week ending 21 FEBRUARY 2014

PHYSICAL REVIEW LETTERS

PRL 112, 076102 (2014)

Enhanced Atomic-Scale Spin Contrast due to Spin Friction S. Ouazi,* A. Kubetzka, K. von Bergmann, and R. Wiesendanger Institute of Applied Physics, University of Hamburg, Jungiusstrasse 11, 20355 Hamburg, Germany (Received 17 December 2013; published 21 February 2014) Atom manipulation with the magnetic tip of a scanning tunneling microscope is a versatile technique to construct and investigate well-defined atomic spin arrangements. Here we explore the possibility of using a magnetic adatom as a local probe to image surface spin textures. As a model system we choose a Néel state with 120° between neighboring magnetic moments. Close to the threshold of manipulation, the adatom resides in the threefold, magnetically frustrated hollow sites, and consequently no magnetic signal is detected in manipulation images. At smaller tip-adatom distances, however, the adatom is moved towards the magnetically active bridge sites and due to the exchange force of the tip the manipulation process becomes spin dependent. In this way the adatom can be used as an amplifying probe for the surface spin texture. DOI: 10.1103/PhysRevLett.112.076102

PACS numbers: 68.35.Af, 68.37.Ef, 71.70.Gm, 81.16.Ta

A single adatom on a surface represents a unique geometry and, in conjunction with a scanning tunneling microscope (STM), it represents a model setup to study frictional phenomena on the atomic scale as well as transport properties through an ultimately narrow junction [1–6]. When the STM tip is close enough to an adatom to form a partial bond, it can induce a directed motion of such an adatom on a substrate [7,8]. With typical energy barriers for diffusion around one hundred meV, the lateral force needed to move an atom is on the order of one hundred pN, e.g., 200 pN to move a Co atom on a Pt(111) surface [9]. Atom manipulation with the tip can be performed during scanning, and the resulting manipulation images then reflect the motion of the manipulated adatom over the surface. The adatom acquires different positions relative to the substrate, exploring the potential landscape and being sensitive to, e.g., the stacking difference of hollow sites [8,10]. Performing such an experiment with a magnetic tip, a magnetic adatom, and a magnetic substrate, magnetic exchange forces can also play a role for the manipulation process leading to spin frictional phenomena [3]. Calculations show that the exchange energy between tip and sample depends sensitively on distance and tip composition, and can reach values of 300 meV, leading to exchange forces of 0.1–0.5 nN [11,12]. Experimentally, exchange energies of up to 50 meV were measured by magnetic exchange force microscopy [13], while the distance dependence was exploited to tune the exchange splitting of a two-impurity Kondo system [6]. For magnetic adatoms on ferromagnetic substrates, the exchange interaction in the relaxed position is on the order of several hundred meV [14]. For more complex spin states, one has to consider an effective exchange coupling between adatom and substrate that results from the sum over all neighbors which may have magnetic moments pointing in different directions; thus the effective magnetic coupling of a magnetic adatom 0031-9007=14=112(7)=076102(4)

to a substrate can vary spatially and even cancel in a magnetically frustrated site. The first example of atomicscale spin friction was demonstrated for the manipulation of a Co adatom over the spin spiral state of the Mn monolayer on W(110) [3]. For this system, the adatom adsorbs in hollow sites, leading to a magnetic coupling to the substrate of 145 meV [15], with alternating direction from site to site. Monte Carlo simulations revealed that the competition between the coupling to the magnetic tip and the substrate leads to significant variations of the adatom motion, a manifestation of spin friction. In this Letter, we show that spin friction can also play a role when the substrate cannot impose a magnetic direction on the adatom in its preferred adsorption position. We study a noncollinear state with magnetically frustrated hollow sites and our model system is the Néel-ordered magnetic state of the hexagonal Fe monolayer on Re(0001). Using a low-temperature spin-polarized scanning tunneling microscope (SP-STM), we perform measurements without and with a magnetic adatom trapped in the tunnel junction. A systematic variation of the tip-sample distance reveals a transition from a mode where the adatom jumps between hollow sites only, to a more complex manipulation mode at smaller distances. Correlated with the change in manipulation mode, a magnetic signal appears in the manipulation images at the positions of the magnetically active bridge sites. As the path of the adatom becomes spin dependent, we observe a magnetic corrugation of up to 35 pm, much larger than usual SP-STM signals. SP-STM images of the pseudomorphic Fe monolayer on Re(0001) pffiffiffi pffiffiffi are shown in Figs. 1(a) and 1(b), revealing a ð 3 × 3ÞR30° superstructure; together with additional experiments [16], we identify this as an in-plane magnetized Néel state [17–19], in agreement with predictions for this system by ab initio calculations [20]. In one case, we were able to obtain atomic resolution together with

