JP2528075B2 - SQUID magnetic sensor - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a SQUID (superconducting quantum interference device) magnetic sensor for accurately measuring weak magnetism.

[0002]

2. Description of the Related Art In recent years, SQUID, which has been researched for detecting weak magnetism, is a high-sensitivity magnetic sensor using a Josephson junction and utilizes the quantum interference effect to generate a magnetic flux quantum φ 0 (1.5 × The magnetic flux of 1 / 100,000 of 10-15 Wb) is converted into voltage and measured. Therefore,
The SQUID is useful for measuring weak magnetism, and is applied to biomagnetic measurement, physical constant measurement, nondestructive inspection, and the like.

Among them, the application to biomagnetic field measurement is a big driving force in the applied research of superconductivity.
For example, it is known that if the QRS wave of the heart or a special magnetic shield is applied, the electroencephalogram can be easily detected by the SQUID. This SQUID includes an rfSQUID having one Josephson junction and a dcSQUID having two Josephson junctions.

[0004]

However, in the conventional SQUID, since the Josephson junction is exposed to the concentrated magnetic flux, the Franhoffer diffraction behavior in the magnetic field-voltage characteristic largely appears and the accuracy varies. There was an inconvenience to do. With the above-mentioned Fran Hoffer diffraction, when a magnetic field is applied to the Josephson junction,
A distribution occurs in the current flowing through the Josephson junction, and the distributed currents interfere with each other to strengthen or weaken each other. When this Franhoffer diffraction occurs in the Josephson junction of the SQUID, the sine wave characteristic, which is the original magnetic flux-voltage characteristic, is overlapped with the swell of a period several to several hundred times longer, and the characteristic Distortion, and SQUID sensitivity is reduced.

The present invention has been made in order to solve the above-mentioned inconvenience, and by reducing the Franhofer diffraction behavior in the magnetic field-voltage characteristic,
An SQUID magnetic sensor capable of measuring a minute magnetic field with uniform accuracy.

[0006]

First, the SQU of the first invention is described.
The ID magnetic sensor is formed of a superconducting thin film, and a hole is formed substantially at the center, and one inner slit toward the outer periphery and one outer slit on both sides of the inner slit are formed around the hole. It has a SQUID layer composed of a washer portion opened in parallel with the slit, and at least one Josephson joint portion formed of a high temperature superconductor thin film and disposed between the inner slit and each outer slit. is there.

The SQUID magnetic sensor of the second invention is formed of the above-mentioned SQUID layer and a superconductor, and a hole is formed at substantially the center, and a slit toward the outer periphery is formed around the hole. SQ with magnetic flux shielding layer
The hole of the UID layer and the hole of the magnetic flux shielding layer are opposed to each other,
The SQUID layer and the magnetic flux shielding layer are laminated so that the slits do not overlap and the Josephson junction is covered with the magnetic flux shielding layer.

[0008]

The SQUID magnetic sensor of the present invention reduces the magnetic flux to S
Concentrate in the hole of the QUID layer. At this time, since the Josephson junction is separated from the hole of the SQUID layer and covered with the magnetic flux shielding layer, it is not exposed to the concentrated magnetic flux.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. Embodiment 1 FIG. 1 is a plan view schematically showing an SQUID magnetic sensor which is a first embodiment of the present invention. In FIG. 1, reference numeral 1 denotes a substrate, which is made of SrTiO3 (100) and has a step difference of 2000 angstrom in height formed on the surface.

Reference numeral 2 denotes an SQUID layer formed on the substrate 1, which is a 2 mm × 2 mm washer portion 3 formed of an oxide superconductor thin film, and a Josephson junction portion 7 formed of a high temperature superconductor thin film. It is composed of. In the washer portion 3, a hole 4 of 25 μm × 25 μm is formed substantially in the center, and an inner slit 5 extending toward the outer periphery and outer slits 6 on both sides of the inner slit 5 are formed around the hole 4.

