arXiv subjects

Branislav K. Nikolic

Publications and source records attributed to Branislav K. Nikolic.

At least 37 records · Page 2Linked to original sources

arXiv2023

Quantum-classical approach to spin and charge pumping and the ensuing radiation in THz spintronics: Example of ultrafast-light-driven Weyl antiferromagnet Mn$_3$Sn

The interaction of fs light pulses with magnetic materials has been intensely studied for more than two decades in order to understand ultrafast demagnetization in single magnetic layers or THz emission from their bilayers with nonmagnetic spin-orbit (SO) materials. Here we develop a multiscale quantum-classical formalism -- where conduction electrons are described by quantum master equation of the Lindblad type; classical dynamics of local magnetization is described by the Landau-Lifshitz-Gilbert (LLG) equation; and incoming light is described by classical vector potential while outgoing electromagnetic radiation is computed using Jefimenko equations for retarded electric and magnetic fields -- and apply it a bilayer of antiferromagnetic Weyl semimetal Mn$_3$Sn with noncollinear local magnetization in contact with SO-coupled nonmagnetic material. Our QME+LLG+Jefimenko scheme makes it possible to understand how fs light pulse generates directly spin and charge pumping and electromagnetic radiation by the latter, including both odd and even high harmonics (of the pulse center frequency) up to order $n \le 7$. The directly pumped spin current then exert spin torque on local magnetization whose dynamics, in turn, pumps additional spin and charge currents radiating in the THz range. By switching on and off LLG dynamics and SO couplings, we unravel which microscopic mechanism contribute the most to emitted THz radiation -- charge pumping by local magnetization of Mn$_3$Sn in the presence of its intrinsic SO coupling is far more important than standardly assumed (for other types of magnetic layers) spin pumping and subsequent spin-to-charge conversion within the neighboring nonmagnetic SO-coupled material.

cond-mat.mes-hall↗

arXiv2023

Comment on "Spin-orbit coupling induced ultrahigh-harmonic generation from magnetic dynamics" with prescriptions on how to validate scientific software for computational quantum transport

In a recent paper [Phys. Rev. B {\bf 105}, L180415 (2022)], Ly and Manchon used open source code {\tt TKWANT} for time-dependent computational quantum transport to predict surprising features in the mature field of current pumping by magnetization dynamics in spintronics -- in the presence of spin-orbit (SO) coupling, the pumped charge current oscillates at both the frequency $ω_0$ of magnetization precession and high harmonics $N=ω/ω_0$, reaching {\em astonishingly high} cutoff \mbox{$N_\mathrm{max} \simeq 1000$} by increasing the SO coupling. However, results in the paper violate two basic theorems of time-dependent quantum transport: ({\em i}) current response to time-periodic external field {\em must be perfectly periodic} itself in the long time limit for a two-terminal device because its active region is attached to two semi-infinite leads bringing continuous energy spectrum; and ({\em ii}) no DC component of charge current is allowed in the left-right symmetric two-terminal devices, or in asymmetric devices its value cannot be changed by simply increasing the SO coupling. We illustrate these two theorems by using completely different calculations applied to one-dimensional two-terminal devices with either ferromagnetic (for which the device is left-right symmetric) and antiferromagnetic (for the device is left-right asymmetric) active region hosting the Rashba SO coupling. We conclude that harmonics in pumped current in the presence of SO coupling do exist, but their ``ultrahigh'' cutoff is an artifact of either ``bugs'' or inadequate algorithms selected within {\tt TKWANT}. Finally, we suggest strategies for {\em validating} time-dependent quantum transport codes, or selection of algorithms by a user within putatively validated (by developers) code, prior to deploying them to produce research papers.

cond-mat.mes-hall↗

arXiv2022

High-harmonic generation in spin and charge current pumping at ferromagnetic or antiferromagnetic resonance in the presence of spin-orbit coupling

