arXiv subjects

A. T. Costa

Publications and source records attributed to A. T. Costa.

At least 37 records · Page 2Linked to original sources

arXiv2016

Microscopic origin of subthermal magnons and the spin Seebeck effect

Recent experimental evidence points to low-energy magnons as the primary contributors to the spin Seebeck effect. This spectral dependence is puzzling since it is not observed on other thermocurrents in the same material. Here, we argue that the physical origin of this behavior is the magnon-magnon scattering mediated by phonons, in a process which conserves the number of magnons. To assess the importance and features of this kind of scattering, we derive the effective magnon-phonon interaction from a microscopic model, including band energy, a screened electron-electron interaction and the electron-phonon interaction. Unlike higher order magnon-only scattering, we find that the coupling with phonons induce a scattering which is very small for low-energy (or subthermal) magnons but increases sharply above a certain energy -- rendering magnons above this energy poor spin-current transporters.

cond-mat.mes-hall↗

arXiv2016

Dynamical current-induced ferromagnetic and antiferromagnetic resonances

We demonstrate that ferromagnetic and antiferromagnetic excitations can be triggered by the dynamical spin accumulations induced by the bulk and surface contributions of the spin Hall effect. Due to the spin-orbit interaction, a time-dependent spin density is generated by an oscillatory electric field applied parallel to the atomic planes of Fe/W(110) multilayers. For symmetric trilayers of Fe/W/Fe in which the Fe layers are ferromagnetically coupled, we demonstrate that only the collective out-of-phase precession mode is excited, while the uniform (in-phase) mode remains silent. When they are antiferromagnetically coupled, the oscillatory electric field sets the Fe magnetizations into elliptical precession motions with opposite angular velocities. The manipulation of different collective spin-wave dynamical modes through the engineering of the multilayers and their thicknesses may be used to develop ultrafast spintronics devices. Our work provides a general framework that probes the realistic responses of materials in the time or frequency domain.

cond-mat.mes-hall↗

arXiv2015

Microscopic theory of Gilbert damping in metallic ferromagnets

We present a microscopic theory for magnetization relaxation in metallic ferromagnets of nanoscopic dimensions that is based on the dynamic spin response matrix in the presence of spin-orbit coupling. Our approach allows the calculation of the spin excitation damping rate even for perfectly crystalline systems, where existing microscopic approaches fail. We demonstrate that the relaxation properties are not completely determined by the transverse susceptibility alone, and that the damping rate has a non-negligible frequency dependence in experimentally relevant situations. Our results indicate that the standard Landau-Lifshitz-Gilbert phenomenology is not always appropriate to describe spin dynamics of metallic nanostructure in the presence of strong spin-orbit coupling.

cond-mat.mes-hall↗

arXiv2013

Origami-based spintronics in graphene

We show that periodically folded graphene sheets with enhanced spin-orbit interaction due to curvature effects can carry spin polarized currents and have gaps in the electronic spectrum in the presence of weak magnetic fields. Our results indicate that such origami-like structures can be used efficiently in spintronic applications.

cond-mat.mes-hall↗

arXiv2013

Spin pumping and interlayer exchange coupling through palladium

The magnetic behaviour of ultrathin ferromagnetic films deposited on substrates is strongly affected by the properties of the substrate. We investigate the spin pumping rate, interlayer exchange coupling and dynamic exchange coupling between ultrathin ferromagnetic films through palladium, a non-magnetic substrate that displays strong Stoner enhancement. We find that the interlayer exchange coupling, both in the static and dynamic versions, is qualitatively affected by the substrate's Stoner enhancement. For instance, the oscillatory behavior that is a hallmark property of the RKKY exchange coupling is strongly suppressed by Stoner enhancement. Although the spin pumping rate of ferromagnetic films atop palladium is only mildly changed by Stoner enhancement the change is large enough to be detected experimentally. The qualitative aspects of our results for palladium are expected to remain valid for any non-magnetic substrate where Coulomb repulsion is large.

cond-mat.mes-hall↗

arXiv2012

Dynamic RKKY interaction in graphene

The growing interest in carbon-based spintronics has stimulated a number of recent theoretical studies on the RKKY interaction in graphene, based on which the energetically favourable alignment between magnetic moments embedded in this material can be calculated. The general consensus is that the strength of the RKKY interaction in graphene decays as 1/D3 or faster, where D is the separation between magnetic moments. Such an unusually fast decay for a 2-dimensional system suggests that the RKKY interaction may be too short ranged to be experimentally observed in graphene. Here we show in a mathematically transparent form that a far more long ranged interaction arises when the magnetic moments are taken out of their equilibrium positions and set in motion. We not only show that this dynamic version of the RKKY interaction in graphene decays far more slowly but also propose how it can be observed with currently available experimental methods.