The inner slit 5 has a width of 3 μm and a length of 25 μm, and the outer slit 6 has a width of 8 μm. A Josephson joint 7 is disposed between the inner slit 5 and the outer slit 6, and the Josephson joint 7 is a stepped portion of the substrate 1.

Next, the fabrication of the SQUID magnetic sensor will be described. First, a YBCO oxide superconductor thin film (film thickness 2000 angstrom) is formed on the substrate 1 having a step difference of 2000 angstrom by a laser deposition method. The film forming conditions are a laser wavelength of 248 nm, a laser power of 300 mJ / Pulse, and a substrate temperature of 670.
℃, film formation rate 500 angstrom / min, oxygen partial pressure 4
00 mTorr. Is.

Next, the washer portion 3, the hole 4, the inner slit 5, and the outer slit 6 are processed and molded by ion milling to form the Josephson joint portion 7 between the inner slit 5 and the outer slit 6. Then, in a similar manner, as a comparative example, a SQUI having a washer portion of 2 mm × 2 mm, a hole of 25 μm × 25 μm, and no inner slit.
D was prepared and the performance was compared.

The SQ of the first embodiment thus obtained
When a measurement system (not shown) was connected to the UID magnetic sensor and the magnetic field resolution was measured in liquid nitrogen, it was 10 pTes.
It was la / Hz1 / 2 at 10 Hz. Then, the Josephson joint portion 7 has the inner slit 5 and the outer slit 6
Since it is no longer exposed to the magnetic flux concentrated by, the Franhoffer diffraction behavior in the magnetic field-voltage characteristic becomes smaller than in the conventional SQUID magnetic sensor without the inner slit 5. Furthermore, since the area of the inner slit 5 is small, the increase in inductance can be suppressed, and the output voltage and the magnetic field resolution hardly change.

Embodiment 2 FIG. 2 is an exploded perspective view schematically showing an exploded SQUID magnetic sensor according to a second embodiment of the present invention, and FIG. 3 shows the SQUID layer and the magnetic flux shielding layer of FIG. FIG. 2 is an explanatory view showing a state in which the same parts as those in FIG. In these figures, 11 indicates a substrate,
It is composed of SrTiO3 (100).

Reference numeral 12 is 11 mm × 1 formed on the substrate 11.
A 1 mm magnetic shield layer is shown, and a hole 13 of 25 μm × 25 μm, which is the same size as the hole 4, is formed in the approximate center, and a slit 1 extending toward the outer periphery is formed around the hole 13.
4 is opened with a width of 5 μm. The magnetic flux shielding layer 1
2 is manufactured according to the SQUID layer 2.

Next, the production of the SQUID magnetic sensor will be described. The hole 4 of the SQUID layer 2 and the hole 13 of the magnetic flux shielding layer 12 produced as described above are overlapped with each other, and the magnetic flux shielding layer 12 is formed on the SQUID layer 2 so that the slit 14 does not overlap the inner slit 5 and the outer slit 6. By stacking and integrating, a SQUID magnetic sensor can be obtained.

The SQ of the second embodiment thus obtained
When a measurement system (not shown) was connected to the UID magnetic sensor and the magnetic field resolution was measured in liquid nitrogen, it was 0.6 pTe.
sla / Hz1 / 2 at 10 Hz, and the Franhoffer diffraction-like behavior in the magnetic field-voltage characteristic completely disappeared.

The reason why the Franchoffer diffraction behavior of the SQUID magnetic sensor of the second embodiment is improved as compared with the first embodiment is that the Josephson junction 7 has the magnetic flux shielding layer 1.
This is because when it is covered with 2, it is not directly exposed to the magnetic field.

