Abstract:
A method for sensing an applied magnetic field uses a superconductor element with superconductive material as a sensor. The element is cooled to its superconducting state and current is supplied to the element. An applied magnetic field is applied to the superconducting material and an output from the superconductor element is fed to a feed back power source. In the feedback power source a comparison is made between an input voltage from the superconductive element and a reference voltage. Current is applied to a coil for applying a bias magnetic field to the superconductor element based on the comparison. With this method, the input voltage and the reference voltage are made as equal as possible. A superconductive device includes a coil for applying an AC power source and a coil for applying a DC power source. The DC power source can be set at various values and the application of the DC power source, which is a bias voltage, changes the output of the superconductive element by changing the electrical wave form outputted from the superconductive element.

Description:
This application is a continuation of application Ser. No. 07/544,899 filed on Jun. 28, 1990 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and device for sensing a magnetic field with use of a magneto-resistive property of a supercoductive material. The superconductive material contains grain boundaries. 
     2. Description of the Prior Art 
     Conventionally, a magnetic sensor which utilizes the Hall effect of magneto-resistive effect in a semiconductor or a magnetic sensor which utilizes the magneto-resistive effect in a magnetic material is widely used for sensing or measuring a magnetic field. These sensors have a limit of resolution of sensing a magnetic field of about 10 -3  gauss. Conventionally, for sensing a weak magnetic field, SQUID (superconductive Quantum Interference Device) has been used. However, SQUID needs the Josephson junction which utilizes thin insulation layer between two thin superconductors. SQUID devices require a high production technology for example because of the thin insulation layer is necessary and it is not easy to produce the SQUID devices. In addition, in order to sense a magnetic field, it is necessary to cool the SQUID, to eliminate noise and to process the output signals from the SQUID, therefore to handle the SQUID for the measurement of the magnetic field is not easy. 
     The inventors of the present invention have found that the superconductive state of a ceramic superconductive element comes to break the superconductive state by application of a small magnetic field due to the grain boundary property of the superconductive material. Also there is a resistance which increases abruptly corresponding to increment of the applied magnetic field and they propose a new magnetic field sensor utilizing the magneto-resistive effect in the U.S. patent application, Ser. No. 226,067, now U.S. Pat. No. 5,011,818 issued Apr. 30, 1991. The magnetic sensor using the ceramic superconductor mentioned above is expected, as the superconductor magnetic sensor having a high sensitivity with easy operation and handling. In addition this sensor is easy to produce partly because the sensor has a large resistivity increasing factor at the time of application of such a magnetic field. Accordingly the superconductive condition is broken and the structure of the device is simple and the control of the sensitivity of the device can be easily made by the application of current, partly because the measurement output can be easily obtained. 
     However, the property of the superconductor magneto-resistive element as mentioned above is non linear in terms of the resistance and the applied magnetic field. Accordingly, it is difficult to calculate an absolute intensity of an applied magnetic field based on the resistance value of the element. 
     Besides, there have been used various kinds of magnetic field sensor using high speed electron mobility semiconductors such as In Sb and In As, and ferro magnetic metal such as Fe-Ni, Co-Ni. 
     However, with the conventional magnetic field sensor using the above material it is difficult to sense a weak magnetic field accurately. 
     SUMMARY OF THE INVENTION 
     An essential object of the present invention is to provide a method of sensing the intensity of an applied magnetic field accurately with a simple magnetic sensor. 
     In order to accomplish the purpose mentioned above, according to the method of measuring magnetic field according to the present invention, a magneto-resistive element having a superconductor magneto-resistive property is provided with means for applying a bias magnetic field using a coil. The intensity of the bias magnetic field is so controlled as to cause the resistance value of the magneto-resistive element to coincide with a set resistance value at which the sensitivity of the element is high. Since it is previously known that the intensity of the magnetic field to make the magneto-resistive element in the set resistance value, it is possible to accurately adjust the bias magnetic field utilizing the property of the magneto-resistive element in the high sensitivity state. The intensity of the magnetic field can be represented by the amount of the current of the coil, whereby the outer magnetic field can be calculated based on the difference of the above mentioned two magnetic fields. 
