Magnetoimpedance Elements Research Articles

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.

A small and wide-range three-phase current sensor using a MI element

This paper describes a compact three-phase electric current sensor using a magneto-impedance (MI) sensor for wide-range current sensing. The MI sensor is composed of a thin-film MI element and a bias coil. Double shields are used for high shielding performance maintaining compact size. Discrete sampling being employed and sensors being intermittently driven in a 3 ms period can achieve a low power consumption of 20 mW. Two kinds of alternating bias are used in the system to achieve a wide measurement range of 1:600. Furthermore, using optimized carrier electric current is shown to decrease the temperature dependence of the MI sensor.

AC biased sub-nano-tesla magnetic field sensor for low-frequency applications utilizing magnetoimpedance in multilayer films

A sensitive magnetic sensor based on the measurement of reflected incident power from magnetoimpedance (MI) multilayer-film element incorporated into 50 /spl Omega/ matching network has been produced. The film system was fabricated by radio-frequency sputtering having two outer magnetic layers (NiFe), insulation layers (AlO/sub 3/), and inner conductor Au. For optimized layer parameters, an MI ratio of 135%, sensitivity 15%/Oe, and linearity within -2.5 and 5 Oe was obtained at a frequency of 434 MHz. An ac sense magnetic field applied to the MI element results in a small amplitude modulation signal due to the impedance variation, which can be measured on the carrier signal. For the sense field frequency of 1 kHz, the resolution of 3.73/spl times/10/sup -7/ Oe was achieved. To improve the field resolution at lower frequencies (<100 Hz), the use of an ac bias technique has been proposed. For a sense field of 20 and 10 Hz with 5 kHz ac bias, field resolutions of 4.78/spl times/10/sup -6/ Oe and 5.27/spl times/10/sup -6/ Oe, respectively, have been obtained.

ＭＩ素子を用いた小形・ワイドレンジ三相電流センサ

This paper describes a three–phase electric current sensor using a magneto–impedance (MI) sensor, which provides compact size and wide–range current sensing. The MI sensor is composed of a thin–film MI element and a bias coil. The MI sensors and electric current bars are placed in u–shaped cores to prevent an unwanted magnetic field from being generated by another phase current. To realize a high shielding performance and a compact size, a double shielding structure is employed. Two kinds of alternate bias are used in the system to implement a wide measurement range of 1:600. The sensor is intermittently driven by discrete sampling to realize a low power consumption of 20 mW. The influence of temperature on the MI sensor is alleviated by supplying an optimum carrier electric current. As a result, a temperature fluctuation ratio of less than ± 1.3% at -25 to 60°C was obtained.

Effect of induced anisotropy on magnetoimpedance characteristics in NiFe∕Au∕NiFe sandwich thin films

The high frequency magnetoimpedance (MI) has been measured in NiFe∕Au∕NiFe narrow sandwich thin films with transverse, longitudinal, and crossed magnetic anisotropies over a frequency range of 50–500MHz. The MI elements have an opened structure with lengths of 2 or 5 mm, widths of 50–200μm and a total film thickness of 1.5μm. The magnetoimpedance characteristics with different field curve shapes, including asymmetrical, have been realized for certain magnetic anisotropy using a dc bias current. Initially the different MI field characteristics have been demonstrated in ferromagnetic microwires with circumferential, helical, and longitudinal anisotropies. Here we have reintroduced these field characteristics for the MI sandwich thin film elements. In an attempt to induce a particular anisotropy in the films the magnetic layers were rf sputtered in the presence of a strong magnetic field, with a further thermal restress treatment of the final MI elements. The use of thin film technology for MI is preferable in many applications because of its compatibility with integrated circuit technology which enables miniaturization, avoiding alignment issues, and wire soldering.

