Non-uniform Exchange Research Articles

Magnetic and Mössbauer study of hydrogen-amorphized GdFe2Hx ferrimagnet

Changes in the magnetic properties of the intermetallic GdFe2 compound during hydrogen-induced amorphization have been studied. A sharp decrease in energy of the GdFe exchange interaction and an increase in the magnetic moment of iron from 1.6 μB in the initial compound to 2.2 μB in an amorphous hydride were established. The results obtained were interpreted from the assumption that a heterophase nanostructure occurs, in which the main amorphous ferrimagnetic phase is enriched in Fe relative to the stoichiometry and is characterized by the non-uniform intersublattice exchange interaction.

Analytical solution of the non-uniform heat exchange in a reactor cooling coil with constant fluid flow

An analytical solution of the partial differential equation describing the enthalpy balance of the cooling fluid in a batch exothermic reactor has been developed for a constant flow. Thanks to an analytical integration on position, the simulation of the process can be performed with one independant variable (time) and without any approximation on the profile of the cooling fluid temperature. Comparisons are made with the well-known approximate method of segmentation into finite volumes at uniform temperature. The proposed method requires the integration of one ordinary differential equation (ODE) instead of the system of N + 1 ODEs used in the N finite volumes method. It is shown in simulations that a small number of finite elements (one to six) leads to a large inaccuracy in the results, however, it is commonly used. Finally the analytic method is applied as a zeroth order approximation to the case of a time varying flow. Comparisons are also made with the finite elements method and shown very good simulation results.

Dynamical critical exponent of a double chain of spins

We study the dynamical critical behaviour of a double chain of spins where we take into account short-range four-spin interactions. This system is exactly soluble only in the thermodynamical equilibrium. We use the initial response rate of the order parameter to establish a lower bound to the dynamical critical exponent z. We show that, for the one- and two-spin-flip Glauber transition rates, the exponent z depends on the microscopic details of the Hamiltonian, that is, the dynamical critical exponent appears non-universal. This type of non-universal behaviour has already been seen in the one-dimensional Ising model with non-uniform exchange interactions.

Micromagnetic computation for wall and Bloch line coercivity in thin films with perpendicular anisotropy

Wall and vertical Bloch line (VBL) coercivity arising from spatial nonuniformity in the material parameters has been investigated for a typical 5-μm bubble garnet film by means of a two-dimensional micromagnetic computation. Two-dimensional sinusoidal modulations in the magnitude of the uniaxial anisotropy were assumed as a model for a compositional nonuniformity. Nonuniformities with a spatial wavelength comparable to the wall width were found to exert the largest pinning effect. The typical computed value for the wall and VBL coercivity were 0.7 and 2.2 Oe, respectively, where 10% variation and wavelength of 0.47 μm were assumed. The tendency, that the VBL coercivity is larger than that for the domain wall, agrees with the experimental results reported previously. The wall and VBL coercivity caused by a nonuniform exchange constant have also been computed and compared to the analytical solutions due to the step-like variation of wall and VBL energies, respectively. The larger VBL coercivity compared to that for the wall was observed for a local modulation with the size less than wall width and period more than 1 μm.

Estimation of electronic and structural influence on the thermal magnetic properties of clusters.

Using an effective Heisenberg model for clusters the spin-wave spectrum is calculated by direct diagonalization. The inclusion of a nonuniform magnetization profile at zero temperature, nonuniform exchange interactions as a result of structural relaxations, longer-range interactions, and of magnetic surface anisotropy have been studied. Very small effects are found on the thermal magnetic properties relative to those predicted for the simple nearest-neighbor Heisenberg model by Hendriksen, Linderoth, and Lindg\aa{}rd [J. Phys. C 31, 5675 (1993); Phys. Rev. B 48, 7259 (1993)].

Magnetodipole mechanism of flexural instability of a moving domain boundary

The steady motion of a one-dimensional domain boundary in a single-axis ferromagnet has been studied. It has been shown that the flat domain boundary becomes unstable with respect to flexural deformation of its structure at a uniform translation velocity that is less than the Walker velocity. Instability of the domain boundary flat state is associated with the nonlocal nature of the magnetodipole interaction. An expression for the limit velocity is determined as a function of the relationship between the nonuniform exchange constants.

