178 research outputs found
Zero-point quantum swing of magnetic couples
Quantum fluctuations are ubiquitous in physics. Ranging from conventional
examples like the harmonic oscillator to intricate theories on the origin of
the universe, they alter virtually all aspects of matter -- including
superconductivity, phase transitions and nanoscale processes. As a rule of
thumb, the smaller the object, the larger their impact. This poses a serious
challenge to modern nanotechnology, which aims total control via atom-by-atom
engineered devices. In magnetic nanostructures, high stability of the magnetic
signal is crucial when targeting realistic applications in information
technology, e.g. miniaturized bits. Here, we demonstrate that zero-point
spin-fluctuations are paramount in determining the fundamental magnetic
exchange interactions that dictate the nature and stability of the magnetic
state. Hinging on the fluctuation-dissipation theorem, we establish that
quantum fluctuations correctly account for the large overestimation of the
interactions as obtained from conventional static first-principles frameworks,
filling in a crucial gap between theory and experiment [1,2]. Our analysis
further reveals that zero-point spin-fluctuations tend to promote the
non-collinearity and stability of chiral magnetic textures such as skyrmions --
a counter-intuitive quantum effect that inspires practical guidelines for
designing disruptive nanodevices
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
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
Impurity-induced orbital magnetization in a Rashba electron gas
We investigate the induced orbital magnetization density in a Rashba electron
gas with magnetic impurities. Relying on classical electrodynamics, we obtain
this quantity through the bound currents composed of a paramagnetic and a
diamagnetic-like contribution which emerge from the spin-orbit interaction.
Similar to Friedel charge ripples, the bound currents and the orbital
magnetization density oscillate as function of distance away from the impurity
with characteristic wavelengths defined by the Fermi energy and the strength of
the Rashba spin-orbit interaction. The net induced orbital magnetization was
found to be of the order of magnitude of its spin counterpart. Besides the
exploration of the impact of the electronic filling of the impurity states, we
investigate and analyze the orbital magnetization induced by an equilateral
frustrated trimer in various non-collinear magnetic states. On the one hand, we
confirm that non-vanishing three-spin chiralities generate a finite orbital
magnetization density. On the other hand, higher order contributions lead to
multiple-spin chiralities affecting non-trivially and significantly the overall
magnitude and sign of the orbital magnetization
A new view on the origin of zero-bias anomalies of Co atoms atop noble metal surfaces
Many-body phenomena are paramount in physics. In condensed matter, their
hallmark is considerable on a wide range of material characteristics spanning
electronic, magnetic, thermodynamic and transport properties. They potentially
imprint non-trivial signatures in spectroscopic measurements, such as those
assigned to Kondo, excitonic and polaronic features, whose emergence depends on
the involved degrees of freedom. Here, we address systematically zero-bias
anomalies detected by scanning tunneling spectroscopy on Co atoms deposited on
Cu, Ag and Au(111) substrates, which remarkably are almost identical to those
obtained from first-principles. These features originate from gaped
spin-excitations induced by a finite magnetic anisotropy energy, in contrast to
the usual widespread interpretation relating them to Kondo resonances. Resting
on relativistic time-dependent density functional and many-body perturbation
theories, we furthermore unveil a new many-body feature, the spinaron,
resulting from the interaction of electrons and spin-excitations localizing
electronic states in a well defined energy.Comment: Supplementary Information include
SysML Model-Driven Approach to Verify Blocks Compatibility
International audienceIn the component paradigm, the system is seen as an assembly of heterogeneous components, where the system reliability depends on these components compatibility. In our approach, we focus on verifying compatibility of components modelled with SysML diagrams. Thus, we model component interactions with sequence diagrams (SDs) and components with SysML blocks. The SDs constitute a good start point for compatibility verification. However, this verification is still inapplicable directly on SDs, because they are expressed in informal language. Thus, to apply a verification method, it is necessary to translate the SDs into formal models, and then verify the wanted properties. In this paper, we propose a high-level model-driven approach which consists of an ATL grammar that automates the transformation of SDs into interface automata. Also, to allow an easy use of Ptolemy tool to verify properties on automata, we have proposed some Acceleo templates, which generate the Ptolemy entry specification
Dynamical amplification of magnetoresistances and Hall currents up to the THz regime
Spin-orbit-related effects offer a highly promising route for reading and
writing information in magnetic units of future devices. These phenomena rely
not only on the static magnetization orientation but also on its dynamics to
achieve fast switchings that can reach the THz range. In this work, we consider
Co/Pt and Fe/W bilayers to show that accounting for the phase difference
between different processes is crucial to the correct description of the
dynamical currents. By tuning each system towards its ferromagnetic resonance,
we reveal that dynamical spin Hall angles can non-trivially change sign and be
boosted by over 500%, reaching giant values. We demonstrate that charge and
spin pumping mechanisms can greatly magnify or dwindle the currents flowing
through the system, influencing all kinds of magnetoresistive and Hall effects,
thus impacting also dc and second harmonic experimental measurements.Comment: 19 pages, 4 figures, Supplementary Informatio
Spin-resolved spectroscopic evidence for spinarons in Co adatoms
Single cobalt atoms on the (111) surfaces of noble metals were for a long
time considered prototypical systems for the Kondo effect in scanning tunneling
microscopy experiments. Yet, recent first-principle calculations suggest that
the experimentally observed spectroscopic zero-bias anomaly (ZBA) should be
interpreted in terms of excitations of the Co atom's spin and the formation of
a novel quasiparticle, the spinaron, a magnetic polaron resulting from the
interaction of spin excitations with conduction electrons, rather than in terms
of a Kondo resonance. Here we present state-of-the-art spin-averaged and
spin-polarized scanning tunneling spectroscopy measurements on Co atoms on the
Cu(111) surface in magnetic fields of up to 12 T, that allow us to discriminate
between the different theoretical models and to invalidate the prevailing
Kondo-based interpretation of the ZBA. Employing extended ab-initio
calculations, we instead provide strong evidence for multiple spinaronic states
in the system. Our work opens a new avenue of research to explore the
characteristics and consequences of these intriguing hybrid many-body states as
well as their design in man-made nanostructures.Comment: 8 pages, 4 figure
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