One of the cornerstone effects in spintronics is spin pumping by dynamical magnetization that is steadily precessing (around, e.g., the $z$-axis) with frequency $ω_0$, due to absorption of low-power microwaves of frequency $ω_0$ under the resonance conditions and in the absence of any applied bias voltage. The two-decades-old "standard model" of this effect, based on the scattering theory of adiabatic quantum pumping, predicts that component $I^{S_z}$ of spin current vector $\big( I^{S_x}(t),I^{S_y}(t),I^{S_z} \big) \propto ω_0$ is time-independent while $I^{S_x}(t)$ and $I^{S_y}(t)$ oscillate harmonically in time with a single frequency $ω_0$; whereas pumped charge current is zero $I \equiv 0$ in the same adiabatic $\propto ω_0$ limit. Here we employ more general than "standard model" approaches, time-dependent nonequilibrium Green's function (NEGF) and Floquet-NEGF, to predict unforeseen features of spin pumping -- precessing localized magnetic moments within ferromagnetic metal (FM) or antiferromagnetic metal (AFM), whose conduction electrons are exposed to spin-orbit coupling (SOC) of either intrinsic or proximity origin, will pump both spin $I^{S_α}(t)$ and charge $I(t)$ currents. All four of these functions harmonically oscillate in time at both even an odd integer multiples $Nω_0$ of the driving frequency $ω_0$. The cutoff order of such high-harmonics increases with SOC strength, reaching $N_\mathrm{max} \simeq 11$ in the chosen-for-demonstration one-dimensional FM or AFM models. Higher cutoff $N_\mathrm{max} \simeq 25$ can be achieved in realistic two-dimensional (2D) FM models defined on the honeycomb lattice, where we provide prescription on how to realize them using 2D magnets and their heterostructures.

cond-mat.mes-hall↗

arXiv2022

Above-room-temperature ferromagnetism in ultrathin van der Waals magnet

Two-dimensional (2D) magnetic van der Waals materials provide a powerful platform for studying fundamental physics of low-dimensional magnetism, engineering novel magnetic phases, and enabling ultrathin and highly tunable spintronic devices. To realize high quality and practical devices for such applications, there is a critical need for robust 2D magnets with ordering temperatures above room temperature that can be created via exfoliation. Here the study of exfoliated flakes of cobalt substituted Fe5GeTe2 (CFGT) exhibiting magnetism above room temperature is reported. Via quantum magnetic imaging with nitrogen-vacancy centers in diamond, ferromagnetism at room temperature was observed in CFGT flakes as thin as 16 nm. This corresponds to one of the thinnest room-temperature 2D magnet flakes exfoliated from robust single crystals, reaching a thickness relevant to practical spintronic applications. The Curie temperature Tc of CFGT ranges from 310 K in the thinnest flake studied to 328 K in the bulk. To investigate the prospect of high-temperature monolayer ferromagnetism, Monte Carlo calculations were performed which predicted a high value of Tc ~270 K in CFGT monolayers. Pathways towards further enhancing monolayer Tc are discussed. These results support CFGT as a promising platform to realize high-quality room-temperature 2D magnet devices.

cond-mat.mtrl-sci↗

arXiv2022

Spin pumping from antiferromagnetic insulator spin-orbit-proximitized by adjacent heavy metal: A first-principles Floquet-nonequilibrium Green's function study

