JPH07198770A - Improved probe device and method for measuring critical superconducting current in non-contacting state - Google Patents

Abstract

PURPOSE:To provide a high-sensitivity probe device and method by which THE local current density of a sample and the critical current density of a superconducting material are measured in a non-contacting state. CONSTITUTION:An improved probe composed of a pair of exciting coils 3 and 10 which are vertically connected in series with a sample in between and form a magnetic field for generating an induced current, detecting coils 5 and 8 which are arranged outside the coils 3 and 10, vertically connected in series with the sample in between, and detect the magnetic field formed by the induced current, and compensating coils 4 and 9 which are arranged outside the coils 3 and 10, vertically connected in series, and automatically cancel the electromotive force induced by the coils 5 and 8. All of the coils 3 and 10, 5 and 8, and 4 and 9 are coaxially arranged. The difference between the electromotive force when a sample exists and that when no sample exists is found as a pure sample output and the nature of the sample is calculated by finding the magnetizing behavior of the sample from the sample output. The detecting sensitivity of this improved probe is remarkably improved, because both the intensity of the exciting magnetic field and the uniformity of the magnetic field sensed by the sample are improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus and method for measuring an induced current density generated in a local portion of a sample, and more particularly to an apparatus and method for measuring a critical current density of a superconductor.

[0002]

2. Description of the Related Art Superconductors have been widely studied and applied due to their unique properties. Therefore, it is becoming particularly important to know the physical properties of superconductors, especially their critical currents, with high precision.

One conventional device for contactlessly measuring the critical current of a superconductor is shown in FIG. In this device, a pair of pickup coils 51 wound in mutually opposite directions are connected in series, a ring-shaped superconductor sample 52 is installed so as to surround the pair of pickup coils, and the pair of pickup coils 51 and the sample are placed. An external coil 53 is provided so as to surround 52 and to be substantially coaxial with the pair of pickup coils, and a magnetic field that changes with time is generated by the external coil 53. The external coil 53 cuts off the internal region of the ring of the sample and develops with the changing magnetic field. Generated superconducting current in the sample, and the superconductivity correlates from the magnitude of the magnetic field generated by the external coil when the voltage of the pickup coil reaches almost zero when the critical current value of the superconductor sample is reached. The critical current value of the body sample is measured.

Another method of non-contact measurement of the critical current is a SQUID magnetometer, and a schematic diagram of the device is shown in FIG. This SQUID magnetometer consists of a superconducting wire search coil 61 and a coil 6 connected to it.
2, a SQUID element 63, a coil 64, an amplifier 65 connected to the coil 64, a superconducting holder 66, and a cryostat 67. Using this magnetometer, the critical current of the superconductor is measured as follows. First, the sample is put in a superconducting state, and a magnetic field is applied to the entire sample. The magnetic field penetrates from the surface of the sample, and its magnetic field distribution depends on the critical current density of the sample. Since this distribution is retained even if the magnetic field is removed, the search coil is brought close to the sample surface and the magnetic field distribution near the surface is measured. Then, the critical current density is derived from the measured magnetic field distribution.

Yet another non-contact measuring device is disclosed in Japanese Patent Laid-Open No. 4-115155. 3 and 4
FIG. 3 is a sectional view of a reflective probe and a transmissive probe according to the present invention, respectively. The reflection type probe 30 of FIG.
A pair of oppositely connected exciting coils 34 and 35 forming a magnetic field for producing an induced current in a limited portion of the adjacent sample 36, and the inside of the excited coil, The detection coil 33 detects the magnetic field formed on the sample, and is arranged such that the axial directions of the excitation coil and the detection coil are both perpendicular to the surface 36 of the sample. On the other hand, FIG.
Of the transmissive probe 40 is formed by a pair of oppositely connected exciting coils 44 and 45 forming a magnetic field for producing an induced current in a confined portion of the sample in close proximity, and the induced current. Detection coil 4 for detecting magnetic field
3, the exciting coil is arranged in the vicinity of the surface of the sample such that its axial direction is perpendicular to the surface 46 of the sample, and the detecting coil has its axis in the vicinity of the opposite side of the sample. It is arranged so as to coincide with the axis of the exciting coil. The critical current density of the superconductor is measured by the following method. When a desired current is applied to the reflective probe 30 of FIG. 3, the magnetic field penetrates into the localized portion of the sample layer 36, an induced current is generated in that portion, and the induced current also forms a magnetic field. The detection coil 33 detects the magnetic field as an induced electromotive force. When this induced current reaches the critical current, the pattern of the magnetic field detected by the detection coil 33 starts to deviate from the common waveform from that moment. The critical current is calculated from the exciting magnetic field and the thickness of the sample at that time.

