Superconductive magneto-resistive device

Abstract: A superconductive magneto-resistive device for use in a sensor system for sensing an external magnetic field which is formed so as to have a predetermined pattern for a current path through which a supplied current flows. The pattern includes portions formed close and paralell to each other so that magnetic fields induced by respective currents flowing through the portions can be cancelled with each other.

Description: The present invention relates to a superconductive magneto-resistive device for a magnetic sensor. Conventionally, magnetic sensors which utilise the Hall or magneto-resistive effect in semiconductors or magnetic sensors which utilise magneto-resistive effects in magnetic materials are widely used for sensing or measuring magnetic fields. The former type of sensor has a sensitivity capable of sensing a magnetic field of about 10⁻² gausses and the latter about 10⁻³ gausses. However, these conventional magnetic sensors have various disadvantages. They have relatively large specific resistance R₀ even where there is no applied magnetic field. Each variation ratio of resistance to the magnetic field is represented by a parabolic curve having a small coefficient, as shown qualitatively in Fig. 1. Since the gain ΔR in the resistance is proportional to the square of the magnetic flux density B of an applied magnetic field, the gain in a weak applied magnetic field of, for example, several tens gausses is very small. The ratio of the gain ΔR to the specific resistance R₀ (ΔR/R₀) is according of the order of 1% at the most. On the contrary, a magnetic sensor using SQUID (Superconductive Quantum Interference Device) which utilizes the Josephson junction is known to have a very high sensitivity capable of sensing a very weak magnetic field of about 10⁻¹⁰ gauss. Structures of tunnel junction, point contact and micro bridge have been known as the Josephson junction. However, magnetic sensors of this type have quite delicate structures and require a complicated operation to be effected in use. They are not, therefore, practical for general use despite their very high sensitivity. In our copending European patent application no. 88307044.3 filed 29th July 1988 and as disclosed in the Technical Digest of the International Electron Devices Meeting, published on 6th December 1987, pp 867-868, IEEE, a superconductive magneto-resistive device is described which is comprised of a superconductive material having grain boundaries acting as weak couplings and means for utilizing a change in the resistance of the material caused when a magnetic field is applied thereto. As shown schematically in Fig. 2, the superconductive material is comprised of superconductive grains 1 and grain boundaries 2 bonding them. These random grain boundaries 2 are considered or supposed to form various weak couplings 3 including tunnel junctions, point contact junctions and micro bridge junctions, as shown by an equivalent network circuit of Fig. 3. In the superconductive phase thereof, individual Cooper pairs can pass freely through weak couplings 3 (Josephson junctions) and, therefore, the resistance becomes zero. When a magnetic field is applied to the superconductor, some of the Josephson junctions 3 are broken and, accordingly, the superconductor has an electric resistance. A Y-Ba-Cu-O ceramic superconductor can be used as a superconductor having grain boundaries. It has a critical temperature of about 90 K. Fig. 4 shows an example of a magnetic sensor system disclosed in the above identified application. In this system, an elongated rectangular device 4 (1 x 7 x 0.7mm³) made of a Y-Ba-Cu-O ceramic superconductive material is prepared and immersed in liquid nitrogen (77 K). A power supply 9 supplies a current through a pair of electrodes 5 and 6 formed on respective ends of the device. The voltage between two electrodes 7 and 8 is measured to detect the change in the resistance of the device on application of a magnetic field B. Fig. 5 shows the result obtained. The resistance of the device 4 changes according to the strength I of the applied current and that of the applied magnetic field B. One of the merits of this system is that the specific resistance of the device is zero in the superconductive phase and another merit is that the change in the resistance of the device is very steep, and, it is therefore, highly sensitive to the applied magnetic field. However, in this system, because of its sensitivity, the magnetic sensor senses a magnetic field induced by the current flowing through the device. In order to avoid this problem, it is desirable to form the device linearly, as shown in Fig. 7. But, such a linear device induces a magnetic field proportional to its length which causes an error in the measurement of the strength of an external magnetic field. It is an aim of the present invention to alleviate at least in part some of the aforementioned