Apparatus for measuring magnetic field using superconductive magneto-resistive element

Abstract: A superconductive magneto-resistive element (1) is connected to a current control device (2-8) and the current flowing the element is controlled corresponding to the magnetic field strength applied to the element so that the voltage generated across the element is constant, whereby the current is converted into the magnetic field strength.

Description: BACKGROUND OF THE INVENTION 〈Field of the Invention〉 The present invention relates to a superconductive magneto-resistive magnetometer, i. e. to an apparatus for measuring the strength of a magnetic field by means of a superconductive magneto-resistive element which can detect a weak magnetic field by utilizing the magneto-resistive effect of a ceramic superconductor. It is noted that in the following description, the word "resistance" means "electric resistance", and "magneto-resistive element" means an "element which changes its electric resistance corresponding to the strength of an applied magnet field". 〈Description of the Prior Art〉 A superconductive magneto-resistive magnetometer comprising the features of the preamble of claim 1 is described in an article by H. Nojima et al. having the title "Galvanomagnetic Effect of an Y-Ba-Cu-O Ceramic Superconductor and Its Application to Magnetic Sensors", as published in Japanese Journal of Applied Physics, Vol. 27, No. 5, May 1988, pp. 746 - 750. As long as the magneto-resistive element in this apparatus has its superconductive properties, practically no voltage develops across the inner electrodes of the element when a current flows between the outer electrodes. However, the superconductive properties are lost, and a voltage can be measured between the inner electrodes, when the strength of an applied magnetic field exceeds a certain threshold, which threshold depends from the flowing current, or when the current exceeds a certain threshold which depends from the strength of the applied magnetic field. This dependence is shown in Fig. 1. According to the teaching of the above mentioned article, the strength of an applied magnetic field is measured by increasing the current till a voltage just develops between the inner electrodes. The apparatus is calibrated by applying magnetic fields of different strengths and by measuring in each case the value of the current at which a voltage across the inner electrodes just appears. From the document EP-A-0 251 023 it is known to bias a non-superconductive magneto-resistive element by applying a constant bias voltage. SUMMARY OF THE INVENTION It is the object of the present invention to provide a very sensitive and accurate apparatus for measuring the strength of an applied magnetic field by using a magneto-resistive element. The apparatus of the present invention is defined by the features of claim 1. In contrast to the prior art apparatus, as described in the above mentioned article, the apparatus of the invention does not measure the current, at which just a voltage develops across the inner electrodes, but it measures the current, at which a certain predetermined voltage larger zero develops. This voltage is fed back to the current adjusting circuit to control the current just in a way to maintain the predetermined voltage. This feedback voltage is an open circuit voltage and not a current driving biasing voltage, as in the case of the apparatus as described in the document EP-A-0 251 023. BRIEF EXPLANATION OF THE DRAWINGS Fig. 1 is a graph for showing the characteristics of the superconductive magneto-resistive element embodying the present invention; Fig. 2 is a graph for showing the characteristics of the magneto-resistive element to describe the operation of the present invention; Fig. 3 is a schematic view for illustrating the essential portion of the present invention; Fig. 4 is a diagram for illustrating the construction of one embodiment of the apparatus according to the present invention; and Fig. 5 shows the manufacturing method of the magneto-resistive element, or the difference in characteristics between the devices different in shape. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment according to the present invention will be described in detail with reference to the accompanying drawings. In this embodiment, a superconductor is employed as the ceramic superconductive magneto-resistive element. The grain boundaries of the superconductor are so formed that the adjacent particles are joined through an extremely thin insulating film or normal electric conducting film, or they are weakly joined by point. In this condition, the superconductive state of the superconductor can be easily collapsed to the normal condition by applying a magnetic field. The superconductor used is of Ba₂Y₁Cu₂Ox type known as a high temperature superconductor. It is produced as follows: BaCO₂ powder, Y₂O₃ powder, and CuO powder are weighed at predetermined values, sufficiently mixed in a mortar, and provisionally calcined at a temperature of 900°C for 5 hours in the air; the baked mixture is repulverized to obtain powder (a particle diameter of 1 µm⊘ or less), which is then pressed under a pressure of 1 ton/cm² into circular pellets; and the circular pellets are heated at a temperature of 1,000°C for 3 hours in the air, and slow-cooled to a temperature of 200°C in 5 hours to obtain the above mentioned superconductors. It is confirmed that the produced ceramic superconductor, shows a mono phase of a rhombic system on the X-ray diffraction observation. Superconductor materials can be produced not only by the calcining method using powder materials, but by the sputtering or CVD process in which the calcining conditions of a filmy superconductor are controlled. The produced superconductor is cut into rectangular chips (1mm wide x 7mm long x 0.7mm thick) as shown by a numeral 1 in Fig. 3. A pair of current electrodes 9 are provided at the respective areas near to the both ends thereof for feeding electric current, and a pair of voltage electrodes 10 are respectively provided at positions on the superconductor 1 inwardly