Superconductive magneto-resistive device for sensing an external magnetic field

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

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a superconductive magneto-resistive device 
for a magnetic sensor. 
2. Description of the Prior Art 
Conventionally, a magnetic sensor which utilizes the Hall effect on 
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. The former sensor has a 
sensitivity capable of sensing a magnetic field of about 10.sup.-2 gausses 
and the latter one has a sensitivity of about 10.sup.-3 gausses. 
However, these conventional magnetic sensors have various disadvantages as 
follows. 
They have relatively large specific resistance R.sub.0 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. 1 
qualitatively. Since a gain .DELTA.R in the resistance is increased 
proportional to the square of the magnetic flux density B of an applied 
magnetic field, the gain related to the application of a weak magnetic 
field of, for example, several tens of gausses is very small and, 
therefore, a ratio of the gain .DELTA.R to the specific resistance R.sub.0 
(.DELTA.R/R.sub.0) is on the order of 1% at the most. 
On the contrary, a magnetic sensor with use of the SQUID (Superconductive 
Quantum Interference Device) which utilizes the Josephson junction is 
known to have a very high sensitivity capable of sensing a very weak 
magnetic field of about 10.sup.-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 a complicated operation to use it. Namely, it 
is not practical for general use although it has a very high sensitivity. 
In a copending application (U.S. Ser. No. 226,067) which was filed in the 
name of KATAOKA et al on Jul. 29, 1988 and will be assigned to SHARP 
KABUSHIKI KAISHA, a superconductive magneto-resistive device is proposed 
which is comprised of a superconductive material having grain boundaries 
acting as weak couplings and means for utilizing a change in the 
resistance of the material caused when a magnetic field is applied 
thereto. 
As shown schematically in FIG. 2, the superconductive material is comprised 
of superconductive grains 1 and grain boundaries 2 bonding them. These 
random grain boundaries 2 are considered or supposed to form various weak 
couplings 3 including tunnel junctions, point contact junctions and micro 
bridge junctions, as shown by an equivalent network circuit of FIG. 3. In 
the superconductive phase thereof, individual Cooper pairs can pass freely 
through weak couplings 3 (Josephson junction) and, therefore, the 
resistance becomes zero. When a magnetic field is applied to the 
superconductor, some of Josephson junction 3 are broken thereby and, 
accordingly, the superconductor has an electric resistance. As a 
superconductor having grain boundaries, a Y-Ba-Cu-O ceramic superconductor 
can be used. The critical temperature thereof is about 90 K. 
FIG. 4 shows an example of the magnetic sensor system disclosed in the 
above identified application. 
In this system, an elongated rectangular device 4 of (1.times.7.times.0.7 
mm.sup.3) which is made of a Y-Ba-Cu-O ceramic superconductive material is 
prepared and is immersed in liquid nitrogen (77 K). A current is supplied 
by a power source 9 through a pair of electrodes 5 and 6 formed on 
respective ends thereof and a voltage between two electrodes 7 and 8 is 
measured to detect a change in the resistance thereof when a magnetic 
field B is applied thereto. 
FIG. 5 shows the result obtained. As is apparent therefrom, the resistance 
of the device 4 changes according to the strength I of the applied current 
and that of the applied magnetic field B. One of the advantages of this 
system is that the specific resistance of the device is zero in the 
superconductive phase and another advantage is that the change in the 
resistance of the device is very steep and, therefore, a very high 
sensitivity to the magnetic field is obtained. 
However, in this system, there is a problem which is that the magnetic 
sensor senses a magnetic field induced by the current flowing through the 
device because of the fine sensitivity thereof. In order to avoid this 
problem, it is desirable to form the superconductive device linearly, as 
shown in FIG. 7. But, such a linear device induces a magnetic field 
proportional to the length thereof which causes an error in the 
measurement of the strength of an external magnetic field to be measured. 
