This title appears in the Scientific Report : 2001 

Non-collinear magnetism at surfaces and in ultrathin films
Kurz, Philipp (Corresponding author)
Elektronische Eigenschaften; IFF-IEE
Jülich Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag 2001
IV, 207 p.
Aachen, Techn. Hoch., Diss., 2000
Dissertation / PhD Thesis
Elektronische Struktur von Festkörpern, Oberflächen und Schichtsystemen
Berichte des Forschungszentrums Jülich 3832
A full-potential linearized augmented plane-wave (FLAPW) electronic structure method was developed to investigate non-collinear magnetism in bulk systems, surfaces, and thin films on the basis of the vector spin-density formulation of the local density approximation (LDA) and the generalized gradient approximation (GGA) to the density functional theory (DFT). To allow the investigation of a large set of relevant magnetic spin-structures, two extensions that go beyond the treatment of periodic and stationary magnetic states were implemented: (i) Arbitrary non-collinear periodic magnetic configurations, which are not the magnetic ground state or a stationary state of the system under consideration, can be treated due to the extension of the density functional equations to constrain the local magnetic moments to any given direction. (ii) Commensurate and incommensurate spiral (or helical) spindensity waves can be treated due the extension of the vector spin-density FLAPW method on the basis of a generalized Bloch theorem. A detailed account of the implementation is given and the importance of various approximations used are discussed. This method was applied to the problem of topological frustration of a two-dimensional antiferromagnet on a triangular lattice. We performed self-consistent calculations for the 3d transition-metal monolayers V, Cr, Mn, and Fe on the (111) oriented surfaces of Cu and Ag, investigating the magnetism, the interlayer relaxation, and the energetics of a nearly complete set of magnetic states. We found an amazing variety of different magnetic ground states: ferromagnetism for Fe/Cu(111) and Fe/Ag(111); row-wise antiferromagnetism for Mn/Ag(111); a coplanar non-collinear periodic 120° Néel structure for V/Ag(111), Cr/Cu(111) and Cr/Ag(111) ; and for Mn/Cu(111) a new complex three-dimensional non-collinear spin structure, a socalled 3Q state, shown on the next page. By comparison with model Hamiltonians we conclude that any realistic description of two-dimensional itinerant antiferromagnets on a triangular lattice requires exchange interactions beyond the nearest neighbors and also exchange interactions beyond the Heisenberg model (i.e. 4-spin and biquadratic interactions). Bulk and surface calculations for hcp Gd and the Gd(0001) surface were performed. Comparing different methods to treat the localized 4f states, which represent a challenge for first-principle theory, we show that it is crucial to remove the unphysical density of states due to the minority 4f electrons at the Fermi energy obtained in both LDA and GGA, in order to predict the magnetic ground state correctly. We carried out spin-spiral calculations to model the effect of magnetic excitations, i.e. temperature, on the electronic structure of the Gd(0001) surface. In the ferromagnetic ground state we found a double peak structure in the local density of states, due to the spin-split d$_{z^{2}}$ surface state of Gd, which is probed by scanning tunneling spectroscopy (STS) experiments. With increasing spin-spiral q-vector, corresponding to increasing temperature, the splitting of the two peaks decreases and finally vanishes, while the valence magnetic moment remains finite. Hence, the vanishing splitting cannot be taken as support for the applicability of a pure Stoner model.