Nowak, Ulrich

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## Suchergebnisse Publikationen

#### 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, Müller, Cord A., Nowak, Ulrich

#### Propagation of thermally induced magnonic spin currents

2014, Ritzmann, Ulrike, Hinzke, Denise, Nowak, Ulrich

The propagation of magnons in temperature gradients is investigated within the framework of an atomistic spin model with the stochastic Landau-Lifshitz-Gilbert equation as underlying equation of motion. We analyze the magnon accumulation, the magnon temperature profile, as well as the propagation length of the excited magnons. The frequency distribution of the generated magnons is investigated in order to derive an expression for the influence of the anisotropy and the damping parameter on the magnon propagation length. For soft ferromagnetic insulators with low damping a propagation length in the range of some micrometers can be expected for exchange driven magnons.

#### 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 breakdown—which usually speeds down the wall—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.

#### Domain walls in thermal gradients : entropic torque and angular momentum transfer

2015, Hinzke, Denise, Schlickeiser, Frank, Yanes Díaz, Rocio, Ritzmann, Ulrike, Selzer, Severin, Nowak, Ulrich

#### 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, Kläui, 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, Kläui, 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.

#### Length Scale of the Spin Seebeck Effect

2015, Kehlberger, Andreas, Ritzmann, Ulrike, Hinzke, Denise, Guo, Er-Jia, Cramer, Joel, Jakob, Gerhard, Onbasli, Mehmet C., Kim, Dong Hun, Ross, Caroline A., Jungfleisch, Matthias B., Hillebrands, Burkard, Nowak, Ulrich, Kläui, Mathias

We investigate the origin of the spin Seebeck effect in yttrium iron garnet (YIG) samples for film thicknesses from 20 nm to 50 μm at room temperature and 50 K. Our results reveal a characteristic increase of the longitudinal spin Seebeck effect amplitude with the thickness of the insulating ferrimagnetic YIG, which levels off at a critical thickness that increases with decreasing temperature. The observed behavior cannot be explained as an interface effect or by variations of the material parameters. Comparison to numerical simulations of thermal magnonic spin currents yields qualitative agreement for the thickness dependence resulting from the finite magnon propagation length. This allows us to trace the origin of the observed signals to genuine bulk magnonic spin currents due to the spin Seebeck effect ruling out an interface origin and allowing us to gauge the reach of thermally excited magnons in this system for different temperatures. At low temperature, even quantitative agreement with the simulations is found.