Using first-principles calculations, we demonstrate that an Fe monolayer can assume very different magnetic phases on hcp ͑0001͒ and fcc ͑111͒ surfaces of 4d-and 5d-transition metals. Due to the substrates' d-band filling, the nearest-neighbor exchange coupling of Fe changes gradually from antiferromagnetic ͑AFM͒ for Fe films on Tc, Re, Ru, and Os to ferromagnetic on Rh, Ir, Pd, and Pt. In combination with the topological frustration on the triangular lattice of these surfaces the AFM coupling results in a 120°Néel structure for Fe on Re and Ru and an unexpected double-row-wise AFM structure on Rh, which is a superposition of left-and right-rotating 90°spin spirals. DOI: 10.1103/PhysRevB.79.094411 PACS number͑s͒: 75.70.Ak, 71.15.Mb Triggered by the discovery of the giant-magnetoresistance effect and the demand to realize spintronic device concepts, 1 magnetic nanostructures on surfaces have been a focus of experimental and theoretical research for more than 20 years now. In particular, there has been a tremendous effort to grow ultrathin transition-metal films on metal surfaces and to characterize and explain their magnetic properties. It is now generally believed that these structurally simple systems are well understood and more complex nanostructures such as atomic chains, clusters, or molecules on surfaces have moved into the spotlight of today's research. [2][3][4][5][6][7] Therefore, it came as a big surprise when it was experimentally shown that the prototypical ferromagnet Fe becomes a two-dimensional ͑2D͒ antiferromagnet on the W͑001͒ surface. 8 Combining spin-polarized scanning tunneling microscopy and first-principles calculations, it has been further demonstrated that complex magnetic order can be obtained even in single monolayer ͑ML͒ magnetic films on nonmagnetic substrates. For example, recently a spin-spiral state was discovered for a Mn ML on W͑110͒ ͑Ref. 9͒ and for a Mn ML on W͑001͒ ͑Ref. 10͒ and a nanoscale magnetic structure was found for an Fe ML on Ir͑111͒. 11 Surfaces of 4d-and 5d-transition metals ͑TMs͒ such as W, Re, Ru, or Ir have been particularly attractive from an experimental point of view as ultrathin 3d-TM films can often be grown pseudomorphically and without intermixing. 12-16 However, there has been controversy in the past about reports concerning dead magnetic layers and absence of magnetic order in ultrathin films on these surfaces. 14,15 The fundamental key to many unresolved puzzles may be the itinerant character of TMs resulting in competing exchange interactions beyond nearest neighbors and higher-order spin interactions beyond the Heisenberg model. The latter interactions have been proposed to play a role in transition metals; however, to our knowledge, no unambiguous proof of their importance has been given.Here, we use first-principles calculations to demonstrate that a hexagonal Fe ML can assume very different magnetic phases on a triangular lattice provided by hcp ͑0001͒ and fcc ͑111͒ surfaces of 4d-and 5d-transition metals, which are also experimentally accessib...

We present an implementation of the ballistic Landauer-Büttiker transport scheme in one-dimensional systems based on density functional theory calculations within the full-potential linearized augmented plane-wave (FLAPW) method. In order to calculate the conductance within the Green's function method, we map the electronic structure from the extended states of the FLAPW calculation to Wannier functions, which constitute a minimal localized basis set. Our approach benefits from the high accuracy of the underlying FLAPW calculations, allowing us to address the complex interplay of structure, magnetism, and spin-orbit coupling and is ideally suited to study spin-dependent electronic transport in one-dimensional magnetic nanostructures. To illustrate our approach, we study ballistic electron transport in nonmagnetic Pt monowires with a single stretched bond including spin-orbit coupling, and in ferromagnetic Co monowires with different collinear magnetic alignment of the electrodes with the purpose of analyzing the magnetoresistance when going from tunneling to the contact regime. We further investigate spin-orbit scattering due to an impurity atom. We consider two configurations: a Co atom in a Pt monowire and vice versa. In both cases, the spin-orbit induced band mixing leads to a change of the conductance upon switching the magnetization direction from along the chain axis to perpendicular to it. The main contribution stems from ballistic spin scattering for the magnetic Co impurity in the nonmagnetic Pt monowire, and for the Pt scatterer in the magnetic Co monowire from the band formed from states with d xy and d x 2 −y 2 orbital symmetry. We quantify this effect by calculating the ballistic anisotropic magnetoresistance, which displays values up to as much as 7% for ballistic spin scattering and gigantic values of around 100% for the Pt impurity in the Co wire. In addition, we show that the presence of a scatterer can reduce as well as increase the ballistic anisotropic magnetoresistance.

We present a first-principles computational scheme for investigating the ballistic transport properties of onedimensional nanostructures with noncollinear magnetic order. The electronic structure is obtained within density functional theory as implemented in the full-potential linearized augmented plane-wave method and mapped to a tight-binding-like transport Hamiltonian via noncollinear Wannier functions. The conductance is then computed based on the Landauer formula using the Green's function method. As a first application, we study the conductance between two ferromagnetic Co monowires terminated by single Mn apex atoms as a function of Mn-Mn separation. We vary the Mn-Mn separation from the contact (about 2.5 to 5Å) to the far tunneling regime (5 to 10Å). The magnetization direction of the Co electrodes is chosen either in parallel or antiparallel alignment and we allow for different spin configurations of the two Mn spins. In the tunneling and into the contact regime, the conductance is dominated by s-d z 2 states. In the close contact regime (below 3.5Å), there is an additional contribution for a parallel magnetization alignment from the d xz and d yz states which give rise to an increase of the magnetoresistance as it is absent for antiparallel magnetization. If we allow the Mn spins to relax, a noncollinear spin state is formed close to contact due to the competition of ferromagnetic coupling between Mn and Co and antiferromagnetic coupling between the Mn spins. We demonstrate that the transition from a collinear to such a noncollinear spin structure as the two Mn atoms approach leaves a characteristic dip in the distance-dependent conductance and magnetoresistance of the junction. We explain this modification of the spin-valve effect due to the noncollinear spin state based on the spin-dependent hybridization between the d xz,yz states of the Mn spins and their coupling to the Co electrodes.

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