arXiv subjects

Markus Weißenhofer

Publications and source records attributed to Markus Weißenhofer.

At least 19 recordsLinked to original sources

arXiv2026

Spurious spin nutation modes arising from truncated memory effects

Spin dynamics is conventionally treated as inertia-free. The inertial Landau-Lifshitz-Gilbert equation (iLLG) extends this with a second-order time derivative, predicting a high-frequency nutational mode that recent experiments have claimed to observe and interpreted as direct evidence for spin inertia. We show that when spin inertia is generated by eliminating additional dynamical degrees of freedom, the resulting exact spin-only description is generally non-Markovian. The iLLG is recovered as the low-frequency, second-order truncation of the underlying memory kernel. Using a minimal spin--cavity model, we show that the exact eigenfrequencies and those obtained via the iLLG differ and that the spin nutation mode predicted by the latter is a truncation-induced high-frequency artifact. This raises a broader question of the applicability of the iLLG obtained from analogous low-frequency truncations, which may likewise generate spurious high-frequency roots. Consequently, agreement of a high-frequency spectral feature with the iLLG nutation frequency does not, by itself, establish a physical nutation mode.

cond-mat.mtrl-sci↗

arXiv2026

Symmetry-Selective Topological Magnon Engineering by Phonon Angular Momentum

Dynamical control of Berry curvature remains an outstanding challenge in the engineering of topological phases. Here, we demonstrate control of magnon band structures via coherently driven phonons, based on \textit{ab initio} spin-lattice coupling and Floquet theory. We show that this control is symmetry selective: linearly polarized phonons leave the spectrum unchanged, whereas circular and elliptical phonons carrying finite phonon angular momentum (PAM) induce chiral interactions that open and tune gaps at Dirac points, generating and reversing topological magnon phases. The gap magnitude and Chern numbers are directly governed by the PAM, enabling handedness-selective topology control. Applied to monolayer CrI$_3$, and supported by symmetry analysis, our results establish driven lattice dynamics as a general route to engineering topological bosonic excitations and a versatile platform for Floquet control of magnetism.

cond-mat.mes-hall↗

arXiv2026

Symmetry-Enforced Chiral Phonons in Altermagnets via Magnon-Phonon Coupling

Chiral phonons are attractive for spintronics applications, however, their zero-field generation in conventional antiferromagnets is forbidden by combined parity and time-reversal ($\mathcal{PT}$) symmetry. Here we demonstrate the emergence of chiral phonons in $\mathcal{PT}$-breaking altermagnetic systems at zero field arising from relativistic magnon-phonon coupling. Focusing on the prototypical altermagnet CrSb, we utilize first-principles methods to calculate the hybridized magnon-polarons across the complete Brillouin zone. We show that this coupling imprints an altermagnetic $g$-wave symmetry directly onto the phonon angular momentum. Furthermore, we demonstrate anomalous spin and phonon angular momentum Nernst responses arising from finite Berry curvatures. These findings establish that chiral lattice dynamics can arise in compensated magnetic ground states without requiring external fields, positioning bulk altermagnets as material candidates for zero-field spin caloritronics and chiral phononics.

cond-mat.mtrl-sci↗

arXiv2026

Huge ultrafast spin Seebeck effect mediated by laser-excited superdiffusive magnon currents

Subpicosecond laser excitation of ferromagnetic metals induces strongly nonequilibrium dynamics involving scattering and transport of electrons, phonons, and magnons. Widely used theoretical approaches, such as the three-temperature model and diffusion equations, are ill-suited to capture these processes on ultrafast timescales. Here, we present an ab initio-parameterized microscopic framework that incorporates nonthermal magnon scattering and transport via the quantum Boltzmann equation. We apply this approach to simulate ultrafast laser-induced demagnetization in bcc Fe films. The model predicts an ultrafast spin Seebeck effect, characterized by a strong burst of fast-moving magnonic spin current reaching technologically relevant amplitudes. Furthermore, we identify a superdiffusive transport regime: a crossover from initially ballistic magnon transport to a diffusive regime at later times. To connect our theoretical predictions to experimentally accessible observables, we calculate the magneto-optical Kerr angles resulting from the predicted depth-resolved magnetization profiles. Our framework provides a route to describe ultrafast nonthermal magnon transport beyond diffusive models and will aid in the design and interpretation of time-resolved spin-transport experiments.

