Giant Magnetoimpedance Element Research Articles

Evaluation of Magnetoimpedance in Narrow NiFe/Al/NiFe Thin Films for Secured Packaging

The purpose of this study is to evaluate the cybersecurity potential of the giant magnetoimpedance (GMI) effect. To fulfill this objective, it was mandatory to develop the narrowest and the most sensitive GMI element together with an uncommon polarization method that consists in injecting an internal dc bias current in the GMI film to generate a transversal magnetic field in the structure. The impedance of a Ni₈₀Fe₂₀/Al/Ni₈₀Fe₂₀ tri-layer was characterized for different thicknesses, widths, and anisotropy orientations. Each combination of these parameters offers an original outcome of the film that may be used as a specific sensor for cybersecurity purposes. This sensor can be used as an active shield that can be put over a device or included in the package in order to protect it against physical attacks, such as fault injection or invasive attacks. If one of these attacks occurs, any modification of the GMI sensing element induces a change in the impedance response allowing the detection of tampering of the secured packaging. Once the detection is confirmed, various actions can be taken, such as erasing sensitive data or resetting the device.

Impact of Adjustment of the Static Working Point on the 1/f Noise in a Negative Feedback GMI Magnetic Sensor

The resolution of giant magneto-impedance (GMI)-based magnetic sensor is limited by its equivalent magnetic noise level. It presents a white noise and a low-frequency 1/ f noise behavior. Previous works have showed that the white noise is mainly determined by the electronic conditioning noise, nevertheless, the 1/ f noise’s origin is not totally clear yet. To optimize the 1/ f noise, in this paper, first, we develop an optimized negative feedback GMI sensor. It contains of a low-distortion oscillator, a high-performance amplitude detector, and a GMI sensing element with adjustable static working point. And then, the sensitivity and the 1/ f noise level of the GMI sensor are studied when considering the electronic conditioning parameters and the static working point. The experiment results show that the adjustment of the static working point has a significant effect on the 1/ f noise at low frequencies. The optimal 1/ f noise is obtained by setting the static working point at the position where the GMI element possesses with the maximum intrinsic sensitivity. Finally, we achieve the GMI sensor noise performance of 20 pT/ $\surd $ Hz at white noise region and 100 pT/ $\surd $ Hz at 1 Hz.

Digital electronic conditioning approach for the high-sensitivity off-diagonal GMI sensors

In this paper, the design and performance of a novel digital electronic conditioning for the off-diagonal Giant Magneto-Impedance (GMI) sensors are presented. The concepts and functions of this digital conditioning were implemented using Field-Programmable-Gate-Array (FPGA) reconfigurable circuits. This implementation consists of a Direct Digital Synthesizer (DDS) for the excitation of the GMI sensitive element and of a real-time quadrature digital demodulation of the voltage across the GMI element. This digital demodulation, which includes digital mixing and decimation, allows preserving important intrinsic characteristics of the off-diagonal configuration of the GMI sensors. In the case of GMI wires with transverse anisotropy as the Co-rich wire used in this study, these characteristics include the intrinsic asymmetric (odd) and the almost linear response of the off-diagonal component of the impedance near the zero-field, without the use of bias magnetic field and without employing additional offset canceling devices in the sensor chain. These features are important for the implementation of a number of linear sensors and magnetometers. The main rules for the implementation of this reconfigurable electronics for such GMI sensors are developed and explained throughout the paper. The first results and performance, in terms of response, sensitivity, equivalent magnetic noise, dynamic range, linearity and slew rate of the developed sensor, are presented and discussed.

High frequency passive micro-magnetic sensor based on surface acoustic wave transponder and giant magnetoimpedance sensitive element

This paper presents a high frequency micro-magnetic field sensor composed of a surface acoustic wave (SAW) transponder and a giant magnetoimpedance (GMI) sensitive element. The coupling-of-mode(COM) method is used to design and optimize the structure of the SAW transponder. An amorphous wire as the GMI element is used to conjunct with the SAW transponder. The experiment result shows the magnetic sensitivity of the sensor of 1.13dB/Oe changes when the magnetic sensitivity does not exceed 2.0Oe. For the two-dimension intensity measurement of the magnetic field, a two-dimension SAW-GMI sensor is developed and tested.