SUMMARY OF THE INVENTION 
One of the objects of the present invention is to provide a superconductive 
magneto-resistive device for a magnetic sensor system having a structure 
in which a magnetic field induced by a current flowing through the device 
does not affect an external magnetic field to be measured. 
Another object of the present invention is to provide a magnetic sensor 
capable of detecting a magnetic field having a non-dimensional or 
two-dimensional distribution. 
In order to achieve these objects, according to the present invention, 
there is provided a superconductive magneto-resistive device for use in a 
sensor system for sensing an external magnetic field. A current is 
supplied to the device while keeping it at a temperature close to the 
critical temperature of the superconductive material forming it. When an 
external magnetic field is applied thereto, the change in the resistance 
thereof caused by the applied magnetic field is detected in order to 
measure the applied magnetic field. The device is formed so as to have a 
predetermined pattern for a current path through which the supplied 
current flows; and that said pattern includes portions formed close and 
parallel to each other so that magnetic fields induced by respective 
currents flowing through said portions can be cancel each other out. 
According to another object of the present invention, there is provided a 
sensor system for sensing an external magnetic field comprising plural 
superconductive magneto-resistive devices, supply means for supplying a 
constant current to each of said devices, means for cooling said devices 
at a temperature close to the critical temperature of a superconductive 
material forming each device, and detection means for detecting a change 
in the resistance of each device. The plural devices are arranged so as to 
form a predetermined pattern.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 6 shows a magnetic sensor system according to the present invention. 
The magnetic sensor system is comprised of a superconductive 
magneto-resistive device 11 housed in a package 12 made of a non-magnetic 
material, a cooling apparatus 13 for cooling the device 11 with use of 
high pressurized N.sub.2 gas so as to keep it in the superconductive 
state, a circuit 14 for generating a constant current to apply it to the 
device 11, a detection circuit 15 for detecting a voltage generated in the 
device 11 and a microcomputer 16 for controlling the constant current 
generation circuit 14 and processing data outputted from the voltage 
detection circuit 15. The processed data is displayed on a display 17. 
When an external magnetic field B is applied to the device 11 in a 
direction indicated by an arrow B, the detection circuit 15 measures the 
voltage generated in the device 11 and the measured voltage is processed 
by the microcomputer 16 in order to give the strength of the applied 
magnetic field. 
FIG. 7 shows the superconductive magneto-resistive device 11. 
The device 11 is comprised of a substrate 21 made of alumina (Al.sub.2 
O.sub.3) and a superconductive magneto-resistive element 22 formed on the 
substrate 21. The element 22 is formed on the substrate 21 as a thin film 
of a Y-Ba-Cu-O ceramic superconductor with use of the spattering method. 
This element 22 has two linear portions 22a and 22b extending parallel to 
each other with a small distance which are connected by a connection 
portion 22c at respective ends thereof. On respective free ends of two 
linear portions 22a and 22b, electrodes 23a and 23b for supplying a 
current to the device 11 are formed by depositing Ti and lead lines 24a 
and 24b are connected to the electrodes 23a and 23b in order to apply the 
constant current from the constant current supplying circuit 14 to the 
device 11. Further, a pair of electrodes 25a and 25b for measuring a 
voltage generated in the device 11 are formed on portions of the device 11 
near the current electrodes 23a and 23b. Two leads 26a and 26b from the 
voltage detection circuit 15 are connected to these electrodes 25a and 
25b, respectively. 
When a constant current I is applied to the current electrode 23a, it flows 
through the first linear portion 22a in a direction indicated by an arrows 
L and, then, returns through the second linear portion 22b to the current 
electrode 23b, via the connecting portion 22c, in a direction indicated by 
an arrow M. 
Since the distance between the first and second linear portions 22a and 22b 
is very small, magnetic fields induced by the currents flowing oppositely 
through the first and second linear portions 22a and 22b cancel with each 
other. Therefore, the external magnetic field B to be detected is not 
affected by these magnetic fields induced along the first and second 
linear portions 22a and 22b. Thus, the device 11 can detect the strength 
of the external magnetic field B exactly. 