problems. One aim of the present invention is to provide a superconductive magneto-resistive device for a magnetic sensor system having a structure in which magnetic fields induced by current flowing through the device do not affect the measurement of external magnetic fields. Another aim of the present invention is to provide a magnetic sensor capable of detecting a magnetic field having a one-dimensional or two-dimensional distribution. In accordance with the present invention, there is provided a magneto-resistive device for a sensor system for sensing an external magnetic field by detecting the change in resistance of a superconductive current-carrying element maintained substantially at its critical temperature, said device including said superconductive element characterised in that said element comprises a plurality of portions which are formed close and parallel to each other and are in series so as to form a path for carrying a current which in use of the device is caused to flow through said portions in respective opposite directions, thereby to provide cancelling of magnetic fields induced by said current in said portions. According to the present invention, there is also provided a sensor system for sensing an external magnetic field comprising one or more superconductive magneto-resistive devices as defined above, supply means for supplying a constant current to each of said devices, means for cooling said devices at a temperature close to the critical temperature of a superconductive material forming each device, and detection means for detecting a change in the resistance of each device. In a preferred embodiment of the invention the superconductive current-carrying elements are formed on a plane, preferably by deposition onto a substrate. Another embodiment of the invention comprises elements formed in a layer structure. Some embodiments of the invention will now be described by way of example with reference to the accompanying drawings as follows: Fig. 1 is a graph showing the magneto-resistive property of a conventional magnetic sensor; Fig. 2 is an enlarged schematical cross-sectional view of a ceramic superconductor for forming a superconductive magneto-resistive device according to the present invention; Fig. 3 is an equivalent circuit for showing a network of weak couplings formed in the ceramic superconductor shown in Fig. 2; Fig. 4 is a perspective view showing a magnetic sensor disclosed in the copending application of US serial no. 226,067; Fig. 5 is a graph showing the magneto-resistive property of the magnetic sensor shown in Fig. 4; Fig. 6 is a perspective view showing a magnetic sensor system according to the present invention; Fig. 7 is an enlarged perspective view of the superconductive magneto-resistive device used in the magnetic sensor system shown in Fig. 6; Fig. 8 and Fig. 9 are plan views showing other examples of the superconductive magneto-resistive device, respectively; Fig. 10 is a perspective view of a superconductive magneto-resistive device having a layered structure; Fig. 11 is an enlarged cross-sectional view of another superconductive magneto-resistive device according to the present invention; Fig. 12 is a graph showing the magneto-resistive property of the device shown in Fig. 11; Figs. 13(I) and 13(II) are schematic plan views showing array structures of plural superconductive magneto-resistive devices for detecting a pattern of an external magnetic field, respectively; Fig. 14 is a schematic perspective view for showing a three-dimensional array of plural superconductive magneto-resistive devices for detecting the strength and the direction of a magnetic field; Fig. 15 is a side view of a magnetic sensor having a cooling means according to the present invention; Fig. 16 is a front view of a magnetic sensor suitable for detecting an orientation of an external magnetic field according to the present invention; Fig. 17 is a front view of a superconductive magneto-resistive device used in the magnetic sensor shown in Fig. 16; and Figs. 18(a), 18(b) and 18(c) are explanative views for showing the action of the magnetic sensor shown in Fig. 16. Fig. 6 shows a magnetic sensor system according to the present invention. The magnetic sensor system includes a superconductive magneto-resistive device 11 housed in a package 12 made of a non-magnetic material, a cooling apparatus 13 for cooling the device 11 with use of high pressurized N₂ gas so as to keep it in the superconductive state, a circuit 14 for generating a constant current for application to the device 11, a detection circuit 15 for detecting a voltage generated in the device 11 and a microcomputer 16 for controlling the constant current generation circuit 14 and processing data outputted from the voltage detection circuit 15. The processed data is displayed on a display 17. When an external magnetic field B is applied to the device 11 in a direction indicated by an