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.

#### Domain wall motion in nanowires

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

We investigated the motion of domain walls in ferromagnetic cylindrical nanowires by solving the Landauâ€“Lifshitzâ€“Gilbert equation numerically for a classical spin model in which energy contributions from exchange, crystalline anisotropy, dipoleâ€“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.

#### 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/Î±Â².

#### Asymmetric Reversal Modes in Ferromagnetic/Antiferromagnetic Multilayers

2003, Beckmann, BjÃ¶rn, Nowak, Ulrich, Usadel, Klaus-Dieter

Experimentally an asymmetry of the reversal modes has been found in certain exchange bias systems. From a numerical investigation of the domain state model evidence is gained that this effect depends on the angle between the easy axis of the antiferromagnet and the applied magnetic field. Depending on this angle the ferromagnet reverses either symmetrically, e.g., by a coherent rotation on both sides of the loop, or the reversal is asymmetric with a nonuniform reversal mode for the ascending branch, which may even yield a zero perpendicular magnetization.

#### Cooling-field dependence of asymmetric reversal modes for ferromagnetic/antiferromagnetic multilayers

2006, Beckmann, BjÃ¶rn, 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.

#### Coercivity and exchange bias of ferromagnetic/antiferromagnetic multilayers

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 NÃ©el 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.

#### Control of exchange bias by diluting the antiferromagnetic layer

2003, Misra, Arkajyoti, Nowak, Ulrich, Usadel, Klaus-Dieter

The domain state model for exchange bias is used for an investigation of recent experiments where the magnitude and direction of the exchange bias was controlled by He ion irradiation of an FeNi/FeMn sample. The defects in the sample which result from the irradiation are modeled as diluting the antiferromagnet (AFM) after the initial cooling procedure. This late dilution, carried out in presence of a field, leads to a rearrangement of the original domain structure of the AFM resulting in an enhancement or reduction in the bias field.

#### 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.

#### Structure of domains in an exchange-bias model

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.

#### Antiferromagnetic layer thickness dependence of the IrMn/Co exchange-bias system

2003, Ali, Mannan, Marrows, Christopher H., Al-Jawad, M., Hickey, Bryan J., Misra, Arkajyoti, Nowak, Ulrich, Usadel, Klaus-Dieter

A study of exchange bias in IrMn/Co systems is presented. Temperature and thickness dependence studies have revealed nonmonotonic behavior in both exchange bias field and coercivity with both variables. In particular the exchange bias field shows a peak for low IrMn thicknesses that is suppressed at temperatures higher than about 200 K. Calculations using the domain state model of exchange biasing are able to describe all the features seen in the experimental data.