Odd spin frustration in Cr(001) films thinner than three nanometers revealed by spin-polarized scanning tunneling microscopy

We have studied the surface structure and morphology of epitaxial Cr films on Au(001) substrate with Cr thickness ($d_{\mathrm{Cr}}$) below 3 nm. We have characterized the films prepared under various growth conditions by using scanning tunneling microscopy and spectroscopy (STM/STS), low-energy electron diffraction, and Auger electron spectroscopy. The good growth conditions of the Cr(001) films, which have atomically flat terraces without surface segregation of Au and with distinct surface states, are obtained by the following two methods. (I) $d_{\mathrm{Cr}}=3$ nm film realized by two-step growth, i.e., first 1.5 nm of Cr deposited at room temperature ($\sim 290$ K) and an additional 1.5 nm Cr deposition at 570 K and (II) $d_{\mathrm{Cr}}=1.5$ nm film by room temperature growth and subsequently post-annealing at 470 K. The magnetic imaging of (I) $d_{\mathrm{Cr}}=3$ nm Cr(001) film was performed by means of spin-polarized STM/STS. The observed magnetic image indicates that the topological antiferromagnetic (AF) order appears in a series of adjacent terraces and two types of spin frustration caused by screw dislocations lead to modifications of the topological AF order. One type of spin frustration is accompanied by narrow domain walls between two screw dislocations. The other type is a complex spin frustration caused by a cluster of an odd number of screw dislocations. The difference of the two types of spin frustration is explained through a micromagnetic simulation. We have also identified a large spin-frustrated area, consisting of a cluster of multiple number of screw dislocations.

Figure 1

(a) Typical layer structure of the sample employed in the experiment. Parallel aligned crystallographic axes in the surface plane depending on the epitaxial films are presented on the right side. (b) Large-scale ($150\times 150 {\mathrm{nm}}^2$) STM topographic image of a clean Au(001) surface after annealing at 570 K. Sample bias voltage ($V_{\mathrm{s}}$) and tunneling current ($I_{\mathrm{t}}$) were 0.3 V and 0.1 nA. Crystallographic direction of the fcc Au(001) surface is depicted in (b). (c) Higher resolution STM image ($30\times 30 {\mathrm{nm}}^2, V_{\mathrm{s}}=-1$ V, $I_{\mathrm{t}}=0.1$ nA) of the clean Au(001) surface.

Figure 2

(a)–(c) Typical LEED patterns of the Cr films obtained at 98.5 eV. (a) $d_{\mathrm{Cr}}=3$ nm (prepared using the two-step growth method: RT growth of half and growth of the other half at 570 K). (b) $d_{\mathrm{Cr}}=1.5$ nm (RT growth and subsequently PA at 470 K). (c) $d_{\mathrm{Cr}}=0.7$ nm (RT growth only).

Figure 3

(a) and (b) STM topographic images ($200\times 200 {\mathrm{nm}}^2$) of the Cr(001) films prepared by using the two-step growth method. (a) $d_{\mathrm{Cr}}=3$ nm ($V_{\mathrm{s}}=0.2$ V, $I_{\mathrm{t}}=0.2$ nA) and (b) $d_{\mathrm{Cr}}=2$ nm ($V_{\mathrm{s}}=0.8$ V, $I_{\mathrm{t}}=0.3$ nA). (c) Cross-sectional line profile along the horizontal white line in (b).

Figure 4

Auger-electron spectra on the Cr(001) films. (a) Sample prepared by using the two-step growth method: RT growth of half and growth of the other half at 570 K ($d_{\mathrm{Cr}}=3$ and 2 nm), and (b) prepared by RT growth and subsequently PA at 470 or 570 K ($d_{\mathrm{Cr}}=1.5$ nm).

