Patent Publication Number: US-7723983-B2

Title: Magnetic sensor and current sensor

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a magnetic sensor capable of sensing a change in magnetic field with high sensitivity and a current sensor capable of sensing a change in current flowing through a conductor with high sensitivity. 
   2. Description of the Related Art 
   Generally, to accurately detect weak control current flowing through a circuit in control equipment, a method of connecting resistors in series in the circuit and measuring a voltage drop in the resistors is used. In this case, however, a load different from that of a control system is applied and there is the possibility that an adverse influence is exerted on the control system. Consequently, a method of performing indirect measurement by detecting the gradient of a current magnetic field generated by the control current is used. As a concrete example, there is a method of winding a line to be measured around a toroidal core, supplying control current to the line to be measured, and detecting a magnetic flux generated in the center portion of the toroidal core by a hall element. 
   With respect to a current sensor realizing the method, however, problems such as difficulty of realizing miniaturization and insufficient linearity or high frequency responsiveness are pointed out. To address the problems, a current sensor has been proposed in which a giant magnetoresistive element (hereinbelow, GMR element) producing giant magnetoresistive effect is disposed in the current magnetic field generated by control current and the gradient of the current magnetic field is detected (refer to, for example, U.S. Pat. No. 5,621,377). In relation to the current sensor, a technique of performing flaw detection on the surface of a metal substrate or the like by using a magnetic sensor having a GMR element is also disclosed. Such a magnetic sensor or a current sensor using such a GMR element has relatively improved detection sensitivity and responsiveness and obtains detection characteristics which are stable even the temperature changes. 
   SUMMARY OF THE INVENTION 
   Recently, a magnetic sensor capable of detecting weaker magnetic flux and a current sensor capable of detecting weaker current, each having a more compact general configuration are being strongly demanded. However, with miniaturization, even a magnetic sensor and a current sensor each using the GMR element are becoming more susceptible to noise from outside (unnecessary magnetic field or the like). Accordingly, there is a challenge to improve stability in detection sensitivity and responsiveness. Although some attempts such as increase in shape anisotropy by setting the shape of the GMR element to an elongated rectangular shape have been made, it cannot be said that the magnetic sensor and the current sensor have achieved the sufficiently stable performance level. 
   The present invention has been achieved in consideration of such problems and it is desirable to provide a magnetic sensor capable of sensing a magnetic field to be detected with high precision and stability while realizing a compact configuration. 
   It is also desirable to provide a current sensor capable of sensing a current magnetic field generated by a current to be detected with high precision and stability while realizing a compact configuration. 
   A magnetic sensor of an embodiment of the invention has: a magnetoresistive element that includes a pinned layer having a magnetization direction pinned in a predetermined direction, a free layer whose magnetization direction changes according to applied magnetic fields, and an intermediate layer sandwiched between the pinned layer and the free layer; and a bias applying means applying a bias magnetic field to the magnetoresistive element, the bias magnetic field having a parallel component parallel to the magnetization direction of the free layer under no magnetic field and an orthogonal component orthogonal to the parallel component. 
   A current sensor of an embodiment of the invention has: first and second magnetoresistive elements each including a pinned layer having a magnetization direction pinned in a predetermined direction, a free layer whose magnetization direction changes according to applied magnetic fields and is parallel to the magnetization direction of the pinned layer under no magnetic field, and an intermediate layer sandwiched between the pinned layer and the free layer, the first and second magnetoresistive elements being disposed along a conductor so that resistance values change in directions opposite to each other in accordance with a current magnetic field generated by a current to be detected flowing through the conductor; a bias applying means applying a bias magnetic field to the first and second magnetoresistive elements, the bias magnetic field having a parallel component parallel to the magnetization direction of the free layer under no magnetic field and an orthogonal component orthogonal to the parallel component; first and second constant current sources that supply constant currents of values equal to each other to the first and second magnetoresistive elements, respectively; and a differential detector that detects the difference between voltage drops occurring in the first and second magnetoresistive elements by the constant current, and the current to be detected is detected on the basis of the difference between the voltage drops. 
   In the magnetic sensor and the current sensor according to an embodiment of the invention, the bias applying means applies a bias magnetic field to the magnetoresistive elements, the bias magnetic field including an orthogonal component orthogonal to the magnetization direction of the free layer under no magnetic field and a parallel component parallel to the magnetization direction of the free layer under no magnetic field. Consequently, uniaxial anisotropy of the free layer is enhanced without using shape anisotropy. 
   In the magnetic sensor and the current sensor of an embodiment of the invention, the bias applying means may be one or more permanent magnets or one or more solenoid coils. 