Nonuniform exchange interaction effect on the structure of ferromagnet domain boundary

In this paper the phase diagrams for uniform states of a rhombic-ferromagnet domain boundary are plotted. The separate domain boundary in a thick ferromagnetic film (the easy axis is in the film plane) is studied. The regions of phase-diagram parameters (nonuniform exchange interaction, rhombic anisotropy, external magnetic field), where the domain boundary has no stable one-dimensional states, are determined. The existence of these regions is a result of the long-range character of magnetic-dipole interaction. It is shown that the transition across the region of multidimensional states by the changes of phase-diagram parameters can lead to the generation of subdomain structure. The expression for the flexural-oscillation spectrum of a plane domain boundary of a rhombic ferromagnet is defined. Both types of relaxation (uniform and exchange) are taken into account in the spectrum of flexural oscillations.

A nonsteady-state two-dimensional model and analysis of nonisothermal state of heat pipe surfaces under nonuniform heat exchange along the perimeter and length

Results of theoretical and experimental studies of heat pipes with nonuniform heat exchange along the perimeter and length are presented.

Stomatal closure fully accounts for the inhibition of photosynthesis by abscisic acid

The partial pressure of intercellular CO2 calculated from gas exchange data for abscisic acid-treated leaves of grapevine (Vitis vinifera L.) and sunflower (Helianthus annuus L.) does not indicate the average intercellular CO2 of the leaf. The latter can be determined from chlorophyll fluorescence quenching information and accurately modelled from gas exchange data. Stomatal closure can fully account for previously assumed non-stomatal inhibition of photosynthesis. Autoradiograms show that abscisic acid induces non-uniform gas exchange over small areas of the leaf.

Thermal stresses in a glass-textolite plate with nonuniform heat exchange

Thermal stresses in a glass-textolite plate with nonuniform heat exchange

THE FOREIGN EXCHANGE CONSTRAINT TARIFFS AND IMPORT SUBSTITUTION

SUMMARY Recent studies have stressed that a foreign exchange constraint can be limiting the rate of output growth in many developing countries. On the basis of a simple constraints model the role of tariffs and related import substitution in relieving a foreign exchange constraint are analyzed. It is first argued that the assumptions necessary for a foreign exchange constraint to be limiting output growth require that the optimal government intervention is through non-uniform exchange rate adjustment implied by trade controls and multiple exchange rates. But, whereas the optimum government intervention is through the establishment of a structure of optimum tariffs and subsidies, such a policy is not feasible in light of data availability and administrative capacity in developing countries. The costs and benefits of a non-optimal intervention through controls are then explored and standards of minimum acceptability of developing countries’ trade policies are established against which actual policy can be compared. It is concluded that trade controls can be used to improve growth performance but only if a number of rigid conditions are met.

A Study of Induced Vorticity in Centrifugal Compressors

Recent investigations have moved a step closer to an understanding of real fluid phenomena in turbomachines by studying rotational flow once it has been established by viscous stresses and nonuniform exchange of energy. The present paper presents a more generalized mathematical treatment of rotational flow (Part I) and makes use of the equations developed to analyze experimentally observed phenomena in the impeller (Part II) and the diffuser (Part III) of a compressor. These studies show that induced vorticity in the impeller produces a nonuniform energy exchange which is the basis for additional rotational effects in the diffuser. Also the amount of vorticity generated varies with flow rate which increases the sensitivity of downstream components to changes in flow rate and thereby reduces compressor range. In the particular diffuser studied, the flow conditions, up to the point of separation, were all established primarily by the induced vorticity while shearing stresses next to the wall seemed to have a negligible influence. Downstream from the beginning of separation, however, an apparent shift in energy distribution is observed which could not be accounted for by the analysis. It is demonstrated that separation is detrimental to vaneless diffuser performance only if it is located at the diffuser discharge. Since the location of separation is a function of the vorticity induced, which varies with the flow rate, a widely spaced diffuser can have substantial variations in performance with a significant influence on the compressor characteristics.

Stresses in Glass Produced by Nonuniform Exchange of Monovalent Ions

A study was made of the rate of diffusion of alkali ions into a soda‐lime glass from nitrate melts, and the distortion of glass disks produced by the stresses thus developed was measured using an interference technique. A maximum of 124,000 psi was produced when Na+ was replaced by K+. Tensile stresses accompanying the replacement of a large ion by a smaller one or by leaching ions out of the surface with water or dilute acid may be large and can destroy the glass.