電歪／磁歪複合振動子による磁界センサ

Magnetic field sensors such as flux gate and magnetoimpedance elements have been sonsidered to be useful tools for indicating the direction of a terrestrial field. Downsizing of these sensors is essential for then to be used in highly integrated electronic devices such as mobile phones and computers. We developed a new type of magnetic field sensor that has a great advantage in this respect. The sensor has a composite structure cosisting of a mechanical resonator with a high Q factor and magnetostrictive layers. With the high sensitivity of the magnetostrictive layers to external fields, the resonance frequency of the resonator is expected to be shifted by the ΔE effect. Since the shift in frequency is independent of the resonator size, appreciable downsizing of the sensor is possible. In this paper we describe our initial experiments to verify that the composite resonator is sufficiently sensitive to detect terrestrial fields. The results show that it has a sensitivity of 40 Hz/Oe for a resonator with a resonance frequency of 120 kHz.

微細化薄膜MI素子

A micromachined magnetoimpedance (MI) element was developed and its MI properties were investigated. Consisting of a SiN/Al/NiFe/SiO2 film, it was prepared on a Si (100) wafer and fabricated by photolithography and etching. Ni80Fe20 film with nearly zero magnetostriction prepared by bias magnetron sputtering under a magnetic field was adopted as a magnetic layer in the MI element, and magnetic anisotropy was induced parallel to the current direction. Sensitivity corresponding to the maximum fractional change in impedance increased with thickness and length, and was greatest for a width of 20 μm. The values for whick the MI element had the best properties were 38 % for the sensitivity, 1446 ppm/°C for the temperature coefficient of impedance and 712 ppm/°C for the temperature coefficient of sensitivity. These values were much better than those of conventional magnetoresistance (MR) sensors. Therefore, these micromachined MI elements have great potential for use in integrated magnetic sensors.

Magneto-impedance effect of a layered CoNbZr amorphous film formed on a polyimide substrate

To produce magnetic field sensors for use on irregular surfaces, thin-film-type elements that can detect a magnetic field by the magneto-impedance (MI) effect were formed on 20-/spl mu/m-thick polyimide substrates by sputtering. The thin-film MI element has a layered configuration with the conductive Cu film surrounded by ferromagnetic CoNbZr magnetic films. Excellent soft magnetic properties could be obtained for the CoNbZr amorphous magnetic films by optimizing preparatory conditions and inserting an SiO/sub 2/ buffer layer between the element and the polyimide sheet. To detect the magnetic field acting on the MI element by means of an impedance change, the element was shaped into a meander pattern. The MI element exhibits an impedance change of more than 100% and shows good temperature stability compared to that of conventional thin-film MI elements fabricated on glass substrates.

Magneto-impedance in multilayer films

This paper reviews results on magneto-impedance (MI) in multilayers consisting essentially of two soft ferromagnetic films (F) and a conductive inner lead (M): F/M/F. Multilayer film MI has a potential to be utilized for developing highly sensitive and quick-responding micro-magnetic sensors for advanced intelligent measurement and control systems and high-density magnetic recording. In materials with a uniform conductivity, a large change in impedance is accountable to a strong skin-effect, requiring the operation frequencies to be of the order of several gigahertz, for a submicron MI element. In the multilayer films F/M/F with sufficiently large difference between the conductivity values of layers M and F, the impedance can vary considerably at relatively low frequencies when the skin effect is not essential. The theory predicts that the MI ratio in multilayers 1-μm thick with a Cu inner lead and soft magnetic amorphous films (CoSiB) or permalloy (NiFe) films is as much as 400–500% at frequencies of 10–100 MHz; in the case of a sandwich 0.1-μm thick, the MI ratio, although drops considerably, is still large being 15–20% at frequencies of 300–400 MHz, which is in good agreement with the experimental results. On the other hand, in narrow films, the flux leakage across the inner conductor may result in a considerable drop in MI ratio if the film width is smaller than some critical value depending on the transverse permeability and the thickness of the M and F layers.

磁場中熱処理による薄膜ＭＩ素子の特性改善

The temperature properties of layered thin film magneto-impedance (MI) elements composed of FeCoSiB/Cu/FeCoSiB were investigated for use in a magnetic sensor that remains stable under temperature change. The properties of the layered thin film MI elements as deposited were changed by increasing the temperature or by establishing a temperature cycle of 25 °C-100 °C. When these MI elements were annealed under a magnetic field at the most suitable temperature, which was 150 °C lower than the curie tempaerature of the magnetic film, the change in the MI properties due to the temperature increase or the temperature cycle was decreased. In particular, the change in inductance due to the temperature increase was small when the Fe4Co74Si8B14 zero magnetostriction magnetic film was used in the layered thin film MI element.

