Patent Publication Number: US-7589612-B2

Title: Current sensor

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a small current sensor capable of sensing with high sensitivity a change in the current flowing through a conductor. 
   2. Description of the Related Art 
   Generally, as a way to accurately sense a weak control current flowing through a circuit of control equipment, resistors are connected in series in the circuit and a voltage drop in the resistors is measured. In this way, however, a load different from that in a control system is applied, which might exert an influence on the control system. For this reason, there is employed an indirect measuring method in which the gradient of a magnetic field generated by a control current is detected. Specifically, a line to be measured is wound around a toroidal core, and a control current is then supplied to the line measured, thereby detecting a magnetic flux generated in the center portion of the toroidal core by Hall element. 
   However, problems with such a current sensor employing the above method such as the difficulty of miniaturization and the insufficiency of linearity or high frequency responsiveness have been pointed out. For this reason, a current sensor has been proposed in which a giant magnetoresistive element (hereinafter referred to as a GMR element) that produces giant magnetoresistive effect is disposed in a magnetic field in order to detect the gradient (see, for example, U.S. Pat. No. 5,621,377). A current sensor using such a GMR element can improve detection sensitivity and responsiveness, and also exhibit detection characteristics that are stable against temperature changes. 
   SUMMARY OF THE INVENTION 
   Recently, there have been demands for a current sensor capable of detecting a weaker current and having a more compact general configuration. In a conventional current sensor using a GMR element, however, the GMR element is disposed adjacent to a line to be measured in an in-plane direction. This makes it difficult to detect a weak current. This is also unfavorable to the miniaturization. 
   The present invention has been achieved in consideration of the above problems, and it is desirable to provide a current sensor that is compact and capable of detecting a magnetic field generated by a current with high sensitivity and high accuracy. 
   A first current sensor of the invention detects a current in the range of 10 mA to 50 mA, and includes the following components: a first conductor which has a first extended portion extending in a first direction in a first layer and being supplied with a current; and a first magnetoresistive element which is disposed at an area corresponding to the first extended portion in a second layer, a resistance value of the first magnetoresistive element varying according to a first magnetic field generated by a current flowing through the first extended portion. In addition, the current sensor satisfies the following conditional expressions (1) and (2):
 
0.4 μm≦D1≦1.0 μm  (1)
 
0.4 μm 2 ≦S1≦3.0 μm 2   (2)
 
where D 1  is a distance between a first extended portion and a first magnetoresistive element; and S 1  is an area of a cross section orthogonal to a first direction in the first extended portion.
 
   A second current sensor of the invention detects a current in the range of 3 mA to 50 mA, and includes a first conductor and a first magnetoresistive element as described above. In addition, the current sensor satisfies the following conditional expressions (3) and (4):
 
0.2 μm≦D1≦0.4 μm  (3)
 
0.4 μm 2 ≦S1≦2.5 μm 2   (4)
 
where D 1  and S 1  are as described above.
 
   In the first and second current sensors, the first conductor and the first magnetoresistive element are disposed in different layers. Therefore, they can be disposed closer than if disposed in the same layer, thereby reducing the overall dimension. Moreover, the first magnetic field based on a current flowing through the first extended portion can be applied more strongly on the first magnetoresistive element. In particular, since the conditional expressions (1) and (2), or the conditional expressions (3) and (4) are satisfied, the first magnetoresistive element is less subjected to the influence of heat generated in the first conductor, so that the first magnetic field can be applied efficiently on the first magnetoresistive element. 
   This permits a compact configuration and efficient detection of the first magnetic field with reduced influence of heat generated from the first conductor. Therefore, a relatively weak current flowing through the first conductor, which is from 10 mA to 50 mA (on the first current sensor), or from 3 mA to 50 mA (on the second current sensor) can be measured with high accuracy and high sensitivity. 
   Preferably, the first magnetoresistive element has a plurality of element patterns which extend in a first direction and are disposed adjacent each other in a second direction orthogonal to the first direction. Preferably, the first conductor is a first thin film coil which winds in the first layer while including a plurality of turn portions as a first extended portion extending in the first direction in correspondence with the element patterns of the first magnetoresistive element, and which applies a first magnetic field on each of the element patterns of the first magnetoresistive element under the supply of a current. In this case, the element patterns in the first magnetoresistive element may be connected to each other in parallel or in series. 
   Especially, when the individual element patterns in the first magnetoresistive element are connected in parallel, the whole resistance value can be held relatively low without decreasing the resistance change rate of the first magnetoresistive element. This reduces the calorific value of the first magnetoresistive element in use. In addition, the influence due to noise (undesired magnetic field) can be reduced to improve an S/N ratio. On the other hand, when the individual element patterns in the first magnetoresistive element are connected in series to each other, the whole extension length of the element patterns functioning as a magnetosensitive part increases without increasing the size in the first direction. This further increases the absolute value of the whole resistance value (impedance) in the first magnetoresistive element. This permits higher-accurate measurement of a weak current. 
   In an alternative, the first and second current sensor may further include a second conductor disposed in a third layer positioned on a side opposite to a first layer with respective to a second layer, the second conductor has a second extended portion extending in a first direction in correspondence with the first magnetoresistive element and generating a second magnetic field through the current supplied, the second magnetic field then applied on the first magnetoresistive element in the same direction as the first magnetic field. In this case, the first current sensor is configured to satisfy the following conditional expressions (5) and (6), and the second current sensor is configured to satisfy the following conditional expressions (7) and (8).
 
