CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national stage of PCT/JP2018/013670, filed on Mar. 30, 2018, which claims priority under 35 U.S.C. § 119(a) and 35 U.S.C. § 365(b) to Japanese Patent Application No. 2017-100227, filed May 19, 2017, the contents of which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a magnetic field measuring device using a magnetic sensor.

BACKGROUND ART

Various magnetic sensors are used according to various applications. There is a Hall element as a widely used magnetic sensor. The Hall element uses a Hall effect. A magnetic field acts on a current flowing in the element, and thus, an electromotive force generated in a direction perpendicular to a direction of the magnetic field is detected, and locality is measured. The Hall element is widely used for detecting a position of a rotating mechanism such as a motor and for an opening and closing switch of a mobile phone.

As a highly sensitive magnetic sensor, there is a magnetic impedance (MI) element. In the MI element, a phenomenon in which impedance when an amorphous alloy wire is driven by a pulse current is changed by a magnetic field is used.

As a magnetic sensor having excellent spatial resolution, there is a magnetoresistance (MR) element. This MR element is widely used for reading out magnetic recording digital data of a magnetic hard disk. The MR element uses a phenomenon in which resistance is changed with respect to the measured magnetic field, and various operating principles are proposed. A giant magnetoresistance (GMR) element, a tunnel magnetoresistance (TMR) element, or an anisotropic magnetoresistance (AMR) element is used depending on the operating principle.

Recently, the Hall element, the MI element, and the MR element are used not only for digital measurement but are often used as the highly sensitive magnetic sensor.

Meanwhile, when the Hall element, the MI element, and the MR element are used for analog measurement, various applications have been developed as a compass for measuring geomagnetism. As characteristics of the magnetic sensor for analog measurement, performance such as linearity in relation to a sensor output value corresponding to magnetic strength, a dynamic range which is a magnetic field strength range, and minimum resolution of the magnetic field capable of being measured is important. Thus, the magnetic sensors are developed in order to improve such performance.

Meanwhile, a superconducting quantum interference device (SQUID) is known as the most sensitive magnetic sensor compared to these magnetic sensors. This SQUID uses a superconducting phenomenon. In a SQUID using a low-temperature superconductor such as Nb, the superconductor is in a superconducting state by being cooled by liquid nitrogen. Since YBCO known as an oxide superconductor has a high superconducting temperature, this superconductor is used in the superconducting state by being cooled by liquid nitrogen. Since the SQUID is very sensitive, the superconductor is used in a device that inspects a very weak magnetic field generated by electrophysiological phenomena such as the brain and heart of a living body.

Sensitivity of the magnetic sensor operated at a room temperature such as the Hall element, the MI element, and the MR element and sensitivity of the magnetic sensor using the superconducting phenomenon such as SQUID are greatly different, and there is no magnetic sensor capable of measuring intermediate sensitivity thereof.

A problem that the SQUID can only periodically respond to the magnetic field is known. That is, in the SQUID, it is necessary to provide a circuit that cancels the measurement magnetic field in order to obtain a linear response to the magnetic field which is important as sensor characteristics. Accordingly, a relative magnetic field change can be measured with high sensitivity even in the SQUID, but an absolute magnetic field, that is, a zero point cannot be determined.

Meanwhile, the magnetic sensor operated at a room temperature is devised such that a sensor output with respect to the measurement magnetic field becomes a linear output, and can measure the absolute magnetic field. However, since there is a drift due to a temperature in such a magnetic sensor, temperature correction of a standard point or sensitivity is required.

Here, the present inventor has reported that the absolute magnetic field can be measured by eliminating the temperature drift and improving the sensitivity by cooling the magnetic sensor operated at a room temperature to a constant temperature by liquid nitrogen (See Non Patent Literature 1). Alternatively, the present inventor has reported a method of disposing a magnetic sensor on a superconducting coil surface as a method of combining a superconducting coil and a magnetic sensor. Here, a direction in which the magnetic sensor performs the measurement is perpendicular to the superconducting coil surface, that is, the same as a direction of the magnetic field detected by the superconducting coil (see Non Patent Literature 2). However, in this configuration, since the magnetic field being measured and the magnetic field generated by the shielding current are regarded as being in the same direction, there is a problem that sufficient sensitivity cannot be obtained due to the mixing of these magnetic fields.

CITATIONS LIST

Non-Patent Literature

Non Patent Literature 1: Keiji Tsukada, Takuya Yasugi, Yatsuse Majima, Kenji Sakai, and Toshihiko Kiwa, “Absolute-magnetic-field measurement using nanogranular in-gap magnetic sensor with second-harmonic and liquid-nitrogen-temperature operation” AIP Advances, 7, 056670 (2017)Non Patent Literature 2: Yasuaki Matsunaga, Ryota Isshiki, Yuta Nakayama, Kenji Sakai, Toshihiko Kiwa, Keiji Tsukada, “Application of HTS-coil with a magnetic sensor to nondestructive testing using a low-frequency magnetic field”, IEEE Transactions on Applied Superconductivity, 27, 1800304 (2017)

SUMMARY OF INVENTION

Technical Problems

Although the sensitivity can be improved by operating the magnetic sensor operated at room temperature at a low temperature, sufficient sensitivity cannot be obtained, and thus, it is necessary to measure the magnetic field with even higher sensitivity.

Solutions to Problems

The present invention provides a highly sensitive magnetic field measuring device, and the magnetic field measuring device includes temperature maintaining means for maintaining a very low temperature state in which a superconductor is in a superconducting state, a magnetic sensor that is provided within the temperature maintaining means to detect a magnetic field, and magnetic field space forming means for forming a magnetic field space specific to the superconducting state by causing the superconductor to be in the superconducting state within the temperature maintaining means. The magnetic sensor is disposed within the magnetic field space.

The magnetic field measuring device of the present invention has the following features.

(1) The magnetic sensor is disposed between first magnetic field space forming means and second magnetic field space forming means.

(2) The magnetic field measuring device includes a base having a slit which allows the magnetic sensor to be inserted, and the first magnetic field space forming means and the second magnetic field space forming means are provided with the slit interposed therebetween.

(3) The first magnetic field space forming means and the second magnetic field space forming means are integrated on the base.

(4) The magnetic sensor is any one of a Hall element, a magnetic impedance element, and a magnetoresistance element.

(5) The magnetic field space forming means is a superconducting coil, and the magnetic sensor measures a magnetic field generated by a shielding current generated in the superconducting coil.

(6) A normal conductor is interposed in a part of the superconducting coil.

Advantageous Effects of Invention

According to the present invention, it is possible to provide the magnetic field measuring device capable of measuring the magnetic field with higher sensitivity.