arrow B, the detection circuit 15 measures the voltage generated in the device 11 and the measured voltage is processed by the microcomputer 16 in order to give the strength of the applied magnetic field. Fig. 7 shows the superconductive magneto-resistive device 11. The device 11 is comprised of a substrate 21 made of alumina (Aℓ₂O₃) and a superconductive magneto-resistive element 22 formed on the substrate 21. The element 22 is formed on the substrate 21 as a thin film of a Y-Ba-Cu-O ceramic superconductor with use of the sputtering method. This element 22 has two linear portions 22a and 22b extending parallel to each other with a small distance which are connected by a connection portion 22c at respective ends thereof. On respective free ends of two linear portions 22a and 22b, electrodes 23a and 23b for supplying a current to the device 11 are formed by depositing Ti and lead lines 24a and 24b are connected to the electrodes 23a and 23b in order to apply the constant current from the constant current supplying circuit 14 to the device 11. Further, a pair of electrodes 25a and 25b for measuring a voltage generated in the device 11 are formed on portions of the device 11 near the current electrodes 23a and 23b. Two lead lines 26a and 26b from the voltage detection circuit 15 are connected to these electrodes 25a and 25b, respectively. When a constant current I is applied to the current electrode 23a, it flows through the first linear portion 22a in a direction indicated by an arrow L and, then, returns through the second linear portion 22b to the current electrode 23b, via the connecting portion 22c, in a direction indicated by an arrow M. Since the distance between the first and second linear portions 22a and 22b is very small, magnetic fields induced by the currents flowing oppositely through the first and second linear portions 22a and 22b cancel with each other. Therefore, the external magnetic field B to be detected is not affected by these magnetic fields induced along the first and second linear portions 22a and 22b. Thus, the device 11 can detect the strength of the external magnetic field B exactly. The superconductive element 22 is made with use of a superconductive material of a Y-Ba-Cu-O oxide having a critical temperature of 90 to 100 K. This material is deposited on the substrate of Aℓ₂O₃ by sputtering in order to form a thin film of the thickness of about 10 µm. This film is heated up to 900 °C in the air and, then, cooled gradually. The obtained component thereof is Y₁Ba₂Cu₃O 7-x (0 < x < 1). The firm is processed by etching to form the element 22 on the substrate 21. This film for the device can be made by various methods such as vacuum evaporation method, CVD method, spray method for spraying a solvent of components of the superconductive material and the like. A substrate made of silicon or Ba₂TiO₄ is usable for the substrate of the device 11. The sensitivity of the superconductive magneto-resistive device is considered to be determined by a radius of grains included therein and the state of grain boundaries. The ceramic superconductive material can be also made by sintering as follows. Powders of Y₂O₃, BaCO₃ and CuO are weighed at a predetermined ratio in order to obtain a component of Y₁Ba₂Cu₃O 7-x ( 0 < x < 1 ) After grinding and mixing these powders, samples formed with the mixture are calcined at 900 °C for 5 hours in air. Then, the samples are crushed and ground into powder comprised of micro particles having a diameter equal to or smaller than 1 µm. Then, the powder is cold-pressed into samples. Finally, these samples are sintered at 1000 °C for 3 hours in air. The sensitivity of the superconductive magneto-resistive device made by sintering as mentioned above is greatly dependent on the radius of crushed micro particles. On the contrary to the sintering method, the diameter of grains forming the superconductive film made by the deposition method is substantially determined by the temperature of the substrate upon depositing the film thereon. In the preferred embodiment, the ceramic superconductive film is formed by sputtering the material on the substrate while keeping it at a temperature of 300 to 400 °C. The deposited film is sintered at 950 °C in air and, thereafter, cooled gradually. The pattern of the device can be formed by irradiating a laser beam, an electron beam or an ion beam onto portions of the film except for the pattern in order to change those into a normal conductive state. Fig. 8 and Fig. 9 show desirable patterns for the device. The pattern shown in Fig. 8 has successive four basic patterns shown in Fig. 7 which are formed parallel. This pattern has a length of current path of four times of that of the basic pattern. Accordingly, an output voltage of four times can be obtained in this pattern with the same current. The number of basic patterns can be changed arbitrarily. The pattern shown in Fig. 9 is formed to have five parallelized