JPH06118150A - Measuring device for superconducting magnetic field - Google Patents

Abstract

PURPOSE:To very accurately measure a weak magnetic field, to miniaturize and to improve operation performance, by measuring an outer field on the basis of an output signal of a superconducting magneto resistance element driven by an AC bias current not less than a fluctuation frequency. CONSTITUTION:A phase and amplitude of a bias current signal, which an AC bias current generating circuit 16 generates, it converted (18) into an AC bias current generating signal, inputted to a minus input of a differential amplifier 19 and adjusted so that an output signal of the amplifier 19 becomes zero when a outer measured magnetic field is zero. The adjusted AC bias current is applied from the circuit 16 to a current electrode of a superconducting magneto resistance element 14. If the AC bias current not less than the fluctuation frequency of the superconducting film is applied to the magneto resistance element 14, the output signal proportioned to an electric resistance value rapidly increasing from the transition point thereof is obtained. The output signal is inputted to a lock-in amplifier 20, only the signal component of the same frequency as the AC bias current is picked out as a DC voltage, and the outer magnetic field is measured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a superconducting magnetic field measuring apparatus for measuring a magnetic field by utilizing a magnetoresistive effect of a superconductor having weak coupling at grain boundaries, and more particularly to a superconducting magnetic resistance by applying an alternating bias current. The present invention relates to a superconducting magnetic field measuring device that utilizes the high sensitivity range of the effect.

[0002]

2. Description of the Related Art Conventionally, a magnetoresistive element using a semiconductor or a magnetic material is generally used for detecting and measuring a magnetic field. In particular, InSb, which is a semiconductor with high electron mobility,
Shape effect in InAs etc. and Fe- which is a ferromagnetic metal
A magnetoresistive element using the orientation effect of Ni, Co-Ni, etc. has been put into practical use, and a magnetic field measuring device equipped with this magnetoresistive element is generally used.

Further, a superconducting magnetic field measuring device has been developed for detecting and measuring a weak magnetic field by utilizing the magnetoresistive effect of the superconductor due to the weak coupling of the oxide superconductor.

[0004]

By the way, the magnetoresistive element using the above-mentioned semiconductor or magnetic material has a small resistance change with respect to the change of the magnetic field when the strength of the magnetic field to be measured is small. It has the drawback of being difficult to measure accurately.

Therefore, by applying a bias magnetic field to the magnetoresistive element with a permanent magnet or the like to shift the measurement range to a region having good sensitivity and characteristic linearity, the magnetic measurement sensitivity is improved. There is a device. However, it was still difficult to accurately measure a weak magnetic field.

Further, a superconducting magnetic field measuring apparatus using the magnetoresistive effect of a superconductor has a low frequency fluctuation phenomenon of 10 Hz or less in its output power, and a bias current having a direct current or a frequency of 10 Hz or less is applied to the superconducting magnetoresistive element. The device described above has a drawback that it is difficult to measure a weak magnetic field, like the magnetoresistive element.

Therefore, in order to measure a weak magnetic field with high accuracy, an AC bias magnetic field of a high frequency which is not affected by the inherent fluctuation frequency is applied to the superconducting magnetoresistive element, so that the sensitivity and characteristic linearity are improved. There is a magnetic field measuring device that shifts the measurement range.

However, this magnetic field measuring apparatus requires an external coil for applying an AC bias magnetic field to the superconducting magnetoresistive element, and thus has a problem in miniaturization and operability of the measuring system.

Therefore, an object of the present invention is to provide a small-sized superconducting magnetic field measuring apparatus which can measure a weak magnetic field with high accuracy and high efficiency.

[0010]

In order to achieve the above object, the present invention provides a superconducting magnetoresistive element including a superconductor having a weakly coupled grain boundary, a current electrode of the superconducting magnetoresistive element, and a superconducting magnetoresistive element. An alternating current applying means for applying an alternating bias current having a frequency of a fluctuation frequency or higher is provided, and the voltage is output from the voltage electrode of the superconducting magnetoresistive element according to the external magnetic field acting on the superconducting magnetoresistive element and the alternating bias current. It is characterized in that the external magnetic field is measured based on the output signal.

