29 research outputs found
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
Chirality-driven orbital magnetic moments as a new probe for topological magnetic structures
When electrons are driven through unconventional magnetic structures, such as
skyrmions, they experience emergent electromagnetic fields that originate
several Hall effects. Independently, ground state emergent magnetic fields can
also lead to orbital magnetism, even without the spin-orbit interaction. The
close parallel between the geometric theories of the Hall effects and of the
orbital magnetization raises the question: does a skyrmion display topological
orbital magnetism? Here we first address the smallest systems with nonvanishing
emergent magnetic field, trimers, characterizing the orbital magnetic
properties from first-principles. Armed with this understanding, we study the
orbital magnetism of skyrmions, and demonstrate that the contribution driven by
the emergent magnetic field is topological. This means that the topological
contribution to the orbital moment does not change under continous deformations
of the magnetic structure. Furthermore, we use it to propose a new experimental
protocol for the identification of topological magnetic structures, by soft
x-ray spectroscopy.Comment: 17 pages, 5 figures, to be published in Nature Communication
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
Unoccupied surface and interface states in Pd thin films deposited on Fe/Ir(111) surface
We present a systematic first-principles study of the electronic surface
states and resonances occuring in thin films of Pd of various thicknesses
deposited on a single ferromagnetic monolayer of Fe on top of Ir(111)
substrate. This system is of interest since one Pd layer deposited on
Fe/Ir(111) hosts small magnetic skyrmions. The latter are topological magnetic
objects with swirling spin-textures with possible implications in the context
of spintronic devices since they have the potential to be used as magnetic bits
for information technology. The stabilization, detection and manipulation of
such non-collinear magnetic entities require a quantitative investigation and a
fundamental understanding of their electronic structure. Here we investigate
the nature of the unoccupied electronic states in Pd/Fe/Ir(111), which are
essential in the large spin-mixing magnetoresistance (XMR) signature captured
using non spin-polarized scanning tunnelling microscopy [Crum et al., Nat.
Commun. {\bf 6} 8541 (2015); Hanneken et al., Nat. Nanotech. {\bf 10}, 1039
(2015)]. To provide a complete analysis, we investigate bare Fe/Ir(111) and
Pd/Fe/Ir(111) surfaces. Our results demonstrate the emergence of
surface and interface states after deposition of Pd monolayers, which are
strongly impacted by the large spin-orbit coupling of Ir surface.Comment: 16 pages, 11 figure
Interplay between Kondo effect and Ruderman-Kittel-Kasuya-Yosida interaction
The interplay between the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction
and the Kondo effect is expected to provide the driving force for the emergence
of many phenomena in strongly correlated electron materials. Two magnetic
impurities in a metal are the smallest possible system containing all these
ingredients and define a bottom up approach towards a long term understanding
of concentrated / dense systems. Here we report on the experimental and
theoretical investigation of iron dimers buried below a Cu(100) surface by
means of low temperature scanning tunnelling spectroscopy (STS) combined with
density functional theory (DFT) and numerical renormalization group (NRG)
calculations. The Kondo effect, in particular the width of the Abrikosov-Suhl
resonance, is strongly altered or even suppressed due to magnetic coupling
between the impurities. It oscillates as function of dimer separation revealing
that it is related to the RKKY interaction mediated by the conduction
electrons. Simulations based on density functional theory support this concept
showing the same oscillation period and trends in the coupling strength as
found in the experiment
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
Comparison of first-principles methods to extract magnetic parameters in ultra-thin films: Co/Pt(111)
We compare three distinct computational approaches based on first-principles
calculations within density functional theory to explore the magnetic exchange
and the Dzyaloshinskii-Moriya interactions (DMI) of a Co monolayer on Pt(111),
namely (i) the method of infinitesimal rotations of magnetic moments based on
the Korringa-Kohn-Rostoker (KKR) Green function method, (ii) the generalized
Bloch theorem applied to spiraling magnetic structures and (iii) supercell
calculations with non-collinear magnetic moments, the latter two being based on
the full-potential linearized augmented plane wave (FLAPW) method. In
particular, we show that the magnetic interaction parameters entering
micromagnetic models describing the long-wavelength deviations from the
ferromagnetic state might be different from those calculated for fast rotating
magnetic structures, as they are obtained by using (necessarily rather small)
supercell or large spin-spiral wave-vectors. In the micromagnetic limit, which
we motivate to use by an analysis of the Fourier components of the domain-wall
profile, we obtain consistent results for the spin stiffness and DMI
spiralization using methods (i) and (ii). The calculated spin stiffness and
Curie temperature determined by subsequent Monte Carlo simulations are
considerably higher than estimated from the bulk properties of Co, a
consequence of a significantly increased nearest-neighbor exchange interaction
in the Co-monolayer (+50%). The calculated results are carefully compared with
the literature
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