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.


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.

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Multiscale modeling of magnetic materials: Temperature dependence of the exchange stiffness

2010, Atxitia, Unai, Hinzke, Denise, Chubykalo-Fesenko, Oksana, Nowak, Ulrich, Kachkachi, Hamid, Mryasov, Oleg N., Evans, Richard Francis L., Chantrell, Roy W.

For finite-temperature micromagnetic simulations the knowledge of the temperature dependence of the exchange stiffness plays a central role. We use two approaches for the calculation of the thermodynamic exchange parameter from spin models: (i) based on the domain-wall energy and (ii) based on the spin-wave dispersion. The corresponding analytical and numerical approaches are introduced and compared. A general theory for the temperature dependence and scaling of the exchange stiffness is developed using the classical spectral density method. The low-temperature exchange stiffness A is found to scale with magnetization as m1.66 for systems on a simple cubic lattice and as m1.76 for an FePt Hamiltonian parametrized through ab initio calculations. The additional reduction in the scaling exponent, as compared to the mean-field theory (A~ m2), comes from the nonlinear spin-wave effects.