Superconductive magneto-resistive device comprising laminated superconductive ceramic films

Abstract: A superconductive magneto-resistive device comprising superconductive ceramic films having a magneto-­ resistive effect is disclosed. In the superconductive magneto-resistive device, the superconductive ceramic films are laminated one by one, and the adjacent superconductive ceramic films are insulated by each electrical insulation film, respectively, except for one portion of each of the adjacent superconductive ceramic films, resulting in that the superconductive ceramic films are connected in series. Furthermore, a pair of electrodes is arranged on the upper-­ most and lower-most superconductive ceramic films.

Description: BACKGROUND OF THE INVENTIO N FIELD OF THE INVENTION The present invention relates to a superconductive magneto-resistive device, and more particularly, to a superconductive magneto-resistive device comprising laminated superconductive ceramic films. DESCRIPTION OF RELATED ART Conventionally, a magnetic sensor utilizing a magneto-resistive device of a semiconductor or a magnetic material has been widely used for measuring a magnetic field. In this sort of magnetic sensor, a device utilizing a form effect such as a semiconductor of InSb, InAs etc. having a high electron mobility and a device utilizing an orientation effect such as a ferromagnetic material of Fe-Ni (permalloy), Co-Ni etc. have been put to practical use. In the conventional magnetic sensor utilizing the magneto-resistive device, the resistance R of the magneto-resistive device increases along a parabolic curve as shown in Fig. 1 when a magnetic field intensity H to be measured is changed in such a state that a constant current I is supplied thereto. Therefore, when the magnetic field intensity H is relatively low, the precision upon measuring a relatively low magnetic field intensity H is lowered since the variation ratio of the resistance to the magnetic field intensity is relatively small. Further, the SQUID (Superconductive Quantum Interference Device) has been used as a magnetic sensor. The SQUID has an extremely high sensitivity to measure a low magnetic field of about 10⁻¹⁴ T (10⁻¹⁰ gauss), and utilizes the Josephson device wherein an extremely thin insulation film is arranged between superconductive films. However, there are such disadvantages that it is difficult to make the Josephson device since it is made by using a high technique and it is necessary to amplify a voltage generated therein since the level of the output voltage is relatively low. Furthermore, a magnetic sensor utilizing a magneto-resistive device of superconductive ceramic films comprising grain boundaries has been proposed by the present inventors. In this sort of magnetic sensor, there is used either a superconductive ceramic film comprising grain boundaries in which either an insulation film where grain boundaries of a superconductor are extremely close to each other or a normal conductive film is arranged, or a superconductive ceramic film comprising grain boundaries which are supposed to form point type weak couplings. Even though a relatively low magnetic field is applied to the ceramic superconductor, some of the superconductive state of the above grain boundaries are broken, resulting in that the ceramic superconductor changes to normal conductive phase from the superconductive phase, and the ceramic superconductor has an electrical resistance. Fig. 2 shows a magneto-resistive characteristic of the above magnetic sensor utilizing the above ceramic superconductor having weak couplings between the grain boundaries, the ceramic superconductor being made by a conventional method for sintering powders of the materials thereof. An shown in Fig. 2, when a relatively low magnetic field is applied to the ceramic superconductor, some of the above weak couplings between the grain boundaries at the superconductive state are broken, and the resistance thereof increases steeply according to the increase of the magnetic field intensity. This implies that the magnetic sensor utilizing the ceramic superconductor has a high sensitivity even though a relatively low magnetic field is applied thereto. For further discussion of this type of magnetic sensor, please refer to Japanese Journal of Applied Physics, Vol. 27, No. 