Nowak, Ulrich


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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.


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.

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Behavior of the antiferromagnetic layer during training in exchange-biased bilayers within the domain state model

2010, Biternas, Andreas G, Chantrell, Roy W., Nowak, Ulrich

An analysis of the antiferromagnetic spins is used to study the training effect in exchange bias bilayers. We use an atomistic model for the magnetic interactions within a classical Heisenberg spin Hamiltonian for the investigation of the temperature dependence of the training effect. Various exchange bias bilayers with different kinds of anisotropy are presented. The spins in the antiferromaget are grouped as stable and unstable according to their behavior during the reversal. It is found that their population changes during the training effect. The stable spins result in a magnetization which is shown to be related to the exchange bias field behavior. The behavior of the interface spins is shown to be determined predominantly by the degree of geometric spin frustration at the interface.