arXiv subjects

Kjetil M. D. Hals

Publications and source records attributed to Kjetil M. D. Hals.

At least 19 recordsLinked to original sources

arXiv2026

Tunable Phonon-Driven Magnon Spin Currents in Altermagnets

Altermagnets have recently attracted considerable interest due to their unique symmetry-governed spintronic properties. Here, we investigate phonon-induced magnon spin currents in a two-dimensional altermagnet. Starting from a microscopic theory of the coupled magnon-phonon system, we derive the nonequilibrium magnon distribution generated by selective phonon excitations. We show that the resulting spin currents exhibit a pronounced d-wave symmetry with respect to the phonon momentum. Moreover, the spin current along the altermagnetic directions can be completely reversed by tuning the phonon frequency. These findings establish altermagnets as promising platforms for realizing highly tunable, phonon-driven coherent terahertz magnon spin currents.

cond-mat.mes-hall↗

arXiv2026

Spin-Seebeck Signatures of Spin Chirality in Kagome Antiferromagnets

Non-collinear antiferromagnets (NCAFMs) are appealing for antiferromagnetic spintronics, as they combine the advantages of collinear antiferromagnets with novel emergent phenomena stemming from their complex spin structures. These phenomena are often associated with the intrinsic spin chirality, which characterizes the handedness of the ground-state spin configuration. Non-collinear antiferromagnets (NCAFMs) are appealing for antiferromagnetic spintronics, as they combine the advantages of collinear antiferromagnets with novel emergent phenomena stemming from their complex spin structures. These phenomena are often associated with the intrinsic spin chirality, which characterizes the handedness of the ground-state spin configuration. Here, we investigate a kagome NCAFM interfaced with a normal metal and demonstrate that the ground-state vector spin chirality can be probed through measurements of the spin Seebeck effect (SSE). Starting from a microscopic spin Hamiltonian, we derive the corresponding bosonic Bogoliubov-de Gennes Hamiltonians for the two chiral configurations. Using linear response theory, we obtain a general expression for the spin current thermally pumped into the normal metal by the SSE. We show that a sizable in-plane spin current emerges exclusively in the negative-chiral state, providing a direct signature for real-time detection of chirality switching in kagome NCAFMs. In addition, we find a field-dependent out-of-plane spin current whose magnitude differs between the two chiralities by about 81%, reflecting their distinct magnon band structures.

cond-mat.mtrl-sci↗

arXiv2025

Strain-induced manipulation of non-collinear antiferromagnets

In recent years, there has been growing interest in harnessing non-collinear antiferromagnets (NCAFMs) for applications in antiferromagnetic spintronics. A key requirement for their practical use is the ability to control the spin order in a reliable and tunable manner. In this work, we investigate how the spin order in kagome antiferromagnets -- an important class of NCAFMs -- can be manipulated via strain. Starting from a microscopic spin Hamiltonian, we derive an effective action for the kagome antiferromagnet that captures the coupling between the spin order and the system's strain tensor. At the microscopic level, this coupling arises from strain-induced modifications of the Dzyaloshinskii-Moriya and exchange interactions. Using this effective description, we explore two strain-driven phenomena: (1) strain-induced switching of the antiferromagnetic spin order and (2) the piezomagnetic response. We numerically show that strain facilitates thermally assisted switching between spin configurations of opposite chirality. Specifically, we find that uniform tensile and compressive strain govern both the average switching time and the preferred switching direction between chiral states. Furthermore, we demonstrate that strain induces a net magnetization and provide an experimentally testable prediction of this effect for a typical NCAFM. Our results provide a theoretical framework for modeling strain-induced manipulation of kagome antiferromagnets, underscoring strain as a promising route for functional control of NCAFMs.

cond-mat.mes-hall↗

arXiv2025

Amplifying the antiferromagnetic spin Seebeck effect through topological magnons

Topological magnons emerge as topologically protected spin wave states at the edges of magnets. Here, we theoretically explore how these surface states can be harnessed to amplify the spin Seebeck effect (SSE) in antiferromagnets (AFMs) interfaced with normal metals (NMs). Based on a microscopic model of a kagome AFM, we demonstrate that broken mirror symmetry, combined with the Dzyaloshinskii-Moriya interaction (DMI), drives the system into a topological phase hosting spin-polarized magnons at the boundaries. Notably, linear response calculations reveal that in AFM/NM heterostructures, the topological magnons exhibit strong coupling to the metal's charge carriers, resulting in a substantial enhancement of the SSE. The relative contribution of the topological magnons is found to be 4-5 times greater than that of the trivial magnon bands. Moreover, our results show that this enhancement is highly sensitive to the strength of the DMI.

