Nowak, Ulrich

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## Suchergebnisse Publikationen

#### 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.

#### Towards multiscale modeling of magnetic materials : Simulations of FePt

2008, Kazantseva, Natalia, Hinzke, Denise, Nowak, Ulrich, Chantrell, Roy W., Atxitia, Unai, Chubykalo-Fesenko, Oksana

The established methods for the numerical evaluation of magnetic material properties exist only in certain limits, including first-principles methods, spin models, and micromagnetics. In the present paper, we introduce a multiscale modeling approach, bridging the gaps between the three approaches above. The goal is to describe thermodynamic equilibrium and nonequilibrium properties of magnetic materials on length scales up to micrometers, starting from first principles. In the first step, we model, as an example, bulk FePt in the ordered Llo phase by using an effective, classical spin Hamiltonian that was constructed earlier on the basis of firstprinciples methods. The next step is to simulate this spin model by using the stochastic Landau-LifshitzGilbert equation. The temperature dependent micromagnetic parameters, which are evaluated with these atomistic simulations, are consequently used to develop a many macrospin micromagnetic approach, based on the Landau-Lifshitz-Bloch equation. As an example, we calculate the magnetization dynamics following a picosecond heat pulse resembling pump-probe experiments.

#### Dynamic approach for micromagnetics close to the Curie temperature

2006, Chubykalo-Fesenko, Oksana, Nowak, Ulrich, Chantrell, Roy W., Garanin, D.

In conventional micromagnetism magnetic domain configurations are calculated based on a continuum theory for the magnetization. This theory assumes that the absolute magnetization value is constant in space and time. Dynamics is usually described with the Landau-Lifshitz-Gilbert (LLG) equation, the stochastic variant of which includes finite temperatures. Using simulation techniques with atomistic resolution we show that this conventional micromagnetic approach fails for higher temperatures since we find two effects which cannot be described in terms of the LLG equation: (i) an enhanced damping when approaching the Curie temperature and, (ii) a magnetization magnitude that is not constant in time. We show, however, that both of these effects are naturally described by the Landau-Lifshitz-Bloch equation which links the LLG equation with the theory of critical phenomena and turns out to be a more realistic equation for magnetization dynamics at elevated temperatures.

#### Exchange bias in ferromagnetic/antiferromagnetic bilayers with imperfect interfaces

2006, Spray, J., Nowak, Ulrich

The influence of an imperfect interface on exchange bias (EB) properties is investigated. Within the framework of the domain state model, the EB field HEB and the coercive field HC are determined using computer simulations, and they are found to depend strongly on the details of the interface structure. This dependence is sensitive to the dilution of the antiferromagnet (AFM) with non-magnetic defects in the bulk. For the optimal interface structure, giving greatest EB, the optimal dilution is found to be much less than that for an ideal-interface system, taking a value in better agreement with experimental results. Even without any defects in the bulk of the AFM the interface roughness leads to EB for thin antiferromagnetic layers, in accordance with the model by Malozemoff. Finally, the thickness dependence of rough-interface systems is found to differ significantly from that of ideal-interface systems.

#### Multiscale modeling of ultrafast element-specific magnetization dynamics of ferromagnetic alloys

2015, Hinzke, Denise, Atxitia, Unai, Carva, Karel, Nieves, Pablo, Chubykalo-Fesenko, Oksana, Oppeneer, Peter M., Nowak, Ulrich

A hierarchical multiscale approach to model the magnetization dynamics of ferromagnetic random alloys is presented. First-principles calculations of the Heisenberg exchange integrals are linked to atomistic spin models based upon the stochastic Landau-Lifshitz-Gilbert (LLG) equation to calculate temperature-dependent parameters (e.g., effective exchange interactions, damping parameters). These parameters are subsequently used in the Landau-Lifshitz-Bloch (LLB) model for multisublattice magnets to calculate numerically and analytically the ultrafast demagnetization times. The developed multiscale method is applied here to FeNi (permalloy) as well as to copper-doped FeNi alloys. We find that after an ultrafast heat pulse the Ni sublattice demagnetizes faster than the Fe sublattice for the here-studied FeNi-based alloys.