Embodiment 3 FIG. 4 is an explanatory view showing a state in which an SQUID layer and a magnetic flux shielding layer constituting an SQUID magnetic sensor according to a third embodiment of the present invention are laminated, and FIGS. The same parts are designated by the same reference numerals and the description thereof will be omitted. In FIG. 4, 13A
Indicates a hole opened in the approximate center of the magnetic flux shielding layer 12 with a size of 40 μm × 40 μm.

Next, the production of the SQUID magnetic sensor will be described. The holes 4 of the SQUID layer 2 manufactured as described above are made to face each other so as to be located in the holes 13A of the magnetic flux shielding layer 12, and the Josephson junction 7 is arranged to face the magnetic flux shielding layer 12.
While covering the magnetic flux shielding layer 12 with S so that the slit 14 does not overlap the inner slit 5 and the outer slit 6.
By stacking and integrating on the QUID layer 2, SQ
It can be a UID magnetic sensor.

The SQ of the third embodiment thus obtained
Also in the UID magnetic sensor, the SQUID of the second embodiment
The same effect as the magnetic sensor was obtained.

Embodiment 4 FIG. 5 is an explanatory view showing a state in which an SQUID layer and a magnetic flux shielding layer constituting an SQUID magnetic sensor according to a fourth embodiment of the present invention are laminated, and FIGS. The same parts are designated by the same reference numerals and the description thereof will be omitted. In FIG. 5, 13B
Indicates a hole opened in the approximate center of the magnetic flux shielding layer 12 with a size of 20 μm × 20 μm.

Next, the production of the SQUID magnetic sensor will be described. The holes 13B of the magnetic flux shielding layer 12 manufactured as described above are made to face each other so as to be located in the holes 4 of the SQUID layer 2, and the Josephson junction portion 7 is disposed.
While covering the magnetic flux shielding layer 12 with S so that the slit 14 does not overlap the inner slit 5 and the outer slit 6.
By stacking and integrating on the QUID layer 2, SQ
It can be a UID magnetic sensor.

The SQ of the fourth embodiment thus obtained
Also in the UID magnetic sensor, the SQUID of the second embodiment
The same effect as the magnetic sensor was obtained.

In the above embodiment, the size of the SQUID layer 2 is a square of 2 mm × 2 mm, but the shape is not limited, but a square, a rectangle close to a square,
Alternatively, it is desirable to make it circular. And SQUI
If the area of the D layer 2 is smaller than 2 mm 2, the magnetic flux concentration efficiency is low and a sufficient magnetic field resolution cannot be obtained.
If it is larger than 2, the position resolution is reduced, so the area of the SQUID layer 2 that effectively obtains the magnetic flux concentration efficiency is 2 m.
m2 to 1000 mm2 is preferable, but 100
It is even more desirable to set it to mm 2 to 400 mm 2.
However, if the area of the SQUID layer 2 is set to 2 mm2 to 6 mm2, the SQUID layer 2 can be manufactured in a single process and can be adapted to mass production.

On the other hand, the size of the hole 4 is 25 μm × 25
Although a square having a size of μm is used, the shape is not limited, but it is desirable that the square, a rectangle close to a square, or a circle be used similarly to the SQUID layer 2. And
If the area of the hole 4 is smaller than 4 μm 2, the amount of trapped magnetic flux is small and the magnetic field resolution is reduced to 2000 μm.
When it is larger than 2, the inductance increases and the output voltage decreases, so the magnetic field resolution is good,
The area of the hole 4 having a small inductance and capable of obtaining a large output voltage is preferably 4 μm 2 to 2000 μm 2, but more preferably 100 μm 2 to 400 μm 2.

Although the width of the inner slit 5 is 3 μm, the smaller the width of the inner slit 5 is, the smaller the area is, which is desirable because the inductance can be reduced. The length of the inner slit 5 is 25 μm, but it is preferably 10 μm to 100 μm. Further, as a method of forming an oxide superconductor thin film on the substrate 1, a laser vapor deposition method, a sputtering method, a vacuum vapor deposition method,
MBE or the like may be applied to form a Y (yttrium) -based, Bi (bismuth) -based, or Tl (thallium) -based oxide superconductor thin film.

