Magnetic doping of 4d transition-metal surfaces: A first-principles study S. Dennler, 1,2, * J. Hafner, 1 M. Marsman, 1 and J. Morillo 2 1 Institut für Materialphysik and Center for Computational Materials Science, Universität Wien, Sensengasse 8, A-1090 Wien, Austria 2 Centre d’Elaboration de Matériaux et d’Etudes Structurales, CNRS UPR 8011, 29, rue Jeanne Marvig, F-31400 Toulouse, France Received 13 July 2004; revised manuscript received 30 November 2004; published 31 March 2005 The doping of 4d transition-metal surfaces by magnetic 3d impurities is studied on the example of Co/Rh111 in the framework of the density functional theory, in its generalized gradient approximation and within the projector-augmented plane-wave method. Full Co monolayers buried close to a free surface in a Rh matrix as well as surface alloys for varying Co concentrations are considered. The magnetic impurities are shown to essentially maintain a bulk-enhanced magnetic moment, even at small concentration. They induce a long-range oscillatory decaying polarization of the Rh adjacent layers whose amplitude is significantly en- hanced at the surface. This behavior leads to a complex magnetic pattern of the surface alloys. Co is prefer- entially accommodated in the subsurface layer and induces substantial magnetic moments on the Rh surface atoms which increase linearly with the local Co coordination and leads to giant effective moments of more than 4 B for low Co coverage. For less stable Co positions, antiparallel induced Rh moments may reduce the total effective moment. Our findings are discussed within the framework of the band theory of magnetism. DOI: 10.1103/PhysRevB.71.094433 PACS numbers: 75.75.+a, 75.70.Rf, 73.22.-f I. INTRODUCTION The magnetic properties of nanostructured materials ul- trathin films and monolayers, nanowires and nanostripes, supported or matrix-isolated clusters have attracted an in- tense research effort for decades. The interest is triggered by their unique properties such as enhanced magnetic moments, modified magnetic anisotropy arising from surface and inter- face phenomena, etc. There is a fascinating interplay be- tween the reduced dimensionality, the geometric structure which is often influenced by the underlying substrate or surrounding matrix, surface effects, electronic hybridization at the interface, and the resulting magnetic properties. The understanding of how the magnetism is governed by the geo- metric structure, the chemical composition, and the elec- tronic structure of the nanostructured ensemble may help to control—or even to design—the magnetic properties of nanostructures, which is of evident importance for advances in magnetic storage technologies and other magnetoelec- tronic applications. 1 On magnetically inert substrates such as noble metals, the reduced dimensionality and reduced coordination lead to strongly enhanced spin and orbital magnetic moments— nanostructures of Co on Cu or Pt substrates Refs. 2–5 or of Fe on Cu Refs. 6–11 are extensively studied model sys- tems. Ultrathin films of Fe grown on Cu100 substrates in particular are an illustrative example of novel structural and magnetic properties imposed by the epitaxial relation be- tween film and substrate. 8,9 Nanowires of Fe grown on vici- nal Cu surfaces 10 illustrate another interesting facet: the mag- netic coupling between nanowires through the conduction electrons of the substrate, although much weaker than the exchange interaction along the wire, leads to a violation of the one-dimensional character of the magnetic wires. As far as the magnetic ordering transition is concerned, Fe/ Cu11n nanowires behave like a strongly anisotropic XY system. For magnetic nanostructures grown on transition-metal substrates, a much stronger adsorbate-substrate interaction has to be expected. The character of this interaction depends strongly on the position of the nonmagnetic substrate ele- ment in the transition-metal series. On a substrate with a less than half-filled d band, the large difference in the surface energies of the substrate and of the adsorbate will stabilize layer-by-layer growth, but the strong hybridization at the in- terface will also strongly affect the magnetism of the ad- sorbed nanostructures. For nanostripes and extended overlay- ers of Fe-Co alloys on flat and stepped W110 surfaces, the complete disappearance of ferromagnetism in Co-rich sys- tems has been reported. 12,13 Recent ab initio calculations 14 have demonstrated that the hybridization of the d bands at the interfaces induces a strong quenching of the magnetic moments and exchange interactions in extended overlayers and may even lead to antiferromagnetic ordering in Fe- Co nanowires. On transition-metal substrates with a filled or nearly completely filled d band, the trend is reversed: the difference in the surface energies now favors a surfactant behavior of the substrate atoms, 2,15 and the strong magnetic polarizabilities of the late transition metals lead to the forma- tion of large induced magnetic moments. For the Fe/ Pd and Fe/ Pt system a “giant” enhancement of the magnetic mo- ment of impurities 16–18 and of embedded Fe monolayers and artificial layered FePt and FePd compounds with an L1 0 structure has been reported. 19–21 Fe atoms polarize the sur- rounding matrix of Pd or Pt atoms. The polarization is ferro- magnetic; it decays almost exponentially at short distances and is oscillatory at larger distances. An isolated Fe impurity carries a slightly enhanced moment of about 3 B , but the complex consisting of the impurity plus the ferromagnetic polarization cloud is recognized as a giant magnetic moment of about 10 B . For Pd n /Fe/Pd n trilayers a strong magnetic polarization of the Pd layers adjacent to the Fe monolayer has been predicted using density-functional theory DFT. 19 The magnetization profile of Pt atoms near a buried Co/ Pt PHYSICAL REVIEW B 71, 094433 2005 1098-0121/2005/719/09443311/$23.00 ©2005 The American Physical Society 094433-1