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.

#### Magnon squeezing in conical spin spirals

2023-04-13, Wuhrer, Dennis, Rózsa, Levente, Nowak, Ulrich, Belzig, Wolfgang

We investigate squeezing of magnons in a conical spin spiral configuration. We find that while the energy of magnons propagating along the k and the −k directions can be different due to the non-reciprocal dispersion, these two modes are connected by the squeezing, hence can be described by the same squeezing parameter. The squeezing parameter diverges at the center of the Brillouin zone due to the translational Goldstone mode of the system, but the squeezing also vanishes for certain wave vectors. We discuss possible ways of detecting the squeezing.

#### 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.

#### 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.

#### 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.

#### 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.

#### Magnetic properties of hematite revealed by an *ab initio* parameterized spin model

2023, Dannegger, Tobias, Deák, András, Rózsa, Levente, Galindez-Ruales, Edgar, Das, Shubhankar, Baek, Eunchong, Kläui, Mathias, Szunyogh, László, Nowak, Ulrich

Hematite is a canted antiferromagnetic insulator, promising for applications in spintronics. Here we present ab initio calculations of the tensorial exchange interactions of hematite and use them to understand its magnetic properties by parametrizing a semiclassical Heisenberg spin model. Using atomistic spin dynamics simulations, we calculate the equilibrium properties and phase transitions of hematite, most notably the Morin transition. The computed isotropic and Dzyaloshinskii–Moriya interactions result in a Néel temperature and weak ferromagnetic canting angle that are in good agreement with experimental measurements. Our simulations show how dipole-dipole interactions act in a delicate balance with first and higher-order on-site anisotropies to determine the material's magnetic phase. Comparison with spin-Hall magnetoresistance measurements on a hematite single crystal reveals deviations of the critical behavior at low temperatures. Based on a mean-field model, we argue that these differences result from the quantum nature of the fluctuations that drive the phase transitions.

#### Inertial effects in ultrafast spin dynamics

2023-08-01, Mondal, Ritwik, Rózsa, Levente, Farle, Michael, Oppeneer, Peter M., Nowak, Ulrich, Cherkasskii, Mikhail

The dynamics of magnetic moments consists of a precession around the magnetic field direction and a relaxation towards the field to minimize the energy. While the magnetic moment and the angular momentum are conventionally assumed to be parallel to each other, at ultrafast time scales their directions become separated due to inertial effects. The inertial dynamics gives rise to additional high-frequency modes in the excitation spectrum of magnetic materials. Here, we review the recent theoretical and experimental advances in this emerging topic and discuss the open challenges and opportunities in the detection and the potential applications of inertial spin dynamics.

#### 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.

#### Nutational switching in ferromagnets and antiferromagnets

2022-07-18T12:47:58Z, Winter, Lucas, Großenbach, Sebastian, Nowak, Ulrich, Rózsa, Levente

It was demonstrated recently that on ultrashort time scales magnetization dynamics does not only exhibit precession but also nutation. Here, we investigate how nutation can contribute to spin switching leading towards ultrafast data writing. We use analytic theory and atomistic spin simulations to discuss the behavior of ferromagnets and antiferromagnets in high-frequency magnetic fields. In ferromagnets, linearly polarized fields align the magnetization perpendicular to the external field, enabling $90^{\circ}$ switching. For circularly polarized fields in the $xy$ plane, the magnetization tilts to the $z$ direction. During this tilting, it rotates around the $z$ axis, allowing $180^{\circ}$ switching. In antiferromagnets, external fields with frequencies higher than the nutation frequency align the order parameter parallel to the field direction, while for lower frequencies it is oriented perpendicular to the field. The switching frequency increases with the magnetic field strength, and it deviates from the Larmor frequency, making it possible to outpace precessional switching in high magnetic fields.