Motivated by recent experiments [P. Vaidya {\em et al.}, Science {\bf 368}, 160 (2020)] on spin pumping from sub-THz radiation-driven uniaxial antiferromagnetic insulator (AFI) MnF$_2$ into heavy metal (HM) Pt hosting strong spin-orbit (SO) coupling, we compute and compare pumped spin currents in Cu/MnF$_2$/Cu and Pt/MnF$_2$/Cu heterostructures. Recent theories of spin pumping by AFI have relied on simplistic Hamiltonians (such as tight-binding) and the scattering approach to quantum transport yielding the so-called interfacial spin mixing conductance (SMC), but the concept of SMC ceases to be applicable when SO coupling is present directly at the interface. In contrast, we use more general first-principles quantum transport approach which combines noncollinear density functional theory with Floquet-nonequilibrium Green's functions in order to take into account: {\em SO-proximitized AFI} as a new type of quantum material, different from isolated AFI and brought about by AFI hybridization with adjacent HM layer; SO coupling at interfaces; and evanescent wavefunctions penetrating from Pt or Cu into AFI layer to make its interfacial region {\em conducting} rather than insulating as in the original AFI. The DC component of pumped spin current $I_\mathrm{DC}^{S_z}$ vs. precession cone angle $θ_{\vb*{l}}$ of the Néel vector $\vb*{l}$ of AFI {\em does not} follow putative $I^{S_z}_\mathrm{DC} \propto \sin^2 θ_{\vb*{l}}$, except for very small angles $θ_{\vb*{l}} \lesssim 10^\circ$ for which we can define an {\em effective} SMC from the prefactor and find that it doubles from MnF$_2$/Cu to MnF$_2$/Pt interface. In addition, the angular dependence $I^{S_z}_\mathrm{DC}(θ_{\vb*{l}})$ differs for opposite directions of precession of the Néel vector, leading to twice as large SMC for the right-handed than for the left-handed chirality of the precession mode.

cond-mat.mes-hall↗

arXiv2021

Wide ferromagnetic domain walls can host both adiabatic reflectionless spin transport and finite nonadiabatic spin torque: A time-dependent quantum transport picture

The key concept in spintronics of current-driven noncollinear magnetic textures, such as magnetic domain walls (DWs), is adiabaticity, i.e., how closely electronic spins track classical localized magnetic moments (LMMs) of the texture. When mistracking occurs nonadiabatic effects arise, the salient of which is nonadiabatic spin transfer torque (STT) where spin angular momentum is exchanged between electrons and LMMs to cause their dynamics and enable DW motion without any current threshold. The microscopic mechanisms behind nonadiabatic STT have been debated theoretically for nearly two decades, but with unanimous conclusion that they should be significant only in narrow DWs. However, this contradicts sharply experiments [O. Boulle {\em et al.}, Phys. Rev. Lett. {\bf 101}, 216601 (2008); C. Burrowes {\em et al.}, Nat. Phys. {\bf 6}, 17 (2010)] observing nonadiabatic STT in DWs much wider than putatively relevant $\sim 1$ nm scale, as well as largely insensitive to further increasing the DW width $w$. Here we employ time-dependent quantum transport for electrons to obtain both nonadiabatic and adiabatic STT from the exact nonequilibrium density matrix and its lowest order as adiabatic density matrix defined by assuming that LMMs are infinitely slow. This allows us to demonstrate that our microscopically, and without any simplifications of prior derivations like effectively static DW, extracted nonadiabatic STT: (i) does not decay, but instead saturates at a finite value, with increasing $w$ of a moving DW ensuring entry into the adiabatic limit, which we characterize by showing that electronic spins do not reflect from the static DW in this limit; and (ii) it has both out-of-DW-plane, as is the case of phenomenological expression widely used in the LLG equation, and in-plane components, where the former remains finite with increasing $w$.

cond-mat.mes-hall↗

arXiv2021

When can localized spins interacting with conduction electrons in ferro- or antiferromagnets be described classically via the Landau-Lifshitz equation: Transition from quantum many-body entangled to quantum-classical nonequilibrium states

