METHOD AND DEVICE FOR SENSING A MAGNETIC FIELD WITH USE OF A MAGNETO-RESISTIVE PROPERTY OF A SUPERCONDUCTIVE MATERIAL

Abstract: There is disclosed a method and device for sensing or responding to a magnetic field using a magneto-resistive property of a superconductive material. A magnetic field (56) is applied to a superconductive material (51) having grain boundaries and the magnetic field produces a change in the resistance of said superconductive material which change is accompanied by a phase transition of said superconductive material from the superconductive phase to the normal conductive phase. This basic technique is employed in the different embodiments of the invention in a variety of different applications. For example in a disclosed position sensor a permanent magnet is arranged to move relative to a piece of the superconductive material and the resulting variation in the magnetic flux causes the resistance change. Other embodiments include one and two dimensional potentiometers, a pick-up head for a record player, a sensor for rotational displacement, a rotary encoder, a microwave controller etc.

Description: The present invention relates to a method and device for sensing a magnetic field using a magneto-resistive property of a superconductive material having grain boundaries therein. Conventionally, a magnetic sensor which utilises the Hall effect or magneto-resistive effect in a semiconductor or a magnetic sensor which utilizes the magneto-resistive effect in a magnetic field is widely used for sensing or measuring a magnetic field. The former sensor has a sensitivity capable of sensing a magnetic field of about 10⁻² gauss and the latter one has a sensitivity of about 10⁻³ gauss. However, these conventional magnetic sensors have various disadvantages as follows. They have relatively large specific resistances R₀ even when no magnetic field is applied to them. Each variation ratio of resistance to the magnetic field is represented by a parabolic curve having a small coefficient, as shown in Fig. 45 qualitatively. Since a gain ΔR in the resistance is increased proportional to the square of the magnetic flux density B of an applied magnetic field, the gain for a weak magnetic field of, for example, several tens gauss is very small and, therefore, a ratio of the gain ΔR to the proper resistance R₀ (ΔR/R₀) is of the order of 1% at the most. On the contrary, a magnetic sensor using the SQUID (Superconductive Quantum Interference Device), which utilizes the Josephson junction, is known to have a very high sensitivity, being 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, the magnetic sensor of this type has a quite delicate structure in manufacturing and requires complicated operation in its use. Therefore, it is not practical for general use although it has a very high sensitivity. One magnetic sensor which uses a superconductor element is known from JP-A-55-37978. The present invention aims to provide a superconductive magneto-resistive device which is sensitive to a weak magnetic field. Another aim of the present invention is to utilize the magneto-resistive property of a superconductive material having grain boundaries. One more aim of the present invention is to provide a practical method and system for sensing a magnetic field with a very high sensitivity by utilizing a magneto-resistive property of a superconductive material having random grain boundaries. A further aim of the present invention is to provide a practical magnetic sensor which is easy to manufacture and handle. One further aim of the present invention is to provide a method and system for controlling the magneto-resistivity of the superconductive magneto-resistive device. In accordance with a first aspect of the invention, there is provided a superconductor device comprising: an element made of a superconductive material which exhibits a magneto-resistive property; means for applying an electric current to said element; and means for detecting a change in the resistance of the element, characterized by: said element being made of a ceramic material having random grain boundaries which act as weak couplings; magnet means for applying a magnetic field to said element, which magnet means is movable so as to change the magnetic field applied to the element; and said change in resistance being dependent upon the position of said magnet means. According to a second aspect, the invention provides a superconductor device comprising: waveguide means; an element made of a superconductive ceramic material having random grain boundaries which act as weak couplings, the material exhibiting a magneto-resistive property; and means for applying a magnetic field to said element to change the resistance thereof, wherein said element is disposed within said waveguide means so that any resistance of the element attenuates a wave passing through the waveguide means. According to a third aspect, the invention provides a method of sensing a magnetic field or the position of a magnetic field applying means, comprising the steps of: applying an electric current to an element made of a superconductive material which exhibits a magneto-resistive property; applying a magnetic field to said element; and detecting a change in the resistance of the element, characterized in that: said element is made of a ceramic material having random grain boundaries which act as weak couplings; and by the step of: making said current zero from time to time in order to overcome hysteresis in said magneto-resistive property. The superconductive magneto-resistive device can be applied to various devices such as magnetic sensor, wave guide, magnetic head for reading magnetic data recorded on a recording medium, one-dimensional, two-dimensional or rotational position sensor and the like. The sensitivity of the device to an external field can be controlled easily by controlling the strength of the magnetic field applied to the superconductive material. The sensitivity of the device to an external field can also be easily controlled by varying the strength of the electric current applied to the superconductive material. In the present invention, there is used a superconductive material such as a ceramic superconductive material which has random grain boundaries 2 among grains 1, as shown schematically in Fig. 1 in an enlarged scale. The inventors of the present invention discovered a magneto-resistive property of a superconductor having random grain boundaries, as shown in Fig. 1 schematically. Namely, as shown in Fig. 2, the resistance of the superconductor