ALBERT

Notizen: A significant loss in the giant magnetoresistive signal of magnetic stacks with antiferromagnetic coupling across nonmagnetic intermediate layers is caused by regions with a ferro- rather than an antiferromagnetic coupling. The impact of these ferromagnetic coupling regions extends itself into the lateral direction due to the bulk exchange coupling. The present micromagnetic model provides a tool by which a detailed quantitative evaluation of the impact of periodic arrays of parallel line defects is possible. These defects have deviating exchange-coupling constants, and/or anisotropy constants or directions, bulk exchange constants, saturation magnetization, etc., in specific regions. Previously, we developed a phenomenological model of trilayers with two magnetic films separated by a nonmagnetic interlayer that contains one such defect. This model, with a relatively small number of free parameters, allows one to trace complete hysteresis curves. A large number of mode branches reveal themselves and jumpwise transitions between these modes frequently occur along the hysteresis loops. The present micromagnetic model requires a sufficiently accurate assessment of the starting magnetization configuration in order to get a convergence of the code. In general, the micromagnetic code is not capable of overcoming the above irreversible mode conversions. The mode branches evaluated by the phenomenological model are applied to provide the micromagnetic model with appropriate starting configurations after meeting a situation of nonstability.The micromagnetic theory of Brown constitutes the basis of the present approach. The micromagnetic effective field is calculated at grid points and the torque exerted by it on the magnetic dipole is made zero at each grid side by an iteration scheme. The long ranging magnetostatic fields are given by convolution integrals and are evaluated in the Fourier space by using two-dimensional fast Fourier transforms. The single defect is micromagnetically studied by zero padding techniques. Depending on the course of the external field, two different wall regions reveal themselves, to wit, the wall core and the so-called Néel tails. These tails were not incorporated into the phenomenological model. Provided that the defects are sufficiently wide spaced, the agreement between both models is rather good in the core regions. The impact on the GMR signal, in particular of the Néel tails, will be discussed with emphasis on systems with weak interlayer coupling, e.g., the decoupled systems.

Notizen: We have measured the thermal conductivity of argon at temperatures from 302 K down to 150.8 K and at densities up to 25 mol L−1. The data were obtained with a steady-state method and we employed a guarded parallel-plate apparatus designed especially for investigating the thermal conductivity of fluids in the critical region. To interpret the data in the critical region a scaled crossover equation of state for argon in the critical region has been constructed. Equations for the thermal conductivity and viscosity of argon as a function of density and temperature are presented.

Notizen: New experimental data are presented for the thermal conductivity of ethane. The thermal conductivity has been measured with a parallel-plate method as a function of temperature along eleven different isochores so as to obtain detailed information on the enhancement of the thermal conductivity in the critical region. The experimental results appear to be consistent with a recently proposed theoretical equation that accounts for both the asymptotic and nonasymptotic critical behavior of the thermal conductivity of fluids.

Notizen: Recently, Zawaideh et al. published a paper [Phys. Fluids B 2, 647 (1990)] asserting that they had obtained, from a more accurate set of fluid equations, physically reasonable plasma sheath solutions of an oscillating nature violating Bohm's criterion. In contrast to this statement, it is shown here that the violation of Bohm's criterion is based on an incorrect interpretation of the plasma sheath concept and that the oscillations are due to an incorrect initial condition in the calculations.

Notizen: The plasma–sheath problem (λD → 0) for an asymmetric warm collision-free plasma is solved in plane parallel geometry. To this end Emmert's model source term [Phys. Fluids 23, 803 (1980)] is generalized to account for a superimposed plasma drift. The quasineutral plasma approximation results in a system of coupled integral equations that can be solved analytically. In contrast to Emmert's symmetric model the upstream facing sheath edge shows the usual field singularity. At the downstream facing sheath edge, a finite field strength and an oversatisfied Bohm criterion are found. These results are in full agreement with general considerations on the Bohm criterion and on the sheath edge field singularity [Phys. Fluids B 1, 961 (1989)].