Nowak, Ulrich

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## Suchergebnisse Publikationen

#### Exchange bias for a ferromagnetic film coupled to a spin glass

2009, Usadel, Klaus-Dieter, Nowak, Ulrich

For a model system consisting of a ferromagnetic layer exchange coupled to a spin glass, extensive Monte Carlo simulations are performed. For the spin glass the standard short-range Gaussian model is used. Exchange bias is observed as a result of a frozen spin-glass state. The exchange bias fields are calculated for different temperatures, cooling fields, and thicknesses of the spin-glass layer and the training effect is investigated. A major result of our simulations is that the bias field decreases with increasing strength of the cooling field in qualitative agreement with recent experiments.

#### Atomistic models of ultrafast reversal

2007, Kazantseva, Natalia, Hinzke, Denise, Nowak, Ulrich, Chantrell, Roy W., Chubykalo-Fesenko, Oksana

It is shown that the physics of reversal on a picosecond timescale requires the use of atomistic models. The basis of atomistic models is outlined. The model is applied to studies of fast laser heating of magnetic materials. In particular it is demonstrated that the magnetisation vanishes in a timescale of picoseconds, whereas the recovery of the magnetisation can take of the order of 1 ns because of the necessity for the magnetisation to grow from a large number of small nuclei. Finally we review the progress in linking atomistic and micromagnetic models in a step toward creating macroscopic models of magnetic materials at temperatures approaching the Curie temperature.

#### Hall magnetometry on Co, Fe, and Py nanowires

2004, Reuter, Dirk, Hoch, Sascha, Wieck, Andreas D., Hausmanns, Britta, Stahlmecke, Burkhard, Dumpich, Guenter, Wieser, Robert, Nowak, Ulrich

We have studied the magnetization reversal behavior of Co, Fe, and Py nanowires by Hall magnetometry at 4.2 K. The wires with a width w ranging from 100 nm to 1 Î¼m were fabricated with one end on a Hall cross prepared from a GaAs/AlxGa1âˆ’xAs heterostructure, containing a two-dimensional electron system. The magnetization reversal behavior differs significantly for the smallest wires (w~100 nm) and for wider wires (w>250 nm). We found that the coercitivity is inversely proportional to the width of the wire, as expected from theoretical considerations.

#### Thermisch aktivierte Ummagnetisierung in klassischen Spinsystemen

2000, Nowak, Ulrich

#### Magnetic Friction and the Role of Temperature

2009, Magiera, Martin P., Wolf, Dietrich E., Brendel, Lothar, Nowak, Ulrich

A magnetic dipole moving parallel to a ferromagnetically interacting surface is subject to a friction force due to the conversion of kinetic energy into spin excitations. This phenomenon is studied in the framework of the classical anisotropic Heisenberg-model, using the stochastic Landau-Lifshitz-Gilbert equation. The friction force is calculated from dissipation rates, which are obtained directly from energy functions. For small velocities, magnetic friction increases linearly (like Stokes' law for laminar flow). The characteristic low- and high-temperature behavior is analyzed and explained by a relaxation time ansatz.

#### A model of damping due to spin-lattice interaction

2007, Karakurt, Serdal, Chantrell, Roy W., Nowak, Ulrich

The dynamic behavior of a spin magnetic moment is often described in terms of the Landau Lifschitz Gilbert (LLG) equation. This contains two terms, the first describing the precession of the spin, and the second providing a damping of the precessional motion. Whereas the precession part is well understood at a fundamental level, only an intuitive knowledge of the damping term exists. The damping term represents the interaction between the spin system and the heat bath, and is included in a phenomenological way in the LLG equation. In order to understand the latter mechanism at a more basic level, we have developed a model in which the heat bath variables are introduced explicitly, with the damping introduced by a term which couples the spins to the heat bath. Specifically, we solve two sets of coupled dynamical equations, one representing the spin dynamics, and the second the underlying mechanical oscillations of the lattice. The former consists of the precessional term only, while for the latter we use a Brownian Dynamic approach, which excites the phonon modes of the lattice. The equations are solved numerically to give the time evolution of the magnetization. It is found that, although in principle the coupling mechanism does not affect the motion of the FMR (k=0) mode, damping of this mode does appear due to non-linearities which scatter energy into magnon modes with non-zero k. We demonstrate that the model gives results similar in form to the LLG equation, i.e. a damped precessional motion of the magnetization into the local field direction. We also present the results of a study of finite size effects, which shows that the effective damping constant is dependent on the system size because of changes in the phonon spectrum.

#### Ferromagnetic resonance in an ensemble of nanoparticles with randomly distributed anisotropy axes

2008, Sukhov, Alexander, Usadel, Klaus-Dieter, Nowak, Ulrich

Spectra of absorbed power as probed by ferromagnetic resonance (FMR) are numerically calculated within a macro-spin model for single domain nanoparticles using Landau-Lifshitz-Gilbert dynamics. Randomly distributed anisotropy axis and a distribution of anisotropy energies result in a significant broadening of the FMR signal as compared to an ensemble of particles all having the same anisotropy. Additionally, a temperature dependence of the shift of the resonance frequency is obtained which is in a qualitative agreement with experimental data on single domain nanoparticles.

#### Testing the domain state model for exchange bias

2004, Beschoten, Bernd, Keller, Janine, Nowak, Ulrich, Usadel, Klaus-Dieter, GÃ¼ntherodt, Gernot

#### Thermally activated magnetization reversal in classical spin chains

2000, Hinzke, Denise, Nowak, Ulrich, Usadel, Klaus-Dieter

We investigate the thermally activated magnetization switching in a classical Heisenberg spin chain driven by an external magnetic field. For small system sizes we expect that the magnetic moments rotate coherently, while in the case of larger system sizes the magnetization reversal is proposed to be due to soliton-antisoliton nucleation. We compare Monte Carlo simulations with the direct integration of the Landau-Lifshitz-Gilbert equation of motion with Langevin dynamics as well as with asymptotic solutions for the escape rates following from the Fokker-Planck equation, finding agreement for low temperatures and high damping. We also discuss deviations in the intermediate temperature regime.