Investigation of Driving Power Dependence on Magnetoimpedance Properties of Thin-Film Elements With Uniaxial Anisotropy

We investigated the dependence of driving ac current on the giant magnetoimpedance (GMI) properties of rectangular elements fabricated using thin films with uniaxial anisotropy. Two types of GMI sensor elements were fabricated. One had the typical GMI configuration, with the easy axis parallel to the width of the sensor, and the other had an inclined easy axis. The impedance changes and its change ratio deteriorated with increasing ac current amplitude in the case of the typical type of the GMI element of narrower width. The observed discontinuous impedance jump was suppressed with increasing ac current amplitude for the GMI elements having an inclined easy axis. These behaviors are explained mainly by partial magnetization saturation due to a large excitation field inside the elements. We also discuss a research direction aimed at gaining higher sensitivity in typical GMI elements and the utilization of the discontinuous jump.

Impedance Variation with Subjecting to Normal Field for the Stepped Giant Magnetoimpedance Element

In this paper, an impedance variation for the stepped giant magnetoimpedance (GMI) element, when subjecting to a strong normal field, is reported. The stepped GMI element is an element which has an impedance-step property, which happens simultaneously with a change of magnetic domain. The strong normal field varies a profile of the sharp dip-point of the impedance-step, that is to say the normal field can control the dynamics of domain change of the element. The application of normal field also arises at a lower impedance state, which is expected as a hidden in-plane inclined Landau-Lifshitz-domain. This lower impedance state disappears by the application of in-plane magnetic field, which is larger than a certain value of threshold in absolute, and does not appear until the next application of the normal field. This element was proposed to apply a sensor with memory function in our prior work. The method reported in this paper would give a special function for the sensor application.

A High Dynamic Range GMI Current Sensor

The design and performances of a high dynamic range DC-AC current sensor utilizing Giant Magneto-Impedance (GMI) are presented. The sensor is based on a GMI element with negative feedback. The sensing element is a 30 μm diameter GMI Co-based amorphous wire. It is curled to a toroidal core of 2 cm diameter. A bias magnetic field of about 650 A/m is applied to the GMI element to obtain an asymmetric GMI effect. A strong negative feedback is introduced to ensure linearity in a wide dynamic range. Analog conditioning electronics was fully developed. This includes a square wave oscillator based on an inverter trigger; a peak detector and a high gain amplifier with zero adjust. The GMI element is driven at a 3 MHz frequency and 5 mA peak-to-peak current. The closed-loop operations are investigated and the performances of the sensor are presented. DC current measurements are performed. The sensor exhibits good sensitivity and very good linearity, free from hysteresis, in a wide dynamic range of ±40 A. The sensitivity is about 0.24 V/A and the linearity error is about 0.02% of the full scale (FS). The hysteresis error is smaller than the measurement accuracy. AC current measurements using the developed sensor have also been successfully achieved. The sensor bandwidth in closed-loop was about 1.7 kHz.

Development of a passive and remote magnetic microsensor with thin-film giant magnetoimpedance element and surface acoustic wave transponder

This paper presents the development of a wireless magnetic field sensor consisting of a three-layer thin-film giant magnetoimpedance sensor and a surface acoustic wave device on one substrate. The goal of this integration is a passive and remotely interrogated sensor that can be easily mass fabricated using standard microfabrication tools. The design parameters, fabrication process, and a model of the integrated sensor are presented together with experimental results of the sensor.

Study of dynamic process of domain change in the stepped giant magnetoimpedance element

A giant magnetoimpedance (GMI) element with stepped impedance property was reported for rectangular amorphous soft magnetic thin film with an in-plane uniaxial easy axis in a controlled direction. This stepped change of impedance occurs simultaneously with a change in a magnetic domain, and its physical basis was explained as the intersection of the level of magnetic energy in each case of these domain structures. In this paper, an induced pulse-voltage caused by a non-linear leap of magnetization in the domain change is studied, for the purpose of making dynamics of the domain change clear. We report the results of detecting a pulse signal by using a planar coil fabricated with the stepped GMI element. Some fundamental properties of the pulse signal are also reported, such as its form, and variations in its height as a function of the frequency of the external magnetic field. As a result we found that the domain change in the stepped GMI element is a kind of Barkhausen jump.

Existence of Three Stable States of Magnetic Domain for the Stepped Giant Magnetoimpedance Element and Proposal for Sensor With Memory Function

In this paper, a giant magnetoimpedance (GMI) element with stepped impedance property was reported for rectangular amorphous soft magnetic thin film with an in-plane uniaxial easy axis in a controlled direction. This stepped change of impedance occurs simultaneously with a change in a magnetic domain, and its physical basis was explained as the intersection of the level of magnetic energy in each case of these domain structures. The variation of stepped impedance as a function of external magnetic field typically has a hysteretic property. The hysteretic property is explained based on an existence of threshold energy barrier which is needed overcoming to change the magnetic domain. In this paper, we will show experimentally both an existence of three stable magnetic domains around zero external magnetic field by using the stepped GMI element, and a possibility to detect these states by impedance measurement. We also propose an application of this phenomenon for a magnetic sensor which has a memory function.

