D.-X. Chen

AC impedance and circular permeability of slab and cylinder

The spectra of internal ac impedance and fluxmetric and energetically effective circular permeabilities are calculated for infinite slab and cylinder in classical and domain models. The domain models assume transverse bar domains for a slab and transverse circular disk domains for a cylinder, with technical magnetization being carried out by domain wall displacements. For each case, a general formula is given together with the low and high-frequency limits. Numerical results are computed accurately from analytical formulas. Quantitative comparisons among different cases are made. Physical concepts and problems of practical significance are discussed on the basis of the results.

Revised core-shell domain model for magnetostrictive amorphous wires

The radial, circular, and axial components /spl sigma//sub r/, /spl sigma//sub /spl phi//, and /spl sigma//sub x/, of the residual stress tensor in amorphous wires made by quenching in rotating water are derived from the classical elasticity theory on the assumption that the density decreases from the wire surface (r=r/sub 0/) as r/sup 8//r/sub 0//sup 8/. After a small reduction in /spl sigma//sub x/ and a small rotation of the easy directions in the core, the current widely applied core-shell domain model is revised. In contrast to the current model, which assumes an axially magnetized core and different core radius r/sub c/ for wires with positive and negative magnetostriction, the revised model assumes: 1) a fixed core radius /spl tau//sub c//spl sim/0.75 r/sub 0/ and 2) a helical core magnetization tilting about /spl plusmn/30/spl deg/ from the axial (circular) directions for wires of positive (negative) magnetostriction. This revision is consistent with the existing basic results of domain observations and magnetic measurements and it is further supported by experiments using a new technique described in this paper.

Transverse demagnetizing factors of long rectangular bars: I. Analytical expressions for extreme values of susceptibility

The demagnetizing problem should be studied numerically in nonellipsoids with nonzero susceptibility χ, except for a few limiting cases where analytical treatment turns out to be practical. In this work, for an infinitely long bar with rectangular cross section 2a×2b and χ=∞ and −1 in a uniform transverse applied field Ha along dimension a or b, analytical expressions for the surface pole or current distributions and the fluxmetric and magnetometric demagnetizing factors Nf,m are derived using a technique of conformal transformation. From the new as well as the existing formulas, Nf,m(χ=∞,0,−1) are evaluated, plotted, and tabulated as functions of a/b.

Comment on “Analysis of asymmetric giant magnetoimpedance in field-annealed Co-based amorphous ribbon” [Appl. Phys. Lett. 75, 2114 (1999)]

For the applied field H to be within 610 Oe, Z(H) is basically reversible. At a low frequency of 0.1 MHz, Z in positive H can be 20% greater than that in negative H if Ha .0.3 Oe; and if the frequency is high, f 510 MHz, Z(H) always shows two peaks located at positive and negative H with Z max .Z max . This high-f feature was further systematically shown in Ref. 2: with increasing Ha from 0.05 to 3 Oe, the difference between Z max and Z max increases from 2% to 20%. In order to understand the high-f results, an analysis was made in Ref. 2 based on domain magnetization rotations. Consider the magnetization Ms rotation in a single domain with a uniaxial anisotropy of anisotropy field Hk in an amorphous core and a unidirectional anisotropy of bias field Hb due to the exchange coupling between the core and a crystallized layer resulting from the field annealing in air. Assume the longitudinal and transverse directions to be along the x and y directions, and Ms , Hk , and Hb to be in the xy plane making angles u, u k , and u b with the y axis. Then, the energy density under a longitudinal field H can be written E5 1 HkMs sin 2 ~u2u k!2HbMs cos~u2u b!

Magnetic and transport eddy-current anomalies in cylinders with core-and-shell regions

Abstract For a conducting magnetic cylinder of radius r0, conductivity σ, and regionally uniform DC permeabilities μ c , μ s , and μb in the core, the shell, and the core–shell border at r=rb

Domain wall propagation in bistable amorphous wires

Magnetostrictive Fe-base amorphous wires spontaneously exhibit square-shaped hysteresis loops that is characterized by a single quite large Barkhausen jump when the switching magnetic field is reached. This jump is ascribed to the propagation of a wall along the wire. In this paper, deeper information is reported on the propagation characteristics in these wires as a function of the amplitude and the frequency of the magnetic field generating such propagation. In particular, the wall velocity and its length are experimentally determined and the results are analysed considering the quasi-planar shape model for the wall that has been introduced recently, whose validity is confirmed.

Magneto-mechanical rotation of magnetostrictive amorphous wires

The mechanical rotation of both positive (FeSiB) and negative (CoSiB) magnetostrictive rapidly quenched amorphous wires, when submitted to an alternating axial magnetic field (Hac) with a frequency of a few kHz, has been investigated. Hac was varied from a few A/m to around 21 kA/m and a laser-based method was implemented to accurately determine the wire rotation frequencies. The appearance of such effect was found to be directly related with both the magnetostrictive nature of these materials and the diameter of the inner tube of the alternating current coil, φ, in which the wires were placed. A dynamical equilibrium of the effect of rotation was only reached for small values of φ. Different frictional arrangements yielded modifications of the spectrum obtained on plotting exciting frequency versus wire rotation frequency (typically several tens of Hz). When rotation started, a directionally controlled axial direct current magnetic field was applied, which eventually made the wires stop rotating. Accordi...

Anomalous asymmetric magneto-inductance in amorphous Co68.2Fe4.3Si12.5B15 wire with shifted hysteresis loop

The quasi-saturated hysteresis M (H ) loop of a nearly zero magnetostrictive CoFeSiB wire, annealed at 470 °C and pre-magnetized with a positive field of 16 kA m-1 , is shifted 0.23 A m-1 in the negative field direction. The magneto-inductance L (H ) curve of this wire is extremely asymmetric, exhibiting a few times greater L in the positive than in the negative H . The shifted loop can be logically explained by considering the magnetostatic interaction between positively aligned hard magnetic crystallites and a soft magnetic matrix. However, a similar explanation for the asymmetry of the magneto-inductance is invalid, and therefore this effect can be regarded as an anomalous behaviour in the technical magnetization of some multiphase ferromagnets.

Enhanced circular susceptibility of nearly-zero magnetostrictive amorphous wires under axial bias field

Abstract The circular susceptibility of nearly zero magnetostrictive amorphous wires with saturation magnetization M s under an axially applied magnetic field H can be one order of magnitude greater than M s / H , the theoretical circular susceptibility deduced from the assumption of zero anisotropies. This phenomenon should be seriously considered in recent studies on magneto-impedance, and it can also become a powerful probe to the micromagnetic structure and local anisotropies in ferromagnetic wires if its further understanding is reached.

Practical model and calculation of AC resistance of long solenoids

In order to study the impedance of long solenoids, a tube model has traditionally been used. The model resistance is expressed in terms of the Bessel functions. We show how to relate the model resistance to the resistance of the actual solenoid, so that the model can be better used in practice. Since the computation accuracy when using the Bessel functions may decrease quickly with increasing the magnitude of their argument, the model results can be calculated only for thin solenoids at low frequencies. This problem has traditionally been solved by using the asymptotic expressions of Bessel functions. We show the appreciable error owing to this and propose a simple approach to make a very accurate correction. Practical formulas for the ac inductance of solenoids are also given.