Magnetic symmetry of the plain domain walls in ferro- and ferrimagnets

Magnetic symmetry of all possible plane domain walls in ferro- and ferrimagnets is considered. Magnetic symmetry classes of non 180° (including 0°) domain walls are obtained. The domain walls degeneracy is investigated. The symmetry classification is applied for research of all possible plane domain walls in crystals of the hexoctahedral crystallographic class.     

Interaction of a moving domain wall with surface magnetoelastic waves in yttrium orthoferrite

The amplitude-frequency characteristics of magnetoelastic surface waves excited by moving domain walls in a lamellar yttrium orthoferrite samples are discovered and measured. The results of analysis of the effect of magnetoelastic surface waves on the dynamics of domain walls in this orthoferrite are considered. The nonlinear interaction between magnetoelastic surface waves accompanying a moving domain wall is analyzed.     

Acoustic streaming with allowance for heat transfer

Acoustic streaming in a gas-filled cavity under vibration is numerically investigated. The case of thermally insulated cavity walls is compared with the case of walls maintained at a constant temperature. A strong influence of heat transfer on the acoustic streaming pattern is revealed for weakly nonlinear processes and vibration frequencies below resonance. Cavities of different diameters are considered.     

Localised spin waves and domain walls stability in four-sublattice antiferromagnet La2CuO4

Domain walls in the four-sublattice antiferromagnet La₂CuO₄ are considered. The spin waves spectrum on the single domain wall is studied. The stability of the walls and the intrawalls phase transitions are analysed.     

Rheology of the Cu-H2O nanofluid in porous channel with heat transfer: Multiple solutions

Dynamics of nanofluid comprising a base fluid (water) with copper (Cu) nanoparticles have been considered in channel with porous walls under magnetic field influence. The channel walls are considered to be permeable in order to analyze the wall mass transfer phenomenon. Relevant mathematical modelling has been performed and the derived PDEs are converted into coupled nonlinear ODEs by using suitable transformations. Computations have been made numerically by employing the shooting technique. It is noted that multiple solutions occur for the variation of suction Reynolds number, solid volume fraction and magnetic parameters which are interpreted in detail.     

Electroacoustic waves confined by a moving domain wall superlattice of a ferroelectric crystal

The dispersion properties of electroacoustic wave modes confined by a superlattice of uniformly moving 180° domain walls in a tetragonal ferroelectric crystal are considered. It is shown that the manifold of partial electroacoustic interfacial waves in the lattice is restricted to the first allowed band, the configuration of which in the plane of spectral variables can significantly vary under the action of the moving domain walls.     

Edge pinning and internal phase transitions in a system of domain walls

A model consisting of an array of flexible self-avoiding domain walls extending across a two-dimensional medium is considered. Adsorption phenomena in the presence of edgepinning forces and rupture, segregation, and order-disorder transitions due to shortrange attractive and repulsive interactions between the domain walls are studied using fermion transfer-matrix methods.Heisenberg Fellow of the Deutsche Forschungsgemeinschaft     

Sound propagation in a planar channel in the presence of a two-layer flow

Expressions describing the field of a point source in a planar channel with admittance walls enclosing a two-layer nonuniform flow are obtained. The dispersion equation that determines the eigenvalues in a wide range of flow velocities in the layers (including supersonic velocities) is studied. The effect of the admittance of the channel walls on the growth rate of unstable disturbances is considered for different frequencies. It is established that the effect of the admittance of the channel walls on the growth rate of the instability waves decreases with increasing frequency and essentially depends on the type of admittance. It is shown that, in the presence of the admittance, new unstable disturbances are formed with a growth rate that can exceed that of the Kelvin—Helmholtz instability wave.     

Domain wall motion in nanowires

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.     

Shielded distribution approximation for a wall-bounded classical fluid

We investigate lower order distribution functions in classical fluids in the presence of large-scale inhomogeneities, in particular those imposed by wall contacts. The consequences of the effective shielding of a wall by the nearest particle of the set being considered are determined in the context of two distribution function hierarchies, kinematic and dynamic in origin. The effects of both flat and spherical, hard and soft walls are considered, as well as those of curved and double walls. A few correction sequences to the basic shielding approximation are discussed.Supported in part by the Department of Energy under contract DE-AC02-76 ERO 3077.     

Influence of magnetic fields on the behavior of bubbles in liquid metals

The paper reviews numerical and experimental investigations concerned with the physics of rising bubbles in conducting liquid metals under the action of a magnetic field. Different situations, characterized by different void fractions ranging from single bubbles to bubble swarms, are considered. The impact of the geometrical arrangement is addressed covering large containers with bubbles far from the walls and narrow containers with bubbles interacting with the walls. It is demonstrated that magnetic fields offer a convenient means to influence bubble dynamics, which makes them interesting for technological applications.     

Effect of insulating walls on the structure of electrodynamic flows in a channel

The effect of the dielectric walls of a channel on the electrohydrodynamic flows in it is considered. The effect in question is analyzed on the basis of numerically simulating a complete set of equations that includes Poisson??s, Navier-Stokes, and Nernst-Planck equations. This led to solutions for models with narrow and wide arrangements of channel walls. The width of the profile of a liquid in the gap between electrodes in a free space is chosen as a parameter of comparison. The distributions of the electric-charge velocities and density are presented for resulting solutions. The channel-width dependences of the efficiency and flow rate are constructed.     

