Verlag des Forschungszentrums Jülich
JUEL-3832
Kurz, Philipp
Non-Collinear Magnetism at Surfaces and in Ultrathin Films
IV, 207 S., 2000
A full-potentiallinearized 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) spin
density 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(lll) and
Fe/ Ag(lll); row-wise antiferromagnetism for Mn/ Ag(lll); a coplanar non-collinear
periodic 120° Neel structure for V / Ag(lll), Cr/Cu(lll) and Cr/ Ag(lll); and for
Mn/Cu(lll) 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 dz2 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.
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