arXiv subjects

Rembert A. Duine

Publications and source records attributed to Rembert A. Duine.

At least 37 records · Page 2Linked to original sources

arXiv2024

Synthetic Altermagnets

Altermagnets, a distinct class of antiferromagnets with electronic structures resembling those of d-wave superconductors, exhibit intriguing properties that have gained significant attention in recent research. In this article, we propose synthetic altermagnets, composed of two anisotropic ferromagnetic layers arranged such that the total net magnetization is zero. We investigate the properties of these synthetic altermagnets, focusing on their electronic band structures, spin current, Berry curvature, and anomalous Hall conductivity. By developing a minimal model for our synthetic altermagnets, we examine the influence of factors such as anisotropic coupling strengths and spin-orbit coupling on the physical phenomena altermagnets manifest. Our findings open a new path for the realization of altermagnetic materials experimentally and highlight their potential applications in magneto-electronics, magneto-optics, and spintronics devices.

cond-mat.mtrl-sci↗

arXiv2024

Position operators and interband matrix elements of scalar and vector potentials in the 8-band Kane model

We diagonalize the 8-band Kane Hamiltonian with a proper inclusion of the interband matrix elements of the scalar and vector potentials. This leads, among other results, to a modification of the conventional expression for the spin-orbit coupling (SOC) strength in narrow-gap semiconductors with the zinc blende symmetry. We find that in GaAs, at low temperatures, the correct expression for the SOC strength is actually twice as large as usually considered. In InSb it is $1.76$ times larger. We also provide a proper treatment of the interband matrix elements of the position operator. We show that the velocity operator in a crystal should be defined as a time-derivative of a fictitious position operator rather than the physical one. We compute the expressions for both these position operators projected to the conduction band of the 8-band Kane model. We also derive an expression for the projected velocity operator and demonstrate that the SOC strength in it differs from the SOC strength in the Hamiltonian. The ratio between them is not equal to $1$, as it is often assumed for the Rashba model. It does not equal $2$ either. The correct result for this ratio is given by a rational function of the parameters of the model. This function takes values between $4(23+3\sqrt{2})/73\approx 1.49$ and $2$. Our findings modify a vast number of research results obtained using the Rashba model and provide a path for a consistent treatment of the latter in future applications.

cond-mat.mes-hall↗

arXiv2024

Single-mode spin-wave laser driven by spin-orbit torque

A central goal in spintronics and magnonics is the use of spin waves rather than electrons for efficient information processing. The key to integrate such spintronic circuits with electronic circuits is the ability to inject, control and detect coherent spin waves with charge currents. Here, we propose a tunable setup consisting of a synthetic antiferromagnet in an inhomogeneous magnetic field in which one of the magnetic layers is thin and biased by spin-orbit torque. We show that for appriopriate conditions single-mode coherent spin waves are emitted in this set-up. The set-up implements coupling of continuum spin waves with a finite region of negative energy spin waves, such that specific frequencies become self-amplified and thus start lasing. We show there exist a large region in parameter space for which the coherent spin wave laser is stabilized by non-linearities and spin-orbit torques. Our findings may lead to new ways of injecting coherent spin waves with direct currents.

cond-mat.mes-hall↗

arXiv2024

Electrical Non-Hermitian Control of Topological Magnon Spin Transport

Magnonic topological phases realize chiral edge spin waves that are protected against backscattering, potentially enabling highly efficient spin transport. Here we show that the spin transport through these magnonic chiral edge states can be electrically manipulated by non-Hermitian control. We consider the paradigmatic magnon Haldane model and show that it is transformed into an effective non-Hermitian magnon Chern insulator by including a sublattice-dependent spin-orbit torque. In linear spin-wave theory, this electrically induced torque reduces the damping of the chiral edge magnons along certain edge directions, leading to an enhancement of the spin-wave amplitude. This prediction is confirmed by numerical simulations based on the Landau-Lifshitz-Gilbert equation. For a spin-wave transport setup, in which magnons are excited by a microwave field and detected with a normal metal conductor, we find that the magnon amplification is remarkably robust against disorder, establishing non-Hermitian control as a promising avenue for topological magnonics.