portions connected one after another. This pattern has a current path of about five times of that of the basic pattern. The pattern having a structure for cancelling magnetic fields induced by respective linear portions can be realized not only by a plane pattern but also by a stacked or layered structure. Fig. 10 shows an example of such a stacked structure. In this example, the device 31 is comprised of first and second elements 32 and 33 and an insulation film 34 inserted inbetween them. Each of the first and second elements 32 and 33 has a linear pattern of a superconductive magneto-resistive material formed on each of substrates 35 and 36, as indicated by a dotted line in Fig. 10. The patterns of the first and second elements 32 and 33 are formed identical with each other. Each one end of these patterns of the first and second elements 32 and 33 are electrically connected with each other by a through hole 37 formed on the insulation film 34. On the other ends of these patterns, electrodes 38 and 39 are, respectively, formed for supplying a constant current from the constant current circuit 14. The electrode 38 is drawn out through a through hole 40 to the upper surface of the first substrate 35. In this structure, the direction of the current flowing through the device is reversed between the pattern of the first element 32 and that of the second element 33 and, therefore, respective magnetic fields induced along the current paths of the first and second elements 32 and 33 are perfectly cancelled with each other. Fig. 11 shows another example of the device 50 having a layered structure. In this structure, six layers from 51 to 56 of a superconductive magneto-resistive material are deposited one by one and, between adjacent layers, an insulation layer 57 is formed so as to insulate them except for one end portion of them. The insulation layer 57 for insulating the upper pair of the adjacent layers is formed so as to extend in a direction opposite to that of the lower pair of the adjacent layers and, thus, a folded current path is formed in the device 50. On the lower-most and upper-most layers 51 and 56, a pair of electrodes 58a and 58b for supplying a constant current I from the current source 14 and a pair of electrodes 59a and 59b for measuring a voltage generated in the device 50 by the detection circuit such as a potentiometer 15 are formed, respectively. Since the current is reversed in the direction thereof between the adjacent layers, magnetic fields induced by respective currents flowing through the adjacent layers are perfectly cancelled with each other. This structure is extremely advantageous in that the output voltage or the resistance to be measured is independent from the strength of the current to be applied to the device because no internal magnetic field is generated in the device 50 and, accordingly, the resistance of the device is determined only by the external magnetic field applied thereto. Fig. 12 is a graph showing the result obtained by measurement with use of the device 50 having the structure shown in Fig. 11. The magneto-resistive characteristic obtained when the current of 0.1 mA is supplied substantially coincides with that obtained when the current of 0.01 mA is supplied. Figs. 13(I) and 13(II) show cue dimensional magnetic array sensor and two dimensional magnetic array sensor, respectively. In the one dimensional magnetic array sensor, a plurality of superconductive magneto-resistive devices from 61-1 to 61-n are connected parallel to each other between lines 62 and 63 connected to a power source. To each of the devices, a resistance 64 is connected serially and each output terminal 65 is drawn out from a portion between each device and each resistance. When a magnetic field having one dimensional pattern is applied to the sensor, the pattern is detected based upon data outputted from individual devices 61-1 to 61-n of the sensor. The device having such a pattern as shown in either of Figs. 7 to 11 is used for each device of the sensor. In the two dimensional magnetic array sensor shown in Fig. 13(II), a plurality of superconductive magneto-resistive devices are arranged in a matrix form. In this case, a two dimensional magnetic pattern can be detected based on data outputted from individual devices. In Fig. 14, a three dimensional magnetic sensor is disclosed. In this three dimensional magnetic sensor, three superconductive magneto-resistive device 91, 92 and 93 each as herein before described are arranged along three orthogonal coordinate axes X, Y and Z, respectively. When an external magnetic field H M is applied to the magnetic sensor, the direction and the strength thereof can be calculated based on respective output data from individual devices 91, 92 and 93. Fig. 15 shows a cooling apparatus for the device 11 utilizing Peltier effect which is formed as a cascade structure of two stages with use of Peltier effect devices. In