Bibliothek

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: 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: Second harmonic generation from magnetic materials is shown to lead to a nonlinear magneto-optical Kerr effect that can be orders of magnitude larger than its linear equivalent. The origin of this effect can be found in the differences between the linear and nonlinear solutions of the optical wave equations and in the symmetry properties of the corresponding optical tensors. Applications for the study of magnetic surfaces, thin films, and multilayers will be discussed. © 1996 American Institute of Physics.

Notizen: A magnetization model is presented that is used to cover trilayers containing two magnetic layers that are exchange coupled via an intermediate nonmagnetic layer and that have different crystalline anisotropies. The interfaces are coupled to the bulk by a twisted magnetization configuration which is evaluated using the Ritz method. By minimizing the total energy, experimental magnetization curves of strongly coupled Co/Ru sandwiches can be reproduced with a good precision and with the same set of parameters in two perpendicular field directions. These physical parameters can be determined with a good reliability and are in agreement with the literature except for the bulk anisotropy of the Co layer first deposited, which is twice as large as the known bulk value. This originates in the magnetoelastic contributions due to lattice misfit and interface roughness. It is shown that the interlayer exchange coupling forces the magnetization of both layers to be along the same axis in the low-field range notwithstanding the opposite sign of the anisotropy constants in most stacks. It is also demonstrated that the differences in the orientations of the moments in one Co layer are modest and depend on the various parameters. In particular, the bulk exchange constant is a decisive parameter that makes the calculated curves close to the experimental ones. © 1996 American Institute of Physics.

Notizen: A self-consistent domain theory, based on micromagnetic principles, is unfolded for two-dimensional solenoidal M distributions present in ideally soft-magnetic plane-parallel thin-film objects at zero external field and in the absence of conduction currents. Two types of domain configurations are distinguished: the basic structures in simply connected regions and the parallel configurations in special types of multiply connected regions—the parallel regions. A decomposition of the objects's area into disjunct subregions, either simply connected or of the parallel type, whose union completely covers the object, is put forward. A procedure for constructing all feasible parallel regions is presented. In each region, the appropriate solenoidal M distribution is specified with which M is taken parallel to the subregion's boundary. Thus, all the domain structures possible in thin-film objects with arbitrary lateral shapes can be constructed. Experimental examples are provided.

Notizen: The self-consistent domain theory, based on micromagnetic principles, is further developed in order to incorporate all possible solenoidal two-dimensional magnetization distributions in plane-parallel thin-film objects with arbitrary lateral shape. A decomposition of the object into a number of disjunct plane-parallel subregions that completely cover the object's area is put forward. In each subregion, a solenoidal M distribution is defined with the M vector parallel to the subregion's boundary, so that the M distributions in adjacent subregions properly link either via a continuous transition, or via a 180° wall at the intermediate boundary. Two types of subregions are distinguished; namely, the simple connected regions and the so-called parallel regions, being a special type of multiple connected region. In the first category, the basic structures as defined in the preceding paper on this subject are present. The parallel regions are closed ringlike configurations that are built of simpler units—the parallel segments. A parallel segment is a region bounded by two orthogonal trajectories of the same set of straight lines, while two of these straight lines close the segment at either end. No points of intersection of members of this family of lines are found inside the segment. In a specific parallel region, the distance between the orthogonal trajectories is the same for all segments. Adjacent segments in a parallel region are separated by a domain wall which is the locus of centers inside the cross section of the segments of circles that touch at corresponding orthogonal edges of both of the segments involved. A systematic procedure is developed for constructing the parallel subregions, and it is shown that, with this, all possible two-dimensional solenoidal M distributions can be recovered.

Notizen: A construction method for determining the domain structure in ideally soft-ferromagnetic cylindrical objects with plane-parallel top and bottom surfaces of arbitrary shape is presented. The self-consistent theory is confined to two-dimensional solenoidal dipole distributions in which the dipoles are parallel to the top and bottom surfaces. It is proved that the basic domain structure is uniquely defined in simply connected objects, while an extra criterion has to be added in order to guarantee the uniqueness in the multiply connected ones. The treatment is based on differential geometrical principles. The object edge is partitioned into segments, in which each segment is situated in between two adjacent edge points where the radii of curvature of convex edge segments are locally minimal. To each edge segment, a region is attributed, in which M is uniquely specified by the course of M along that edge segment. In the cross section of regions corresponding to different edge segments, domain walls provide an adequate separation of the dipole distribution imposed by these segments. The extremities of these domain walls are found in the singular points of the evolute corresponding to the extremities of the edge segments and in the points where a number of walls meet. It is proved that the basic domain structure is the locus of centers of all circles inside the object that touch the object edge at at least two points. A number of experimentally observed basic structures are given, and the relevance of the definition of basic structures in multiply connected objects is examined. OFF

Notizen: A method for constructing three-dimensional solenoidal magnetization distributions, with invariant magnitude of m, in arbitrarily shaped objects is presented. The formalism harks back to the theory developed by van den Berg for two-dimensional m distributions. The space within a general object Ω is partitioned into i subspaces Ωi, described by a family of surfaces to which the magnetization is tangent. A characteristic equation which defines the course of m at each of the surfaces is derived. A boundary condition for m arises naturally, or can be chosen to determine m at the surface. Within the above framework an infinite number of solutions are generated that, in general, exhibit singularities. Special attention, also from the topological point of view, is paid to the m distributions having point defects only.

Notizen: A description is given of the construction and operating procedure of a parallel-plate apparatus for the measurement of the thermal conductivity coefficient of fluids. This instrument can be operated at pressures up to 150 MPa and at temperatures down to 77 K. The use of platinum resistance temperature sensors allows measurements with temperature differences between upper and lower plate as small as 1 mK, which together with the small plate separation of 155 μm, makes the instrument suitable for both the normal fluid region as well as for the region very close to the critical point. The complete working equations for the instrument are presented and evaluated.

Notizen: We have studied the magnetic domain structure of a thin polycrystalline Co film by magnetic force microscopy (MFM). Domain walls of the cross-tie type have been observed for a Co film of 50 nm thickness. Due to the high lateral resolution of MFM we have been able to study the magnetic structure of a single cross tie. We have determined locations of Bloch lines within a domain wall comparing the experimental data with a theoretical model of a cross-tie wall. In order to explain our experimental results we have proposed a model for the interaction between a MFM tip and a cross-tie wall. © 1996 American Institute of Physics.