cond-mat.mes-hall↗

arXiv2024

Entangled magnon-pair generation in a driven synthetic antiferromagnet

Understanding, manipulating, and using magnons - the quanta of spin waves - for energy-efficient applications is one of the primary goals of magnonics. In this paper, we consider a synthetic antiferromagnet in which one of the ferromagnetic layers is driven by spin-orbit torque. We find that under specific conditions for the magnitude of the spin-orbit torque and field, magnon pairs are spontaneously produced by quantum fluctuations in a way that is similar to Hawking pair production near black-hole horizons. One of the magnons is generated near the interface with the spacer layer in one of the magnetic layers of the synthetic antiferromagnet, while the other magnon is produced in the other magnetic layer. We compute the magnon current due to these spontaneously generated magnon pairs and estimate the temperature below which they should become observable. Additionally, we find that the magnons are entangled, which makes them interesting for future applications in quantum magnonics.

cond-mat.mes-hall↗

arXiv2024

Negative-energy spin waves in antiferromagnets for spin-current amplification and analogue gravity

Magnonic black holes-analogue event horizons for the spin-wave collective excitations of ordered magnets-can be used for fundamental research, for example for investigating Hawking radiation, but also for technological applications of spin waves. Here we show how to engineer a horizon for spin waves in antiferromagnets, which have the attractive feature of fast magnetization dynamics and linear dispersion relation. We propose a set-up with spatially varying exchange interaction with spin transfer torque to implement the horizon and a second set-up for the amplification of spin waves consisting of an antiferromagnet subject to a spatially varying external magnetic field that is driven by spin orbit torque. We compute the values of parameters needed to implement the horizon and to have amplification of spin waves. We develop the corresponding Klein-Gordon equation and quantify the amplification. Our work paves the way for investigation of Hawking radiation of spin waves and for antiferromagnet-based spin-waves amplifiers.

cond-mat.mes-hall↗

arXiv2024

Magnon spin capacitor

In this work we show that a magnon spin capacitor can be realized at a junction between two exchange coupled ferromagnets. In this junction, the buildup of magnon spin over the junction is coupled to the difference in magnon chemical potential, realizing the magnon spin analogue of an electrical capacitor. The relation between magnon spin and magnon chemical potential difference directly follows from considering the magnon density-density interaction between the two ferromagnets. We analyse the junction in detail by considering spin injection and detection from normal metal leads, the tunneling current across the junction and magnon decay within the ferromagnet, showing that such a structure realizes a magnon spin capacitor in series with a spin resistor. Choosing yttrium iron garnet as the ferromagnet, we numerically calculate the capacitance, which ranges from picofarad to microfarad, depending on the area of the junction. We therefore conclude that the magnon spin capacitor could directly be of use in applications.

cond-mat.mes-hall↗

arXiv2024

Kubo formula for dc conductivity: generalization to systems with spin-orbit coupling

We revise the Kubo formula for the electric dc conductivity in the presence of spin-orbit coupling (SOC). We discover that each velocity operator that enters this formula differs from $\partial H/\partial \boldsymbol p$, where $H$ is the Hamiltonian and $\boldsymbol p$ is the canonical momentum. Moreover, we find an additional contribution to the Hall dc conductivity from noncommuting coordinates that is missing in the conventional Kubo-Streda formula. This contribution originates from the "electron-positron" matrix elements of the velocity and position operators. We argue that the widely used Rashba model does in fact provide a finite anomalous Hall dc conductivity in the metallic regime (in the noncrossing approximation) if SOC-corrections to the velocity and position operators are properly taken into account. While we focus on the response of the charge current to the electric field, linear response theories of other SOC-related effects should be modified similarly.

cond-mat.mes-hall↗

arXiv2023

Charge conservation in spin torque oscillators leads to a self-induced torque

Spin torque oscillators are conventionally described by the Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation. However, at the onset of oscillations, the predictions of the conventional LLGS equation differ qualitatively from experimental results and thus appear to be incomplete. In this work we show that taking charge conservation into account leads to a previously-overlooked self-induced torque, which modifies the LLGS equation. We show that the self-induced torque originates from the pumping current that a precessing magnetization drives through a magnetic tunnel junction. To illustrate the importance of the self-induced torque, we consider an in-plane magnetized nanopillar, where it gives clear qualitative corrections to the conventional LLGS description.

