JP2846507B2 - Gradiometer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a superconducting quantum interference device (SQUID: Supercon
The present invention relates to an improvement of a gradiometer provided with a superconducting pickup coil which is magnetically coupled to the element (a conductive Quantum Interence Device).

[0002]

2. Description of the Related Art Conventionally, SQUIDs utilizing the Josephson effect have been known as one of superconductor devices. By coupling the superconducting pickup coil to the SQUID by magnetic flux, it is possible to obtain a gradiometer for measuring an extremely weak magnetic flux such as magnetism accompanying a small current flowing in a living body. This gradiometer can measure weak magnetic flux, which is extremely lower than the level of geomagnetism, so it can be used to diagnose the human heart and lungs, or
It is attracting attention as being effective in various ranges such as brain wave measurement.

Conventionally manufactured gradiometers use Nb, NbN, Pb, etc. as a superconductor. However, these metal-based superconductors generally have a very low superconducting critical temperature Tc, and in fact, did not exhibit effective characteristics unless cooled with extremely expensive liquid helium.

On the other hand, in 1986 [La, Sr] ₂ Cu
An oxide sintered body such as O ₄ has been found to be a superconducting material with a high Tc, followed by Y ₁ Ba ₂ Cu ₃ O
It was confirmed that the oxide high-temperature superconductor having the composition represented by _7-x exhibited effective superconductivity in the temperature range of liquid nitrogen temperature or higher. Materials that exhibit superconducting properties at such high temperatures can use inexpensive liquid nitrogen as a cooling medium, so practical applications of superconducting technology have been considered, and high-temperature oxide superconducting materials have been used. Various attempts have been made to realize such a gradiometer.

[0005]

However, when a gradiometer is manufactured using the above-described oxide high-temperature superconductor, there are the following problems. SQ as gradiometer
In order to use UID, in addition to the junction, as a flux transformer (Flux Transformer), a pickup coil that converts the magnetic flux gradient to be measured into an electric signal, a magnetic flux transmission circuit that transmits the signal, a magnetic flux that converts the electric signal again An input coil is required to convert and couple to the SQUID ring, and these must be composed of low noise superconductors for efficient transmission.

Under these circumstances, as a pickup coil for measuring a primary axial magnetic flux gradient, there is a conventional coil in which a superconducting wire such as Nb is wound around a bobbin 101 as shown in FIG. The gradiometer was manufactured by connecting by welding or the like. However, when manufacturing a gradiometer as described above using an oxide high-temperature superconductor, a low-noise oxide high-temperature superconductor wire capable of forming a pickup coil is not available. Further, since the surface of the oxide high-temperature superconductor is deteriorated, there is a problem that even when pressure welding or the like is performed, the oxide high-temperature superconductors cannot be superconductively connected to each other.

For example, if an Ag sheath wire is used,
Nyquist noise (heat noise) due to sheath material and Ag
Due to the weak coupling of the superconductor in the sheath, and if a filament without a sheath material is used, the superconducting material constituting the wire is weakly coupled to the superconductor due to the polycrystalline material, so that it can be used as an SQUID. A pickup coil with low noise that cannot be endured cannot be manufactured. Therefore, the pickup coil is connected to the SQUI with low noise and no connection.
A method for coupling to D is desired.

Conventionally, as a flux transformer for magnetically coupling a magnetic flux gradient detected by a pickup coil to a SQUID, there is a laminated type in which a SQUID portion and an input coil are laminated on the same substrate. For example, FIG.
Square washer (S
(quare Washer) type plane dc-SQUID. In the figure, a superconductor thin film (N
b) An input coil 103 (Nb) made of a superconducting thin film is laminated on an intervening insulating layer, and a Josephson junction (a counter superconductor 10) is formed in a portion surrounded by a dotted line in the drawing.
6b; Pb) is formed. This laminated flux
Transformers are characterized by high coupling efficiency.

By the way, in order to constitute the above-mentioned laminated flux transformer, a multilayer technique of a thin film is necessary. However, the above technique has not been established for an oxide high-temperature superconductor. That is, when the above-mentioned laminated flux transformer is made of an oxide high-temperature superconductor, it exhibits such bad noise characteristics that it cannot be used for a gradiometer. Therefore, there is a demand for the development of a gradiometer having a structure in which oxide high-temperature superconductors can be formed in multiple layers or without intersecting.