Figure 5

STM images of the Cr(001) films ($d_{\mathrm{Cr}}=1.5$ nm) prepared by RT growth and PA at 570 or 470 K. The scan size is (a) and (b) $200\times 200 {\mathrm{nm}}^2$, and (c) $50\times 50 {\mathrm{nm}}^2$. (a) PA at 570 K ($V_{\mathrm{s}}=-0.2$ V, $I_{\mathrm{t}}=0.2$ nA), (b) and (c) PA at 470 K [(b) $V_{\mathrm{s}}=0.5$ V, $I_{\mathrm{t}}=0.2$ nA and (c) $V_{\mathrm{s}}=0.2$ V, $I_{\mathrm{t}}=0.5$ nA]. (d) Cross-sectional line profile along the horizontal white line in (c). 0.14 nm is the monolayer height of Cr(001).

Figure 6

STM images of the Cr(001) films ($d_{\mathrm{Cr}}=0.7$ nm) prepared by RT growth. The scan size is (a) $200\times 200 {\mathrm{nm}}^2$ and (b) $50\times 50 {\mathrm{nm}}^2$. (c) Cross-sectional line profile along the horizontal white line in (b). (d) Auger-electron spectrum on the Cr(001) film prepared by RT growth.

Figure 7

Averaged tunneling $dI/dV$ spectra of the Cr(001) film surfaces. (a) Sample prepared by using the two-step growth method: RT growth of half and growth of the other half at 570 K. ($d_{\mathrm{Cr}}=3$ and 2 nm), (b) prepared by RT growth and subsequently PA at 470 or 570 K ($d_{\mathrm{Cr}}=1.5$ nm), and (c) prepared by RT growth ($d_{\mathrm{Cr}}=0.7$ nm).

Figure 8

STM images ($235\times 200 {\mathrm{nm}}^2$) of (a) the topography and (b) $dI/dV$ map obtained simultaneously from the same area of two-step grown Cr(001) film surface ($d_{\mathrm{Cr}}=3$ nm) with the use of Fe coated W tip ($V_{\mathrm{s}}=-0.2$ V, $I_{\mathrm{t}}=0.2$ nA). Crystallographic direction of the bcc Cr(001) surface is shown in (a). Steps and screw dislocations are indicated in (b) for clarity purpose by the white lines and orange dots, respectively. Magnetization directions in the individual region are shown by the black and white arrows (representing opposite directions). The representative AF domain wall A and the noticeable spin-frustrated area B [designated B in Fig. 8] are indicated by green- and red-dotted triangular lines, respectively. (c) Cross-sectional line profile of height along the horizontal white line in (a). 0.144 nm is the monolayer height of Cr(001). (d) Cross-sectional line profile of the magnetic dI/dV signal along the full green line in the region A in Fig. 8. Red dashed curve shows a fit by Eq. (3)

Figure 9

The experimentally derived $dI/dV$ magnetic images around region A (a) ($140\times 110 {\mathrm{nm}}^2$) and region B (c) ($170\times 180$ ${\mathrm{nm}}^2$), where different types of spin frustrations are observed. The directions of the magnetization are represented by the black and white arrows in (a) and (c). Steps and screw dislocations are represented by white lines and orange dots, respectively in (a) and (c). The calculated magnetic structures of (b) and (d) correspond to (a) and (c), respectively. In (b) and (d) the red (blue) contrasts and arrows indicate the parallel (antiparallel) magnetization relative to the [100] direction, respectively. Screw dislocations are represented by yellow or light blue dots in (b) and (d). The white regions in (b) and (d) correspond to the spin frustration region.

Figure 10

STM images of the topography (a) and (c), and magnetic $dI/dV$ map (b) and (d) obtained simultaneously from the same area of Cr(001) film surfaces prepared by using the two-step growth method: (a) and (b) are for the $d_{\mathrm{Cr}}=3$ nm film ($300\times 175 {\mathrm{nm}}^2, V_{\mathrm{s}}=-0.2$ V, $I_{\mathrm{t}}=0.2$ nA), (c) and (d) are for the $d_{\mathrm{Cr}}=9$ nm film ($275\times 175 {\mathrm{nm}}^2, V_{\mathrm{s}}=-0.1$ V, $I_{\mathrm{t}}=0.1$ nA). Steps and screw dislocations are indicated in (b) and (d) for clarity purposes by the white lines and orange dots, respectively. Spin-frustrated area in (b) is indicated by red broken lines.