JP3005628B2 - Magnetic field strength measurement method - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a magnetic field strength measuring method, and particularly to a Bi-Pb-Sr
-Ca-Cu-O system, Bi-Sr-Ca-Cu-O system, Y-Ba-Cu-
The present invention relates to a method for measuring magnetic strength using a magnetoresistive effect using an O-based, Tl-Ba-Ca-Cu-O-based oxide superconducting polycrystalline thin film as a magnetic sensor.

[Conventional technology]

2. Description of the Related Art Conventionally, methods for measuring magnetic field strength can be broadly divided into two methods: a method for measuring a weak magnetic field and a method for measuring a magnetic field that is not so weak.

As a high-sensitivity magnetic sensor for a weak magnetic field, an SQUID using the superconducting quantum interference effect is known, and has a sensitivity of about 10 to ¹⁰ Gauss.

On the other hand, to measure a magnetic field strength of about 1 Gauss or more,
A magnetic sensor based on a method utilizing the magnetoresistance effect of a semiconductor or a magnetic material has been used. InSb, InAs for semiconductors
For example, Fe-Ni permalloy, CoNi, and the like have been used as magnetic materials.

Here, the magnetoresistance effect is a phenomenon in which the resistance increases as the strength of the magnetic field increases.

Recently, a magnetoresistive element using a ceramic superconductor has attracted attention. For example, JP-A-1-138770 discloses that
A superconducting material having crystal grain boundaries is applied at a temperature lower than its critical temperature and a bias magnetic field slightly larger than the magnetic field that breaks the weak coupling between the crystal grains is applied. A magnetic field detection device for measuring has been proposed.

[Problems to be solved by the invention]

In the above method, the SQUID has a problem that the structure is complicated, the price and the maintenance cost are high, and the magnetic sensor using a semiconductor or a magnetic material has low sensitivity. The method proposed in Japanese Patent Application Laid-Open No. 1-138770 is excellent in sensitivity, but requires a bias magnetic field and is useful for measurement of magnetic change, but is equivalent to detection of weak magnetism. The presence of an amount of bias magnetism can be problematic.

An object of the present invention is to provide a method for detecting a weak magnetism with high sensitivity without using a bias magnetic field with a simple structure.

[Means for solving the problem]

The present inventors have investigated the relationship between the magnetic sensitivity of the oxide superconducting thin film and the applied current value and found that the magnetic sensitivity depends on the applied current value, and that the magnetic sensitivity increases when an applied current of a certain level or more is applied. They have found and completed 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 the magnetic field strength. A magnetic field strength measuring method characterized by applying a current lower than the critical temperature and higher than the critical current value of the thin film at that temperature.

Since the oxide superconductor is a ceramic, it has a crystal grain boundary which is an insulating phase. For this reason, when a magnetic field is applied, resistance is generated, and the critical current density sharply decreases. By utilizing this property, a magnetic field can be measured.

The measurement of the magnetic field is usually performed by a four-terminal method in which a current electrode and a voltage electrode are provided on a superconducting thin film strip as shown in FIG. 1, and a voltage output is measured by a resistance generated between the voltage electrodes due to the magnetic field. . By adjusting the width of the thin film,
The critical current value (Ic) of the thin film can be controlled to an appropriate value, for example, about 0.2 mA.

In this case, when the current applied to the thin film is smaller than the critical current, the above-described resistance does not occur unless the external magnetic field is large. In addition, when a current having a critical current value at that temperature 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, Moreover, when the applied current is at least 10 times the critical current, the sensitivity is further improved.

FIG. 2 shows the change in the 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 with respect to the critical current value. The magnetic sensitivity to a weak magnetic field is high, and And an applied current magnification of 11 to 100 indicates excellent magnetic sensitivity.

The sensitivity to a weak magnetic field also depends on the critical current density.
A superconducting film of 0 A / cm ² or less is preferable as the magnetically sensitive element.
It is considered that a superconducting film having a high critical current density has a small grain boundary phase and is apt to capture magnetic flux, so that the sensitivity to a weak magnetic field is rather lowered.

Fig. 3 compares the magnetic sensitivities of the superconducting thin films of various critical current densities at the applied current magnification, which showed the highest sensitivity at 77K, and the critical current density (Jc) was 100 A / cm ^2.
The following thin films show the highest magnetic sensitivity.