Further, a signal having the same frequency as the alternating current generated by the alternating current applying means is outputted, and the signal generating means capable of adjusting the phase and amplitude of the signal and the voltage electrode of the superconducting magnetoresistive element are outputted. It is desirable to have an output signal and a differential amplifier to which the output signal of the signal generating means is input, and to measure the external magnetic field based on the output signal output from the differential amplifier.

[0012]

The superconducting magnetic field measuring apparatus of the present invention includes a superconducting magnetoresistive element including a superconductor having weak coupling at grain boundaries. Then, when an AC bias current having a fluctuation frequency of a superconductor or more is applied to the superconducting magnetoresistive element from the AC current applying unit, the superconductor transits to a normal conducting state and increases rapidly from the transition point. An output signal proportional to the electric resistance value is output from the superconducting magnetoresistive element.

The external magnetic field is measured based on the output signal of the superconducting magnetoresistive element driven by an AC bias current having a relatively high frequency higher than the fluctuation frequency.

That is, in the superconducting magnetic field measuring apparatus of the present invention, the magnetoresistive effect of the superconductor can be obtained by applying an alternating bias current having a fluctuation frequency of the superconductor or more to the current electrode of the superconducting magnetoresistive element. It is used in the frequency band above the fluctuation frequency. Therefore, by detecting the resistance change of the superconducting magnetoresistive element due to the change of the external measurement magnetic field as a voltage signal from the voltage electrode of the superconducting magnetoresistive element, the weak magnetic field is not affected by the inherent low frequency fluctuation of the superconductor. It enables high sensitive measurement of.

Further, according to the present invention, the size can be reduced and the operability is remarkably improved as compared with the conventional example which requires the external coil.

Further, a signal having the same frequency as the alternating current generated by the alternating current applying means is outputted, and the signal generating means capable of adjusting the phase and amplitude of the signal is outputted from the voltage electrode of the superconducting magnetoresistive element. An output signal and a differential amplifier to which the output signal of the signal generating means is input, and based on the output signal output from the differential amplifier, when the external magnetic field is measured, When the magnetic field to be measured is zero, the phase and amplitude of the signal output by the signal generating means are adjusted so that the output signal of the differential amplifier becomes zero. The bias of the magnetic field can be canceled.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below based on the illustrated embodiments.

First, the structure of the superconducting magnetoresistive element 14 included in the superconducting magnetic field measuring apparatus of this embodiment will be described in detail with reference to FIG.

In the superconducting magnetoresistive element 14 shown in FIG. 2, a superconducting film 2 made of a weakly-bonded aggregate that is bonded in a point-like manner is formed on a non-magnetic substrate 1, and this film 2 is mechanically processed into a meandering shape. Then, titanium (Ti) is vapor-deposited on the film 2 by a vapor deposition method to form current electrodes 3a, 3b and voltage electrodes 4a, 4b.

The superconducting film 2 may be formed by forming fine oxide superconductor particles on the non-magnetic substrate 1 via an extremely thin insulating film.

FIG. 2A is a plan view of the superconducting magnetoresistive element 14. 2A shows that when the element 14 is used, the current source 5 is connected to the current electrodes 3a and 3b and the voltage electrode 4a is connected.
, 4b is connected to the output voltage measuring device 6.

FIG. 2B is a sectional view of the element 14.

FIG. 3 shows an outline of an apparatus for producing the superconducting film 2 shown in FIG. 2 by a spray pyrolysis method.