cond-mat.mes-hall↗

arXiv2011

Spin Currents in Metallic Nanostructures; Explicit Calculations

In ultrathin ferromagnets deposited on metallic substrates, excitation of precessional motion of the spins produces a spin current in the substrate that transports angular momentum out of the film. This phenomenon is referred to as spin pumping, and is a source of damping of the spin motion. Spin pumping enters importantly in the description of spin dynamics in other nanoscale and subnanoscale systems as well. In this paper, we present an approach based on the Kubo formalism that allows the explicit calculation of this spin current and its spatial variation. We use the formalism to explore features of the spin current generated by spin motions in a simple model system.

cond-mat.mes-hall↗

arXiv2011

Anomalously large g-factor of single atoms adsorbed on a metal substrate

We have performed inelastic scanning tunneling spectroscopy (ISTS) on individual Fe atoms adsorbed on a Ag(111) surface. ISTS reveals a magnetization excitation with a lifetime of about 400 fsec which decreases linearly upon application of a magnetic field. Astoundingly, we find that the g-factor, which characterizes the shift in energy of the excitation in a magnetic field, is g = 3.1 instead of the regular value of 2. This enhancement can be understood when considering the complete electronic structure of both the Ag(111) surface state and the Fe atom, as shown by ab initio calculations of the magnetic susceptibility.

cond-mat.mes-hall↗

arXiv2011

Graphene as a non-magnetic spin-current lens

In spintronics, the ability to transport magnetic information often depends on the existence of a spin current traveling between two different magnetic objects acting as source and probe. A large fraction of this information never reaches the probe and is lost because the spin current tends to travel omni-directionally. We propose that a curved boundary between a gated and a non-gated region within graphene acts as an ideal lens for spin currents despite being entirely of non-magnetic nature. We show as a proof of concept that such lenses can be utilized to redirect the spin current that travels away from a source onto a focus region where a magnetic probe is located, saving a considerable fraction of the magnetic information that would be otherwise lost.

cond-mat.mes-hall↗

arXiv2010

Dynamical magnetic excitations of nanostructures from first-principles

Within time-dependent density functional theory, combined with the Korringa-Kohn-Rostoker Green functions, we devise a real space method to investigate spin dynamics. Our scheme enables one to deduce the Coulomb potential which assures a proper Goldstone mode is present. We illustrate with application to 3$d$ adatoms and dimers on Cu(100).

cond-mat.mtrl-sci↗

arXiv2010

Spin waves in zigzag graphene nanoribbons and the stability of edge ferromagnetism

We study the low energy spin excitations of zigzag graphene nanoribbons of varying width. We find their energy dispersion at small wave vector to be dominated by antiferromagnetic correlations between the ribbon's edges, in accrodance with previous calculations. We point out that spin wave lifetimes are very long due to the semi-conducting nature of the electrically neutral nanoribbons. However, application of very modest gate voltages cause a discontinuous transition to a regime of finite spin wave lifetime. By further increasing doping the ferromagnetic alignments along the edge become unstable against transverse spin fluctuations. This makes the experimental detection of ferromagnetism is this class of systems very delicate, and poses a difficult challenge to the possible uses of these nanoribbons as basis for spintronic devices.

cond-mat.mes-hall↗

arXiv2010

Theory of Local Dynamical Magnetic Susceptibilities from the Korringa-Kohn-Rostoker Green Function Method

Within the framework of time-dependent density functional theory combined with the Korringa-Kohn-Rostoker Green function formalism, we present a real space methodology to investigate dynamical magnetic excitations from first-principles. We set forth a scheme which enables one to deduce the correct effective Coulomb potential needed to preserve the spin-invariance signature in the dynamical susceptibilities, i.e. the Goldstone mode. We use our approach to explore the spin dynamics of 3d adatoms and different dimers deposited on a Cu(001) with emphasis on their decay to particle-hole pairs.

cond-mat.mes-hall↗

arXiv2010

Spin Orbit Coupling and Spin Waves in Ultrathin Ferromagnets: The Spin Wave Rashba Effect