of this type is kept at zero in the superconductive phase until the flux density B of a magnetic field applied thereto is increased up to a threshold value Bth, but it increases very steeply when the flux density B is increased over the threshold value Bth. This implies that the ratio of ΔR to R₀ is infinitely large since the specific resistance R₀ of the superconductor is substantially zero. This phenomenon can be understood by assuming that individual grain boundaries act as weak couplings in the superconductor of this type. Namely, a ceramic superconductor is a crystal having an orthorhombic structure of single phase according to a macroscopic analysis using X-ray diffraction and, when examined with an electron microscope, is a crystal being composed of superconductive grains and grain boundaries bonding them. These grain boundaries are considered to be formed randomly with an insulative material or an electrically resistant material. This microscopic structure suggests that those random grain boundaries form various weak couplings including tunnel junction type, point contact type and micro bridge type. If this presumption is correct, a network including a lot of Josephson junctions 3 of various types is considered to be formed in the ceramic superconductor, as shown in Fig. 3. In the superconductive phase thereof, individual Cooper pairs of electrons can pass freely through the Josephson junctions 3 (weak couplings) and, therefore, the resistance becomes zero. When a magnetic field is applied to the superconductor, some of the Josephson junctions 3 are broken thereby and, therefore, the superconductor acquires an electrical resistance. The resistance thereof is increased as the number of broken Josephson junctions increases with increase of the magnetic field applied. It is to be noted that weak couplings are formed by grain boundaries themselves in the superconductor of this type. This gives a great merit when compared with known weak couplings made artificially. The present invention will become more apparent when the preferred embodiments are described below in detail with reference of accompanied drawings, in which: Fig. 1 is an enlarged schematical cross-sectional view of a ceramic superconductor; Fig. 2 is a graph showing the magneto-resistive property of a ceramic superconductor of a type suitable for use in a device according to the present invention; Fig. 3 is a schematic model of the ceramic superconductor for showing a network of weak couplings assumed to be formed therein; Fig. 4 is a graph showing the resistance property of the ceramic superconductor as a function of the absolute temperature; Fig. 5 is a perspective view of a first superconductor device; Fig. 6 is a graph showing the magneto-resistive property obtained from the device of Fig. 5; Fig. 7 is a perspective view showing a second superconductor device; Fig. 8 is a graph showing the magneto-resistive property obtained by varying the thickness of the device of Fig. 7; Fig. 9 is a graph showing the magneto-resistive property obtained by varying the current to be applied to the device shown in Fig. 7; Fig. 10 is a graph showing the dependency of the threshold magnetic flux density Bth on the strength of current applied to the device shown in Fig. 7; Fig. 11 is a schematic plan view of a variation of the first or second devices for adjusting the magneto-resistive property of the device; Fig. 12 is a graph showing the change in the magneto-resistive property obtained by varying the value of resistance connected parallel to the device in the variation shown in Fig. 11; Fig. 13 is a schematic perspective view of a position sensor as one preferred embodiment of the present invention; Fig. 14 is a graph showing changes in the resistance of the device measured by varying the distance d in the vertical direction and the position ℓ in the horizontal direction in the first preferred embodiment of the present invention; Fig. 15 is a schematic perspective view of another one-dimensional position sensor according to a preferred embodiment of the present invention; Fig. 16 is a graph showing a relation between the output voltage Vout and the position ℓ in the horizontal direction; Fig. 17 is a schematic plan view showing a two-dimensional position sensor as another preferred embodiment of the present invention; Fig. 18 is a schematic plan view of a circuit desirable for the two-dimensional position sensor shown in Fig. 17; Figs. 19(a) and 19(b) are examples of the device intended to increase the resistance of unit length, respectively; Fig. 20 shows an application of the two-dimensional position sensor to a pick up head for a record player; Fig. 21 is a schematic plan view showing a potentiometer for detecting a rotational displacement as a variation of the two-dimensional position sensor; Figs. 22(a) and 22(b) show one half of another potentiometer for detecting a rotational displacement according to the present invention, wherein Fig. 22(a) is a vertical cross-sectional view thereof and Fig. 22(b) is a cross-sectional view along the line X - X of Fig. 22(a); Figs. 23(a) and 23(b) show another half of said another potentiometer, wherein Fig. 23(a) is a vertical cross-sectional view thereof and Fig. 23(b) is a cross-sectional view along the line X - X of Fig. 23(a); Fig. 24 is a schematic front view showing an application of the device to a variable resistance or a displacement transformer; Fig. 25 is a graph showing a relation between the resistance of the device and the distance d from the magnet to the device in Fig. 24; Fig. 26 is a schematic plan view showing an application of the device to a rotary encoder; Fig. 27 shows an output voltage Vout obtained according to the rotation of the gear 262 shown in Fig. 26; Fig. 28 is a schematic perspective view showing another preferred embodiment of the present invention which provides means for applying a variable magnetic field to the device; Fig. 29 is a schematic perspective view showing an electro-magnetic relay as a variation of the embodiment shown in Fig. 28; Fig. 30 is a schematic front view showing another electro-magnetic relay as another variation of the embodiment shown in Fig. 28; Fig. 31 is a schematic cross-sectional view showing a further preferred embodiment of the present invention as an application to a microwave guide; Fig. 32 is a graph showing a relation between the magnetic flux density and the intensity of microwave measured when the former is varied; Figs 33(a) and 33(b) show another example of the application to the microwave guide; Fig. 34 is a schematic perspective