Structural and magnetic properties of $3d$ transition metal oxide chains on the (001) surfaces of Ir and Pt

We present a survey of the structural and magnetic properties of submonolayer transition metal dioxides on the (001) surfaces of the heavy face-centered cubic noble metals Ir and Pt performed by spin-averaged scanning tunneling microscopy (STM) and spin-polarized (SP) STM. Our STM results confirm that deposition of Co, Fe, Mn, and Cr on the $(2\times 1)$ oxygen-reconstructed Ir(001) surface leads to the formation of quasi-one-dimensional chains with a $(3\times 1)$ unit cell. As recently predicted by density functional theory [P. Ferstl et al., Phys. Rev. Lett. 117, 046101 (2016)], our SP-STM images of ${\mathrm{FeO}}_2$ and ${\mathrm{MnO}}_2$ on Ir(001) show a twofold periodicity along the chains which is characteristic for an antiferromagnetic coupling along the chains. In addition, these two materials also exhibit spontaneous, permanent, and long-range magnetic coupling across the chains. Whereas we find a ferromagnetic interchain coupling for ${\mathrm{FeO}}_2$/Ir(001), the magnetic coupling of ${\mathrm{MnO}}_2$ on Ir(001) appears to be a noncollinear ${120}^{\circ }$ spin spiral, resulting in a $(9\times 2)$ magnetic unit cell. On Pt(001), patches of $(3\times 1)$-reconstructed oxide chains could only be prepared by transition metal (Co, Fe, and Mn) deposition onto the cold substrate and subsequent annealing in an oxygen atmosphere. Again SP-STM on ${\mathrm{MnO}}_2$/Pt(001) reveals a very large, $(15\times 2)$ magnetic unit cell which can tentatively be explained by a commensurate ${72}^{\circ }$ spin spiral. Large-scale SP-STM images reveal a long-wavelength spin rotation along the ${\mathrm{MnO}}_2$ chain.

Figure 1

Structure model of transition metal oxide chains on Ir(001) as proposed in Ref. [10]. The transition metal atom chains sit above empty substrate rows, held in place by the oxygen atoms. The interchain spacing corresponds to $3a_{\mathrm{Ir}}$.

Figure 2

(a) Overview scan of a sample with coexisting $(3\times 1)$-ordered ${\mathrm{CoO}}_2$ chains and $(2\times 1)$ oxygen-reconstructed areas on Ir(001). (b) Higher-magnification image of the area marked by a white square in (a) revealing an orthogonal orientation row orientation of $(2\times 1)$- and $(3\times 1)$-ordered areas. The line profile in the bottom panel confirms a $3a_{\text{Ir}}$ periodicity perpendicular to the ${\mathrm{CoO}}_2$ chains. (c) Atomic-resolution scan of the area marked by a box in (b). The line profile verifies the $\times 1$ periodicity along the ${\mathrm{CoO}}_2$ chains. Scan parameters: (a), (b) $U=1\mathrm{V}, I=300\mathrm{pA}$; (c) $U=50\mathrm{mV}, I=1\mathrm{nA}$. All scans performed with a nonmagnetic W tip.

Figure 3

(a) Overview scan of ${\mathrm{FeO}}_2$/Ir(001) with chains running in the [110] direction on the upper and $\left[1\overline{1}0\right]$ direction on the lower terrace. (b) Higher-resolution scan on the lower terrace. (c) Line profile along the blue line in (b). The periodicity of the stripes amounts to $(855\pm 50)\mathrm{pm}$. Scan parameters: (a), (b) $U=-1\mathrm{V}, I=300\mathrm{pA}$, nonmagnetic W tip.

Figure 4

(a) Atomic-resolution scan of ${\mathrm{FeO}}_2$ chains on Ir(001) obtained with a nonmagnetic W tip. The atoms along the chains are separated by one Ir lattice constant $a_{\text{Ir}}=271\mathrm{pm}$. (b) SP-STM image obtained with an out-of-plane Cr/W tip. Comparison to (a) reveals period along the chains corresponding to $2a_{\text{Ir}}=542\mathrm{pm}$ resulting in a $(3\times 2)$ magnetic unit cell. (c) Line profiles measured along the chains at the position indicated by the arrows in (a) and (b). (d) Schematic illustration of the $(3\times 2$) collinear magnetic order. While the magnetic coupling is AFM along the chains, the interaction of adjacent chains is FM. Scan parameters: (a) $U=-10\mathrm{mV}, I=20\mathrm{nA}$; (b) $U=50\mathrm{mV}, I=5\mathrm{nA}$.

Figure 5

(a) Overview scan of ${\mathrm{MnO}}_2$ chains on Ir(001). Domains with stripes of the $(3\times 1)$ structure orientated along the $\left[\overline{1}10\right]$ and the [110] direction can be recognized. (b) Spin-averaged STM image of an atomically flat surface area showing a few defects only. (c) Line profile measured along the blue line in (b) confirming an interchain distance of $3a_{\text{Ir}}$. Scan parameters: $U=1\mathrm{V}, I=300\mathrm{pA}$, nonmagnetic W tip.