5, May 1988, pp. 746-750: "Galvano-magnetic Effect of an Y-Ba-Cu-O Ceramic Superconductor and its Application to Magnetic Sensors", H. Nojima et al. SUMMARY OF THE INVENTION An essential object of the present invention is to provide a superconductive magneto-resistive device having a high sensitivity for measuring a relatively low magnetic field. In order to accomplish the above object, according to the present invention, there is provided a superconductive magneto-resistive device which includes a superconductive ceramic element comprising a plurality of superconductive ceramic films stacked with interleaved electrical insulation films, each intermediate superconductive film being connected to its neighbouring said superconductive films in the stack at respective opposite end portions thereof so that said element forms a convoluted zig-zag current path between electrodes connected to the outermost said superconductive films. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and features of the present invention will become clear from the following description of a preferred embodiment thereof with reference to the accompanying drawings, in which: Fig. 1 is a graph showing a magnetic field detection characteristic of a conventional magnetic sensor utilizing a magneto-resistive device; Fig. 2 is a graph showing a magnetic field detection characteristic of a conventional magnetic sensor utilizing a superconductive magneto-resistive device which is made by a method for sintering powders of compounds of a superconductor so as to form a superconductive element to be used for a magnetic sensor; Fig. 3 is a schematic view showing a spray pyrolysis method for forming a superconductive ceramic film to be used for a magnetic sensor of a preferred embodiment according to the present invention; Figs. 4a to 4e are cross sectional views showing a process for making a superconductive magneto-resistive device having a lamination structure of the present preferred embodiment; Fig. 5 is a perspective view showing a magnetic sensor utilizing a superconductive magneto-resistive device having a lamination structure of the present preferred embodiment according to the present invention; Fig. 6 is a graph showing a magnetic field detection characteristic of the magnetic sensor shown in Fig. 5; Fig. 7 is a graph showing a magnetic field detection characteristic of a magnetic sensor of a comparative example; and Fig. 8 is a perspective view of a superconductive ceramic film of a modification which is shaped into a zigzag configuration. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment according to the present invention will be described below with reference to the attached drawings. Fig. 5 shows a magnetic sensor using a superconductive magneto-resistive film of a preferred embodiment according to the present invention. The superconductive magneto-resistive film of Y-Ba-Cu-O ceramic compounds is made by a spray pyrolysis method as shown in Fig. 3 as follows. In the present preferred embodiment, materials of Y(NO₃)₃·6H₂O, Ba(NO₃)₂ and Cu(NO₃)₂·3H₂O are used to form the above-mentioned superconductive magneto-resistive film, and materials of Y(NO₃)₃·6H₂O, Ba(NO₃)₂, Cu(NO₃)₂·3H₂O and Zn(NO₃)·6H₂O are used to form an electrical insulation film. Powders of Y(NO₃)₃·6H₂O, Ba(NO₃)₂ and Cu(NO₃)₂·3H₂O are weighed at a predetermined ratio. Each powder is sufficiently dissolved into pure water so as to prepare an aqueous solution 4 containing Y, Ba, Cu nitrates. The aqueous solution 4 made as described above is put into a container 3 of a spray gun 2, and is sprayed over a stabilized zirconia substrate 1 which is heated by a heating lamp 6, with use of the spray gun 2 using compressed air 5, with sufficient stirring so as to form a superconductive ceramic film on the substrate 1 in a well known decomposition and oxidation process of the aqueous solution 4. Similarly, an electrical insulation film is formed thereon by using powders of Y(NO₃)₃·6H₂O, Ba(NO₃)₂, Cu(NO₃)₂·3H₂O and Zn(NO₃)·6H₂O. Figs. 4a to 4e show a process for making superconductive films having a lamination structure to be used for a magnetic sensor. First of all, after powders of Y(NO₃)₃·6H₂O, Ba(NO₃)₂ and Cu(NO₃)₂·3H₂O are weighed so that a composition ratio of Y, Ba and Cu becomes 1 : 2 : 3. Each powder is sufficiently dissolved into