Asymmetric Magnetoimpedance Research Articles

Asymmetric magnetoimpedance in exchange-biased systems

Magnetic systems with competing anisotropies generally exhibit asymmetry between the maximum amplitudes of the right and left maxima in a magnetoimpedance curve. Small errors in positioning the samples at the experimental setup may also produce such asymmetry. In this work, we present a study on the sources of the asymmetry between magnetoimpedance peaks in systems that present the exchange bias phenomenon, by comparing a phenomenological model to experimental data. A set of samples with different repetitions of the NiFe/FeMn exchange-biased bilayer was used in this study. From the frequency evolution of the asymmetry, together with magnetization curves, we were able to identify the sources for the observed magnetoimpedance asymmetry found on our experimental data.

On the nature of the interlayer magnetic interactions in biphase ferromagnetic films

We report on the nature of the interlayer magnetic interactions in NiFe/Cu/Co films. By probing the quasi-static and dynamic magnetic properties of biphase ferromagnetic films, with soft and hard ferromagnetic phases intermediated by a non-magnetic layer, we address aspects of the coupling between magnetic layers. Our results demonstrate the nature of the interlayer magnetic coupling in biphase films. We also disclose the asymmetric magnetoimpedance effect as a fingerprint of the nature of the magnetic interlayer interactions playing key role in the magnetization dynamics of the system. We revisit in literature data and ideas on the asymmetric magnetoimpedance and the nature of the magnetic interactions in biphase ferromagnetic systems. Then, we compare our findings with results for biphase ribbons and microwires. Our observations raise the fundamental similarities and differences in the asymmetric magnetoimpedance of these structures.

Asymmetric magnetoimpedance effect and dipolar interactions of FINEMET/SiO2/FePd composite ribbons

The dipolar interactions are investigated through the asymmetric magneto-impedance in FINEMET/SiO2/FePd composite ribbons. The interface between the hard (FePd layer) phase and soft (FINEMET ribbon) phase is coherent by SiO2 layer in FINEMET/SiO2/FePd composite ribbons, which effectively induces dipolar interactions. The contribution of dipolar interaction to the bias field (H b) by asymmetrical giant magneto-impedance and magnetic properties is analyzed. The results show that H b response decreases with the increase of the SiO2 layer thickness, indicating that the linear region near-zero field can be tuned by the thickness of SiO2 layer. These results allow the GMI ratio (58%) and characteristic frequency (500 kHz) to be optimized. The transverse and longitudinal magnetic domain structures of FINEMET ribbon and FePd film are confirmed, respectively. The composite ribbons with high GMI ratio and low frequency can be applied to linear magnetic sensors.

Influence of Bias Current on Off-Diagonal Magnetoimpedance in Composite Wires

The off-diagonal magnetoimpedance in a composite wire consisting of a highly conductive inner core covered by a soft magnetic layer is studied theoretically. In the presence of bias current, the field dependence of the off-diagonal magnetoimpedance becomes asymmetric due to the combination of the circular magnetic field produced by the bias current and a helical anisotropy in the magnetic layer of the composite wire. The dependences of the asymmetric off-diagonal magnetoimpedance on the external field, frequency, and bias current are investigated. It is demonstrated that the off-diagonal magnetoimpedance in composite wires has high field sensitivity and exhibits nearly linear behavior at low external fields. The influence of the wire parameters such as the anisotropy axis angle, the magnetic layer thickness, and conductivity on the field sensitivity of the off-diagonal magnetoimpedance is analyzed. The results obtained may be useful for development of weak magnetic-field sensors.

Asymmetric Magnetoimpedance in Ferromagnetic Biphase Films: Effects of Magnetostatic Coupling and Number of Layers

A model to describe the asymmetric magnetoimpedance in biphase soft/hard magnetic multilayered film is proposed. It is shown that a magnetostatic coupling changes the magnetization distribution in soft magnetic layers and leads to asymmetry in the field dependence of the impedance. The influence of the number of the repetitions of the base three-layered film structure having the soft and hard magnetic layers separated by non-ferromagnetic metallic spacer on the asymmetric magnetoimpedance is analyzed. The results of modelling describe main features of the asymmetric magnetoimpedance effect observed in bimagnetic multilayered films. The analysis shows that the multilayers may be attractive for use in miniature sensors of a weak magnetic field.

