JPH04118572A - Measuring process of magnetic field intensity - Google Patents

Abstract

PURPOSE:To measure the intensity of a weak magnetic field stably at high sensitivity to magnetism by setting the temperature of a thin film at less than a critical temperature, and applying a current more than a critical current value for the thin film at that temperature. CONSTITUTION:A film is formed on a substrate of MgO monocrystals by using 1) Bi0.5Pb0.5Ox (mixed powder of Bi2O3 and PbO), 2) CaCu0.75Ox (950 deg.C baked powder of CaCO3 and CuO), 3) SrCu0.75Ox (950 deg.C baked powder of SrCO3 and CuO) as sputtering target. Such a deposition layer as a single layer is laminated for 200 times to obtain a thin film of about 1mum. This is temporarily baked for two hours at 780 deg.C, and heat processes are applied for 110 hours at 832 deg.C, 835 deg.C, 839 deg.C respectively, so three sorts of thin films A - C of different heat process temperatures are obtained. The sensitivity of the obtained thin film to magnetism is measured in a four-terminal method by changing an applied current. By detecting to the order of 0.1muV in this method, an external magnetic field can be measured at a sensitivity to the order of 10<-5> gauss.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for measuring the field strength of B1-1.
Pb-3r-Ca-Cu-0 system, B1-3r-Ca-
Cu-0 series, Y-Ba-Cu-0 series, Tl-Ba-Ca
The present invention relates to a method for measuring magnetic strength using a magnetoresistance effect using an oxide superconducting polycrystalline thin film such as -Cu-0 system as a magnetic sensor.

[Prior art] Conventionally, there are two methods for measuring magnetic field strength: a method of measuring a weak magnetic field and a method of measuring a not-so-weak magnetic field.
It can be broadly divided into

As a highly sensitive magnetic sensor for weak magnetic fields, 5QUID, which utilizes the superconducting quantum interference effect, is known.
``It has a sensitivity of Gaussian level.

On the other hand, to measure magnetic field strengths of about 1 Gauss or more, magnetic sensors have been used that utilize the magnetoresistive effect of semiconductors or magnetic materials. As a semiconductor, InSb,
Magnetic materials such as InAs include Fe-Ni permalloy,
CoNi etc. were used.

The magnetoresistive effect referred to here is a phenomenon in which resistance increases as the strength of the magnetic field increases.

Recently, magnetoresistive elements using ceramic superconductors have attracted attention. For example, in Japanese Patent Application Laid-Open No. 1-138770, a superconducting material having crystal grain boundaries is A magnetic field detection device has been proposed that measures an external weak magnetic field using the magnetoresistive effect while applying a bias magnetic field that is slightly larger than the magnetic field that breaks the weak coupling state.

[Problem to be Solved by the Invention 1] Among the above methods, QUID has a complicated structure, high price and maintenance costs, and magnetic sensors using semiconductors or magnetic materials have low sensitivity. Also, JP-A-1-13877
The method proposed in Publication No. 0 is superior in terms of sensitivity, but requires a bias magnetic field, and although it is useful for measuring magnetic changes, it requires a considerable amount of bias magnetic field to detect 1 weak magnetism. Existence can be a problem.

The present invention aims to provide a method for detecting pure weak magnetism with high sensitivity using a simple structure and without using a bias magnetic field.

[Means for Solving the Problems 1 The present inventors have discovered the magnetic sensitivity and application of oxide superconducting thin films 11
As a result of investigating the relationship between current values, the present inventors found that magnetic sensitivity depends on the applied current value, and that magnetic sensitivity increases when an applied current above a certain level is applied, leading to the completion of the present invention.

That is, the present invention uses an oxide superconducting polycrystalline thin film produced by a physical method as a magnetic sensor to measure magnetic field strength by lowering the temperature of the thin film below a critical temperature,
The magnetic field strength measuring method is characterized in that a current higher than the critical current value of the thin film at that temperature is applied.

Since oxide superconductors are ceramics, they have crystal grain boundaries that are insulating phases, and therefore, when a magnetic field is applied, resistance occurs and the critical current density sharply decreases. By utilizing this property, it becomes possible to measure magnetic fields.

As shown in Figure 1, magnetic fields are usually measured using the four-terminal method in which current electrodes and voltage electrodes are provided on a strip of superconducting thin film, and the voltage output due to the resistance generated between the voltage electrodes due to the magnetic field is measured. . By adjusting the width of the thin film, the critical current value (Icl) of the thin film can be adjusted to an appropriate value, for example 0.2 mA.
It can be controlled to a certain degree.

In this case, when the current applied to the thin film is smaller than the critical current, the resistance explained above will not occur unless the external magnetic field is large, and the critical 1! When a current with a current value is applied, the sensitivity to a weak magnetic field is low, and to measure a weak magnetic field, it is necessary to apply a current larger than the critical current to the thin film, and the applied current is When the current is 10 times or more, the sensitivity is even better.

