Planar magnetism sensor utilizing a squid of oxide superconductor

Abstract: A magnetism sensor comprises a substrate (5), a SQUID (1) including a washer of an oxide superconductor thin film formed on a principal surface of the substrate, a similarly shaped hole at the center of the washer, a slit formed between one side of the washer and the hole and a Josephson junction which connects portions of the washer at both sides across the slit. The magnetism sensor further comprises a superconducting flux transformer (2) of an oxide superconductor thin film including an input coil (3) arranged on the SQUID but isolated from the SQUID and a pickup coil (4) arranged on the principal surface of the substrate far from the SQUID, which are connected in parallel and a magnetism shield (30) which excludes flux penetrating into the Josephson junction of the SQUID.

Description: Background of the Invention Field of the invention The present invention relates to a planar magnetism sensor utilizing a SQUID (Superconducting QUantum Interference Device) of an oxide superconductor, and more specifically to a sensitive planar magnetism sensor utilizing a SQUID of an oxide superconductor. Description of related art A SQUID is a sensitive magnetism sensor utilizing a Josephson junction, which is capable of measuring extremely weak flux less than flux quantum  0 (= h /2e , h : Planck's constant, e : charge of an electron) by converting the flux into voltage. The SQUID has a superconducting loop and one or two Josephson junctions which are inserted to the superconducting loop. The SQUID which has one Josephson junction is called a RF-SQUID and the one which has two Josephson junctions is called a DC-SQUID. When a SQUID is used for a magnetism sensor, the SQUID is usually associated with a superconducting flux transformer in order to increase its sensitivity. The superconducting flux transformer is constituted of two superconducting coils of different sizes connected in parallel, which are called a pickup coil and an input coil. The input coil is the smaller one which has a size close to that of the SQUID and is placed on the SQUID. The pickup coil is the larger one which detects flux and is arranged apart from the SQUID. The flux detected by the pickup coil is concentrated to the input coil and the SQUID measures the concentrated flux. By this, the substantial sensitivity of the SQUID increases in relative to the self conductance ratio of the pickup coil to the input coil. Figure 1 shows a partial view of a conventional magnetism sensor utilizing a planar DC-SQUID, which is disclosed by M. B. Kechen et al. in Appl. Phys. Lett., 44, 736 (1982). In Figure 1, the magnetism sensor includes a substrate 5, a SQUID 1 formed on the substrate 5 and a superconducting flux transformer of which input coil 3 is arranged on the SQUID 1. The SQUID 1 is mainly constituted of a washer 10 of a square superconducting thin film, which has a square hole 11 at the center. An input coil 3 of the superconducting flux transformer is placed on the washer 10 of the SQUID 1 but isolated from the SQUID 1. The superconducting thin film of the washer 10 overlaps at a portion 19 and the overlap portion 19 is connected to the under layer through two Josephson junctions (not shown) connected in series. The washer 10 has two tongue portions 14 and 15, in order to connect the SQUID 1 to a signal processor (not shown). The input coil 3 is formed of a helical rectangular shaped superconducting coil. The input coil 3 is connected to the pickup coil (not shown) of the superconducting flux transformer, which is formed on the substrate 5 far from the SQUID 1. Namely, the SQUID 1 and the pickup coil is arranged on the same plane. The SQUID 1 measures 100 µm square and the pickup coil measures 1 mm square. Therefore, when a magnetic field is measured by using the above-mentioned conventional planar magnetism sensor, flux penetrates not only the pickup coil but also the SQUID. The flux penetrating the Josephson junctions of the SQUID has an adverse effect on the tunnel current flowing through the Josephson junction, so that the voltage generated by the SQUID decreases. The drop of the voltage can reduce substantial sensitivity of the magnetism sensor. A magnetic field sensor corresponding to the preamble of claim 1 is known from EP-A-0 399 499. The paper published in Appl. Phys. Lett. 60(9), 2 March 1992, pages 1141-1143; J.S. Marten et al.: "Ti-Ca-Ba-Cu-O step edge Josephson junctions" discusses the use of high critical temperature compounds for realising step edge type junctions. