Thin-film Magnetoimpedance Element Research Articles

Realization of Different Anisotropy Direction for Each Thin-Film Magnetoimpedance Element on the Same Substrate

In this study, we have attempted to realize different magnetic anisotropy locally on the same substrate, i.e., different directions of the easy axis in each area by combining simple Joule heating and the application of a magnetic field with permanent magnets. Orthogonal arranged thin-film magnetoimpedance elements were fabricated on the same substrate, and the easy axis was successfully induced parallel to the width direction of each element, respectively. Each element shows the double-peak impedance profiles after Joule heating which is important to obtain higher sensitive magnetic field sensor. The results enable local anisotropy control, which has been difficult to realize previously, and contribute to the integration of multifunctional thin-film magnetic devices on a substrate.

Controlling the Magnetoimpedance Property of Thin-Film Elements Using Joule Heating

Optimizing the performance of magnetoimpedance (MI) requires controlling the anisotropy of material which is often done with field annealing. However, field annealing needs large-scale equipment with a vacuum environment and consumes a large amount of electricity for heating and generating a large magnetic field. Conversely, Joule heating has advantages of simple, compact, and possible-to-use small current. Thus, we attempted controlling the magnetic anisotropy on thin-film MI elements using Joule heating and the applied field by magnets. Before annealing, the element has an easy axis with a direction parallel to the longitudinal direction of the element owing to shape anisotropy. After Joule heating, the MI behavior showed a double peak profile, which is a typical impedance profile when large impedance changes are obtained. The peak height in the impedance and inductance increases with the heating current up to 100 mA and then becomes constant or slightly increases. The field at which the impedance and inductance have a peak increases monotonically with increasing heat current. We confirm that the magnetic anisotropy direction changes with Joule heating and with a field applied by magnets based on observing the magnetic domain structure using a Kerr effect microscope.

Effects of parallel and meander configuration on thin-film magnetoimpedance element

We investigated the effects of configuration of thin-film magnetoimpedance element on impedance profile. The parallel type and meander type sensor element were fabricated. The parallel line configuration has advantages to reduce a bias field and to enhance total inductance attributable to positive mutual inductance, while a large number of lines brings a decrement of impedance changes. The meander type induces a large demagnetizing effect and decrement of the total inductance due to negative mutual inductance. However, this configuration can utilize the space of the element, which contributes to enhance spatial resolution.

Impedance Change Ratio and Sensitivity of Micromachined Single-Layer Thin Film Magneto-Impedance Sensor

Thin-film techniques are important in the miniaturization of electronic devices. We studied a thin-film magneto-impedance element made with Co-based amorphous soft magnetic material and developed a highly sensitive magnetic field sensor with high spatial resolution. Thin films are several micrometers thick, and show significant impedance changes as frequencies change from the megahertz to the gigahertz region. Multilayer systems are often used to achieve a large impedance change, but they typically require a complex thin-film structure and a length of several millimeters. Our thin-film sensors have lengths between 30 µm and 1 mm in a simple monolayer configuration and successfully overcome the demagnetizing field. The impedance change ratio was 300% for a 1 mm long element at 1 GHz and between 20–30% at 1 GHz for a miniaturized element of 30 µm. The sensors had a maximum sensitivity of 33.4%/Oe.