0.4 μm≦D2≦1.0 μm  (5)
 
0.4 μm 2 ≦S2≦3.0 μm 2   (6)
 
0.2 μm≦D2≦0.4 μm  (7)
 
0.4 μm 2 ≦S2≦2.5 μm 2   (8)
 
where D 2  is a distance between a second extended portion and a first magnetoresistive element; and S 2  is an area of a cross section orthogonal to a first direction in the second extended portion.
 
   In particular, the second conductor is preferably a second thin film coil which winds in a third layer while including a plurality of turn portions as a second extended portion extending in a first direction in correspondence with the element patterns of the first magnetoresistive element, and which generates and applies a second magnetic field on each of the element patterns of the first magnetoresistive element under the supply of a current. 
   The first current sensor satisfying the conditional expressions (5) and (6), and the second current sensor satisfying the conditional expressions (7) and (8) enable a composite magnetic field of the first and second magnetic fields to be applied on the first magnetoresistive element, so that the absolute value of the resistance value in the first magnetoresistive element can be further increased, resulting in further improved measuring accuracy of a current. 
   In another alternative, the first and second current sensors may further include, in addition to a first magnetoresistive element and a first conductor, (i) a third conductor disposed in a first layer, the third conductor having a third extended portion extending in a first direction in an area other than where the first conductor is formed and being supplied with a current; and (ii) a second magnetoresistive element disposed in the second layer in an area corresponding to the third extended portion and commonly connected to the first magnetoresistive element, and the resistance value of the second magnetoresistive element varying according to a third magnetic field generated by a current flowing through the third extended portion. In this case, the first current sensor satisfies the following conditional expressions (11) and (12), and the second current sensor satisfies the following conditional expressions (9) and (10).
 
0.4 μm≦D3≦1.0 μm  (11)
 
0.4 μm 2 ≦S3≦3.0 μm 2   (12)
 
0.2 μm≦D3≦0.4 μm  (9)
 
0.4 μm 2 ≦S3≦2.5 μm 2   (10)
 
where D 3  is a distance between a third extended portion and a second magnetoresistive element; and S 3  is an area of a cross section orthogonal to a first direction in the third extended portion.
 
   The presence of the first and second magnetoresistive elements permits greater accurate measurements of a current. With such a configuration that the resistance value of the second magnetoresistive element varies according to the third magnetic field in the direction opposite to resistance value variations in the first magnetoresistive element which can be brought by the first current magnetic filed, a current can be measured with greater accuracy based on a difference in voltage drop which can be brought when the same constant current is allowed to flow through the first and second magnetoresistive elements. 
   It may be arranged to further include: (i) a second conductor disposed in a third layer located in a side opposite to a first layer with respective to a second layer, the second conductor having a second extended portion extending in the first direction in correspondence with the first magnetoresistive element and generating a second magnetic field through the current supplied, the second magnetic field then applied on the first magnetoresistive element in the same direction as the first magnetic field; and (ii) a fourth conductor disposed in the third layer located in an area other than where the second conductor is formed, the fourth conductor having a fourth extended portion extending in the first direction in correspondence with the second magnetoresistive element and generating a fourth magnetic field through the current supplied, the fourth magnetic field then applied on the second magnetoresistive element in the same direction as the third magnetic field. In this case, the first current sensor satisfies the following conditional expressions (17) to (20), and the second current sensor satisfies the following conditional expressions (13) to (16).
 