   In the magnetic sensor of an embodiment of the invention, preferably, the magnetization direction of the free layer under no magnetic field is parallel to that of the pinned layer. The expression “parallel” is a concept that, desirably, the angle formed by the magnetization directions is strictly zero but a deviation of the angle of about an error that occurs in manufacture is permitted. In this case, the free layer has an axis of easy magnetization parallel to the magnetization direction of the pinned layer. Preferably, magnetic flux density of a composite magnetic field lies in the range from 22×10 −4  tesla (T) to 52×10 −4  tesla (T), the composite magnetic field being composed of an exchange bias magnetic field and the parallel component of the bias magnetic field, the exchange bias magnetic field being generated between the pinned layer and the free layer. The magnetic flux density of the orthogonal component of the bias magnetic field lies, preferably, in the range from 15×10 −4  tesla (T) to 45×10−4 tesla (T). 
   The magnetic sensor of an embodiment of the invention may be also constructed so that the magnetization direction of the free layer under no magnetic field is orthogonal to that of the pinned layer. 
   In the current sensor of an embodiment of the invention, desirably, the free layer has an axis of easy magnetization parallel to the magnetization direction of the pinned layer. Preferably, magnetic flux density of a composite magnetic field lies in the range from 22×10 −4  tesla (T) to 52×10−4 tesla (T), the composite magnetic field being composed of an exchange bias magnetic field and the parallel component of the bias magnetic field, the exchange bias magnetic field being generated between the pinned layer and the free layer. Preferably, magnetic flux density of the orthogonal component of the bias magnetic field lies in the range from 15×10 −4  tesla (T) to 45×10 −4  tesla (T). 
   The magnetic sensor of an embodiment of the invention has the bias applying means applying a bias magnetic field to the magnetoresistive element, and the bias magnetic field includes a parallel component parallel to the magnetization direction of the free layer under no magnetic field and an orthogonal component orthogonal to the free layer. Consequently, uniaxial anisotropy of the free layer can be enhanced without using shape anisotropy. Therefore, the magnetic field to be detected can be detected with high precision and stability irrespective of the shapes of the magnetoresistive elements, and the invention is favorable for miniaturization. 
   The current sensor of an embodiment of the invention has the bias applying means applying a bias magnetic field to first and second magnetoresistive elements disposed along a conductor so that resistance values change in directions opposite to each other in accordance with a current magnetic field generated by a current to be detected which flows through the conductor, and the bias magnetic field includes a parallel component parallel to the magnetization direction of the free layer under no magnetic field and an orthogonal component orthogonal to the parallel component. Consequently, uniaxial anisotropy of the free layer can be enhanced without using shape anisotropy. Further, the current to be detected is detected on the basis of the difference between a voltage drop which occurs in the first magnetoresistive element and a voltage drop which occurs in the second magnetoresistive element when constant currents of the same value are supplied. Thus, the current magnetic field generated by the current to be detected can be detected with high precision and stability while realizing a compact configuration. 
   Other and further objects, features, and advantages of the invention will appear more fully from the following description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view showing the configuration of a current sensor according to an embodiment of the invention. 
       FIG. 2  is an explanatory diagram illustrating a state of a bias magnetic field in the current sensor shown in  FIG. 1 . 
       FIG. 3  is a circuit diagram of the current sensor illustrated in  FIG. 1 . 
       FIG. 4  is an exploded perspective view showing the configuration of a magnetoresistive element as a main part of the current sensor illustrated in  FIG. 1 . 
       FIG. 5  is a perspective view showing the configuration of a part of the magnetoresistive element illustrated in  FIG. 4 . 
       FIG. 6  is a schematic diagram showing spin distribution in a free layer of the magnetoresistive element illustrated in  FIG. 4 . 
       FIG. 7  is a characteristic diagram showing magnetic field dependency of a resistance change ratio in the magnetoresistive element illustrated in  FIG. 4   
       FIG. 8  is another exploded perspective view showing the configuration of the magnetoresistive element as a main part of the current sensor illustrated in  FIG. 1 . 
       FIG. 9  is a plan view showing a schematic configuration of a modification of the current sensor illustrated in  FIG. 1 . 
       FIG. 10  is a first characteristic diagram showing magnetic field dependency of a resistance value R in the magnetoresistive element of the current sensor illustrated in  FIG. 1 . 
       FIG. 11  is a second characteristic diagram showing magnetic field dependency of the resistance value R in the magnetoresistive element of the current sensor illustrated in  FIG. 1 . 
       FIG. 12  is a third characteristic diagram showing magnetic field dependency of the resistance value R in the magnetoresistive element of the current sensor illustrated in  FIG. 1 . 
       FIG. 13  is a fourth characteristic diagram showing magnetic field dependency of the resistance value R in the magnetoresistive element of the current sensor illustrated in  FIG. 1 . 
       FIG. 14  is a fifth characteristic diagram showing magnetic field dependency of the resistance value R in the magnetoresistive element of the current sensor illustrated in  FIG. 1 . 
       FIG. 15  is a characteristic diagram showing magnetic field dependency of a gap GP due to hysteresis in the magnetoresistive element of the current sensor illustrated in  FIG. 1 . 
       FIG. 16  is a characteristic diagram showing magnetic field dependency of linearity in the magnetoresistive element of the current sensor illustrated in  FIG. 1 . 
       FIG. 17  is a characteristic diagram showing magnetic field dependency of sensibility in the magnetoresistive element of the current sensor illustrated in  FIG. 1 . 