Although each of the above-mentioned methods is non-contact, there is a four-terminal method as a method of directly providing a terminal on the sample and measuring the critical current. In this method, a sample is provided with a terminal, a transport current is passed through the sample through the terminal, and a potential drop in the sample is measured by a voltage terminal provided on the sample.

[0007]

In the method of measuring the critical current density by the device shown in FIG. 5, the sample must be processed into a predetermined shape. Therefore, such a method cannot be adopted when it is necessary to measure the critical current density of a large sample or various parts of the sample.

The SQUID magnetometer shown in FIG. 6 can measure the critical current density and the like for each part of the sample, but it is necessary to magnetize the entire sample in order to perform the measurement. Therefore, it is necessary to form a magnetic field for stable magnetization until the time of measurement.

Furthermore, since the external magnetic field penetrates from the entire surface of the sample, the heat generated by the induced current accompanying it is generated from the entire sample. Therefore, in order to prevent the destruction of the sample due to this heat generation, the strength of the external magnetic field and the magnetic field application time must be limited.

In the apparatus shown in FIGS. 3 and 4, since the magnetic field sensed by the sample increases and decreases depending on the distance between the exciting coil and the sample surface, a step of flattening the sample surface is required, and strict alignment is required. Work is required.

In the four-terminal method, the terminals must be directly provided on the sample, and since a transport current is passed through the sample, the size thereof is limited. Furthermore, measurements on each part of the sample are almost impossible.

Therefore, an object of the present invention is to provide a method and an apparatus for stably measuring the current density flowing through a sample without processing the sample and without contact.

Another object of the present invention is to provide a method and apparatus for measuring the current density for each local portion of the sample without contacting the sample.

Still another object of the present invention is to provide an apparatus and method for measuring the critical current value of a superconductor with higher sensitivity.

Still another object of the present invention is to provide an apparatus and method for measuring current density which does not require precise alignment of samples.

[0016]

In order to achieve the above object, a probe for measuring the current density of a sample in a non-contact manner of the present invention is a sample which is connected in series vertically with a sample interposed therebetween. A pair of exciting coils that form a magnetic field for generating an induced current in the formed part, and a magnetic field formed by the induced current that is arranged outside the exciting coil and is connected in series vertically with the sample sandwiched therebetween. And a detection coil for detecting, and the axial directions of the excitation coil and the detection coil are both perpendicular to the surface of the sample.

The pair of exciting coils are coaxially arranged,
It is desirable that the coils have the same specifications wound in the same direction. Similarly, it is desirable that the pair of detection coils be coaxially arranged, and that the axis line be coaxial with the axis line of the exciting coil. Further, it is desirable that the detection coil is a coil of the same specification wound in the same direction and is arranged near the sample.

Here, in the probe of the present invention, the diameter of the exciting coil and the interval between the upper and lower coils are selected so as to satisfy the relationship of giving a uniform magnetic field to the sample.

Further, the probe of the present invention is arranged coaxially outside the exciting coil, and is for canceling the voltage due to the exciting magnetic field generated in the detecting coil, which is connected in series vertically and coaxially with the sample sandwiched therebetween. It may include a coil.
In that case, it is desirable that the canceling coil is arranged away from the sample so that the magnetic flux generated by the sample is not felt.

In order to change the maximum value and the minimum value of the magnetic field formed by the exciting coil, a triangular wave current or a sinusoidal wave current may be supplied to the exciting coil.

The apparatus for measuring a critical current value of a superconductor according to the present invention comprises the probe, a variable mutual inductance system for canceling a voltage due to an exciting magnetic field generated in the detection coil, and an exciting power supply for operating the exciting coil of the probe. When,
The detection amplifier is connected to the detection coil of the probe and amplifies the detected voltage, and a processor for calculating the critical current value of the superconductor from the signal amplified by the detection amplifier. Here, the probe may or may not include the cancellation coil. The detection amplifier is preferably an integrator.