Figure 6

(a) Atomic-resolution scan of the ${\mathrm{MnO}}_2$ chains on Ir(001) measured with a nonmagnetic W tip showing the $(3\times 1)$ unit cell. (b) SP-STM image of ${\mathrm{MnO}}_2$/Ir(001) obtained with an in-plane sensitive Cr-coated W tip. The period along the stripes corresponds to $2a_{\text{Ir}}$ indicating AFM order along the chain. The magnetic contrast of adjacent chains is modulated by phase and corrugation changes resulting in a $(9\times 2)$ magnetic unit cell. (c) Line profile measured along three adjacent chains in (a) without (black) and in (b) with magnetically sensitive tips (colored). Scan parameters: (a) $U=-300\mathrm{mV}, I=3\mathrm{nA}$; (b) $U=200\mathrm{mV}, I=300\mathrm{pA}$.

Figure 7

(a) Schematic illustration of the $(9\times 2)$ magnetic unit cell of ${\mathrm{MnO}}_2$ chains on Ir(001). The arrows indicate the spin direction of the corresponding Mn atoms. (b) Sketch of the SP-STM signal expected for the proposed chiral ${120}^{\circ }$ spin spiral. The corrugation and the phase shift observed in Fig. 6 can be explained by an angle $\mathrm{\Theta }={(15\pm 6)}^{\circ }$ between tip polarization (black arrow) and the TMO chain with highest corrugation (blue line).

Figure 8

(a) Overview and (b) higher-resolution scan on ${\mathrm{CrO}}_2$ chains on Ir(001). (c)–(g) Bias-dependent STM images showing the same sample area. A cluster (marked by an arrow) serves as a position marker. A corrugation reversal is observed between panels (e) and (f), i.e., between $U=200\mathrm{mV}$ and $U=-200\mathrm{mV}$. (h) Line profiles measured along lines corresponding to the blue line in (c). Scan parameters: (a), (b) $U=1\mathrm{V}, I=300\mathrm{pA}$; (c) $I=2\mathrm{nA}$; (d), (g) $I=1.5\mathrm{nA}$; (e), (f) $I=0.5\mathrm{nA}$.

Figure 9

Growth of ${\mathrm{CoO}}_2$ chains on Pt(001). (a) Topography of Co/Pt(001) after evaporation on the warm substrate. (b) After oxidation of sample (a) no $(3\times 1)$ structure is observed. (c) Topography of Co/Pt(001) after evaporation on the cold substrate. (d) After oxidation of sample (c) a coexistence of Pt reconstruction and domains of ${\mathrm{CoO}}_2$ chains is observed. (e) Higher-resolution scan of the chains with line profile across the stripes. (f) Atomic-resolution scan with a nonmagnetic W tip confirms the structural $(3\times 1)$ unit cell. Scan parameters: (a)–(d) $U=1\mathrm{V}, I=300\mathrm{pA}$; (e) $U=-100\mathrm{mV}, I=1.5\mathrm{nA}$; (f) $U=50\mathrm{mV}, I=5\mathrm{nA}$. All data measured with a nonmagnetic W tip.

Figure 10

(a) Overview scan of oxidized Fe on Pt(001). (b) A higher-resolution scan shows that terraces and islands exhibit a stripe pattern with the expected $3a_{\text{Pt}}$ periodicity. (c) Atomic-scale image of oxidized Fe on Pt(001). Along the stripes the corrugation has a period of twice the atomic-lattice vector. Within the circle the order across the stripes changes from aligned to a shifted position. (d) Line profiles across (top) and along the stripes (bottom) at the positions of the blue line in (b) and at the position of the arrows in (c). Scan parameters: (a), (b) $U=1\mathrm{V}, I=300\mathrm{pA}$; (c) $U=100\mathrm{mV}, I=1\mathrm{nA}$. Nonmagnetic W tip.

Figure 11

(a) Overview scan of Pt(001) after Mn deposition and subsequent annealing in oxygen. In addition to reconstructed clean Pt(001) one also observes disordered (nr) and $(3\times 1)$-reconstructed surface regions. (b) Higher-resolution scan at the position of the stripes. (c) Line profile measured along the blue line in (b) confirming the $3a_{\text{Pt}}$ periodicity. Scan parameters: $U=1\mathrm{V}, I=300\mathrm{pA}$, nonmagnetic W tip.

Figure 12

(a) Atomic-resolution scan of ${\mathrm{MnO}}_2$ chains on Pt(001) performed with a W tip. (b) Spin-polarized measurement taken with a Cr-coated W tip. A periodic modulation of the chain-specific corrugation can clearly be seen. (c) Line profiles measured along five adjacent chains marked by correspondingly colored arrows as measured with a nonmagnetic (top) and a magnetic tip (bottom). Whereas a doubling of the period characteristic for an intrachain AFM coupling is observed on most of the chains, a pronounced modulation of the contrast is also visible across the chains. Scan parameters: (a) $U=-5\mathrm{mV}, I=20\mathrm{nA}$; (b) $U=1\mathrm{V}, I=300\mathrm{pA}$.

Figure 13

(a) Schematic representation of a $(15\times 2)$ magnetic unit cell. A cycloidal spin spiral with out-of-plane rotating spins. The model in (b) is based on the magnetic corrugation in Fig. 12. From this a tilt $\mathrm{\Theta }={(-15\pm 5)}^{\circ }$ of the tip polarization with respect to the light blue chain with highest corrugation is seen.

Figure 14

Large SP-STM overview scan performed with a magnetic tip on ${\mathrm{MnO}}_2$ chains on Pt(001). The intrachain coupling is not strictly collinear, as can be recognized by inspecting the ${\mathrm{MnO}}_2$ chains marked by black arrows. Whereas no significant magnetic contrast is detected in the bottom part, a strong magnetic contrast with a $2a_{\text{Pt}}$ periodicity is clearly visible in the upper part of the image.