     Referring to FIG. 4, there is illustrated a typical characteristics of a supercoductor magneto-resistive element. In FIG. 4, the horizontal axis represents the intensity of the magnetic field applied to the element and the curve (a) in a solid line represents the resistance (ohm) of the element scaled by the left vertical axis and the curve (b) in dotted lines represents the increase rate (ohm/gauss) of the element against the applied magnetic field scaled by the right vertical axis. 
     The curve (a) shows that the magneto-resistive element has no resistance when the applied magnetic field is extremely weak and the element has a resistance suddenly when the applied magnetic field exceeds a predetermined intensity. The resistance increases abruptly as the applied magnetic field increases. This characteristic shows that a high sensitive measurement of a magnetic field can be realized by using the region in the curve (b) where the increase rate of the resistance of the element is abrupt. 
     Moreover, the curve (a) shows that the relation between the resistance of the element and the intensity of the applied magnetic field is not linear and that it is difficult to calculate the absolute value of the intensity of the applied magnetic field accurately. 
     In another aspect of the present invention, there is provided a device for sensing a magnetic field using a magneto-resistive element comprising a superconductor member having weak coupling grain boundaries. There is provided a means for applying A.C. bias magnetic field to said element, and means for obtaining an output voltage of the element generated by application of the bias magnetic field as the signal representing the intensity of the applied magnetic field. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram showing an arrangement for performing a first embodiment of the method of sensing a magnetic field according to the present invention, 
     FIG. 2 is a schematic diagram showing an arrangement for performing a second embodiment of the method of sensing a magnetic field according to the present invention, 
     FIG. 3 is a schematic diagram showing an arrangement for performing a third embodiment of the method of sensing a magnetic field according to the present invention, 
     FIG. 4 is a schematic diagram showing characteristic curves of the superconductor magneto-resistive element, 
     FIG. 5 is a schematic diagram showing an example of an arrangement for producing a ceramic superconductor film used in the embodiments of the present invention, 
     FIG. 6 is a plan view of a superconductor magneto-resistive element employed in the embodiments, 
     FIG. 7 shows characteristic curves of the superconductor magneto-resistive elements, 
     FIG. 8 is a schematic diagram showing a general structure of the superconductor magneto-resistive element according to the present invention, 
     FIG. 9 is a plan view showing an embodiment of the superconductor magneto-resistive element, 
     FIG. 10 is a cross sectional view of FIG. 9, 
     FIG. 11 is a characteristic curve between D.C. bias and output of the superconductor magneto-resistive element used in the embodiments, 
     FIG. 12 is characteristic curves between D.C. bias magnetic field and noise of the superconductor magneto-resistive element, 
     FIGS. 13(a) to 13(f) are schematic diagrams showing output wave forms against the D.C. bias values, 
     FIG. 14 is a schematic diagram showing the operation point of FIG. 13, 
     FIG. 15 is a circuit diagram of a lock in amplifier, and 
     FIG. 16 is an output characteristic curve of the lock in amplifier against the D.C. magnetic field. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First, a method of the production of a ceramic superconductor magneto-resistive element used in the following embodiments and its structure and property are explained. 
     FIG. 5 shows a summary of the production method of the superconductor magneto-resistive element by a spray pylorysis. 