Experiments in spintronics and magnonics operate with macroscopically large number of localized spins within ferromagnetic (F) or antiferromagnetic (AF) materials, so that their nonequilibrium dynamics is standardly described by the Landau-Lifshitz (LL) equation treating localized spins as classical vectors of fixed length. However, spin is a genuine quantum degree of freedom, and even though quantum effects become progressively less important for spin value $S>1$, they exist for all $S < \infty$. While this has motivated exploration of limitations/breakdown of the LL equation, by using examples of F insulators, analogous comparison of fully quantum many-body vs. quantum (for electrons)-classical (for localized spins) dynamics in systems where nonequilibrium conduction electrons are present is lacking. Here we employ quantum Heisenberg F or AF chains of $N=4$ sites, whose localized spins interact with conduction electrons via $sd$ exchange interaction, to perform such comparison by starting from unentangled pure (at zero temperature) or mixed (at finite temperature) quantum state of localized spins as the initial condition. This reveals that quantum-classical dynamics can faithfully reproduce fully quantum dynamics in the F metallic case, but only when spin $S$, Heisenberg exchange between localized spins and $sd$ exchange are sufficiently small. Increasing any of these three parameters can lead to substantial deviations, which are explained by the dynamical buildup of entanglement between localized spins and/or between them and electrons. In the AF metallic case, substantial deviations appear even at early times, despite starting from unentangled Néel state, which therefore poses a challenge on how to rigorously justify wide usage of the LL equation in phenomenological modeling of antiferromagnetic spintronics experiments.

cond-mat.str-el↗

arXiv2021

Quantum many-body states and Green functions of nonequilibrium electron-magnon systems: Localized spin operators vs. their mapping to Holstein-Primakoff bosons

The operators of localized spins within a magnetic material commute at different sites of its lattice and anticommute on the same site, so they are neither fermionic nor bosonic operators. Thus, to construct diagrammatic many-body perturbation theory, the spin operators are usually mapped to the bosonic ones with Holstein-Primakoff (HP) transformation being the most widely used in magnonics and spintronics literature. However, to make calculations tractable, the square root of operators in the HP transformation is expanded into a Taylor series truncated to some low order. This poses a question on the range of validity of truncated HP transformation when describing nonequilibrium dynamics of localized spins interacting with each other or with conduction electron spins. Here we apply exact diagonalization techniques to Hamiltonian of fermions (i.e., electrons) interacting with HP bosons vs. Hamiltonian of fermions interacting with the original localized spin operators in order to compare their many-body states and one-particle equilibrium or nonequilibrium Green functions. The Hamiltonian of fermions interacting with HP bosons gives incorrect ground state and electronic spectral function, unless large number of terms are retained in truncated HP transformation. Furthermore, tracking nonequilibrium dynamics of localized spins over longer time intervals requires progressively larger number of terms in truncated HP transformation. Finally, we show that recently proposed [M. Vogl et al., Phys. Rev. Research 2, 043243 (2020); J. König et al., SciPost Phys. 10, 007 (2021)] resummed HP transformation resolves the trouble with truncated HP transformation, while allowing us to derive an exact (manifestly Hermitian) Hamiltonian consisting of finite and fixed number of boson-boson and electron-boson interacting terms.

cond-mat.str-el↗

arXiv2021

Annihilation of topological solitons in magnetism with spin wave burst finale: The role of nonequilibrium electrons causing nonlocal damping and spin pumping over ultrabroadband frequency range

We not only reproduce burst of short-wavelength spin waves (SWs) observed in recent experiment [S. Woo et al., Nat. Phys. 13, 448 (2017)] on magnetic-field-driven annihilation of two magnetic domain walls (DWs) but, furthermore, we predict that this setup additionally generates highly unusual} pumping of electronic spin currents in the absence of any bias voltage. Prior to the instant of annihilation, their power spectrum is ultrabroadband, so they can be converted into rapidly changing in time charge currents, via the inverse spin Hall effect, as a source of THz radiation of bandwidth $\simeq 27$ THz where the lowest frequency is controlled by the applied magnetic field. The spin pumping stems from time-dependent fields introduced into the quantum Hamiltonian of electrons by the classical dynamics of localized magnetic moments (LMMs) comprising the domains. The pumped currents carry spin-polarized electrons which, in turn, exert backaction on LMMs in the form of nonlocal damping which is more than twice as large as conventional local Gilbert damping. The nonlocal damping can substantially modify the spectrum of emitted SWs when compared to widely-used micromagnetic simulations where conduction electrons are completely absent. Since we use fully microscopic (i.e., Hamiltonian-based) framework, self-consistently combining time-dependent electronic nonequilibrium Green functions with the Landau-Lifshitz-Gilbert equation, we also demonstrate that previously derived phenomenological formulas miss ultrabroadband spin pumping while underestimating the magnitude of nonlocal damping due to nonequilibrium electrons.