view of another preferred embodiment of the present invention; Fig. 35 is a schematic perspective view showing a variation of the embodiment of Fig. 34; Fig. 36 is a graph showing the way for determining the strength of the biasing magnetic field in the composition shown in Fig. 35; Fig. 37 is a plan view showing an example for increasing the resistance of unit length of the device; Fig. 38 is a graph showing the hysteresis in the magneto-resistive property of a superconductive element which can be used in the present invention; Fig. 39 shows various desirable waveforms of the current to be applied to the element; Fig. 40 is a graph showing the magneto-resistive property of another superconductor suitable for use in the present invention; Fig. 41 is a graph showing the resistance of said another superconductor as a function of the absolute temperature; Fig. 42 is an explanatory view showing one manufacturing method for a superconductor device according to the present invention; Fig. 43 is a graph showing the magneto-resistive property of the device manufactured by the method shown in Fig. 42; Fig. 44 is a graph showing an X-ray diffraction pattern of the device manufactured by the method shown in Fig. 42; and Fig. 45 is a graph showing the magneto-resistive property of a conventional magnetic sensor. As a superconductor having grain boundaries, a Y-Ba-Cu-O ceramic superconductor can be used. This ceramic superconductor is produced as follows. Powders of Y₂O₃, BaCo₃ and CuO are weighed at a predetermined ratio in order to obtain a component of Y₁Ba₂Cu₃O 7-x . 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 composed of micro particles having a diameter equal to or smaller than 1 µm. Then, the powder is cold-pressed into disc-like pellets of about 9 mm diameter and 1 mm thick with a press force of 1 ton/cm². Finally, these pellets are sintered at 1,000°C for 3 hours in air and, then, the temperature is gradually lowered to 200°C for 5 hours. The superconductor thus produced has an orthothrombic crystal system having a composition of Y₁Ba₂Cu₃O 7-x when examined macroscopically with use of X-ray diffraction and, when examined microscopically with use of an electron microscope, is composed of superconductive grains having grain boundaries which can be deemed as weak couplings formed among superconductive grains. As shown in Fig. 4, the electrical resistance of the superconductor begins to lower steeply from 97K and becomes zero at 83K. As shown in Fig. 5, a thin rectangular device 51 of (1 x 7 x 0.7mm) is prepared by cutting the pellet. Electrodes 52 and 53 are formed at each end of the device 51 by evaporating Ti with use of electron beam at room temperature and at a pressure of about 10⁻⁶ Torr. Lead lines 54 and 55 are fixed onto Ti electrodes 52 and 53 respectively by Ag paste. This structure of electrode is very advantageous since Ti shows a tight contact with the superconductor and an excellent ohmic property. The device 51 is immersed in liquid nitrogen (77K) and a constant electric current is supplied, via the lead lines 54, 55 and Ti electrodes 52, 53, by a stabilized power source (not shown) in order to measure the electrical resistance of the device 51. The measurement thereof is done by measuring a voltage generated between the two Ti electrodes 52 and 53. When no magnetic field is applied to the device 51, the device 51 has no resistance. On the contrary, when a magnetic field 56 is applied to the device 51, while supplying the constant current thereto, the resistance appears suddenly. As shown in Fig. 6, the resistance appears when the flux density B of magnetic field is raised up to about 10 gauss while supplying the constant current of 50 mA, and increases steeply as the magnetic flux density B increases. When the current is 10 mA, the resistance appears at a flux density B of about 100 gauss. Further, when the current is 100 mA, it appears at a flux density B of about 5 gauss. The device shows a stable reappearance regarding this property and the latter is not dependent on a direction of application of the magnetic field. In this specification, the device is referred to as the superconductive magneto-resistive device. This excellent superconductive magneto-resistive property suggests various applicability for electronic devices. One important application is a magnetic sensor since the device has an excellent sensitivity to magnetic field. Further, the sensitivity of the device is isotropic because of the randomness of grain boundaries. This property enhances merits as a magnetic sensor since it is impossible to obtain in a conventional magnetic sensor utilizing a magneto-resistive property of a semiconductor or the SQUID. Further, the device shows both properties as a digital device and an analogue device since it is critical to the magnetic field and has such a magneto-resistive property that the resistance increases as the intensity of the magnetic field increases. This is also an advantageous feature which is impossible to obtain in a conventional magnetic sensor as mentioned above. This property suggests an application for a magnetic head for picking up a digital or analogue magnetic signal. In order to measure the magnetic field more exactly, it is desirable to form electrodes for supplying a constant current and electrodes for measuring a voltage separately, as shown in Fig. 7. Namely, a pair of current electrodes 72 connected to a power source 77 is formed at respective end portions of the device and a pair of voltage electrodes 73 is formed inside of respective current electrodes 72. The pair of voltage electrodes 73 is connected to a potentiometer 78 for measuring a voltage caused between the pair of voltage electrodes 73. Conditions for producing the ceramic superconductive material give various influences to the superconductive magneto-resistive property thereof. For example, the filling density of the ceramic superconductive material has a great influence on the magneto-resistive property since it relates to formation of grain boundaries. The filling density can be changed by varying a press force for forming green pellets. Also, the average diameter of micro particles in a ceramic powder to be sintered is related to the formation of grain boundaries. Further, the roughness of surface of the device has a great influence on the magneto-resistive property thereof since it relates to the penetration depth of the magnetic field applied thereto. In this context, the thickness of the device also relates to the penetration depth of the magnetic field. For example, when