Nowak, Ulrich

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## Suchergebnisse Publikationen

#### Roles of heating and helicity in ultrafast all-optical magnetization switching in TbFeCo

2018-07-16, Lu, Xianyang, Zou, Xiao, Hinzke, Denise, Liu, Tao, Wang, Yichuan, Wu, Jing, Ostler, Thomas A., Cai, Jianwang, Nowak, Ulrich, Xu, Yongbing

Using the time-resolved magneto-optical Kerr effect method, helicity-dependent all-optical magnetization switching (HD-AOS) is observed in ferrimagnetic TbFeCo films. Our results reveal the individual roles of the thermal and nonthermal effects after a single circularly polarized laser pulse. The evolution of this ultrafast switching occurs over different time scales, and a defined magnetization reversal time of 460 fs is shown—the fastest ever observed. Micromagnetic simulations based on a single macro-spin model, taking into account both heating and the inverse Faraday effect, are performed which reproduce HD-AOS demonstrating a linear path for magnetization reversal.

#### Fundamentals and applications of the Landau–Lifshitz–Bloch equation

2017-01-25, Atxitia, Unai, Hinzke, Denise, Nowak, Ulrich

The influence of thermal excitations on magnetic materials is a topic of increasing relevance in the theory of magnetism. The Landau–Lifshitz–Bloch equation describes magnetisation dynamics at finite temperatures. It can be considered as an extension of already established micromagnetic methods with a comparable numerical effort. This review is a brief summary of this new field of research, with a focus on the fundamentals of the Landau–Lifshitz–Bloch equation, its connection with the stochastic Landau–Lifshitz equation, and its applications in modern magnetism.

#### 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.

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

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

#### Magnetisation switching of FePt nanoparticle recording medium by femtosecond laser pulses

2017-12, John, Robin, Berritta, Marco, Hinzke, Denise, Müller, Jakob C., Santos, Tiffany, Ulrichs, Henning, Nieves, Pablo, Walowski, Jakob, Mondal, Ritwik, Chubykalo-Fesenko, Oksana, McCord, Jeffrey, Oppeneer, Peter M., Nowak, Ulrich, Münzenberg, Markus

Manipulation of magnetisation with ultrashort laser pulses is promising for information storage device applications. The dynamics of the magnetisation response depends on the energy transfer from the photons to the spins during the initial laser excitation. A material of special interest for magnetic storage are FePt nanoparticles, for which switching of the magnetisation with optical angular momentum was demonstrated recently. The mechanism remained unclear. Here we investigate experimentally and theoretically the all-optical switching of FePt nanoparticles. We show that the magnetisation switching is a stochastic process. We develop a complete multiscale model which allows us to optimize the number of laser shots needed to switch the magnetisation of high anisotropy FePt nanoparticles in our experiments. We conclude that only angular momentum induced optically by the inverse Faraday effect will provide switching with one single femtosecond laser pulse.

#### 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.

#### Disparate ultrafast dynamics of itinerant and localized magnetic moments in gadolinium metal

2015-09-10, Frietsch, Björn, Bowlan, John, Carley, Robert, Teichmann, Martin, Wienholdt, Sönke, Hinzke, Denise, Nowak, Ulrich, Carva, Karel, Oppeneer, Peter M., Weinelt, Martin

The Heisenberg-Dirac intra-atomic exchange coupling is responsible for the formation of the atomic spin moment and thus the strongest interaction in magnetism. Therefore, it is generally assumed that intra-atomic exchange leads to a quasi-instantaneous aligning process in the magnetic moment dynamics of spins in separate, on-site atomic orbitals. Following ultrashort optical excitation of gadolinium metal, we concurrently record in photoemission the 4f magnetic linear dichroism and 5d exchange splitting. Their dynamics differ by one order of magnitude, with decay constants of 14 versus 0.8 ps, respectively. Spin dynamics simulations based on an orbital-resolved Heisenberg Hamiltonian combined with first-principles calculations explain the particular dynamics of 5d and 4f spin moments well, and corroborate that the 5d exchange splitting traces closely the 5d spin-moment dynamics. Thus gadolinium shows disparate dynamics of the localized 4f and the itinerant 5d spin moments, demonstrating a breakdown of their intra-atomic exchange alignment on a picosecond timescale.

#### Modeling ultrafast all-optical switching in synthetic ferrimagnets

2017-06-28, Gerlach, Stefan, Oroszlany, Laszlo, Hinzke, Denise, Sievering, Steffen, Wienholdt, Sönke, Szunyogh, Laszlo, Nowak, Ulrich

Based on numerical simulations, we demonstrate thermally induced magnetic switching in synthetic ferrimagnets composed of multilayers of rare-earth and transition metals. Our findings show that deterministic magnetization reversal occurs above a certain threshold temperature if the ratio of transition-metal atoms to rare-earth atoms is sufficiently large. Surprisingly, the total thickness of the multilayer system has little effect on the occurrence of switching. We further provide a simple argument to explain the temperature dependence of the reversal process.

#### Role of temperature-dependent spin model parameters in ultra-fast magnetization dynamics

2017, Deák, András, Hinzke, Denise, Szunyogh, László, Nowak, Ulrich

In the spirit of multiscale modelling magnetization dynamics at elevated temperature is often simulated in terms of a spin model where the model parameters are derived from first principles. While these parameters are mostly assumed temperatureindependent and thermal properties arise from spin fluctuations only, other scenarios are also possible. Choosing bcc Fe as an example, we investigate the influence of different kinds of model assumptions on ultra fast spin dynamics, where following a femtosecond laser pulse, a sample is demagnetized due to a sudden rise of the electron temperature. While different model assumptions do not affect the simulational results qualitatively, their details do depend on the nature of the modelling.

#### 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.