Off-diagonal Magnetoimpedance Research Articles

Off-Diagonal Magnetoimpedance in Annealed Amorphous Microwires with Positive Magnetostriction: Effect of External Stresses

It was observed recently that the giant magnetoimpedance (GMI) effect in Fe-rich glass-coated amorphous microwires with positive magnetostriction can be improved significantly by means of post-annealing. The increase in the GMI is attributed to the induced helical magnetic anisotropy in the surface layer of the microwire, which appears after the annealing. The application of external stresses to the microwire may result in changes in its magnetic structure and affect the GMI response. In this work, we study theoretically the influence of the tensile and torsional stresses on the off-diagonal magnetoimpedance in annealed amorphous microwires with positive magnetostriction. The static magnetization distribution is analyzed in terms of the core–shell magnetic structure. The surface impedance tensor is obtained taking into account the magnetoelastic anisotropy induced by the external stresses. It is shown that the off-diagonal magnetoimpedance response exhibits strong sensitivity to the magnitude of the applied stress. The obtained results may be useful for sensor applications of amorphous microwires.

A Core–Shell Model for Magnetoimpedance in Stress-Annealed Fe-Rich Amorphous Microwires

The magnetoimpedance effect in a stress-annealed Fe-rich amorphous microwire with induced magnetic anisotropy in a surface region is studied theoretically. The static magnetization distribution within the microwire is described in the framework of the core–shell magnetic structure. It is assumed that the microwire has an axial magnetic anisotropy in the inner core and a helical anisotropy in the external shell appearing due to the stress-annealing. Both the diagonal and off-diagonal magnetoimpedance responses are found in terms of the surface impedance tensor. The influence of the bias current on the magnetoimpedance is analyzed. The obtained theoretical dependences are shown to be in a qualitative agreement with the results of recent experimental studies of the magnetoimpedance effect in stress-annealed Fe-rich glass–coated amorphous microwires.

Scanning magnetic microscope based on magnetoimpedance sensor for measuring of local magnetic fields

A scanning magnetic microscope based on an off-diagonal magnetoimpedance sensor is presented. As a sensor, we use a 4 mm segment of glass covered microwire, having a metallic core diameter of 13.5 μm and CoFeCrSiB composition and a pick-up coil wound around the microwire containing 70 turns. The magneto-impedance sensor is fixed perpendicular to the surface of the sample, in such a way that the distance between the tip of the microwire and the sample is about 200 μm. The relative movement of the sample and the GMI sensor is carried out using a non-magnetic X-Y positioner. Measurements are taken inside the magnetic screen. For test measurements, samples in the form of the letters IWMW – of the conference abbreviation are used. The samples were made based on thin copper wire, Fe-rich amorphous ferromagnetic microwires and printed by laser-jet printer. Clear magnetic images were obtained on all samples, which demonstrates the high practical potential of the proposed method.

Effect of the temperature on the off-diagonal Giant Magneto-Impedance (GMI) sensors

Effect of the temperature on the off-diagonal Giant Magneto-Impedance (GMI) sensors

Offset Reduction in GMI-Based Device by Using Double-Frequency Bias-Current Modulation

In this paper, we investigate the effect of temperature variations on the off-diagonal giant magneto-impedance (GMI) response. This paper is based on a theoretical model describing the GMI-based sensor response. The modeling has been extended to include the resistivity dependence versus the temperature. It allows to calculate both temperature sensitivity and intrinsic magnetic field sensitivity, whose ratio leads to the determination of the magnetic field offset drift versus the temperature. Thanks to an original conditioning circuitry based on a double ac current excitation instead of the classical one, we achieved an improvement of the long-term stability and minimized the sensor temperature dependence. Experimentally obtained results have been compared with the expected offset drift for the classical GMI, and with the expected fluctuation reduction while adopting this technique, showing a relatively good agreement with the proposed modeling.

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.

Analysis of the off-diagonal component of giant magnetoimpedance effect in Co-based (as-cast and stress-annealed) amorphous ribbons

We investigated the effect of the application of dc bias current IB on the off-diagonal magnetoimpedance (MI) components at 50 MHz in nearly-zero magnetostrictive Co66.5Fe3.5Si12.0B18.0 amorphous ribbons in as-cast and stress-annealed state. The off-diagonal MI component is anti-symmetrical with near-linear behavior within a certain field interval. For the off-diagonal response, the dc bias current is necessary to eliminate transverse domains present in the ribbon. A highly asymmetry of the off-diagonal dependence with the axial magnetic field was obtained when the bias current is applied along the ribbon. A rather high slope at the zero-field point was obtained, being more pronounced in the stress-annealed samples. This is probably related with the domain structure reorganization under the effect of the bias current. This behavior is ideal for a practical sensor circuit design.