pure water so as to prepare a first aqueous solution containing Y, Ba, Cu nitrates. The first aqueous solution made as described above is sprayed over a stabilized zirconia substrate 18 having a width of 5 mm and a length of 20 mm which is heated at a temperature in the range from 300°C to 600°C, and thereafter, it is heated at a temperature in the range from 900°C to 1000°C in air at 1.01 x10⁵ Nm⁻²(one atmospheric pressure) so as to form a superconductive ceramic film 20 of Y₁Ba₂Cu₃O y (6.8 ≦ y ≦ 7.0) on the substrate 18 as shown in Fig. 4a. After powders of Y(NO₃)₃·6H₂O, Ba(NO₃)₂, Cu(NO₃)₂·3H₂O and Zn(NO₃)·6H₂O are weighed so that a composition ratio of Y, Ba, Cu and Zn becomes 1 : 2 : 2.5 : 0.5, each powder is sufficiently dissolved into pure water so as to prepare a second aqueous solution containing Y, Ba, Cu, Zn nitrates. The second aqueous solution is sprayed over the top surface of the superconductive ceramic film 20 by the above spray pyrolysis method in such a state that the right and left side portions 20a and 20b of the film 20 are covered by mask films 51a and 51b, respectively, as shown in Fig. 4b, so as to form an electrical insulation film 21 of Y₁Ba₂Cu 2.5 Zn 0.5 O y (6.8 ≦ y ≦ 7.0) having an electrical resistance of several MΩ or more thereon except for the right and left side portions 20a and 20b of the film 20. After removing the mask films 51a and 51b, the first aqueous solution which is made as described above is sprayed over the top surface of the insulation film 21 and the top surface of the right side portion 20b of the superconductive ceramic film 20 by the above spray pyrolysis method in such a state that the left side portion 20a of the superconductive ceramic film 20 is covered by a mask film 52, as shown in Fig. 4c, so as to form a superconductive ceramic film 22 thereon except for the left side portion 20a of the film 20. Thereafter, the second aqueous solution which is made as described above is sprayed over the top surface of the superconductive ceramic film 22 by the above spray pyrolysis method in such a state that the left side portion 22a of the superconductive ceramic film 22 is covered by a mask film 53, as shown in Fig. 4d, so as to form an insulation film 23 thereon except for the left side portion 22a of the film 22. After removing the mask film 53, as shown in Fig. 4e, a superconductive ceramic film 24 is formed in the manner similar to that of the process for making the superconductive ceramic film 22 shown in Fig. 4c. Similarly, the process for forming the superconductive ceramic film and the insulation film is repeated so as to form the superconductive ceramic films having a lamination structure to be used for the magnetic sensor shown in Fig. 5. In the structure of the magnetic sensor, ten superconductive ceramic films 20, 22, 24, 26, 28, 30, 32, 34, 36 and 38 are laminated one by one, and nine electrical insulation films 21, 23, 25, 27, 29, 31, 33, 35 and 37 are formed between the adjacent superconductive ceramic films so as to insulate them except for one end portion of each. The insulation film for insulating the upper pair of the adjacent superconductive ceramic films is formed so as to extend in a direction opposite to that of the lower pair of the adjacent films, resulting in that a folded current path is formed in the magnetic sensor shown in Fig. 5. Furthermore, respective superconductive ceramic films 20, 22, 24, 26, 28, 30, 32, 34, 36 and 38 which have been made in the above process are electrically connected in series to each other. The lattice constant of each superconductive ceramic film is substantially same as that of each insulation film, and the thermal expansion coefficient of each superconductive ceramic film is substantially same as that of each insulation film. Therefore, in the magnetic sensor, any defect due to the mismatching between the lattice does not occur and any peel between the films does not occur, resulting in that there is obtained the superconductive ceramic films having a lamination structure of a good quality to be used for the magnetic sensor. On the lower-most and upper-most films 20 and 38, a pair of current electrodes 8 and 12 of Ti and a pair of voltage electrodes 9 and 13 of Ti are formed by the vapour deposition method, respectively. The pair of current electrodes 8 and 12 is