cond-mat.mes-hall↗

arXiv2023

Zero-frequency chiral magnonic edge states protected by non-equilibrium topology

Topological bosonic excitations must, in contrast to their fermionic counterparts, appear at finite energies. This is a key challenge for magnons, as it prevents straightforward excitation and detection of topologically-protected magnonic edge states and their use in magnonic devices. In this work, we show that in a non-equilibrium state, in which the magnetization is pointing against the external magnetic field, the topologically-protected chiral edge states in a magnon Chern insulator can be lowered to zero frequency, making them directly accessible by existing experimental techniques. We discuss the spin-orbit torque required to stabilize this non-equilibrium state, and show explicitly using numerical Landau-Lifshitz-Gilbert simulations that the edge states can be excited with a microwave field. Finally, we consider a propagating spin wave spectroscopy experiment, and demonstrate that the edge states can be directly detected.

cond-mat.mes-hall↗

arXiv2023

Oscillatory Edge Modes in Two Dimensional Spin-Torque Oscillator Arrays

Spin torque oscillators (STOs) are dissipative magnetic systems that provide a natural platform for exploring non-Hermitian phenomena. We theoretically study a two-dimensional (2d) array of STOs and show that its dynamics can be mapped to a 2d, non-Hermitian Su-Schrieffer-Heeger (SSH) model. We calculate the energy spectrum and identify the one-dimensional (1d) edge states of our model, corresponding to auto-oscillation of STOs on the boundary of the system while the bulk oscillators do not activate. We show that tuning the Gilbert damping, injected spin current, and coupling between STOs allows for exploring the edge state properties under different parameter regimes. Furthermore, this system admits 1d edge states with non-uniform probability density, and we explore their properties in systems of different sizes. Additional symmetry analysis indicates that these states are not topologically protected but are nevertheless confined to the edge of the system, as the bulk is protected by PT-symmetry. These results indicate that 2d arrays of STOs may be useful to explore novel edge state behavior in dissipative systems.

cond-mat.mes-hall↗

arXiv2023

Magnon bundle in a strongly dissipative magnet

Hybrid quantum systems based on magnetic platforms have witnessed the birth and fast development of quantum spintronics. Until now, most of the studies rely on magnetic excitations in low-damping magnetic insulators, particularly yttrium iron garnet, while a large class of magnetic systems is ruled out in this interdisciplinary field. Here we propose the generation of a magnon bundle in a hybrid magnet-qubit system, where two or more magnons are emitted simultaneously. By tuning the driving frequency of qubit to match the detuning between magnon and qubit mode, one can effectively generate a magnon bundle via super-Rabi oscillations. In contrast with general wisdom, magnetic dissipation plays an enabling role in generating the magnon bundle, where the relaxation time of magnons determines the typical time delay between two successive magnons. The maximal damping that allows an antibunched magnon bundle can reach the order of 0.1, which may break the monopoly of low-dissipation magnetic insulators in quantum spintronics and enables a large class of magnetic materials for quantum manipulation. Further, our finding may provide a scalable and generic platform to study multi-magnon physics and benefit the design of magnonic networks for quantum information processing.

cond-mat.mes-hall↗

arXiv2022

The fractional Landau-Lifshitz-Gilbert equation

The dynamics of a magnetic moment or spin are of high interest to applications in technology. Dissipation in these systems is therefore of importance for improvement of efficiency of devices, such as the ones proposed in spintronics. A large spin in a magnetic field is widely assumed to be described by the Landau-Lifshitz-Gilbert (LLG) equation, which includes a phenomenological Gilbert damping. Here, we couple a large spin to a bath and derive a generic (non-)Ohmic damping term for the low-frequency range using a Caldeira-Leggett model. This leads to a fractional LLG equation, where the first-order derivative Gilbert damping is replaced by a fractional derivative of order $s \ge 0$. We show that the parameter $s$ can be determined from a ferromagnetic resonance experiment, where the resonance frequency and linewidth no longer scale linearly with the effective field strength.

cond-mat.mes-hall↗

arXiv2022

Antimagnonics

Magnons are the quanta of collective spin excitations in magnetically-ordered systems and manipulation of magnons for computing and information processing has witnessed the development of ``magnonics". A magnon corresponds to an excitation of the magnetic system from its ground state and the creation of a magnon thus increases the total energy of the system. In this perspective, we introduce the antiparticle of a magnon, dubbed the antimagnon, as an excitation that lowers the magnetic energy. We investigate the stability and thermal occupation of antimagnons and verify our theory by micromagnetic simulations. Furthermore, we show how the concept of antimagnons yields a unified picture to understand the magnonic analog of the Klein effect, magnonic black-hole horizons, and magnonic black-hole lasing. Our work may stimulate fundamental interest in antimagnons, as well as their applications to spintronic devices.

