2011-02-10, Nowak, Ulrich, Misra, Arkajyoti, Usadel, Klaus-Dieter

The domain state model for exchange bias consists of a ferromagnetic layer exchange coupled to an antiferromagnetic layer. In order to model a certain degree of disorder within the bulk of the antiferromagnet, the latter is diluted throughout its volume. Extensive Monte Carlo simulations of the model were performed in the past. Exchange bias is observed as a result of a domain state in the antiferromagnetic layer which develops during the initial field cooling, carrying a remanent domains state magnetization which is partly irreversible during hysteresis. A variety of typical effects associated with exchange bias like, e. g., its dependence on dilution, positive bias, temperature and time dependences as well as the dependence on the thickness of the antiferromagnetic layer can be explained within this model.

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.

2005-01, Wieser, Robert, Nowak, Ulrich, Usadel, Klaus-Dieter

We investigated the motion of domain walls in ferromagnetic cylindrical nanowires by solving the Landau–Lifshitz–Gilbert equation numerically for a classical spin model in which energy contributions from exchange, crystalline anisotropy, dipole–dipole interactions, and a driving magnetic field are considered. Depending on the diameter, either transverse domain walls or vortex walls are found. A transverse domain wall is observed for diameters smaller than the exchange length of the given system. In this case, the system effectively behaves one dimensionally and the domain wall velocity agrees with the result of Slonczewski for one-dimensional walls. For larger diameters, a crossover to a vortex wall sets in which enhances the domain wall velocity drastically. For a vortex wall the domain wall velocity is described by the Walker formula.

2004, Misra, Arkajyoti, Nowak, Ulrich, Usadel, Klaus-Dieter

The structure of domains in the interface monolayer of the antiferromagnet in an exchange-bias system is investigated in the framework of the domain state model. These interface domains carrying remanent magnetization provide the bias field and are strongly influenced by the bulk. The stable part of the spin configurations at the interface, which is responsible for exchange bias, is identified. The stability analysis of the interface domains leads to an explanation of the nontrivial dependence of the bias field on thickness and anisotropy of the antiferromagnet.

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.

2006, Beckmann, Björn, 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.

2005, Scholten, Gregor, Usadel, Klaus-Dieter, Nowak, Ulrich

For a model system consisting of a ferromagnetic layer exchange coupled to an antiferromagnetic layer with a compensated interface, detailed mean-field-type calculations are performed. Both the coercive field and the exchange bias field are calculated. For the coercive field, a rather broad enhancement around the Néel temperature T N of the antiferromagnetic layer is found irrespective of whether the antiferromagnetic layer is structurally disordered or not, while exchange bias is only found for disordered systems. We show that the observed enhancement of the coercivity around T N also found experimentally and the occurrence of exchange bias are of different origin.

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.

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.

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