arXiv subjects

A. S. Nunez

Publications and source records attributed to A. S. Nunez.

18 recordsLinked to original sources

arXiv2026

A Topological Magnonic Black-White Hole Crystal

The low-energy dynamics of antiferromagnetic spin waves map onto a massive Klein-Gordon scalar field, providing a solid-state platform for analogue-horizon physics. We show that an inhomogeneous spin current generates an effective magnon flow capable of forming black- and white-hole horizons. Horizon-pair cavities exhibit resonant transport and superradiant-like amplification mediated by magnon-antimagnon mixing. Our central result is that a periodic arrangement of submagnonic and supermagnonic regions forms a non-reciprocal magnonic crystal in which entering the supermagnonic regime makes negative-norm propagating channels available and enables the bulk gap to close and reopen. This drives a topological transition characterized by a change in the Zak phase and the emergence of a hybrid magnon-antimagnon edge state. These results establish a direct connection between analogue-horizon physics and topological magnonics, providing an electrically tunable route for controlling magnonic band topology.

cond-mat.mtrl-sci↗

arXiv2023

Chiral spin-transfer torque induced by curvature gradient

This work analyzes the propagation of a transverse domain wall (DW) motion under the action of an electric current along a nanowire (NW) with a curvature gradient. Our results evidence that the curvature gradient induces a chiral spin-transfer torque (CSTT) whose effect on the DW motion depends on the direction along which the DW points. The origin of the CSTT is explained in terms of a position and phase-dependent effective field associated with the DW profile and the electric current direction. Finally, our results reveal that this chiral mechanism can also affect the behavior of other magnetization collective modes, such as spin waves. This work shows the emergence of curvature-induced chiral spin transport and highlights a new phenomenon to be considered for designing spintronic devices.

cond-mat.mes-hall↗

arXiv2020

Magnon Polarons induced by a magnetic field gradient

In this work, we report the theoretical possibility of generating magnon polaron excitations through a space-varying magnetic field. The spatial dependence of the magnetic field in the Zeeman interaction gives rise to a magnon-phonon coupling when a magnetic field gradient is applied, and such a coupling depends directly on the strength of the gradient. It is also predicted that the direction of the magnetic field gradient allows control over which phonon polarization couples to the magnons in the material. Here we develop the calculations of the magnon-phonon coupling for an arbitrary (anti)ferromagnet, which are later used to numerically study its consequences. These results are compared to the ones obtained with the phenomenological magnetoelastic coupling in YIG, where we show that the magnon polaron bandgap seen in YIG can be also obtained with a magnetic field gradient of $\sim 0.1$T/m which can be achieved with the current experimental techniques. Our results propose a new way of controlling the magnetoelastic coupling in an arbitrary material and open a new route to exploit the magnon-phonon interaction in magnonic and spintronic devices.

cond-mat.mes-hall↗

arXiv2018

Spin-wave amplification and lasing driven by inhomogeneous spin transfer torques

We show that an inhomogeneity in the spin-transfer torques in a metallic ferromagnet under suitable conditions strongly amplifies incoming spin waves. Moreover, at nonzero temperatures the incoming thermally occupied spin waves will be amplified such that the region with inhomogeneous spin transfer torques emits spin waves spontaneously, thus constituting a spin-wave laser. We determine the spin-wave scattering amplitudes for a simplified model and set-up, and show under which conditions the amplification and lasing occurs. Our results are interpreted in terms of a so-called black-hole laser, and could facilitate the field of magnonics, that aims to utilize spin waves in logic and data-processing devices.

cond-mat.mes-hall↗

arXiv2017

Piezospintronic effect in honeycomb antiferromagnets

The emission of pure spin currents by mechanical deformations, the piezospintronic effect, in antiferromagnets is studied. We characterize the piezospintronic effect in an antiferromagnetic honeycomb monolayer in response to external strains. It is shown that the strain tensor components can be evaluated in terms of the spin Berry phase. In addition, we propose an experimental setup to detect the piezo-spin current generated in the piezospintronic material through the inverse spin Hall effect. Our results apply to a wide family of two-dimensional antiferromagnetic materials without inversion symmetry, such as the transition-metal chalcogenophosphates materials MPX3 (M=V, Mn; X=S, Se, Te) and NiPSe3.

cond-mat.mes-hall↗

arXiv2016

Soliton-like magnetization textures in non-collinear antiferromagnets

We show that proper control of magnetization textures can be achieved in noncollinear antiferromagnets. This opens the versatile toolbox of domain-wall manipulation in the context of a different family of materials. In this way, we show that noncollinear antiferromagnets are a good prospect for applications in the context of antiferromagnetic spintronics. As in many noncollinear antiferromagnets, the order parameter field takes values in SO(3). By performing a gradient expansion in the energy functional we derive an effective theory that accounts for the physics of the magnetization of long-wavelength excitations. We apply our formalism to static and dynamic textures such as domain walls and localized oscillations, and identify topologically protected textures that are spatially localized. Our results are applicable to the exchange-bias materials Mn3X, with X = Ir, Rh, Pt.