The present invention has been made in view of the above points, and a main object of the present invention is to provide a superconducting pickup coil, a pickup coil, and a SQUID in a gradiometer.
By improving the configuration for magnetically coupling the ID,
By forming a magnetic flux coupling with the pickup coil using an oxide high-temperature superconductor epitaxial thin film with low noise characteristics without using a connection structure or multilayer structure between the oxide high-temperature superconductors, It is an object of the present invention to provide a low-noise gradiometer structure that effectively exhibits the excellent superconducting characteristics described above.

[0011]

A gradiometer according to the present invention comprises a superconductor loop, a pickup coil for detecting a magnetic flux gradient interlinking in the loop, and a superconducting quantum interference device magnetically coupled to the pickup coil. In order to achieve the above object, the loop of the pickup coil is formed of an oxide superconductor thin film epitaxially grown on a pair of parallel surfaces of a substantially rectangular solid single crystal, An input coil loop for magnetically coupling a magnetic flux gradient detected by the pickup coil to the superconducting quantum interference device has an oxide epitaxially grown on a side surface of the single crystal body sandwiched between the pair of parallel surfaces. The loop of the pickup coil and the loop of the input coil are formed on the surface of the single crystal body by a superconductor thin film. Takisharu consisting grown oxide superconductor thin film and wiring through a continuous, it said superconducting quantum interference device is one that is arranged to be superimposed on the loop of the input coil.

[0012]

The high-temperature oxide superconductor exhibits higher 1 / f noise characteristics than low-temperature superconductors such as Nb and Pb. The present inventors have studied the origin of this noise by measuring the flux noise of the oxide high-temperature superconductor. As a result, 1. Oxide high-temperature superconductor is 77
At operating temperatures near Tc for K, high 1
/ F noise, and the noise significantly increases with an increase in operating temperature. 2. This 1 / f noise causes high noise when the gradiometer is used at liquid nitrogen temperature. 3.1 / f noise is due to the movement of magnetic flux trapped in the superconductor, especially at grain boundaries. There was found. Furthermore, the noise due to the movement of the magnetic flux is reduced by improving the microstructure of the superconductor. In particular, when providing a high-temperature oxide superconductor thin film on a single crystal substrate, a single crystal thin film or a thin film grown epitaxially is used. As a result, it has been found that noise is reduced, and the present invention has been achieved.

It has also been found that the current lamination and connection technologies for high-temperature oxide superconductors cannot reduce the noise associated with the multilayer structure and connection. That is, in the gradiometer of the present invention, all the superconductor circuits are formed by an oxide high-temperature superconductor thin film epitaxially grown on a single crystal body, and do not have a multilayer structure or a connection structure between superconductors. , Low noise gradiometer can be achieved.

Here, in the present invention, the pickup coil is formed not on the bobbin of the air core but on the single crystal body as in the conventional example, so that the single crystal body is made of a material having a high dielectric constant such as SrTiO _3. By installing a pickup coil, the distribution of the measured magnetic flux is affected. Therefore, the measured magnetic flux gradient is calculated from the measured magnetic flux gradient,
It is necessary to perform the correction determined by the boundary condition. However, since the change in the measured magnetic flux distribution due to the single crystal is a change in a direction in which the sensitivity is improved, it does not matter that the pickup coil is not air-core. In particular, when it is desired to avoid the influence on the magnetic flux distribution due to the single crystal, LaAlO ₃
For example, a single crystal of a material having a relative dielectric constant close to 1 may be used.

In the present invention, since a substantially rectangular parallelepiped single crystal is used as the bobbin of the coil, the superconducting loop constituting the SQUID and the pickup coil for detecting the magnetic flux gradient can be installed at an accurate right angle. It is possible to prevent the magnetic flux in the measurement direction from interfering with the SQUID.