       FIG. 18  is a schematic view showing a configuration example as a second modification of the current sensor illustrated in  FIG. 1 . 
       FIG. 19  is a schematic view showing a configuration example as a third modification of the current sensor illustrated in  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An embodiment of the present invention will be described in detail hereinbelow with reference to the drawings. 
   First, the configuration of a current sensor as an embodiment of the invention will be described by referring to  FIGS. 1 to 3 .  FIG. 1  is a schematic view illustrating a perspective configuration of a current sensor  10  according to the embodiment.  FIG. 2  is a schematic view showing a plan view configuration of the current sensor  10  of  FIG. 1 .  FIG. 3  shows a circuit configuration of the current sensor  10  of  FIG. 1 . All of arrows of a current Im to be detected, a compensation current Id, a current magnetic field Hm, a compensation current magnetic field Hd, bias magnetic fields Hb 1  and Hb 2 , and a current I 0  (which will be described later) in  FIG. 3  show directions relative to first and second magnetoresistive elements. 
   The current sensor  10  is an ammeter for measuring the current Im to be detected supplied to a conductor  4  formed on a substrate  3 , and has first and second magnetoresistive elements  1 A and  1 B connected to each other at a first connection point P 1  (hereinbelow, simply called the magnetoresistive elements  1 A and  1 B). The magnetoresistive elements  1 A and  1 B are disposed line-symmetrically with respect to a center line CL as an axis of symmetry passing through an intermediate point on a virtual straight line connecting the magnetoresistive elements  1 A and  1 B and extend along the center line CL (X-axis direction in the diagram). The conductor  4  includes a U-shaped portion having a pair of parallel parts  4 A and  4 B. The parallel parts  4 A and  4 B of the pair are disposed in line symmetry with respect to the center line CL as an axis of symmetry in a plane parallel to a plane including the magnetoresistive elements  1 A and  1 B, and extend in parallel with each other along the center line CL. With the configuration, as shown by the arrows in  FIG. 3 , the current magnetic field Hm generated by the current Im to be detected is applied in the −Y direction to the magnetoresistive element  1 A and is applied in the +Y direction to the magnetoresistive element  1 B. Specifically, the magnetoresistive elements  1 A and  1 B are disposed so that the current magnetic fields Hm make resistance values R 1  and R 2  of the magnetoresistive elements  1 A and  1 B change in the directions opposite to each other at the time of driving of the current sensor  10 . Moreover, the distance from the parallel part  4 A to the magnetoresistive element  1 A is equal to that from the parallel part  4 B to the magnetoresistive element  1 B in the conductor  4 , so that absolute values of the current magnetic fields Hm applied to the magnetoresistive elements  1 A and  1 B are equal to each other. Both of the magnetoresistive elements  1 A and  1 B are formed on a circuit board  5  provided over the conductor  4 . 
   The current sensor  10  also has a constant current source  2 A (first constant current source) and a constant current source  2 B (second constant current source) whose one ends are connected to each other at a second connection point P 2 . The constant current source  2 A is connected to an end on the side opposite to the first connection point P 1  in the magnetoresistive element  1 A at a third connection point P 3 . The other constant current source  2 B is connected to an end on the side opposite to the first connection point p 1  in the magnetoresistive element  1 B at a fourth connection point P 4 . More concretely, the magnetoresistive element  1 A and the constant current source  2 A are connected to each other in series along the center line CL, and the magnetoresistive element  1 B and the constant current source  2 B are connected to each other in series along the center line CL. They are disposed so as to align in the direction (Y axis direction) orthogonal to the center line CL and connected in parallel with each other. The constant current sources  2 A and  2 B are constructed so as to supply the constant currents I 0  whose values are equal to each other to the magnetoresistive elements  1 A and  1 B, respectively. The constant current sources  2 A and  2 B are formed in a circuit board  6  provided over the substrate  3 . 
   Permanent magnets HHM 1  and HHM 2  as bias applying means are disposed so as to face each other while sandwiching the magnetoresistive elements  1 A and  1 B in the X·Y plane (refer to  FIGS. 1 and 2 ). As shown in  FIG. 2 , the permanent magnets HHM 1  and HHM 2  are parallel with each other so that their north poles NP and south poles SP are directed in the same directions, thereby enhancing their magnetic fluxes. The permanent magnets HHM 1  and HHM 2  apply bias magnetic fields Hb to the magnetoresistive elements  1 A and  1 B (refer to  FIG. 2 ). The bias magnetic field Hb applied to the magnetoresistive elements  1 A and  1 B can be divided into a parallel component Hxb parallel to a magnetization direction J 13  of a free layer  13  (which will be described later) and an orthogonal component Hyb orthogonal to the magnetization direction J 13  of the free layer  13 . In this case, the orthogonal component Hyb corresponds to the induction direction of each of the magnetoresistive elements  1 A and  1 B. The magnetic flux density of a composite magnetic field between an exchange bias magnetic field Hin (which will be described later) generated between a pinned layer  11  and the free layer  13  (which will be described later) and the parallel component Hxb in the bias magnetic field Hb is preferably in the range from 22×10 −4  tesla (T) to 52×10 −4  tesla (T). On the other hand, the magnetic flux density of the orthogonal component Hyb in the bias magnetic field is preferable in the range from 15×10 −4  tesla (T) to 45×10 −4  tesla (T). 