cond-mat.mes-hall↗

arXiv2022

Disentangling electrical switching of antiferromagnetic NiO using high magnetic fields

Recent demonstrations of the electrical switching of antiferromagnets (AFs) have given an enormous impulse to the field of AF spintronics. Many of these observations are plagued by non-magnetic effects that are very difficult to distinguish from the actual magnetic ones. Here, we study the electrical switching of thin (5 nm) NiO films in Pt/NiO devices using magnetic fields up to 15 T to quantitatively disentangle these magnetic and non-magnetic effects. We demonstrate that these fields suppress the magnetic components of the electrical switching of NiO, but leave the non-magnetic components intact. Using a monodomainization model the contributions are separated, showing how they behave as a function of the current density. These results show that combining electrical methods and strong magnetic fields can be an invaluable tool for AF spintronics, allowing for implementing and studying electrical switching of AFs in more complex systems.

cond-mat.mes-hall↗

arXiv2022

Non-linear dynamics of the non-Hermitian Su-Schrieffer-Heeger model

We numerically determine the robustness of the lasing edge modes in a spin-torque oscillator array that realizes the non-Hermitian Su-Schrieffer-Heeger model. Previous studies found that the linearized dynamics can enter a topological regime in which the edge mode is driven into auto-oscillation, while the bulk dynamics are suppressed. Here we investigate the full non-linear and finite-temperature dynamics, whose understanding is essential for spin-torque oscillators-based applications. Our analysis shows that the lasing edge mode dynamics persist in the non-linear domain for a broad range of parameters and temperatures. We investigate the effects of perturbations relevant to experimental implementations and discuss which ones might be detrimental to the stability of the lasing edge mode. Finally, we map our model onto a photonic model. Our analysis has the potential to shed light onto the dynamics of a plethora of non-Hermitian systems with non-linearities.

cond-mat.mes-hall↗

arXiv2022

Master equation approach to magnon relaxation and dephasing

There has been a recent upsurge of interest in the quantum properties of magnons for quantum information processing. An important issue is to examine the stability of quantum states of magnons against various relaxation and dephasing channels. Since the interaction of magnons in magnetic systems may fall in the ultra-strong and even deep-strong coupling regimes, the relaxation process of magnon states is quite different from the more common quantum optical systems. Here we study the relaxation and dephasing of magnons based on the Lindblad formalism and derive a generalized master equation that describes the quantum dynamics of magnons. Employing this master equation, we identify two distinct dissipation channels for squeezed magnons, i.e., the local dissipation and collective dissipation, which play a role for both ferromagnets and antiferromagnets. The local dissipation is caused by the independent exchange of angular momentum between the magnonic system and the environment, while the collective dissipation is dressed by the parametric interactions of magnons and it enhances the quantumness and thermal stability of squeezed magnons. Further, we show how this formalism can be applied to study the pure dephasing of magnons caused by four-magnon scattering and magnon-phonon interactions. Our results provide the theoretical tools to study the decoherence of magnons within a full quantum-mechanical framework and further benefit the use of quantum states of magnons for information processing.

cond-mat.mes-hall↗

arXiv2022

Probing Laser-Induced Spin-Current Generation in Synthetic Ferrimagnets Using Spin Waves

Several rare-earth transition-metal ferrimagnetic systems exhibit all-optical magnetization switching upon excitation with a femtosecond laser pulse. Although this phenomenon is very promising for future opto-magnetic data storage applications, the role of non-local spin transport in these systems is scarcely understood. Using Co/Gd and Co/Tb bilayers we isolate the contribution of the rare-earth materials to the generated spin currents by using the precessional dynamics they excite in an adjacent ferromagnetic layer as a probe. By measuring THz standing spin-waves as well as GHz homogeneous precessional modes, we probe both the high- and low-frequency components of these spin currents. The low-frequency homogeneous mode indicates a significant contribution of Gd to the spin current, but not from Tb, consistent with the difficulty in achieving all-optical switching in Tb-containing systems. Measurements on the THz frequency spin waves reveal the inability of the rare-earth generated spin currents to excite dynamics at the sub-ps timescale. We present modelling efforts using the $s$-$d$ model, which effectively reproduce our results and allow us to explain the behavior in terms of the temporal profile of the spin current.

cond-mat.mes-hall↗