We present theoretical studies of the influence of spin orbit coupling on the spin wave excitations of the Fe monolayer and bilayer on the W(110) surface. The Dzyaloshinskii-Moriya interaction is active in such films, by virtue of the absence of reflection symmetry in the plane of the film. When the magnetization is in plane, this leads to a linear term in the spin wave dispersion relation for propagation across the magnetization. The dispersion relation thus assumes a form similar to that of an energy band of an electron trapped on a semiconductor surfaces with Rashba coupling active. We also show SPEELS response functions that illustrate the role of spin orbit coupling in such measurements. In addition to the modifications of the dispersion relations for spin waves, the presence of spin orbit coupling in the W substrate leads to a substantial increase in the linewidth of the spin wave modes. The formalism we have developed applies to a wide range of systems, and the particular system explored in the numerical calculations provides us with an illustration of phenomena which will be present in other ultrathin ferromagnet/substrate combinations.

cond-mat.mes-hall↗

arXiv2010

Graphene-based spin-pumping transistor

We demonstrate with a fully quantum-mechanical approach that graphene can function as gate-controllable transistors for pumped spin currents, i.e., a stream of angular momentum induced by the precession of adjacent magnetizations, which exists in the absence of net charge currents. Furthermore, we propose as a proof of concept how these spin currents can be modulated by an electrostatic gate. Because our proposal involves nano-sized systems that function with very high speeds and in the absence of any applied bias, it is potentially useful for the development of transistors capable of combining large processing speeds, enhanced integration and extremely low power consumption.

cond-mat.mes-hall↗

arXiv2010

Carbon nanotube: a low-loss spin-current waveguide

We demonstrate with a quantum-mechanical approach that carbon nanotubes are excellent spin-current waveguides and are able to carry information stored in a precessing magnetic moment for long distances with very little dispersion and with tunable degrees of attenuation. Pulsed magnetic excitations are predicted to travel with the nanotube Fermi velocity and are able to induce similar excitations in remote locations. Such an efficient way of transporting magnetic information suggests that nanotubes are promising candidates for memory devices with fast magnetization switchings.

cond-mat.mes-hall↗

arXiv2009

Emergence of local magnetic moments in doped graphene-related materials

Motivated by recent studies reporting the formation of localized magnetic moments in doped graphene, we investigate the energetic cost for spin polarizing isolated impurities embedded in this material. When a well-known criterion for the formation of local magnetic moments in metals is applied to graphene we are able to predict the existence of magnetic moments in cases that are in clear contrast to previously reported Density Functional Theory (DFT) results. When generalized to periodically repeated impurities, a geometry so commonly used in most DFT-calculations, this criterion shows that the energy balance involved in such calculations contains unavoidable contributions from the long-ranged pairwise magnetic interactions between all impurities. This proves the fundamental inadequacy of the DFT-assumption of independent unit cells in the case of magnetically doped low-dimensional graphene-based materials. We show that this can be circumvented if more than one impurity per unit cell is considered, in which case the DFT results agree perfectly well with the criterion-based predictions for the onset of localized magnetic moments in graphene. Furthermore, the existence of such a criterion determining whether or not a magnetic moment is likely to arise within graphene will be instrumental for predicting the ideal materials for future carbon-based spintronic applications.

cond-mat.mtrl-sci↗

arXiv2008

Indirect exchange coupling between localized magnetic moments in carbon nanotubes: a dynamic approach

Magnetic moments dilutely dispersed in a metallic host tend to be coupled through the conduction electrons of the metal. This indirect exchange coupling, known to occur for a variety of magnetic materials embedded in several different metallic structures, is of rather long range, especially for low-dimensional structures like carbon nanotubes. Motivated by recent claims that the indirect coupling between magnetic moments in precessional motion has a much longer range than its static counterpart, here we consider how magnetic atoms adsorbed to the walls of a metallic nanotube respond to a time-dependent perturbation that induces their magnetic moments to precess. By calculating the frequency-dependent spin susceptibility we are able to identify resonant peaks whose respective widths provide information about the dynamic aspect of the indirect exchange coupling. We show that by departing from a purely static representation to another in which the moments are allowed to precess, we change from what is already considered a long range interaction to another whose range is far superior. In other words, localized magnetic moments embedded in a metallic structure can feel each other's presence more easily when they are set in precessional motion. We argue that such an effect can have useful applications leading to large-scale spintronics devices.

cond-mat.str-el↗

arXiv2007

Non-collinear coupling between magnetic adatoms in carbon nanotubes

The long range character of the exchange coupling between localized magnetic moments indirectly mediated by the conduction electrons of metallic hosts often plays a significant role in determining the magnetic order of low-dimensional structures. In addition to this indirect coupling, here we show that the direct exchange interaction that arises when the moments are not too far apart may induce a non-collinear magnetic order that cannot be characterized by a Heisenberg-like interaction between the magnetic moments. We argue that this effect can be manipulated to control the magnetization alignment of magnetic dimers adsorbed to the walls of carbon nanotubes.

cond-mat.other↗