#### Slow recovery of the magnetisation after a sub-picosecond heat pulse

2008, Kazantseva, Natalia, Nowak, Ulrich, Chantrell, Roy W., Hohlfeld, Julius, Rebei, Adnan

The response of a magnetic spin system to pulsed laser heating on time scales in the picosecond regime is investigated using an atomic level classical spin Hamiltonian, the dynamics of which are based on the stochastic Landau-Lifshitz-Gilbert equation. It is found that the ferro- to paramagnetic phase transition can occur in less than one picosecond, in agreement with published experimental data. Calculating the spin temperature via the internal energy of the spin system we find that the system does not necessarily fully demagnetize even for spin temperatures above the Curie temperature. Our findings suggest that the spin system is far from thermal equilibrium so that the concept of a spin temperature has to be questioned on the time scale of picoseconds. Most importantly, the time for recovery of the magnetization can vary by orders of magnitude depending on the magnetic state after heating, a prediction which is verified by supporting pumpprobe experiments.

#### Cooling-field dependence of asymmetric reversal modes for ferromagnetic/antiferromagnetic multilayers

2006, Beckmann, BjÃ¶rn, Usadel, Klaus-Dieter, Nowak, Ulrich

A numerical investigation of exchange coupled ferromagnetic/antiferromagnetic multilayers with a twinned crystal structure for the antiferromagnet is presented. Motivated by recent experimental findings we focus on the influence of the directions of the magnetic field during the initial cooling procedure and during the hysteresis. Upon variation of these directions the ferromagnet displays different reversal modes or even an asymmetric reversal with different kinds of reversal mechanisms for the decreasing and increasing branch of a single hysteresis loop. These findings can be explained within the context of the domain state model for exchange bias.

#### Training effect of exchange-bias bilayers within the domain state model

2009, Biternas, Andreas G., Nowak, Ulrich, Chantrell, Roy W.

An investigation of the temperature dependence of the training effect of various exchange coupled bilayers with different types of anisotropy is presented. We use an atomistic model for the magnetic interactions within a classical Heisenberg spin Hamiltonian. In general, the behavior of the exchange-bias field is separated into low- and high-temperature regions. This separation is made according to the trend of exchange-bias field after the second hysteresis loop and the parameters of the power-law fit for these fields. It is found that with increasing antiferromagnetic thickness, systems follow the same temperature trend but with lower values of the exchange-bias field and a weaker training effect. This is due to the fact that thicker antiferromagnetic layers lead to increased stability of the antiferromagnetic domains. Also, the behavior of the coercive fields is investigated, concluding that the training effect occurs predominantly in the first half of the hysteresis loop.

#### Domain wall properties of FePt : From Bloch to linear walls

2008, Hinzke, Denise, Kazantseva, Natalia, Nowak, Ulrich, Mryasov, Oleg N., Asselin, Pierre, Chantrell, Roy W.

An investigation of the orientation and temperature dependence of domain wall properties in FeP! is presented. We use a microscopic, atomic model for the magnetic interactions within an effective, classical spin Hamiltonian constructed on the basis of spin density functional theory. We find a significant dependence of the domain wall width and the domain wall energy on the orientation of the wall with respect to the crystal lattice. Investigating the temperature dependence, we demonstrate the existence of elliptical as well as linear domain walls in FePt. The calculation and further analysis of the domain wall free energy results in the evaluation of a thermodynamic exchange stiffness and anisotropy constant.

#### Thermodynamic behavior of nanomagnets with a vortex configuration

2006, Wieser, Robert, Usadel, Klaus-Dieter, Nowak, Ulrich

The thermodynamic behavior of flat circular nanomagnets with a vortex domain configuration is studied using Langevin dynamics simulations for the dynamical behavior as well as local mean-field calculations for equilibrium properties. Our studies show that the vortex core becomes thermally unstable with increasing temperature, acting like a superparamagnetic system. On time scales where the vortex core remains within one of the metastable states it still has a stronger temperature dependence than the magnetization far away in the bulk of a domain.