Invariance of the magnetic behavior and AMI in ferromagnetic biphase films with distinct non-magnetic metallic spacers

We investigate the quasi-static magnetic, magnetotransport, and dynamic magnetic properties in ferromagnetic biphase films with distinct non-magnetic metallic spacer layers. We observe that the nature of the non-magnetic metallic spacer material does not have significant influence on the overall biphase magnetic behavior, and, consequently, on the magnetotransport and dynamic magnetic responses. We focus on the magnetoimpedance effect and verify that the films present asymmetric magnetoimpedance effect. Moreover, we explore the possibility of tuning the linear region of the magnetoimpedance curves around zero magnetic field by varying the probe current frequency in order to achieve higher sensitivity values. The invariance of the magnetic behavior and the asymmetric magnetoimpedance effect in ferromagnetic biphase films with distinct non-magnetic metallic spacers place them as promising candidates for probe element and open possibilities to the development of lower-cost high sensitivity linear magnetic field sensor devices.

Asymmetric magnetoimpedance in two-phase ferromagnetic film structures

The magnetoimpedance of a film trilayer consisting of magnetically soft and magnetically hard films separated by a high-conductivity nonmagnetic spacer is theoretically investigated. A model for describing the dependences of the sample impedance on the external magnetic field and frequency is proposed, which is based on simultaneous solution of the linearized Maxwell and Landau–Lifshitz equations and takes into account the magnetostatic interaction between the magnetic layers. It is demonstrated that the magnetostatic interaction changes the distribution of magnetization of the magnetically soft film and leads to the occurrence of asymmetry in the field dependence of the impedance. The results obtained can be used in the development of weak magnetic field sensors.

A model for asymmetric magnetoimpedance effect in multilayered bimagnetic films

The magnetoimpedance in three-layered bimagnetic film structure is studied theoretically. The structure consists of the soft and hard magnetic films separated by highly conductive non-magnetic layer. A model to describe the magnetoimpedance effect in the film structure based on a simultaneous solution of linearized Maxwell equations and Landau–Lifshitz equation is proposed. It is shown that magnetostatic coupling between the magnetic layers results in the asymmetry in the field dependence of the film impedance. The magnetostatic coupling is described in terms of an effective bias field appearing in the soft magnetic layer. The calculated field and frequency dependences of the film impedance are shown to be in a qualitative agreement with previous results of experimental studies of the asymmetric magnetoimpedance in NiFe/Cu/Co film structures. The results obtained may be useful for development of weak magnetic-field sensors.

Asymmetric magnetoimpedance effect in CoFeSiB amorphous ribbons by combination of field and current annealing for sensor applications

The roles of applied magnetic field during the current annealing of Co68.15Fe4.35Si12.5B15 soft magnetic amorphous ribbons are studied. Samples heat treated by Joule heating effect in open air and simultaneously in the present of longitudinal external magnetic field showed asymmetric magnetoimpedance (AMI) behavior. The AMI profile can be related to the exchange bias interaction between the soft magnetic amorphous material and a harder magnetic crystalline phase formed on the surface of the ribbon. This effect stems from thermal effect, the transverse Oe field generated from the annealing current which is thickness dependent and the longitudinal external field. The single peak AMI with the field sensitivity of 101%/Oe for DC annealing current is achieved. Our results could address a simple way to achieve the AMI response toward developing high sensitive magnetic field sensors.

Bias Current Effect on Second Harmonic in Asymmetric Magnetoimpedance Response in Amorphous Microwires

The influence of bias current on the second harmonic in the nonlinear magnetoimpedance in an amorphous microwire with a helical anisotropy is studied theoretically. The voltage response of the microwire is found in the framework of a rotational model. It is shown that the application of the bias current leads to the asymmetry in the field dependence of second harmonic. The second harmonic amplitude is analyzed as a function of the external field, current amplitude and the value of the bias current. The conditions of maximum field sensitivity of the second harmonic are found.