The superconductive element 22 is made with use of a superconductive 
material of a Y-Ba-Cu-O oxide having a critical temperature of 90 to 100 
K. This material is deposited on the substrate of Al.sub.2 O.sub.3 by 
spattering in order to form a thin film of the thickness of about 10 
.mu.m. This film is heated up to 900.degree. C. in the air and, then, 
cooled gradually. The obtained component thereof is Y.sub.1 Ba.sub.2 
Cu.sub.3 O.sub.7-x (0&lt;x&lt;1). The film is processed by etching to form the 
element 22 on the substrate 21. 
This film for the device can be made by various methods such as vacuum 
evaporation method, CVD method, spray method for spraying a solvent of 
components of the superconductive material and the like. A substrate made 
of silicon or Ba.sub.2 TiO.sub.4 is usable for the substrate of the device 
11. 
The sensitivity of the superconductive magneto-resistive device is 
considered to be determined by a radius of grains included therein and the 
state of grain boundaries. 
The ceramic superconductive material can be also made by sintering as 
follows. 
Powders of Y.sub.2 O.sub.3, BaCO.sub.3 and CuO are weighed at a 
predetermined ratio in order to obtain a component of Y.sub.1 Ba.sub.3 
Cu.sub.3 O.sub.7-x (0&lt;x&lt;1) After grinding and mixing these powders, 
samples formed with the mixture are calcined at 900.degree. C. for 5 hours 
in air. Then, the samples are crushed and ground into powder comprised of 
micro particles having a diameter equal to or smaller than 1 .mu.m. Then, 
the powder is cold-pressed into samples. Finally, these samples are 
sintered at 1000.degree. C. for 3 hours in air. 
The sensitivity of the superconductive magneto-resistive device made by 
sintering as mentioned above is greatly dependent on the radius of crushed 
micro particles. 
On the contrary to the sintering method, the diameter of grains forming the 
superconductive film made by the deposition method is substantially 
determined by the temperature of the substrate upon depositing the film 
thereon. 
In the preferred embodiment, the ceramic superconductive film is formed by 
spattering the material on the substrate while keeping it at a temperature 
of 300.degree. to 400.degree. C. The deposited film to sintered at 
950.degree. C. in air and, thereafter, cooled gradually. 
The pattern of the device can be formed by irradiating a laser beam, an 
electron beam or an ion beam onto portions of the film except for the 
pattern in order to change those into a normal conductive state. 
FIG. 8 and FIG. 9 show desirable patterns for the device. 
The pattern shown in FIG. 8 has successive four basic patterns shown in 
FIG. 7 which are formed parallel. This pattern has a length of current 
path of four times of that of the basic pattern. Accordingly, an output 
voltage of four times can be obtained in this pattern with the same 
current. The number of basic patterns can be changed arbitrarily. 
The pattern shown in FIG. 9 is formed to have five parallelized portions 
connected one after another. This pattern has a current path of about five 
times of that of the basic pattern. 
The pattern having a structure for cancelling magnetic fields induced by 
respective linear portions can be realized not only by a plane pattern but 
also by a stacked or layered structure. 
FIG. 10 shows an example of such a stacked structure. 
In this example, the device 31 is comprised of first and second elements 32 
and 33 and an insulation film 34 inserted inbetween them. 
Each of the first and second elements 32 and 33 has a linear pattern of a 
superconductive magneto-resistive material formed on each of substrates 35 
and 36, as indicated by a dotted line in FIG. 10. The patterns of the 
first and second elements 32 and 33 are formed identical with each other. 
Each one end of these patterns of the first and second elements 32 and 33 
are electrically connected with each other by a through hole 37 formed on 
the insulation film 34. 