無電解Ｆｅ‐Ｎｉ系合金めっきしたＣｕ細線磁気インピーダンス素子の作製と評価

We prepared magneto-impedance (MI) elements of Cu wires electrolessly-deposited with Fe-Ni-P and Fe-Ni-B film layers, and investigated the dependence of their MI effect on the frequency (f) and the film thickness (t). A maximum MI sensitivity of about 100 %/Oe was obtained for the MI element of an 11.3 wt.% Fe-Ni-P film with t = 5 μm at f = 10 MHz. High MI sensitivity was obtained with an increase in the film thickness of the element, owing to the larger maximum MI ratio (ηmax) and the smaller anisotropy field (Hk).

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.

Recent advances of micro magnetic sensors and sensing application

Principle and basic performance of micro magnetic sensors such as the Hall element, the magneto-resistance (MR) element, the giant magneto-resistance (GMR) element, the thin-film fluxgate sensor (FGS), and the magneto-impedance (MI) element are summarized. Their basic properties are compared with each other considering a new sensing target in various modern industrial fields. A new micro-sized magnetic field sensor with a high sensitivity and a quick response is developed using the MI effect in amorphous wires and thin films. The MI sensor having a micro-sized zero-magnetostrictive amorphous wire head of about 1 mm installed in self oscillation circuits such as the Colpitts oscillator and a multivibrator circuit shows a high sensitivity with a resolution of 10−6 Oe for ac fields and 10−5 Oe for dc fields, quick response with a cutt-off frequency of about 1 MHz, and a high temperature stability of less than 0.05%FS °C −1 up to 70°C. A very low power consumption and highly stable MI sensor is constructed using a CMOS multivibrator. A stable measurement for a surface-flux distribution around a 2000-pole ring magnet with a diameter of 19 mm for a high density rotary encoder was carried out using the MI sensor. A small pin hole of 100 mm diameter in an iron sheet was detected at non destructive testing (NDT) using a gradient-field detection type MI sensor cancelling back ground disturbance fields. A sample of clustered magnetic particles was magnetically detected using the MI sensor probe in a simulation of detection of a brain tumor at which an injected sol was induced with the magnetic particles.

Asymmetrical Magneto-Impedance Effect in an Amorphous Wire with a Coil

A new method of obtaining the asymmetrical magneto-impedance (MI) effect in a zero-magnetostrictive FeCoSiB amorphous wire is presented, where in a wire coil is used to apply a wire pulse current. The mechanism for the generation of the asymmetrical MI effect is qualitatively explained by means of a magnetization rotation model for the surface layer of a wire with a spiral magnetization. A linear magnetic-field sensor circuit uses the amorphous-wire asymmetrical MI element magnetized with a sharp pulse train current in a C-MOS multivibrator circuit.

High frequency (1–1200 MHz) magnetoimpedance in CoFeSiB amorphous wires (abstract)

Magnetoimpedance (MI) in Co-based amorphous alloys has been shown to be a very sensitive, quick-response new method for measurements of magnetic fields. At present, the experimental results on MI in amorphous wires are obtained for frequencies f&amp;lt;200 MHz. For quick-response magnetic heads used in high density magnetic recording, the carrier current frequency of the MI element is needed to be increased up to 1 GHz to detect recorded signals of 50–100 MHz. Here we present the experimental data on MI in CoFeSiB amorphous wires for a broad waveband of 1–1200 MHz. The experimental technique is based on the measurement of the complex reflection coefficient from a coaxial waveguide having an amorphous wire as an internal conductor. This method avoids the radiation effects, inevitable at high frequencies, f≳100 MHz, in simpler techniques utilizing an oscilloscope or an impedance analyzer with a lead wire. From the data on reflection coefficient, the real and imaginary parts of the wire impedance Z and permeability μ are found as functions of a frequency and an external longitudinal field. The impedance versus field behavior changes with increasing the frequency. For f&amp;lt;1 MHz the absolute value of the impedance ‖Z‖ decreases with increasing the field. As the frequency is increased, a maximum appears in the impedance-field dependence. In the case of high frequencies, f≳800 MHz, ‖Z‖ increases with the field. For all frequencies, a higher sensitivity is seen in small fields less than 2 Oe. The sensitivity has a maximum of about 100%/Oe at the frequency of 600 MHz, and it is still very high (∼20%/Oe) up to f∼1 GHz. These results are in satisfactory agreement with the theoretical ones based on the skin effect in a magnetic wire with a tensor rotational permeability.