cond-mat.mes-hall↗

arXiv2025

Chiral Phonons Arising From Chirality-Selective Magnon-Phonon Coupling

Chiral phonons are desirable for applications in spintronics but their generation and control remains a challenge.Here we demonstrate the emergence of truly chiral phonons from selective magnon-phonon coupling in inversion-symmetric magnetic systems. Considering bcc Fe as example, we quantitatively calculate hybridized magnon-phonon quasiparticle states across the entire Brillouin zone utilizing first-principles calculations. Our findings challenge conventional magneto-elastic interpretations and reveal finite zero-point phonon angular momentum and strong anomalous thermal Hall responses linked to finite (spin) Berry curvatures. Our results further establish that the existence of chiral phonons, particularly along high-symmetry directions, is common in many magnetic materials, offering promising avenues for novel spintronic and phononic devices.

cond-mat.mtrl-sci↗

arXiv2025

Generation of Phonons with Angular Momentum During Ultrafast Demagnetization

A major question in the field of femtosecond laser-induced demagnetization is whereto the angular momentum lost by the electrons is transferred. Recent ultrafast electron diffraction measurements [Tauchert \textit{et al.}, Nature {\bf 602}, 73 (2022)] suggest that this angular momentum is transferred to the rotational motion of atoms on a sub-picosecond timescale, but a theory confirmation of this proposition has yet to be given. Here we investigate the coupled electron-nuclear dynamics during ultrafast demagnetization of L1$_0$ FePt, using Ehrenfest nuclear dynamics simulations combined with the time-dependent density functional theory (TDDFT) framework. We demonstrate that atomic rotations appear, i.e., the generation of phonons carrying finite angular momentum following ultrafast demagnetization. We further show that both ultrafast demagnetization and the generation of phonons with angular momentum arise from symmetry constraints imposed by the spin-orbit coupling, thus providing insight in spin-phonon interaction at ultrafast timescales.

cond-mat.mtrl-sci↗

arXiv2024

Element-selective probing of ultrafast ferromagnetic--antiferromagnetic order dynamics in Fe/CoO bilayers

The ultrafast magnetization dynamics of an epitaxial Fe/CoO bilayer on Ag(001) is examined in an element-resolved way by resonant soft-x-ray reflectivity. The transient magnetic linear dichroism at the Co L2 edge and the magnetic circular dichroism at the Fe L3 edge measured in reflection in a pump-probe experiment with 120 fs temporal resolution show the loss of antiferromagnetic and ferromagnetic order in CoO and Fe, respectively, both within 300 fs after excitation with 60 fs light pulses of 800 and 400 nm wavelengths. Comparison to spin-dynamics simulations using an atomistic spin model shows that direct energy transfer from the laser-excited electrons in Fe to the magnetic moments in CoO provides the dominant demagnetization channel in the case of 800-nm excitation.

cond-mat.mtrl-sci↗

arXiv2024

Atomistic spin dynamics simulations of magnonic spin Seebeck and spin Nernst effects in altermagnets

Magnon band structures in altermagnets are characterized by an energy splitting of modes with opposite chirality, even in the absence of applied external fields and relativistic effects, due to an anisotropy in the Heisenberg exchange interactions. We perform quantitative atomistic spin dynamics simulations based on ab initio electronic structure calculations on rutile RuO$_2$, a prototypical "d-wave" altermagnet, to study magnon currents generated by thermal gradients. We report substantial spin Seebeck and spin Nernst effects, i.e., longitudinal or transverse spin currents, depending on the propagation direction of the magnons with respect to the crystal, together with a finite spin accumulation associated with non-linearities in the temperature profile. Our findings are consistent with the altermagnetic spin-group symmetry, as well as predictions from linear spin wave theory and semiclassical Boltzmann transport theory.

cond-mat.mtrl-sci↗

arXiv2024

Ultrafast Demagnetization through Femtosecond Generation of Non-thermal Magnons

Ultrafast laser excitation of ferromagnetic metals gives rise to correlated, highly non-equilibrium dynamics of electrons, spins and lattice, which are, however, poorly described by the widely-used three-temperature model (3TM). Here, we develop a fully ab-initio parameterized out-of-equilibrium theory based on a quantum kinetic approach--termed (N+2) temperature model--that describes magnon occupation dynamics due to electron-magnon scattering. We apply this model to perform quantitative simulations on the ultrafast, laser-induced generation of magnons in iron and demonstrate that on these timescales the magnon distribution is non-thermal: predominantly high-energy magnons are created, while the magnon occupation close to the center of the Brillouin zone even decreases, due to a repopulation towards higher energy states via a so-far-overlooked scattering term. We demonstrate that the simple relation between magnetization and temperature computed at equilibrium does not hold in the ultrafast regime and that the 3TM greatly overestimates the demagnetization. The ensuing Gilbert damping becomes strongly magnon wavevector dependent and requires a description beyond the conventional Landau-Lifshitz-Gilbert spin dynamics. Our ab-initio-parameterized calculations show that ultrafast generation of non-thermal magnons provides a sizable demagnetization within 200fs in excellent comparison with experimentally observed laser-induced demagnetizations. Our investigation emphasizes the importance of non-thermal magnon excitations for the ultrafast demagnetization process.