cond-mat.mes-hall↗

arXiv2021

Quantum spin torque driven transmutation of antiferromagnetic Mott insulator

The standard model of spin-transfer torque (STT) in antiferromagnetic spintronics considers exchange of angular momentum between quantum spins of flowing electrons and noncollinear-to-them localized spins treated as classical vectors. These vectors are assumed to realize Néel order in equilibrium, $\uparrow \downarrow \ldots \uparrow \downarrow$, and their STT-driven dynamics is described by the Landau-Lifshitz-Gilbert (LLG) equation. However, many experimentally employed materials (such as archetypal NiO) are strongly electron-correlated antiferromagnetic Mott insulators (AFMI) where localized spins form a ground state quite different from the unentangled Néel state $|\!\! \uparrow \downarrow \ldots \uparrow \downarrow \rangle$. The true ground state is entangled by quantum spin fluctuations, leading to expectation value of all localized spins being zero, so that LLG dynamics of classical vectors of fixed length rotating due to STT cannot even be initiated. Instead, a fully quantum treatment of both conduction electrons and localized spins is necessary to capture exchange of spin angular momentum between them, denoted as quantum STT. We use a recently developed time-dependent density matrix renormalization group approach to quantum STT to predict how injection of a spin-polarized current pulse into a normal metal layer coupled to AFMI overlayer via exchange interaction and possibly small interlayer hopping -- which mimics, e.g., topological-insulator/NiO bilayer employed experimentally -- will induce nonzero expectation value of AFMI localized spins. This new nonequilibrium phase is a spatially inhomogeneous ferromagnet with zigzag profile of localized spins. The total spin absorbed by AFMI increases with electron-electron repulsion in AFMI, as well as when the two layers do not exchange any charge.

cond-mat.str-el↗

arXiv2020

Magnon versus electron mediated spin-transfer torque exerted by spin currents across antiferromagnetic insulator to switch magnetization of adjacent ferromagnetic metal

The recent experiment [Y. Wang et al., Science 366, 1125 (2019)] on magnon-mediated spin-transfer torque (MSTT) was interpreted in terms of a picture where magnons are excited within an antiferromagnetic insulator (AFI), by applying nonequilibrium electronic spin density at one of its surfaces, so that their propagation across AFI deprived of conduction electrons eventually leads to reversal of magnetization of a ferromagnetic metal (FM) attached to the opposite surface of AFI. We employ a recently developed time-dependent nonequilibrium Green functions combined with the Landau-Lifshitz-Gilbert equation (TDNEGF+LLG) formalism to evolve conduction electrons quantum-mechanically while they interact via self-consistent back-action with localized magnetic moments described classically by atomistic spin dynamics solving a system of LLG equations. Upon injection of square current pulse as the initial condition, TDNEGF+LLG simulations of FM-polarizer/AFI/FM-analyzer junctions show that reversal of localized magnetic moments within FM-analyzer is less efficient, in the sense of requiring larger pulse height and its longer duration, than conventional electron-mediated STT (ESTT) driving magnetization switching in standard FM-polarizer/normal-metal/FM-analyzer spin valve. Since both electronic, generated by spin pumping from AFI, and magnonic, generated by direct transmission from AFI, spin currents are injected into the FM-analyzer, its localized magnetic moments will experience combined MSTT and ESTT. Nevertheless, by artificially turning off ESTT we demonstrate that MSTT plays a dominant role whose understanding, therefore, paves the way for all-magnon-driven magnetization switching devices with no electronic parts.