On the other ends of these patterns, electrodes 38 and 39 are, 
respectively, formed for supplying a constant current from the constant 
current circuit 14. The electrode 38 is drawn out through a through hole 
40 to the upper surface of the first substrate 35. 
In this structure, the direction of the current flowing through the device 
is reversed between the pattern of the first element 32 and that of the 
second element 33 and, therefore, respective magnetic fields induced along 
the current paths of the first and second elements 32 and 33 are perfectly 
cancelled with each other. 
FIG. 11 shows another example of the device 50 having a layered structure. 
In this structure, six layers from 51 to 56 of a superconductive 
magneto-resistive material are deposited one by one and, between adjacent 
layers, an insulation layer 57 is formed so as to insulate them except for 
one end portion of them. The insulation layer 57 for insulating the upper 
pair of the adjacent layers is formed so as to extend in a direction 
opposite to that of the lower pair of the adjacent layers and, thus, a 
folded current path is formed in the device 50. On the lower-most and 
upper-most layers 51 and 56, a pair of electrodes 58a and 58b for 
supplying a constant current I from the current source 14 and a pair of 
electrodes 59a and 59b for measuring a voltage generated in the device 50 
by the detection circuit such as a potentiometer 15 are formed, 
respectively. 
Since the current is reversed in the direction thereof between the adjacent 
layers, magnetic fields induced by respective currents flowing through the 
adjacent layers are perfectly cancelled with each other. 
This structure is extremely advantageous in that the output voltage or the 
resistance to be measured is independent from the strength of the current 
to be applied to the device because no internal magnetic field is 
generated in the device 50 and, accordingly, the resistance of the device 
is determined only by the external magnetic field applied thereto. 
FIG. 12 is a graph showing the result obtained by measurement with use of 
the device 50 having the structure shown in FIG. 11. The magneto-resistive 
characteristic obtained when the current of 0.1 mA is supplied 
substantially coincides with that obtained when the current of 0.01 mA is 
supplied. 
FIGS. 13(I) and 13(II) show one dimensional magnetic array sensor and two 
dimensional magnetic array sensor, respectively. 
In the one dimensional magnetic array sensor, a plurality of 
superconductive magneto-resistive devices from 61-l to 61-n are connected 
parallel to each other between lines 62 and 63 connected to a power 
source. 
To each of the devices, a resistance 64 is connected serially and each 
output terminal 65 is drawn out from a portion between each device and 
each resistance. 
When a magnetic field having one dimensional pattern is applied to the 
sensor, the pattern is detected based upon data outputted from individual 
devices 61-l to 61-n of the sensor. The device having such a pattern as 
shown in either of FIGS. 7 to 11 is desirably used for each device of the 
sensor. However, the superconductive magneto-resistive device as shown in 
FIG. 4 can be used for the device of the sensor. 
In the two dimensional magnetic array sensor shown in FIG. 13(II), a 
plurality of superconductive magneto-resistive devices are arranged in a 
matrix form. 
In this case, a two dimensional magnetic pattern can be detected based on 
data outputted from individual devices. 
FIG. 13(III) shows another example of the two dimensional magnetic sensor. 
In this magnetic sensor, plural column line devices 71-l to 71-n are formed 
on a substrate (not shown) at a predetermined pitch and plural row lines 
devices 81-l to 81-n are formed at a predetermined pitch so as to form a 
lattice together with the column line devices 71-l to 71-n. Each of the 
row line devices and each of the column line devices are insulated with 
each other at the crossing portion between them. 
Individual one ends of the column line devices are connected to a power 
source line 75 and individual one ends of the row line devices are 
connected to another power source line 85. The other end of each of the 
column line devices is connected to each of other source terminals 76-l to 
76-m via a resistance 77. Also, the other end of each of the row line 
devices is connected to each of other power source terminals 86-l to 86-n 
via a resistance 87. Each of output terminals 78-l to 78-m is drawn out 
from a portion between each of the column line devices and the resistance 
77. Also, each of output terminals 88-l to 88-n is drawn out from a 
portion between each of the row line devices and the resistance 87. 