The non-contact method for measuring the critical current value of a superconductor according to the present invention is such that the sample is placed between the upper and lower exciting coils and the detecting coil so that the axis of the coil is perpendicular to the sample surface. A step of closely disposing at equidistant positions,
The step of supplying an alternating exciting current of a magnitude and constant amplitude to the exciting coil such that the magnetization of the sample is sufficiently saturated, and a canceling coil or variable mutual-instance so that the detection voltage when there is no sample becomes zero volt. Adjusting the system, calculating the device constant from the magnetization measurement of the metal sample, calculating the saturation magnetization using the above device constant from the magnetization measurement of the superconductor sample, and using the magnetization curve of the sample And the step of obtaining the current density corresponding to the saturation magnetization.

Here, in the method according to the present invention, when the superconducting substance and the ordinary metal are mixed in the sample, the frequency is sufficiently lowered to separate the magnetization of the superconductor, and the magnetization of the ordinary metal is reduced. The method may further include a step of excluding the influence.

[0024]

According to the invention, the magnetic field for the excitation has a local spatial distribution, so that it can be applied to a limited part of the sample and thus produces an induced current for that limited part. be able to.

[0025]

1 is a schematic cross-sectional view of a preferred embodiment of the improved non-contact probe according to the present invention. Two upper and lower core rods 2 and 11 positioned along the axis of the probe 1.
Exciting coils 3 and 10 are wound so as to surround the tip of each. These exciting coils are connected in series,
It is preferably a coil of the same specification wound in the same direction.
Further, a pair of upper and lower detection coils 5 and 8 are wound coaxially and coaxially with the sample 6 so as to surround the exciting coils outside the exciting coils 3 and 10. The pair of detection coils are connected in series and preferably have the same specifications and are wound in the same direction. Further, the exciting coils 3 and 1
A pair of upper and lower canceling coils 4 and 9 are wound coaxially and apart from the sample 6 so as to surround the exciting coil outside 0. The pair of canceling coils are connected in series, and are coils of the same specifications wound in the opposite direction to the detection coil, and preferably in the same direction. However, these canceling coils 4 and 9 may be omitted when the variable mutual inductance system described later is used.

The rod 2, the exciting coil 3, and the detecting coil 5 are integrally molded (as indicated by the one-dot chain line in FIG. 1) so that the tips thereof are flush with each other.
The rod 11, the excitation coil 10 and the detection coil 8 are also molded in the same manner. By thus integrating the detection coil and the excitation coil, the probe can be moved relative to the sample.

The ratio between the diameters of the exciting coils 3 and 10 of the probe 1 and the distance between the upper and lower probes is selectable.
Each sample can be adjusted to energize the desired uniform magnetic field. Generally, the magnetic field experienced by the sample is a function of the position such that it increases as it approaches the exciting coil, but in the present invention, the sample is properly sandwiched between the upper and lower coils, so to speak, inside the coil. Since it is something like that, the magnetic field felt by the sample is constant even if it moves up and down to some extent.

The sample is placed between the upper and lower excitation coils and the detection coil so that the axis of the coil is perpendicular to the sample surface and equidistant from the upper and lower coils. A current is applied to the exciting coils 3 and 10 so as to form a desired magnetic field on the sample layer 6. Since the exciting coils 3 and 10 form a localized magnetic field, a uniform magnetic field can be applied only to a limited part of the sample layer 6.

A time-varying current is passed through the exciting coils 3 and 10 in order to generate an induced current in the sample layer 6. It is desirable to apply a sinusoidal current to a sample having a relatively weak critical current, but it is desirable to apply a current by LCR discharge to a sample having a relatively large critical current.

When such an intermittently changing current is passed through the exciting coil, the exciting coil generates heat due to its resistance in the energized state, but is (naturally) cooled in the de-energized state. Therefore, since heating and cooling of the exciting coil alternate, even if the exciting coil rises to a temperature at which stable operation cannot be performed due to the flow of a very large current in a short time, it is cooled next and the exciting coil as a whole operates. It is possible to suppress the heat generation and to supply a large current to the exciting coil. Also in the sample layer, in the non-superconducting state, an induced current is generated when the exciting coil is energized to generate heat, but since it is cooled in the next non-energized state, heat generation can be suppressed and a large magnetic field can be applied.