Study of Large Barkhauzen Sensor with Application of Step-like Giant Magnetoimpedance Element

A giant magneto-impedance element with step-like impedance properties was obtained for rectangular amorphous Co85Nb12Zr3 soft magnetic thin film with an in-plane uniaxial easy axis at approximately 60 degrees relative to the width of the element. This step-like impedance phenomenon occurred simultaneously with a change in the magnetic domain. We found through experiments that the magnetic field with changes in step-like impedance occurs had an almost constant value, with a standard deviation in the micro-tesla (μT) range, when an alternating magnetic field was applied. There is a possibility of applying this phenomenon to a large Barkhausen sensor with a constant magnetic field where a pulse signal appears. This paper reports a first trial where a step-like GMI element was applied to a large Barkhausen sensor. We present the results of measuring the MH loop for the step-like GMI element with resulting properties of a non-linear leap in magnetization, and also present the results of detecting a pulse signal by using a planar coil fabricated with the step-like GMI element. Some fundamental properties of the pulse signal are also reported, such as its form, variations in its height as a function of the frequency of the externa magnetic field, and variations in the pulse interval as a function of the frequency of the external magnetic field. We found that the domain change in the step-like GMI element is a kind of Barkhausen leap, and can be applied to a large Barkhausen sensor.

不連続的ＧＭＩ素子におけるインピーダンス不連続点の外部磁界変動に対する安定性

A giant magnetoimpedance element with a step-like impedance property was obtained in the case of amorphous Co85Nb12Zr3 soft magnetic thin film in a rectangular shape with an in-plane uniaxial easy axis at an angle of approximately 60 degrees relative to the width direction of the element. It was shown experimentally that the magnetic field in which the step-like impedance change occurs has an almost constant value, with a standard deviation in the micro tesla (μT) range, when an alternating magnetic field of constant amplitude is applied. There is a possibility of applying this phenomenon to a magnetic switch. This application requires a constant impedance step-point uninfluenced by the route or amplitude of the external magnetic field. In this paper, a variation of the magnetic field in which the impedance step occurs as a function of the range of the alternating magnetic field is studied. As a result, a stability of less than 20 mOe (1.6 A/m) at about 1200 mOe (96 A/m) is obtained for the impedance step-point, with consideration given to every experimental condition of the range of applying magnetic field. This result means that it is possible to make a magnetic switch with a switching field accuracy of less than 2%.

Analysis of steplike change of impedance for thin-film giant magnetoimpedance element with inclined stripe magnetic domain based on magnetic energy

The phenomenon of steplike impedance change for thin-film giant magnetoimpedance (GMI) element was reported. The steplike change happens simultaneously with the appearance or the disappearance of stripe magnetic domain. This phenomenon is obtained for amorphous Co85Nb12Zr3 soft magnetic thin film in a rectangle shape with an in-plane uniaxial easy axis in the direction nearly 60° against the short-side axis of the element. The steplike change of impedance appears in a certain magnitude of external magnetic field. A high-sensitive magnetic field sensor can be realized by applying this phenomenon. In this study, a mechanism of the appearance or the disappearance of inclined stripe magnetic domain is discussed based on an analysis of magnetic domain energy, for the purpose of applying to a high-sensitivity magnetic field sensor. We assume these extremely different magnetic domains as (I) stripe domain with closure domain and (II) single domain, based on experimentally observed domain structures. The result of analysis shows that the magnetic energy of these two phases intersects each other in a certain external magnetic field, which means that the basis of steplike GMI of thin-film element is revealed as a structural change of magnetic domain based on magnetic energy.

Impedance-step phenomenon of thin-film GMI element with inclined magnetic stripe domain

An extraordinary impedance-step phenomenon of GMI (giant magnetoimpedance) effect was studied. It is observed for a thin-film GMI element with an inclined magnetic stripe domain. The “inclined stripe domain” means a domain structure that has inclined domain walls against the width direction of the GMI strip. An observation of magnetic domain reveals that a sudden change of domain structure, such as stripe-domain vanishing or occurrence, happens simultaneously with the impedance step. A theoretical consideration, based on the bias-susceptibility model of a single-domain magnetic thin film with uniaxial anisotropy, qualitatively shows that the existence of an inclined stripe domain makes the impedance lower than that characteristic of a single-domain structure.