0.44 μm≦D2≦1.0 μm  (17)
 
0.4 μm≦D4≦1.0 μm  (18)
 
0.4 μm 2 ≦S2≦3.0 μm 2   (19)
 
0.4 μm 2 ≦S4≦3.0 μm 2   (20)
 
0.2 μm≦D2≦0.4 μm  (13)
 
0.2 μm≦D4≦0.4 μm  (14)
 
0.4 μm 2 ≦S2≦2.5 μm 2   (15)
 
0.4 μm 2 ≦S4≦2.5 μm 2   (16)
 
where D 2  and S 2  are as described above; D 4  is a distance between a fourth extended portion and a second magnetoresistive element; and S 4  is an area of a cross section orthogonal to a first direction in the fourth extended portion.
 
   With such a configuration that the first magnetoresistive element detects a composite magnetic field of the first and second magnetic fields, and the second magnetoresistive element detects a composite magnetic field of the third and fourth magnetic fields, the presence of the first and second magnetoresistive elements permits much more accurate measurements of a current flowing through the first to fourth conductors, while maintaining a compact configuration. 
   Preferably, it is configured such that the direction of a resistance value variation of the second magnetoresistive element brought through the third and fourth magnetic fields is opposite to resistance value variations in the first magnetoresistive element which can be generated by the first and second magnetic fields. 
   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 a first embodiment of the invention. 
       FIG. 2  is a sectional view taken along the line II-II of the current sensor shown in  FIG. 1 . 
       FIG. 3  is a sectional view showing in enlarged dimension a key part of the current sensor shown in  FIG. 2 . 
       FIG. 4  is a schematic diagram for explaining the heat transfer amount from a turn portion to an element pattern in the current sensor shown in  FIG. 1 . 
       FIG. 5  is a plot of characteristics showing the relationship between a cross-sectional area of the turn portion shown in  FIG. 4  and a generated magnetic field. 
       FIG. 6  is an exploded perspective view showing the configuration of a magnetoresistive element that forms a key part of the current sensor shown in  FIG. 1 . 
       FIG. 7  is a perspective view showing the configuration of a part of the magnetoresistive element shown in  FIG. 6 . 
       FIG. 8  is a plot of characteristics showing magnetic field dependency of a resistance change rate in the magnetoresistive element shown in  FIG. 6 . 
       FIG. 9  is another exploded perspective view showing the configuration of the magnetoresistive element that forms a key part of the current sensor shown in  FIG. 1 . 
       FIG. 10  is another sectional view showing in enlarged dimension a key part of  FIG. 2 . 
       FIG. 11  is a perspective view showing the configuration of a current sensor according to a second embodiment of the invention. 
       FIG. 12  is a sectional view taken along the line XII-XII of the current sensor shown in  FIG. 11 . 
       FIG. 13  is a sectional view showing in enlarged dimension a key part of  FIG. 12 . 
       FIG. 14  is a perspective view showing the configuration of a current sensor according to a third embodiment of the invention. 
       FIG. 15  is a sectional view taken along the line XV-XV of the current sensor shown in  FIG. 14 . 
       FIG. 16  is a sectional view showing in enlarged dimension an important part of  FIG. 15 . 
       FIG. 17  is a circuit diagram of the current sensor shown in  FIG. 14 . 
       FIG. 18  is a perspective view showing the configuration of a current sensor according to a fourth embodiment of the invention. 
       FIG. 19  is a sectional view taken along the line XIX-XIX of the current sensor shown in  FIG. 18 . 
       FIG. 20  is a sectional view showing in enlarged dimension a key part of  FIG. 19 . 
       FIG. 21  is a perspective view showing the configuration of a modification of the current sensor shown in  FIG. 18 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments of the invention will be described in detail hereinbelow with reference to the drawings. 
   First Embodiment 
   Reference to  FIGS. 1 and 2 , the configuration of a current sensor as a first embodiment of the invention will be described.  FIG. 1  is a schematic view illustrating a perspective configuration of a current sensor  1  according to the embodiment.  FIG. 2  shows a cross-sectional configuration taken along line II-II in the current sensor  1  of  FIG. 1 , looking in the direction of the appended arrows (−X-axis direction). The current sensor  1  is mounted on, for example, communication equipment, and used to accurately detect and measure a current of a control signal. In particular, current in the range of 10 mA to 50 mA is detected here. To distinguish the current sensor of the first embodiment from that in embodiments to be described later, the current sensor in the first embodiment is hereinafter referred to as a current sensor  1 A. 
