Nowak, Ulrich

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## Suchergebnisse Publikationen

#### Magnonic Proximity Effect in Insulating Ferro- and Antiferromagnetic Trilayers

2022, Brehm, Verena, Evers, Martin, Ritzmann, Ulrike, Nowak, Ulrich

The design of spin-transport based devices such as magnon transistors or spin valves will require multilayer systems composed of different magnetic materials with different physical properties. Such layered structures can show various interface effects, one class of which being proximity effects, where a certain physical phenomenon that occurs in the one layers leaks into another one. In this work a magnetic proximity effect is studied in trilayers of different ferro- and antiferromagnetic materials within an atomistic spin model. We find the magnetic order in the central layer - with lower critical temperature - enhanced, even for the case of an antiferromagnet surrounded by ferromagnets. We further characterize this proximity effect via the magnon spectra which are specifically altered, especially for the case of the antiferromagnet in the central layer.

#### Terahertz spin dynamics driven by a field-derivative torque

2019-08-23, Mondal, Ritwik, Donges, Andreas, Ritzmann, Ulrike, Oppeneer, Peter M., Nowak, Ulrich

Efficient manipulation of magnetization at ultrashort timescales is of particular interest for future technology. Here, we numerically investigate the influence of the so-called field-derivative torque, which was derived earlier based on relativistic Dirac theory [R. Mondal et al., Phys. Rev. B 94, 144419 (2016)], on the spin dynamics triggered by ultrashort laser pulses. We find that only considering the THz Zeeman field can underestimate the spin excitation in antiferromagnetic oxide systems such as, e.g., NiO and CoO. However, accounting for both the THz Zeeman torque and the field-derivative torque, the amplitude of the spin excitation increases significantly. Studying the damping dependence of the field-derivative torque we observe larger effects for materials having larger damping constants.

#### Thermally induced magnon accumulation in two-sublattice magnets

2017, Ritzmann, Ulrike, Hinzke, Denise, Nowak, Ulrich

We present a temperature dependent study of the thermal excitation of magnonic spin currents in two-sublattice magnetic materials. Using atomistic spin model simulations, we study the local magnetization profiles in the vicinity of a temperature step in antiferromagnets, as well as in ferrimagnets. It is shown that the strength of the excitation of the spin currents in these systems scales with the derivative of the magnetization with respect to the temperature.

#### Magnonic spin currents: localization, propagation, and accumulation.

2015, Hinzke, Denise, Ritzmann, Ulrike, Evers, Martin, MÃ¼ller, Cord A., Nowak, Ulrich

#### Ultrafast coherent all-optical switching of an antiferromagnet with the inverse Faraday effect

2021, Dannegger, Tobias, Berritta, Marco, Carva, Karel, Selzer, Severin, Ritzmann, Ulrike, Oppeneer, Peter M., Nowak, Ulrich

We explore the possibility of ultrafast, coherent all-optical magnetization switching in antiferromagnets by studying the action of the inverse Faraday effect in CrPt, an easy-plane antiferromagnet. Using a combination of density-functional theory and atomistic spin dynamics simulations, we show how a circularly polarized laser pulse can switch the order parameter of the antiferromagnet within a few hundred femtoseconds. This nonthermal switching takes place on an elliptical path, driven by the staggered magnetic moments induced by the inverse Faraday effect and leading to reliable switching between two perpendicular magnetic states.

#### Inertia-Free Thermally Driven Domain-Wall Motion in Antiferromagnets

2016-08-29, Selzer, Severin, Atxitia, Unai, Ritzmann, Ulrike, Hinzke, Denise, Nowak, Ulrich

Domain-wall motion in antiferromagnets triggered by thermally induced magnonic spin currents is studied theoretically. It is shown by numerical calculations based on a classical spin model that the wall moves towards the hotter regions, as in ferromagnets. However, for larger driving forces the so-called Walker breakdownâ€”which usually speeds down the wallâ€”is missing. This is due to the fact that the wall is not tilted during its motion. For the same reason antiferromagnetic walls have no inertia and, hence, no acceleration phase leading to higher effective mobility.