the filling density is reduced lower than 90 % by reducing the press force to 0.5 ton/cm² and the roughness of surface of the device is increased more than 1 µm, the sensitivity to the magnetic field is observed to increase considerably. It is also increased by increasing continuous porosities formed therein while decreasing the filling density lower than 85 % or by forming micro porosities therein with use of laser beam. Fig. 8 shows the dependency of the sensitivity on the thickness of device which is measured at 77K. As shown clearly therein, the device formed to have the thickness of 600 µm shows an excellent sensitivity when compared with that of the device of 1 mm thickness. These devices are formed by cutting the disc-like pellet produced under conditions the same as those mentioned above. Fig. 9 shows the magneto-resistive property measured at 77K when the grading of a ceramic powder to be sintered is increased up to 2 to 5 µm while keeping other production conditions the same as those mentioned above. As is clear from comparison of Fig. 9 with Fig. 6, the sensitivity to the magnetic field is amazingly increased. Further, the dependency on the current applied to the device is greatly improved. This suggests that the sensitivity to the magnetic field can be controlled by varying the current to be applied thereto. Fig. 10 shows the dependency of the threshold magnetic flux density Bth on the applied current I. Bth is defined as a threshold value from which the resistance appears. Bth is 30 gauss at 1 mA, 5.5 gauss at mA and is reduced to 0.2 gauss at 10 mA. Reasons why such a great difference is caused between to devices are considered as follows. Since the grading of the starting ceramic material is different from each other though other conditions are the same, coupling states among superconductive grains, namely, weak couplings are formed different from each other. Weak couplings formed in the latter device having a larger grading are considered to be more sensitive to the magnetic field than those of the former device. As is apparent from the above, it is rather difficult to obtain devices having a uniform magneto-resistive property since there is a high sensitivity to production conditions such as the grading of the starting material and/or dimensions thereof. Fig. 11 shows a resolution for adjusting or regulating the magneto-resistive property of the device. In this preferred embodiment, the resistance 114 is connected parallel to the device 111 having two electrodes 112 and 113. If the device has a magneto-resistive property as indicated by a characteristic curve A in Fig. 12, it is varied to a characteristic curve B by connecting a resistance 114 of, for instance, 50mΩ parallel to the device 111. If the value of the resistance 114 is chosen at a value smaller than 50 mΩ, the characteristic curve B is shifted in a direction indicated by an arrow b. Also, it is shifted in a direction indicated by an arrow a when the value of the resistance is chosen at a value larger than 50 mΩ. Thus, the characteristic curve of the device can be adjusted or controlled by choosing the value of the resistance 114 suitably. It is to be noted that the threshold Bth of magnetic flux density is not varied by the resistance 114. The resistance 114 can be connected between either one pair of current electrodes 72 and voltage electrodes 73 of the device 71 shown in Fig. 7. Fig. 13 shows an application of the device as a displacement sensor. In this preferred embodiment, a permanent magnet 134 is provided as means for applying a magnetic field to the device 131 having two terminals 132 and 133. When the magnet 134 is moved relative to the device 131 kept at 77K in a vertical or a horizontal direction as indicated by an arrow d or ℓ, the magnetic density flux B applied to the device 131 is varied according to the relative movement thereof. Therefore, it is possible to detect a relative position of the magnet 134 by measuring a variation of the resistance of the device 131. Fig. 14 shows variation curves of the resistance in the vertical and horizontal directions d and ℓ. Fig. 15 shows a potentiometer utilizing the device 151 having three terminals 152, 153 and 154. Between two current terminals 152 and 154, a power source 156 such as a battery is connected and a potentiometer 157 is connected between the centre terminal 153 and one current terminal 154 in order to measure a voltage generated therebetween. If a permanent magnet 155 locates above the portion P of the device 151 defined between the left current electrode 152 and the centre terminal 153, a resistance R₂₃ is generated along the portion P due to the magnetic field applied thereto by the magnet 155. However, no resistance is generated along the portion Q defined between the centre terminal 153 and the right current terminal 154 since the magnetic field applied thereto is too weak to break weak couplings in the portion Q. Therefore, an output voltage Vout between the terminals 153 and 154 is zero. When the magnet 155 is moved so as to locate above the centre terminal 153, the magnetic field is applied equally to both of the portions P and Q and, therefore, the superconductive phase in each half portion of respective portions P and Q is broken and even resistances R₂₃ and R₃₄ are generated in respective portions P and Q. Accordingly, the output voltage Vout between two terminals 153 and 154 becomes equal to one half of the input voltage Vin applied between the terminals 152 and 154. Further, when the magnet 155 is moved to locate above the portion Q, R₂₃ becomes zero while R₃₄ has a certain value. In this state, Vout becomes equal to Vin. As is apparent from the above, the output voltage Vout varies proportional to an amount ℓ of displacement of the magnet 155 when it is moved parallel to the lengthwise direction of the device 151. This relation is shown by a line a in Fig. 16. Thus, a non-contact potentiometer with a linear property can be obtained. If the cross section of the device is continuously varied along the lengthwise direction thereof, a logarithmic relation b or an exponential relation c may be obtained between the output voltage Vout and the displacement ℓ as shown in Fig. 16. The potentiometer of this type is noiseless and stable since the magnet 155 and the device 151 do not contact one another. Fig. 17 shows an application of the principle of the non-contact potentiometer to two-dimensional potentiometer. In this embodiment, the device is formed to have a configuration of cross. Four end electrodes 172, 173, 175 and 176 are formed at