cond-mat.mes-hall↗

arXiv2016

Magnonic black holes

We show that the interaction between spin-polarized current and magnetization dynamics can be used to implement black-hole and white-hole horizons for magnons - the quanta of oscillations in the magnetization direction in magnets. We consider three different systems: easy-plane ferromagnetic metals, isotropic antiferromagnetic metals, and easy-plane magnetic insulators. Based on available experimental data, we estimate that the Hawking temperature can be as large as 1 K. We comment on the implications of magnonic horizons for spin-wave scattering and transport experiments, and for magnon entanglement.

cond-mat.mes-hall↗

arXiv2015

Topological spin waves in the atomic-scale magnetic skyrmion crystal

We study the spin waves of the triangular skyrmion crystal that emerges in a two dimensional spin lattice model as a result of the competition between Heisenberg exchange, Dzyalonshinkii-Moriya interactions, Zeeman coupling and uniaxial anisotropy. The calculated spin wave bands have a finite Berry curvature that, in some cases, leads to non-zero Chern numbers, making this system topologically distinct from conventional magnonic systems. We compute the edge spin-waves, expected from the bulk-boundary correspondence principle, and show that they are chiral, which makes them immune to elastic backscattering. Our results illustrate how topological phases can occur in self-generated emergent superlattices at the mesoscale.

cond-mat.mes-hall↗

arXiv2015

Antiferromagnetic Magnonic Crystals

We describe the features of magnonic crystals based upon antiferromagnetic elements. Our main results are that with a periodic modulation of either magnetic fields or system characteristics, such as the anisotropy, it is possible to tailor the spin wave spectra of antiferromagnetic systems into a band-like organization that displays a segregation of allowed and forbidden bands. The main features of the band structure, such as bandwidths and bandgaps, can be readily manipulated. Our results provide a natural link between two steadily growing fields of spintronics: antiferromagnetic spintronics and magnonics.

cond-mat.mes-hall↗

arXiv2008

Effective temperature and Gilbert damping of a current-driven localized spin

Starting from a model that consists of a semiclassical spin coupled to two leads we present a microscopic derivation of the Langevin equation for the direction of the spin. For slowly-changing direction it takes on the form of the stochastic Landau-Lifschitz-Gilbert equation. We give expressions for the Gilbert damping parameter and the strength of the fluctuations, including their bias-voltage dependence. At nonzero bias-voltage the fluctuations and damping are not related by the fluctuation-dissipation theorem. We find, however, that in the low-frequency limit it is possible to introduce a voltage-dependent effective temperature that characterizes the fluctuations in the direction of the spin, and its transport-steady-state probability distribution function.

cond-mat.mes-hall↗

arXiv2007

Current-Induced Torques in Magnetic Metals: Beyond Spin Transfer

Current-induced torques on ferromagnetic nanoparticles and on domain walls in ferromagnetic nanowires are normally understood in terms of transfer of conserved spin angular momentum between spin-polarized currents and the magnetic condensate. In a series of recent articles we have discussed a microscopic picture of current-induced torques in which they are viewed as following from exchange fields produced by the misaligned spins of current carrying quasiparticles. This picture has the advantage that it can be applied to systems in which spin is not approximately conserved. More importantly, this point of view makes it clear that current-induced torques can also act on the order parameter of an antiferromagnetic metal, even though this quantity is not related to total spin. In this informal and intentionally provocative review we explain this picture and discuss its application to antiferromagnets.

cond-mat.mes-hall↗

arXiv2007

Functional Keldysh Theory of Spin Torques

We present a microscopic treatment of current-induced torques and thermal fluctuations in itinerant ferromagnets based on a functional formulation of the Keldysh formalism. We find that the nonequilibrium magnetization dynamics is governed by a stochastic Landau-Lifschitz-Gilbert equation with spin transfer torques. We calculate the Gilbert damping parameter $α$ and the non-adiabatic spin transfer torque parameter $β$ for a model ferromagnet. We find that $β\neq α$, in agreement with the results obtained using imaginary-time methods of Kohno, Tatara and Shibata [J. Phys. Soc. Japan 75, 113706 (2006)]. We comment on the relationship between $s-d$ and isotropic-Stoner toy models of ferromagnetism and more realistic density-functional-theory models, and on the implications of these relationships for predictions of the $β/α$ ratio which plays a central role in domain wall motion. Only for a single-parabolic-band isotropic-Stoner model with an exchange splitting that is small compared to the Fermi energy does $β/α$ approach one. In addition, our microscopic formalism incorporates naturally the fluctuations needed in a nonzero-temperature description of the magnetization. We find that to first order in the applied electric field, the usual form of thermal fluctuations via a phenomenological stochastic magnetic field holds.

