Magnetoimpedance In Amorphous Wires Research Articles

Development of pico tesla resolution amorphous wire magneto-impedance sensor for bio-magnetic field measurements

For achieving the room temperature magnetoencephalography (MEG) and magnetocardiography (MCG) measurements in an unshielded environment, we have developed a high performance, pico-tesla resolution magnetoimpedance (MI) sensor system with the peak-to-peak voltage detector, by synchronized switching. We have achieved a high field detection sensitivity with good linearity, and a noise level lower than 1 pT/Hz1/2, by utilizing the time-differential measurement in each MI element for suppressing the low frequency noise components and increasing the stability of sensor system. We have successfully developed a Pk–pk VD-type MI sensor system aiming to measure the room-temperature real-time alpha rhythm signals, and demonstrated a clear sensitivity for spontaneous brain activity with SNR of about 8. The considerable enhancement of alpha rhythm measured by new MI sensor system in each eyes-closed condition is further evidence that these recorded signals are emanating from spontaneous brain activity. Meanwhile, we have also achieved the real time MCG measurements via pk-pk VD-type MI sensor system, with a high real time SNR of about 14 dB, and successfully measured the T wave and R wave of MCG signals, in averaging over 10 cycles, with a high SNR of more than 20 dB.

Off-diagonal magnetoimpedance in amorphous wires and its application in miniature sensors of weak magnetic fields

Off-diagonal magnetoimpedance in amorphous cobalt-based microwire with helical magnetic anisotropy is investigated using a DC bias current. It is demonstrated that the circular magnetic field generated by the bias current improves the sensitivity to an axial external magnetic field, raises the output voltage peak, and expands the sensed field in which the peak is observed. However, a further increase in the bias current reduces the sensitivity. The experimental results agree qualitatively with the theory for large bias fields when the hysteresis in static magnetization is eliminated.

Amplification of even Harmonics in Nonlinear Magnetoimpedance Response of Amorphous Wires in Presence of Longitudinal Alternating Magnetic Field

A model to describe the influence of longitudinal alternating magnetic field on the nonli-near magnetoimpedance in amorphous wires is proposed. The appearance of even harmonics in the voltage response is shown to arise from the asymmetry in the magnetization reversal process in the wire due to the presence of the longitudinal alternating field. The behavior of even harmonics is analyzed as a function of the external field, alternating field amplitude and current amplitude.

Investigation of Magnetic End Effects in Amorphous Wires and Ribbons and Simulation of the Resulting Magnetoimpedance

The existing experimental results have shown that the magnetic characteristics of amorphous wires in different length or in different positions at the end are significantly varied. To study the end effect, the magnetic field distribution at the end of amorphous ribbon of infinite-length and finite-width is analyzed by means of conformal mapping. Because the internal magnetic field intensity of the amorphous ribbon is impossible to exceed that of the external, the model of three magnetic zones is proposed, which consists of the magnetic uniform zone in the middle, the magnetic in-zone and out-zone at the two ends. And the formula for calculating the critical length of magnetic end effect is derived. Due to the great difficulty in obtaining the analytic solution of the end magnetic field of amorphous wire, the assumption in the distribution of magnetic charges and the numerical computation method are applied to obtain the magnetic field distributing form at the end of amorphous wire. The three-zone model is also suitable for the distribution of the amorphous wire and its critical length formula of the magnetic end effect can be deduced. Considering the magnetic end effect, a simple simulation modeling method to study the magnetoimpedance of amorphous wires and ribbons is presented based on the fitting magnetization curve and the local magnetization analysis, which can well explain the existing experimental phenomena.

Comparison of ferromagnetic resonance between amorphous wires and microwires

Measurements of magnetoimpedance in amorphous wires and microwires at the GHz region are presented here. The maxima observed in the magnetoimpedance of different samples in the high frequency range are attributed to the ferromagnetic resonance (FMR) that occurs when a sample is submitted to a longitudinal static magnetic field and an oscillating transversal field. While the appearance of a peak on the resistance and the drop of the inductance is explained by means of FMR, the dependence of the width of that resonance with different parameters (magnetic field, stress distribution, sample size, etc.) is not clearly understood, and therefore additional works to explain the value of the resonance width are needed. It is interesting to consider firstly, the influence of the diameter of the sample and thus the stress distribution on the FMR. The size and position of the FMR is found to be completely different for diameters ranging from 24 to 171 μm. The dependence of the frequency value at which the peak of the resistance is found can be explained by the different values of the anisotropy field in the sample. The width of the FMR changes drastically with the diameter of the sample and with the applied magnetic field and they will be discussed in this paper.

A model for torsion-stress effect on nonlinear magnetoimpedance in amorphous wires with negative magnetostriction

A model to describe the influence of torsional stress on nonlinear magnetoimpedance in amorphous wires with negative magnetostriction is proposed. The nonlinear voltage response is found in the framework of the low-frequency approximation taking into account the spatial distribution of the circular magnetic field and the magnetoelastic anisotropy induced by the torsional stress. It is demonstrated that the application of torsional stress results in an increase of the second harmonic amplitude in voltage due to a reinforcement of helical anisotropy in the wire. The second harmonic amplitude is analyzed as a function of external field, torsional stress and current amplitude. The ranges of torsional stress and current amplitude to achieve maximal field sensitivity of the second harmonic are found.

