An apparatus and method for magnetic measurement using a superconductive magneto-resistive device

Abstract: An apparatus for magnetic measurement using superconductive magneto-resistive film which is disposed in a magnetic field with the surface of the superconductive magneto-­ resistive film(1) directed in parallel with the direction of the magnetic field so that the magnetic hysterisis property of the film can be avoided and the accuracy of the measurement can be much improved.

Description: The present invention relates to a method and an apparatus using a superconductive magneto-resistive element to measure the strength of a weak magnetic field. In an article 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, H. Nojima et al. describe a method and an apparatus using a superconductive magneto-resistive element prepared by sintering a mixture of oxide powders. Due to this manufacturing method, the element has isotropic properties. Anisotropic properties, and with this higher sensitivity, has an element as it is described in the not prepublished document EP-A-0 301 902 which bases on an application having an earlier priority date than the present application. This document describes a method and an apparatus of using such an anisotropic element. The magnetic sensor as described in the mentioned document is simply constructed by a ceramic superconductor which comprises a layer of grains of a superconductive magneto-resistive material grown on a plane substrate in a way that the crystalloqraphic c-axis of the crystalline grains is essentially perpendicular to the substrate plane, current electrodes, and voltage electrodes. The dependence between the electrical resistance and the strength of an applied, weak magnetic field is as shown in Fig. 2. However, this element has hysteresis properties as shown in Fig. 4. To prevent errors caused by these hysteresis properties when measuring the strength of a changing weak magnetic field, the mentioned document proposes to decrease the current to zero before each measurement, and then to increase it again, to always measure along the same line of the hysteresis curve. Accordingly, the use of the said element is troublesome. SUMMARY OF THE INVENTION The present invention has been developed to overcome the above mentioned problems which the driving system of the conventional superconductive magneto-resistive device has been suffered, and the object of the present invention is to provide a magnetic measuring apparatus using a superconductive magneto-resistive film which can measure a magnetic value with a good operability without any affection of hysteresis. It is considered that the magneto-resistive device of the above mentioned ceramic superconductor causes hysteresis as follows. As shown in Fig. 10, the ceramic superconductor has such a structure that a great number of superconductive grains 14 are coupled by sintering and the superconductor can be considered as aggregate organism of the superconductive particles weakly coupled by grain boundaries 13 formed of either extremely thin insulation films, normal conductive films or point like coupling. Fig. 11 shows an equivalent circuit of the aggregated structure which forms an aggregation of Josephson devices. When a magnetic field is applied to the ceramic superconductor, the superconductive state is collapsed from the weakly-coupled portions therein, so that the superconductor shows electrical resistance. Accordingly, as shown in Fig. 2, the resistance is increased rapidly by applying a weak magnetic field. The reason why this superconductive magneto-resistive element shows the hysteresis feature shown in Fig. 4 is as follows. With the growing of an initially applied magnetic field, a cyclic superconducting current flowing through the weak coupling to form a diamagnetic field collapses the superconductive state from the weak junctions, and permits magnetic flux to invade the superconductor. Furthermore, with reducing the magnetic field having had a strength enabling magnetic flux to invade the superconductor, superconducting current flows into the weak junctions, thereby forming a cyclic superconducting current causing magnetic flux opposite to that occurring when the magnetic field strength is increased. The difference described above affects the interior magnetic field of the superconductor. To prevent the device from having hysteresis property, the superconductor must not permit a cyclic current to flow thereinto nor magnetic flux to be caught. These requirements, it was found out, could be satisfied when a magnetic field was applied to the device in parallel to the superconductive film surface of the device. Namely, a magnetic field was measured by using the device of a filmy superconductor, and the magnetic field was found out to have dependence on the applying direction of the magnetic field. As a result, it was discovered that the hysteresis property of the device could be dramatically reduced when a magnetic field was applied to the device in parallel to the film surface thereof. In the present invention, the magneto-resistive element is composed