When a magnetic field as indicated by a dotted circle H is applied to the 
sensor, only two output terminals of the second column line terminals 78-2 
and the second row line terminal 88-2 output data corresponding thereto. 
Accordingly, a two dimensional magnetic field can be detected by scanning 
the column and row line terminals sequentially. 
It is also possible to apply the power to an arbitrary pair of the column 
line device and the row line device by providing respective switching 
means for selectively switching on either of the column line devices and 
for selectively switching either one of the row line devices. If a pair of 
i-th column line device 71-i and j-th row line device 81-j are switched 
on, the magnetic field induced by a current flowing through either one of 
them is applied to the other device as a bias magnetic filed reciprocally. 
Due to this, it becomes possible to detect an external magnetic field 
applied to one of crossing points selectively by the application of the 
internal bias magnetic field thereto. 
In FIG. 14, a three dimensional magnetic sensor is disclosed. In this 
sensor, three superconductive magneto-resistive device 91, 92 and 93 are 
arranged along three orthogonal coordinate axes X, Y and Z, respectively. 
When an external magnetic field H.sub.M is applied to the magnetic sensor, 
the direction and the strength thereof can be calculated based on 
respective output data from individual devices 91, 92 and 93. 
FIG. 15 shows a cooling apparatus for the device 11 utilizing Peltier 
effect which is formed as a cascade structure of two stages with use of 
Peltier effect devices. 
In FIG. 15, reference numeral 41 denotes a heat radiation metal plate, 
reference numeral 42 denotes a cooling metal plate, reference numeral 43 
denotes an insulator, reference numerals 44a and 44b denote p-type and 
n-type semiconductor devices, respectively, and reference numeral 45 
denotes a heat radiation substrate. 
FIG. 16 shows another example of a magnetic sensor 101. 
This sensor 101 is comprised of two rod-like elements 102 and 102' made of 
a material having a high permeability and a superconductive 
magneto-resistive device 103 inserted between two elements 102 nd 102'. As 
shown in FIG. 17, the device 103 is comprised of a substrate 105 and a 
folded linear pattern element 106 of a superconductive magneto-resistive 
material deposited thereon. A constant current is applied from a pair of 
electrodes 107a and 107b formed on respective end portions of the pattern 
element 106. 
When the magnetic sensor 101 is directed parallel to the magnetic flux of a 
magnetic field as shown in FIG. 18(a), the magnetic flux is converged into 
rod-like element 102 or 102' and, therefore, a strong magnetic field is 
applied to the device 103 to generate a high resistance in the device. 
On the contrary, if the magnetic sensor 101 is inclined to the magnetic 
flux of a magnetic field by an angle .alpha. as shown in FIG. 18(b), the 
magnetic flux is not converged so much into the rod-like element 102 and 
102'. Therefore, a magnetic field applied to the device 103 becomes 
considerably weak. And, if the magnetic sensor 101 is directed 
perpendicularly to the magnetic flux of a magnetic field, as shown in FIG. 
18(c), all of the magnetic flux pass freely through each of the rod-like 
elements 102 and 102' and, therefore, none of the magnetic field is 
applied to the device 103. Thus the direction of a magnetic field relative 
to the magnetic sensor can be detected based on data outputted from the 
device 103. 
It is understood that various other modifications will be apparent to and 
can be readily made by those skilled in the art without departing from the 
scope and spirit of the present invention. Accordingly, it is not intended 
that the scope of the claims appended hereto be limited to the description 
as set forth herein, but rather that the claims be construed as 
encompassing all of the features of patentable novelty that reside in the 
present invention, including all features that would be treated as 
equivalents thereof by those skilled in the art to which the present 
invention pertains.