cond-mat.mtrl-sci↗

arXiv2023

Enhanced thermally-activated skyrmion diffusion with tunable effective gyrotropic force

Magnetic skyrmions, topologically-stabilized spin textures that emerge in magnetic systems, have garnered considerable interest due to a variety of electromagnetic responses that are governed by the topology. The topology that creates a microscopic gyrotropic force also causes detrimental effects, such as the skyrmion Hall effect, which is a well-studied phenomenon highlighting the influence of topology on the deterministic dynamics and drift motion. Furthermore, the gyrotropic force is anticipated to have a substantial impact on stochastic diffusive motion; however, the predicted repercussions have yet to be demonstrated, even qualitatively. Here we demonstrate enhanced thermally-activated diffusive motion of skyrmions in a specifically designed synthetic antiferromagnet. Suppressing the effective gyrotropic force by tuning the angular momentum compensation leads to a more than 10 times enhanced diffusion coefficient compared to that of ferromagnetic skyrmions. Consequently, our findings not only demonstrate the gyro-force dependence of the diffusion coefficient but also enable ultimately energy-efficient unconventional stochastic computing.

cond-mat.mtrl-sci↗

arXiv2023

Defect-enhanced diffusion of magnetic skyrmions

Defects, i.e. inhomogeneities of the underlying lattice, are ubiquitous in magnetic materials and can have a crucial impact on their applicability in spintronic devices. For magnetic skyrmions, localized and topologically non-trivial spin textures, they give rise to a spatially inhomogeneous energy landscape and can lead to pinning, resulting in an exponentially increased dwell time at certain positions and typically a strongly reduced mobility. Using atomistic spin dynamics simulations, we reveal that under certain conditions defects can instead enhance thermal diffusion of ferromagnetic skyrmions. By comparing with results for the diffusion of antiferromagnetic skyrmions and using a quasi-particle description based on the Thiele equation, we demonstrate that this surprising finding can be traced back to the partial lifting of the impact of the topologigal gyrocoupling, which governs the dynamics of ferromagnetic skyrmions in the absence of defects.

cond-mat.mtrl-sci↗

arXiv2023

Calculating spin-lattice interactions in ferro- and antiferromagnets: the role of symmetry, dimension and frustration

Recently, the interplay between spin and lattice degrees of freedom has gained a lot of attention due to its importance for various fundamental phenomena as well as for spintronic and magnonic applications. Examples are ultrafast angular momentum transfer between the spin and lattice subsystems during ultrafast demagnetization, frustration driven by structural distortions in transition metal oxides, or in acoustically driven spin-wave resonances. In this work, we provide a systematic analysis of spin-lattice interactions for ferro- and antiferromagnetic materials and focus on the role of lattice symmetries and dimensions, magnetic order, and the relevance of spin-lattice interactions for angular momentum transfer as well as magnetic frustration. For this purpose, we use a recently developed scheme which allows an efficient calculation of spin-lattice interaction tensors from first principles. In addition to that, we provide a more accurate and self consistent scheme to calculate ab initio spin lattice interactions by using embedded clusters which allows to benchmark the performance of the scheme introduced previously.

cond-mat.mtrl-sci↗

arXiv2023

Néel-Vector Switching and THz Spin-Wave Excitation in Mn$_2$Au due to Femtosecond Spin-Transfer Torques

Efficient and fast manipulation of antiferromagnets has to date remained a challenging task, hindering their application in spintronic devices. For ultrafast operation of such devices, it is highly desirable to be able to control the antiferromagnetic order within picoseconds - a timescale that is difficult to achieve with electrical circuits. Here, we demonstrate that bursts of spin-polarized hot-electron currents emerging due to laser-induced ultrafast demagnetization are able to efficiently excite spin dynamics in antiferromagnetic Mn$_2$Au by exerting a spin-transfer torque on femtosecond timescales. We combine quantitative superdiffusive transport and atomistic spin-model calculations to describe a spin-valve-type trilayer consisting of Fe$|$Cu$|$Mn$_2$Au. Our results demonstrate that femtosecond spin-transfer torques can switch the Mn$_2$Au layer within a few picoseconds. In addition, we find that spin waves with high frequencies up to several THz can be excited in Mn$_2$Au.