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PHYSICAL REVIEW LETTERS

PRL 112, 076102 (2014)

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FIG. 1 (color online). Spin-resolved constant-current measurements of the Fe monolayer on Re(0001) revealing the magnetic superstructure with (a) typical and (b) exceptionally high magnetic corrugation (Measurement parameters: (a) R ¼ 4 mV=42 nA ¼ 95 kΩ, (b) R ¼ 2 mV=92 nA ¼ 22 kΩ). Model of the magnetic Néel state and simulated SP-STM measurement are presented below and to the right the corresponding one-dimensional FFT images are shown. (c) Line profiles as indicated in (a),(b).

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Controlled conditions for atom manipulation with a magnetic tip can be achieved by characterizing a selected area by SP-STM, see Fig. 2(a). Magnetic contrast with a corrugation of about 1.5 pm is visible. The same tip is then positioned above a Cr atom in the vicinity and the same area (see buried defect in the top left corner) is scanned again in manipulation mode, Fig. 2(b). Remarkably, the corrugation is of the order of 60 pm. For the chosen gap parameters, the magnetic contrast in the manipulation mode is very weak and the image is dominated by the atomic lattice periodicity. Note that these two images have been measured with similar gap parameters (gap resistance R, defined as the ratio of the bias voltage U to the tunnel current I), and the striking difference is entirely due to the presence of the Cr atom trapped by the tip potential.

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magnetic contrast, see Fig. 1(b). Atomic and magnetic unit cells are shown by yellow and red diamonds, respectively. This measurement reveals the strict commensurability of the noncollinear magnetic state to the atomic lattice, which is also evident from the one-dimensional fast Fourier transform (FFT) to the right of the images. A model of the Néel state as well as SP-STM simulations [21] are displayed below the experimental images. The parameters like tip height and its magnetization direction have been chosen to reproduce the measurements [16]. The line profiles in Fig. 1(c) show good agreement between the measurement and the simulation with an experimental magnetic corrugation of about 3 and 7 pm. Such small signals can be resolved only with an appropriate stability of the experimental setup.

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FIG. 2 (color online). (a) SP-STM in constant-current mode and (b) measurement with a Cr adatom manipulated over the same surface area for direct comparison (R ¼ 2 mV=69 nA ¼ 29 kΩ and R ¼ 2 mV=87 nA ¼ 23 kΩ, respectively). (c) Manipulation images in constant-current mode with a Cr adatom over Fe=Reð0001Þ for different gap resistances (R ¼ 4 mV=70 nA ¼ 57 kΩ; R ¼ 3 mV=70 nA ¼ 43 kΩ; R ¼ 2 mV=60 nA ¼ 33 kΩ; R ¼ 2 mV=70 nA ¼ 29 kΩ; R ¼ 2 mV=87 nA ¼ 23 kΩ). Δz gives the relative tip-sample distance variation. The right panels show the one-dimensional FFT of the topography. (d) Line profiles of the apparent height along the colored lines in (c).

