L. G. C. Melo

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.

Optimization of the magnetic noise and sensitivity of giant magnetoimpedance sensors

The performance of magnetic field sensors based upon the giant magnetoimpedance (GMI) effect in soft magnetic wires is investigated in the MHz frequency range. The performance of the sensor is based on its sensitivity, voltage noise level and (voltage) noise-to-sensitivity ratio, or intrinsic magnetic noise level. Optimization of the sensitivity and noise response of the sensor through variation of the sample anisotropy direction and external applied axial field and dc bias current suggest an intrinsic noise level in the fT/Hz level. Qualitative agreement is obtained between theory and experiment on a CoFeSiB microwire, for the maximum sensitivity and the corresponding noise, as a function of the external field and dc bias current.

Equivalent Magnetic Noise Limit of Low-Cost GMI Magnetometer

We present a noise analysis of a giant magnetoimpedance (GMI) sensor using a peak detector at the optimal magnetic field working bias point of a sensor wire, by considering internal noise sources (intrinsic GMI device associated noise sources and conditioning electronic noise sources). An expression is obtained for the theoretical expected noise for known electronic design parameters and physical characteristics of the GMI wire. The most significant contributions to noise in a GMI measurement, using two basic oscillators (either a simple discrete RC oscillator or quartz oscillator) along with a peak detector, are presented. We discuss the expected extrinsic equivalent magnetic noise limit.

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.

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.

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.

Formalism to Optimize Magnetic Noise in Giant Magnetoimpedance-Based Devices

This paper describes a theoretical study of the sensitivity and intrinsic noise response in a longitudinal giant magnetoimpedance sensor of a magnetically soft microwire. We show that the ratio of the intrinsic magnetic noise voltage spectral density to the sensitivity is proportional to epsiv/sinthetas, where epsiv depends upon the internal field and frequency and thetas is the angle between the equilibrium magnetization and the axial applied static field. The model allows us to calculate the influence of the easy axis angle and the applied fields on sensor response and to obtain a working point. We show the conditions necessary to reduce the intrinsic noise level to the fT/radicHz level

Study of the complex permeability of amorphous wires using microwave impedance spectroscopy

We have measured (10 MHz-6 GHz) the magnetoimpedance of ferromagnetic amorphous wires placed as internal conductors in shorted coaxial lines. Three types of materials have been investigated: CoFeSiB wires from Unitika (Japan), cast by rapid quenching in water, and NiCoFeSiBMn and CoFeSiNbB wires from MXT (Canada), cast by melt extraction. In order to investigate magnetic properties, we used a complex permeability formalism directly obtained from impedance by means of a transformation which leads to an exchange in components from real to imaginary and vice-versa. From the analysis of Cole-Cole plots of the relative permeability, ferromagnetic resonance was identified as the magnetization process occurring when an axial dc magnetic field of several hundred oersteds was applied to the line. An equivalent circuit of the magnetic wire was used to demonstrate that the field variation of the inverse of the equivalent resistance was quite similar to those of the line width vs resonance frequency ratio and of the maximum of the imaginary part of the relative permeability.

Hysteretic and asymmetric magnetoimpedance in cobalt-rich amorphous wires

Summary form only given. The magnetization processes in Co-rich amorphous wires prepared by melt extraction were investigated for tensile stresses applied to the wire in the 0-200 MPa range. The magneto-impedance (MI) responses - /spl Delta/Z/Z vs. the longitudinal d.c. field, H - were measured using an impedance analyzer at constant frequency (10 MHz) and in the field range of

Perspectives in Giant Magnetoimpedance Magnetometry

4 to + 4 Oe. The wire was saturated with a 200 Oe longitudinal field before each measurement. Circumferential hysteresis loops, m (h/sub /spl phi//), where h/sub /spl phi// is the rf circular field generated by the current flowing through the wire, were also obtained by time integration of the voltage signal picked-up across the wires.