Fig. 4 shows the results of measuring the magnetoresistance effect of the superconducting thin film having a critical current density of 4000 A / cm ^{2 at} an output voltage of Ic = 1.5 mA and an applied current of 60 mA (40 times). Shows substantially no magnetoresistance until it reaches about 1.5 gauss, and the magnetic sensitivity is not so high for magnetic fields above that.

The superconducting thin film is manufactured by a physical method such as a sputtering method and a vapor deposition method. The number of sputtering targets for producing a thin film by a sputtering method is not limited.

When liquid nitrogen (boiling point 77K) is used as the 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, Bi-based Bi _a Pb _b Sr _1.00
In Ca _c Cu _d O _x , it is preferable that a, b, c, d are in the range of 0.5 <a <1.2 b <1.0 0.6 <c <1.2 1.4 <d <2.0.

If Bi is less than 0.5, it is difficult to synthesize a superconductor, and if it is more than 1.2, the magnetic sensitivity is deteriorated. When Pb is more than 1.0, the film is easily melted, and thus a semiconductor phase is easily generated. Ca
If it is less than 0.6, a semiconductor phase is easily generated, and if it is more than 1.2,
Although a 110K phase is generated, a large amount of an insulating phase is precipitated between the superconducting particles, so that the path of the superconducting current is disturbed, and it is difficult to become a superconductor. If Cu is less than 1.4, it is difficult to synthesize a superconductor, and if it is more than 2.0, the film is easily melted, so that a semiconductor phase is easily generated.

In the Y-Ba-Cu-O system, YBa ₂ Cu ₃ O _x , Tl-Ba-Ca
In -cu-O system may be a _{Tl 2 Ba 2 Ca 2 Cu 3} O x.

Substrates include MgO, SrTiO ₃ , LaGaO ₃ , LaAlO ₃ and other oxide single crystals, Ag, Au, Pt, Cu with an insulating buffer layer
And polycrystalline metals such as Si and CaAs.

In addition, as a target in sputtering or vapor deposition, inorganic compound powders such as oxides, carbonates, and nitrates, ceramics obtained by sintering these powders, or metal alloys are used.

The produced thin film is crystallized by heat treatment in the following temperature range.

Bi-Pb-Sr-Ca-Cu-O: 820-850 ° C Bi-Sr-Ca-Cu-O: 850-880 ° C Y-Ba-Cu-O: 900-1000 ° C Tl-Ba-Ca -Cu-O-based: 900-1000 ° C If calcined at 700-800 ° C for 2-10 hours before heat treatment, the characteristics will be stable. After the heat treatment, it is gradually cooled in the furnace.

[Action]

The superconductor has zero resistance below its critical temperature, and the magnetoresistance effect does not appear unless a certain magnetic field is applied, but when a current greater than the critical current value is passed,
Even without applying a bias magnetic field, a resistance is generated in the superconductor, and this resistance shows a highly sensitive magnetoresistance effect even with a weak external magnetic field.

This effect is due to the existence of a grain boundary phase in the superconducting thin film. In a thin film having a high critical current density with a small grain boundary phase, the sensitivity due to the magnetoresistance effect is rather reduced.

In the magnetoresistance effect according to the present invention, as is apparent from FIG. 2, the output voltage curve becomes gentle as the applied magnetic field increases, so that the application of the bias magnetic field has the effect of lowering the sensitivity.

〔Example〕

Examples 1 to 15 As sputtering targets, Bi _0.5 Pb _0.5 O _x (mixed powder of Bi ₂ O ₃ and PbO) CaCu _0.75 O _x (950 ° C. baked powder of CaCO ₃ and CuO) SrCu _0.75 O _x (SrCO ₃ and CuO 950 ° C. baked powder) to form a film on an MgO single crystal substrate. The deposition time for each target is as follows.