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a high sensitive planar magnetism sensor utilising a SQUID composed of an oxide superconductor material, which has overcome the above-mentioned defects of the conventional ones. Another object of the present invention is to provide a planar magnetism sensor utilising a SQUID which has a larger output voltage. The above and other objects of the present invention are achieved in accordance with the present invention by a magnetic sensor comprising: (A) a substrate; (B) a SQUID; (C) a superconducting flux transformer of a superconductor thin film; and (D) a magnetic shield; wherein said SQUID comprises: (B1) a washer of a superconductor thin film formed on a principal surface of the substrate; (B2) a hole at the centre of the washer; (B3) a slit formed between one side of the washer and the hole; and (B4) at least one Josephson junction by which portions of the washer at each side of the slit are connected across the slit; wherein said superconducting flux transformer comprises: (C1) an input coil arranged on the SQUID but isolated from the SQUID; and (C2) a pickup coil arranged on the principal surface of the substrate remote from the SQUID, the pickup coil and the input coil being connected in parallel so that magnetic flux that penetrates the pickup coil is transferred to the input coil so that the magnetic flux penetrates the SQUID; characterised in that said hole at the centre of the washer has a shape similar to the washer; in that said thin films forming said flux transformer and said washer comprise an oxide superconductor; and in that said magnetic shield covers the Josephson junction of the SQUID without covering the input coil on the SQUID to exclude magnetic flux from penetrating into the Josephson junction of the SQUID. In the magnetic sensor in accordance with the present invention, no flux penetrates the Josephson junction of the SQUID, so that the tunnel current flowing through the Josephson junction is not affected by the flux and output voltage of the SQUID is improved. Therefore, the sensitivity of the magnetic sensor in accordance with the present invention is substantially improved. In a preferred embodiment, the magnetic shield of the magnetism sensor in accordance with the present invention is formed of an oxide superconductor thin film. In this case, when the magnetic sensor is cooled and the oxide superconductor thin films of the SQUID and the superconducting flux transformer are in the superconducting state, the magnetic shield of the oxide superconductor thin film is also in the superconducting state. Therefore, flux penetrating into the Josephson junction is excluded by the Meissner effect at the magnetic shield, so that the tunnel current flowing through the Josephson junction is not affected by the flux. Preferably, the washer and the hole of the SQUID of the magnetic sensor in accordance with the present invention are rectangular. If their shapes have poor symmetry, the SQUID has an isotropic characteristic or sensitivity which will become an obstacle for the practical application. In this case, it is preferable that the input coil and the pickup coil of the superconducting flux transformer are rectangular. According to the present invention, the Josephson junction of the SQUID of the magnetic sensor is of weak link type constituted of a grain boundary of the oxide superconductor. Since the weak link type Josephson junction can be easily formed without a extremely fine processing by utilizing a grain boundary of the oxide superconductor crystals, it is favorable that the Josephson junction of the SQUID of the magnetism sensor of the present invention is constituted of a grain boundary of the oxide superconductor. In one preferred embodiment, the oxide superconductor thin films are formed of a high-T c (high critical temperature) oxide superconductor, particularly, formed of a high-T c copper-oxide type compound oxide superconductor for example a Y-Ba-Cu-O type compound oxide superconductor material, a Bi-Sr-Ca-Cu-O type compound oxide superconductor material, and a Tl-Ba-Ca-Cu-O type compound oxide superconductor material. In addition, the substrate can be formed of a single crystal oxide substrate such as a MgO (100) substrate, a SrTiO 3 (110) substrate, an yttrium stabilized zirconia (YSZ) substrate, etc. According to one preferred embodiment of the present invention, the magnetism sensor comprising a substrate having a rectangular parallelepipedic concavity on a surface, a SQUID including a square washer having two tongue portions of an oxide superconductor thin film formed on the surface of the substrate, a square hole at the center of the washer, a slit formed between the tongue portions to the hole, and a member of an oxide superconductor thin film including two Josephson junctions connected in series, which connects the two tongue portions across the slit, the magnetic sensor further including a superconducting flux transformer of an oxide superconductor thin film including a rectangular input coil arranged on the SQUID but isolated from the SQUID and a rectangular pickup coil arranged on the principal surface of the substrate far from the SQUID, which are connected in parallel and a magnetism shield which excludes flux penetrating into the Josephson junctions of the SQUID in which the Josephson junctions of the SQUID are constituted of grain