cond-mat.mtrl-sci↗

arXiv2022

Rotationally invariant formulation of spin-lattice coupling in multi-scale modeling

In the spirit of multi-scale modeling, we develop a theoretical framework for spin-lattice coupling that connects, on the one hand, to ab initio calculations of spin-lattice coupling parameters and, on the other hand, to the magneto-elastic continuum theory. The derived Hamiltonian describes a closed system of spin and lattice degrees of freedom and explicitly conserves the total momentum, angular momentum and energy. Using a new numerical implementation that corrects earlier Suzuki-Trotter decompositions we perform simulations on the basis of the resulting equations of motion to investigate the combined magnetic and mechanical motion of a ferromagnetic nanoparticle, thereby validating our developed method. In addition to the ferromagnetic resonance mode of the spin system we find another low-frequency mechanical response and a rotation of the particle according to the Einstein-de-Haas effect. The framework developed herein will enable the use of multi-scale modeling for investigating and understanding a broad range of magneto-mechanical phenomena from slow to ultrafast time scales.

cond-mat.mtrl-sci↗

arXiv2022

Angular momentum transfer via relativistic spin-lattice coupling from first principles

The transfer and control of angular momentum is a key aspect for spintronic applications. Only recently, it was shown that it is possible to transfer angular momentum from the spin system to the lattice on ultrashort time scales. In an attempt to contribute to the understanding of angular momentum transfer between spin and lattice degrees of freedom we present a scheme to calculate fully-relativistic spin-lattice coupling parameters from first-principles. By treating changes in the spin configuration and atomic positions at the same level, closed expressions for the atomic spin-lattice coupling parameters can be derived in a coherent manner up to any order. Analyzing the properties of these parameters, in particular their dependence on spin-orbit coupling, we find that even in bcc Fe the leading term for the angular momentum exchange between the spin system and the lattice is a Dzyaloshiskii-Moriya-type interaction, which is due to the symmetry breaking distortion of the lattice.

cond-mat.mes-hall↗

arXiv2021

Skyrmions as quasiparticles: free energy and entropy

The free energy and the entropy of magnetic skyrmions with respect to the collinear state are calculated for a (Pt$_{0.95}$Ir$_{0.05}$)/Fe bilayer on Pd(111) via atomistic spin model simulations. The simulations are carried out starting from very low temperatures where the skyrmion number is conserved up to the range where skyrmions are constantly created and destroyed by thermal fluctuations, highlighting their quasiparticle nature. The higher entropy of the skyrmions at low temperature leads to a reduced free energy, such that the skyrmions become energetically preferred over the collinear state due to entropic stabilization as predicted by linear spin-wave theory. Going beyond the linear spin-wave approximation, a sign change is shown to occur in the free energy as well as the entropy at elevated temperature.

cond-mat.stat-mech↗

arXiv2020

Spin waves in skyrmionic structures with various topological charges

Equilibrium properties and localised magnon excitations are investigated in topologically distinct skyrmionic textures. The observed shape of the structures and their orientation on the lattice is explained based on their vorticities and the symmetry of the crystal. The transformation between different textures and their annihilation as a function of magnetic field is understood based on the energy differences between them. The angular momentum spin-wave eigenmodes characteristic of cylindrically symmetric structures are combined in the distorted spin configurations, leading to avoided crossings in the magnon spectrum. The susceptibility of the skyrmionic textures to homogeneous external fields is calculated, revealing that a high number of modes become detectable due to the hybridization between the angular momentum eigenmodes. These findings should contribute to the observation of spin waves in distorted skyrmionic structures via experiments and numerical simulations, widening the range of their possible applications in magnonic devices.

cond-mat.mtrl-sci↗

arXiv2020

Anisotropic skyrmion diffusion controlled by field-induced symmetry breaking

Diffusion of particles has wide repercussions ranging from particle-based soft matter systems to solid state systems with particular electronic properties. Recently, in the field of magnetism, diffusion of magnetic skyrmions, topologically stabilized quasi-particles, has been demonstrated. Here we show that by applying a magnetic in-plane field and therefore breaking the symmetry of the system, the skyrmion diffusion becomes anisotropic with faster diffusion parallel to the field axis and slower diffusion perpendicular to it. We furthermore show that the absolute value of the applied field controls the absolute values of the diffusion coefficients so that one can thereby uniquely tune both the orientation of the diffusion and its strength. Based on the stochastic Thiele equation, we can explain the observed anisotropic diffusion as a result of the elliptical deformation of the skyrmions by the application of the in-plane field.

cond-mat.mtrl-sci↗