JPH0282178A - Superconducting magnetic sensor - Google Patents

Abstract

PURPOSE:To realize a new type sensor of simple structure with high sensitivity and high reliability by utilizing Josephson junction characteristics generated at the grain boundary part that a superconductor has. CONSTITUTION:The superconductor 1 where the grain boundary serves as Josephson junction is arranged in a solenoid coil 2. The superconductor 1 is fitted with two current supply electrodes and two voltage measuring electrodes and a current I1 is supplied from a stabilized power source 3. The coil 2 can controls its magnetic field extremely accurately, its internal magnetic field is uniform, and a current I2 is supplied from an AC power source (several Hz - tens of Hz) 4. A stationary magnetic field H is applied from outside as shown by an arrow. The intensity of the maximum magnetic field in one cycle which is generated by the coil 2 is larger than that of the magnetic field H. The sensor of this structure uses a material which has a small lower critical magnetic field and a material which is large grain size and this sensor is used almost at superconductive transition temperature to obtain 0.1Oe detection sensitivity.

Description

Detailed Description of the Invention Field of Application of the Invention The present invention relates to a magnetic sensor using a superconductor having grain boundaries. A method for making the sensor more sensitive will be described.The highly sensitive, simple-structured, and inexpensive magnetic sensor of the present invention is useful both academically and industrially, and can be used in various fields. Be expected.

rPrior Art j A quantum interference magnetic sensor, so-called 5QUID, has been known as a high-sensitivity magnetic sensor. This conventional magnetic sensor has very high sensitivity, but its structure is complex;
The disadvantage was that the price and maintenance costs were high.

On the other hand, sensitivity is not required so much, but it is easy to use,
There was also demand for a type of magnetic sensor that was inexpensive. Semiconductor magnetic sensors were available as magnetic sensors that met such requirements, but their sensitivity was only 50 (Oe) (
The arrival of a magnetic sensor with a detection sensitivity of 1 (Oe) or less, which is slightly more sensitive than this, has been eagerly awaited.

In recent years, oxide superconductors YBCO and B5CC0 have been discovered, and in order to investigate their physical properties, T
It has been reported that there is a significant decrease in c or critical current value.

This phenomenon occurs because the grain boundaries of the superconductor act as Josephson junctions.

The present invention utilizes this property to use superconductors as magnetic sensors.

In other words, if a current slightly lower than the critical current is passed through a superconductor (in this state, no voltage is generated in the superconductor) and a small magnetic field is applied to this superconductor, the superconducting state is partially broken. , a voltage is generated.

Since there is an l-to-l correspondence between the generated voltage and the magnetic field as shown in FIG. 1, the magnitude of the magnetic field can be determined by reading the voltage. However, such magnetic sensors have poor reliability because their characteristics vary depending on the manufacturing conditions even if they are made from the same material.

Therefore, the present inventor has developed this magnetic sensor to provide a magnetic sensor with higher sensitivity, higher reliability, and easier industrial application.

Its structure and operation are shown below according to FIG.

Figure 2 is a conceptual diagram. Four electrodes, i.e., the same as those used when measuring normal resistivity,
The structure is such that a superconductor (1), to which two current supply electrodes and two voltage measurement electrodes are attached, is located within a solenoid coil (2). Solenoid coil (
2) allows the magnetic field to be controlled extremely accurately;
The magnetic field inside it is uniform. A current 11 is passed through the superconductor by a stabilized power supply (3).The solenoid coil (2) is supplied with an alternating current power (frequency is from several Hz to several tens of Hz).
) (4) causes current ■2 to flow. A steady magnetic field H is applied from the outside from the left side of the figure.

It is assumed that the magnitude of the maximum magnetic field in one cycle generated by the solenoid coil (2) is larger than the magnetic field H. AC power supply (4)
The magnitude and direction of the magnetic field generated by the solenoid coil (2) change periodically, and by combining with the magnetic field H, the magnitude of the magnetic field inside the solenoid coil (2) periodically becomes zero or remains zero. It may disappear. At this time,
Using the oscilloscope (5), check the solenoid coil (2).
), the horizontal axis represents the voltage generated in the superconductor (1), and the spectrum shown in Figure 3 depends on the value of the current 11 flowing through the superconductor (1). can get.

Three types of spectra are depicted in Figure 3, (a
) is when the current ■1 is small, (b) is when it is just right, (
C) is too large.

These spectra indicate that the current I2 flowing through the solenoid coil (2) is I. When , the magnetic field in the solenoid coil (2) becomes zero and the voltage of the superconductor (1) is zero or reaches a minimum value, and at other times a voltage is generated due to the magnetic field-voltage effect. It shows. (b
) is the resolution of this magnetic sensor.

This is determined by the superconductor (1) if the solenoid coil (2) is made with sufficient precision.

