44 research outputs found
Mapping the magnetic exchange interactions from first principles: Anisotropy anomaly and application to Fe, Ni, and Co
Mapping the magnetic exchange interactions from model Hamiltonian to density
functional theory is a crucial step in multi-scale modeling calculations.
Considering the usual magnetic force theorem but with arbitrary rotational
angles of the spin moments, a spurious anisotropy in the standard mapping
procedure is shown to occur provided by bilinear-like contributions of high
order spin interactions. The evaluation of this anisotropy gives a hint on the
strength of non-bilinear terms characterizing the system under investigation.Comment: 11 pages, 1 figur
Ballistic Spin Injection and Detection in Fe/Semiconductor/Fe Junctions
We present {\it ab initio} calculations of the spin-dependent electronic
transport in Fe/GaAs/Fe and Fe/ZnSe/Fe (001) junctions simulating the situation
of a spin-injection experiment. We follow a ballistic Landauer-B\"uttiker
approach for the calculation of the spin-dependent dc conductance in the
linear-responce regime, in the limit of zero temperature. We show that the bulk
band structure of the leads and of the semiconductor, and even more the
electronic structure of a clean and abrupt interface, are responsible for a
current polarisation and a magnetoresistance ratio of almost the ideal 100%, if
the transport is ballistic. In particular we study the significance of the
transmission resonances caused by the presence of two interfaces.Comment: 13 pages, 9 figure
Surface state scattering by adatoms on noble metals
When surface state electrons scatter at perturbations, such as magnetic or
nonmagnetic adatoms or clusters on surfaces, an electronic resonance, localized
at the adatom site, can develop below the bottom of the surface state band for
both spin channels. In the case of adatoms, these states have been found very
recently in scanning tunneling spectroscopy experiments\cite{limot,olsson} for
the Cu(111) and Ag(111) surfaces. Motivated by these experiments, we carried
out a systematic theoretical investigation of the electronic structure of these
surface states in the presence of magnetic and non-magnetic atoms on Cu(111).
We found that Ca and all 3 adatoms lead to a split-off state at the bottom
of the surface band which is, however, not seen for the elements Ga and
Ge. The situation is completely reversed if the impurities are embedded in the
surface: Ga and Ge are able to produce a split-off state whereas the 3
impurities do not. The resonance arises from the s-state of the impurities and
is explained in terms of strength and interaction nature (attraction or
repulsion) of the perturbing potential.Comment: 6 pages, 5 figure
RKKY-like contributions to the magnetic anisotropy energy: 3d adatoms on Pt(111) surface
The magnetic anisotropy energy defines the energy barrier that stabilizes a
magnetic moment. Utilizing density functional theory based simulations and
analytical formulations, we establish that this barrier is strongly modified by
long-range contributions very similar to Frieden oscillations and
Rudermann-Kittel-Kasuya-Yosida interactions. Thus, oscillations are expected
and observed, with different decaying factors and highly anisotropic in
realistic materials, which can switch non-trivially the sign of the magnetic
anisotropy energy. This behavior is general and for illustration we address
transition metals adatoms, Cr, Mn, Fe and Co deposited on Pt(111) surface. We
explain in particular the mechanisms leading to the strong site-dependence of
the magnetic anisotropy energy observed for Fe adatoms on Pt(111) surface as
revealed previously via first-principles based simulations and inelastic
scanning tunneling spectroscopy (A. A. Khajetoorians et al. Phys. Rev. Lett.
111, 157204 (2013)). The same mechanisms are probably active for the
site-dependence of the magnetic anisotropy energy obtained for Fe adatoms on Pd
or Rh(111) surfaces and for Co adatoms on Rh(111) surface (P. Blonski et al.
Phys. Rev. B 81, 104426 (2010)).Comment: published manuscript with additional figures and comment
Quantum well states and amplified spin-dependent Friedel oscillations in thin films
Electrons mediate many of the interactions between atoms in a solid. Their
propagation in a material determines its thermal, electrical, optical, magnetic
and transport properties. Therefore, the constant energy contours
characterizing the electrons, in particular the Fermi surface, have a prime
impact on the behavior of materials. If anisotropic, the contours induce strong
directional dependence at the nanoscale in the Friedel oscillations surrounding
impurities. Here we report on giant anisotropic charge density oscillations
focused along specific directions with strong spin-filtering after scattering
at an oxygen impurity embedded in the surface of a ferromagnetic thin film of
Fe grown on W(001). Utilizing density functional theory, we demonstrate that by
changing the thickness of the Fe films, we control quantum well states confined
to two dimensions that manifest as multiple flat energy contours, impinging and
tuning the strength of the induced charge oscillations which allow to detect
the oxygen impurity at large distances ( 50nm).Comment: This paper has an explanatory supplemen
Spin-polarization of platinum (111) induced by the proximity to cobalt nanostripes
We measured a spin polarization above a Pt (111) surface in the vicinity of a
Co nanostripe by spin-polarized scanning tunneling spectroscopy. The spin
polarization is exponentially decaying away from the Pt/Co interface and is
detectable at distances larger than 1 nm. By performing self-consistent
ab-initio calculations of the electronic-structure for a related model system
we reveal the interplay between the induced magnetic moments within the Pt
surface and the spin-resolved electronic density of states above the surface.Comment: 19 pages, 6 figure
Theory of real space imaging of Fermi surfaces
A scanning tunneling microscope can be used to visualize in real space Fermi
surfaces with buried impurities far below substrates acting as local probes. A
theory describing this feature is developed based on the stationary phase
approximation. It is demonstrated how a Fermi surface of a material acts as a
mirror focusing electrons that scatter at hidden impurities.Comment: 10 pages, 4 figure
Changing the Magnetic Configurations of Nanoclusters Atom-by-Atom
The Korringa-Kohn-Rostoker Green (KKR) function method for non-collinear
magnetic structures was applied on Mn and Cr ad-clusters deposited on the
Ni(111) surface. By considering various dimers, trimers and tetramers, a large
amount of collinear and non-collinear magnetic structures is obtained.
Typically all compact clusters have very small total moments, while the more
open structures exhibit sizeable total moments, which is a result of the
complex frustration mechanism in these systems. Thus, as the motion of a single
adatom changes the cluster structure from compact to open and vice versa, this
can be considered as a magnetic switch, which via the local exchange field of
the adatom allows to switch the cluster moment on and off, and which might be
useful for future nanosize information storage.Comment: 7 page