Figure 2 shows the change in output voltage due to the applied magnetic field, based on the output voltage when the magnetic field is zero, by changing the magnification of the applied current to the critical current value. The applied current magnification is 11 to 100.
It shows excellent magnetic sensitivity.

In addition, the sensitivity to weak magnetic fields also depends on the critical current density, and the critical current density in zero magnetic field at the measurement temperature is 20
A superconducting film of 0 A/cm" or less is preferable as a magnetic sensing element. A superconducting film with a high critical current density has few grain boundary phases,
It is thought that because the magnetic flux is easily captured, the sensitivity to weak magnetic fields is rather reduced.

Figure 3 shows superconducting thin films with various critical current densities.
This is a comparison of the magnetic sensitivity at the applied current magnification that showed the highest sensitivity in 77.
The highest magnetic sensitivity is shown for thin films of less than /cm".

In addition, Figure 4 shows the results of measuring the magnetoresistive effect in terms of output voltage for a superconducting thin film with a critical current density of 400 OA/c2 at Ic = 1.5 mA and an applied t4 current of 60 mA (40 times). , it shows virtually no magnetoresistive effect until the applied magnetic field reaches about 1.5 Gauss, and its magnetic sensitivity is not very high even for magnetic fields higher than that.

The method used to manufacture superconducting thin films is sputtering.

The number of sputtering targets is not limited when manufacturing a thin film by a sputtering method, which is performed by a physical method such as a vapor deposition method.

When liquid nitrogen (boiling point 77K) is used as a refrigerant, the strength of the magnetic field is measured using a superconducting thin film having a critical temperature equal to or higher than the boiling point.

Such a superconducting thin film is, for example, BiaPbm made of Bi thread.
Sr+, oocaccuaL, a, b, c, d
is preferably in the following ranges: 0.5 < a < 1.2 b < 1.0 0 J < c < 1.2 1.4 < d < 2.0.

If Bi is less than 0.5, it will be difficult to synthesize a superconductor.
．． When the number is more than 2, magnetic sensitivity deteriorates. When Pb is more than 1.0, the film tends to melt and a semiconductor phase is easily generated. When Ca is less than 0.6, a semiconductor phase is easily generated, and when it is more than 1.2, an ILOK phase is generated. , because a large amount of insulating phase precipitates between superconducting particles, the path of superconducting current is obstructed, making it difficult to become a superconductor, C
If u is less than 1.4, it will be difficult to synthesize a superconductor; 2.
When the amount is more than 0, the film tends to melt and a semiconductor phase tends to be generated.

In addition, in the Y-Ba-Cu-0 system, YBazCuxO-,
In the Tj-BaCa-Cu-0 system, TjJazCaiCu
Jm is fine.

As the substrate, MgO, 5rTiOz, LaGa0i,
Laue.

Oxide single crystals such as Ag with an insulating buffer layer,
Polycrystalline metals such as Au, Pt, Cu and Si, Ga
A semiconductor such as As is used.

Further, as targets for sputtering and vapor deposition, powders of inorganic compounds such as oxides, carbonates, and nitrates, ceramics made by sintering these powders, or metal alloys are used.

The produced thin film is heat-treated in the temperature range shown below to crystallize it.

B1-Pb-5r-Ca-Cu-0 series-820-850
℃B1-5r-Ca-Cu-0 system・850~880℃Y
-Ba-Cu-0 system -900~1000℃T Ba
-Ca-Cu-0 system - Properties will be stabilized if pre-calcined at 700-800°C for 2-10 hours before heat treatment at 900-1000°C. After heat treatment, it is slowly cooled in a furnace.

[Operation] A superconductor has zero resistance below its critical temperature;
The magnetoresistive effect does not appear unless a certain degree of magnetic field is applied, but if a current exceeding the critical current value is passed, resistance will occur in the superconductor even if no bias magnetic field is applied, and this resistance will be caused by a weak external magnetic field. It also shows a highly sensitive magnetoresistive effect.

This effect is due to the presence of a grain boundary phase in the superconducting thin film, and in a thin film with a high critical current density with few grain boundary phases, the sensitivity due to the magnetoresistive effect is rather reduced.

Furthermore, as is clear from FIG. 2, the magnetoresistive effect according to the present invention has an output voltage curve that becomes gentler as the applied magnetic field increases, so that the application of a bias magnetic field has the effect of lowering the sensitivity.

[Example 1 Examples 1 to I5 As sputtering targets, ■Bio, 5Pbo, so++ fBizOi and p
mixed powder of bo) ■CaCuo, ys low fCaCO-
and CuO 950 “C fired powder) ■5rCuo, ?
950'C calcined powder of IL fsrcOi and CuO)
was used to form a film on an MgO single crystal substrate. The deposition time for each target is as follows.