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A magnetic signal during atom manipulation is not naturally expected, as the magnetic exchange interactions of the adatom to the substrate are frustrated when it is adsorbed in the hollow site. In search for conditions where a magnetic signal is detectable, we systematically vary the gap resistance from the threshold of manipulation down to almost contact between tip and adatom. This range corresponds to a variation of tip-sample distance of 60  5 pm. The resulting images are shown in Fig. 2(c) and illustrate the drastic role of R. To disentangle the atomic and magnetic contributions, we show their one-dimensional FFT to the right. The first two images, at 57 kΩ and 43 kΩ, show the atomic lattice with hexagonal cells in the real space image (indicated in yellow) and a line at the atomic periodicity in the one-dimensional FFTs to the right (yellow arrow). In the manipulation image, each hexagon is the manifestation of the Cr adatom in a preferred adsorption position and we can conclude that this is one of the two inequivalent hollow sites. Line profiles taken at different locations with respect to the hexagons are presented in Fig. 2(d) (green and orange curves), showing abrupt jumps spaced by the nearest-neighbor distance. As already mentioned, for our model system the effective magnetic coupling of an adatom in a hollow site to the three nearest neighbor substrate atoms cancels, meaning that the substrate cannot align the magnetic moment of the adatom, and consequently no magnetic contrast is obtained. Starting from 33 kΩ, the appearance of the manipulation images changes and the pattern becomes more complex. This suggests, that at these smaller tip-sample distances the adatom does not only reside in the preferred hollow sites anymore, but is forced to stay in closer vicinity to the tip, thereby exploring also other positions. In this regime, the line at the atomic periodicity in the one-dimensional FFT is interrupted. Additional spots appear at one third of the atomic periodicity (red arrow), i.e., the magnetic periodicpffiffiffi p ffiffiffi ity, and also in the real space images the ð 3 × 3ÞR30° magnetic superstructure is observed. Depending on position, the line profiles are dominated by the atomic or magnetic periodicity, see blue and red curves in Fig. 2(d). In both cases, the corrugations are larger than at smaller resistance and they almost reach the apparent adatom height of 60  15 pm. The occurrence of magnetic contrast is correlated with the more complex manipulation mode: whereas at the threshold of manipulation the images are dominated by the lattice periodicity, bringing the tip closer reveals the magnetic Néel ordered state. To understand the origin of magnetic signal generation, we need to identify the positions of the substrate Fe atoms in the manipulation images. Figure 3(a) shows again a constant-current manipulation image at R ¼ 29 kΩ [cf. Fig. 2(c)]. The simultaneously recorded spatially resolved current, i.e., the error signal, is presented in Fig. 3(b). Here, the different lattice sites can be attributed due to their symmetry: the center of the hexagonal areas

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(yellow hexagons) is the preferential, threefold hollow adsorption site of the Cr adatom. Enhanced current noise (red cross) indicates unstable positions of the adatom, i.e., the other hollow site [8]. Green dots at the intersection of the hexagons are the position of the Fe atoms. As seen in the constant-current image (a), several lines merge at the on-top position. To have access to all the information in one measurement channel, we also measure the manipulation image in constant-height mode, i.e., no feedback loop, and record the current, Fig. 3(c). The resulting image qualitatively resembles the constant-current image (a). In the onedimensional FFT to the right, we can identify the position from where the magnetic signal originates and bring it together with the positions of the underlying Fe atoms: when the tip moves on a line between the two inequivalent hollow sites (yellow line), the lattice periodicity dominates, whereas when the tip moves across the sample over top sites and connecting bridge sites (red line) we observe the magnetic periodicity. This means that at this tip-sample distance it is favorable for the adatom to move out of the frustrated hollow site towards an adjacent bridge site, where magnetic exchange coupling to the substrate does play a role, see diamonds in Fig. 2(c). We observe a magnetic signal for bridge sites in all three equivalent directions; however, with this tip the magnetic corrugation is largest for the horizontally connected bridge sites [16]. The modification of the potential landscape due to the competition of magnetic exchange interactions from adatom to substrate and tip is different compared to the previous work [3]. In contrast to

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structures. We have shown that this technique is applicable for a surface where the adsorption site is magnetically frustrated, proving that spin friction is generic to all complex spin textures. Beyond the aspect of magnetic signal amplification, spin-dependent atom manipulation is a tool to study adatom-surface and tip-adatom exchange forces, as these forces determine the details of the manipulation process.

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The authors thank B. Wolter and A. Schwarz for discussions and the DFG (SFB668) for financial support.