cond-mat.mes-hall↗

arXiv2020

Scattering-induced and highly tunable by gate damping-like spin-orbit torque in graphene doubly proximitized by two-dimensional magnet Cr$_2$Ge$_2$Te$_6$ and WS$_2$

Graphene sandwiched between semiconducting monolayers of ferromagnet Cr$_2$Ge$_2$Te$_6$ and transition-metal dichalcogenide WS$_2$ acquires both spin-orbit (SO), of valley-Zeeman and Rashba types, and exchange couplings. Using first-principles combined with quantum transport calculations, we predict that such doubly proximitized graphene within van der Waals heterostructure will exhibit SO torque driven by unpolarized charge current. This system lacking spin Hall current, putatively considered to be necessary for efficient damping-like (DL) SO torque that plays a key role in magnetization switching, demonstrates how DL torque component can be generated solely by skew-scattering off spin-independent potential barrier or impurities in purely two-dimensional electronic transport due to the presence of proximity SO coupling and its spin texture tilted out-of-plane. This leads to current-driven nonequilibrium spin density emerging in all spatial directions, whose cross product with proximity magnetization yields DL SO torque, unlike the ballistic regime with no scatterers in which only field-like (FL) SO torque appears. In contrast to SO torque on conventional metallic ferromagnets in contact with three dimensional SO-coupled materials, the ratio of FL and DL torque can be tuned by more than an order of magnitude via combined top and back gates.

cond-mat.mes-hall↗

arXiv2020

Robustness of quantized transport through edge states of finite length: Imaging current density in Floquet topological vs. quantum spin and anomalous Hall insulators

The theoretical analysis of topological insulators (TIs) has been traditionally focused on infinite homogeneous crystals with band gap in the bulk and nontrivial topology of their wavefunctions, or infinite wires whose boundaries host surface or edge metallic states. However, experimental devices contain finite-size topological region attached to normal metal (NM) leads, which poses a question about how precise is quantization of longitudinal conductance and how electrons transition from topologically trivial NM leads into the edge states. This is particularly pressing issues for recently conjectured two-dimensional (2D) Floquet TI where electrons flow from time-independent NM leads into time-dependent edge states---the very recent experimental realization of Floquet TI using graphene irradiated by circularly polarized light did not exhibit either quantized longitudinal or Hall conductance. Here we employ charge conserving solution for Floquet-nonequlibrium Green functions (NEGFs) of irradiated graphene nanoribbon to compute longitudinal two-terminal conductance, as well as spatial profiles of local current density as electrons propagate from NM leads into the Floquet TI. In the case of Floquet TI both bulk and edge local current densities contribute equally to total current, which leads to longitudinal conductance below the expected quantized plateau that is slightly reduced by edge vacancies. We propose two experimental schemes to detect coexistence of bulk and edge current densities within Floquet TI: (i) drilling a nanopore in the interior of irradiated region of graphene will induce backscattering of bulk current density, thereby reducing longitudinal conductance by $\sim 28$%; (ii) imaging of magnetic field produced by local current density using diamond NV centers.

cond-mat.mes-hall↗

arXiv2020

Spin-orbit-proximitized ferromagnetic metal by monolayer transition metal dichalcogenide: Atlas of spectral functions, spin textures and spin-orbit torques in Co/MoSe$_2$, Co/WSe$_2$ and Co/TaSe$_2$ heterostructures