relative to the current electrodes for detecting voltage developed therein. These electrodes are made by deposition Ti metal which shows an excellent electrical connection to ceramics or the like. Lead wires are connected to the respective current electrodes 9 and voltage electrodes 10 made of Ti by Ag paste, and the respective lead wires are connected to a constant current source 11, and a voltage indicator 12 respectively. The ceramic superconductor 1 is kept at a liquid nitrogen temperature (77K) bellow its critical temperature Tc, and voltage developed across the voltage electrodes 10 is measured by the voltage indicator 12 so as to control the current from the constant current source 11 through the current electrodes 9. Under the above mentioned conditions, when any magnetic field is not applied to the superconductor 1, any voltage is not developed unless a current of several tens mA is fed to the superconductor 1, however, the superconductor has a feature that even when a current of several mA is fed thereto, voltage is abruptly generated with the application of a relatively weak magnetic field of several tens oersted, and is rapidly increased as shown in Fig. 2. A superconductor is different from the conventional semiconductor and magnetic material in that the magneto-resistive effect of the superconductor shows a radical variation in resistance with the application of a small magnetic field as mentioned above, while the magneto-resistive element of a semiconductor or magnetic material shows resistance variation extending along a quadratic curve. The reason why the superconductor shows such a radical resistance variation is because the superconducting state is collapsed beginning from the grain boundaries in the superconductor by the application of a weak magnetic field as described above, so that the superconductor abruptly has the electrical resistive property. Owing to this magneto-resistive effect, by increasing current fed to the superconductor 1 as shown in Fig. 3, the strength of a magnetic field for inducing the voltage across the superconductor, can be decreased as shown in Fig. 1. As apparent from Fig. 1, when a current of 1mA is fed to the superconductor 1, voltage is induced by the application of a magnetic field of 25 oersted (hereinafter referred to as Oe); when a current of 5mA is fed thereto, voltage is induced by the application of a magnetic field of 7.5 Oe; and when a current of 10mA is fed thereto, voltage is induced by the application of a magnetic field of 3 Oe. Fig. 2 shows the magnetic field measurement according to the present invention which utilizes the above mentioned magneto-resistive effect of a superconductor. On a condition that no magnetic field is applied to the superconductor 1, in a direction perpendicular to the surface 1a of the superconductor 1 by means of a magnetic source 20. It is noted that the magnetic field in any direction can be applied to the superconductor 1. In order to generate a voltage at the point A (0.5 µV), a current of 21mA is fed thereto through the current electrodes 9µ. When a magnetic field is applied to the superconductor under the condition, the resistance of the superconductor is increased, thereby increasing output voltage of the superconductor 1. For this reason, the current is decreased to lower the output voltage to a predetermined value. When a magnetic field of 8 Oe is applied to the superconductor 1, the current is decreased to 5mA so as to shift the voltage value to the B. When a magnetic field of 29 Oe is applied to the superconductor 1, the current is decreased to 1mA so as to shift the voltage value to the C. In this connection, at the points B and C, the output voltage is specified to 0.5 µV. As shown in Fig. 2, by preparing the relational characteristic graph or data of the current fed thereto, an applied magnetic field strength, and output voltage of the respective ceramic superconductive magneto-resistive element, an applied magnetic field strength can be calculated based on the current value corresponding to the predetermined output voltage. Fig. 4 is a circuit diagram for effecting the magnetic field measuring operations by the ceramic superconductive magneto-resistive element according to the present invention. An output voltage setting circuit is shown in the left side of Fig. 4. Referring to Fig. 4, a constant voltage circuit 4 made of an integrated circuit (IC) and a semi-variable resistor 8 for voltage division are connected in parallel and are connected to a power source Vc through a resistor 6. Output voltage is set by adjusting the semi-variable resistor 8. In the right side of Fig. 4, a magneto-resistive element composed of a superconductor 1, a transistor 7, and an output resistor 5 are serially connected to the power source Vc. In this circuit connection, the output voltage from the voltage electrodes 10 of the magneto-resistive element is inputted to a differential amplifier 2, the output terminal of which is connected to the negative input terminal of an operational amplifier 3, and the output of the above mentioned output voltage setting circuit is connected to the positive terminal of the operational amplifier 3. An output of the operational amplifier 3 is connected to the base of the transistor 7. The operational amplifier 3 compares the predetermined voltage value of the positive input terminal with the output voltage of the magneto-resistive element of the negative input terminal, and inputs, to the transistor 7, a signal indicating the difference therebetween, and the transistor 7 controls the current flowing the magneto-resistive element 1 so as to make the voltage of the positive input terminal of the amplifier 3 and that of the negative input equal. By the above mentioned circuit operation, the stabilized current value is fetched as voltage through the resistor 5. This is adopted to the embodiment as described in connection with Fig. 2. When the resistor 5 is set to 100 Ω, and the predetermined voltage to 0.5 µv, currents of 21 mA, 5 mA, and 1 mA flow respectively corresponding to the respective magnetic fields of 0 Oe, 8 Oe, and 