Improving the sensitivity of asymmetric magnetoimpedance in exchange biased NiFe/IrMn multilayers

We investigate the asymmetric magnetoimpedance effect in exchange biased NiFe/IrMn multilayers and explore the possibility of tailoring the linear region of the curves around zero magnetic field by playing with the thickness of the ferromagnetic layers, probe current frequency, and the orientation between external magnetic field and exchange bias field direction. We quantify the sensitivity by calculating the |dZ/dH|H=0 in a wide range of frequencies. The highest sensitivity is observed for the thicker multilayer, reaching ∼160mΩ/Oe at ∼2.0GHz. The results place exchange biased magnetoimpedance multilayers as attractive candidates for the development of auto-biased linear magnetic field sensors.

Asymmetric magnetoimpedance effect in ferromagnetic multilayered biphase films

We investigate the magnetoimpedance effect in ferromagnetic multilayered biphase films. We verify that the films present asymmetric magnetoimpedance behavior and, by playing with the number of the repetitions of the base structure of the multilayer and the probe current frequency, we explore the possibility of tailoring the linear region around zero magnetic field in order to achieve higher sensitivity values. The highest sensitivity is observed for the thicker film, reaching ∼15mΩ/Oe at ∼0.52GHz. Thus, the results open the possibilities for application of ferromagnetic multilayered biphase films with asymmetric magnetoimpedance effect in sensors devices.

Tunable asymmetric magnetoimpedance effect in ferromagnetic NiFe/Cu/Co films

We investigate the magnetization dynamics through the magnetoimpedance effect in ferromagnetic NiFe/Cu/Co films. We observe that the magnetoimpedance response is dependent on the thickness of the non-magnetic Cu spacer material. We verify asymmetric magnetoimpedance in films with biphase magnetic behavior and explore the possibility of tuning the linear region of the magnetoimpedance curves around zero magnetic field by varying the thickness of the spacer and probe current frequency. We discuss the experimental results in terms of the different mechanisms governing the magnetization dynamics at distinct frequency ranges, quasi-static magnetic properties, thickness of the spacer, and the kind of the magnetic interaction between the ferromagnetic layers. The results place films with biphase magnetic behavior exhibiting asymmetric magnetoimpedance effect as very attractive candidates for application as probe element in the development of auto-biased linear magnetic field sensors.

Angular dependence of asymmetric magnetoimpedance in exchange biased NiFe/IrMn multilayers

We investigate the angular dependence of asymmetric magnetoimpedance of NiFe/IrMn multilayers and explore the possibility of tuning the linear region of the magnetoimpedance curves by modifying the angle between the applied external magnetic field and exchange bias field, and probe current frequency. We quantify the sensitivity by calculating the ratio |ΔZ|/|ΔH| at low magnetic fields as a function of the frequency for samples cut with different orientations and show that considerable values in the linear region around zero field can be reached. The results extend the possibilities for application of exchange biased magnetoimpedance multilayers as probe element for the development and improvement of auto-biased linear magnetic field sensors.

The effect of mechanical polishing on current annealed Co67Fe5Si15B13amorphous ribbons: magnetoimpedance response

A systematic current annealing treatment to achieve a high magnetoimpedance (MI) response in CoFeSiB amorphous ribbons is demonstrated. After current annealing in the present of air, although transverse permeability can increase but because of a significant surface oxidization, condition to reach a high MI response cannot be achieved. Additionally, this process leads to asymmetric MI response. In this paper, the effect of mechanical polishing of samples after current annealing is studied. We show a significant MI change by removing the hard magnetic phase on sample surface after current annealing. Results are useful and promising to be employed to reach high MI response for sensing applications.

Abcès pulmonaires compliqués : chirurgie ou drainage ?