The bilayer heterostructures composed of an ultrathin ferromagnetic metal (FM) and a material hosting strong spin-orbit (SO) coupling are principal resource for SO torque and spin-to-charge conversion nonequilibrium effects in spintronics. We demonstrate how hybridization of wavefunctions of Co layer and a monolayer of transition metal dichalcogenides (TMDs)---such as semiconducting MoSe$_2$ and WSe$_2$ or metallic TaSe$_2$---can lead to dramatic transmutation of electronic and spin structure of Co within some distance away from its interface with TMD, when compared to the bulk of Co or its surface in contact with vacuum. This is due to proximity induced SO splitting of Co bands encoded in the spectral functions and spin textures on its monolayers, which we obtain using noncollinear density functional theory (ncDFT) combined with equilibrium Green function (GF) calculations. In fact, SO splitting is present due to structural inversion asymmetry of the bilayer even if SO coupling within TMD monolayer is artificially switched off in ncDFT calculations, but switching it on makes the effects associated with proximity SO coupling within Co layer about five times larger. Injecting spin-unpolarized charge current through SO-proximitized monolayers of Co generates nonequilibrium spin density over them, so that its cross product with the magnetization of Co determines SO torque. The SO torque computed via first-principles quantum transport methodology, which combines ncDFT with nonequilibrium GF calculations, can be used as the screening parameter to identify optimal combination of materials and their interfaces for applications in spintronics. In particular, we identify heterostructure two-monolayer-Co/monolayer-WSe$_2$ as the most optimal.

cond-mat.mtrl-sci↗

arXiv2020

Spintronics meets nonadiabatic molecular dynamics: Geometric spin torque and damping on noncollinear classical magnetism due to electronic open quantum system

We analyze a quantum-classical hybrid system of steadily precessing slow classical localized magnetic moments, forming a head-to-head domain wall, embedded into an open quantum system of fast nonequilibrium electrons. The electrons reside within a metallic wire connected to macroscopic reservoirs. The model captures the essence of dynamical noncollinear and noncoplanar magnetic textures in spintronics, while making it possible to obtain the exact time-dependent nonequilibrium density matrix of electronic system and split it into four contributions. The Fermi surface contribution generates dissipative (or damping-like in spintronics terminology) spin torque on the moments, and one of the two Fermi sea contributions generates geometric torque dominating in the adiabatic regime. When the coupling to the reservoirs is reduced, the geometric torque is the only nonzero contribution. Locally it has both nondissipative (or field-like in spintronics terminology) and damping-like components, but with the sum of latter being zero, which act as the counterparts of geometric magnetism force and electronic friction in nonadiabatic molecular dynamics. Such current-independent geometric torque is absent from widely used micromagnetics or atomistic spin dynamics modeling of magnetization dynamics based on the Landau-Lifshitz-Gilbert equation, where previous analysis of Fermi surface-type torque has severely underestimated its magnitude.

cond-mat.mes-hall↗

arXiv2020

Magnon-driven chiral charge and spin pumping and electron-magnon scattering from time-dependent quantum transport combined with atomistic spin dynamics theory

Using newly developed quantum-classical hybrid framework, we investigate interaction between spin-polarized conduction electrons and a single spin wave (SW) coherently excited within a metallic ferromagnetic nanowire. When the nanowire hosting SW is attached to two normal metal (NM) leads, with no dc bias voltage applied between them, the SW pumps chiral electronic charge and spin currents into the leads---their direction is tied to the direction of SW propagation and they scale linearly with the frequency of the precession. This is in contrast to: standard pumping by the uniform precession mode with identical spin currents flowing in both directions and no accompanying charge current; or experimentally observed [C.~Ciccarelli et al., Nat. Nanotech. 10, 50 (2014); M.~Evelt et al., Phys. Rev. B 95, 024408 (2017)] magnonic charge pumping which requires spin-orbit coupling spin-orbit coupling. The mechanism behind our prediction is nonadiabaticity due to time-retardation effects---motion of localized magnetic moment affects conduction electron spin in a retarded way, so that it takes a finite time until the electron spin reacts to the motion of the classical vector. Upon injecting dc spin-polarized charge current from the left NM lead, electrons interact with SW where outflowing charge and spin current into the right NM lead are changed due to both scattering off time-dependent potential generated by the SW and superposition with the currents pumped by the SW itself. Using Lorentzian voltage pulse to excite leviton out of the Fermi sea, which carries one electron charge with no accompanying electron-hole pairs and behaves as soliton-like quasiparticle, we describe how a single electron interacts with a single SW.