29 Oe, and hence, voltages of 2.1 V, 0.5 V, and 0.1 V are generated in the output voltage terminal OUT through the resistor 5. The voltage on the terminal OUT is applied to a data processor 21 in which calculation to convert the current value into the strength of the magnetic field is performed. The foregoing description is made with respect to the magnetic measuring operation according to the present invention, the measuring precision is, however, improved by employing a superconductive magneto-resistive element which is the most suitable for an applied magnetic field strength by selecting from the magneto-resistive elements of different sensitivity. The ceramic superconductive magneto-resistive elements have different sensitivities to accomplish the above mentioned object. Superconductors employed for the ceramic superconductive magneto-resistive devices can have different sensitivities by varying the production conditions. For example, a ceramic superconductor is produced by calcining the before mentioned powder materials. When the pressing force is only changed from 1 ton/cm² to 0.5 ton/cm², the packing density is reduced to 90% or less of that produced under a pressure of 1 ton/cm², the surface roughness is increased to 1 µm or more, and the sensitivity as the magneto-resistive element is, however, improved. Furthermore, the semiconductor sensitivity is improved by decreasing the packing density of the superconductor to make a porous one, by forming fine pores on the superconductor surface with a laser or the like, or by varying the junction conditions in the grain boundaries of the superconductor through a thermal or mechanical process. The sensitivity characteristics of the magnetic resistive device obtained through the above mentioned process have reproducibility, are not varied with time, and can be kept stably to a predetermined value. Fig. 5 is the modification of the before described embodiment, in which a superconductor is varied in thickness to change the sensitivity of the magneto-resistive element. In this embodiment, the ceramic superconductor is formed in a rectangular chip of 1mm wide x 7mm long, and the graph I represents the original thickness 1mm thereof when cut, and the graph II represents the thickness 0.6mm after mechanical abrasion. Currents fed to them are 1 mA. As understood from the above mentioned descriptions, the sensitivity of the magneto-resistive element is varied depending on the production conditions of the superconductor, and current fed to the device can be arbitrarily controlled in accordance with the size, shape or other design of the device. Therefore, there can be manufactured a desirable magneto-resistive element being capable of matching with the measuring conditions such as an applied magnetic field strength, magnetic and spatial resolving powers or the like as required, so that a precise magnetism measurement can be effected. When a magneto-resistive element is selected from those having various characteristics for use, or when the predetermined voltage value of the measuring circuit is varied, an apparatus for generating a predetermined magnetic field is provided for quickly preparing the calibrating data therefor, so that calibrating operation taken into account the conditions of the magnetic resistive device and the driving circuit therein can be readily carried out. Accordingly, a magnetic field strength can be calculated precisely under the most suitable conditions. According to the present invention, the ceramic superconductive magneto-resistive element which has a high sensitivity to a weak magnetic field because of the weak grain boundary junction is used for measuring the magnetic field adopting such properties that the current is remarkably changed against variation in a magnetic field strength. In other words, the superconductive magneto-resistive element can measure the magnetic field under such a condition of the element that the current value with a small electric resistance, whereby the current fed thereto can be controlled with a high accuracy. Therefore, by measuring this controlled current, a magnetic field strength can be measured in the most operative conditions of the ceramic superconductive magneto-resistive element.

Claims: An apparatus for measuring the strength of a magnetic field, comprising a superconductive magneto-resistive element (1) which changes its electric resistance corresponding to the strength of an applied magnetic field; a pair of outer electrodes (9, 9) arranged on said element; a pair of inner electrodes (10, 10) arranged along the current path between said outer electrodes; a current control means (11; 3-8) connected to said outer electrodes to direct an electric current through said element, whereby the value of the adjusted current is the measure for the strength of the applied magnetic field; and a voltage indicator (12; 2) connected to said inner electrodes to measure the voltage which develops across said inner electrodes, when a current flows between said outer electrodes; characterized in that said current control means (11; 3-8) receives the voltage as measured by said voltage indicator (12; 2) as a feedback value to perform feedback control of the current in a way to adjust the voltage developed across the inner electrodes to a predetermined voltage larger than zero. The apparatus as claimed in claim 1, which further comprises means for applying a predetermined constant magnetic field to said superconductive magneto-resistive element (1) for calibrating the apparatus. The apparatus as claimed in claim 1 or claim 2, comprising a plurality of said superconductive magneto-resistive elements, each being different from the other ones in sensitivity with respect to the strength of the applied magnetic field, for selective use corresponding to the strength of said applied magnetic field.

Inventors: HASHIZUME NOBUO, KATAOKA SHOEI, SHINTAKU HIDETAKA, TSUCHIMOTO SHUHEI, HASHIZUME, NOBUO, KATAOKA, SHOEI, SHINTAKU, HIDETAKA, TSUCHIMOTO, SHUHEI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1989
Main classification: G01R33
IPC codes: H01L39, G01R33