We have developed a common-mode magnetic field rejection-type magneto-impedance (MI) gradiometer to reduce the common magnetic field applied to the sensing-and reference-type MI elements. Compared with a general-type MI gradiometer, the magnetic noise spectral density was lower and the noise at 60 Hz related to the power source line was reduced by 1/8. Using the developed sensor, we successfully detected a microscopic 5 nT magnetic signal in a common-mode magnetic field that was 14 times larger. We expect this will lead to a simpler method of detecting microscopic magnetic signals such as biomagnetism without the need for magnetic shielding.
We have developed a precise off-diagonal magnetoimpedance (MI) gradiometer that can operate in an unshielded environment and at room temperature with 200 pT root-mean-square noise in a 100 Hz bandwidth. The MI sensor probe is compact and easy to handle. The achieved noise level corresponds approximately to the maximum magnetocardiography (MCG) signal reported so far. We have performed MCG measurements using the developed gradiometer system in an unshielded environment, and a real-time signal like MCG can be identified by the MI gradiometer when the distance between the sensor head and chest surface is less than 3 mm. However, the signal seems to be affected by the movement of the chest surface caused by the heartbeat. A peak magnetic signal of 100 pT (corresponding to conventional MCG) was observed when the sensor head was set 10 mm apart from the chest surface to avoid the influence of the chest movement. Under such conditions, the signal needed to be averaged over more than 50 cycles to identify the peak magnetic signal.
A first-order gradiometer-type MI sensor that can reduce environmental magnetic noise without a magnetic shield was developed. The noise spectral density of this sensor is 20 pT/Hz 1/2 at 1-30 Hz, which is as good as a commercial fluxgate sensor, and the noise at 60 Hz relating to the power source line is reduced. We detected SUS304 austenitic stainless steel balls with the developed MI gradiometer to simulate a system for detecting metallic contaminants using the MI sensor. The variation of magnetic field with moving SUS304 balls corresponds to the theoretical value obtained for the model of a steel ball. This MI sensor without a magnetic shield system provides a compact and simple method of detecting metallic contaminants.
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