Superconductor technology: apparatus – material – process – High temperature devices – systems – apparatus – com- ponents,... – Measuring or testing system or device
Reexamination Certificate
1998-11-12
2001-05-01
Strecker, Gerard R. (Department: 2862)
Superconductor technology: apparatus, material, process
High temperature devices, systems, apparatus, com- ponents,...
Measuring or testing system or device
C324S248000, C505S702000, C505S846000
Reexamination Certificate
active
06226538
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic sensor utilizing a superconductor.
2. Related Background Art
A conventional sensor using a SQUID (superconducting quantum interference device) is disclosed, for example, in “Jpn. J. Appl. Phys. vol. 32, p. 662-664 (1993).” For improving the magnetic field resolution of a SQUID, it is necessary to increase the area by which the SQUID effectively captures a magnetic flux (effective flux capturing area). To this end, the size of the SQUID is enhanced so as to increase the effective magnetic flux capturing area in the above-mentioned publication.
SUMMARY OF THE INVENTION
Enhancing the size of SQUID, however, reduces the number of SQUIDs that can be produced from a single wafer substrate. Namely, while 9 pieces of 5-mm square SQUIDs can be made on a 20-mm square substrate, only one 17-mm square SQUID can be made thereon, thus increasing the manufacturing cost by a factor of 9. Therefore, in “Appl. Phys. Lett. vol. 63, p. 3630-3632(1993),” a single-layer flux transformer for introducing a magnetic flux to be detected is opposed to the SQUID, so as to enhance the effective magnetic flux capturing area. In this publication, a YBCO superconducting thin layer is formed on a YSZ substrate, so as to constitute a flux transformer. Since the YSZ substrate is expensive, however, it has not been possible to enhance the size of the magnetic transformer so as to increase the effective magnetic flux capturing area and realize a high-performance, low-cost magnetic sensor. In view of this problem, it is an object of the present invention to provide a high-performance, low-cost magnetic sensor.
In order to solve the above-mentioned problem, the magnetic sensor in accordance with the present invention comprises a flux transformer having a superconducting thin film formed on a sapphire substrate, and a SQUID disposed on the flux transformer opposite thereto. Since this magnetic sensor uses, for a flux transformer, a sapphire substrate which can be obtained in a large size at a low cost, even when the SQUID is made smaller, the magnetic flux introduced from the flux transformer into the SQUID can be enhanced so as to increase the effective magnetic flux capturing area, whereby the detecting performance is improved, while the manufacturing cost can be reduced due to the smaller size of the SQUID. Here, from the viewpoint of manufacturing cost, the superconducting thin film is preferably a single layer.
As a result of diligent studies concerning superconducting materials which can be formed on a sapphire substrate, the inventors have found that Ho oxide superconductors can be formed as a superconducting thin film on the sapphire substrate. In particular, when the superconducting thin film comprises an HoBaCuO thin film, and it is formed on the sapphire substrate with a cerium oxide layer interposed therebetween, its crystal state and characteristics would become favorable. When the flux transformer is formed on the sapphire substrate as mentioned above, the SQUID can be made smaller. Namely, the SQUID is preferably shaped like a washer having an annular part made of a superconducting material, and the annular part of the SQUID preferably has an outside diameter not greater than 5 mm. This annular part can detect, at its opening, the magnetic flux from the magnetic transformer. Here, the outside diameter of the SQUID is defined by the square root of the area of the part of SQUID constituting the superconducting closed loop of the SQUID, i.e., the area of the annular part.
Also, it is preferred that a predetermined portion of the annular part form a Josephson junction by covering a step edge which traverses thereunder. The Josephson junction in the present invention is formed by utilizing a weak junction formed at the step edge portion when a superconducting material covers the step edge. When such a step edge type Josephson junction is included in the annular part, a quantum mechanical interference effect occurs due to the macroscopic quantum effect of superconductivity, thus allowing the detecting magnetic field sensitivity to increase.
Preferably, in the magnetic sensor of the present invention, a predetermined portion of the annular part forms a Josephson junction, the annular part has a slit cut toward a position between a predetermined outer edge point and a predetermined inner edge point of the annular part from a point different from the predetermined outer edge point in the outer edge, the Josephson junction traverses between the predetermined inner edge point side of a ridge defining the slit and the inner edge of the annular part, the superconducting thin film of the flux transformer has a pickup coil for acquiring a magnetic flux of a subject and an input coil connected to the pickup coil, and the input coil is disposed opposite to the annular part of the SQUID and covers the whole region of the slit.
The weak junction due to the Josephson junction is yielded by forming a grain boundary within the superconducting thin film with the aid of a step edge. For more favorably forming the weak junction, it is preferred that a slit be formed as mentioned above, such that the width of the Josephson junction is narrowed. Since the input coil of the magnetic transformer is connected to the pickup coil, the magnetic flux detected by the pickup coil is condensed within the input coil, so as to be introduced into the annual part of the SQUID opposed to the input coil. Since the slit is open at its proximal end side, the magnetic field to be introduced into the opening of the annual part of the SQUID from the input coil partially leaks out of the slit. Therefore, the present invention employs a structure in which, as mentioned above, the input coil is opposed to the annular part of the SQUID and covers the whole region of the slit, thereby suppressing the leaking phenomenon and improving the detecting sensitivity.
Further, when the inside diameter of the input coil is greater than the outside diameter of the annular part, the magnetic flux may partially leak out as in the case of the above-mentioned slit. Therefore, the present invention employs a structure in which the superconducting thin film of the flux transformer has a pickup coil for acquiring a magnetic flux of a subject and an input coil connected to the pickup coil, and the input coil is opposed to the annular part and has an inside diameter smaller than the outside diameter of the annular part, thereby suppressing the leakage and introducing a larger amount of magnetic field into the SQUID. Here, the inside diameter of the input coil is defined by the square root of the area of its opening portion.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.
REFERENCES:
patent: 4801882 (1989-01-01), Daalmans
patent: 4804915 (1989-02-01), Hoenig
patent: 5012190 (1991-04-01), Dossel
patent: 5139192 (1992-08-01), Simmonds
patent: 5218297 (1993-06-01), Nakane et al.
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patent: 5767043 (1998-06-01), Cantor et al.
Properties of YBa2Cu3O7-yLarge Washer SQUID; Jpn. J. Appl. Phys. vol. 32 (1993) pp. L662-L664.
High performance dc SQUID magnetometers with single layer YBa2Cu3O7-xflux transformers; Appl. Phys. Lett. vol. 63 (26), Dec. 27, 1993 pp. 3630-3632.
Itozaki Hideo
Kugai Hirokazu
Nagaishi Tatsuoki
Smith Gambrell & Russell
Strecker Gerard R.
Sumitomo Electric Industries Ltd.
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