Fig. 15, reference numeral 41 denotes a neat radiation metal plate, reference numeral 42 denotes a cooling metal plate, reference numeral 43 denotes an insulator, reference numerals 44a and 44b denote p-type and n-type semiconductor devices, respectively, and reference numeral 45 denotes a heat radiation substrate. Fig. 16 shows another example of a magnetic sensor 101. This sensor 101 is comprised of two rod-like elements 102 and 102′ made of a material having a high permeability and a superconductive magneto-resistive device 103 inserted between two elements 102 and 102′. As shown in Fig. 17, the device 103 is comprised of a substrate 105 and a folded linear pattern element 106 of a superconductive magneto-resistive material deposited thereon. A constant current is applied from a pair of electrodes 107a and 107b formed on respective end portions of the pattern element 106. When the magnetic sensor 101 is directed parallel to the magnetic flux of a magnetic field as shown in Fig. 18(a), the magnetic flux is converged into rod-like element 102 or 102′ and, therefore, a strong magnetic field is applied to the device 103 to generate a high resistance in the device. On the contrary, if the magnetic sensor 101 is inclined to the magnetic flux of a magnetic field by an angle  as shown in Fig. 18(b), the magnetic flux is not converged so much into the rod-like element 102 and 102′. Therefore, a magnetic field applied to the device 103 becomes considerably weak. And, if the magnetic sensor 101 is directed perpendicularly to the magnetic flux of a magnetic field, as shown in Fig. 18(c), all of the magnetic flux pass freely through each of the rod-like elements 102 and 102′ and, therefore, none of the magnetic field is applied to the device 103. Thus the direction of a magnetic field relative to the magnetic sensor can be detected based on data outputted from the device 103. 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 and spirit of the present invention.

Claims: A magneto-resistive device (11; 31; 50) for a sensor system for sensing an external magnetic field by detecting the change in resistance of a superconductive current-carrying element (22; 32, 33) maintained substantially at its critical temperature, said device including said superconductive element (22; 32, 33), characterised in that said element comprises a plurality of portions (22a, 22b; 51, ... 56) which are formed close and parallel to each other and are in series so as to form a path for carrying a current which in use of the device (11; 31; 50) is caused to flow through adjacent ones of said portions (22a, 22b; 51, ... 56) in respective opposite directions, thereby to provide cancelling of magnetic fields induced by said current in said portions (22a, 22b; 51, ..., 56). A superconductive magneto-resistive device (11; 31) as claimed in claim 1, in which said element (22; 32, 33) is formed in a plane. A superconductive magneto-resistive device (11; 31) as claimed in claim 1 or claim 2, in which said element (22; 32, 33) is formed on a substrate (21; 35, 36) by deposition. A superconductive magneto-resistive device (50) as claimed in claim 1, in which said element is formed by a layered structure (32, 33, 34; 51,..., 56) when seen in the cross-section thereof. A sensor system for sensing an external magnetic field comprising one or more superconductive magneto-resistive devices (11; 31; 50) as claimed in any preceding claim, and supply means for supplying a current to each of said devices (11; 31, 50), means for maintaining said devices (11; 31, 50) at a temperature close to the critical temperature of the superconductive elements forming each device (11; 31; 50), and detection means for detecting a change in the resistance of each device (11; 31; 50). A sensor system as claimed in claim 5 comprising a plurality of devices arranged so as to form a predetermined pattern. A sensor system as claimed in claim 6, in which said pattern is a one-dimensional array (61,1...61,n) A sensor system as claimed in claim 6, in which said pattern is a two-dimensional array (61,1...61,''n) A sensor system as claimed in claim 6, in which said pattern is comprised of a three-dimensional array. An apparatus for sensing an external magnetic field characterised in that the apparatus comprises an array of devices according to any of claims 1-4 comprising respective superconductive magneto-resistive elements (22; 32, 33; 61-1...n; 71-1...m, 81-1...n) in which the changes of resistance due to the external field can be individually detected while the array is maintained substantially at the critical temperature of said elements (22; 32, 33; 61-1...n; 71-1...m, 81-1...n).

Inventors: KATAOKA SHOEI, KITA RYUSUKE, NOJIMA HIDEO, SAITOH SUSUMU, TSUCHIMOTO SHUHEI, KATAOKA, SHOEI, KITA, RYUSUKE, NOJIMA, HIDEO, SAITOH, SUSUMU, TSUCHIMOTO, SHUHEI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1988
Main classification: G01R33
IPC codes: G01R33