Nowak, Ulrich

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## Suchergebnisse Publikationen

#### Two levels of topology in skyrmion lattice dynamics

2024-01-25, Schick, Daniel, Weißenhofer, Markus, Rózsa, Levente, Rothörl, Jan, Virnau, Peter, Nowak, Ulrich

Skyrmions are localized, topological spin structures that can be described as quasiparticles. Skyrmions in thin films are an ideal model system to study Brownian motion and lattice formation in two dimensions. They follow an equation of motion, the Thiele equation, which includes a topology-dependent chiral term, linear in velocity, causing a skyrmion Hall effect and a drastic reduction of the diffusion coefficient for individual skyrmions, as compared to normal Brownian particles. Using Brownian dynamics simulations, we show that this topological suppression of the diffusion can be partially lifted in two-dimensional lattices of skyrmions. Counterintuitively, this causes enhanced diffusive properties with increasing particle density, similar to odd-diffusive Brownian particles. We show how the topological charge of the skyrmions influences the dynamics of topological lattice defects, which also affects the dynamics of the phase formation.

#### Defect-enhanced diffusion of magnetic skyrmions

2023-10-17, Rieger, Philipp, Weißenhofer, Markus, Nowak, Ulrich

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 nontrivial 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 quasiparticle 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 topological gyrocoupling, which governs the dynamics of ferromagnetic skyrmions in the absence of defects.

#### 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.

#### 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.

#### Discovery of ultrafast spontaneous spin switching in an antiferromagnet by femtosecond noise correlation spectroscopy

2023-11-29, Weiss, Marvin, Herbst, Andreas J., Schlegel, Julius, Dannegger, Tobias, Evers, Martin, Donges, Andreas, Leitenstorfer, Alfred, Goennenwein, Sebastian T. B., Nowak, Ulrich, Kurihara, Takayuki

Owing to their high magnon frequencies, antiferromagnets are key materials for future high-speed spintronics. Picosecond switching of antiferromagnetic spin systems has been viewed a milestone for decades and pursued only by using ultrafast external perturbations. Here, we show that picosecond spin switching occurs spontaneously due to thermal fluctuations in the antiferromagnetic orthoferrite Sm_{0.7}Er_{0.3}FeO_{3} . By analysing the correlation between the pulse-to-pulse polarisation fluctuations of two femtosecond optical probes, we extract the autocorrelation of incoherent magnon fluctuations. We observe a strong enhancement of the magnon fluctuation amplitude and the coherence time around the critical temperature of the spin reorientation transition. The spectrum shows two distinct features, one corresponding to the quasi-ferromagnetic mode and another one which has not been previously reported in pump-probe experiments. Comparison to a stochastic spin dynamics simulation reveals this new mode as smoking gun of ultrafast spontaneous spin switching within the double-well anisotropy potential.

#### THz spontaneous magnon fluctuations and room-temperature spin switching in the orthoferrite Sm0.7Er0.3FeO3

2023-09-17, Kurihara, Takayuki, Weiss, Marvin, Herbst, Andreas J., Schlegel, Julius, Dannegger, Tobias, Evers, Martin, Donges, Andreas, Goennenwein, Sebastian T. B., Nowak, Ulrich, Leitenstorfer, Alfred

Ultrafast dynamics of correlated spins in the picosecond to femtosecond timescales have so far been pursued mainly with pump-probe schemes by perturbing the system and measuring the induced changes. Here, we reveal the femtosecond time-resolved autocorrelation function of the sub-THz spontaneous spin fluctuation dynamics in an orthoferrite, existent under the ambient condition due to thermal occupation. The correlation dynamics shows a significant enhancement of the amplitude and coherence time around the spin reorientation transition temperature, which is ascribed to the spontaneous switching of spin moment between the energetically degenerate states.

#### 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.

#### Magnon squeezing in conical spin spirals

2023-11-06, 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.

#### 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.

#### 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.