-STM images (Fe tip, $V_{\mathrm{s}}=50\mathrm{mV}, I_{\mathrm{T}}=100\mathrm{pA}$, smoothed by Gaussian filter two points); scale bars are 2 nm. Color bar: (a) 0–56 pm; (b) 0–5 pm; (c) and (d): 0–76 pm. The Fe adatom (the white circle) was moved in between the dark and the bright spin spirals. The reference atom $R$ rests at the same position (see the Supplemental Material [26]). (e) Apparent height $\mathrm{\Delta }h$ of the adatom versus the distance $\mathrm{\Delta }{d}_y$ to the reference atom $R$ when moved along a line in between the dark and the bright spin spirals. The $y$ direction is defined in (c).