cond-mat.mes-hall↗

arXiv2023

Voltage-Controlled High-Bandwidth Terahertz Oscillators Based On Antiferromagnets

Producing compact voltage-controlled frequency generators and sensors operating in the terahertz (THz) regime represents a major technological challenge. Here, we show that noncollinear antiferromagnets (NCAFM) with kagome structure host gapless self-oscillations whose frequencies are tunable from 0 Hz to the THz regime via electrically induced spin-orbit torques (SOTs). The auto-oscillations' initiation, bandwidth, and amplitude are investigated by deriving an effective theory, which captures the reactive and dissipative SOTs. We find that the dynamics strongly depends on the ground state's chirality, with one chirality having gapped excitations, whereas the opposite chirality provides gapless self-oscillations. Our results reveal that NCAFMs offer unique THz functional components, which could play a significant role in filling the THz technology gap.

cond-mat.mes-hall↗

arXiv2023

Manipulating Vortices with Domain Walls in Superconductor-Ferromagnet Heterostructures

Vortices are point-like topological defects in superconductors whose motion dictates superconducting properties and controls device performance. In superconductor-ferromagnet heterostructures, vortices interact with topological defects in the ferromagnet such as line-like domain walls. While in previous heterostructure generations, vortex-domain wall interactions were mediated by stray fields; in new heterostructure families, more important become exchange fields and spin-orbit coupling. However, spin-orbit coupling's role in vortex-domain wall interactions remains unexplored. Here we uncover, via numerical simulations and Ginzburg-Landau theory, that Rashba spin-orbit coupling induces magnetoelectric interactions between vortices and domain walls that crucially depend on the wall's winding direction$-$its helicity. The wall's helicity controls whether vortices are pushed or dragged by Néel walls, and their gliding direction along Bloch walls. Our work capitalizes on interactions between topological defects from different order parameters and of different dimensionality to engineer enhanced functionality.

cond-mat.supr-con↗

arXiv2022

Dzyaloshinskii-Moriya Induced Spin-Transfer Torques in Kagome Antiferromagnets

In recent years antiferromagnets (AFMs) have become very promising for nanoscale spintronic applications due to their unique properties such as THz dynamics and absence of stray fields. Manipulating antiferromagnetic textures is currently, however, limited to very few exceptional material symmetry classes allowing for staggered torques on the magnetic sublattices. In this work, we predict for kagome AFMs with broken mirror symmetry a new coupling mechanism between antiferromagnetic domain walls (DWs) and spin currents, produced by the relativistic Dzyaloshinskii-Moriya interaction (DMI). We microscopically derive the DMI's free energy contribution for the kagome AFMs. Unlike ferromagnets and collinear AFMs, the DMI does not lead to terms linear in the spatial derivatives but instead renormalizes the spin-wave stiffness and anisotropy energies. Importantly, we show that the DMI induces a highly nontrivial twisted DW profile that is controllable via two linearly independent components of the spin accumulation. This texture manipulation mechanism goes beyond the concept of staggered torques and implies a higher degree of tunability for the current-driven DW motion compared to conventional ferromagnets and collinear AFMs.

cond-mat.mtrl-sci↗

arXiv2021

Spin pumping in noncollinear antiferromagnets

The ac spin pumping of noncollinear antiferromagnets is theoretically investigated. Starting from an effective action description of the spin system, we derive the Onsager coefficients connecting the spin pumping and spin-transfer torque associated with the dynamics of the SO(3)-valued antiferromagnetic order parameter. Our theory is applied to a kagome antiferromagnet resonantly driven by a uniform external magnetic field. We demonstrate that the reactive (dissipative) spin-transfer torque parameter can be extracted from the pumped ac spin current in-phase (in quadrature) with the driving field. Furthermore, we find that the three spin-wave bands of the kagome AF generate spin currents with mutually orthogonal polarization directions. This offers a unique way of controlling the spin orientation of the pumped spin current by exciting different spin-wave modes.