#### Unveiling domain wall dynamics of ferrimagnets in thermal magnon currents : competition of angular momentum transfer and entropic torque

2020, Donges, Andreas, Grimm, Niklas, Jakobs, Florian, Selzer, Severin, Ritzmann, Ulrike, Atxitia, Unai, Nowak, Ulrich

Control of magnetic domain wall motion holds promise for efficient manipulation and transfer of magnetically stored information. Thermal magnon currents, generated by temperature gradients, can be used to move magnetic textures, from domain walls to magnetic vortices and skyrmions. In the past several years, theoretical studies have focused on ferro- and antiferromagnetic spin structures, where domain walls always move toward the hotter end of the thermal gradient. Here we perform numerical studies using atomistic spin dynamics simulations and complementary analytical calculations to derive an equation of motion for the domain wall velocity in ferrimagnets. We demonstrate that in ferrimagnets, domain wall motion under thermal magnon currents shows a much richer dynamics. Below the Walker breakdown, we find that the temperature gradient always pulls the domain wall toward the hot end by minimizing its free energy, in agreement with the observations for ferro- and antiferromagnets in the same regime. Above Walker breakdown, the ferrimagnetic domain wall can show the opposite, counterintuitive behavior of moving toward the cold end. We show that in this case, the motion to the hotter or the colder ends is driven by angular momentum transfer and therefore strongly related to the angular momentum compensation temperature, a unique property of ferrimagnets where the intrinsic angular momentum of the ferrimagnet is zero while the sublattice angular momentum remains finite. In particular, we find that below the compensation temperature the wall moves toward the cold end, whereas above it toward the hot end. Moreover, we find that for ferrimagnets, there is a torque compensation temperature at which the domain wall dynamics shows similar characteristics to antiferromagnets, that is, quasi-inertia-free motion and the absence of Walker breakdown. This finding opens the door for fast control of magnetic domains as given by the antiferromagnetic character while conserving the advantage of ferromagnets in terms of measuring and control by conventional means such as magnetic fields.

#### Magnon detection using a ferroic collinear multilayer spin valve

2018-03-14, Cramer, Joel, Fuhrmann, Felix, Ritzmann, Ulrike, Gall, Vanessa, Niizeki, Tomohiko, Ramos, Rafael, Qiu, Zhiyong, Hou, Dazhi, Nowak, Ulrich, KlÃ¤ui, Mathias

Information transport and processing by pure magnonic spin currents in insulators is a promising alternative to conventional charge-current-driven spintronic devices. The absence of Joule heating and reduced spin wave damping in insulating ferromagnets have been suggested for implementing efficient logic devices. After the successful demonstration of a majority gate based on the superposition of spin waves, further components are required to perform complex logic operations. Here, we report on magnetization orientation-dependent spin current detection signals in collinear magnetic multilayers inspired by the functionality of a conventional spin valve. In Y_{3}Fe_{5}O_{12}|CoO|Co, we find that the detection amplitude of spin currents emitted by ferromagnetic resonance spin pumping depends on the relative alignment of the Y_{3}Fe_{5}O_{12}and Co magnetization. This yields a spin valve-like behavior with an amplitude change of 120% in our systems. We demonstrate the reliability of the effect and identify its origin by both temperature-dependent and power-dependent measurements.

#### Magnetic field control of the spin Seebeck effect

2015, Ritzmann, Ulrike, Hinzke, Denise, Kehlberger, Andreas, Guo, Er-Jia, KlÃ¤ui, Mathias, Nowak, Ulrich

The origin of the suppression of the longitudinal spin Seebeck effect by applied magnetic fields is studied. We perform numerical simulations of the stochastic Landau-Lifshitz-Gilbert equation of motion for an atomistic spin model and calculate the magnon accumulation in linear temperature gradients for different strengths of applied magnetic fields and different length scales of the temperature gradient. We observe a decrease of the magnon accumulation with increasing magnetic field and we reveal that the origin of this effect is a field dependent change of the frequency distribution of the propagating magnons. With increasing field the magnonic spin currents are reduced due to a suppression of parts of the frequency spectrum. By comparison with measurements of the magnetic field dependent longitudinal spin Seebeck effect in YIG thin films with various thicknesses, we find qualitative agreement between our model and the experimental data, demonstrating the importance of this effect for experimental systems.