For example, when forming the superconducting film 2 made of a Y—Ba—Ca—O type superconductor, first, the raw materials Y (NO ₃ ) ₃ .6H ₂ O and Ba (NO ₃ ) ₂ and Ca (NO ₃ ) ₂
3H ₂ O is weighed to a predetermined composition ratio (YBa ₂ Cu ₃ ) to prepare an aqueous solution 7. Next, the aqueous solution 7 is put in a container 8 of a spray gun 9, compressed air is sent from a pipe 10, the aqueous solution 7 is sprayed from the spray gun 9 little by little, and the substrate 13 is heated to about 600 ° C. by a heater 12. Spray on. FIG. 3 shows a state in which the aqueous solution 7 sprayed on the substrate 13 is made into ceramic by thermal decomposition.

The superconducting film 2 produced as described above is
The thickness was set to about 10 μm, and heat treatment was performed in air. The superconducting film 2 may have another composition, different manufacturing conditions, or another manufacturing method. Good results were obtained when the film thickness was between 1 and 10 μm.

The output characteristics of the superconducting magnetoresistive element 14 having the above-mentioned superconducting film 2 and configured as shown in FIG. 2 were measured as shown in FIG. That is, the external magnetic field applying coil 15 is applied with a DC bias current of 10 mA applied to the element 14 via the current electrode 3 of the superconducting magnetoresistive element 14.
An external direct-current magnetic field was applied to the element 14 by using, and the output of the superconducting magnetoresistive element 14 was measured. Figure 5 shows the measurement results.
Shown in. 5, the vertical axis represents the output of the element 14 and the horizontal axis represents the strength of the external DC magnetic field.

Next, FIG. 6 shows noise characteristics indicating the magnitude of noise contained in the output of the element 14. FIG. 6 shows the magnitude of noise contained in the output of the element 14 for each frequency of the noise when the strength of the external DC magnetic field is changed like the horizontal axis under the measurement conditions described in FIGS. It is a thing.
The vertical axis represents the noise level. As can be seen from FIG. 6, the noise from the element 14 has little change in the strength of the applied magnetic field and is large at low frequencies of several Hz or less. This indicates that it is difficult to measure the direct current and the minute magnetic field of the low-frequency magnetic field with high accuracy when the element 14 is supplied with the direct current bias current of 10 mA as described above.

Next, FIG. 1 shows a block diagram of a system of the superconducting magnetic field measuring apparatus of the above embodiment. In FIG. 1, 1
6 is an AC bias current generation circuit, 14 is a superconducting magnetoresistive element, 17 is an amplifier, 18 is an AC bias current generation signal conversion circuit, 19 is a differential amplifier, and 20 is a lock-in amplifier.

A block diagram showing an outline of the lock-in amplifier 20 is shown in FIG. As shown in FIG. 9, the lock-in amplifier 20 includes an amplifier 92 connected to a phase comparator 91.
, A PLL circuit 93, and a low-pass filter 95 connected to the output side of the phase comparator 91.

The superconducting magnetoresistive element 14 having the structure shown in FIG. 2 is arranged at the position of the superconducting magnetoresistive element 14 in the block diagram shown in FIG. An alternating bias current is applied from the alternating bias current generating circuit 16 to the current electrode 3 of the superconducting magnetoresistive element 14 arranged in this way. In this embodiment, the AC bias current has a frequency of 1 KHz,
A sine wave current having a current value of 20 mApp (± 10 mA) is applied to the current electrode 3. By applying this AC bias current,
A voltage signal corresponding to the magnetic resistance of the element 14 at that time is output from the voltage electrode 4 of the element 14.

When an AC bias current of 1 kHz higher than the fluctuation frequency of the superconducting film 2 is applied from the AC bias current generating circuit 16 to the superconducting magnetoresistive element 14, the superconducting film 2 transitions to the normal conducting state, An output signal proportional to the electrical resistance value that rapidly increases from the transition point,
It is output from the superconducting magnetoresistive element 14.

Then, the external magnetic field is measured based on the output signal of the superconducting magnetoresistive element 14 driven by an AC bias current having a relatively high frequency higher than the fluctuation frequency.