FIG. 2 is a schematic block diagram of a preferred embodiment of a sample current density measuring apparatus using the above-mentioned probe. The variable mutual inductance system 25 includes, on the one hand, the excitation coils 21, 23 and the detection coil 22, of the probe 20.
24, and on the other hand, is connected to an excitation power supply 26 and a magnetization signal processing system 28. The variable mutual inductance system 25 automatically cancels the induced electromotive force generated in the detection coils 22 and 24 by the exciting magnetic field generated by the exciting coils 21 and 23. The excitation power source 26 is connected to the excitation coils 21 and 23 via the variable mutual inductance system 25 and energizes the coils. The data of the current value flowing through the exciting coils 21 and 23 is sent to the arithmetic unit 29. Detection amplifier 28
Are connected to the detection coils 22 and 24 via a variable mutual inductance system 25. The amplifier 28 is preferably an integrator, so the data obtained from the amplifier 28 will be the magnetic field induced in the sample 6. The data is also sent to the arithmetic unit 29. The arithmetic unit 29 and the data processing unit 30 arithmetically process the data of the excitation power supply 26 and the amplifier 28 to derive the current density, electric resistance, critical current, and the like. Further, a control unit may be provided and the excitation power supply 26 and the variable mutual inductance system 25 may be controlled by a signal from the data processing unit 30. The probe used in the embodiment of FIG. 2 may or may not include the cancellation coils 4, 9 of FIG.

Next, a method of measuring the critical current value of the superconductor according to the present invention will be described. The canceling coil or the variable mutual inductance system completely cancels the electromotive force in the detection coil due to the magnetic field generated by the exciting coil, and the output voltage is purely the electromotive force in the detection coil due to the magnetic field generated by the sample.

In this state, first, the magnetization of a normal metal, preferably a flat metal, is measured. When a sinusoidal current that changes with time as an exciting current is supplied to the exciting coil, a magnetization M that changes in an AC manner is generated on the sample surface. In the case of a flat plate metal, the magnetization M is expressed as M∝σDω and is proportional to the product of the conductivity σ of the sample, the thickness D and the angular frequency ω. Further, the output V is the magnetization M
Since it is proportional to
And the proportionality constant R of the magnetization M can be calculated as a device constant.

Next, the magnetization of the superconductor sample is measured. In the case of a superconductor, the maximum amplitude of the magnetization M is determined by the critical current density J _C , so that the output V is proportional to the saturation magnetization M ₀ (J _C ). Here, using the above device constant R,
The relational expression V = 4RM ₀ is derived. The saturation magnetization M ₀ is obtained from this equation, and the corresponding J _C is derived from the magnetization curve of the sample.

In this method, a normal metal is added to the sample,
It can be applied when superconductors are mixed. As mentioned above, the magnetization of the metal is proportional to ω, but the magnetization of the superconductor does not depend on the frequency. Therefore, if ω is made sufficiently small, the influence of the magnetization of the metal can be excluded, and Only the output of can be separated.

Further, the above method can be applied to the measurement of the resistivity of ordinary metals. The signal can be increased by increasing the frequency of the exciting current.

Furthermore, in the above method, since the variation of the sample empirical magnetic field depending on the sample position is small, it is easy to regenerate the relational expression of the exciting magnetic field versus the exciting current.

As described above, according to the present invention, the critical current value and the like of the superconductor can be measured for a desired portion of the sample. Therefore, it is possible to measure the entire sample surface by moving the probe or by arranging it in a matrix, and thus a distribution map of the critical current values can be created.

[0039]

[Effect] The improved probe of the present invention makes it possible to measure the current density without processing the sample and without contact.

With the improved probe of the present invention, the magnetic field strength of the exciting magnetic field is about twice that of the conventional one, and the detection sensitivity is improved.

With the improved probe of the present invention, the magnetic field sensed by the sample was made extremely uniform, and the positional accuracy of the sample in the Z direction became less of a problem.

The superconducting critical current measuring apparatus of the present invention outputs only the electromotive force induced by the magnetic field produced by the sample, and the detection level is improved to about four times that of the conventional method.

