62 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
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
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
Fermi-surface origin of skyrmion lattices in centrosymmetric rare-earth intermetallics
We show from first-principles that barrel-shaped structures within the Fermi
surface of the centrosymmetric intermetallic compounds GdRuSi and
GdPdSi give rise to Fermi surface nesting, which determines the
strength and sign of quasi-two-dimensional Ruderman-Kittel-Kasuya-Yosida
pairwise exchange interactions between the Gd moments. This is the principal
mechanism leading to their helical single- spin-spiral ground states,
providing transition temperatures and magnetic periods in good agreement with
experiment. Using atomistic spin-dynamic simulations, we draw a direct line
between the subtleties of the three-dimensional Fermi surface topology and the
stabilization of a square skyrmion lattice in GdRuSi at applied
magnetic fields as observed in experiment
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
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
Ab initio investigation of impurity-induced in-gap states in BiTe and BiSe
We investigate in-gap states emerging when a single transition metal
impurity is embedded in topological insulators (BiTe and BiSe).
We use a combined approach relying on first-principles calculations and an
Anderson impurity model. By computing the local density of states of Cr, Mn, Fe
and Co embedded not only in surfaces of BiTe and of BiSe but
also in their bulk phases, we demonstrate that in-gap states originate from the
hybridization of the electronic states of the impurity with bulk bands and not
with the topological surface states as it is usually assumed. This finding is
analyzed using a simplified Anderson impurity model. These observations are in
contradiction with the prevailing models used to investigate the magnetic
doping of topological insulators [R. R. Biswas et al. PRB 81, 233405 (2010), A.
M. Black-Schafer et al. PRB 91, 201411 (2015)], which attribute the origin of
the in-gap states to the hybridization with the topological surface states
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