respective free ends of four arms of the device 171 and one electrode 174 is formed at the centre thereof. The predetermined constant current I is applied to each pair of end electrodes 172, 173 and 175, 176. As indicated by a dotted square in Fig. 17, a magnetic field M having a square uniform flux density is applied to the device 171 kept at 77K. Assuming that all four arms have the same structure, no voltages are generated between two terminals 172 and 173 and between two terminals 175 and 176 if the centre of the magnetic field M is located so as to coincide with that of the device 171. However, if the magnetic field M is moved in the transverse direction without any accompanying movement in the longitudinal direction, a voltage Vab proportional to the displacement is generated between two terminals 172 and 173. But no voltage is generated between two terminals 175 and 176. Similarly, if the magnetic field M is moved in the longitudinal direction without any accompanying movement in the transverse direction, a voltage Vcd is generated between two terminals 175 and 176 while no voltage is generated between the other two terminals 172 and 173. If the centre of the magnetic field M locates at an arbitrary position, voltages Vab and Vcd have respective values proportional to respective displacements in the transverse and longitudinal directions. Thus, the two-dimensional potentiometer of non-contact type is obtained. Generally speaking, all of the arms need not have the same structure and all of the currents to be applied to respective terminals need not have the same value. In such a case, a position of the magnetic field M at which no voltages Vab and Vcd are generated is sought as the standard or neutral position of the potentiometer. Fig. 18 shows an example of a circuit desirable for a practical use of the two-dimensional potentiometer shown in Fig. 17. As shown in Fig. 18, each of four end terminals 172, 173, 175 and 176 is connected, via a resistance 187 having a high value, to a common line 188 and, between the centre electrode (terminal) 174 and the common line 188, a power source 189 having a constant voltage is connected. The potentiometer of cross configuration can be formed with use of four devices of the type shown in Fig. 5. In this case, they are arranged to form a cross and a respective one end of each device is electrically connected to the same end of each of the other devices by a suitable connecting means to form a centre electrode. As shown in Figs. 19(a) and 19(b), each arm 191 between two electrodes 193 and 194 can be formed as a folded line in order to increase the resistance of unit length. Fig. 20 shows an application of the two dimensional potentiometer to a pick up head for a record player. In this case, a needle 202 is fixed to the two dimensional potentiometer 201 so as to contact with a groove 203 of a record 204. When the needle 202 is shifted by the guide of the groove, two dimensional displacement of the potentiometer 201 relative to a constant magnetic field applied thereto is detected in order to obtain recorded sound information. Fig. 21 shows an application to a potentiometer for detecting a rotational displacement. In this embodiment, a device 211 is formed to have a configuration of ring cut off in a radial direction at a position. At each free end of the device 211 and at a position diametrically opposite, electrodes 212, 213 and 214 are formed. A constant current I is applied to each of the free end electrodes 212 and 213. As indicated by a dotted line in Fig. 21, a magnetic field M having a half circular cross section which is concentric with the device 211 kept at 77K is applied to the latter. If the magnetic field M is applied symmetrically with respect to the centre electrode 214, the voltage Vac generated between the two electrodes 212 and 214 is equal to the voltage Vbc generated between the two electrodes 213 and 214. However, if the magnetic field M is rotated by Δϑ around the centre axis of the device 211, the balance between Vac and Vbc is broken to generate a voltage between the two free end electrodes 212 and 213 proportional to Δϑ. Figs. 22(a), 22(b), 23(a) and 23(b) show another example of a potentiometer for detecting a rotational displacement. This potentiometer comprises a bottom member 221 and a cap member 231. The bottom member 221 is made of magnetic material and is of disc-like configuration, as shown in Figs. 22(a) and 22(b). At the centre portion of the bottom thereof, a circular superconductive device 222 is fixed by an insulation element 223. The cap member 231 made of a magnetic material is formed to define a closed space between the bottom member 221 and is guided by a guide ring 224 of the bottom member 221 to rotate around the centre axis 232 thereof. A permanent magnet 233 having a half circular cross-section is fixed inside thereof so as to be opposite the device 222 with a small gap. When the cap member 231 is rotated relative to the bottom member 221, the relative phase of the magnet 233 to the device 222 is varied. Accordingly, a potentiometer is realized by detecting the relative phase of the magnet 233. In this potentiometer, the magnetic flux density is maintained permanently since there is not any factor which decreases the magnetic flux density. Fig. 24 shows an application of the device as a variable resistance or a displacement transformer. In this example, a permanent magnet 241 is displaced relative to an elongated superconductive device 242 having two terminals 243 in the vertical direction while applying a homogeneous magnetic field to the device 242 along the whole length thereof. When the distance d between the magnet 241 and the device reaches a certain value, a resistance appears in the device 242 and increases as the magnet 241 approaches the device. This relation is shown in Fig. 25. This relation can be utilized as a variable resistance or a displacement converter which outputs a voltage between two terminals 243 in accordance with the distance d. Fig. 26 shows an application to a rotary encoder. A superconductive device 261 is arranged near to a flat gear 262 which is magnetized permanently. When the gear 262 is rotated around the centre axis thereof together with a rotating body (not shown), the magnetic field applied to the device 261 is varied periodically in accordance with a pitch of teeth of the gear 262. That is, when the top of each tooth approaches the device 261, the magnetic field is increased while it is decreased when the bottom of each tooth approaches thereto. Accordingly, an output voltage