Effect of direct current on higher harmonic generation in the frequency spectrum of magnetoimpedance of amorphous wires with circular anisotropy

The effect of direct current on the nonlinear response of magnetoimpedance in circularly anisotropic amorphous wires is investigated. A model is proposed for calculating the voltage across the ends of the wire in the case of a weak skin effect in the sample. It is shown that a direct current passing through the wire can substantially increase the amplitude of even harmonics of the voltage. The conditions in which the sensitivity of the second harmonic to the applied magnetic field is maximal are determined.

Noise Characterization of Coil Detection Type Magnetic Field Sensor Utilizing Pulse Excitation Amorphous Wire Magneto-Impedance Element

In this experiment, we investigated noise characteristics of a pulse magneto-impedance sensor which is based on an amorphous magnetic wire with a pickup coil. For a helical static magnetization, the ac current i passing the wire induces a coil voltage Vc in the coil around the wire. The pulsed excitation, combining both high and low frequency harmonic, produces the voltage Vc with dc current biasing. We describe improvements of pulse excitation conditions for an amorphous magnetic wire to reduce magnetic noises. The relation between the time interval of a current pulse train T and magnetic noises was investigated. The magnetic noise is suppressed clearly when T is smaller than 1.5 μs for pulse width 10 ns. Under the optimal conditions, the rms noise of 1-cm-long an amorphous wire magneto-impedance sensor with 600 turns of the pickup was estimated as 1pT/Hz1/2M at 20 Hz∼500 Hz.

Biomagnetic field detection using very high sensitivity magnetoimpedance sensors for medical applications

AbstractThis paper demonstrates the performance of magnetoimpedance (MI) sensors for biomagnetic field recording. The rms noise of an amorphous wire MI sensor with 600 turns of pick‐up coil was estimated as approximately 3 pT/Hz1/2 at 1 Hz. Here, we demonstrate that this sensitivity is enough to clearly detect magnetic cardiogram signals. We also measured the biomagnetic field around a smooth muscle tissue sample taken from a guinea‐pig. We discovered that the magnetic‐field waveform of a small tissue sample is similar to the voltage waveform measured by a microelectrode array in a smooth muscle sheet sample. The results suggest that the localized biomagnetic field generated by smooth muscle cells can be detected by an MI sensor. We concluded that the MI sensor is a promising device for use as a noninvasive tool to test cell activity or function through magnetic‐field measurement. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Advances of amorphous wire magnetics over 27 years

AbstractAn overview of advances of the amorphous wire magnetics over the past 27 years with discoveries of extremely sensitive magnetic effects such as the large Barkhausen effect, the Matteucci effect and the magnetoimpedance effect and their successive applications to security sensor tags, pen‐inputting computer tablets and electronic compasses for mobile phones in commercial productions is presented. An important new topic of biocell magnetic field sensing in vitro and human physiological magnetic field sensing in vivo using a 10 pT amorphous wire magnetoimpedance sensor is also shown that would be improved using thinner glass‐covered amorphous wires. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of torsional stresses on the magnetoimpedance of amorphous wires with negative magnetostriction

A model to describe the effect of torsional stresses on the magnetoimpedance of amorphous wires with negative magnetostriction is suggested. An approximate expression for the impedance with regard to the spatial distribution of magnetoelastic anisotropy induced by torsional stresses is derived. It is shown that the relative variation of the impedance is maximal near a critical stress value at which the surface magnetic structure of the wire changes. The calculated dependences of the impedance on the external magnetic field and torsional stress are in good qualitative agreement with experimental data for amorphous wires with negative magnetostriction.

A model for exchange-biased asymmetric giant magneto-impedance in amorphous wires

A model describing the exchange-biased asymmetric giant magneto-impedance in Joule-heated amorphous wires is proposed. It is assumed that the Joule heating results in the formation of a surface hard magnetic crystalline layer, and the asymmetric giant magneto-impedance is related to the exchange coupling between the amorphous and crystalline phases. The coupling between the surface layer and the amorphous bulk is described in terms of an effective bias field. The model is based on a simultaneous solution of linearized Maxwell equations and the Landau–Lifshitz equation. The calculated field and frequency dependences of the wire impedance are in qualitative agreement with the results of the experimental study of the asymmetric giant magneto-impedance in Joule-heated Co-based amorphous wires.