     The ceramic superconductor film is made of composition of YBa 2  Cu 3  O 7- δ and a substrate 7 made of stabilized zilconia on which the ceramic superconductor film is formed is placed on a base plate 8 heated at 350° C. by a heater 9. On the other hand, compositions of nitrate of elements of the ceramic superconductor represented by Y(NO 3 ) 3 .6H 2  O, Ba(NO 3 ) 2  and Cu(NO 3 ) 2 .H 2  O were measured and solved in solution and the solution was saved in a tank of a sprayer 10. The saved solution was made to fine moisture under high pressure air or high pressure nitrogen gas by the sprayer 10 and sprayed to the substrate 7. The moisture reaching the substrate 7 formed a ceramic layer by the heat of the substrate 7. The ceramic layer was annealed in air at 950° C. for 5 minutes, whereby the film having superconductor characteristic were obtained. The superconductor layer thus obtained showed a resistance property that was decreased from 100K and the resistance was lost at 81K and brought into the superconductor state. 
     The superconductor film formed as mentioned above is made in a meander shape by cutting the superconductor film by slits 103 by a machine as shown in FIG. 6, and current terminals 101a and 101b and voltage electrodes 102a and 102b are formed on the superconductor film by vaporing Ti. This film is installed and sealed in a package made of anti magnetic material with dry nitrogen gas so that the superconductor magneto-resistive element 1 is obtained. 
     The current terminals 101a and 101b are connected to a stabilized current source 2 to flow a constant current and the resistance generated in the superconductor magneto-resistive element 1 is measured by a voltage generated across the voltage terminals 102a and 102b which is measured by a volt meter 104. 
     FIG. 7 shows a characteristic by changing the bias current of the superconductor magneto resistive element 1. FIG. 7 is a graph measured when the element is held at 77K of liquid helium temperature, the horizontal axis corresponds to the intensity of the magnetic field applied to the element 1 and the vertical axis to the resistance generated in the element 1. The current values attached to the respective curves are the bias currents applied to the element 1 through the current terminals 101a and 101b. As understood from the graphs FIG. 7, the magneto-resistance generated in the element 1 increases corresponding to the increase of the bias current and the critical magnetic field which the resistance occurs is decreased. 
     First embodiment 
     FIG. 1 shows the first embodiment according to the present invention which uses the superconductor magneto-reistive element 1. 
     The embodiment is such that the output voltage of the element 1 is being measured and a feedback current is flown through a bias magnet field coil so as to hold the output voltage of the element 1 constant, whereby the external magnetic field is measured by the bias current value. 
     In the embodiment shown in FIG. 1, the element 1 is cooled at 77K and a current 1 mA is supplied to the element 1 from the stabilized current source 2. The output voltage obtained by the application of the external magnetic field is inputted to the feed back source 3. The input voltage is compared to a reference voltage and the power source 3 controls the current flowing through a coil 4 which applies a bias magnetic field to the element 1 so that the difference between the input voltage and the reference voltage is as small as possible. 
     The coil 4 is a copper wire that is wound ten turns so that there can be obtained 10 gauss/A of the magnetic field applied to the element 1 against the current passed through the coil 4. 
     In the measuring device as mentioned above, the standard output voltage of the element 1 was set at 2 mV and the current to the coil 4 for applying the magnetic field was controlled so as to decrease the difference from the standard voltage by operating the feed back power source 3. In the arrangement of the first embodiment, since the current resolution was 10 μA, due to the noise level, the magnetic resolution in the first embodiment was 10 -4  gauss. 
     Second embodiment 
     The second embodiment of the present invention is shown in FIG. 2. 
     The second embodiments is substantially the same as the first embodiment except that a modulation magnetic coil 5 is added so as to apply a A.C. bias magnetic field to the element in addition to the D.C. bias magnetic field and that only the A.C. voltage component of the output voltage of the element 1 is supplied to the feedback source 3 so that the current to the coil 4 for applying the bias magnetic field is controlled so as to hold the output voltage of the element 1 is equal to the standard voltage. The operation of the device is substantially the same as that in the first embodiment except for the above two points and the explanation of the same portions is herein omitted. 
     In the second embodiment, output voltage of amplitude of A.C. 2 mV were obtained in the element 1 when A.C. current of 1 mA of peak value, 100 Hz was supplied to the modulation coil 5. The A.C. output could be amplified in synchronism with the period of the modulation magnetic field and the the effect of noise could be reduced, whereby the magnetic solution could be made to 10 -5  gauss. 