   The current sensor  1 A includes a first thin film coil  31  as a first conductor disposed in a first layer L 1 , and a first magnetoresistive element  21  disposed in a second layer L 2  (hereinafter referred to simply as a thin film coil  31 ). The first magnetoresistive element  21  has two element patterns  21 A and  21 B, extending in a first direction (an X-axis direction). The thin film coil  31  is configured so as to wind while including turn portions  31 A and  31 B as a first extended portion extending in the X-axis direction in correspondence to the element patterns  21 A and  21 B, respectively. Specifically, as shown in  FIG. 2 , the current sensor  1 A has a structure in which the second layer L 2  including the first magnetoresistive element  21 , and the first layer L 1  including the thin film coil  31  are stacked in the order listed via an underlayer  3  made of aluminum oxide (Al 2 O 3 ) or the like on a substrate  2  made of silicon (Si) or the like. In the cross section of  FIG. 2 , the first magnetoresistive element  21  and the thin film coil  31  are covered with insulating films Z 1  and Z 2  made of Al 2 O 3  or the like, respectively, and they are electrically isolated from each other. Moreover, a plurality of electrode films  41  to  44  are provided over the insulating film Z 2 , as shown in  FIG. 1 . 
   The thin film coil  31  is a thin film pattern made of high conductivity metal material such as copper (Cu), to which a current Im, for example, a control signal current is supplied. One end  31 S of the thin film coil  31  is connected via a contact hole (not shown) to an electrode film  41 , and the other end  31 E is connected via a contact hole (not shown) to an electrode film  42  (see  FIG. 1 ). The current sensor  1 A is set such that a current Im flows from the end  31 S to the end  31 E. 
   The element patterns  21 A and  21 B detect a first magnetic field Hm 1  (hereinafter referred to simply as a magnetic field Hm 1 ) which can be generated by a current Im and are provided, in the stacking direction, in areas corresponding to the turn portions  31 A and  31 B of the thin film coil  31 , respectively. The element patterns  21 A and  21 B are disposed such that they extend in an X-axis direction and are adjacent each other in a Y-axis direction (a second direction) orthogonal to the X-axis direction, and are connected in parallel to each other by electrode patterns  4  and  5 . The electrode pattern  4  is connected via a contact hole (not shown) to the electrode film  43 , and the electrode pattern  5  is connected via a contact hole (not shown) to the electrode film  44 . The element patterns  21 A and  21 B are formed in a thickness of, for example, 0.8 μm by using sputtering method or the like. When a read current is allowed to flow through the element patterns  21 A and  21 B, they undergo resistance value variations according to a magnetic field Hm 1  which can be caused by a current Im flowing through the thin film coils  31 A and  31 B. 
     FIG. 3  is a sectional view showing in enlarged dimension an important part of  FIG. 2 . In  FIG. 3 , the turn portion  31 A and the element pattern  21 A are shown as representatives. The turn portion  31 A and the element pattern  21 A are arranged such that their respective center positions in the Y-axis direction match with each other (namely, both of the center positions exist on a virtual center line CL extending in the Z-axis direction), and they are disposed apart a distance D 1  in the Z-axis direction. The distance D 1  is in the range of 0.4 μm to 1.0 μm (Conditional expression  1 ). Specifically, if the insulating film Z 1  separating the turn portion  31 A from the element pattern  21 A has a thickness of more than or equal to 0.4 μm, it can withstand momentary application of a voltage of 1000V, which is suitable to practical use. Since the smallest possible dimension is preferable in the interest of compactness, the distance D 1  should be less than or equal to 1.0 μm. 
   The turn portion  31 A is shaped as a rectangle defined by a width MX 1  along a Y-axis and a thickness MY 1  along a Z-axis in a YZ cross section (a cross section orthogonal to the X-axis direction), and has a cross-sectional area S 1  (=MX 1 *MY 1 ). Specifically, it is arranged such that the width MX 1  is less than or equal to 3.0 μm, and the cross-sectional area S 1  is in the range of 0.4 μm 2  to 3.0 μm 2  (Conditional expression 2). In consideration of accuracy during the process of formation, it is desirable to set the thickness MY 1  at more than or equal to 0.2 μm and to be equal to or less than the width MX 1 . 
   When the cross-sectional area S 1  is less than 0.4 μm 2 , a current Im (=10 mA to 50 mA) flowing through the turn portion  31 A might cause excess temperature rise (for example, exceeding 2.0° C.) in the element pattern  21 A, resulting in poor accuracy of detection. A temperature change exceeding 2.0° C. in the element pattern  21 A may cause output variations exceeding about 0.2%, which is undesirable because the reliability of the current sensor is lost. A calorific value P per unit length (1 m) of the turn portion  31 A can be expressed by the following equation (A):
 
 P=Im   2 *(ρ/ S )  (A)
 
where Im is a current value; ρ is the specific resistance of copper; and SI is a cross-sectional area.
 
