Effect of exchange interaction on the spin-polarized bound states on metal surfaces: Ab initio study O. O. Brovko, 1 V. S. Stepanyuk, 1 and P. Bruno 1,2 1 Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, D06120 Halle, Germany 2 European Synchrotron Radiation Facility, BP 220, F38043 Grenoble Cedex, France Received 3 July 2008; revised manuscript received 5 September 2008; published 15 October 2008 The exchange interaction between magnetic adatoms on a metal surface is known to significantly influence the magnetic characteristics of the system. In present study we analyze the effect of the exchange interaction on the spin-dependent localization of the surface state. Our ab initio study of spin-polarized bound states arising at pairs of magnetic adatoms shows that both the position and the shape of the bound-state peak are strongly affected by the spin coupling in the system. Moreover we demonstrate that the spin splitting of the bound-state peak can serve as a suitable tool for probing the exchange coupling in the system. By the example of Co pairs on a Cu111 surface at different interatomic separations, we demonstrate that the presence of the spin splitting of the bound-state peak is a signature of a ferromagnetic coupling while its absence signifies an antiferromagnetic configuration of the system spins. Also, the amount of splitting can be regarded as a measure of the interaction’s strength. DOI: 10.1103/PhysRevB.78.165413 PACS numbers: 73.20.At, 73.20.Fz, 72.10.Fk, 71.15.Mb Recent advances in experimental techniques concerning fabrication and manipulation of nanostructures brought the designers of spin-based devices very close to the singe atomic limit. Fortunately, modern preparation and analysis techniques, such as the scanning tunneling microscopy STM, 1,2 allow for simultaneous manipulation of adsorbed nanostructures with atomic precision and measurement of their structural and electronic properties. This has greatly shifted the focus of theoretical and experimental studies to the subject of electronic and magnetic properties of atomic- scale systems such as adatom or cluster groups on surfaces. One of the key roles in these systems is played by magnetic interaction between constituent parts. The origin of interac- tions involved can be very different: direct interaction due to orbitals overlapping, direct magnetic coupling, or indirect coupling mediated by substrate or host, e.g., Ruderman- Kittel-Kasuya-Yosida RKKY. Thus, it is vital for spintron- ics to be able to tune these interactions. This realization brought about the appearance of several methods of control- ling the exchange interaction in subnanoscale systems utiliz- ing precise geometry control 3 and quantum confinement of system’s electrons within artificially assembled nanostructures. 4 However, the task of directly measuring the magnetic in- teraction between single adatoms remains rather poorly ex- plored. Until recently, a direct measurement of the magnetic interaction between individual atoms has been impossible. First steps in this field were made by Hirjibehedin et al. 5 as they managed to experimentally probe the spin-exchange in- teraction in linear manganese chains through spin-flip experi- ments. Another method, based on the analysis of the Kondo resonance, was proposed by Chen et al. 6 for cobalt dimers on a Au111 surface, and successfully extended and improved by Wahl et al. 7 for Co on Cu111. Moreover only recently a state of the art experiment by Meier et al. 8 allowed direct mapping of the exchange coupling of single adatoms to their magnetization curves. The first two methods utilize the fact that the exchange interaction has its roots in the spin- dependent scattering of conduction electrons of the system by interacting impurities. Thus it is logical to assume that other scattering-related phenomena might as well be affected by the exchange interaction of impurities on the surface. It is well known that 111 surfaces of some noble metals produce an electronic surface state, which in its properties resemble a free two-dimensional electron gas 2DEG. The origin of the surface state is the trapping of electrons between the vacuum barrier and the band gap of the metal bulk. An impurity immersed in such a 2DEG presents an additional potential for the surface-state electrons. The Simon theorem predicts that any two-dimensional 2D attractive potential should have a bound state. 9 Truly enough, it has been shown 10–12 that the localization of a 2D Shockley surface state on an impurity potential manifests itself as a split-off bound state just below the surface-state band bottom. Similar states have been observed at nonmagnetic Cu chains. 13 In present paper we study the interaction of surface-state electrons with Co adatoms adsorbed on a Cu111 surface. We resort to first-principles calculations to prove that the bound state is strongly affected by the spin coupling in the system and that the spin splitting of the bound state can be utilized to probe the exchange coupling between single mag- netic adatoms. Our calculations for Co pairs on a Cu111 surface at various separations reveal that the ferromagnetic FM alignment of adatom spins results in strong spin split- ting of the bound-state peak while an antiferromagnetic AFM alignment causes the bound-state peak to become spin degenerate. Our ab initio calculations were done in the framework of the density-functional theory in local spin-density approxi- mation. The Korringa-Kohn-Rostoker KKR Green’s- function method in atomic spheres approximation 14,15 was used to obtain the solution of Kohn-Sham equations in terms of Green’s functions. The KKR approach exploits the prop- erties of the Green’s function of the Kohn-Sham operator, in particular the fact that the electronic density can be ex- pressed through the imaginary part of the energy-dependent PHYSICAL REVIEW B 78, 165413 2008 1098-0121/2008/7816/1654135 ©2008 The American Physical Society 165413-1 TH-2008-21