connected to a constant-current regulated power source 16 for supplying a constant current I through lead wires 10 and 15 bonded thereon with use of Ag paste. The pair of voltage electrodes 9 and 13 is connected to a voltmeter 17 for measuring a voltage generated in the magnetic sensor through lead wires 11 and 14 bonded thereon with use of Ag paste. Fig. 6 is a graph showing the result obtained by measurement with use of the magnetic sensor having the ten laminated superconductive ceramic films shown in Fig. 5, which is immersed in liquid nitrogen so as to be cooled at 77K. The thickness of each superconductive ceramic film is 10 µm , and a current of 1 mA is supplied to the magnetic sensor. Fig. 7 is a graph showing the result obtained by measurement with use of a magnetic sensor of a comparative example having one superconductive ceramic film which is made in the manner similar to that of the magnetic sensor of the present preferred embodiment shown in Fig. 5. The thickness of the superconductive ceramic film of the magnetic sensor of the comparative example is 10 µm , and a current of 1 mA is supplied thereto. As is apparent from the comparison between Figs. 6 and 7, when a magnetic filed of 7958 A/m (100 Oe) is applied in parallel to the plane of the superconductive ceramic film or films of each magnetic sensor, in the magnetic sensor of the preferred embodiment, a voltage of about 14 mV is generated when a measuring current of 1 mA is supplied thereto, and then, the resistance thereof is 14 Ω. On the other hand, in the magnetic sensor of the comparative example, as shown in Fig. 7, a voltage of 1.4 mV is generated when a measuring current of 1 mA is supplied thereto, and then, the resistance thereof is 1.4 Ω. As described above, the magnetic sensor of the preferred embodiment according to the present invention generates ten times the voltage generated by the magnetic sensor of the comparative example having only one superconductive ceramic film, resulting in that a magnetic sensor having a high sensitivity is obtained. In the present preferred embodiment, the superconductive ceramic films of Y₁Ba₂Cu₃O y (6.8 ≦ y ≦ 7.0) and the electrical insulation films of Y₁Ba₂Cu 2.5 Zn 0.5 O y (6.8 ≦ y ≦ 7.0) are used. However, superconductive ceramic films of Y₁Ba₂Cu₃O y (6.5 ≦ y ≦ 6.8) and electrical insulation films of Y₁Ba₂Cu 2.5 Zn 0.5 O y (6.5 ≦ y ≦ 6.8) may be used. In this case, the threshold temperature of the superconductive ceramic films of Y₁Ba₂Cu₃O y (6.5 ≦ y ≦ 6.8) at which they change from the normal conductive phase to the superconductive phase is smaller than the threshold temperature of the superconductive ceramic films of Y₁Ba₂Cu₃O y (6.8 ≦ y ≦ 7.0). In the present preferred embodiment, the magnetic sensor having the superconductive ceramic films of Y-Ba-Cu-O compounds is described; however, the present invention is not so limited. Another magnetic sensor having the superconductive ceramic films of compounds such as Bi₂Sr₂Ca n-1 Cu n O 2n+4 (n = 1, 2, 3), Tℓ₂Ba₂Ca n-1 Cu n O 2n+4 (n = 1, 2, 3, 4, 5, 6), Tℓ₁Ba₂Ca n-1 Cu n O 2n+3 (n = 1, 2, 3, 4, 5, 6, 7), which comprise superconductive grain boundaries, can be applied to the present invention. In the present preferred embodiment, the spray pyrolysis method is used for forming a superconductive ceramic film; however, the present invention is not so limited. The other methods for forming a superconductive ceramic film such as a spattering method, a CVD method may be used. It is to be noted that a magnetic sensor comprising the superconductive ceramic films respectively having a smaller thickness has a higher sensitivity. However, when the thickness thereof is equal to or smaller than 0.1 µm , a constant magneto-resistive property of the superconductive ceramic film is not obtained because the grains thereof have grown irregularly due to the heating process after forming the superconductive film. Furthermore, when the thickness of the superconductive ceramic film is equal to or larger than 50 µm , the smoothness of the surface thereof and the sensitivity of the magnetic sensor are lowered because the grains of the superconductor have grown excessively. In the present preferred embodiment, the insulation film of Y₁Ba₂Cu 2.5 Zn 0.5 O y is used for insulating the adjacent superconductive ceramic films; however, the