   The current sensor  1 A of this embodiment is aimed at detecting a control signal of communication equipment or the like, and therefore the magnitude of a current Im is 50 [mA] (=5×10 −2  [A]) in maximum. The specific resistance of copper is 1.92×10 −8  [Ω*m]. Substitution of these into Equation (A) yields: 
   
     
       
         
           
             
               
                 
                   
                     
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   The element pattern  21 A, which is disposed apart by the insulating film Z 1  from the turn portion  31 A that generates the calorific value P so obtained, will receive a heat transfer amount Q per unit length (1 m), which can be expressed by the following equation (C). For sake of simplicity, there is replaced by such an approximate model as shown in Fug.  4 , in which the cross section of the turn portion  31 A is a circle having a radius r 1  (=(S 1 /π) 0.5  [m], and the insulating film Z fills a range over a radius r 2  [m] around this circle. In this case, the calorific value P generated from the turn portion  31 A is transferred uniformly to the surroundings and passed through the insulating film Z having the radius r 2 , and then released from the surface of the insulating film Z to the exterior. At this time, the element pattern  21 A in the vicinity of the turn portion  31 A will receive part of the heat transfer amount Q, so that the temperature of the element pattern  21 A is raised approximately as much as a surface temperature increment ΔT [° C.] of the turn portion  31 A. This can be expressed as follows by letting λ be the heat conductivity of aluminum oxide (Al 2 O 3 ) constituting the insulating film Z 1 .
 
 Q =λ(2π/1 n ( r 2/ r 1))*Δ T   (C)
 
where the heat conductivity λ is 30 [W/m/° C.]. It is desirable that the temperature change ΔT is less than or equal to 2.0° C. Substitution of these into Equation (C) yields the following equation (D):
 
                       Q   ≦       ⁢     30   *     (     2   ⁢     π   /   1     ⁢     n   ⁡     (     r   ⁢           ⁢     2   /   r     ⁢           ⁢   1     )         )     ×   2.0                   ≦       ⁢     120   ⁢     π   /   1     ⁢     n   ⁡     (     r   ⁢           ⁢     2   /   r     ⁢           ⁢   1     )           )                 (   D   )               
Here, Q=P, and the following equation (E) is obtained from Equations (B) and (D).
 48.0*10 −12   /S 1≦120π/1 n ( r 2/ r 1)  (E) 
   If the cross-sectional area “S 1 ” is replaced with “π* (r 1 ) 2 ”, the following equation (F) is obtained. 
   
     
       
         
           
             
               
                 
                   
                     
                       
