Spin Friction Observed on the Atomic Scale Boris Wolter, 1 Yasuo Yoshida, 1, * Andre ´ Kubetzka, 1 Saw-Wai Hla, 2 Kirsten von Bergmann, 1 and Roland Wiesendanger 1,† 1 Institute of Applied Physics, University of Hamburg, Jungiusstrasse 11, D-20355 Hamburg, Germany 2 Nanoscale & Quantum Phenomena Institute, Physics & Astronomy Department, Ohio University, Athens, Ohio 45701, USA (Received 16 May 2012; published 10 September 2012) With the advent of scanning probe microscopy techniques that involve a tip and a sample in relative motion in the contact or noncontact regime, the microscopic aspects of friction have become a major branch of research called nanotribology. A significant number of recent studies in this field have concentrated on the distinction between electronic and phononic contributions to friction. Here, we are using the combination of spin-polarized scanning tunneling microscopy and single-atom manipulation in order to move individual magnetic atoms over a magnetic template. By monitoring the spin-resolved manipulation traces and comparing them with results of Monte Carlo simulations, we are able to reveal the characteristic friction force variations resulting from the occurrence of spin friction on the atomic scale. DOI: 10.1103/PhysRevLett.109.116102 PACS numbers: 68.35.Af, 05.10.Ln, 71.70.Gm, 81.16.Ta Friction is one of the oldest phenomena known and utilized by mankind. Frictional heating was already used by our prehistorical ancestors more than 400 000 years ago for the lighting of fires. While the force of friction had already been recognized and studied by Aristotle, it was Leonardo da Vinci who performed the first quantitative studies of interacting surfaces in relative motion, thereby pioneering the field of tribology [1]. Related phenomena such as triboluminescence, i.e., the emission of light when a material is stressed to the point of fracture [2], or tribo- chemistry, i.e., the acceleration of reaction rates by friction [3], have also been known since the 16th century. The first two laws of (macroscopic) friction, generally known as Amontons’ laws, governed the field of tribology for a long time. Recently, advanced experimental methods, such as the quartz-crystal microbalance, the surface forces appa- ratus, or friction force microscopy, have allowed for deeper insight into the microscopic origin of frictional phenomena [4]. For instance, by making use of the transition between normal and superconducting states, it has been demon- strated that electronic friction is the main dissipative chan- nel in the metallic state, while phononic friction dominates in the superconducting state [5–8]. Since electronic de- grees of freedom govern frictional phenomena for normal metals, it is natural to consider a possible spin-dependence of frictional forces in the case of magnetic materials [9,10]. Because of the fact that the experiments used previously to measure friction-related forces on the atomic scale [11] cannot resolve spin structures, the experimental method of choice is then spin-polarized scanning tunneling micros- copy (SP-STM) [12,13] involving a magnetic tip and a magnetic sample. By operating under extreme conditions, i.e., tiny tunnel gap resistances [14], atomic manipulation experiments have recently become feasible while still being sensitive to the spin degree of freedom [15–17]. Here, we present a combined experimental and theoretical study of frictional phenomena occurring when a single magnetic atom is moved over a magnetic surface by means of an SP-STM tip, thereby revealing the importance of the spin degree of freedom in frictional phenomena. Our experiments were carried out under ultrahigh vac- uum conditions at a temperature of 8 K, using a home-built SP-STM setup [18]. As substrate we chose a Mn mono- layer on W(110) [19], which exhibits a spin spiral with roughly 170  between adjacent atomic rows as visualized in Fig. 1(a). Figure 1(b) shows a spin-resolved constant- current STM image of this substrate obtained with a mag- (c) (d) (a) 0 150 pm z-scale × ×5 1 nm [110] [001] - (b) FIG. 1 (color). Magnetic template and STM measurements. (a) Visualization of the Mn=Wð110Þ spin spiral. (b) SP-STM image of Mn=Wð110Þ. (c) and (d) Manipulation images of a Co adatom moved across the substrate (c) exhibiting the symmetry of the atomic lattice and (d) reflecting the magnetic order of the underlying spin spiral, obtained with a spin-polarized tip. (measurement parameters: (b) I ¼ 80 nA, U ¼8 mV, (c) I ¼ 50 nA, U ¼5 mV, (d) I ¼ 80 nA, U ¼5 mV). PRL 109, 116102 (2012) Selected for a Viewpoint in Physics PHYSICAL REVIEW LETTERS week ending 14 SEPTEMBER 2012 0031-9007= 12=109(11)=116102(5) 116102-1 Ó 2012 American Physical Society