     Third embodiment 
     FIG. 3 shows the third embodiment of the present invention. The third embodiment is substantially the same as the first embodiment except that A.C. modulated current is added to the bias current and that only the A.C. component is taken from the element 1 as same as in the second embodiment. The output from the element 1 was synchronizing amplified, and the magnetic field solution was 5×10 -5  gauss by the noise by the bias current of the element 1. 
     The above embodiments are mentioned only as examples of performing the method of the magnetic field sensing using the superconductor magneto-resistive effect according to the present invention, but the method according to the present invention is not limited to the method by the shape and connection in the above embodiments. 
     The ceramic superconductor film of the superconductor magneto-resistive element in the above embodiments is explained by Y - Ba - Cu - O film made by the spray pyrolysis method, the materials may be any superconductor materials of Bi - Sr - Ca - Cu - O and Tl - Ba - Ca - Cu - O so long as the materials have weak coupling at the grain boundaries and a high non linear sensitivity against the magnetic field and there may be used the sputter method, electron beam method or CVD method as the film forming method. 
     Fourth embodiment 
     FIG. 9 shows a further example of the superconductor magneto-resistive element used in the fourth embodiment of the present invention in which the element 1 per se is substantially the same as the element shown in FIG. 6, except that the stabilized current source 2 is connected to the element 1 through a current controller 6. The like parts in FIG. 6 and FIG. 9 are numbered by the like reference numerals and the detailed explanation thereof is herein omitted. 
     The ceramic superconductor film in this embodiment was made in such a manner that Y(NO 3 ) 3 .6H 2  O, Ba(NO 3 ) 2  and Cu(NO 3 ) 2 .H 2  O were measured and solved in solution and the solution was saved in a tank of a sprayer 10. The saved solution was made to fine moisture under high pressure air or high pressure nitrogen gas by the sprayer 10 and sprayed to the substrate 7. The moisture reaching the substrate 7 formed a ceramic layer by the heat of the substrate 7 which is heated at 600° C. The superconductor film was made as 10 μm thick and was subjected to a thermal treatment in air. 
     A good result can be obtained when the superconductor film is made with 1 μm to 10 μm thick. 
     The superconductor magneto-resistive element 1 shown in FIG. 9 is set at a center of two coils 14 and 15, as shown in FIG. 8, which apply bias magnetic fields in the same direction. The measurement by the arrangement shown in FIG. 8 was made in a magnetically shielded room. 
     The coil 15 is connected to a A.C. power source 17 and the coil 16 is connected to a D.C. power source 18 so as to apply the A.C. magnetic field and the D.C. magnetic field to the element 1. 
     FIG. 11 shows one example of the output characteristic of the element 1 as mentioned above. The output characteristic in FIG. 11 was obtained under such a condition that a bias current of 10 mA was supplied to the current terminals 101a and 101b of the element 1 and the D.C. bias magnetic field is applied by the coil 16. The vertical axis of FIG. 11 shows the output of the element 1 and the horizontal axis of FIG. 11 shows the intensity of the D.C. bias magnetic field. 
     In FIG. 12, the vertical axis of the graph shows intensity of noise included in the output of the element 1 in various frequencies when the element is operated under the same condition as shown in FIG. 11 with the D.C. bias magnetic field changed as shown in the horizontal axis in FIG. 12. 
     From FIG. 12, it is understood that the change of the noise of the element 1 by the change of the intensity of the applied magnetic field is small but the noise of the low frequency below several hertz is relatively large, whereby FIG. 12 shows that a precise measurement of D.C. and low frequency magnetic field is difficult. 
     According to the present embodiment, D.C. and low frequency magnetic field can be precisely measured without the effect of the noise by the measurement method shown in the various embodiments already mentioned. 