by forming a ceramic superconductor film on a substrate; and it is installed in parallel to a magnetic field to be applied, thereby measuring the magnetic field strength. A ceramic superconductor, in which the grain boundaries have a hysteresis feature and show a high sensitivity to a magnetic field, is formed into a thin film; to reduce the invasion and capture of magnetic flux causing the hysteresis to a negligibly small degree, the superconductor film is provided in parallel to an applied magnetic field to be free from the affection of hysteresis, and also to utilize the superconductor features more efficiently, so that even a quick variation in a weak magnetic field can be measured precisely. BRIEF EXPLANATION OF THE DRAWINGS Figs. 1 and 2 are graphs for showing the magneto-resistive effects of the conventional magneto-resistive device and ceramic superconductive magneto-resistive device, Fig. 3 is a perspective view for illustrating an embodiment of the magnetic measuring apparatus according to the present invention, Fig. 4 is a graph for showing hysteresis occurred in the magneto-resistive effect, Figs. 5 and 6 are diagrams for showing the difference in the affects of magnetic fields of different applied directions, Fig. 7 is a graph for showing the magneto-resistive effect when a magnetic field is applied to the superconductive film perpendicularly thereto, Fig. 8 is a graph for showing the magneto-resistive effect when a magnetic field is applied to the superconductive film in parallel thereto, Fig. 9 is a schematic view of an apparatus for manufacturing the ceramic superconductive film employed in the embodiment of the present invention, Fig. 10 shows the composition of the ceramic superconductor, Fig. 11 is a diagram for showing the equivalent circuit of the ceramic superconductor shown in Fig. 10 and Fig. 12 is a graph for showing the characteristics of the ceramic superconductive film employed in the embodiment of the present invention, which is observed by the X-ray diffraction. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is described more. in detail taken in conjunction with preferred embodiments with reference to the accompanying drawings. Fig. 3 shows an embodiment according to the present invention, in which a ceramic superconductor 1 is formed as a thin film having a generally elongated rectangular shape, and a pair of current electrodes 2 and 3 are provided near to both longitudinal ends thereof, and a pair of voltage terminals 4 and 5 are provided on the ceramic superconductor 1 separated inwardly form the current electrodes. A predetermined current I is fed to the device from a constant current source through the current electrodes 2 and 3, and a predetermined magnetic field H is applied to the superconductor 1. In such a state, the applied magnetic field is measured based on voltage V developed between the voltage terminals 4 and 5. Next, description is given to the method for manufacturing the superconductor film 1 shown in Fig. 3. The superconductor is made with a film forming apparatus for ceramic superconductor films which is shown schematically in Fig. 9. Referring to Fig. 9, a substrate made of a Y-stabilized zirconia (YSZ) is fixed to a base 8 of a large heat capacity heated by a heater 9. The substrate is kept to 400°C, while an aqueous solution is sprayed intermittently with an atomizer 11. As the aqueous solution, nitrate as the compositional element of the ceramic superconductor, namely, Y(NO₃)₃.6H₂O, Ba(NO₃)₂, and Cu(NO₃)₂.3H₂O are weighed and mixed at a rate of Y₁Ba₂Cu₃, and they are dissolved in water to prepare the aqueous solution of the present invention. As a result, a uniform film of a thickness of 5 µm is obtained by the aid of the substrate temperature, the thermal decomposition of the nitrate, and the spray pyrolisis process which grows the Y₁Ba₂Cu₃O₇-x. The prepared film was heated at 950°C for 60 minutes in the air, and then annealed at 500°C for 10 hours in the air to adjust the composition ratio of oxygen. The resistance of the obtained superconductor film started decreasing from 100K (T co ), reaching to zero ohm at 88K (T ci ) which was called a critical temperature. Fig. 12 is a diffraction chart for showing the measurements of the Y₁Ba₂Cu₃O₇-x film by the X-ray diffraction. As apparent from this chart, it can be seen that the superconductor film grows so that the c axis of the crystalline grains be essentially perpendicular to the substrate 7. Next, titanium, which shows a suitable property to a ceramic superconductor, was deposited to the superconductor film to form the current electrodes 2 and 3, and voltage electrodes 4 and 5 as shown in Fig. 3. Thus, a ceramic superconductive magneto-resistive device 6 was completed. The device 6 was 5mm in width, and 10mm in length, and the distance between the voltage electrodes 4 and 5 was 5mm. Current of 0.1 mA was fed to the device 6 through the current electrodes 2 and 3 for the below-mentioned measurements. The ceramic superconductive magneto-resistive device 6 was chilled to a liquid nitrogen temperature of 77K at which it was brought