56 research outputs found
Magnetoelastic mechanism of spin-reorientation transitions at step-edges
The symmetry-induced magnetic anisotropy due to monoatomic steps at strained
Ni films is determined using results of first - principles relativistic
full-potential linearized augmented plane wave (FLAPW) calculations and an
analogy with the N\'eel model. We show that there is a magnetoelastic
anisotropy contribution to the uniaxial magnetic anisotropy energy in the
vicinal plane of a stepped surface. In addition to the known spin-direction
reorientation transition at a flat Ni/Cu(001) surface, we propose a
spin-direction reorientation transition in the vicinal plane for a stepped
Ni/Cu surface due to the magnetoelastic anisotropy. We show that with an
increase of Ni film thickness, the magnetization in the vicinal plane turns
perpendicular to the step edge at a critical thickness calculated to be in the
range of 16-24 Ni layers for the Ni/Cu(1,1,13) stepped surface.Comment: Accepted for publication in Phys. Rev.
Quantum Monte Carlo simulation of thin magnetic films
The stochastic series expansion quantum Monte Carlo method is used to study
thin ferromagnetic films, described by a Heisenberg model including local
anisotropies. The magnetization curve is calculated, and the results compared
to Schwinger boson and many-body Green's function calculations. A transverse
field is introduced in order to study the reorientation effect, in which the
magnetization changes from out-of-plane to in-plane. Since the approximate
theoretical approaches above differ significantly from each other, and the
Monte Carlo method is free of systematic errors, the calculation provides an
unbiased check of the approximate treatments. By studying quantum spin models
with local anisotropies, varying spin size, and a transverse field, we also
demonstrate the general applicability of the recent cluster-loop formulation of
the stochastic series expansion quantum Monte Carlo method.Comment: 9 pages, 12 figure
Step-induced unusual magnetic properties of ultrathin Co/Cu films: ab initio study
We have performed ab initio studies to elucidate the unusual magnetic
behavior recently observed in epitaxial Co films upon absorption of
submonolayers of Cu and other materials. We find that a submonolayer amount of
Cu on a stepped Co/Cu (100) film changes dramatically the electronic and
magnetic structure of the system. The effect is mainly due to hybridization of
Co and Cu -electrons when copper forms a ``wire'' next to a Co step at the
surface. As a result, a non-collinear arrangement of magnetic moments
(switching of the easy axis) is promoted. [PACS 75.70.Ak,75.70.-i]Comment: 10 pages, RevTeX 3.0, 4 PostScript figures available on request from
A. Bratkovsky at [email protected]
Sodium atoms and clusters on graphite: a density functional study
Sodium atoms and clusters (N<5) on graphite (0001) are studied using density
functional theory, pseudopotentials and periodic boundary conditions. A single
Na atom is observed to bind at a hollow site 2.45 A above the surface with an
adsorption energy of 0.51 eV. The small diffusion barrier of 0.06 eV indicates
a flat potential energy surface. Increased Na coverage results in a weak
adsorbate-substrate interaction, which is evident in the larger separation from
the surface in the cases of Na_3, Na_4, Na_5, and the (2x2) Na overlayer. The
binding is weak for Na_2, which has a full valence electron shell. The presence
of substrate modifies the structures of Na_3, Na_4, and Na_5 significantly, and
both Na_4 and Na_5 are distorted from planarity. The calculated formation
energies suggest that clustering of atoms is energetically favorable, and that
the open shell clusters (e.g. Na_3 and Na_5) can be more abundant on graphite
than in the gas phase. Analysis of the lateral charge density distributions of
Na and Na_3 shows a charge transfer of about 0.5 electrons in both cases.Comment: 20 pages, 6 figure
Ferromagnetism and Temperature-Driven Reorientation Transition in Thin Itinerant-Electron Films
The temperature-driven reorientation transition which, up to now, has been
studied by use of Heisenberg-type models only, is investigated within an
itinerant-electron model. We consider the Hubbard model for a thin fcc(100)
film together with the dipole interaction and a layer-dependent anisotropy
field. The isotropic part of the model is treated by use of a generalization of
the spectral-density approach to the film geometry. The magnetic properties of
the film are investigated as a function of temperature and film thickness and
are analyzed in detail with help of the spin- and layer-dependent quasiparticle
density of states. By calculating the temperature dependence of the
second-order anisotropy constants we find that both types of reorientation
transitions, from out-of-plane to in-plane (``Fe-type'') and from in-plane to
out-of-plane (``Ni-type'') magnetization are possible within our model. In the
latter case the inclusion of a positive volume anisotropy is vital. The
reorientation transition is mediated by a strong reduction of the surface
magnetization with respect to the inner layers as a function of temperature and
is found to depend significantly on the total band occupation.Comment: 10 pages, 8 figures included (eps), Phys Rev B in pres
Fabrication of one-dimensional Ag/multiwalled carbon nanotube nano-composite
Composite made of multiwalled carbon nanotubes coated with silver was fabricated by an electroless deposition process. The thickness of silver layer is about 40 to 60 nm, characterized as nano-crystalline with (111) crystal orientation along the nanotube's axial direction. The characterization of silver/carbon nanotube [Ag/CNT] nanowire has shown the large current carrying capability, and the electric conductivity is similar to the pure silver nanowires that Ag/CNT would be promising as building blocks for integrated circuits
Systematic theoretical study of the spin and orbital magnetic moments of 4d and 5d interfaces with Fe films
Results of ab initio calculations using the relativistic Local Spin Density
theory are presented for the magnetic moments of periodic 5d and 4d transition
metal interfaces with bcc Fe(001). In this systematic study we calculated the
layer-resolved spin and orbital magnetic moments over the entire series. For
the Fe/W(001) system, the Fe spin moment is reduced whilst its orbital moment
is strongly enhanced. In the W layers a spin moment is induced, which is
antiparallel to that of Fe in the first and fourth W layers but parallel to Fe
in the second and third W layers. The W orbital moment does not follow the spin
moment. It is aligned antiparallel to Fe in the first two W layers and changes
sign in the third and fourth W layers. Therefore, Hund's third rule is violated
in the first and third W layers, but not in the second and fourth W layers. The
trend in the spin and orbital moments over the 4d and 5d series for multilayers
is quite similar to previous impurity calculations. These observations strongly
suggest that these effects can be seen as a consequence of the hybridization
between 5d (4d) and Fe which is mostly due to band filling, and to a lesser
extent geometrical effects of either single impurity or interface
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