That is, in the superconducting magnetic field measuring apparatus of this embodiment, the current electrodes 3a and 3b of the superconducting magnetoresistive element 14 are used.
In addition, the magnetoresistive effect of the superconducting film 2 is utilized in a frequency band equal to or higher than the peculiar fluctuation frequency by applying an AC bias current of the fluctuation frequency or higher of the superconducting film 2. Therefore, the change in the resistance of the superconducting magnetoresistive element 14 due to the change in the external measurement magnetic field is prevented from occurring.
By detecting the voltage signal from the voltage electrode 4 of No. 4, it is possible to perform a highly sensitive measurement of a weak magnetic field without being affected by the low-frequency fluctuation peculiar to the superconducting film 2.

Further, according to this embodiment, the size can be reduced and the operability is remarkably improved as compared with the conventional example which requires the external coil.

The output signal of the element 14 differs depending on the state of the external environmental magnetic field (environmental magnetic field not to be measured) to which the superconducting magnetoresistive element 14 is exposed.

Here, let us consider a case where the external environmental magnetic fields are at Q ₀ and Q ₁ , as shown in FIG. 7, for example.

Q ₀ is the case where the external environmental magnetic field is zero.
FIG. 8 shows an example of the relationship between the AC bias current and the output voltage signal when a DC magnetic field is externally applied by the method shown in FIG. 4 in the state of Q ₀ .

FIG. 8A shows an AC bias current signal.
A waveform B1 shown in FIG. 8B is a waveform of the output voltage signal of the element 14 when the externally applied DC magnetic field is zero, and a waveform B2 is an output signal of the element 14 when the externally applied DC magnetic field is a positive magnetic field. The waveform B3 is the waveform of the output signal of the element 14 when the externally applied DC magnetic field is a negative magnetic field. In this example, the positive magnetic field and the negative magnetic field have the same magnitude.

Next, the case where the external environmental magnetic field is at Q _1, that is, when there is some external environmental magnetic field in advance.
In this case, the element 14 when a DC magnetic field is applied from the outside
The output voltage signal waveform of is shown in FIG. 8 (C). In FIG. 8 (C), the waveform C1 is the output voltage signal waveform when the externally applied DC magnetic field is zero, and the absolute value of this signal is, of course, larger than that in the case of Q ₀ where the external environmental magnetic field is zero.

Waveform C2 is a signal waveform when the externally applied DC magnetic field is a positive magnetic field, and C3 is a signal waveform when the externally applied DC magnetic field is a negative magnetic field.

Generally, the external environmental magnetic field to which the superconducting magnetoresistive element 14 is exposed is not Q _0, that is, the zero magnetic field in FIG. 7, but Q ₁ , that is, some external environmental magnetic field exists in advance. The main one is geomagnetism, and its value is about 0.5 × 10 ^-4 T. Therefore, the output signal of the superconducting magnetoresistive element 14 becomes the signal of FIG. The output signal of the element 14 passes through the amplifier 17 shown in FIG. 1 and is input to the plus input of the differential amplifier 19 in the next stage. When the external magnetic field to be measured is zero, the input signal to the plus input has a waveform proportional to the waveform C1 shown in FIG.

On the other hand, the phase and amplitude of the AC bias current signal generated by the AC bias current generating circuit 16 are appropriately converted by the AC bias current generating signal converting circuit 18,
By inputting to the negative input of the differential amplifier 19,
When the external measurement magnetic field is zero, the output signal of the differential amplifier 19 is adjusted to zero. By doing this,
The output signal of the differential amplifier 19 is an output signal that changes only with the magnetic field to be measured. In this way, the magnetic field to be measured can be accurately measured while avoiding the influence of the external environmental magnetic field.

Then, this output signal becomes the input signal of the lock-in amplifier 20.

The operating principle of the lock-in amplifier 20 will be described below.

First, the input signal V of the lock-in amplifier 20
s and the reference signal Vr are expressed as shown in the following equations 1 and 2.