   Moreover, the current sensor  10  has a compensation current line C, and the compensation current Id based on a potential difference between the third connection point P 3  and the fourth connection point P 4  when a voltage is applied across the first and second connection points P 1  and P 2  is supplied to the compensation current line C (refer to  FIG. 3 ). The input side of the compensation current line C is connected to the output side of a differential amplifier AMP as a differential detector connected to the third and fourth connection points P 3  and P 4 , and the other end is grounded via a resistor RL. On the side of the differential amplifier AMP in the resistor RL, a compensation current detecting means S is connected at a connection point T 1 . The compensation current line C is constructed so that the compensation current Id gives the compensation current magnetic fields Hd to the magnetoresistive elements  1 A and  1 B. The direction of the compensation current magnetic field Hd is opposite to that of the current magnetic field Hm generated by the current Im to be detected. That is, the compensation current magnetic fields Hd generated by the compensation current Id are applied, as shown by arrows in  FIG. 3 , in the +Y direction to the magnetoresistive element  1 A and in the −Y direction to the magnetoresistive element  1 B. 
   Next, referring to  FIG. 4 , the configuration of the magnetoresistive elements  1 A and  1 B will be described more specifically.  FIG. 4  is an exploded perspective view of the magnetoresistive elements  1 A and  1 B. 
   As shown in  FIG. 4 , the magnetoresistive elements  1 A and  1 B are GMR elements each having a spin valve structure in which a plurality of function films including magnetic layers are stacked. Each of the magnetoresistive elements  1 A and  1 B includes a pinned layer  11  having a magnetization direction J 11  pinned in a direction (the −X direction in the diagram) along the center line CL, a free layer  13  whose magnetization direction changes according to external magnetic fields H such as the current magnetic fields Hm, and an intermediate layer  12  which is sandwiched between the pinned layer  11  and the free layer  13  and does not show any specific magnetization direction. The free layer  13  is made of a soft magnetic material such as nickel iron alloy (NiFe). The intermediate layer  12  is made of copper (Cu), whose top face is in contact with the pinned layer  11  and whose bottom face is in contact with the free layer  13 . The intermediate layer  12  can be made of not only copper but also a nonmagnetic metal having high electric conductivity such as gold (Au). Each of the top face of the pinned layer  11  (the face on the side opposite to the intermediate layer  12 ) and the under face of the free layer  13  (the face on the side opposite to the intermediate layer  12 ) is provided with a not-shown protection film. Between the pinned layer  11  and the free layer  13 , exchange bias magnetic fields Hin in the magnetization direction J 11  (hereinbelow, simply called “exchange bias magnetic fields Hin”) are generated and act each other via the intermediate layer  12 . The strength of the exchange bias magnetic field Hin changes as the spin direction in the free layer  13  rotates according to the interval between the pinned layer  11  and the free layer  13  (that is, the thickness of the intermediate layer  12 ). In this case, the direction of the exchange bias magnetic field Hin is the −X direction. Although  FIG. 4  shows a configuration example of the case where the free layer  13 , the intermediate layer  12 , and the pinned layer  11  are stacked in order from the bottom, the invention is not limited to the configuration. The layers may be stacked in the opposite order. 
     FIG. 5  shows a detailed configuration of the pinned layer  11 . The pinned layer  11  has a configuration in which a magnetization pinned film  14  and an antiferromagnetic film  15  are stacked in order from the side of the intermediate layer  12 . The magnetization pinned film  14  is made of ferromagnetic material such as cobalt (Co) or cobalt iron alloy (CoFe). The magnetization direction of the magnetization pinned film  14  is the magnetization direction J 11  of the pinned layer  11  as a whole. The antiferromagnetic film  15  is made of antiferromagnetic material such as platinum manganese alloy (PtMn) or iridium manganese alloy (IrMn). The antiferromagnetic film  15  is in the state where the spin magnetic moment in the −X direction and that in the opposite direction (+X direction) completely cancel out each other, and functions so as to pin the magnetization direction J 11  of the magnetization pinned film  14 . 
   In the magnetoresistive elements  1 A and  1 B constructed as described above, by application of the current magnetic field Hm, the magnetization direction J 13  of the free layer  13  rotates, thereby changing a relative angle between the magnetization directions J 13  and J 11 . The relative angle is determined according to the magnitude and the direction of the current magnetic field Hm. 
     FIG. 4  shows a no load state where the current magnetic field Hm is zero (Hm=0) and the bias magnetic fields Hb generated by the permanent magnets HHM 1  and HHM 2  are not applied (that is, the state where the external magnetic field H is zero). Since an easy magnetization axis direction AE 13  of the free layer  13  is set in parallel to the magnetization direction J 11  of the pinned layer  11 , all of the easy magnetization axis direction AE 13  and the magnetization directions J 13  and J 11  are parallel to each other along the −X direction in this state. Consequently, the spin directions of magnetic domains in the free layer  13  align in almost the same direction.  FIG. 6  is a conceptual diagram schematically showing the spin directions of the magnetic domains in the free layer  13 . The free layer  13  has, as shown in  FIG. 6 , a plurality of domains  13 D partitioned by domain walls  13 W and whose spins  23 S align in almost the same direction (the magnetization direction J 13 ). 
   When the magnetic field H are applied to the magnetoresistive elements  1 A and  1 B each having the free layer  13  in which the spin directions align as described above in a direction (+Y direction or −Y direction) orthogonal to the magnetization direction J 11 , the characteristic shown in  FIG. 7  is obtained.  FIG. 7  shows the relation between the magnetic field H and the resistance change rate ΔR/R when the magnetic field H in the +Y direction is assumed to be positive. The relation is the smallest (ΔR/R=0) when the magnetic field H is zero, and is expressed by a curve C 1  which hardly shows hysteresis. In this case, 1/f noise caused by hysteresis is extremely small, so that high-sensitive and stable sensing can be performed. 