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   Considering the average thickness of the insulating film Z in the vicinity of the element pattern  21 A, it is possible to regard this embodiment as the case where the insulating film Z covers the area over the radius r 2  (=5+r 1 ) πm. Hence, from Equation (F), the radius r 1  is approximately more than or equal to 0.36 μm. It is therefore desirable that the cross-sectional area S is more than or equal to 0.4 μm 2 . 
   On the other hand, when the cross-sectional area S 1  exceeds 3.0 μm 2 , the strength of the magnetic field Hm 1  is lowered, so that it is difficult for the element pattern  21 A to perform excellent detecting operation.  FIG. 5  plots the relationship between the cross-sectional area S 1  and an average magnetic field Hm 1 , where a distance D 1  is 1.0 μm, and a current Im is 10 mA. This indicates that the strength of the magnetic field Hm 1  applied to the element pattern  21 A is lowered as the cross-sectional area S 1  is increased. Since the strength of substantially more than or equal to 50 e (=5*10 3 /4π [A/m]) is necessary for the element pattern  21 A to perform stable detecting operation, the cross-sectional area S 1  is preferably less than or equal to 3.0 μm 2 . 
   The turn portion  31 A having the above cross-sectional area S 1  is configured with, for example, a width MX in the range of 0.8 μm to 3.0 μm and a thickness MY in the range of 0.2 μm to 1.4 μm in the YZ cross section orthogonal to the X-axis direction. A width MW in the Y-axis direction of the element pattern  21 A is preferably less than or equal to 2.0 μm, in order to provide the element pattern  21 A with a magnetic field Hm 1  being sufficiently uniform over the whole in the Y-axis direction. On the other hand, the lower limit of the width MW is preferably 0.5 μm in order to achieve uniform film formation in the Y-axis direction. Although  FIG. 3  shows the configuration (dimension and arrangement) of only the turn portion  31 A and the element pattern  21 A, the same is true for the turn portion  31 B and the element pattern  21 B. 
   Reference to  FIGS. 6 to 9 , the configuration of the element patterns  21 A and  21 B will next be described in more detail.  FIG. 6  is an exploded perspective view showing the configuration of the element patterns  21 A and  21 B. Provided the proportion of dimension is different from that of a real thing. 
   As shown in  FIG. 6 , the element patterns  21 A and  21 B have a spin valve structure in which a plurality of function films including magnetic layers are stacked. Each of the element patterns  21 A and  21 B includes a pinned layer  11  having a magnetization direction J 11  pinned in the +X direction, a free layer  13  whose magnetization direction J 13  varies according to external magnetic fields H such as the magnetic field Hm 1 , and an intermediate layer  12  that 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), the top face of which is in contact with the pinned layer  11  and the under face is in contact with the free layer  13 . The intermediate layer  12  can be made of, instead of copper, nonmagnetic metal having high conductivity such as gold (Au). The top face of the pinned layer  11  (the surface on the side opposite to the intermediate layer  12 ) and the under face of the free layer  13  (the surface on the side opposite to the intermediate layer  12 ) are respectively protected by a protection film (not shown). Between the pinned layer  11  and the free layer  13 , exchange bias magnetic fields Hin in the magnetization direction J 11  (hereinafter referred to simply as “exchange bias magnetic fields Hin”) are generated and interact with each other via the intermediate layer  12 . The strength of the exchange bias magnetic field Hin varies as the spin direction of 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 ). Although  FIG. 6  shows an example of the configuration where the free layer  13 , the intermediate layer  12 , and the pinned layer  11  are stacked in bottom-to-top order, the invention is not limited to this configuration. These layers may be stacked in the reverse order. 
   The element patterns  21 A and  21 B are configured such that a length ML in the X-axis direction (longitudinal dimension) is in the range of 10 times to 200 times a width MW in the Y-axis direction (width dimension). Specifically, the length ML is preferably in the range of, for example, in the range of 20 μm to 100 μm. Thus, the element patterns  21 A and  21 B are shaped as a stripe having a large length ML with respect to the width MW, thereby exhibiting shape magnetic anisotropy along the Y-axis direction. Such a strip-shaped configuration improves the linearity of variations in resistance change rate with respect to variations in the external magnetic field H applied in +Y or −Y direction. If the length ML (longitudinal dimension) is less than 10 times the width MW in the Y-axis direction (width dimension), ample shape magnetic anisotropy is unobtainable. In contrast, any dimensional ratio exceeding 100 times is undesirable because no improvement of shape magnetic anisotropy can be expected and noise due to an increased resistance value may be generated. 
     FIG. 7  shows a detailed configuration of the pinned layer  11 . The pinned layer  11  has such a structure that 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 a 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 an 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 it functions to pin the magnetization direction J 11  of the magnetization pinned film  14 . 
   In the element patterns  21 A and  21 B so configured, upon the application of the magnetic field Hm 1 , the magnetization direction J 13  of the free layer  13  rotates thereby to change a relative angle between the magnetization directions J 13  and J 11 . The relative angle is determined according to the magnitude and direction of the magnetic field Hm 1 . 