Vout generated between two terminals of the device 261 is varied periodically as shown in Fig. 27. Thus, the rotation angle of the gear 262 can be measured by counting a number of pulses obtained from the output voltage Vout. Further, it is possible to apply this system to a detecting system for detecting a rotational angular velocity by counting the number of pulses for a predetermined time interval. If the gear is replaced by a rack (not shown) which moves parallel to the device 261, displacement or velocity of a linear motion can be detected by counting a number of pulses output as Vout. Fig. 28 shows an example utilizing an electromagnet as means for applying a magnetic field to a superconductive device. In this case, the electromagnet 284 is arranged opposed to the device 281 having two terminals 282 and 283. When a current I is applied to the electromagnet 284, a resistance appears in the device 281 kept at 77K if the magnetic field generated thereby has a strength enough for causing the resistance. Accordingly, this system can be applied to an electro-magnetic relay by connecting it to a load and a power source serially or connecting it to a control terminal of a device having a high input impedance such as a MOS transistor. As shown in Fig. 29, it is also possible to constitute an electro-magnetic relay by inserting a superconductive device 291 having two terminals 292 and 293 in a solenoid 294. Further, as shown in Fig. 30, such an electro-magnetic relay can be constituted by arranging a device 301 having two terminals 302 and 303 in a gap of an electromagnet 304 of a doughnut-like configuration. When a current is supplied to a solenoid 305 of the electromagnet 304, a magnetic field is applied to the device 301 which causes a resistance in the device. The electro-magnetic relay of this type can be operated with a high credibility since any mechanical vibration such as a chattering which occurs in a conventional device is not caused and it is free from imperfect contact due to dust or oxidation of contacts. Further, it is advantageous in that it never generates noise or mechanical sounds. It is also to be noted that the resistance of the device can be varied by controlling the current applied to the electromagnet. This suggests an applicability to a variable resistor of non-contact type. The system shown in either one of Figs. 28, 29 and 30 can be also applicable to a potentiometer or a displacement sensor of non-contact type. Fig. 31 shows an example of application to a control system for microwaves. As shown therein, a superconductive device 311 is fixed onto an inside wall of a wave guide 312 and an electromagnet 313 is provided for applying a magnetic field B to the device 311 kept at 77K from the outside of the wave guide 312. In this system composition, input microwave is propagated in the wave guide 312 without any loss since no resistance is generated when no current is applied to the solenoid of the electromagnet 313. However, when a current is supplied thereto, the device 311 has a resistance due to the magnetic field applied thereto. The microwave being propagated in the wave guide 312 is absorbed by the device 313 partially. Due to this, the intensity of the microwave is reduced. If the intensity of the magnetic field B is increased by increasing the current to be supplied to the electromagnet 311, the microwave is attenuated as shown schematically in Fig. 32. Namely, the system shown in Fig. 31 constitutes a variable attenuator for microwave free from mechanical structures. Figs. 33(a) and 33(b) show another application of a superconductive device to the microwave system. In this example, a superconductive device 331 having two terminals 332 and 333 is arranged in a wave guide 334 so as to be perpendicular to the axial direction of the wave guide 334. Each of terminals 332 and 333 is connected to the inner wall of the wave guide 334. If no magnetic field is applied to the device 331, a microwave incident into the waveguide 334 is reflected by the device 331 since it is in the perfect anti-magnetic state, as shown in Fig. 33(a). However, if a magnetic field B is applied to the device 331 as shown in Fig. 33(b), the device 331 becomes a resistive body in the wave guide 334. Accordingly, an incident microwave is propagated in the wave guide 334 and output from the exit thereof although it is absorbed partially by the device 331. Thus, this system functions as a microwave switch. Further, it functions as a variable attenuator for microwave by varying the intensity of the magnetic field applied to the device. Fig. 34 shows a detection system for detecting a magnetic field with use of a superconductive device 341 having two current electrodes 342 and two voltage electrodes 343. To this device 341, a permanent magnet 344 is provided for applying a magnetic field B₀ as a bias which has an intensity slightly larger than that necessary for breaking weak couplings in the device 341. When a weak magnetic field B s is applied to the device 341 from a source 345 of weak magnetic signal, an output voltage V out generated between the two voltage electrodes 343 is increased sensitively to the applied magnetic field B s . The permanent magnet 344 can be replaced by an electromagnet 347 as shown in fig. 35. In this case, it becomes possible to control the bias magnetic field by adjusting a current applied to the electromagnet 347. Accordingly, it is always possible to set a suitable bias magnetic field in accordance with the dimension and configuration of the device to be used. This system may be applied to a reading head for reading a magnetic tape or to a processing head for processing image signal for the like of a superconductive material which can operate at room temperature is developed. In the above embodiment, the bias magnetic field B₀ is chosen carefully in order to obtain a high sensitivity to a weak magnetic signal. Namely, it is chosen so as to utilize a steep variation in the resistance of a device, as shown in Fig. 36. In order to obtain a large variation in the output voltage Vout or the resistance, it is advantageous to form a superconductive portion of the device in a zigzag configuration by the ion plating method for the laser irradiation, as shown in Fig. 37. This enables the resistance to be increased up to an order of 1 kΩ. In this example, the resistance varies by about 270 mΩ to a variation of 1 gauss in the magnetic signal and, therefore, the output voltage Vout varies by 27 mV to the current of 100 mA. According to an experiment done by the present inventors, the device