[0046]

[Formula 1] Vr = Acos (ωr · t + θ)

[0047]

[Formula 2] Vs = cos (ωs · t)

In Equation 1, A is a constant, ωr is the angular velocity of the reference signal, θ is the phase angle, and in Equation 2, ωs is the angular velocity of the input signal. The two signals Vr and Vs are compared by the phase comparator 91. When multiplied by (Phase Sensitive Detector), a signal Vpsd represented by the following equation 3 and equation 4 is obtained.

[0049]

[Equation 3] Vpsd = Acos (ωr · t + θ) cos (ωs · t)

[0050]

Vpsd = A / 2cos [(ωr + ωs) t + θ] + A / 2cos [(ωr−ωs) t + θ]

In Expressions 3 and 4, since ωr = ωs, the second term on the right side of Expression 4 is a DC component. Further, since the low-pass filter 95 removes the AC component of the first term on the right-hand side of Expression 4, the output signal VLP from the low-pass filter 95 is expressed by the following Expression 5.

[0052]

[Formula 5] VLP = A / 2 cos θ

As is clear from Equation 5, the signal VLP becomes maximum when the phase difference θ between the reference signal Vr and the input signal Vs is zero. Therefore, the signal VLP can be maximized by adjusting the lock-in amplifier 20 so that the phase difference θ between the reference signal Vr and the input signal Vs becomes zero.

As described above, only the signal component having the same frequency as the AC bias current can be extracted as the DC voltage from the output signal of the superconducting magnetoresistive element 14.

Therefore, as the reference input of the lock-in amplifier 20, the sine wave 1 of the AC bias current generating waveform is used.
By using KHz, only the 1 KHz component is extracted from the output signal of the element 14, the effective value of noise is suppressed low, and the minute magnetic field can be measured very accurately.

FIG. 10 specifically shows the relational characteristics between the DC applied magnetic field from the outside to the superconducting magnetoresistive element 14 and the output signal of the lock-in amplifier 20. In FIG. 10, the differential magnetic sensitivity at the operating point due to the applied DC magnetic field is the lock-in amplifier 2
Measured as 0 output. In the minute magnetic field region where the externally applied DC magnetic field is near zero, the output signal of the lock-in amplifier 20 is in the linear region. Using this linear region, the time constant of the low-pass filter 95 of the lock-in amplifier 20 is set to 1
By setting it to 00 msec, a magnetic field of several Hz from direct current could be measured with a resolution of 10 ^-9 T.

The above is a description of the embodiments, but the present invention is not limited to the embodiments, and the magnitude and frequency of the AC bias current to the superconducting magnetoresistive element are changed depending on the range and accuracy of the magnetic field measurement. It is possible.

Further, in the above-mentioned embodiment, the external environmental magnetic field to which the superconducting magnetoresistive element is exposed is merely an example of geomagnetism, but in the above-mentioned embodiment, a permanent magnet or a coil which generates a DC magnetic field is used. By appropriately changing the external environmental magnetic field, it is possible to select the external environmental magnetic field that matches the range and accuracy of the magnetic field measurement.

Furthermore, by changing the time constant of the low-pass filter of the lock-in amplifier, it is possible to measure the magnetic field changing over several Hz.

[0060]

As is apparent from the above description, the superconducting magnetic field measuring apparatus of the present invention includes a superconducting magnetoresistive element including a superconductor having a weakly coupled grain boundary, a current electrode of the superconducting magnetoresistive element, and An alternating current applying means for applying an alternating bias current having a frequency not less than the fluctuation frequency of the superconductor, and applying an alternating bias current not less than the fluctuation frequency of the superconductor to the current electrode of the superconducting magnetoresistive element. The magnetoresistive effect of is used in a frequency band higher than the peculiar fluctuation frequency (several Hz).

Therefore, by detecting the resistance change of the superconducting magnetoresistive element due to the change of the externally measured magnetic field as a voltage signal from the voltage electrode of the superconducting magnetoresistive element, the low frequency fluctuation peculiar to the superconductor is not affected. It enables highly sensitive measurement of weak magnetic field.

