Norikazu Ohta

Thin film magnetic field sensor utilizing Magneto Impedance effect

Abstract Recently, the Magneto Impedance effect found in amorphous wires with soft magnetic properties is noticeable as a new principle for sensing magnetic filed. According to this effect, the impedance of the wire in the range of high frequencies over 10 MHz changes remarkably with the external magnetic field. This effect is expected to be promising for magnetic field sensor with high sensitivity. Therefore, we have attempted to introduce this effect into amorphous thin films to extend application fields, and a novel thin film sensor sensitive to small magnetic field based on the Magneto Impedance effect has been proposed. The sensor consists of half bridge of the individual detecting element with FeCoSiB/Cu/FeCoSiB multi-layer, which exhibits the large impedance change ratio more than 100% when an external magnetic field is applied. By the optimization of the operating point due to bias field and the signal processing with a synchronous rectifier circuit, no hysteresis, good linearity and good stability against temperature variation as well as high sensitivity in the sensor characteristics have been achieved. The variation of the sensor output with the temperature is largely reduced to one-third, compared to the conventional thin film sensor we developed formerly. The detection resolution of 10−3 Oe order higher than those of any other conventional thin film sensors is obtained.

Sensing of passing vehicles using a lane marker on a road with built-in thin-film MI sensor and power source

In a field test, we demonstrated the sensing of passing vehicles, using onroad lane markers with a built-in magneto impedance (MI) sensor and a self-contained power source. The vehicle-sensing rate was 100% in the field test and we found that the lane marker could detect vehicles with a high degree of accuracy. A thin-film MI sensor, which is very sensitive to small external magnetic fields, an analog circuit for driving/processing, and a digital circuit with EEPROM memory for storing the number of passing vehicles were incorporated into a thin aluminum plate marker. The lane marker also had a self-contained power source, consisting of a secondary battery and a solar battery, which eliminated the need for an external power supply.

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.

Micromachined Thin Film MI Element for Integrated Magnetic Sensor

A micromachined thin film magnetoimpedance (MI) element was developed and its MI properties were investigated. Consisting of a SiN/Al/NiFe/SiO2 film, it was formed 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 driving current direction. Sensitivity defined as the maximum fractional change in impedance increased with thickness and length, and was greatest for a width of 20 mum. The values for which the MI element had the best properties were 38% for the sensitivity, 1446 ppm/degC for the temperature coefficient of impedance TCZ and 712 ppm/degC for the temperature coefficient of sensitivity TCS at the driving frequency of 100 MHz. These values were much better than those of conventional magnetoresistance (MR) sensors. Therefore, these micromachined thin film MI elements have great potential for use in high sensitive integrated magnetic sensors.

Micromachined layered thin film magnetoimpedance element

This paper presents a micromachined layered magnetoimpedance (MI) element consisting of a CoNbZr/Al/CoNbZr film and its MI properties were investigated. Sensitivity defined as maximum fractional change in impedance changed with thickness and dimension. The values for which MI element had the best properties were 45% for the sensitivity, 5% for the fractional change in impedance dependent on temperature. These values were much better than those of integrated magnetoresistance (MR) sensors. Therefore, these micromachined elements have great potential use in integrated magnetic sensors.