of this kind shows a hysteresis in the superconductive magneto-resistive property when measured by varying the magnetic field applied to the device while keeping the current applied thereto constant, as is shown in Fig. 38. This hysteresis phenomena causes an error upon detection of a weak magnetic signal of an order of about several tens gauss. This phenomenon is considered as the result that weak couplings are maintained in the broken state when the magnetic field is reduced while keeping the current constant. In order to avoid the hysteresis which causes an error upon detecting a weak magnetic field, it is desirable to drive the device with use of a current which becomes zero once. Fig. 39 shows examples of such a current to be applied to the device. Namely, in the example (a), the current is kept at zero for a while and then raised up to a predetermined level. In the example (b), square pulses are applied to the device. Also, in the examples (c) and (d), an alternating current and a saw tooth current are used to drive the device, respectively. When the current to be applied to the device is reduced to zero, the device resumes its superconductive state thereof and, therefore, the hysteresis does not affect the detection of a weak magnetic signal. This driving method enhances the stability and credibility of the system according to the present invention. Although the superconductive material mentioned above shows the superconductivity at 77K (temperature of the liquid nitrogen), a superconductive material having a magneto-resistive property which shows a small intrinsic resistance R₀ at 77K, as shown in Fig. 40, can be used in the present invention so long as the resistance increases steeply as the magnetic flux density increases. The superconductive material having the magneto-resistive property mentioned above is produced as follows. Powders of Y₂O₃, BaCO₃ and CuO are weighed at a predetermined ratio in order to obtain a superconductive material having a composition Y 1.2 Ba 1.8 Cu₃O 7-x . 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 composed of micro particles having a diameter equal to or smaller than 1 µm. Then, the powder is pressed into disc-like pellets of 1 mm thick with a press force of 1 ton/cm². Finally, these pellets are sintered at 1000°C for 3 hours in air and, then, the temperature is gradually lowered to 200°C for 5 hours. This superconductive material has a resistance which begins to fall down steeply at 82K and has a very small but finite intrinsic resistance R₀ (equal to 12 mΩ) at 78K, as shown in Fig. 41. The resistance becomes zero at 38K. The device cut from the pellet is immersed into liquid nitrogen and the magneto-resistive property is measured while applying a constant current of 5 mA to the device. This measured magneto-resistive property is shown in Fig. 40. As is apparent from comparison of Fig. 40 with Fig. 9, the superconductive material of this kind can be utilized as a device sensitive to a magnetic field since it has a high sensitivity to a weak magnetic field similarly to the superconductive material having the composition Y₁Ba₂Cu₃O 7-x . The production method of the device is not limited to the sintering method mentioned above. A thin device can be easily produced according to the spray-pyrolysis method, as shown in Fig. 42. Y(NO₃)₃·6H₂O, Ba(NO₃)₂ and Cu(NO₃)₂·3H₂O are prepared and weighed at a predetermined ratio in order to obtain a component of Y₁Ba₂Cu₃O 7-x . Then, they are dissolved into pure water completely. The solvent 421 is poured into a container 422 of a spray gun apparatus 423 and sprayed by a small amount from the spray gun on a crystal substrate 444 of stabilized zirconia which is heated at about 600°C by a heater 445. The solvent is sprayed by pressurized air as mists on the substrate. This spraying is done by a small amount in order to avoid possible lowering of the temperature of the substrate 444 and repeated about one hundred times to obtain a thin film 446 of about 10 µm thickness. In place of air, N₂ or O₂ gas can be used for spraying. The thickness of the film can be controlled from 0.5 µm to 50 µm. The temperature of the substrate can be altered in a range from 300 to 600°C. The obtained film is heated at 950°C for 30 minutes and cooled gradually to room temperature. The sintering temperature can be varied in a range from 900 to 1,000°C and the sintering time can be varied in a range from 1 to 60 minutes in accordance with the thickness thereof. The magneto-resistive property of the device thus produced is shown in Fig. 43. As is apparent from comparison of Fig. 43 with Fig. 9 which shows the magneto-resistive property of the device produced by the powder-sintering method, the former device has an amazingly high sensitivity (resistivity) when compared with that of the latter one. It is also possible to calcine a thin film in air at a temperature in a range from 750 to 850°C whenever the thickness of the sprayed layer becomes a predetermined thickness in a range from 1 to 3 µm. A substrate of SrTiO₃ having very few defects can be used in place of the crystal substrate of zirconia. It is also possible to form a buffer layer on the substrate. Such a buffer layer can be formed with use of sapphire, an oxide such as MgO, Aℓ₂O₃, a semiconducting material such as GaAs, Si, or a metal such as Ag, Pt, Y or Ba. The superconductive film has such an orientation of crystal axis that the direction of growth is oriented in the direction of c axis on the substrate. Fig. 44 shows an X-ray diffraction pattern of the superconductive film. As shown clearly therein, diffraction peaks appear only at diffraction angles having indexes of c axis. Although only Y-Ba-Cu-O ceramic superconductive material is described in the above, a ceramic superconductive material composed of a group IIIa element, group IIa element, Cu and O, such as La-Ba-Cu-O or Y-Sr-Ba-Cu-O (or part of O elements is replaced by F elements), or a superconductive material having a multi-crystal structure which has random grain boundaries acting as weak couplings, is also suitable. Further, the production method of the superconductive device is not limited to the powder-sintering method and/or the spray-pyrolysis method and various methods such as the sputtering method, CVD method, the screen method and doctor-blade method can be utilized. The preferred embodiments described herein are illustrative and not restrictive, the scope of the invention being indicated by the appended claims and all variations which come within the meanings of the claims are intended to be embraced herein.