Moreover, according to the present invention, since the performance of the element alone can be obtained without the need for an external coil or the like, the size can be reduced and the operability is remarkably improved as compared with the conventional example requiring an external coil. To do. Furthermore, it is very advantageous for measuring the spatial distribution of the weak magnetic field. Therefore, measurement with high resolution becomes possible, and it can be used in various fields such as medical treatment and nondestructive inspection.

Further, a signal having the same frequency as the alternating current generated by the alternating current applying means is outputted, and the signal generating means capable of adjusting the phase and amplitude of the signal is outputted from the voltage electrode of the superconducting magnetoresistive element. An output signal and a differential amplifier to which the output signal of the signal generating means is input, and based on the output signal output from the differential amplifier, when the external magnetic field is measured, When the magnetic field to be measured is zero, the phase and amplitude of the signal output by the signal generating means are adjusted so that the output signal of the differential amplifier becomes zero. The bias of the magnetic field can be canceled, and accurate magnetic field measurement can be performed.

[Brief description of drawings]

FIG. 1 is a system block diagram of an embodiment of a superconducting magnetic field measuring apparatus of the present invention.

FIG. 2 is a structural diagram including a front view and a cross-sectional view of the superconducting magnetoresistive element of the above embodiment.

FIG. 3 is an explanatory diagram of a ceramic manufacturing method for manufacturing the superconducting film of the above-described example by a spray pyrolysis method.

FIG. 4 is a diagram illustrating a method for measuring the characteristics of the superconducting magnetoresistive element of the above example.

FIG. 5 is an output characteristic diagram of the superconducting magnetoresistive element of the above-described embodiment with respect to a DC applied magnetic field.

FIG. 6 is a noise characteristic diagram of the superconducting magnetoresistive element with respect to a DC applied magnetic field.

FIG. 7 is a diagram showing operating points of the superconducting magnetoresistive element.

FIG. 8 is an output waveform diagram of the superconducting magnetoresistive element with respect to a DC applied magnetic field.

FIG. 9 is a block diagram of the lock-in amplifier of the above embodiment.

FIG. 10 is a characteristic diagram of a lock-in amplifier output with respect to a DC magnetic field in the above-described embodiment.

[Explanation of symbols]

1, 13 ... Substrate, 2 ... Superconducting film, 3 ... Current electrode, 4 ... Voltage electrode, 5 ... Current source, 6
… Voltage meter, 7… Aqueous solution,
8 ... container, 9 ... spray gun,
10 ... pipe, 11 ... spray,
12 ... Heater, 14 ... Superconducting magnetoresistive element, 15 ... External magnetic field applying coil, 16 ...
AC bias current generation circuit, 17 ... Amplifier, 1
8 ... AC bias current generation signal conversion circuit, 19 ... Differential amplifier, 20 ... Lock-in amplifier.

Claims (2)

[Claims] 1. A superconducting magnetoresistive element including a superconductor having a weakly coupled grain boundary, and an alternating current application for applying an alternating bias current having a frequency higher than a fluctuation frequency of the superconductor to a current electrode of the superconducting magnetoresistive element. And a means for measuring the external magnetic field based on an output signal output from the voltage electrode of the superconducting magnetoresistive element according to the external magnetic field acting on the superconducting magnetoresistive element and the AC bias current. A superconducting magnetic field measuring device characterized in that 2. The superconducting magnetic field measuring apparatus according to claim 1, further comprising a signal generating means for outputting a signal having the same frequency as the alternating current generated by the alternating current applying means and adjusting the phase and amplitude of the signal. An output signal output from the voltage electrode of the superconducting magnetoresistive element, and a differential amplifier to which the output signal of the signal generating means is input, based on the output signal output from the differential amplifier, A superconducting magnetic field measuring device characterized by measuring an external magnetic field.