cond-mat.mes-hall↗

arXiv2019

Effective spin-mixing conductance of topological-insulator/ferromagnet and heavy-metal/ferromagnet spin-orbit-coupled interfaces: A first-principles Floquet-nonequilibrium-Green-function approach

The spin mixing conductance (SMC) is a key quantity determining efficiency of spin transport across interfaces. Thus, knowledge of its precise value is required for accurate measurement of parameters quantifying numerous effects in spintronics, such as spin-orbit torque, spin Hall magnetoresistance, spin Hall effect and spin pumping. However, the standard expression for SMC, provided by the scattering theory in terms of the reflection probability amplitudes, is inapplicable when strong spin-orbit coupling (SOC) is present directly at the interface. This is the precisely the case of topological-insulator/ferromagnet and heavy-metal/ferromagnet interfaces of great contemporary interest. We introduce an approach where first-principles Hamiltonian of these interfaces, obtained from noncollinear density functional theory (ncDFT) calculations, is combined with charge conserving Floquet-nonequilibrium-Green-function formalism to compute {\em directly} the pumped spin current $I^{S_z}$ into semi-infinite left lead of two-terminal heterostructures Cu/X/Co/Cu or Y/Co/Cu---where X=Bi$_2$Se$_3$ and Y=Pt or W---due to microwave-driven steadily precessing magnetization of the Co layer. This allows us extract an effective SMC as a prefactor in $I^{S_z}$ vs. precession cone angle $θ$ dependence, as long as it remains the same, $I^{S_z} \propto \sin^2 θ$, as in the case where SOC is absent. By comparing calculations where SOC in switched off vs. switched on in ncDFT calculations, we find that SOC consistently reduces the pumped spin current and, therefore, the effective SMC.

cond-mat.mes-hall↗

arXiv2019

First-principles theory of proximity spin-orbit torque on a two-dimensional magnet: Current-driven antiferromagnet-to-ferromagnet reversible transition in bilayer CrI$_3$

The recently discovered two-dimensional (2D) magnetic insulator CrI$_3$ is an intriguing case for basic research and spintronic applications since it is a ferromagnet in the bulk, but an antiferromagnet in bilayer form, with its magnetic ordering amenable to external manipulations. Using first-principles quantum transport approach, we predict that injecting unpolarized charge current parallel to the interface of bilayer-CrI$_3$/monolayer-TaSe$_2$ van der Waals heterostructure will induce spin-orbit torque (SOT) and thereby driven dynamics of magnetization on the first monolayer of CrI$_3$ in direct contact with TaSe$_2$. By combining calculated complex angular dependence of SOT with the Landau-Lifshitz-Gilbert equation for classical dynamics of magnetization, we demonstrate that current pulses can switch the direction of magnetization on the first monolayer to become parallel to that of the second monolayer, thereby converting CrI$_3$ from antiferromagnet to ferromagnet while not requiring any external magnetic field. We explain the mechanism of this reversible current-driven nonequilibrium phase transition by showing that first monolayer of CrI$_3$ carries current due to evanescent wavefunctions injected by metallic transition metal dichalcogenide TaSe$_2$, while concurrently acquiring strong spin-orbit coupling (SOC) via such proximity effect, whereas the second monolayer of CrI$_3$ remains insulating. The transition can be detected by passing vertical read current through the vdW heterostructure, encapsulated by bilayer of hexagonal boron nitride and sandwiched between graphite electrodes, where we find tunneling magnetoresistance of $\simeq 240$%.

cond-mat.mes-hall↗