   As is evident from  FIG. 7 , however, a linear change may not be obtained in a range around zero of the magnetic field H (H=0). Therefore, in the case of actually measuring the current magnetic field Hm, the bias magnetic field Hb generated by the permanent magnets HHM 1  and HHM 2  is applied to the magnetoresistive elements  1 A and  1 B and, as shown in  FIG. 8 , the magnetization direction J 13  is turned and slightly tilted so as to include a component in the +Y direction or a component in the −Y direction ( FIG. 8  shows the case where the magnetization direction J 13  is tilted to the +Y direction). In such a manner, in linear areas L 1  and L 2  whose centers are bias points BP 1  and BP 2 , respectively, shown in  FIG. 7 , a change in the current magnetic field Hm can be detected with high precision. In the current sensor  10 , the magnetization directions J 11  in the pinned layers  11  of the magnetoresistive elements  1 A and  1 B are the same direction (−X direction) and the bias magnetic field Hb having the orthogonal component Hyb in the +Y direction is applied to both of the magnetoresistive elements  1 A and  1 B. Consequently, when the current magnetic field Hm is detected, a resistance value R 1  of the magnetoresistive element  1 A and the resistance value R 2  of the magnetoresistive element  1 B change in directions opposite to each other. For example, the bias magnetic field Hb having the orthogonal component Hyb whose strength corresponds to that in the bias point BP 1  (refer to  FIG. 7 ) is preliminarily applied in the +Y direction to the magnetoresistive elements  1 A and  1 B. When the current Im to be detected is passed as shown in  FIG. 3 , the current magnetic field Hm in the −Y direction is applied to the magnetoresistive element  1 A. Consequently, as is evident from  FIG. 7 , the resistance change ratio decreases (the resistance value R 1  decreases). In contrast, the current magnetic field Hm in the +Y direction is applied to the magnetoresistive element  1 B. Consequently, as is obvious from  FIG. 7 , the resistance change ratio increases (the resistance value R 2  increases). 
   At this time, it is preferable to equalize the magnetic flux densities of the bias magnetic fields Hb applied to the magnetoresistive elements  1 A and  1 B to make the sensibilities of the magnetoresistive elements  1 A and  1 B equal to each other. For example, it is sufficient to adjust the magnetic flux densities of the bias magnetic fields Hb passing through the magnetoresistive elements  1 A and  1 B by properly moving the permanent magnets HHM 1  and HHM 2  so as to be apart from each other along their extending directions while maintaining the parallel state. Specifically, the permanent magnets HHM 1  and HHM 2  are moved so that the center position of the permanent magnet HHM 1  (or HHM 2 ) and the center position of the magnetoresistive element  1 A (or  1 B) in a relatively close position become closer to each other. Concretely, in the case where a straight line LO passing an intermediate point O between the magnetoresistive elements  1 A and  1 B and orthogonal to the extending directions of the permanent magnets HHM 1  and HHM 2  is regarded as a reference, it is preferable to move the permanent magnet HHM 1  in a direction in which the center position of magnetoresistive element  1 A is deviated (that is, to the north pole NP side) and to move the permanent magnet HHM 2  in a direction in which the center position of the magnetoresistive element  1 B is deviated (that is, to the south pole SP side). 
   Further, the bias magnetic field Hb also includes the parallel component Hxb parallel to the magnetization direction J 13  of the free layer  13 . The parallel component Hxb enhances uniaxial anisotropy of the free layer  13  as a component corresponding to the anisotropic magnetic field and contributes to stabilization of the magnetic field detecting operation in the magnetoresistive elements  1 A and  1 B. Hitherto, shape anisotropy is increased by elongating the shape of the magnetoresistive element itself to an elongate shape. Further, by disposing a plurality of elongated magnetoresistive elements in parallel at predetermined intervals, the resistance change ratio is increased. In this case, however, a relatively large space is necessary and the compensation current line becomes larger. In the current sensor  10  of the embodiment, however, the shape anisotropy is not used, so that the magnetoresistive elements  1 A and  1 B have a high degree of flexibility in shape and each of the magnetoresistive elements  1 A and  1 B does not have be divided into a plurality of elements. Therefore, the detection of the current magnetic field can be performed with high precision and sufficient stability while realizing a more compact configuration. 
   In the current sensor  10  having such a configuration, the compensation current Id based on the potential difference V 0  between the third and fourth connection points P 3  and P 4  (the difference between voltage drops occurring in the magnetoresistive elements  1 A and  1 B) when voltage is applied across the first and second connection points P 1  and P 2  flows in the compensation current line C via the differential amplifier AMP as a differential detector. The compensation current Id is detected by the compensation current detecting means S. The differential amplifier AMP adjusts the compensation current Id so that the difference V 0  becomes zero. 
   A method of measuring the current magnetic field Hm generated by the current Im to be detected will be described hereinbelow with reference to  FIG. 3 . 
   In  FIG. 3 , constant currents from the constant current sources  2 A and  2 B when a predetermined voltage is applied across the first and second connection points P 1  and P 2  are expressed as I 0 , and the resistance values of the magnetoresistive elements  1 A and  1 B are expressed as R 1  and R 2 , respectively. In the case where no current magnetic field Hm is applied, the potential V 1  at the third connection point is expressed as follows.
 