The oxide high-temperature superconducting material that can be used in the gradiometer according to the present invention is Y ₁ Ba ₂ Cu ₃ O.
_7-x and a composite oxide Tl having a composition in which Y of the composite oxide is substituted with a lanthanoid element such as Ho or Er.
₂ Ba ₂ Ca ₁ Cu ₂ O _x , Tl ₂ Ba ₂ Ca ₂ Cu ₃
O _10-y or Bi ₂ Sr ₂ Ca ₁ Cu ₂ O _x , Bi
_{2 Sr 2 Ca 2 Cu 3 O} 10-y and that the addition of Pb to these composite oxides, can be exemplified, and the like.

The single crystals that can be used in the gradiometer according to the present invention include MgO, SrTiO ₃ , LaAlO.
₃ , LaGaO _{3 and the} like, but are not particularly limited to the exemplified materials.
Any material can be used as long as it is a material of a single crystal substrate that can be epitaxially grown, that is, a material in which the single crystal and the oxide high-temperature superconductor have close lattice constants and thermal expansion coefficients to each other, and a material in which a rectangular solid can be obtained. .

The shape of a single crystal usable for the gradiometer according to the present invention is basically a rectangular parallelepiped (including a cube). However, if the required size cannot be obtained if the obtained single crystal is too small due to the polishing of the single crystal or the shape of the obtained single crystal is too small, due to the superconducting wiring connecting the loops, Although there is noise characteristic deterioration,
The single crystal body can be used in the gradiometer of the present invention as long as it is a polygon having at least a pair of parallel planes and a plane perpendicular to the plane. Further, in the case where the gradiometer according to the present invention is formed on a rectangular parallelepiped single crystal, in order to prevent the superconductor wiring from being cut on the sides of the rectangular parallelepiped, FIG. Like the coil loop 2a, the input coil loop 3, and the wiring 4),
It is desirable to chamfer the sides of the rectangular parallelepiped. In the present invention, it is preferable that the plane of the single-crystal rectangular parallelepiped is a simple exponential plane such as 100 or 110 because the epitaxial growth conditions can be easily obtained.

In the present invention, the shape of the loop of the input coil formed on the side surface of the single crystal body (the surface sandwiched between the two loops of the pickup coil)
In order to increase the coupling efficiency when the UID is superimposed and the magnetic flux gradient detected by the pickup coil is magnetically coupled to the SQUID, it is desirable that the SUID has the same shape as the superconductor loop.

[0020]

An embodiment of the present invention will be described. In the examples described below, Y ₁ Ba ₂ Cu ₃ O was used as the oxide high-temperature superconductor.
_Although the example of _{7-x is} an example in which an MgO single crystal is used as a single crystal body, it goes without saying that other materials may be used as long as fabrication is possible in the process. 1 and 2 show the configuration of a magnetic flux detecting coil section of a gradiometer according to an embodiment of the present invention. FIG. 1 is a perspective view of a single crystal body on which each coil is formed, and FIGS. 1 (a) and 1 (b) are front views of respective surfaces of the single crystal body of FIG. Note that FIGS. 1 and 2 are not drawn to a constant scale so that the pattern is easy to see.

In the drawing, pickup coil loops 2a and 2b are provided on two parallel surfaces (upper surface and bottom surface in the illustrated example) of a rectangular parallelepiped single crystal 1, and a side surface (pickup coil) of the single crystal 1 is provided. (A surface between the upper surface and the bottom surface where the loops 2a and 2b are formed) is provided with a loop 3 of an input coil, and the loop of each coil is connected to a wiring 4
Is continuous through. Loop 2 of each of these coils
The a, 2b, 3 and the wiring 4 are composed of an oxide high-temperature superconductor thin film epitaxially grown on the single crystal body 1.

Next, FIG. 5 shows the configuration of an oxide high-temperature superconductor thin film growing apparatus used in the embodiment of the present invention. In this apparatus, since the sputter guns 9a and 9b are provided in two sets and the single crystal 1 can be rotated, the oxide high-temperature superconductor can be grown on five surfaces of the single crystal 1 at a time. At this time, in order to make the growth rate of the bottom surface equal to the growth rate of the side surface, the RF of the sputter gun 9a installed on the side surface
The output is set to the sputter gun 9b installed opposite to the single crystal body 1.
4 times the RF output of Single crystal 1 is heater 8
To the desired temperature. Further, the cylinder 10 for supplying oxygen to the thin _{film, O 2, O 3, N} 2 O, NO 2
Gas is introduced into the chamber via the valve 11 and the mass flow meter 12,
It is sprayed through the nozzle 16 to the vicinity of the single crystal body 8. Reference numeral 13 denotes an exhaust pump, and reference numeral 14 denotes a chamber pressure adjusting valve.