present invention is not so limited. The ratio of composition of elements constituting the ceramic superconductor may be changed so as to form an insulation superconductive ceramic film suitable for making the magnetic sensor comprising the superconductive ceramic films having a lamination structure. For example, as the superconductive films, a material of compounds such as A₁Ba₂Cu₃O y (6.5 ≦ y ≦ 7.0) or A₁Ba 2-x Sr x Cu₃O y (0 ≦ x ≦ 0.5, 6.5 ≦ y ≦ 7.0) may be used, wherein the above element A is either Lu, Yb, Tm, Er, Y, Ho, Dy, Gd, Eu, Sm or Nd. On the other hand, as the electrical insulation films, a material of compounds such as Q₁Ba₂Cu₃O y (6.5 ≦ y ≦ 7.0), Q₁Ba₂Cu 3-z M z O y (6.5 ≦ y ≦ 7.0, z ≧ 0.5), Q₁Ba 2-x Sr x Cu₃O y (x > 0.5, 6.5 ≦ y ≦ 7.0), Q₁Ba 2-x Sr x Cu 3-z M z O y (x > 0.5, 6.5 ≦ y ≦ 7.0, z ≧ 0.5), Y₁Ba₂Cu 3-z M z O y (6.5 ≦ y ≦ 7.0, z ≧ 0.5), Y₁Ba 2-x Sr x Cu₃O y (x > 0.5, 6.5 ≦ y ≦ 7.0), Y₁Ba 2-x Sr x Cu 3-z M z O y (x > 0.5, 6.5 ≦ y ≦ 7.0, z ≧ 0.5) may be used, wherein the above element Q is either Sc, Ce, Pr or Tb, and the above element M is either Fe, Co, Zn, Cr, Ga or Ni. The shape of the superconductive ceramic film according to the present invention is not limited to that of the present preferred embodiment shown in Fig. 5. For example, each superconductive ceramic film may be shaped into a zigzag configuration as shown in Fig. 8 by using a photolithographic method etc. and an etching method or a masked film deposition method. In this case, the number of the grain boundaries which are in contact in series can be further increased, resulting in that a magnetic sensor having a higher sensitivity can be obtained. As described above, there can be obtained the magnetic sensor utilizing the magneto-resistive device constituted by the superconductive ceramic films having a lamination structure according to the present invention, which has an extremely high sensitivity and acts at an extremely low consumption power. It is understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope of the present invention as defined by the appended claims.

Claims: A superconductive magneto-resistive device which includes a superconductive ceramic element comprising a plurality of superconductive ceramic films (20,22,24,26, 28,30,32,34,36,38) stacked with interleaved electrical insulation films (21,23,25,27,29,31,33,35,37), each intermediate superconductive film being connected to its neighbouring said superconductive films in the stack at respective opposite end portions thereof so that said element forms a convoluted zig-zag current path between electrodes (8,9;12,13) connected to the outermost said superconductive films (20;38). The superconductive magneto-resistive device as claimed in claim 1, wherein the thickness of each of said superconductive ceramic films is in the range from 0.1 µm to 50 µm. The superconductive magneto-resistive device as claimed in claim 1, wherein said electrical insulation film is made by adding one element to the composition of said superconductive ceramic film. The superconductive magneto-resistive device as claimed in claim 1, wherein said electrical insulation film is made by replacing at least one element of the composition of said superconductive ceramic film as a whole or partially with another element. The superconductive magneto-resistive device as claimed in claim 1, wherein said electrical insulation film is made by changing the ratio of the elements of the composition of said superconductive ceramic film. The superconductive magneto-resistive device as claimed in claim 3, wherein said superconductive ceramic film is made of Y₁Ba₂Cu₃O y , and said electrical insulation film is made of Y₁Ba₂Cu 2.5 Zn 0.5 O y . The superconductive magneto-resistive device as claimed in claim 1, wherein said superconductive ceramic film is made of a Bi-Sr-Ca-Cu-O compound. The superconductive magneto-resistive device as claimed in claim 1, wherein said superconductive ceramic film is made of a Tl-Ba-Ca-Cu-O compound.

Inventors: NAGATA MASAYA, NOJIMA HIDEO WAKAKUSA-HAITSU, OHNO EIZO - HEIJYO DAI- -DANCH, TSUCHIMOTO SHUHEI, NAGATA, MASAYA, NOJIMA, HIDEO 203, WAKAKUSA-HAITSU, OHNO, EIZO 37-305, HEIJYO DAI-2-DANCHI, TSUCHIMOTO, SHUHEI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1989
Main classification: H01L39
IPC codes: G01R33, H01L39, H01L43