 V 1 =I 0 ·R 1
 
The potential V 2  at the fourth connection point P 4  is expressed as follows.
 
 V 2 =I 0 ·R 2
 
Therefore, the potential difference between the third and fourth connection points P 3  and P 4  is obtained by the following equation.
 
   
     
       
         
           
             
               
                 
                   
                     
                       
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                 2 
                 ) 
               
             
           
         
       
     
   
   As already described above, since the magnetoresistive elements  1 A and  1 B are disposed so that the current magnetic fields Hm make the resistance values R 1  and R 2  change in directions opposite to each other, the signs (positive and negative signs) of the change amounts ΔR 1  and ΔR 2  are accordingly different from each other. Therefore, in Equation (2), while the resistance values R 1  and R 2  before application of the current magnetic fields Hm cancel out each other, the change amounts ΔR 1  and ΔR 2  are maintained as they are. 
   When it is assumed that the magnetoresistive elements  1 A and  1 B have the same characteristics, that is, R 1 =R 2 =R and ΔR 1 =−ΔR 2 =ΔR, Equation (3) is changed as follows. 
                         V   ⁢           ⁢   0     =     I   ⁢           ⁢     0   ·     (       R   ⁢           ⁢   1     +     Δ   ⁢           ⁢   R   ⁢           ⁢   1     -     R   ⁢           ⁢   2     -     Δ   ⁢           ⁢   R   ⁢           ⁢   2       )                     =     I   ⁢           ⁢     0   ·     (     R   +     Δ   ⁢           ⁢   R     -   R   +     Δ   ⁢           ⁢   R       )                     =     I   ⁢           ⁢     0   ·     (     2   ⁢   Δ   ⁢           ⁢   R     )                       (   4   )               
Therefore, by using the magnetoresistive elements  1 A and  1 B whose relations between the resistance change amounts and magnetic field applied are grasped in advance, the magnitude of the current magnetic field Hm can be measured, and the magnitude of the current Im to be detected which generates the current magnetic field Hm can be estimated. In this case, since sensing is performed by using the two magnetoresistive elements  1 A and  1 B, the resistance change amount twice as large as that of the case of performing sensing using the magnetoresistive element  1 A or  1 B singly can be obtained. Thus, the invention is favorable for higher precision of a measurement value. In addition, variations in the characteristics among the magnetoresistive elements, variations in connection resistance, and the like can be suppressed more than the case of performing sensing using a bridge circuit constructed by four magnetoresistive elements. Therefore, even in the case of using high-sensitive magnetoresistive elements, balance adjustment is easy. Moreover, the number of magnetoresistive elements themselves can be also reduced and, accordingly, the number of terminals decreases, so that the invention is favorable for reduction in space.
 