アモルファス磁性ワイヤＭＩ素子によるコルピッツ自己発振形高感度・高速応答電流センサ

A new high-performance micro-current sensor with circular amorphous wire magneto-impedance (MI) element installed in a Colpitts oscillator is presented. A zeromagnetostrictive amorphous wire ((Fe0.06Co0.94)72.5 Si12.5B15, λs = -10-7) was used to make a circular head with a 10 mm diameter set around a conductor. The current sensor is of the amplitude modulation-demodulation type with a strong negative feedback loop. It has a sensing performance of a high linearity (a non-linearity of less than 0.5%FS (± 1.5 A)), quick response with a cur-off frequency of about 1 MHz due to a high oscillation frequency of 30 MHz, and a high sensitivity of about ±10μA(ƒ=1kHz). This sensor works stably in uniform-distrubance magnetic field such as the terrestrial magnetic field (≅0.3 Oe), because of the cancelling in the circular MI head.

アモルファスワイヤＭＩヘッド磁石変位センサによるまばたき波形の検出と眠気度計測

Two highly sensitive magnet-displacement sensors with a resonant-multivibrator and a Colpitts oscillator, respectively,were constructed by using amorphous wire magnetoimpedance elements. The sensor output is independent of the magnetic pole sign on account of the magnetoimpedance characteristics, which allow highly operable measurement of eyelid motions such as blinks. Small magnets were attached to the eyelids of several subjects, and the movements of the magnets when the subjects blinked were detected by the sensor. Sleepiness was clearly estimated by measuring the length of time that the eyelids were closed during blinking, the results were independent of subjects.

Pulse response bistable magneto-impedance effect in amorphous wires

A bistable magneto-impedance (MI) effect has been found in twisted amorphous wires magnetized with a sharp pulse current, in which the induced wire voltage jumps from a higher value to a lower one at some positive critical magnetic field and conversely for a negative field, exhibiting two clearly defined residual states. The bistable MI element has a potential for application in a digital magnetic memory cell of nonvolatile and nondestructive readout for a neural network memory. It can also operate as switch type field sensors such as a sensitive micro-sized proximity sensor. The basic properties of the pulse response bistable MI effect and its analysis based on the hysteresis in an ac rotational permeability are also presented.

アモルファス磁気‐インピーダンス素子共振マルチバイブレータ形磁石変位センサによる両眼けん振動検出と眠気度

A highly sensitive magnet-displacement sensor was constructed by using a pair of amorphous wire magnetoimpedance elements as the head of a resonant-multivibrator- type field sensor. The sensor output is independent of the magnetic pole sign on account of the magnetoimpedance characteristics, which allow highly operable measurement of eyelid motion. Sleepiness was sensed by noncontact detection of the movements of both eyelids. Two small magnets were attached to the eyelids and their movements were detected by using a field sensor fixed to a glass frame. Sleepiness was clearly detected for a special oscillation pattern, specific to each subject, of a pursuit target.

Magneto-Impedance Element Using Amorphous Micro Wire

A large magneto-impedance (MI) effect was found in amorphous magnetic wires while passing a high-frequency current. The amplitude of the induced voltage between the ends of a wire changes by 40 to 50% when an external magnetic field of about 10 Oe (approximately 800 A/m) is applied parallel to the wire axis. This paper reports the principles and basic properties of the MI effect in very small amorphous wires of composition (Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.06</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.94</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">72.5</sub> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12.5</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sub> and magnetostriction -10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> , and describes an application to rotary encoder heads using very small MI elements 30 μm in diameter and 1 mm in length.