Incident Power Influence on Magnetoimpedance Element With Domain-Wall Resonance

Magnetoimpedance(MI) sensor utilizes permeability changes of the material through skin effect and ferromagnetic resonance when the sensor is applied to a magnetic field, and it has a higher sensitivity [1]. The sensors, which are composed of amorphous wires, are currently commercialized as compasses in mobile phones, and researches advance their sensitivity still continue for application of biomedical research [2] and nondestructive testing [3]. On the other hand, thin-film MI elements which has the compatibility with miniaturized integrated electronic devices such as driving and detecting circuits, contributes to the miniaturization of sensor device with higher spatial resolution. However, one problem of using thinfilm elements is higher operating frequency. Typically, operating frequency is above 100 MHz to GHz range for elements with several μm thickness; this is unsuitable for general-purpose driving and detecting circuits, which normally operate below 20–30 MHz. During our investigations about thinfilm MI elements, we found an interesting behavior that the frequency profile of impedance shows double peak depending on the applied bias field, that is, another peak appears at lower frequency region [4]. Then we analyzed this profile based on the domain wall equation and indicated the changes in impedance is attributed to the domain wall resonance (DWR) [5]. We also showed a potential of the phenomenon applied for developing a highly sensitive sensor operated at frequencies around the dozen megahertz region. In such cases, more detailed investigations about incident power is required because the behavior of impedance changes strongly depends on input high frequency power and this behavior is not still clear. Thus, in this study, we evaluated experimentally the impedance profile around frequency where the DWR occurs when the incident ac power modulated, and discussed about the sensitivity of impedance changes.

Effects of DC Bias Current on Behavior and Sensitivity of Thin-Film Magnetoimpedance Element

We investigated the behaviors and sensitivity of thin-film magnetoimpedance elements having an easy axis angle of 0°–45° when applying dc bias current directly to the elements. All elements show symmetric impedance profiles with respect to the impedance axis without dc bias current, while their profiles become asymmetric with dc bias current. This appearance of the asymmetric property on the impedance profiles indicates that the shape of cross section of the element has asymmetric configuration. On the other hand, when the easy axis angle is relatively small, the sensitivity for field detection is enhanced with a small dc bias level, while a stronger bias level is required for the element with a larger easy axis angle. The obtained results show a potential to optimize the sensor properties by dc bias current with small intensity in case that design properties are not obtained in the fabrication process.

Analysis of thin-film magnetoimpedance behavior in low MHz regions based on domain wall equation and bias susceptibility theory

An interesting behavior of thin-film magnetoimpedance elements in the relatively low megahertz (MHz) region is found experimentally when the width of the element is narrow and the thickness of elements is several micrometers. The impedance peaks and inductance shows a rapid drop at around 10 MHz when a bias field is applied to such elements. The impedance profiles of the elements were analyzed on the basis of a domain wall motion equation and bias susceptibility theory. Calculation of the domain wall motion equation when accounting for domain resonance explains the impedance behavior in the low MHz region. The rapid drop in inductance and peak in impedance can be attributed to the domain wall resonance. At a relatively higher frequency, above 100 MHz, the calculation of bias susceptibility while considering ferromagnetic resonance thoroughly explains the experimental behavior.

Behavior of sensitivity at edge of thin-film magnetoimpedance element

We fabricated thin-film magnetoimpedance elements in which an impedance of each 100 μm section of element can be examined, to investigate impedance changes of each section subjected to a DC magnetic field. The field strength where the impedance peaks shows a larger value at the edge and it decreases toward the center of the element, while the sensitivity is small at the end of the elements and increases toward the center of the element. The obtained results can be explained on a basis of magnetic field simulation and simple impedance model taking into account a distribution of demagnetizing field. A uniformity of demagnetizing field is significant to obtain a higher sensitivity, and intensity of the demagnetizing field strongly affects a magnetic field strength when the impedance peaks. We also clarified an ellipsoidal shape uniform the distribution of demagnetizing field within the element, which contributes to improve the sensitivity of the MI sensor, especially near edge part.

Sensing of Magnetic Field and Strain utilizing Magneto-Impedance Effect

A Magneto-Impedance (MI) effect which exhibited the strong dependence of impedance on external magnetic field was found in amorphous wire by Mohri et.al. fifteen years ago. The magneto-impedance (MI) sensors were developed as a new magnetic field detection principle and achieved to magnetic sensors and strain sensors with high performance. In this review, we describe the layered thin film MI elements and the high resolution sensing of magnetic field and strain utilizing them.

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.

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.

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.

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.