Claims: A superconductor device comprising: an element (151) made of a superconductive material which exhibits a magneto-resistive property; means (152, 154) for applying an electric current to said element; and means (153, 154) for detecting a change in the resistance of the element, characterized by: said element being made of a ceramic superconductive material having random grain boundaries (2) which act as weak couplings (3); magnet means (155) for applying a magnetic field to said element, which magnet means is movable so as to change the magnetic field applied to the element; and said change in resistance being dependent upon the position of said magnet means. A superconductor device according to claim 1, wherein said element (151) is elongate and said magnet means (155) is movable parallel to the lengthwise dimension of the element. A superconductor device according to claim 2, wherein respective opposite ends of said element are connected to a voltage source (156) which supplies said current, and a lengthwise portion of the element is connected to a voltage detecting means (157) for detecting said change in resistance. A superconductor device according to claim 3, wherein the cross-section of said element is made non-uniform to provide a desired relationship between the position of said magnet means and the voltage across said lengthwise portion. A superconductor device according to claim 1, wherein said element (211) comprises an incomplete annulus and said magnet means (M) is rotatable about an axis through the centre of the annulus to apply said magnetic field to at least a portion of the element. A superconductor device according to claim 1, wherein said magnetic means (233; 262) comprises a rotatable member which applies a magnetic field to said element (222; 261) whose strength is dependent upon the angular position of the rotatable member. A superconductor device according to claim 6, wherein said rotatable member comprises a toothed wheel (262) made of a magnetized material. A superconductor device according to claim 1, wherein said magnet means comprises an elongate member which provides a magnetic field whose strength varies along the lengthwise dimension of the member. A superconductor device according to claim 8, wherein said elongate member comprises a toothed member made of a magnetized material. A superconductor device according to any one of the preceding claims, including a resistance (114) connected to said element (111) to receive at least a portion of said current so that the magneto-resistive characteristics of said element are modified. A superconductor device comprising: waveguide means (312; 334); an element (311, 331) made of a superconductive ceramic material having random grain boundaries which act as weak couplings, the material exhibiting a magneto-resistive property; and means (313) for applying a magnetic field to said element to change the resistance thereof, wherein said element is disposed within said waveguide means so that any resistance of the element attenuates a wave passing through the waveguide means. A superconductor device according to claim 11, wherein said element (331) is disposed within said waveguide means so that when the element has no resistance it reflects a wave incident on the element. A superconductor device according to claim 11, wherein said element (311) is elongate and disposed parallel to the longitudinal axis of said waveguide means (312). A superconductor device according to claim 12, wherein said element (331) is elongate and disposed transverse to the longitudinal axis of said waveguide means (334). A superconductor device according to any one of the preceding claims, wherein said superconductive ceramic material is a IIIa-IIa-Cu-O material. A method of sensing a magnetic field or the position of a magnetic field applying means, comprising the steps of: applying an electric current to an element made of a superconductive material which exhibits a magneto-resistive property; applying a magnetic field to said element; and detecting a change in the resistance of the element, characterized in that: said element is made of a ceramic material having random grain boundaries which act as weak couplings; and by the step of: making said current zero from time to time in order to overcome hysteresis in said magneto-resistive property. The method of claim 16, wherein said current is made zero periodically. The method of claim 16 or claim 17, wherein said current is a pulse current. The method of claim 17, wherein said current has a sawtooth waveform. The method of any of claims 16 to 19, including the step of applying a bias magnetic field (B o ) to said element. The method of claim 20, wherein said bias magnetic field (B o ) is of sufficient strength to bring about a transition of the superconductive material from the superconductive phase to the normal conductive phase. The method of any one of claims 16 to 21, wherein said magnetic field is applied to said element by magnet means which is movable so as to change the magnetic field applied to the element.

Inventors: KATAOKA, SHOEI, KITA, RYUSUKE, NAGATA, MASAYA, NOJIMA, HIDEO, TSUCHIMOTO, SHUHEI
Applicants: SHARP KABUSHIKI KAISHA
Application year: 1988
Main classification: G01R33
IPC codes: G11B5, G01R33