Measurement of magnetic noise in magnetoimpedance sensing element

A method for measurement of magnetic noise in magnetoimpedance sensing element is proposed. Glass-coated Co-based amorphous microwires with a slightly negative magnetostriction were used as the sensing element. The operation principle of the sensing element was based on the nonlinear off-diagonal magnetoimpedance, when field dependent higher harmonic components appeared in voltage in the pick-up coil wound around the microwire. The magnetic noise for the second harmonic in the pick-up coil voltage was studied. The dependences of the magnetic noise and the signal-to-noise ratio in the sensitive element on the current amplitude were analyzed. The noise performance of the second harmonic in the pick-up coil voltage was compared to that for a flux-gate sensing element.

Noise Performance of SDR-Based Off-Diagonal GMI Sensors

In this paper, we propose a general model to evaluate the equivalent magnetic noise for an off-diagonal giant magneto-impedance (GMI) sensor utilizing a software-defined radio (SDR) as a digital receiver. Based on this SDR approach, an analog-to-digital conversion (ADC) of the voltage induced by the sensing element is directly performed. The signal processing is digitally achieved in real time. It includes a digital quadrature demodulation, a decimation, and a filtering. The approach allows to give the key rules for quantifying the white noise level of this digital implementation. The voltage noise spectral density of the ADC and of each SDR stage is given. The white noise level expected by the modeling was in good agreement with the measurements. With a sensing element composed of a 400 turn coil wound around a co-rich GMI wire, equivalent magnetic noise levels of 220 and 4.2 pT/ $\sqrt {Hz}$ were obtained at 1 Hz and in the white noise region, respectively. The potential improvement of these performances, allowed by this digital conditioning, as well as limitations is also discussed.

Development of precise off-diagonal magnetoimpedance gradiometer for magnetocardiography

We have developed a precise off-diagonal magnetoimpedance (MI) gradiometer that can operate in an unshielded environment and at room temperature with 200 pT root-mean-square noise in a 100 Hz bandwidth. The MI sensor probe is compact and easy to handle. The achieved noise level corresponds approximately to the maximum magnetocardiography (MCG) signal reported so far. We have performed MCG measurements using the developed gradiometer system in an unshielded environment, and a real-time signal like MCG can be identified by the MI gradiometer when the distance between the sensor head and chest surface is less than 3 mm. However, the signal seems to be affected by the movement of the chest surface caused by the heartbeat. A peak magnetic signal of 100 pT (corresponding to conventional MCG) was observed when the sensor head was set 10 mm apart from the chest surface to avoid the influence of the chest movement. Under such conditions, the signal needed to be averaged over more than 50 cycles to identify the peak magnetic signal.

Multicore Off-Diagonal Magnetoimpedance Sensors Utilising Amorphous Wires

Magnetoimpedance (MI) sensors are attractive for realising a localised detection of weak magnetic fields owing to micro-size and high sensitivity of the MI elements based on Co-rich amorphous wires of diameter 10-30 microns. The MI sensors in off-diagonal configuration when the signal is taken from the coil mounted on the wire have a number of advantages including linearity, enhanced output voltage, and resonance detection circuitry. In this work, the off-diagonal impedance of a number of Co-rich glass-coated amorphous wires placed in the same coil is investigated. The wires are connected in parallel and excited by a harmonic current with optimised dc bias which is used to remove the domain structure. The best sensitivity of about 350mV/Oe is obtained for two closely spaced wires at the excitation frequency of 20MHz which corresponds to the resonance conditions for the detection circuit having only 55 turns. The full scale and linearity range are also increased almost twice in comparison with those when only one wire is used. The response drops if the distance between the wires is increased which is due to a strong effect of the magnetostatic interaction between the wires. We conclude that the off-diagonal MI sensors with multiple wires can provide higher transfer function per one turn and could be advantageous for localised field detection.

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.

Magnetoimpedance hysteresis in amorphous microwires induced by core–shell interaction

We report on magneto-impedance (MI) hysteresis at MHz and GHz frequencies in amorphous microwires subject to a sufficiently high applied axial magnetic field HE. We show that this hysteresis originates from the magnetic hysteresis of the inner core which biases the outer shell causing a shift of the MI curve along the HE axis. Combined experiments (longitudinal and off-diagonal MI Z(HE), hysteresis loops M(HE) by vibrating sample magnetometer, and induction method) reveal the details of the magnetization reversal process in these microwires: Partial field dependencies Z(HE) and M(HE) are shown to be practically anhysteretic but shifted to the left or to the right about the origin by a residual magnetic field of the inner core HC. This shift can vary in the range from −35 to +35 A/m, depending on the magnetic history of the sample. We demonstrate that the hysteresis can be suppressed by application of a high enough axial magnetic field that saturates the magnetization of the inner core. A potential application of this hysteresis for memory devices is also proposed.

Improvement of the off-diagonal magnetoimpedance sensor white noise

This work deals with the analysis of noise sources of the off-diagonal magneto-impedance sensor with stress-annealed amorphous ribbon. This experimental analysis shows that the equivalent magnetic noise spectral density is mostly due to the electronic conditioning circuitry that operates the sensor. The electronic noise was then reduced with employment of an improved excitation-detection technique for our sensor and an equivalent magnetic noise spectral density below 1pT/Hz in white noise region has been reached. Indeed, the equivalent magnetic white noise level of the output signal decreased from 5pT/Hz to 560fT/Hz. Nevertheless, this noise level is still limited by the electronic conditioning, leaving room for further improvement.

