Nowak, Ulrich

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## Suchergebnisse Publikationen

#### Enhanced thermally-activated skyrmion diffusion with tunable effective gyrotropic force

2023-09-11, Dohi, Takaaki, Weißenhofer, Markus, Kerber, Nico, Kammerbauer, Fabian, Ge, Yuqing, Raab, Klaus, Zázvorka, Jakub, Syskaki, Maria-Andromachi, Nowak, Ulrich, Kläui, Mathias

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.

#### Temperature dependence of current-driven and Brownian skyrmion dynamics in ferrimagnets with compensation point

2023-02-24, Weißenhofer, Markus, Nowak, Ulrich

Magnetic skyrmions are topological spin textures and promising candidates for novel spintronic applications. Recent studies on the current-driven dynamics of ferromagnetic (FM) skyrmions revealed that they exhibit an undesirable transverse motion, the skyrmion Hall effect. For antiferromagnetic (AFM) skyrmions, a vanishing skyrmion Hall effect was predicted, along with faster dynamics. However, their zero net magnetization obstructs efficient detection. Ferrimagnetic (FI) materials promise to combine both advantages: fast, AFM-like dynamics and easy read-out via stray fields. Here, we investigate the current-driven and Brownian dynamics of skyrmions in a FI with a compensation point. We perform atomistic spin dynamics simulations based on a model Hamiltonian and the stochastic Landau-Lifshitz-Gilbert equation supplemented with spin-orbit torques, accompanied by analytical calculations based on a collective coordinate approach. Our results unveil a nonmonotonic temperature dependence of the velocities and the diffusion coefficient with a strong enhancement at the angular momentum compensation temperature, due to scaling from FM- to AFM-like dynamics. These findings open up a new pathway for the efficient manipulation of skyrmion dynamics via temperature.

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

2022-03-30T08:35:09Z, Mankovsky, Sergiy, Polesya, Svitlana, Lange, Hannah, Weißenhofer, Markus, Nowak, Ulrich, Ebert, Hubert

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.

#### Anisotropic Skyrmion Diffusion Controlled by Magnetic-Field-Induced Symmetry Breaking

2021, Kerber, Nico, Weißenhofer, Markus, Raab, Klaus, Litzius, Kai, Zázvorka, Jakub, Nowak, Ulrich, Kläui, Mathias

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.

#### Rotationally invariant formulation of spin-lattice coupling in multiscale modeling

2023-08-23, Weißenhofer, Markus, Lange, Hannah, Kamra, Akashdeep, Mankovsky, Sergiy, Polesya, Svitlana, Ebert, Hubert, Nowak, Ulrich

In the spirit of multiscale 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 magnetoelastic 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 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 multiscale modeling for investigating and understanding a broad range of magnetomechanical phenomena from slow to ultrafast timescales.

#### Néel vector switching and terahertz spin-wave excitation in Mn_{2}Au due to femtosecond spin-transfer torques

2023, Weißenhofer, Markus, Foggetti, Francesco, Nowak, Ulrich, Oppeneer, Peter M.

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.

#### Walker breakdown of Brownian domain wall dynamics

2022, Weißenhofer, Markus, Selzer, Severin, Nowak, Ulrich

The Brownian motion of domain walls in uniaxial and biaxial ferromagnetic nanowires is studied, comparing spin dynamics simulations with analytical calculations within the framework of a collective coordinate approach. Our results demonstrate that the interplay between spatial and angular dynamics gives rise to a complex time dependence of the MSD in biaxial nanowires and to a drastically reduced diffusion coefficient in uniaxial nanowires, analogous to magnetic skyrmions. This diffusion suppression is also responsible for the peculiar temperature dependence of the diffusion coefficient in biaxial systems: while it is found to scale linearly with temperature up to a certain threshold, the emergence of a Walker breakdown of Brownian motion is responsible for a reduction of the diffusion coefficient with increasing temperature above this threshold.

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

2023-03-31, Lange, Hannah, Mankovsky, Sergiy, Polesya, Svitlana, Weißenhofer, Markus, Nowak, Ulrich, Ebert, Hubert

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 us to benchmark the performance of the scheme introduced previously.

#### Topology dependence of skyrmion Seebeck and skyrmion Nernst effect

2022-04-26, Weißenhofer, Markus, Nowak, Ulrich

We explore the dynamics of skyrmions with various topological charges induced by a temperature gradient in an ultra-thin insulating magnetic film. Combining atomistic spin simulations and analytical calculations we find a topology-dependent skyrmion Seebeck effect: while skyrmions and antiskyrmions move to the hot regime, a topologically trivial localized spin structure moves to the cold regime. We further reveal the emergence of a skyrmion Nernst effect, i.e. finite, topology-dependent velocities transverse to the direction of the temperature gradient. These findings are in agreement with accompanying simulations of skyrmionic motion induced by monochromatic magnon currents, allowing us to demonstrate that the magnonic spin Seebeck effect is responsible for both, skyrmion Seebeck and Nernst effect. Furthermore we employ scattering theory together with Thiele's equation to identify linear momentum transfer from the magnons to the skyrmion as the dominant contribution and to demonstrate that the direction of motion depends on the topological magnon Hall effect and the topological charge of the skyrmion.

#### Skyrmion Dynamics at Finite Temperatures : Beyond Thiele’s Equation

2021, Weißenhofer, Markus, Rózsa, Levente, Nowak, Ulrich

Magnetic textures are often treated as quasiparticles following Thiele’s equation of motion. We demonstrate via spin model simulations of the current-driven and Brownian motion of ferromagnetic skyrmions that the existing theory based on Thiele’s equation is insufficient to describe the dynamics of skyrmions at finite temperatures. We propose an extended equation of motion that goes beyond Thiele’s equation by taking into account the coupling of the skyrmion to the magnonic heat bath leading to an additional dissipative term that is linear in temperature. Our results indicate that this so-far-neglected magnon-induced friction dominates for finite temperatures and Gilbert damping values typical for thin films and multilayers.