Hereinafter, a method of manufacturing the gradiometer according to the present embodiment will be described. First, a Y-based oxide superconductor thin film was formed on an MgO single crystal cube 1 having a size of 10 mm × 10 mm × 10 mm, each plane being a (100) plane, using the above-described oxide high-temperature superconductor thin film growth apparatus. As the target, a sintered target having a composition of Y ₁ Ba ₂ Cu ₃ O _7-x was used. The single crystal body 1 was fixed to the heater 5 with an Ag paste, and the set temperature was 700 ° C. A YBCO film was formed under the growth conditions of a film formation rate of 1 ° / sec and RF power of 50 W and 200 W. During the growth, Ar / O ₂ = 10
400 mT with SCCM / 10 SCCM gas flow rate
orr pressure was maintained. After the sputtering is completed, oxygen at 1 atm is introduced into the vacuum chamber and cooled to room temperature. By this operation, YBCO is added to five of the six faces of single crystal cube 1 at a time.
A film is formed. The resulting film has a glossy black color and a smooth surface. The thickness is 0.3 to 0.6 μm. X
When the crystal structure was examined by a line analysis method, only a diffraction peak of (00l) was observed, and the Y-system of the 1-2-3 structure in which this thin film was grown with the c axis oriented in a direction perpendicular to each surface of the single crystal cube 1 was obtained. It was confirmed that it was an oxide superconductor. When the surface structure of each surface was observed by RHEED, it was found that YB
It was found that the CO film was grown epitaxially on the single crystal body 1.

Next, a resist is applied on the YBCO thin film according to an ordinary method. After pre-baking, the resist is exposed and developed using a photomask, and then subjected to post-baking to form a pattern. This is repeated for each surface, and the single crystal 1 has two loops (2a, 2b) for pickup coils, one loop (3) of the input coil constituting the flux transformer, and the wiring 4 connecting the loops. Is formed. Subsequently, the YBCO thin film is patterned by a plasma etching method using SF ₆ gas. At this time, the gas pressure in the chamber is set to 10 mTorr, a high-frequency power of 200 W is applied to ionize the SF ₆ gas, and the YBCO thin film on the single crystal cube 1 is selectively used as a mask using the previously formed resist film as a mask. Etch. In order to prevent damage due to ion bombardment, 4
It is desirable to apply a high-frequency power of 00 W or less. Subsequently, the resist used as the mask was removed, and a magnetic flux detecting coil portion of the gradiometer as shown in FIG. 1 was manufactured.

Next, a SQUID operating at the temperature of liquid nitrogen
Square-washer-type grain boundary junction dc-SQU in which a Josephson junction is formed on a junction line of a bicrystal substrate
An ID was prepared. FIG. 4 is a perspective view of the SQUID. First, a 0.5 μm superconductor thin film (YBCO thin film) was grown on a 10 mm × 10 mm MgO single crystal substrate 5 by the RF magnetron sputtering method described above. At this time, the critical temperature of the produced superconductor thin film was 89
K, the critical current density was 10 ⁶ A / cm ² . Next, the obtained film is square-washer type (superconductor loop 6; Josephson junction 7 is formed at the portion extended on the bonding line of substrate 5) by the plasma etching method using SF ₆ gas described above. Is patterned. Next, electrodes (not shown) formed by Au evaporation were formed on the superconductor thin film as SQUID bias current terminals and voltage measurement terminals, and dc-SQUIDs as shown in FIG. 4 were produced. Finally, the magnetic flux detection coil unit and the dc-SQUID are overlapped so that the input coil loop 3 of the magnetic flux detection coil unit (FIG. 1) and the superconductor loop 6 of the SQUID (FIG. 4) match. A gradiometer was prepared.