The influence of torsional stress on the asymmetric magnetoimpedance in a glass-coated negative magnetostrictive amorphous microwire due to bias current is studied theoretically. The longitudinal and off-diagonal impedance components are found assuming a simplified spatial distribution of the magnetoelastic anisotropy induced by the torsional stress. The asymmetry in the field dependence of the impedance components is attributed to the combination of the circular magnetic field produced by the bias current and a helical anisotropy induced by the torsional stress. The asymmetry in the magnetoimpedance and the low-field hysteresis are analyzed as a function of the bias current and torsional stress. It is shown that the application of torsional stress significantly changes the value of the bias current required to suppress the hysteresis effect. The results obtained may be useful for applications in magnetic-field and stress sensors.

Asymmetric magnetoimpedance in amorphous microwires due to bias current: Effect of torsional stress

The influence of torsional stress on the asymmetric magnetoimpedance in a glass-coated negative magnetostrictive amorphous microwire due to bias current is studied theoretically. The longitudinal and off-diagonal impedance components are found assuming a simplified spatial distribution of the magnetoelastic anisotropy induced by the torsional stress. The asymmetry in the field dependence of the impedance components is attributed to the combination of the circular magnetic field produced by the bias current and a helical anisotropy induced by the torsional stress. The asymmetry in the magnetoimpedance and the low-field hysteresis are analyzed as a function of the bias current and torsional stress. It is shown that the application of torsional stress significantly changes the value of the bias current required to suppress the hysteresis effect. The results obtained may be useful for applications in magnetic-field and stress sensors.

Expanding the longitudinal magnetoimpedance sensor range by direct bias current

We investigated the effects of induced helical anisotropy and application of dc bias current IB on longitudinal magnetoimpedance (MI) in amorphous microwires both separately and together. We demonstrated that when both parameters are present, i.e., a dc bias current IB is applied to the microwire with induced helical anisotropy, the longitudinal MI sensor range can be considerably extended up to the fields considerably higher than its anisotropy field as the slope of MI curve dZ/dHE remains rather high. A highly asymmetric longitudinal MI dependence with a rather high slope at the zero-field point was obtained. Reversing the bias current IB causes reversal of the bias field direction and results in a mirroring of the MI dependence. This gives a possibility to determine both the sign and the magnitude of the external magnetic field. The obtained results can be used in development of the longitudinal MI sensors with increased sensitivity and/or range.

Asymmetric magnetoimpedance in self-biased layered CoFe/CoNi microwires

The magnetoimpedance (MI) effect has been investigated in a family of multilayer microwires with biphase magnetic behavior consisting of a soft nucleus (CoFeSiB), an intermediate nonmagnetic insulating layer, and a hard outer shell (CoNi). The MI response of the soft phase can be tailored by its magnetostatic coupling with the hard phase. The hard outer shell, in its remanence state, creates a bias field in the soft nucleus that shifts the magnetization process and results in an asymmetric MI response. The amplitude of that bias field is determined by the geometric characteristics and the magnetic state of the hard phase. Furthermore, a near linear MI behavior with high sensitivity was realized around zero operation field point, with the advantage of not employing external biasing fields and additional coils. This makes biphase microwires exhibiting self-bias and asymmetric MI very attractive as sensing elements in magnetic-field sensor devices and materials.

Design of a double core linear magnetometer based on asymmetric magnetoimpedance effect in nanostructured Finemet ribbons

Abstract The techniques used to produce magnetic sensors encompass many aspects of physics and electronics and are going to develop with each passing year. In this paper, we introduce a double core giant magnetoimpedance sensor. The sensing elements are provided by heat treating the FeCuNbSiB (Finemet) ribbons at 560 °C for 1 h to achieve a high magnetic permeability associated with the nanostructured state. Sensor cores are exposed to a dc current bias in order to provide asymmetric giant magnetoimpedance (AGMI) behavior. The sensor detects both direction and amplitude of the magnetic field with a sensibility of 2 mV/Oe.