cond-mat.mes-hall↗

arXiv2021

Steering Majorana braiding via skyrmion-vortex pairs: a scalable platform

Majorana zero modes are quasiparticles that hold promise as building blocks for topological quantum computing. However, the litmus test for their detection, the observation of exotic non-abelian statistics revealed by braiding, has so far eluded experimental efforts. Here we take advantage of the fact that skyrmion-vortex pairs in superconductor-ferromagnet heterostructures harboring Majorana zero modes can be easily manipulated in two spatial dimensions. We adiabatically braid the hybrid topological structures and explicitly confirm the non-abelian statistics of the Majorana zero modes numerically using a self-consistent calculation of the superconducting order parameter. Our proposal of controlling skyrmion-vortex pairs provides the necessary leeway toward a scalable topological quantum computing platform.

cond-mat.str-el↗

arXiv2021

Spin-wave driven bidirectional domain wall motion in kagome antiferromagnets

We predict a mechanism to controllably manipulate domain walls in kagome antiferromagnets via a single linearly polarized spin-wave source. We show by means of atomistic spin dynamics simulations of antiferromagnets with kagome structure that the speed and direction of the domain wall motion can be regulated by only tuning the frequency of the applied spin-wave. Starting from microscopics, we establish an effective action and derive the corresponding equations of motion for the spin-wave-driven domain wall. Our analytical calculations reveal that the coupling of two spin-wave modes inside the domain wall explains the frequency-dependent velocity of the spin texture. Such a highly tunable spin-wave-induced domain wall motion provides a key component toward next-generation fast, energy-efficient, and Joule-heating-free antiferromagnetic insulator devices.

cond-mat.mes-hall↗

arXiv2020

Large Surface Magnetization in Noncentrosymmetric Antiferromagnets

Thin-film antiferromagnets (AFs) with Rashba spin-orbit coupling are theoretically investigated. We demonstrate that the relativistic Dzyaloshinskii-Moriya interaction (DMI) produces a large surface magnetization and a boundary-driven twist state in the antiferromagnetic N\' eel vector. We predict a magnetization on the order of $2.3\cdot 10^4$~A/m, which is comparable to the magnetization of ferromagnetic semiconductors. Importantly, the magnetization is characterized by ultra-fast terahertz dynamics and provides new approaches for efficiently probing and controlling the spin dynamics of AFs as well as detecting the antiferromagnetic DMI. Notably, the magnetization does not lead to any stray magnetic fields except at corners where weak magnetic monopole fields appear.

cond-mat.mes-hall↗

arXiv2018

Twists in Ferromagnetic Monolayers With Trigonal Prismatic Symmetry

Two-dimensional materials such as graphene or hexagonal boron nitride are indispensable in industry. The recently discovered 2D ferromagnetic materials also promise to be vital for applications. In this work, we develop a phenomenological description of non-centrosymmetric 2D ferromagnets with trigonal prismatic crystal structure. We chose to study this special symmetry group since these materials do break inversion symmetry and therefore, in principle, allow for chiral spin structures such as magnetic helices and skyrmions. However, unlike all non-centrosymmetric magnets known so far, we show that the symmetry of magnetic trigonal prismatic monolayers neither allow for an internal relativistic Dzyaloshinskii-Moriya interaction (DMI) nor a reactive spin-orbit torque. We demonstrate that the DMI only becomes important at the boundaries, where it modifies the boundary conditions of the magnetization and leads to a helical equilibrium state with a helical wavevector that is inherently linked to the internal spin orientation. Furthermore, we find that the helical wavevector can be electrically manipulated via dissipative spin-torque mechanisms. Our results reveal that 2D magnets offer a large potential for unexplored magnetic effects.

cond-mat.mes-hall↗

arXiv2018

The effect of boundary-induced chirality on magnetic textures in thin films

In the quest for miniaturizing magnetic devices, the effects of boundaries and surfaces become increasingly important. Here we show how the recently predicted boundary-induced Dzyaloshinskii-Moriya interaction (DMI) affects the magnetization of ferromagnetic films with a $C_{\infty v}$ symmetry and a perpendicular magnetic anisotropy. For an otherwise uniformly magnetized film, we find a surface twist when the magnetization in the bulk is canted by an in-plane external field. This twist at the surfaces caused by the boundary-induced DMI differs from the common canting caused by internal DMI observed at the edges of a chiral magnet. Further, we find that the surface twist due to the boundary-induced DMI strongly affects the width of the domain wall at the surfaces. We also find that the skyrmion radius increases in the depth of the film, with the average size of the skyrmion increasing with boundary-induced DMI. This increase suggests that the boundary-induced DMI contributes to the stability of the skyrmion.

