P. Ciureanu

Giant magnetoimpedance in a cylindrical magnetic conductor

A rigorous treatment of the giant magnetoimpedance (GMI) in soft magnetic wires is presented. A small-signal approximation is used for a cylindrical magnetic conductor which is saturated along its axis by a static magnetic field. The general analysis of GMI includes a discussion of the influence of different parameters on the GMI and of how the calculation can be extended to nonsaturating fields. The comparison with high frequency impedance spectra of CoFeSiB wires measured with a network analyzer, including the observation of the ferromagnetic resonance peaks, confirms that the proposed model gives a satisfactory explanation for the linear GMI effect over a broad frequency range and opens the way to more refined calculations.

CALCULATIONS OF GIANT MAGNETOIMPEDANCE AND OF FERROMAGNETIC RESONANCE RESPONSE ARE RIGOROUSLY EQUIVALENT

It is simply demonstrated that the giant magnetoimpedance (GMI) response of a plate or ribbon is rigorously equivalent to the response of the same sample in ferromagnetic resonance (FMR) experiment. Thus, all of the solutions for FMR response behavior of metals may be applied to the description of GMI. For situations which have not been studied before, the methods which have been developed over the past 40 years for theoretical description of FMR in metals may be applied to predict the GMI behavior.

Magnetoimpedance measurements of ferromagnetic resonance and antiresonance

We report the observation of both ferromagnetic resonance and antiresonance in a magnetic metal using a magnetoimpedance technique. In this experiment, the magnetoimpedance was measured as the frequency was swept from 30 MHz to 11 GHz at constant magnetic fields ranging up to 1.1 kOe (88 kA/m). The sample was an amorphous NiCo-rich soft-magnetic wire with a saturation magnetization sufficiently small to meet both the resonance and antiresonance conditions at frequencies below 10 GHz. A saturation magnetization, very close to that obtained through magnetometry, was deduced using a simultaneous fit to the field dependence of the resonance and antiresonance frequencies. This experiment clearly demonstrates that magnetoimpedance provides a powerful tool for characterizing the intrinsic properties of magnetic metals, with several advantages compared to standard ferromagnetic resonance techniques.

Physical models of magnetoimpedance

We recall the methods for the rigorous calculation of the electromagnetic behavior of magnetic metallic samples and their application to the modeling of ferromagnetic resonance and of giant magnetoimpedance experiments. We explain the effect of various approximations and simplifications, particularly of the neglect of the exchange-conductivity effect, which has been the subject of confusion and of misconceptions in the literature, as have questions of domain wall motion and of nonlinear behavior. We show that the rigorous treatment provides a satisfactory description of experimental results, while the simplifications can only do so under limited circumstances.

HIGH FREQUENCY BEHAVIOR OF SOFT MAGNETIC WIRES USING THE GIANT MAGNETOIMPEDANCE EFFECT

We have investigated the high frequency properties of several amorphous and polycrystalline wires mounted as inner conductors in coaxial lines. A static magnetic field was applied along the wire axis. The impedance spectra of the wires, measured using a network analyzer, show peaks in the real part of the impedance, which shift to higher frequency with the strength of the static field, a behavior typical of ferromagnetic resonance. The theoretical resonance condition predicts a straight line on an f02−H0 plot, where f0 is the resonance frequency and H0 is the resonant field, whose slope depends only on the saturation magnetization, Ms, of the material. All our wires obey this relation, and the values of Ms calculated from the slopes are in good agreement with those measured directly using a vibrating sample magnetometer.

Modeling of domain structure and anisotropy in glass-covered amorphous wires

Giant magnetoimpedance (GMI) at 10 MHz and longitudinal magnetization curves are used to investigate the anisotropy of Co68.15Fe4.35Si12.5B15 wires, glass-covered and after glass removal. The high resolution GMI response to the field shows hysteresis and large Barkhausen jumps, in good agreement with those observed in the magnetization curves. These are modeled through superposition of the response of the inner core and outer shell of the wires. The GMI response is calculated using the differential susceptibility deduced from the model, thus relating the domain structure to the observed magnetoimpedance.

Modeling the magnetoimpedance in anisotropic wires

We have developed a theory of giant magnetoimpedance (GMI) in ideal anisotropic magnetic wires, which is valid over a broad field and frequency range. The emphasis is put on the moderate frequency GMI response in the low field region, where the wire is not saturated. The model agrees with experimental data on amorphous CoFeSiB wires, over broad frequency and field ranges, but does not correspond to an experiment at low field.

Microwave studies of magnetic anisotropy of Co nanowire arrays

The effect of magnetocrystalline anisotropy and dipolar interactions in Co nanowire arrays is studied by ferromagnetic resonance (FMR). Microwave measurements performed by the microstripline method are reported for two series of crystalline hcp Co (with the c axis nominally perpendicular [Co(c⊥)] and parallel to the wires [Co(c∥)]) and an amorphous alloy with Co as the main component—Co94Fe5B1. Extrapolation of the high field linear part of the resonance curve (frequency versus dc field) permitted an evaluation of the effective anisotropy fields for saturated samples, as well as of the intrinsic fields HK, showing that the great differences between the three series are due to the magnetocrystalline anisotropy. The HK values for the two series of Co are discussed in terms of a model which accounts for the effect of the distributions of the c axis orientation in systems of uniaxial ferromagnets. The observed dependence of the effective anisotropy fields on the array geometry (wire length and diameter) is in...

Influence of surface anisotropy on magnetoimpedance in wires

The variation of the amplitude of the giant magnetoimpedance maxima for a magnetic cylindrical conductor in the range of 1<f<300 MHz has been investigated. Emphasis is put on the effect of the surface anisotropy, Ks, which was neglected in previous studies. The calculation of the impedance of a perfect anisotropic, nonsaturated wire with a helical magnetic structure also includes exchange-conductivity effects and Landau–Lifshitz damping. The results are compared with experimental data on CoFeSiB amorphous wires. It is found that other factors must also be taken into consideration to describe the data.

Coupled core–shell model of magnetoimpedance in wires

Magnetoimpedance (MI) has been studied extensively in soft magnetic wires and plates. Although a general theoretical basis has evolved, several details remain poorly understood. In particular, the amplitude of the effect in the low field region has proven impossible to fit within current models which assume a uniform static magnetization within the material. In this article, we present magnetization and MI data on CoFeSiBNb melt-extracted wires and conclude that the behavior of these materials can be analyzed on the basis of a core–shell magnetic structure. This approach introduces a nonuniform magnetization into the MI theory in such wires. We calculate the static magnetic configuration in the presence of an exchange coupling between the two regions and use it to solve for the dynamical magnetization of the outer shell using the Landau–Lifshitz and Maxwell equations to obtain the impedance as a function of the applied field and frequency. The agreement for the MI between theory and experiment is greatly ...