Bi _0.5 Pb _0.50 O _x … 6 seconds CaCu _0.75 O _x … 58 seconds SrCu _0.75 O _x … 34 seconds This deposit was laminated 200 times to obtain a thin film of about 1 μm. The composition of the obtained thin film was EPMA. As a result of analysis by, it was (Bi + Pb) _1.00 Sr _1.00 Ca _0.96 Cu _1.95 O _x . This was calcined at 780 ° C for 2 hours, and the heat treatment temperature was set to 832
C., 835.degree. C. and 839.degree. C. for 110 hours to obtain three kinds of thin films A to C having different heat treatment temperatures. The critical temperature (Tc), critical current density (Jc) and critical current value (Ic) at 77K are as follows: Thin film heat treatment temperature Tc Jc Ic A 832 ° C 80K 14A / cm ² 0.2mA B 836 ° C 93K 7A / cm ² 0.2 mA C 839 ° C 100K 7 A / cm ² 0.2 mA. However, the critical current value (Ic) is controlled to 0.2 mA by adjusting the film width of each thin film as a sensor.

The magnetic sensitivity of the obtained thin film was measured by a four-terminal method while changing the applied current (0.4 to 30 mA). The measurement was performed at 77K. The magnetic sensitivity is measured with a DC current flowing through the thin film.
When a magnetic field is applied (0-2 Gauss), the resistance of the thin film increases. The increase in the resistance value was evaluated based on the increase in the output voltage caused by the resistance. That is, the measurement result is expressed in μV / Gauss, and a larger value indicates better magnetic sensitivity. The length between the electrodes at the measurement voltage was 2 mm.
Tables 1 to 3 show the measurement results for each thin film.

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

Examples 16 to 19, Comparative Examples 1 and 2 Bi _0.5 Pb _0.5 O _x (mixed powder of Bi ₂ O ₃ and PbO) CaCu _0.75 O _x (a 950 ° C. baked powder of CaCO ₃ and CuO) as a sputtering target SrCu _0.75 O _x (SrCO ₃ and CuO calcined powder at 950 ° C.) was used to form a film on a MgO single crystal substrate. The deposition time for each target is as follows.

Bi _0.5 Pb _0.50 O _x … 6 seconds CaCu _0.75 O _x … 58 seconds SrCu _0.75 O _x … 34 seconds This deposit was laminated 400 times to obtain a thin film of about 2 μm. The composition of the obtained thin film was EPMA. As a result of analysis by, it was (Bi + Pb) _1.00 Sr _1.00 Ca _0.96 Cu _1.95 O _x . This was calcined at 780 ° C for 2 hours and then heat-treated at 815 to 844 ° C for 110 hours.

After the heat treatment, the magnetic sensitivity of the obtained thin film was measured at 77 K by a four-terminal method at a measurement interval of 2 mm.

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

As is clear from the results of Tables 1 to 4, according to the present invention,
By detecting up to the order of 0.1 μV,
0 ^{can -5} measured at a sensitivity of Gaussian order.

[Effects of the Invention] According to the present invention, by applying a current having a larger critical current value to a superconductor, it is possible to stably measure the intensity of a weak magnetic field with a high magnetic sensitivity of the order of 10 ^-5 Gauss. Can be.

Further, since no bias magnetic field is applied, it is possible to capture absolute magnetism in consideration of terrestrial magnetism.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram of a quadrupole magnetic sensor used for measuring a magnetic field in the present invention. FIG. 2 is a change curve of the 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 with respect to the critical current value. Figure 3 shows superconducting thin films of various critical current densities.
It is a comparison of the magnetic sensitivity at the applied current magnification which showed the highest sensitivity at 77K. FIG. 4 is a change curve of an output voltage for a superconducting thin film having a critical current density of 4000 A / cm ² . FIG. 5 shows the temperature change of the resistivity of various superconducting thin films.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-2-159589 (JP, A) JP-A-64-82576 (JP, A) JP-A-2-264879 (JP, A) JP-A 63-82 233387 (JP, A) JP-A-1-286978 (JP, A) JP-A-2-205784 (JP, A) JP-A-1-250875 (JP, A) JP-A-1-138770 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) G01R 33/00-33/18 H01L 39/22

Claims (2)

(57) [Claims] When a magnetic field strength is measured using an oxide superconducting polycrystalline thin film prepared by a physical method as a magnetic sensor, a crystal grain boundary of the thin film is used as a magnetic field sensing portion, and the temperature of the thin film is measured. A method for measuring a magnetic field strength, comprising applying a current lower than the critical temperature and not less than the critical current value of the thin film at that temperature. 2. The critical current density at the measurement temperature is 200 A / cm ^2.
The method according to claim 1, wherein the following thin film is used.