   Further, in the current sensor  10 , both of the potential V 1  detected at the third connection point P 3  and the potential V 2  detected at the fourth connection point P 4  are supplied to the differential amplifier AMP, and the compensation current Id by which the difference (the potential difference V 0 ) becomes zero is output. The compensation current Id from the differential amplifier AMP flows in the direction opposite to the current Im to be detected near the magnetoresistive elements  1 A and  1 B, and generates the compensation current magnetic fields Hd in the direction opposite to that of the current magnetic fields Hm, thereby acting so as to cancel an error caused by variations in the connection resistance in the circuit, variations in the characteristics between the magnetoresistive elements  1 A and  1 B, bias in the temperature distribution, an interference magnetic field from the outside, or the like. As a result, the magnitude of the compensation current Id becomes proportional only to the current magnetic field Hm. Therefore, the compensation current detecting means S can compute the current magnetic field Hm more accurately by measuring the output voltage V 0  out and calculating the compensation current Id from the relation with the known resistor RL and, accordingly, can estimate the magnitude of the current Im to be detected with high precision. 
   As described above, the current sensor  10  of the embodiment has: the magnetoresistive elements  1 A and  1 B disposed along the conductor  4  so that resistance values R 1  and R 2  change in directions opposite to each other in accordance with the current magnetic field Hm generated by the current Im to be detected flowing through the conductor  4 , and connected in parallel with each other; the constant current source  2 A connected in series with the magnetoresistive element  1 A at the third connection point P 3 ; and the constant current source  2 B connected in series with the magnetoresistive element  1 B at the fourth connection point P 4 . The constant current sources  2 A and  2 B are connected to each other at the second connection point P 2 . The current Im to be detected is detected on the basis of the potential difference V 0  between the third and fourth connection points P 3  and P 4  when voltage is applied across the first and second connection points P 1  and P 2 . With the configuration, an offset value can be adjusted at the zero magnetic field more easily as compared with the case of using four magnetoresistive elements, and magnetoresistive elements having higher sensitivity can be used as the magnetoresistive elements  1 A and  1 B. Further, equal and stable currents can be supplied to the magnetoresistive elements  1 A and  1 B. Therefore, the current magnetic field Hm generated even by the weak current Im to be detected can be detected with high sensitivity and high precision. By providing the constant current sources  2 A and  2 B, balance adjustment including the magnetoresistive elements  1 A and  1 B becomes necessary but it can be electrically controlled. Consequently, as compared with the case of using four magnetoresistive elements, the balance adjustment can be performed more easily. 
   In particular, the current sensor  10  further includes the compensation current line C to which the compensation current Id based on the difference V 0  between the potential V 1  detected at the third connection point P 3  and the potential V 2  detected at the fourth connection point P 4  (the difference between the voltage drops which occur in the magnetoresistive elements  1 A and  1 B) is supplied. The compensation current Id gives the compensation current magnetic field Hd in the direction opposite to that of the current magnetic field Hm to each of the magnetoresistive elements  1 A and  1 B. Therefore, a change in the output voltage Vout caused by variations in characteristics between the magnetoresistive elements, variations in connection resistance in the circuit, a temperature distribution, and the like can be cancelled. Thus, the current magnetic field Hm can be detected with higher sensitivity and higher precision. 
   Further, in the current sensor  10 , the magnetoresistive elements  1 A and  1 B, the constant current sources  2 A and  2 B, and the compensation current line C are provided so as to be line-symmetrical with respect to the center line CL as a symmetry axis, and the pair of parallel parts  4 A and  4 B in the conductor  4  are disposed line-symmetrically with respect to the center line CL as a symmetry axis in a plane parallel to a plane including the magnetoresistive elements  1 A and  1 B and extend in parallel with each other along the center line CL. Thus, a temperature distribution which is symmetrical with respect to the center line CL can be obtained. Therefore, a zero point drift depending on the temperature distribution can be suppressed. 
   Modification 
   A current sensor  10 A as a modification in the embodiment will be described hereinbelow with reference to  FIG. 9 . 
   In the foregoing embodiment, as shown in  FIG. 2 , the ends of each of the permanent magnets HHM 1  and HHM 2  in the longitudinal direction are set as the north pole NP and the south pole SP Alternately, shown in  FIG. 9 , ends in the width direction of each of the permanent magnets HHM 1  and HHM 2  may be set as the north pole NP and the south pole SP. In this case, the permanent magnets HHM 1  and HHM 2  are disposed parallel with each other so that the north pole NP of one of them (the permanent magnet HHM 1 ) and the south pole SP of the other permanent magnet (the permanent magnet HHM 2 ) face each other. With such a configuration as well, the bias magnetic field Hb can be applied to the magnetoresistive elements  1 A and  1 B. Moreover, by changing the relative positions (distance and angle) between the magnetoresistive elements  1 A and  1 B and the permanent magnets HHM 1  and HHM 2 , the magnetic flux densities of the parallel component Hxb and the orthogonal component Hyb applied to the magnetoresistive elements  1 A and  1 B can be adjusted. 
   Example 
   Next, an example of the invention will be described hereinbelow. 
   First, in the current sensor of the embodiment, a change in the resistance value R was measured in the case where each of a magnetic field Hx parallel to the magnetization direction of the pinned layer and a magnetic field Hy orthogonal to the magnetization direction of the pinned layer was applied to the magnetoresistive element. At that time, the bias magnetic field Hb including the parallel component Hxb and the orthogonal component Hyb was also applied.  FIGS. 10 to 14  show the results. In this case, the positive/negative sign of the magnetic field Hx and that of the parallel component Hxb coincide with each other and, similarly, the positive/negative sign of the magnetic field Hy and that of the orthogonal component Hyb coincide with each other. 
   In each of  FIGS. 10 to 14 , the horizontal axis indicates the magnetic flux densities of the magnetic fields Hx and Hy and the vertical axis indicates the resistance value R. In each of diagrams, a change in the resistance value R at the time of applying the magnetic field Hx is indicated by a curve R-Hx and a change in the resistance value R at the time of applying the magnetic field Hy is indicated by a curve R-Hy.  FIG. 10  shows results corresponding to the case where each of the parallel component Hxb and the orthogonal component Hyb was set to 10×10 −4  T. Similarly,  FIGS. 11 to 14  show results corresponding to the cases where the parallel component Hxb and the orthogonal component Hyb were set to 20×10 −4  T, 30×10 −4  T, 40×10 −4  T, and 50×10 −4  T. 
   As evident from the results in  FIGS. 10 to 14 , the resistance values R hardly showed hysteresis for changes in the magnetic fields Hy (that is, changes of the magnetic fields in the induction direction) but showed hysteresis for changes in the magnetic fields Hx in each of the cases. However, by increasing the parallel component Hxb of the bias magnetic field Hb, a deviation in the numerical value when the magnetic field Hx is zero (a gap GP caused by hysteresis) was reduced. A change in the gap GP with respect to the parallel component Hxb is shown in  FIG. 15 . 
   In  FIG. 15 , the horizontal axis shows a magnetic density Hxb+Hin of a composite magnetic field obtained by combining the parallel component Hxb and the exchange bias magnetic field Hin, and the vertical axis shows the gap GP corresponding to  FIGS. 10 to 14 . The magnetic flux density of the exchange bias magnetic fields Hin was 7×10 −4  T. The case where the orthogonal component Hyb of the bias magnetic fields Hb was set to 10×10 −4  T, 20×10 −4  T, 30×10 −4  T, 40×10 −4  T, and 50×10 −4  is shown. It was understood from the result of  FIG. 15  that when the magnetic flux density Hxb+Hin of the composite magnetic field is equal to or higher than 22×10 −4  T, noise which becomes a problem does not occur and high-sensitive and stable sensing can be performed. 
     FIG. 16  shows the relation between linearity of the compensation current Id (in the current circuit of  FIG. 3 ) and the magnetic flux density Hxb+Hin of the composite magnetic field between the parallel component Hxb and the exchange bias magnetic field Hin. Concretely, the linearity is expressed as follows.
 (linearity %)=[( Im−m−·Id )/ Im (max)]max  (5) 
where, Im indicates a current to be detected, Im(max) indicates a maximum current to be detected, and “m” indicates a constant of proportionality that is expressed as Im(max)/Id. In  FIG. 16 , the horizontal axis indicates the magnetic flux density Hxb+Hin(10 −4  T) of the composite magnetic field obtained by combining the parallel component Hxb and the exchange bias magnetic field Hin, and the vertical axis indicates the linearity (%) computed from Equation (5). The case where the magnetic flux density of the exchange bias magnetic field Hin was set to 7×10 4  T and the orthogonal component Hyb of the bias magnetic field Hb was set to 10×10 −4  T, 20×10 −4  T, 30×10 −4  T, 40×10 −4  T, and 50×10 −4  T in a manner similar to  FIG. 15 . It was understood from the result of  FIG. 16  that stable linearity can be obtained in each of the cases when the magnetic flux density Hxb+Hin of the composite magnetic field is equal to 52×10 −4  T or less.
 