■Blo, 1lPbo, sad, -” 6 seconds ■C
aCuo, ysO, -58 seconds 5rCua, tsO, -34 seconds This deposition was laminated 200 times as one layer to obtain a thin film of approximately 1 μ-.As a result of analyzing the composition of the obtained thin film by EPMA, it was found that (Bi+Pbl +, oosrl, ooca
a, *aCu+, *sOm. This is 78
After calcination at 0℃ for 2 hours, the heat treatment temperature was changed to 832℃ and 832℃, respectively.
Heat treatment was performed at 835°C and 839°C for 110 hours to obtain three types of thin HA-C with different heat treatment temperatures. The respective critical temperature (Tc), critical current density 1Jcl, and critical current value flcl at 77 are determined by the thin film heat treatment temperature Tc
Jc IcA332℃ 80K 14A/
am" 0.2mAB 836℃
93K 7A/c+w” 0.2mA
C839℃ 100K 7A/am”
It was 0.2s+A. However, critical current value fIc
l is adjusted to 0.2 in each thin film as a sensor.
■It is controlled by A.

The magnetic sensitivity of the obtained thin film was measured by a four-terminal method while changing the applied current (0.4 to 30A+A). Regarding magnetic sensitivity, when a magnetic field is applied (0 to 2 Gauss) while a direct current is flowing through the thin film, the resistance value of the thin film increases. This increase in resistance value was evaluated by the amount of increase in output voltage caused by the resistance. That is, the measurement results are expressed in μV/Gauss, and the larger the value, the better the magnetic sensitivity.The length between the electrodes for the measurement voltage was set to 2. The measurement results for each thin film are shown in Tables 1 to 3.

In addition, regarding Examples 11 to 15, the change curve of the output voltage with respect to the applied magnetic field is shown in FIG.

Table-1 Thin film A (Tc: 80, Jc: 14A/cm
"1 Table-2 Thin film B (Tc: 93, JC near A/CI+"1 Table-3 Thin film C (Tc: 100K, Jc near A/cm21 Examples 16-19, Comparative examples 1-2 As a sputtering target, ■Mixed powder of Bla, 5Pbo, sO Bi, O, and pbo) ■CaCuo, tsL fcacOz and C
uO 950℃ calcined powder)■5rCuo, 750m+
(SrCOi and CuO powder fired at 950°C) was used to form a film on an MgO single crystal substrate. The deposition time for each target is as follows.

■Bio, 5Pbo, 5ons-6 seconds■CaCuo
, t, 0 m - 58 seconds ■ 5 rCuo, ysO++ = -34 seconds This deposition was made into one layer and laminated 400 times to obtain a thin film of about 2μ intestine. As a result of analyzing the composition of the obtained thin film by EPMA, fB1+
Pt1l +, oosr+ oocao, 5scu
+, *sOx. This was calcined at 780°C for 2 hours and then heat treated at 815-844°C for 110 hours.

After the heat treatment, the magnetic sensitivity of the obtained thin film was measured at 77°C using a four-terminal method at measurement intervals of 2■■.

The temperature change in resistivity measured for each thin film is shown in Figure 5, and the measurement results for magnetic sensitivity are shown in Table 4.

As is clear from the results in Tables 1 to 4, the present invention
By detecting down to the order of 0.1 iV, external magnetic fields can be measured with a sensitivity on the order of 10-' Gauss.

(Left below) Table 4 [Effect of the invention 1] According to the present invention, by applying a current with a larger critical current value to a superconductor, it is possible to reduce the strength of a weak magnetic field with a high magnetic sensitivity on the order of 10-' Gauss. can be measured stably.

Furthermore, since no bias magnetic field is applied, it is possible to capture the absolute magnetism including the earth's magnetism.

[Brief explanation of drawings]

FIG. 1 is a conceptual diagram of a magnetic sensor based on the quadrupole method used for magnetic field measurement in the present invention. FIG. 2 shows a change curve of the output voltage due to the applied magnetic field, which is based on the output voltage when the magnetic field is zero, by changing the magnification of the applied current with respect to the critical current value. Figure 3 shows superconducting thin films with various critical current densities.
The magnetic sensitivity is compared at the applied current magnification that showed the highest sensitivity in No. 77. Figure 4 shows a change curve of output voltage for a superconducting thin film with a critical current density of 4000 A/c2. Figure 5 shows the temperature change in resistivity of various superconducting thin films.

Claims (2)

[Claims] (1) When measuring the magnetic field strength using an oxide superconducting polycrystalline thin film prepared by a physical method as a magnetic sensor, the temperature of the thin film must be lower than the critical temperature, and the critical temperature of the thin film at that temperature must be lowered. A magnetic field strength measuring method characterized by applying a current greater than a current value. (2) Critical current density at measurement temperature is 200A/cm
Claim (1) characterized in that a thin film of ^2 or less is used.
The method described in.