Search

The dc and ac flux density vs magnetic field B(H) loops of Epstein electrical strips are measured in an IEC type-A permeameter with a high-quality electrical strip wound double yokes of inside length l0 = 0.2 m and inside height h0 = 0.1 m and in a long solenoid. The relevant demagnetizing and eddy-current effects are analyzed, modeled, and discussed. It is concluded that demagnetizing corrected solenoid measurement developed for determining dc B(H) loops of the material cannot be used for the ac case, owing to complicated eddy-current demagnetizing effects. Permeameter-measured ac B(H) loops with H detected by a flat H-coil of length less than l0/2 touching the strip's middle surface may be considered representative of the actual material because H is very uniform along the strip within 3l0/4. Strips with ac B(H) loops thus determined should be used to calibrate the effective magnetic path length lm of Epstein measurements, where a very nonuniform field is applied to the strips. [ABSTRACT FROM AUTHOR]

The effective magnetic path length lm for a 25 cm Epstein frame test is studied on the basis of a relevant demagnetizing model and determined by a comparative measurement of dc and ac hysteresis loops at each value of peak flux density Bp, with the field recorded by H-coils placed above the mid-section of the limbs or calculated from the magnetizing current. The obtained Bp and frequency dependences of lm for testing core loss Ps and peak field Hp of an electrical steel are explained in terms of differential susceptibility along the measured loops and demagnetizing and eddy-current effects on local magnetization processes. [ABSTRACT FROM AUTHOR]

A reliable model of the heat transfer at the air bearing surface of a flying head slider is important for treating thermomechanical aspects of the molecular gas lubrication system in today’s hard disk drives. This article proposes a new model for heat transfer in the head disk interface, which considers both the heat conduction and viscous dissipation. The conduction heat flux based on this model shows better agreement with the numerical results of the linearized Boltzmann equation than existing models derived from the temperature jump theory. The viscous dissipation of plane Couette flow as well as that of plane Poiseuille flow in the gas film is analyzed using the energy conservation equation instead of the linearized Boltzmann equation, which is incapable of calculating the viscous dissipation at the boundaries. The new model gives simple analytical expressions for the heat flux contributed by heat conduction and viscous dissipation, and it is applicable to numerical thermomechanical simulations of the slider’s performance. [ABSTRACT FROM AUTHOR]

Complex ac susceptibility of magnetic colloids of nanospheres, each consisting of Fe3O4 nanoparticles densely and uniformly embedded in a polystyrene matrix, is measured as a function of frequency. A data analysis based on a model and the directly measured size distribution shows that each spherical aggregate of nanoparticles carries a small residual magnetic moment, whose average is basically independent of the sphere size. The mentioned model assumes that magnetic spheres undergo rotational Brownian motions obeying Debye’s theory and nanoparticles undergo Néel magnetization rotations. A discussion is made on the superparamagnetism and the nanoparticle interactions in the present case, in order to justify the applicability of the model and to look for the mechanism of the detected sphere-size independent magnetic moment. [ABSTRACT FROM AUTHOR]

Levitation force is studied in a two-dimensional system consisting of an infinitely long superconductor immersed in the magnetic field created by different arrangements of infinitely long parallel permanent magnets, based on the critical-state model using the magnetic energy minimization procedure. Results on force, stability, and losses in the levitation process in this system are analyzed. [ABSTRACT FROM AUTHOR]

Fluxmetric and magnetometric demagnetizing factors Nf and Nm have been calculated using a finite-element method for rectangular prism 2a×2b×2c with susceptibility χ=-1, in the completely shielded state. Assuming the field is applied along the c dimension, the results are plotted and tabulated as functions of their aspect ratios c/a and b/max[c,a]. [ABSTRACT FROM AUTHOR]

For an infinitely long bar with rectangular cross section 2a×2b and constant susceptibility χ in a uniform transverse applied field H[sub a] along dimension a, the fluxmetric and magnetometric demagnetizing factors N[sub f,m] as functions of a/b and χ are accurately computed using a finite-elements method. By comparison of the computed results with exact analytical results for χ=-1,0,∞ and with a set of conjugate relations derived in this work, the elements distributions have been optimized to give a minimum discretization error. This error is further greatly reduced in the final results of N[sub f,m] by using an error correction approach previously proposed in a similar work for cylinders. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents a study that calculated the fundamental complex susceptibilities from the symmetric critical-state hysteresis loops for an infinitely long hard superconductor with a rectangular cross section. Discussion on the magnetic properties of hard superconductors; Overview of the general procedure in deriving critical-state susceptibilities.

The levitation force and stability in a superconductor-magnet system with maglev geometry are studied as function of the critical-current density of the superconductor, from a realistic model based on minimizing the magnetic energy. Results show that although the force is an increasing function of the critical-current density Jc, an overhigh Jc may result in instability. Therefore, the optimum critical-current density in the superconductor for a given levitation system should be an intermediate value which depends on the system dimensions and the magnetic field of the magnet. [ABSTRACT FROM AUTHOR]

The unique properties of superconductors make the design of levitation devices possible. In some of them, lateral displacements of the superconducting levitating part often result in a decrease in the levitation force, which can cause a critical malfunction of the system. In this work, the authors theoretically analyze the levitation force of a system consisting of an infinitely long superconductor over an infinitely long permanent-magnet guideway using a realistic model based on a magnetic-energy minimization procedure, from which they give the physical keys for understanding the levitation force reduction after lateral displacements and how to minimize these effects. [ABSTRACT FROM AUTHOR]

The force and stability of a levitating system consisting of an infinitely long superconductor and different arrangements of infinitely long parallel permanent magnets are theoretically analyzed for two different cooling processes, field and zero-field cooling, by using a model based on the critical-state model and a magnetic energy minimization procedure. The authors find that by cooling the superconductor near the magnets, one can obtain improved lateral stability as compared to the case of zero-field cooling. [ABSTRACT FROM AUTHOR]

From a theoretical analysis based on an energy minimization procedure and the critical-state model, the authors analyze the levitation of an infinitely long superconductor above infinitely long permanent magnets with different arrangements. A discussion on optimum geometries showing large levitation force and good stability is presented from the model results. [ABSTRACT FROM AUTHOR]