     FIG. 17  shows a change in sensibility of the magnetoresistive element. The “sensibility” indicates here a resistance change ratio of the magnetoresistive element, that corresponds to a gradient at the magnetic field Hy=0 in the curve R-Hy shown in each of  FIGS. 10 to 14 . The horizontal axis indicates the magnetic flux density of the orthogonal component Hyb and the vertical axis indicates the sensibility obtained from  FIGS. 10 to 14 .  FIG. 17  shows the correlation between the magnetic flux density of the orthogonal component Hyb and sensitivity with respect to the case where the parallel component Hxb of the bias magnetic field Hb was set to 10×10 −4  T, 20×10 −4  T, 30×10 −4  T, 40×10 −4  T, and 50×10 −4  T. As a result, it was known that almost constant sensibility can be obtained when the magnetic flux density of the orthogonal component Hyb lies in the range from 15×10 −4  T to 45×10 −4  T. 
   Although the invention has been described above by the embodiments and the example, the invention is not limited to the foregoing embodiments and the example but can be variously modified. For example, although the magnetoresistive element constructed so that the magnetization direction of the free layer under no magnetic field is parallel to that of the pinned layer is employed, the invention is not limited to the configuration. For example, the magnetoresistive element constructed so that the magnetization direction of the free layer under no magnetic field is orthogonal to that of the pinned layer may be used. 
   Although the permanent magnets are disposed so as to face each other as means for applying the bias magnetic field in the foregoing embodiment and the like, the means is not limited to the magnets. For example, as shown in  FIG. 18 , solenoid coils BC 1  and BC 2  (hereinbelow, simply called coils BC 1  and BC 2 ) may be provided near the magnetoresistive elements  1 A and  1 B, respectively. For example, the orthogonal component Hyb may be generated by the coil BC 1 , the parallel component Hxb may be generated by the coil BC 2 , and the bias magnetic field Hb as a whole may be applied to the magnetoresistive elements  1 A and  1 B. In the case, solenoid coils BC 3  and BC 4  (hereinbelow, simply called coils BC 3  and BC 4 ) may be provided so as to face the coils BC 1  and BC 2 , respectively, while sandwiching the magnetoresistive elements  1 A and  1 B as shown in  FIG. 19 . With the configuration, a stabler orthogonal component Hyb can be generated by the coils BC 1  and BC 3 , and a stabler parallel component Hxb can be formed by the coils BC 2  and BC 4 . 
   Although the direction of the exchange bias magnetic field Hin and that of the parallel component Hxb are set to be the same in the foregoing embodiment, the directions may be opposite to each other. However, the case where the directions are set to the same is preferable since the detecting operation becomes stabler. 
   Although the invention has been described by using, as an example, the current sensor measuring the current magnetic field generated by the current flowing through the conductor in the foregoing embodiment and the like, the invention is not limited to the current sensor. The invention also includes a magnetic sensor detecting a change in a magnetic flux itself in the environment thereof without having a conductor to which current is passed. The magnetic sensor of the invention can be also applied to an eddy current inspection technique for detect a defective in printed wiring and the like. In this case, a number of magnetoresistive elements are disposed linearly on an object and a change in an eddy current is captured as a change in a magnetic flux. 
   Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.