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Mn=Wð110Þ, where the depth of the adsorption potential for the magnetic adatom alternates, for Fe=Reð0001Þ the energy barrier to move the adatom to an adjacent hollow site via the bridge sites is modulated. To quantify the magnetic effect and its variation with tipsample distance, in Fig. 4 we plot the magnetic corrugation versus the gap resistance and compare it to the structural corrugation. While with decreasing R the latter keeps increasing up to 55 pm at the smallest gap resistance measured (23 kΩ), the magnetic corrugation is found to be nonmonotonic with a maximum of 35 pm at 29 kΩ. At this distance the interplay of magnetic exchange interactions between adatom and tip and adatom and sample modulate its motion over the substrate with magnetic periodicity, and the magnetic corrugation is as large as the structural corrugation. As the tunnel current depends exponentially on the distance, a movement of the adatom due to its spin can result in changes of the tunnel current that are much larger compared to typical contributions from a spin-polarized current. Spindependent atom manipulation is thus a means to enhance atomic-scale magnetic contrast. Surprisingly, for 23 kΩ we observe a significant reduction of the magnetic corrugation to about 10 pm, even though the manipulation mode does not change qualitatively (cf. Fig. 2(c)). A similar effect of vanishing magnetic contrast towards smaller tip-sample distances was observed in exchange force microscopy measurements on NiO(001) [22] and interpreted as a surface-induced change in the tip magnetization direction. An alternative explanation is a nonmonotonic distancedependent exchange energy as reported in [11,12]. In conclusion, magnetic adatoms can be used as a local probe to enhance the magnetic signal of atomic-scale spin

[email protected] [1] M. Ternes, C. Gonzalez, C. P. Lutz, P. Hapala, F. J. Giessibl, P. Jelinek, and A. J. Heinrich, Phys. Rev. Lett. 106, 016802 (2011). [2] K.-F. Braun and S.-W. Hla, Phys. Rev. B 75, 033406 (2007). [3] B. Wolter, Y. Yoshida, A. Kubetzka, S.-W. Hla, K. von Bergmann, and R. Wiesendanger, Phys. Rev. Lett. 109, 116102 (2012). [4] L. Limot, J. Kröger, R. Berndt, A. Garcia-Lekue, and W. A. Hofer, Phys. Rev. Lett. 94, 126102 (2005). [5] M. Ziegler, N. Nïel, C. Lazo, P. Ferriani, S. Heinze, J. Kröger, and R. Berndt, New J. Phys. 13, 085011 (2011). [6] J. Bork, Y.-h. Zhang, L. Diekhöner, L. Borda, P. Simon, J. Kroha, P. Wahl, and K. Kern, Nat. Phys. 7, 901 (2011). [7] D. M. Eigler and E. K. Schweizer, Nature (London) 344, 524 (1990). [8] J. A. Stroscio and R. J. Celotta, Science 306, 242 (2004). [9] M. Ternes, C. P. Lutz, C. F. Hirjibehedin, F. J. Giessibl, and A. J. Heinrich, Science 319, 1066 (2008). [10] Y.-h. Zhang, P. Wahl, and K. Kern, Nano Lett. 11, 3838 (2011). [11] C. Lazo, V. Caciuc, H. Hölscher, and S. Heinze, Phys. Rev. B 78, 214416 (2008). [12] C. Lazo and S. Heinze, Phys. Rev. B 84, 144428 (2011). [13] R. Schmidt, C. Lazo, U. Kaiser, A. Schwarz, S. Heinze, and R. Wiesendanger, Phys. Rev. Lett. 106, 257202 (2011). [14] Y. Yayon, V. W. Brar, L. Senapati, S. C. Erwin, and M. F. Crommie, Phys. Rev. Lett. 99, 067202 (2007). [15] D. Serrate, P. Ferriani, Y. Yoshida, S.-W. Hla, M. Menzel, K. von Bergmann, S. Heinze, A. Kubetzka, and R. Wiesendanger, Nat. Nanotechnol. 5, 350 (2010). [16] See Supplemental Material at http://link.aps.org/ supplemental/10.1103/PhysRevLett.112.076102 for additional information. [17] D. Wortmann, S. Heinze, P. Kurz, G. Bihlmayer, and S. Blügel, Phys. Rev. Lett. 86, 4132 (2001). [18] C. L. Gao, W. Wulfhekel, and J. Kirschner, Phys. Rev. Lett. 101, 267205 (2008). [19] M. Wasniowska, S. Schröder, P. Ferriani, and S. Heinze, Phys. Rev. B 82, 012402 (2010). [20] B. Hardrat, A. Al-Zubi, P. Ferriani, S. Blügel, G. Bihlmayer, and S. Heinze, Phys. Rev. B 79, 094411 (2009). [21] S. Heinze, Appl. Phys. A 85, 407 (2006). [22] F. Pielmeier and F. J. Giessibl, Phys. Rev. Lett. 110, 266101 (2013).

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Enhanced atomic-scale spin contrast due to spin friction.

Atom manipulation with the magnetic tip of a scanning tunneling microscope is a versatile technique to construct and investigate well-defined atomic s...
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