boundaries of the oxide superconductor at steps between the surface of the substrate and the bottom of the concavity. The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings. Brief Description of the Drawings Figure 1 is a partial schematic view a conventional planar magnetism sensor utilizing a SQUID; Figure 2A is a diagrammatic plane view of a preferred embodiment of the planar magnetism sensor in accordance with the present invention; Figure 2B is a diagrammatic sectional view taken on line A-A of Figure 2A; Figure 3 is a diagrammatic plane view of the DC-SQUID which is used in the magnetism sensor shown in Figure 2; and Figure 4 is an enlarged detail of the DC-SQUID shown in Figure 3. Description of the Preferred embodiments Figure 2A shows a diagrammatic plan view of an embodiment of the planar magnetism sensor in accordance with the present invention. The magnetism sensor includes a MgO (100) substrate 5, a DC-SQUID 1 of a c -axis orientated Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film formed on the substrate 5, a superconducting flux transformer 2 of which an input coil 3 is arranged on the DC-SQUID 1 and a magnetism shield 30 which covers Josephson junctions of the DC-SQUID 1. The superconducting flux transformer 2 includes a rectangular input coil 3 arranged on the DC-SQUID 1 but isolated from it and a rectangular pickup coil 4 arranged on the substrate 5 far from the DC-SQUID 1. The superconducting flux transformer 2 is formed of a patterned c -axis orientated Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film having a thickness of a few hundreds nanometers, for example 300 nanometers, and the input coil 3 and the pickup coil 4 are connected in parallel. The shapes of the input coil 3 and the pickup coil 4 are not limited to rectangles, and can be helical rectangular shapes, as shown in Figure 1. The superconducting flux transformer concentrates the flux detected by the pickup coil into the input coil so that the DC-SQUID measures the concentrated flux . The magnetism shield 30 is also formed of a Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film having a thickness of a few hundreds nanometers, for example 300 nanometers, which covers Josephson junctions of the DC-SQUID 1. Figure 2B shows a diagrammatic sectional view taken on line A-A of Figure 2A. In Figure 2B, an insulating layer 31 of MgO is formed on the Josephson junctions 12 and 13 and the magnetism shield 30 is formed on the insulating layer 31. The magnetism shield 30 is isolated from the DC-SQUID 1. When the magnetism sensor is cooled and the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin films of the DC-SQUID 1 and the superconducting flux transformer are in the superconducting state, the magnetism shield 30 of the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film is also in the superconducting state. Therefore, flux penetrating into the Josephson junctions 12 and 13 is excluded by the Meissner effect of the magnetic shield 30, so that the tunnel current flowing through the Josephson junctions 12 and 13 is not affected by the flux. By this, the magnetism sensor has a stable performance. It can be considered that the magnetic shield 30 covers the input coil 3 and the entire SQUID 1. However, in order to stabilize the operation of the Josephson junctions 12 and 13 and increase output voltage of the SQUID 1, the magnetic shield according to the present invention covers the Josephson junctions 12 and 13 rather than covering the input coil 3 and the entire SQUID. The magnetic shield 30 may be formed of any material other than oxide superconductor, if flux penetrating into the Josephson junctions can be excluded. Figure 3 shows a diagrammatic plan view of the DC-SQUID 1. In Figure 3, the DC-SQUID 1 includes a washer 10 of a square shaped c-axis orientated Y 1 Ba 2 Cu 3 O 7-8 oxide superconductor thin film formed on the substrate 5, at the center of which a square hole 11 is formed. The oxide superconductor thin film has a thickness of 500 nanometers. The washer 10 has tongue portions 14 and 15 on its one side and a slit 16 formed between the tongue portions 14 and 15 which reaches the hole 11. The leading edges of the tongue portions 14 and 15 are connected by a member 17 of the oxide superconductor thin film, which has two bridge portions 181 and 182 at which Josephson junctions 12 and 13 of weak link type are respectively formed. In other words, the leading edges of the tongue portions 14 and 15 are connected through the Josephson junctions 12 and 13. The DC-SQUID 1 is connected to a signal processor (not shown) through the tongue portions 14 and 15. Figure 4 is an enlarged detail of a part of the DC-SQUID shown in Figure 3. In Figure 4, the member 17, the bridge portions 181 and 182, and the Josephson junctions 12 and 13 are shown in detail. The member 17 connects the leading edges of the tongue portions 14 and 15, which has two bridge portions 181 and 182. A center portion of the member 17 is arranged at the