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 subs- trate, 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 compo- sition, 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 depend- ence 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 sub- strate 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 to a substrate can vary spatially and even cancel in a magnetically frustrated site. The first example of atomic- scale 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 micro- scope (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 manipu- lation 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) are shown in Figs. 1(a) and 1(b), revealing a ð ffiffi 3 p × ffiffi 3 p ÞR30° superstructure; together with additional experiments [16], we identify this as an in-plane magnet- ized 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 PRL 112, 076102 (2014) PHYSICAL REVIEW LETTERS week ending 21 FEBRUARY 2014 0031-9007=14=112(7)=076102(4) 076102-1 © 2014 American Physical Society