In fact, since the coil can be made extremely precisely, the properties of the superconductor (1) will influence the properties of the magnetic sensor.

In a superconductor whose grain boundaries function as Josephson junctions, the minimum magnetic field H0 to which this superconductor reacts is given by:

Here φ. is the magnetic flux quantum and 2. o7XIO-'Oecm"
, λ is the magnetic field penetration length of the superconductor, and L is the typical length of the junction. This Ho becomes the half width W of the spectrum.

Therefore, to increase the sensitivity of the magnetic sensor, λ
Just make the scarecrow bigger. Since λ is determined by the material, sensitivity can be increased by using a material with a large λ, that is, a material with a small lower critical magnetic field.

Furthermore, since λ increases near the superconducting transition temperature, sensitivity can be increased by using a magnetic sensor near the superconducting transition temperature.

Regarding L, in the case of ceramics, L is considered to be the particle size, so the sensitivity increases when a material with a large particle size is used.

As a result of the above efforts, we were able to achieve a detection sensitivity of 0.1 (Oe). EXAMPLES The present invention will be explained in more detail with reference to Examples below.

r Example j The superconductor used was YBCO with nickel added as an impurity. Nickel acts to lower the superconducting transition temperature.

In this example, since the magnetic sensor is used at liquid nitrogen temperature, nickel was added to bring the superconducting transition temperature close to the liquid nitrogen temperature, as described above.

This made it possible to improve the sensitivity of the magnetic sensor.

A normal solid phase reaction was used to produce the superconducting oxide in this example. The raw materials are yttrium oxide, barium carbonate, copper oxide, and nickel oxide powder (all purity is 3N).
) was used.

The ratio of yttrium, barium, copper, and nickel in the raw materials was 1:2:3. After mixing well as 9:o and 1 and calcining in the atmosphere at 900°C for 112 hours, 10XIX0.7
Formed into a rectangular parallelepiped of mm3 and heated again at 900°C1 in the atmosphere.
After firing for 12 hours, it was cooled at a rate of 1"C per minute.

The reason for adding more copper than the stoichiometric ratio is to increase the grain size (thus increasing sensitivity). The Tc (zero resistance temperature) of the superconductor thus prepared is 79. Electrodes are attached to this superconducting material, and a solenoid coil (
2).

This magnetic sensor device is placed in liquid nitrogen, and a superconductor (
1), a current of 50 mA was applied. The relationship between the current flowing through the solenoid coil (2) and the voltage generated across the superconductor (1) at this time is shown in FIG. When the current flowing through the coil was 1 mA, the magnetic field generated in the coil was 0.5 (Oe).

Also, since Io was 2.2 mA, an external magnetic field fI equal to the magnetic field generated by this solenoid coil (2)
The magnitude was 1.1 (Oe), and the detection sensitivity of the magnetic sensor was 0.1 (Oe).

In this example, an oxide superconductor was used as the superconductor, but other superconductors can be similarly applied.

"Effect" In the present invention, regardless of the manufacturing conditions and type of superconductor, all superconductors that have grain boundaries can:
The absolute value of the same magnetic field is obtained.

That is, it is not necessary to create characteristic curves and the like depending on the manufacturing conditions and types of superconductors. However, the detection sensitivity depends on the manufacturing conditions.
Although it depends on the type, it only needs to exceed a certain standard and will not cause any problems in mass production. The present invention has made it possible to create a low-cost, highly sensitive magnetic sensor. The industrial benefits of this are enormous. In the present invention, the principle of operation remains the same whether the superconductor is in the form of a plate or a thin film. Although not described in the examples, it is possible to create a magnetic sensor with higher sensitivity by forming a superconductor as a thin film on a suitable substrate and reducing its cross-sectional area.

[Brief explanation of the drawing]

FIG. 1 shows an example of the magnetovoltage effect in a superconductor with grain boundaries. Horizontal axis: Applied magnetic field Vertical axis: Voltage generated in the superconductor Figure 2 shows a conceptual diagram of a magnetic sensor. FIG. 3 shows an example of the characteristics of the magnetic sensor of the present invention. Horizontal axis: Current flowing through the solenoid coil Vertical axis: Voltage generated in the superconductor (a) I, = 10mA (b) I+ = 50mA (c) I+ = 200mA 119. Superconductor 230. solenoid coil

Claims (1)

[Claims] 1. A magnetic sensor that utilizes the Josephson junction characteristics that occur at the grain boundaries of a superconductor. 2. A solenoid coil, a means for passing a current through the solenoid coil to generate a magnetic field, a superconductor placed in the solenoid coil, a means for passing a constant current through the superconductor, and a voltage generated in the superconductor. A superconducting magnetic sensor having means for measuring . 3. A superconducting magnetic sensor, characterized in that the superconductor used in claim 1 or 2 has a superconducting transition temperature near the operating temperature of the magnetic sensor.