     FIG. 6  shows an unloaded state in which the magnetic field Hm 1  is zero (Hm=0) and other magnetic fields (such as a bias magnetic field) are not applied (namely, the state in which the external magnetic field H is zero). An easy magnetization axis direction AE 13  of the free layer  13  is parallel to the magnetization direction J 11  of the pinned layer  11 . Thereby, in this state, all of the easy magnetization axis direction AE 13  and the magnetization directions J 11  and J 13  are parallel to each other along the +X direction, so that the spin directions of magnetic domains in the free layer  13  are aligned in almost the same direction. In the case where the external magnetic field H is applied to the element patterns  21 A and  21 B in the direction orthogonal to the magnetization direction J 11  (+Y direction or −Y direction), such characteristics as shown in  FIG. 8  are obtainable.  FIG. 8  shows the relationship between the external magnetic field H and the resistance change rate ΔR/R, on the assumption that the external magnetic field H in the +Y direction is positive. The relationship between the two becomes local minimum (ΔR/R=0) when the external magnetic field H is zero. This can be expressed by a curve C 1  which remains practically hysteresis-free. In this case, 1/f noise caused by hysteresis is minimized, permitting high-sensitive and stable sensing. 
   As apparent from  FIG. 8 , however, a linear change is unobtainable in the neighborhood of zero in the external magnetic field H (H=0). Therefore, in the practical measuring of a magnetic field Hm 1 , by applying a bias magnetic field arising from a permanent magnet (not shown) in a direction orthogonal to the magnetization direction J 11 , the magnetization direction J 13  is rotated and slightly tilted so as to include a component in the +Y direction or a component in the −Y direction, as shown in  FIG. 9  (exemplifying the case of being tilted in the −Y direction). This enables variations in the magnetic field Hm 1  to be detected with high accuracy in linear areas LA 1  and LA 2  around bias points BP 1  and BP 2  shown in  FIG. 8 , respectively. 
   In sensing with the current sensor  1 A having the above configuration, first, a sense current is allowed to flow through the element patterns  21 A,  21 B via the electrode films  43 ,  44 . It is arranged such that a current Im is supplied via the electrode films  41  and  42  to the thin film coil  31  such that the element patterns  21 A and  21 B can detect a magnetic field Hm 1  generated from the turn portions  31 A and  31 B, respectively. For example, if a current Im is allowed to flow from the end  31 S of the thin film coil  31  to the end  31 E, as shown in  FIG. 10 , the current Im will flow in the −X direction (from near side to far side as seen in  FIG. 10 ). As a result, the magnetic field Hm 1  is generated which winds (in a clockwise as seen in  FIG. 10 ) the surroundings of the turn portions  31 A and  31 B, respectively, according to corkscrew rule. Hence, the magnetic field Hm 1  toward the −Y direction is applied on each of the element patterns  21 A and  21 B, so that their respective resistance values will vary. At this time, the dimension of the current Im can be estimated by detecting a variation in voltage drop (a variation in resistance value) between the electrode patterns  4  and  5 . 
   Thus, the current sensor  1 A of this embodiment is configured to have: (i) the first magnetoresistive element  21  extending in the X-axis direction in the second layer L 2  and including the element patterns  21 A and  21 B that are disposed adjacent each other in the Y-axis direction orthogonal to the X-axis direction and connected in parallel to each other; and (ii) the thin film coil  31  which winds in the first layer L 1  while including the turn portions  31 A and  31 B that extend in the X-axis direction in correspondence with the element patterns  21 A and  21 B, respectively, and which applies the magnetic field Hm 1  on each of the element patterns  21 A and  21 B under the supply of a current Im. With this configuration, the first magnetoresistive element  21  and the thin film coil  31  can be brought into closer than the case where they are disposed adjacent each other in an in-plane direction, for example, they are disposed within the same layer. 
   Moreover, by virtue of the turn portions  31 A and  31 B of the thin film coil  31 , the magnetic field Hm 1  can be applied separately on the element patterns  21 A and  21 B of the magnetoresistive element  21  which correspond to the turn portions  31 A and  31 B, respectively. Therefore, from the viewpoint of the heat generated from the coil and the current efficiency of the coil magnetic field strength, the location and cross-sectional dimension in the cross section orthogonal to the first direction can be optimized easily. The magnetic field Hm 1  can be applied on the element patterns  21 A and  21 B with higher efficiency compared to the case of applying on each element pattern a magnetic field formed only by a current flowing through a single conductor (a turn portion). This permits high-sensitivity detection of the current Im. 
   In particular, the parallel connection of the element patterns  21 A and  21 B enables the entire resistance value in the first magnetoresistive element  21  to be held relatively low without reducing resistance change rate, thereby reducing the calorific value during use. Furthermore, the influence due to noise from the exterior (undesired magnetic field) can be reduced to improve the S/N ratio. For the above reasons, the current sensor  1 A that is compact permits high-accuracy measurement of a current Im flowing through the thin film coil  31 . 
   Additionally, setting the distance D 1  in thickness direction (Z-axis direction) between the turn portions  31 A,  31 B and the element patterns  21 A,  21 B in the range of 0.4 μm to 1.0 μm, and the cross-sectional area S 1  of the turn portion  31 A in the range of 0.4 μm 2  to 3.0 μm 2  enable the magnetic field Hm 1  to be detected efficiently with reduction of the influence of heat generated from the thin film coil  31 . This permits high-accuracy measurement of a relatively weak current Im flowing through the thin film coil  31  which is in the range of 10 mA to 50 mA. 