bottom of a rectangular parallelepipedic concavity 20 of the substrate 5 and the bridge portions 181 and 182 are formed so as to cross the steps between the surface of the substrate 5 and the bottom of the concavity 20. The Josephson junctions 12 and 13 are formed at the bridge portions 181 and 182, which are constituted of grain boundaries of the oxide superconductor formed at the step portions. The oxide superconductor thin film which constitutes the above magnetism sensor is preferably formed of a high-T c (high critical temperature) oxide superconductor material, particularly a high-T c copper-oxide type compound oxide superconductor material, for example, a Bi-Sr-Ca-Cu-O type compound oxide superconductor material, or a Tl-Ba-Ca-Cu-O type compound oxide superconductor material other than a Y-Ba-Cu-O type compound oxide superconductor material. The above mentioned magnetism sensor was manufactured by the following process. At first, a c -axis orientated Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film having a thickness of 500 nanometers was deposited by a high frequency magnetron sputtering process on a surface of an MgO (100) single crystal substrate 5 which had a concavity 20 on the surface. The conditions of the sputtering process were as follows; Temperature of substrate 630°C Sputtering gases Ar 8 sccm O 2 4 sccm Pressure 6.6 Pa (5 × 10 -2 Torr) The oxide superconductor thin film was formed in such a manner that the concavity 20 was covered with the oxide superconductor thin film near the center of one side of the oxide superconductor thin film. Grain boundaries were generated in the oxide superconductor thin film at the steps between the surface of the substrate and the bottom of the concavity, which constituted weak links of Josephson junctions. Then, the oxide superconductor thin film was etched by an ion milling using Ar-ions so that a square washer 10 and a projecting portion on one side, which covered the concavity 20 and would be shaped into the tongue portions 14 and 15. Thereafter, by an ion milling using Ar-ions, a square hole 11 was formed at the center of the washer and a member 17 was formed at the edge of the projecting portion on the concavity 20 in such a manner that the member 17 had narrow bridge portions 181 and 182 crossing the steps at which grain boundaries had been formed. After then, a slit 16 which divided the projecting portion into two tongue portions 14 and 15 was formed to the hole 11. By this, the DC-SQUID was completed. Thereafter, an MgO thin film is deposited on the DC-SQUID 1 by a vacuum evaporation. The MgO thin film was formed so as to have an enough thickness for constituting an insulating layer, for example 100 nanometers. Then, a c -axis orientated Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film having a thickness of 300 nanometers was deposited on the MgO thin film and the substrate 5, and the oxide superconductor was processed into the superconducting flux transformer 2 and the magnetism shield 30. By this, the embodiment of the magnetism sensor in accordance with the present invention was completed. By using the above mentioned magnetism sensor in accordance with the present invention and a conventional magnetism sensor which did not have a magnetism shield on the Josephson junctions of the DC-SQUID, a faint magnetic field was measured while the magnetism sensors were chilled by liquid nitrogen. The magnetism sensor in accordance with the present invention had a dV/d of 20 µv/ i and a magnetic field sensitivity of 5 × 10 -5  0 /(Hz) 1/2 at 1 Hz. On the other hand, the conventional magnetism sensor had a dV/d of 10 µv/ i and a magnetic field sensitivity of 1 × 10 -4  0 /(Hz) 1/2 at 1 Hz. Thus, the magnetism sensor in accordance with the present invention had twice output voltage and twice magnetic field sensitivity of the conventional one. As explained above, the above mentioned magnetism sensor in accordance with the present invention has an improved output voltage and sensitivity. This high sensitivity is realized by a magnetism shield which excludes flux penetrating into the Josephson junction of the SQUID. In addition, if the magnetism sensor is manufactured in accordance with the above mentioned process, the limitation in the fine processing technique required for manufacturing the magnetism sensor is relaxed. In the above process, of course, the film deposition process of the oxide superconductor is not limited to a high frequency magnetron sputtering process, but any film deposition process such as an MBE (Molecular Beam Epitaxy), a laser ablation, a vacuum evaporation can be employed, so long as a high quality oxide superconductor thin film can be deposited. The invention has thus been shown and described with reference to the specific embodiments. However, it should be noted that the present invention is in no way limited to the details of the illustrated structures but converts and modifications may be made within the scope of the appended claims.