Manipulating the magnetoimpedance by dc bias current in amorphous microwire

We have studied the effect of the internal circumferential magnetic field HB created by the dc bias current IB on longitudinal and off-diagonal magnetoimpedance (MI) in amorphous microwire with helical anisotropy and experimentally demonstrated that by changing the dc current IB it is possible to considerably change the MI dependencies. The bias current applied to such microwire transforms the symmetric and hysteretic MI curve to asymmetrical and anhysteretic. The minimum of longitudinal MI curve shifts from the zero-field point. Reversing the bias current causes reversal of the bias field direction and results in a mirroring of the MI curves. It is proposed to apply a cross-checking of two MI curves with IB of different polarity for magnetic field sensing. In particular, this method allows to overcome the drawbacks usually associated with longitudinal MI—namely the impossibility to determine the direction of an external axial magnetic field HE and the low sensitivity near the zero-field point. Moreover, the operating range of the longitudinal MI sensor, in contrast to the off-diagonal one, can be much extended as it exhibits a quite high sensitivity in the field range up to one order of magnitude higher than the anisotropy field.

Correlation of surface domain structure and magneto-impedance in amorphous microwires

The correlation between surface domain structure (SDS) and high frequency magneto-impedance (MI) in amorphous microwires has been systematically studied. First, we applied the magneto-optical polarizing microscopy to determine the SDS and found that it is considerably different in unstressed microwire and in microwires to which tensile and torsional stress were applied. Then, we measured the longitudinal and off-diagonal MI in these microwires and also observed quite different MI dependencies. We analyzed the experimental MI curves and their dependence on the SDS with the help of a simple model that nevertheless yields good qualitative agreement with experiment. We have demonstrated that the analysis of the MI dependencies, especially the off-diagonal one, can reveal the SDS in the microwires. The obtained results can also be useful for magnetic and stress sensing applications.

Magnetoimpedance in soft magnetic amorphous and nanostructured wires

The surface impedance tensor approach has been used to review the impedance response in a variety of amorphous and nanocrystallized wires. An experimental study on the torsion annealing effect on the magnetoimpedance (MI) behaviour for positive and negative magnetostriction amorphous wires of FeSiB and CoSiB compositions, respectively, has been carried out. Moreover, the influence of the onset nanocrystallization on the MI behaviour in Finemet-type alloys, with particular attention focussed on the case that the wires are annealed under applied torsional stress, is also presented. The analysis of the MI and torsion-impedance (TI) effects allows us to compare the different magnetic characteristics observed in a variety of wires. Special attention is paid to new results of the off-diagonal MI huge responses of these ferromagnetic samples, which are also a very useful source of information on the magnetic properties of the wires.

Off-Diagonal Magnetoimpedance Dependence of Magnetostriction and Anisotropy in Co-Based and Fe-Based Amorphous Ribbons

The object of this work is the comparison of domain structure and off-diagonal magnetoimpedance effect in amorphous ribbons with different magnetostriction coefficient. The Co66Fe4Ni1Si15B14 and Fe80B20 samples were obtained by melt-spinning. During the quenching procedure a 0.07 T transverse magnetic field was applied to some of the samples. Domain patterns obtained by the Bitter technique confirm that the differences on the samples are related to the different anisotropy and magnetostriction coefficient, and the quenching procedure. Small changes on the anisotropy distribution and the magnetostriction coefficient can be detected by the off-diagonal impedance spectra as a consequence of the different permeability values of the samples.

Off-diagonal GMI sensor with stress-annealed amorphous ribbon

A novel type of magnetic field sensing device using a single amorphous ribbon is presented. The sensor operation is based on a non-vanishing off-diagonal magnetoimpedance in ribbons with creep-induced anisotropy when the ribbon is driven by high frequency current with a dc component. The sensor was examined in two regimes. First, it was operated by laboratory instruments in the open loop connection. Afterwards, simple electronics was developed to operate the sensor in the closed feedback loop. Noise RMS value of 210 pT and 410 pT in 10 Hz band and noise spectral density of 17 pT√Hz at 1 Hz and 70 pT√Hz at 1 Hz were achieved when sensor operated in open and closed feedback loop, respectively. Temperature offset drift of −67 nT/K and linearity error 0.2% FSR in the range ±75 μT were achieved in the feedback loop connection.

The surface domain structure and off-diagonal magnetoimpedance in glass-coated amorphous microwires

We have studied the response voltage in a coil wound on a glass-coated amorphous microwire, through which an ac current is passed. The amplitudes of the first and second harmonics of the response voltage have been studied as functions of the applied magnetic field. The first harmonic amplitude, which corresponds to a linear off-diagonal magnetoimpedance, is negligibly small, while the second harmonic is highly sensitive to the magnetic field. The behavior of harmonics in the frequency spectrum of the response voltage can be explained by the existence of a regular domain structure in the surface layer of the microwire exposed to a weak magnetic field.