into a superconductive state. When a magnetic field was applied in an the x axial direction which is perpendicular to the main surface la of the superconductive film 1 as shown in Fig. 3, the device 6 showed a hysteresis property of the resistance R in which the resistance R changes different way depending on whether the applied magnetic field H is increasing or decreasing as shown in Fig. 7. With reference to Fig. 5, the reason why the hysterisis property of the resistance R occurs can be considered that the magnetic flux easily passes through the film surface la and is caught by the superconductor 1, when the magnetic field is applied to the superconductive film 1 in a direction perpendicular to the surface la. In contrast, when the magnetic field is applied to the superconductive film 1 in a Y axial direction which is in parallel to the surface 1a as shown in Fig. 3, the resistance R developed in the device 6 showed the same values against the same amount of the magnetic field despite that the applied magnetic field H is increasing or decreasing. Within the permissible errors in measurement, the hysteresis-property of the resistance R of the device 6 did not appear. The value ϑ in Fig. 8 represents an angle formed by the direction of the X axis which is perpendicular to the surface 1a of the superconductive film 1 and the direction H of the magnetic field as shown in Fig. 3. The above described superconductive magneto-resistive device 6 is an embodiment according to the present invention, and as for the ceramic superconductor, the same effects can be obtained from other film-forming process such as the sputtering process, the MOCVD process or the electron beam deposition process, not only from the spray pyrolisis process. Though this embodiment employs a superconductor of Y-Ba-Cu-O system, other superconductors may be used for obtaining the same effects, for example, superconductbrs as of Bi-Sr-Ca-Cu-O system, T1-Ba-Ca-Cu-O system or the like in which the weak coupling of the grains can be formed in the grain boundaries. It is not necessary to apply the magnetic field always in the horizontal direction against the superconductive film 1 of the device 6, but same effects can be also obtained so far as the magnetic field is applied inclined from the horizontal direction within a predetermined angular range. The angle ϑ defined by the axis X vertical to the surface 1a of the superconductive film 1 and the applied magnetic field H affects the sensitivity of the superconductive film, which is apparent from the comparison between the graphs of Figs. 7 and 8, so that the sensitivity of the magnetic measurement of the device 6 can be easily controlled simply by adjusting the angle ϑ. Description has been so far given to the method in which the superconductive film 1 is provided in parallel with the magnetic field to be measured. However, in case where the magnetic field is not parallel with the superconductive film 1, it is possible to use a high magnetic permeable substance such as a silicon steel or permalloy, or by utilizing an absolute diamagnetic property of the superconductor, so that the magnetic field is applied in parallel to the superconductive film 1 for measurement. In this way, the strength of the applied magnetic fields of various directions relative to the superconductive film can be measured. In the present invention, a magnetic field can be measured by the magneto-resistive device composed of a ceramic superconductive film having a high magnetic sensitivity with simple construction and excellent operability. Particularly it is possible to measure the magnetic field with a high accuracy without the effect of the hysteresis of the magneto-resistive by selecting the angle between the superconductor film and an applied magnetic field.

Claims: Method of using a superconducting magneto-resistive element for measuring the strength of a magnetic field applied to said element, which element comprises a layer of grains of a superconducting magneto-resistive material grown on a plane substrate in a way that the crystallographic c-axis of the crystalline grains is essentially perpendicular to the substrate plane, whereby the plane of said element is arranged substantially parallel to the direction of the magnetic fields to be measured, to reduce the measurement hysteresis, as it develops when measuring a field of changing strength, to a predetermined amount, which hysteresis is essentially zero when the element plane is arranged exactly parallel to the field direction. An apparatus comprising a superconducting magneto-resistive element for measuring the strength of a magnetic field applied to said element, which element comprises a layer of grains of a superconductive magneto-resistive material grown on a plane substrate in a way that the crystallographic c-axis of the crystalline grain is essentially perpendicular to the substrate plane, and including a highly magnetically permeable substance which applies the magnetic field to be measured essentially parallel to the plane of said element.

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