FIG. 6 shows the result of measuring the relationship between the magnetic flux gradient and the generated voltage by cooling the gradiometer manufactured as described above to 77K. The applied magnetic flux gradient is the unit magnetic flux φ
₀ and the distance between the pickup coil loops 2a and 2b 10
of mm, it is standardized in φ ₀ / 10mm. The periodic structure of FIG. 7 (the generated voltage changes with the period of the strength of the magnetic field that generates the unit magnetic flux quantum in the SQUID superconductor loop 6)
Could be observed up to a magnetic flux gradient corresponding to 100 magnetic flux quanta. No hysteresis was observed in the relationship between the magnetic flux gradient and the generated voltage. FIG. 7 shows the result of measuring the power spectrum of noise at 77K of the gradiometer according to the present embodiment. Noise level is 1 × 10 ^-30
J / Hz (at 10 KHz) and 1 / f noise were up to 50 Hz, and it was confirmed that the gradiometer according to the present embodiment had low noise.

[0027]

As described above, in the present invention, the pickup coil, the input coil, and the wiring are all formed of the oxide high-temperature superconductor thin film epitaxially grown on the surface of the substantially rectangular solid single crystal. Because it does not use a multilayer structure or connection between superconductors,
A gradiometer having a noise level low enough to withstand practical use and reduced 1 / f noise can be realized. The gradiometer of the present invention effectively exhibits the excellent superconducting characteristics of the oxide high-temperature superconductor, shows good noise characteristics, can accurately measure extremely weak magnetic flux, and is very useful in various fields. It is.

[Brief description of the drawings]

FIG. 1 is a perspective view of a magnetic flux detecting coil unit of a gradiometer according to an embodiment of the present invention.

2 (a) is a front view of a pickup coil surface of the magnetic flux detecting coil unit of FIG. 1, and FIG. 2 (b) is a front view of an input coil surface.

FIG. 3 is a perspective view showing that a single crystal body is chamfered with respect to the magnetic flux detection coil unit of FIG. 1;

FIG. 4 is a SQUID of a gradiometer according to an embodiment of the present invention.
It is a perspective view of D part.

FIG. 5 is a configuration diagram of a film forming apparatus used in an embodiment of the present invention.

FIG. 6 is a graph showing a result of measuring a relationship between a magnetic flux gradient and a generated voltage of the gradiometer according to the embodiment of the present invention.

FIG. 7 is a graph showing a result of measuring a noise power spectrum of the gradiometer according to the present embodiment.

FIG. 8 is a configuration diagram of a conventional pickup coil.

FIG. 9 is a plan view of a conventional laminated flux transformer.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Single crystal body 2a, 2b Pickup coil 3 Loop of input coil 4 Wiring

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Shoji Yoshihara 2-6-1 Marunouchi, Chiyoda-ku, Tokyo Inside Furukawa Electric Co., Ltd. (56) References JP-A-1-147387 (JP, A) JP-A-Hei 2-253670 (JP, A) Japanese Utility Model Hei 3-27375 (JP, U) (58) Fields surveyed (Int. Cl. ⁶ , DB name) H01L 39/00 H01L 39/22-39/24 G01R 33 / 035

Claims (1)

(57) [Claims] 1. A gradiometer comprising a superconducting loop, a pickup coil for detecting a magnetic flux gradient interlinked in the loop, and a superconducting quantum interference device flux-coupled to the pickup coil. Is formed by an oxide superconductor thin film epitaxially grown on a pair of parallel surfaces of a substantially rectangular parallelepiped single crystal, and a magnetic flux gradient detected by the pickup coil is applied to the superconducting quantum interference device by a magnetic flux. The loop of the input coil for coupling
An oxide superconductor thin film epitaxially grown on a side surface of the single crystal body sandwiched between the pair of parallel surfaces is formed, and a loop of the pickup coil and a loop of the input coil are formed on the surface of the single crystal body. A gradiometer, which is continuous through a wiring made of an epitaxially grown oxide superconductor thin film, wherein the superconducting quantum interference device is arranged so as to be superimposed on a loop of the input coil.