Claims: A magnetic sensor comprising: (A) a substrate (5); (B) a SQUID (1); (C) a superconducting flux transformer (2) of a superconductor thin film; and (D) a magnetic shield (30); wherein said SQUID comprises: (B1) a washer (10) of a superconductor thin film formed on a principal surface of the substrate; (B2) a hole (11) at the centre of the washer (1); (B3) a slit (16) formed between one side of the washer and the hole; and (B4) at least one Josephson junction (13) by which portions of the washer at each side of the slit are connected across the slit; wherein said superconducting flux transformer comprises: (C1) an input coil (3) arranged on the SQUID but isolated from the SQUID; and (C2) a pickup coil (4) arranged on the principal surface of the substrate remote from the SQUID, the pickup coil and the input coil being connected in parallel so that magnetic flux that penetrates the pickup coil is transferred to the input coil so that the magnetic flux penetrates the SQUID; characterised in that said hole (11) at the centre of the washer (10) has a shape similar to the washer; in that said thin films forming said flux transformer (2) and said washer (10) comprise an oxide superconductor; and in that said magnetic shield (30) covers the Josephson junction (13) of the SQUID to exclude magnetic flux from penetrating into the Josephson junction of the SQUID, said magnetic shield being interposed between the input coil and the Josephson junction (13). A magnetic sensor as claimed in claim 1, wherein the magnetic shield (30) is formed of an oxide superconductor thin film. A magnetic sensor as claimed in claims 1 or 2, wherein the washer (10) and the hole (11) of the SQUID are rectangular. A magnetic sensor as claimed in claim 3, wherein the input coil (3) and the pickup coil (4) of the superconducting flux transformer (2) are rectangular. A magnetic sensor as claimed in any one of claims 1 to 4, wherein the Josephson junction (13) of the SQUID (1) is constituted of a grain boundary of the oxide superconductor. A magnetic sensor as claimed in any one of claims 1 to 5, wherein the SQUID (1) includes two Josephson junctions (13) so as to constitute a DC-SQUID. A magnetic sensor as claimed in any one of claims 1 to 6, wherein the oxide superconductor thin films are formed of a high-T c (high critical temperature) oxide superconductor, preferably of a high-T c copper-oxide type compound oxide superconductor. A magnetic sensor as claimed in claim 7, wherein the oxide superconductor thin films are formed of an oxide superconductor material selected from the group consisting of a Y-Ba-Cu-O type compound oxide superconductor material, a Bi-Sr-Ca-Cu-O type compound oxide superconductor material, and a Tl-Ba-Ca-Cu-O type compound oxide superconductor material. A magnetic sensor as claimed in any one of claims 1 to 8, wherein the substrate (5) is formed of a material selected from the group consisting of a MgO (100) substrate, a SrTiO 3 (100) substrate and an yttrium stabilised zirconia (YSZ) substrate. A magnetic sensor according to any one of claims 1 to 9, wherein: said substrate (5) has a rectangular parallelepipedic concavity on a surface, said SQUID (1) has a square washer (10) having two tongue portions (14, 15) of an oxide superconductor thin film formed on the surface of the substrate, said hole (11) at the centre of the washer being correspondingly square, said slit (16) being formed between said tongue portions to the hole, and a member of an oxide superconductor thin film including two Josephson junctions connected in series, which connects the two tongue portions across the slit, wherein said Josephson junctions of the SQUID are constituted of grain boundaries of the oxide superconductor at steps between the surface of the substrate and the bottom of a concavity; and wherein the superconducting flux transformer of an oxide superconductor thin film has a rectangular input coil arranged on the SQUID, but isolated from the SQUID, and a rectangular pickup coil arranged on the principal surface of the substrate far from the SQUID, the input and the pickup coil being connected in parallel.

Inventors: ITOZAKI HIDEO, MATSUURA TAKASHI, ITOZAKI, HIDEO, MATSUURA, TAKASHI, ITOZAKI, HIDEO, C/O ITAMI WORKS, MATSUURA, TAKASHI, C/O ITAMI WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1993
Main classification: G01R33
IPC codes: G01R33, H01L39