32 research outputs found
Relativistic dynamical spin excitations of magnetic adatoms
We present a first-principles theory of dynamical spin excitations in the
presence of spin-orbit coupling. The broken global spin rotational invariance
leads to a new sum rule. We explore the competition between the magnetic
anisotropy energy and the external magnetic field, as well as the role of
electron-hole excitations, through calculations for 3-metal adatoms on the
Cu(111) surface. The spin excitation resonance energy and lifetime display
non-trivial behavior, establishing the strong impact of relativistic effects.
We legitimate the use of the Landau-Lifshitz-Gilbert equation down to the
atomic limit, but with parameters that differ from a stationary theory.Comment: 8 pages, 3 figures, accepted in PR
Role of Dzyaloshinskii-Moriya interaction for magnetism in transition-metal chains at Pt step-edges
We explore the emergence of chiral magnetism in one-dimensional monatomic Mn,
Fe, and Co chains deposited at the Pt(664) step-edge carrying out an ab-initio
study based on density functional theory (DFT). The results are analyzed
employing several models: (i) a micromagnetic model, which takes into account
the Dzyaloshinskii-Moriya interaction (DMI) besides the spin stiffness and the
magnetic anisotropy energy, and (ii) the Fert-Levy model of the DMI for diluted
magnetic impurities in metals. Due to the step-edge geometry, the direction of
the Dzyaloshinskii vector (D-vector) is not predetermined by symmetry and
points in an off-symmetry direction. For the Mn chain we predict a long-period
cycloidal spin-spiral ground state of unique rotational sense on top of an
otherwise atomic-scale antiferromagnetic phase. The spins rotate in a plane
that is tilted relative to the Pt surface by towards the upper step
of the surface. The Fe and Co chains show a ferromagnetic ground state since
the DMI is too weak to overcome their respective magnetic anisotropy barriers.
Beyond the discussion of the monatomic chains we provide general expressions
relating ab-initio results to realistic model parameters that occur in a
spin-lattice or in a micromagnetic model. We prove that a planar homogeneous
spiral of classical spins with a given wave vector rotating in a plane whose
normal is parallel to the D-vector is an exact stationary state solution of a
spin-lattice model for a periodic solid that includes Heisenberg exchange and
DMI. The validity of the Fert-Levy model for the evaluation of micromagnetic
DMI parameters and for the analysis of ab-initio calculations is explored for
chains. The results suggest that some care has to be taken when applying the
model to infinite periodic one-dimensional systems.Comment: 21 pages, 9 figure
Renormalization of electron self-energies via their interaction with spin excitations: A first-principles investigation
Access to magnetic excitation spectra of single atoms deposited on surfaces
is nowadays possible by means of low-temperature inelastic scanning tunneling
spectroscopy. We present a first-principles method for the calculation of
inelastic tunneling spectra utilizing the Korringa-Kohn-Rostoker Green function
method combined with time-dependent density functional theory and many-body
perturbation theory. The key quantity is the electron self-energy describing
the coupling of the electrons to the spin excitation within the adsorbate. By
investigating Cr, Mn, Fe and Co adatoms on a Cu(111) substrate, we
spin-characterize the spectra and demonstrate that their shapes are altered by
the magnetization of the adatoms, of the tip and the orbital decay into vacuum.