     FIGS. 13(a) to (f) show A.C. wave forms in one example of the present invention with the arrangement shown in FIG. 8 in which a sinusoidal magnetic field of ±100 miligauss of 100 Hz as shown in FIG. 13(a) is applied to the element 1 by the coil 15. 
     Under the condition that the A.C. current is supplied to the coil 15, a predetermined D.C. current is supplied to the coil 16. When various intensities of the D.C. bias magnetic field shown by the points A, B, C, D and E in FIG. 14 having the contents explained in relation to FIG. 11 is applied, the wave forms generated at the output of the element 1 corresponding to the above intensities of the magnetic field are as shown in FIGS. 13(b), (c), (d), (e) and (f). 
     The outputs as mentioned above and the A.C. magnetic field generating signals are inputted to a lock-in amplifier, and since only 100 Hz component only is taken out with a narrow band, it becomes possible to suppress the effective value of the noise. 
     The general structure of the lock-in amplifier is shown in FIG. 16. The output from the voltage terminals 102a and 102b of the element 1 is amplified by 20 times with an amplifier 31 and inputted to the lock-in amplifier. On the other hand, the 100 Hz signals of the sine wave generator is used as the reference input. 
     A reference voltage is supplied to the phase lockloop denoted as 32. 
     The principle of the lock-in amplifier is as follows. 
     It is assumed that the input signal Vs and the reference signal Vr are represented by the following equations. 
     
         Vr=A cos (ωr t+θ)                              (1) 
    
     
         Vs=cos (ωs t)                                        (2) 
    
     wherein ω: constant, ωr: angular speed of the reference signal, θ: phase angle, ωs: angular speed of the input signal. 
     By multiplying the above two equations in a phase sensitive detector 33, the following signal Vpsd can be obtained, ##EQU1## Since ωr=ωs, the second term of the equation (3) becomes D.C. component. Since the A.C. component of the first term of the equation (3) is eliminated by a low pass filter 34, the output V LP  from the low pass filter 34 is 
     
         V.sub.LP =A/2 cos θ. 
    
     In order to make V LP  maximum, the lock-in amplifier is adjusted so as to make the phase difference of the reference signal and the input signal 0. As mentioned above, it is possible to take out the D.C. component from the frequency component by the A.C applied magnetic field. 
     By taking the horizontal axis as the intensity of the D.C. bias magnetic field when the current supplied to the coil 16 is changed and taking the vertical axis as the output of the lock-in amplifier, the result of the measurement is shown in FIG. 16. In FIG. 16, the differential magnetic sensitivity of the operation point by the applied D.C. magnetic field is measured as the lock-in amplifier output, however, when the D.C. magnetic field is nearly zero, the wave forms are depicted as FIGS. 13(c), (d), (e) by the output property of the superconductor magneto-resistive element 1, it is found that there is a liner portion in the present embodiment. In the device of the present invention, magnetic fields of D.C to several hertz can be measured in the solution of 0.1 mili gauss by setting the time constant of the low pass filter 34 as 100 milisecond using the linear portion. 
     The present invention is not limited to the above embodiment but it is possible to change the measurement range of the magnetic field to be measured and the accuracy of the measurement by adjusting the bias current to the superconductor magneto resistive element, intensity of the A.C. bias current, its frequency or presence and absence of the D.C bias magnetic field and change of the intensity of the D.C. bias magnetic field. 
     The A.C bias magnetic field and D.C bias magnetic field may be generated by one coil by supplying A.C current and D.C. current to one coil. 
     Also, it is possible to sense a magnetic field which changes more than several hertzs by adjusting the time constant of the low pass filter of the lock-in amplifier. 
     It is possible to provide the coil for applying the bias magnetic fields in the thin film shape on the substrate on which the superconductor magnetic resistive element is provided, whereby it is possible to stabilize the magnetic field sensing operation and to facilitate manufacturing of the magnetic field sensing device.