Mathematical simulation of nonstationary regimes of natural convection in a cubical enclosure with finite-thickness heat-conducting walls

Nonstationary regimes of conjugate thermal-gravitational convection in a cubical enclosure in the conditions of horizontal temperature difference is numerically analyzed. The external surfaces of two opposite walls were at constant different temperatures, the rest external edges were considered adiabatic. The mathematical model based on Oberbeck-Bussinesq equations is formulated in dimensionless natural velocity-pressure variables. Typical temperature and velocity fields, which represent the effect of the nonstationarity factor, Prandtl number, and thermophysical characteristics of the enclosure solid walls on the flow and heat transfer, are obtained.     

Modeling of methane steam reforming in a microchannel subject to multicomponent diffusion

Catalytic methane steam reforming in a slot microchannel under external heat supply to the mixture reacting on walls is considered based on numerical simulation of a complete system of Navier-Stokes equations. Three ways of heat supply to channel walls are represented, namely, a uniform heat flux, a heat flux linearly decreasing in channel length, and a heat flux following the reaction rate profile of the main reaction. The thermophysical parameters of the mixture depend on its temperature and composition. Two diffusion models are considered, namely, models with equal and different diffusion coefficients for each mixture component. It is shown that consideration of multicomponent diffusion does not practically affect the concentration of the components and the methane reforming at the outlet. For the above-mentioned ways of heat supply, the methane reforming with a heat flux linearly decreasing in channel length is most significant.     

Interactions of glide dislocations in a channel of a persistent slip band

Two unlike dislocations gliding in parallel slip planes in a channel of a persistent slip band are considered. Initially they are kept apart in straight screw positions. As the dislocations are pushed by the applied stress between two walls in the opposite directions, they bow out and attract one another forming a dipole. With the increasing stress the dislocations become more and more curved, until they separate. The walls of the channel are represented by elastic fields of rigid edge dipoles. The dislocations are modelled as planar curves approximated by moving polygons. The objective of the simulations is to determine the stress in the channel needed for the dislocations to escape one another. The stress and strain controlled regimes considered provide upper and lower estimates of the escape stress. The results are compared with the studies by Mughrabi and Pschenitzka, and Brown and the recent dislocation dynamics estimates. Problems encountered in the dislocation dynamics evaluation of the escape stress are analyzed.     

On the thickness of domain walls in ferroelectrics and ferroelastics

Ferroelastic and ferroelectric domain walls are commonly described by wall profiles of the tanh(x/w)-type. We argue that this profile is still a good approximation if higher-order gradient energies are considered. Such energies are relevant for phase transitions close to structural incommensurations and also for phase transitions with dominant elastic interactions. Their effect on the wall profile is to influence the effective wall thickness. Positive gradient energies tend to widen domain walls beyond the values predicted in classic Landau-Ginzburg theory.     

On the possibility of implementation of Kohn’s theorem in the case of ellipsoidal quantum dots

An electron gas in a strongly oblated ellipsoidal quantum dot with impenetrable walls is considered. Influence of the walls of the quantum dot is assumed to be so strong in the direction of the minor axis (the OZ axis) that the Coulomb interaction between electrons in this direction can be neglected and considered as two-dimensional, coupled. On the basis of geometric adiabaticity we show that in the case of a few-particle gas a powerful repulsive potential of the quantum dot walls has a parabolic form and localizes the dot in the geometric center of the structure. Due to this fact, conditions occur to implement the generalized Kohn theorem for this system.     

Second-order Percus-Yevick theory for a confined hard-sphere fluid

A fluid of hard spheres confined between two hard walls and in equilibrium with a bulk hard-sphere fluid is studied using a second-order Percus-Yevick approximation. We refer to this approximation as second-order because the correlations that are calculated depend upon the position of two hard spheres in the confined fluid. However, because the correlation functions depend upon the positions of four particles (two hard spheres and two walls treated as giant hard spheres), this is the most demanding application of the second-order theory that has been attempted. When the two walls are far apart, this calculation reduces to our earlier second-order approximation calculations of the properties of hard spheres near a single hard wall. Our earlier calculations showed this approach to be accurate for the single-wall case. In this work we calculate the density profiles and the pressure of the hard-sphere fluid on the walls. We find, by comparison with grand canonical Monte Carlo results, that the second-order approximation is very accurate, even when the two walls have a small separation. We compare with a singlet approximation (in the sense that correlation functions that depend on the position of only one hard sphere are considered). The singlet approach is fairly satisfactory when the two walls are far apart but becomes unsatisfactory when the two walls have a small separation. We also examine a simple theory of the pressure of the confined hard spheres, based on the usual Percus-Yevick theory of hard-sphere mixtures. Given the simplicity of the latter approach the results of this simple (and explicit) theory are surprisingly good.     

On Current Distribution through the Cross-Section of an Arc Moving in a Narrow Insulating Slot

Current distribution across the arc plasma cross-section constructed in a narrow slot between isolating walls and other factors affecting the distribution are considered. Experimental data on current density distribution of the arc moving at subsonic velocity are analyzed. Data available on the mean current density in an arc moving at supersonic velocity and under phase transition conditions on an active surface of the walls (gas-gen erating) are summarized. Possible influence of conditions of heat transfer with walls and pressure inside the column on the extent of non-uniformity of current distribution on the arc cross-section are shown. The problems under consideration are of practical application for avoiding thermal overload of arc chambers of magnetic-blast breakers.