Stress effect on magnetoimpedance in amorphous wires at gigahertz frequencies and application to stress-tunable microwave composite materials

The effect of tensile stress on magnetoimpedance (MI) in CoMnSiB amorphous wires at microwave frequencies (0.5–3GHz) is investigated both experimentally and theoretically. In the presence of the dc bias magnetic field of the order of the anisotropy field, the impedance shows very large and sensitive change when the wire is subjected to a tensile stress: 100% and 60% per 180MPa for frequencies 500MHz and 2.5GHz, respectively. It is demonstrated that this behavior owes mainly to the directional change in the equilibrium magnetization caused by the applied stress and field, which agrees well with the theoretical results for the surface impedance. This stress effect on MI is proposed to be used for creating microwave stress-tunable composite materials containing short magnetic wires. The analysis of the dielectric response from such materials shows that depending on the stress level in the material, the dispersion of the effective permittivity can be of a resonant or relaxation type with a considerable change in its values (up to 100% at 600MPa). This media can be used for structural stress monitoring by microwave contrast imaging.

Magnetoimpedance in amorphous wires and multifunctional applications: from sensors to tunable artificial microwave materials

Magnetoimpedance in amorphous wires and multifunctional applications: from sensors to tunable artificial microwave materials

Magnetoimpedance in amorphous wires and multifunctional applications: from sensors to tunable artificial microwave materials

This paper is aimed to give a generalized approach to magnetoimpedance (MI) in amorphous wires in relation to the applications in sensors and microwave materials, raising new ideas and concepts. The impedance change ratio per Oersted in Co–Fe wires runs into more than 100%/Oe at MHz frequencies. This makes wire elements unique for various sensing applications. The emphasis required here is on induced asymmetric and off-diagonal effects to produce highly sensitive and linear responses with respect to the sensed external forces (magnetic field, stress). The field sensitivity of the impedance in wires with a circumferential anisotropy remains very high even at the GHz range. The combination of microwave properties of wires with electrodynamic behavior of wire-composite materials is foreseen as resulting in completely new effects such as magnetic field-dependent dielectric response and microwave activity. Thus, the MI effect can be useful to design microwave composites with tunable properties including negative permittivity, band-gap structures and waveguides.

Magnetic after-effect of the giant magnetoimpedance in cobalt-based amorphous wires

Abstract Time dependence of the giant magnetoimpedance of amorphous wires is studied here. Magnetoimpedance measurements were performed for different frequencies from 1 kHz to 30 MHz and temperatures ranging from 80 to 400 K. The highest after-effect percent was found for temperatures close to 350 K and 100 kHz in frequency. Temperature dependence of the magnetic after-effect suggests the existence of long range diffusion processes governing the dynamics of the system.

Giant magneto-impedance effect in Fe 4.5Co 67.5Mn 0.5Si 12B 15 amorphous wires

The Giant magneto-impedance of amorphous wires with a nominal composition of Fe 4.5Co 67.5Nb 0.5Mn 0.5Si 12B 15 has been investigated using a HP4192A impedance analyzer in the frequency range from 5 Hz to 13 MHz. The results show that there exists an optimum frequency where the GMI shows a maximum for all samples investigated, and the maximum GMI values of the as-cast wire and the wire annealed at 350°C are 73 and 71%, respectively. GMI effect for the as-cast wire at the frequency range <1 MHz is much higher than those for the annealed samples, and in the case of frequency range >1 MHz GMI effect is better than that of the as-cast state. GMI( Z) max shows a maximum with annealing temperature and optimum annealing temperature is 350°C in the study. Both magneto-resistance and magneto-reactance of the as-cast wire have also been investigated.

Impedance Spectroscopy in Ferromagnetic Materials

AbstractImpedance spectroscopy (IS) is a powerful characterization methodology for a wide range of electric materials such as ceramics, ferroelectrics, ion conductors, piezoelectrics, etc. In this paper, an extension of IS to the case of ferromagnetic materials is presented. In particular, the resolution of the most common magnetization processes (domain wall bulging, domain wall displacement and spin rotation) is analyzed. IS is applied to ferrites and giant magnetoimpedance in amorphous wires.

Integrated thin film magneto-impedance sensor head using plating process

An integrated thin film MI (magneto-impedance) sensor head constructed with a combination of a thin film NiFe MI element and thin film bias and negative feedback coils using a micro-plating technology, has been developed in order to solve the problems of an amorphous wire MI head. The sensor head showed a very high field-detection sensitivity of about 0.41%/A/m (33%/Oe) magnetized with pulse current and a small hysteresis for the change of an external field. We have fabricated a differential type sensor module using a pair of the thin film MI sensor heads. The output voltage of the sensor has a good linearity within the range of /spl plusmn/80 A/m, and the voltage was 25mV/A/m(2.0V/Oe), and has small thermal drift within 0.1 A/m/spl deg/C. The consumption power of the sensor module was 165 mW.

Nondiagonal impedance of amorphous wires with circular magnetic anisotropy

The effect of the nondiagonal magnetoimpedance of amorphous wires with low magnetostriction is studied experimentally and theoretically. The nondiagonal effect was observed and investigated. For a wire with circumferential magnetic anisotropy the tensor of magnetic permeability is obtained as a function of external magnetic field amplitude H and other parameters. A strong dependence of this quantity on H is shown in the domain of the external magnetic field, H⩽ H A where H A is the anisotropy field. The nondiagonal component of the surface impedance is responsible for the measured effect.