Our method also predicts spectral features more complex than the steps obtained
by simpler models for the adsorbate (e.g., localized spin models)
Engineering elliptical spin-excitations by complex anisotropy fields in Fe adatoms and dimers on Cu(111)
We investigate the dynamics of Fe adatoms and dimers deposited on the Cu(111)
metallic surface in the presence of spin-orbit coupling, within time-dependent
density functional theory. The \textit{ab initio} results provide
material-dependent parameters that can be used in semiclassical approaches,
which are used for insightful interpretations of the excitation modes. By
manipulating the surroundings of the magnetic elements, we show that elliptical
precessional motion may be induced through the modification of the magnetic
anisotropy energy. We also demonstrate how different kinds of spin precession
are realized, considering the symmetry of the magnetic anisotropy energy, the
ferro- or antiferromagnetic nature of the exchange coupling between the
impurities, and the strength of the magnetic damping. In particular, the normal
modes of a dimer depend on the initial magnetic configuration, changing
drastically by going from a ferromagnetic metastable state to the
antiferromagnetic ground state. By taking into account the effect of the
damping into their resonant frequencies, we reveal that an important
contribution arises for strongly biaxial systems and specially for the
antiferromagnetic dimers with large exchange couplings. Counter intuitively,
our results indicate that the magnetic damping influences the quantum
fluctuations by decreasing the zero-point energy of the system
Transverse dynamical magnetic susceptibilities from regular static density functional theory: Evaluation of damping and g-shifts of spin-excitations
The dynamical transverse magnetic Kohn-Sham susceptibility calculated within
time-dependent density functional theory shows a fairly linear behavior for a
finite energy window. This observation is used to propose a scheme where the
computation of this quantity is greatly simplified. Regular simulations based
on static density functional theory can be used to extract the dynamical
behavior of the magnetic response function. Besides the ability to calculate
elegantly damping of magnetic excitations, we derive along the way useful
equations giving the main characteristics of these excitations: effective
-factors and the resonance frequencies that can be accessed experimentally
using inelastic scanning tunneling spectroscopy or spin-polarized electron
energy loss spectroscopy.Comment: 7 pages, 1 figur
Perpendicular Reading of Single Confined Magnetic Skyrmions
Thin-film sub-5 nm magnetic skyrmions constitute an ultimate scaling
alternative for future digital data storage. Skyrmions are robust non-collinear
spin-textures that can be moved and manipulated by small electrical currents.
We show here an innovative technique to detect isolated nanoskyrmions with a
current-perpendicular-to-plane geometry, which has immediate implications for
device concepts. We explore the physics behind such a mechanism by studying the
atomistic electronic structure of the magnetic quasiparticles. We investigate
how the isolated skyrmion local-density-of-states which tunnels into the
vacuum, when compared to the ferromagnetic background, is modified by the
site-dependent spin-mixing of electronic states with different relative canting
angles. Local transport properties are sensitive to this effect, as we report
an atomistic conductance anisotropy of over 20% for magnetic skyrmions in
Pd/Fe/Ir(111) thin-films. In single skyrmions, engineering this spin-mixing
magnetoresistance possibly could be incorporated in future magnetic storage
technologies
Theoretical probing of inelastic spin-excitations in adatoms on surfaces
We review our recent work on the simulation, description and prediction of
spin-excitations in adatoms and dimers deposited on metallic surfaces. This
work done together with Douglas L. Mills, is an extension of his seminal
contribution (with Pascal Lederer) published 50 years ago on the spin-dynamics
of transition metal impurities embedded in transition metal hosts [P. Lederer,
D.L. Mills, Phys. Rev. {\bf 160}, 590 (1967)]. The main predictions of his
model were verified experimentally with state of the art inelastic scanning
tunneling spectroscopy on adatoms. Our formalism, presented in this review, is
based on time-dependent density functional theory, combined with the
Korringa-Kohn-Rostoker Green function method. Comparison to experiments is
shown and discussed in detail. Our scheme enables the description and
prediction of the main characteristics of these excitations, \emph{i.e.} their
resonance frequency, their lifetime and their behavior upon application of
external perturbations such as a magnetic field.Comment: 24 pages, invited review to the special issue "Spins at Surfaces" in
Surface Scienc
Spin excitations of individual Fe atoms on Pt(111): impact of the site-dependent giant substrate polarization
We demonstrate using inelastic scanning tunneling spectroscopy (ISTS) and
simulations based on density functional theory that the amplitude and sign of
the magnetic anisotropy energy for a single Fe atom adsorbed onto the Pt(111)
surface can be manipulated by modifying the adatom binding site. Since the
magnitude of the measured anisotropy is remarkably small, up to an order of
magnitude smaller than previously reported, electron-hole excitations are weak
and thus the spin-excitation exhibits long lived precessional lifetimes
compared to the values found for the same adatom on noble metal surfaces