Further, in order to process / mold each hole 4, 13 and each slit 5, 6, 14, ion milling or chemical etching may be applied. The Josephson junction 7 may be of a weak coupling type utilizing the step of the substrate 1, a bicrystal type, a tunnel type, or the like. The magnetic flux shielding layer 12 is SQU.
The insulating layer may be formed on the ID layer 2 and then formed on the insulating layer.

When the SQUID magnetic sensor is constructed in this manner, the adhesion between the SQUID layer 2 and the magnetic flux shielding layer 12 is improved, so that the behavior of the magnetic field-voltage characteristic in a Franchoffer diffraction can be more effectively prevented. It is possible to more effectively prevent the leakage of magnetic flux in the outer slit 6. Although the SQUID layer 2 is the dcSQUID, it may be the rfSQUID. Furthermore, although the superconductor thin film and the superconductor are oxides, it goes without saying that they may be other materials.

[0031]

As described above, according to the present invention, by opening the inner slit and the outer slit, and by covering the Josephson junction with the magnetic flux shielding layer, the magnetic flux concentrated in the Josephson junction is increased. Since it is not exposed to the magnetic field, the Franhoffer diffraction behavior in the magnetic field-voltage characteristic becomes small. Therefore, a minute magnetic field can be measured with uniform accuracy.

Further, by reducing the area of the inner slit, the area of the holes in the SQUID layer including the area of the inner slit does not change significantly, so that the increase of the inductance can be suppressed and the output voltage and the magnetic field resolution are almost changed. You can turn it off. Further, by covering the inner slit and / or the outer slit with the magnetic flux shielding layer, leakage of magnetic flux in the inner slit and / or the outer slit is eliminated, so that the sensitivity can be improved and the measurement can be performed with higher accuracy. .

[Brief description of drawings]

FIG. 1 is a plan view schematically showing an SQUID magnetic sensor according to a first embodiment of the present invention.

FIG. 2 is an exploded perspective view schematically showing an exploded SQUID magnetic sensor according to a second embodiment of the present invention.

FIG. 3 is an explanatory diagram showing a state in which an SQUID layer and a magnetic flux shielding layer of FIG. 2 are laminated.

FIG. 4 is an explanatory diagram showing a state in which an SQUID layer and a magnetic flux shielding layer that constitute an SQUID magnetic sensor according to a third embodiment of the present invention are laminated.

FIG. 5 is an explanatory diagram showing a state in which an SQUID layer and a magnetic flux shielding layer that constitute an SQUID magnetic sensor according to a fourth embodiment of the present invention are laminated.

[Explanation of symbols]

 1 Substrate 2 SQUID Layer 3 Washer Part 4 Hole 5 Inner Slit 6 Outer Slit 7 Josephson Junction 11 Substrate 12 Magnetic Flux Shielding Layer 13 Hole 13A, 13B Hole 14 Slit

Claims (2)

(57) [Claims] 1. A hole made of a superconducting thin film, having a hole formed substantially in the center, and having one inner slit toward the outer periphery and one outer slit on each side of the inner slit, which is formed around the hole. A SQUID layer formed of a washer portion opened in parallel with the inner slit, and at least one Josephson junction portion formed of a high-temperature superconductor thin film and arranged between the inner slit and each of the outer slits. An SQUID magnetic sensor having. 2. The hole of the SQUID layer according to claim 1, wherein a magnetic flux shielding layer is formed of a superconductor, and a hole is formed substantially in the center, and a slit toward the outer periphery is formed around the hole. And the hole of the magnetic flux shielding layer are opposed to each other so that the slits do not overlap each other, and the SQUID layer and the magnetic flux shielding layer are laminated so as to cover the Josephson junction with the magnetic flux shielding layer. A SQUID magnetic sensor characterized by the following.