The new local critical current measuring method of the present invention shortens the measuring time and facilitates the separation of the sample.

[Brief description of drawings]

FIG. 1 is a partial cross-sectional view of an improved probe of the present invention.

FIG. 2 is a schematic block diagram of a current density measuring device using the probe of the present invention.

FIG. 3 is a partial cross-sectional view of a conventional reflective probe.

FIG. 4 is a partial cross-sectional view of a conventional transmission probe.

FIG. 5 shows a conventional critical current measuring device for a superconductor.

FIG. 6 shows a conventional Squid magnetometer.

[Explanation of symbols]

 1 Improved probe 2 Core rod 3 Excitation coil 4 Canceling coil 5 Detection coil 6 Specimen layer 8 Detection coil 9 Canceling coil 10 Excitation coil 11 Core rod

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[Procedure amendment]

[Submission date] June 17, 1994

[Procedure Amendment 2]

[Document name to be corrected] Drawing

[Correction target item name] All drawings

[Correction method] Change

[Correction content]

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[Figure 5]

[Figure 6]

Claims (13)

[Claims] 1. A probe for contactlessly measuring a current density of a sample, which comprises a pair of vertically connected magnetic fields for generating an induced current in the sample. An exciting coil; and a detecting coil arranged outside the exciting coil and connected in series vertically with the sample sandwiched therebetween, for detecting a magnetic field formed by the sample by the induced current, the exciting coil and the detecting coil The probe, wherein both axis directions of are perpendicular to the plane of the sample. 2. The probe according to claim 1, wherein the pair of exciting coils are coaxially arranged and are coils of the same specification wound in the same direction. 3. The probe according to claim 2, wherein the pair of detection coils are coaxially arranged, and the axis is coaxial with the axis of the exciting coil. 4. The probe according to claim 3, wherein the pair of detection coils are coils of the same specifications wound in the same direction and are arranged near the sample. 5. The probe according to claim 4, wherein the diameter of the exciting coil and the distance between the upper and lower coils are selected so as to satisfy the relationship of giving a uniform magnetic field to the sample. 6. The probe according to claim 5, further comprising: an excitation generated in a detection coil, which is coaxially arranged outside the excitation coil and is connected in series vertically and coaxially with the sample interposed therebetween. A probe including a cancellation coil for canceling the voltage due to the magnetic field. 7. The probe according to claim 6, wherein the canceling coil is arranged away from the sample so that a magnetic field generated by the sample is not sensed. 8. The probe according to claim 1, wherein the exciting coil is energized by supplying a triangular wave current. 9. The probe according to claim 1, wherein the exciting coil is energized by supplying a sinusoidal current. 10. A device for measuring a critical current value of a superconductor, comprising: the probe according to claim 1; and a variable mutual-instance system for canceling a voltage due to an exciting magnetic field generated in the detection coil. An exciting power source for operating the exciting coil of the probe; a detection amplifier connected to the detection coil of the probe for amplifying the detected voltage; and a critical value of the superconductor from the signal amplified by the detection amplifier. A device comprising a processor for calculating a current value 11. The apparatus of claim 10, wherein the detecting amplifier is an integrator. 12. A method for measuring a critical current value of a superconductor using the apparatus according to claim 10 or 11, wherein a sample is placed between the excitation coil and the detection coil above and below, and an axis line of the coil. And a step of arranging them so that the sample surface is perpendicular and equidistant from the upper and lower coils, and an alternating exciting current of a magnitude and a constant amplitude such that the magnetization of the sample is sufficiently saturated is applied to the exciting coil. , Adjusting the cancellation coil or the variable mutual inductance system so that the detection voltage becomes zero volts when there is no sample, calculating the device constant from the magnetization measurement of the metal sample, and the superconductor. The process of calculating the saturation magnetization from the measurement of the magnetization of the sample using the above device constant, and the step of obtaining the current density corresponding to the saturation magnetization as the critical current density using the magnetization curve of the sample. A method consisting of 13. The method according to claim 12, wherein when the superconducting substance and the ordinary metal are mixed in the sample, the frequency is sufficiently lowered to further separate the magnetization of the superconductor, and the ordinary metal is used. A method comprising the step of excluding the effect of magnetization due to.