cond-mat.mes-hall↗

arXiv2006

Ab-initio GMR and current-induced torques in Au/Cr multilayers

We report on an {\em ab-initio} study of giant magnetoresistance (GMR) and current-induced-torques (CITs) in Cr/Au multilayers that is based on non-equilibrium Green's functions and spin density functional theory. We find substantial GMR due primarily to a spin-dependent resonance centered at the Cr/Au interface and predict that the CITs are strong enough to switch the antiferromagnetic order parameter at current-densities $\sim 100$ times smaller than typical ferromagnetic metal circuit switching densities.

cond-mat.mes-hall↗

arXiv2006

Current Induced Order Parameter Dynamics: Microscopic Theory Applied to Co/Cu/Co spin valves

Transport currents can alter alter order parameter dynamics and change steady states in superconductors, in ferromagnets, and in hybrid systems. In this article we present a scheme for fully microscopic evaluation of order parameter dynamics that is intended for application to nanoscale systems. The approach relies on time-dependent mean-field-theory, on an adiabatic approximation, and on the use of non-equilibrium Greens function (NEGF) theory to calculate the influence of a bias voltage across a system on its steady-state density matrix. We apply this scheme to examine the spin-transfer torques which drive magnetization dynamics in Co/Cu/Co spin-valve structures. Our microscopic torques are peaked near Co/Cu interfaces, in agreement with most previous pictures, but suprisingly act mainly on Co transition metal $d$-orbitals rather than on $s$-orbitals as generally supposed.

cond-mat.mes-hall↗

arXiv2006

Inelastic scattering in ferromagnetic and antiferromagnetic metal spintronics

We use a ferromagnetic voltage probe model to study the influence of inelastic scattering on giant magnetoresistance and current-induced torques in ferromagnetic and antiferromagnetic metal spin valves. The model is based on the Green's function formulation of transport theory and represents spin-dependent and spin-independent inelastic scatterers by interior voltage probes that are constrained to carry respectively no charge current and no spin or charge current. We find that giant magnetoresistance and spin transfer torques in ferromagnetic metal spin valve structures survive arbitrarily strong spin-independent inelastic scattering, while the recently predicted analogous phenomena in antiferromagnetic metal spin valves are partially suppressed. We use toy-model numerical calculations to estimate spacer layer thickness requirements for room temperature operation of antiferromagnetic metal spin valves.

cond-mat.mes-hall↗

arXiv2006

Thermally-Assisted Current-Driven Domain Wall Motion

Starting from the stochastic Landau-Lifschitz-Gilbert equation, we derive Langevin equations that describe the nonzero-temperature dynamics of a rigid domain wall. We derive an expression for the average drift velocity of the domain wall as a function of the applied current, and find qualitative agreement with recent magnetic semiconductor experiments. Our model implies that at any nonzero temperature the average domain-wall velocity initially varies linearly with current, even in the absence of non-adiabatic spin torques.

cond-mat.mtrl-sci↗

arXiv2006

Spin transfer in an antiferromagnet

An electrical current can transfer spin angular momentum to a ferromagnet. This novel physical phenomenon, called spin transfer, offers unprecedented spatial and temporal control over the magnetic state of a ferromagnet and has tremendous potential in a broad range of technologies, including magnetic memory and recording. Recently, it has been predicted that spin transfer is not limited to ferromagnets, but can also occur in antiferromagnetic materials and even be stronger under some conditions. In this paper we demonstrate transfer of spin angular momentum across an interface between ferromagnetic and antiferromagnetic metals. The spin transfer is mediated by an electrical current of high density (~10^12 A/m^2) and revealed by variation in the exchange bias at the ferromagnet/antiferromagnet interface. We find that, depending on the polarity of the electrical current flowing across the interface, the strength of the exchange bias can either increase or decrease. This finding is explained by the theoretical prediction that a spin polarized current generates a torque on magnetic moments in the antiferromagnet. Current-mediated variation of exchange bias can be used to control the magnetic state of spin-valve devices, e.g., in magnetic memory applications.

cond-mat.mtrl-sci↗

arXiv2005

Antiferromagnetic Metal Spintronics

Spintronics in ferromagnetic metals is built on a complementary set of phenomena in which magnetic configurations influence transport coefficients and transport currents alter magnetic configurations. In this Letter we propose that corresponding effects occur in circuits containing antiferromagnetic metals. The critical current for switching can be smaller in the antiferromagnetic case because of the absence of shape anisotropy and because spin torques act through the entire volume of an antiferromagnet. Our findings suggest that current-induced order parameter dynamics can be used to coarsen the microstructure of antiferromagnetic thin films.

cond-mat.mes-hall↗