We analyzed and organized the reasons why the amorphous wire CMOS IC magneto-impedance sensor (MI sensor) has rapidly been mass-produced as the electronic compass chips for the smart phones, mobile phones, and the wrist watches. Comprehensive advantageous features regarding six terms of (1) microsizing and ultralow power consumption, (2) high linearity without any hysteresis for the magnetic field detection, (3) high sensitivity for magnetic field detection with a Pico-Tesla resolution, (4) quick response for detection of magnetic field, (5) high temperature stability, and (6) high reversibility against large disturbance magnetic field shock are based on the magneto-impedance effect in the amorphous wires. We have detected the biomagnetic field using the Pico-Tesla resolution MI sensor at the room temperature such as the magneto-cardiogram (MCG), the magneto-encephalogram (MEG), and the self-oscillatory magnetic field of guinea-pig stomach smooth muscles (in vitro) that suggest the origin of the biomagnetic field is probably pulsive flow of Ca2+ through the muscle cell membrane.
We have found a new electromagnetic phenomenon in the amorphous wire in 1993 and named it as “the magneto-impedance effect” [
Electronic compasses have been developed and mass-produced by Aichi Steel Co., Japan, supported by the Japan Science and Technology Agency (JST) using the 3-axis amorphous wire MI sensor chips for the mobile phones since 2005, the smart phones since 2011, and the wrist watch since 2013.
We have also developed a highly sensitive magnetic sensor with 1 Pico-Tesla resolution MI sensor and applied it to detect the biomagnetic field at the room temperature such as for an in vitro biopsy fragment of guinea-pig stomach [
Figure
A comparison of seven sensor features among magnetic sensors.
Figure
Domain model for amorphous wire [
The magnetic noises are mainly due to the fluctuating magnetic domain wall displacement of the spike domains (Barkhausen noises) in the inner core. Therefore, we should avoid magnetizing the inner core region to realize the highly sensitive magnetic sensor. The demagnetizing field
Figure
Frequency characteristics of the impedance of an amorphous wire with a sinusoidal current for external static magnetic field
The impedance
When
When
The sensitivity of the magneto-impedance effect is
A theoretical analysis of a minimum magnetic noise spectral density
Considering the typical parameters of the Co-rich amorphous wire, we could estimate the value of
Although a high sensitive micromagnetic sensor would be possible using the magneto-impedance effect expressed in (
We developed an amorphous wire CMOS IC multivibrator type microlinear magnetic field sensor for industrial usage MI sensor chip as illustrated in Figure
Amorphous wire CMOS IC multivibrator type MI sensor for industrial usage.
We adopted the CMOS IC inverter multivibrator generally used as the timing pulse circuit in the personal computer as a pulse train generator for amorphous wire magnetization via a differentiation circuitry and two time delay inverters. A pulse current train is illustrated in a photograph in Figure
Pulse train current for amorphous magnetization in (a) and explanatory illustration for a correspondence between a pulse current and a sinusoidal current biased with a dc current in (b).
Magnetization rotation model in the surface layer of an amorphous wire at strong skin effect;
In Figure
Then
Measured results of the magneto-impedance characteristics in the amorphous wire magnetized with the pulse current have been represented in Figure
Measured results of the magneto-impedance characteristics in a 30
A highly sensitive magneto-impedance effect showing an impedance change rate (
We have set a pick-up coil around the amorphous wire to constitute a highly linear magnetic field sensor.
When a sufficient large pulse magnetic field
Thus we have created a highly linear without any hysteresis micromagnetic field sensor using a simple digital type circuitry without any negative feedback circuit as illustrated in Figure
Figure
A high resolution micromagnetic sensor with a noise level (nonlinearity) of around 0.0125% of the dynamic range full scale was first developed, which is available for industrial mass production with a high temperature stability for –40~85-degree C operating temperature range.
The industrial mass production type magneto-impedance element (MI element) is fabricated by Aichi Steel Co., as illustrated in Figure
Mass production type amorphous wire MI element fabricated by Aichi Steel Co.
An amorphous wire is set by a microrobot on a patterned coil under part and after that an upper part patterned coil is plated followed by forming the coil. Two electrodes of the amorphous wire ends are formed with plating. A coil pitch of the plated patterned coil is possible to be up to 2
The amorphous wire CMOS IC MI sensor shows almost 100% directionality as represented in Figure
Directionality of an amorphous wire CMOS IC MI sensor.
Figure
Electronic compass chips using 3-axis amorphous wire CMOS IC MI sensor.
The electronic compass chips using the amorphous wire CMOS IC MI sensor have also been installed in the wrist watch since 2013, in which the performance of the MI sensor chip (AMI 306) is the geomagnetic field detection of 160 nT resolution for a dynamic range of ±1.2 mT (12 Oe) and a low current consumption of 150
Highly sensitive sensor chips have also been developed as the motion sensor chips with the gyro (rotation angle velocity) sensing performance of the amorphous wire CMOS MI sensor.
A handy ultrahigh sensitive magnetic sensor with a resolution of around 1 Pico-Tesla (10−8 Gauss) using the amorphous wire CMOS MI sensor has been developed [
Figure
Biomagnetic field sensing using a Pico-Tesla resolution amorphous wire CMOS IC MI sensor.
Guinea-pig stomach biopsy fragment (in vitro)
Human chest magneto-cardiogram
Human back magneto-cardiogram before and after a magnetic stimulation
In (a), an amorphous wire sensor head of 30
In the physiology, sequential electromagnetic activities in the smooth muscle organs such as the heart and other viscera have been observed as follows:
We consider a possibility to detect physiological information differs from the electrocardiogram (ECG) in the magneto-cardiogram (MCG) on the basis of these measured results. We may assume that the back MCG reflects the Ca2+ flow (electric current) into the muscle cells membrane showing a pulse time lag against the pulse action potential as observed in Figures
We may clearly estimate the progress processes of the amorphous wire CMOS IC sensitive micromagnetic sensors for the industrial and personal usages on the basis of the abovementioned comprehensive advantageous features as follows: Sensitive micromagnetic sensor chips in the electronic compasses for the smart phones and mobile phones, the wrist watches, the tablets, and various personal information terminals connected to the Internet. Various applications for social security systems such as the car parking controls and the personal daily behavior monitoring are widely under development. Ultrahigh sensitive magnetic sensors for the biomagnetic sensing for estimation and prediction of the ELF physiological magnetic stimulation (PMS) effects for the arousal preventing drowsy driving without “the rebound sleep” and the blood flow activation for health [ GHz high-frequency magneto-impedance effects [
We have analytically reviewed recent advances of the sensitive linear micromagnetic sensors adopted in the mobile phones, smart phones, and the wrist watches as the electronic compass chips. We evaluated a comprehensive feature of magnetic sensors suitable to be adopted in the electronic compass chip considering seven features of the microsizing, the small power consumption, the sensitivity, the linearity and directionality, the maximum operating temperature, the reliability and reversibility of antishock magnetic field, and the response speed, in which a quick response magnetic sensor is under development. The amorphous wire CMOS IC magneto-impedance sensor has a maximum area surrounded with feature lines on a seven-radial-axis sensor evaluation figure (Figure
We have not yet established a construction method of a quick response magnetic sensor such as a GHz response sensor although electronic compass chips have been mass-produced. The high-frequency operation sensor electronic circuitry needs the distributed parameter circuit technology with the impedance matching suppressing distributed power losses. The surface magnetic smoothness of amorphous wires sensitively affects the sensitivity and the power losses in high-frequency magneto-impedance sensors with very strong skin effect.
The authors declare that there is no conflict of interests regarding the publication of this paper.