cond-mat.mes-hall↗

arXiv2017

Boundary-Driven Twist States in Systems with Broken Spatial Inversion Symmetry

A full description of a magnetic sample includes a correct treatment of the boundary conditions (BCs). This is in particular important in thin film systems, where even bulk properties might be modified by the properties of the boundary of the sample. We study generic ferromagnets with broken spatial inversion symmetry and derive the general micromagnetic BCs of a system with Dzyaloshinskii-Moriya interaction (DMI). We demonstrate that the BCs require the full tensorial structure of the third-rank DMI tensor and not just the antisymmetric part, which is usually taken into account. Specifically, we study systems with $C_{\infty v}$ symmetry and explore the consequences of the DMI. Interestingly, we find that the DMI already in the simplest case of a ferromagnetic thin-film leads to a purely boundary-driven magnetic twist state at the edges of the sample. The twist state represents a new type of DMI-induced spin structure, which is completely independent of the internal DMI field. We estimate the size of the texture-induced magnetoresistance effect being in the range of that of domain walls.

cond-mat.mes-hall↗

arXiv2017

Magnetoelectric coupling in superconductor-helimagnet heterostructures

The Ginzburg-Landau free energy of a conventional superconductor coupled to a helimagnet is microscopically derived using functional field integral techniques. We show that the spin texture leads to a Lifshitz invariant in the free energy, which couples the momentum density of the superconducting condensate to the magnetization of the helimagnet. For helimagnets with a conical texture, the Lifshitz invariant yields a spatial modulation of the superconducting phase along the helical wavevector of the magnetic texture. Based on self-consistent numerical calculations, we verify the theoretical formalism by investigating a superconductor that contains a helical Yu-Shiba-Rusinov (YSR) chain. We demonstrate that the texture-induced magnetoelectric coupling produces a strong supercurrent along the YSR chain, which induces a detectable magnetic field.

cond-mat.mes-hall↗

arXiv2016

Composite Topological Excitations in Ferromagnet-Superconductor Heterostructures

We investigate the formation of a new type of composite topological excitation -- the skyrmion-vortex pair (SVP) -- in hybrid systems consisting of coupled ferromagnetic and superconducting layers. Spin-orbit interaction in the superconductor mediates a magnetoelectric coupling between the vortex and the skyrmion, with a sign (attractive or repulsive) that depends on the topological indices of the constituents. We determine the conditions under which a bound SVP is formed, and characterize the range and depth of the effective binding potential through analytical estimates and numerical simulations. Furthermore, we develop a semiclassical description of the coupled skyrmion-vortex dynamics and discuss how SVPs can be controlled by applied spin currents.

cond-mat.mes-hall↗

arXiv2016

Supercurrent-Induced Spin-Orbit Torques

We theoretically investigate the supercurrent-induced magnetization dynamics of a two-dimensional lattice of ferromagnetically ordered spins placed on a conventional superconductor with broken spatial inversion symmetry and strong spin-orbit coupling. We develop a phenomenological description of the coupled dynamics of the superconducting condensate and the spin system, and demonstrate that supercurrents produce a reactive spin-orbit torque on the magnetization. By performing a microscopic self-consistent calculation, we show that the spin-orbit torque originates from a spin-polarization of the Cooper pairs due to current-induced spin-triplet correlations. Interestingly, we find that there exists an intrinsic limitation for the maximum achievable spin-orbit torque, which is determined by the coupling strength between the condensate and the spin system. In proximitized hole-doped semiconductors, the maximum achievable spin-orbit torque field is estimated to be on the order of $0.16$ mT, which is comparable to the critical field for current-induced magnetization switching in ferromagnetic semiconductors.

cond-mat.mes-hall↗

arXiv2015

Nonlocal Damping of Helimagnets in One-Dimensional Interacting Electron Systems

We investigate the magnetization relaxation of a one-dimensional helimagnetic system coupled to interacting itinerant electrons. The relaxation is assumed to result from the emission of plasmons, the elementary excitations of the one-dimensional interacting electron system, caused by slow changes of the magnetization profile. This dissipation mechanism leads to a highly nonlocal form of magnetization damping that is strongly dependent on the electron-electron interaction. Forward scattering processes lead to a spatially constant damping kernel, while backscattering processes produce a spatially oscillating contribution. Due to the nonlocal damping, the thermal fluctuations become spatially correlated over the entire system. We estimate the characteristic magnetization relaxation times for magnetic quantum wires and nuclear helimagnets.

cond-mat.mes-hall↗