Nowak, Ulrich

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## Suchergebnisse Publikationen

#### 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.

#### 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.

#### Spin dynamics of magnetic nanoparticles : Beyond Brown s theory

2005, Nowak, Ulrich, Mryasov, Oleg N., Wieser, Robert, Guslienko, Konstantin, Chantrell, Roy W.

An investigation of thermally induced spin dynamics of magnetic nanoparticles is presented. We use an atomistic model for the magnetic interactions within an effective, classical spin Hamiltonian constructed on the basis of first-principles calculations for L1 0 FePt. Using Langevin dynamics we investigate how the internal degrees of freedom affect the switching behavior at elevated temperatures. We find significant deviations from a single-spin model, arising from the temperature dependence of the intrinsic properties, from longitudinal magnetization fluctuations, and from both thermal and athermal finite-size effects. These findings underline the importance of atomistic simulations for the understanding of fast magnetization dynamics.

#### Universality in three dimensional random-field ground states

1999, Hartmann, Alexander K., Nowak, Ulrich

We investigate the critical behavior of three-dimensional random-field Ising systems with both Gauss and bimodal distribution of random fields and additional the three dimensional diluted Ising antiferromagnet in an external field. These models are expected to be in the same universality class. We use exact ground-state calculations with an integer optimization algorithm and by a finite-size scaling analysis we calculate the critical exponents v, β, and γ. While the random-field model with Gauss distribution of random fields and the diluted antiferromagnet appear to be in same universality class, the critical exponents of the random-field model with bimodal distribution of random fields seem to be significantly different.

#### Constrained Monte Carlo Method and Calculation of the Temperature Dependence of Magnetic Anisotropy

2010, Asselin, Pierre, Evans, Richard Francis L., Barker, Joe, Chantrell, Roy W., Yanes Díaz, Rocio, Chubykalo-Fesenko, Oksana, Hinzke, Denise, Nowak, Ulrich

We introduce a constrained Monte Carlo method which allows us to traverse the phase space of a classical spin system while fixing the magnetization direction. Subsequently we show the method's capability to model the temperature dependence of magnetic anisotropy, and for bulk uniaxial and cubic anisotropies we recover the low-temperature Callen-Callen power laws in M. We also calculate the temperature scaling of the 2-ion anisotropy in L10 FePt, and recover the experimentally observed M2.1 scaling. The method is newly applied to evaluate the temperature dependent effective anisotropy in the presence of the N'eel surface anisotropy in thin films with different easy axis configurations. In systems having different surface and bulk easy axes, we show the capability to model the temperature-induced reorientation transition. The intrinsic surface anisotropy is found to follow a linear temperature behavior in a large range of temperatures.

#### 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.

#### Domain wall mobility in nanowires : Transverse versus vortex walls

2004, Wieser, Robert, Nowak, Ulrich, Usadel, Klaus-Dieter

The motion of domain walls in ferromagnetic, cylindrical nanowires is investigated numerically by solving the Landau-Lifshitz-Gilbert equation for a classical spin model in which energy contributions from exchange, crystalline anisotropy, dipole-dipole interaction, and a driving magnetic field are considered. Depending on the diameter, either transverse domain walls or vortex walls are found. The transverse domain wall is observed for diameters smaller than the exchange length of the given material. Here, the system behaves effectively one dimensional and the domain wall mobility agrees with a result derived for a one-dimensional wall by Slonczewski. For low damping the domain wall mobility decreases with decreasing damping constant. With increasing diameter, a crossover to a vortex wall sets in which enhances the domain wall mobility drastically. For a vortex wall the domain wall mobility is described by the Walker formula, with a domain wall width depending on the diameter of the wire. The main difference is the dependence on damping: for a vortex wall the domain wall mobility can be drastically increased for small values of the damping constant up to a factor of 1/α².

#### Temperature dependence of the current-induced domain wall motion from a modified Landau-Lifshitz-Bloch equation

2009, Schieback, Christine, Hinzke, Denise, Kläui, Mathias, Nowak, Ulrich, Nielaba, Peter

We employ the Landau-Lifshitz-Bloch (LLB) equation to investigate current-induced domain wall motion at finite temperatures by numerical micromagnetic simulations. We extend the LLB equation with spin torque terms that account for the effect of spin-polarized currents and we find that the velocities depend strongly on the interplay between adiabatic and nonadiabatic spin torque terms. As a function of temperature, we find nonmonotonous behavior, which might be useful to determine the relative strengths of the spin torque terms experimentally.

#### Transition to Linear Domain Walls in Nanoconstrictions

2005, Kazantseva, Natalia, Wieser, Robert, Nowak, Ulrich

Domain walls in nanoconstrictions are investigated with a focus on thermal properties. In general, the magnetization component perpendicular to the easy axis which in a domain wall usually occurs has a value different from the easy-axis bulk magnetization value with a separate phase transition at a critical temperature below the Curie temperature. Since this effect is the more pronounced the smaller the domain wall width is, we investigate it especially in domain walls with a confined geometry, using analytical arguments, mean-field theory, and Monte Carlo simulations. Our findings may contribute to the understanding of magnetoresistive effects in domain walls with sizes of only a few atomic layers, as, e.g., in nanocontacts or nanoconstrictions.

#### Magnetization switching in nanowires : Monte Carlo study with fast Fourier transformation for dipolar fields

2000, Hinzke, Denise, Nowak, Ulrich

For the investigations of thermally activated magnetization reversal in systems of classical magnetic moments numerical methods are desirable. We present numerical studies which base on time quanti"ed Monte Carlo methods where the long-range dipole}dipole interaction is calculated with the aid of fast Fourier transformation. As an example, we study models for ferromagnetic nanowires comparing our numerical results for the characteristic time of the reversal process also with numerical data from Langevin dynamics simulations where the fast Fourier transformation method is well established. Depending on the system geometry different reversal mechanism occur like coherent rotation, nucleation, and curling.