   Modification 1 
   Although the first embodiment describes the case of measuring a current Im which is in the range of 10 mA to 50 mA, it is possible to configure a current sensor so as to measure a weaker current Im, for example, in the range of 3 mA to 50 mA. In this case, the distance D 1  as shown in the sectional view of  FIG. 3  is in the range of 0.2 μm to 0.4 μm (Conditional expression 3). This is because the turn portion  31 A and the element pattern  21 A need to be closer in order to detect a magnetic field Hm 1  formed by a weaker current Im as low as less than or equal to 10 mA. On the other hand, setting at more than or equal to 0.2 μm makes it possible to withstand an application of a surge voltage of 700 V. This is practically favorable. 
   The cross-sectional area S 1  (=MX*MY 1 ) in the turn portion  31 A is in the range of 0.4 μm 2  to 2.5 μm 2  (Conditional expression 4). When the cross-sectional area S 1  is less than 0.4 μm 2 , a current Im flowing through the turn portion  31 A causes an excessive temperature rise (for example, exceeding 2.0° C.) in the element pattern  21 A, and the accuracy of detection might be deteriorated. On the other hand, when the cross-sectional area S 1  is above 2.5 μm 2 , the strength of the magnetic field Hm 1  may be lowered, making it difficult for the element pattern  21 A to perform excellent detecting operation. 
   Thus, the current sensor  1 A as a modification, which is configured to satisfy Conditional expressions 3 and 4, permits efficient measurement of a magnetic field Hm 1  with reduction of the influence of heat generated from the thin film coil  31 . This permits high-accuracy measurement of a current Im flowing through the thin film coil  31 , which is in the range of 3 mA to 50 mA. 
   Second Embodiment 
   Reference to  FIGS. 11 to 13 , a current sensor  1 B as a second embodiment of the invention will be described below. 
     FIG. 11  is a perspective view showing a perspective configuration of the current sensor  1 B.  FIG. 12  shows a cross-sectional configuration in the direction indicated by the arrows (−X direction) along the line XII-XII of the current sensor  1 B shown in  FIG. 11 . The current sensor  1 B can be obtained by adding a second thin film coil  32  (hereinafter referred to simply as a thin film coil  32 ) as a second conductor, to the current sensor  1 A of the above embodiment. 
   Specifically, with respect to a second layer L 2 , a third layer L 3  is provided on the side opposite to a first layer L 1  in the current sensor  1 B, and the thin film coil  32  is formed so as to be buried in an insulating film Z 3  in the third layer L 3 . In other words, as shown in  FIG. 12 , the current sensor  1 B has such a structure that the third layer L 3  including the thin film coil  32 , the second layer L 2  including a first magnetoresistive element  21 , and the first layer L 1  including a thin film coil  31  are stacked in the order listed, via an underlayer  3  made of Al 2 O 3  or the like, on a substrate  2  made of silicon or the like. In the cross section of  FIG. 12 , the thin film coil  32 , the first magnetoresistive element  21 , and the thin film coil  31  are covered with the insulating films Z 3 , Z 1 , and Z 2 , respectively, which are made of Al 2 O 3  or the like, and they are electrically isolated from each other. 
   Like the thin film coil  31 , the thin film coil  32  is a thin film pattern made of a high conductive metal material such as copper. The thin film coil  32  is configured so as to wind while including turn portions  32 A and  32 B extending in the X-axis direction in correspondence with element patterns  21 A and  21 B of the first magnetoresistive element  21 , and also apply a second magnetic field Hm 2  (hereinafter referred to simply as a magnetic field Hm 2 ), which is the same in direction as a magnetic field Hm 1 , on each of the element patterns  21 A and  21 B under the supply of a current Im. One end  31 E of the thin film coil  31  is connected via a contact hole (not shown) to one end  32 S of the thin film coil  32 , and the other end  32 E of the thin film coil  32  is connected via a contact hole (not shown) to an electrode film  42 . Consequently, the thin film coils  31  and  32  configure a single conductor line in the circuit. 
   In the current sensor  1 B so configured, single stream of current Im flowing through the thin film coils  31  and  32  causes the two magnetic fields Hm 1  and Hm 2  to act on the first magnetoresistive element  21 , as shown in  FIG. 13 .  FIG. 13  shows in enlarged dimension part of  FIG. 12 , and it is an explanatory diagram for explaining the actions of the magnetic fields Hm 1  and Hm 2  on the element patterns  21 A and  21 B, respectively. Here, the turn portions  31 A,  31 B and the element pattern  21 A,  21 B are disposed apart a distance D 1  in the Z-axis direction, and the turn portions  32 A,  32 B and the element patterns  21 A,  21 B are disposed apart a distance D 2  in the Z-axis direction. Each of the distances D 1  and D 2  is in the range of 0.4 μm to 1.0 μm (Conditional expressions 1 and 5). Each of the turn portions  31 A and  31 B has a rectangle cross section defined by a width MX 1  and a thickness MY 1 , and has a cross-sectional area S 1  (=MX 1 *MY 1 ). Similarly, the turn portions  32 A and  32 B are shaped as a rectangle defined by a width MX 2  along the Y-axis direction and a thickness MY 2  along the Z-axis direction, and have a cross-sectional area S 2  (=MX 2 *MY 2 ). Specifically, it is arranged such that each of the widths MX 1  and MX 2  is less than or equal to 3.0 μm, and each of the cross-sectional areas S 1  and S 2  is in the range of 0.4 μm 2  to 3.0 μm 2  (Conditional expressions 2 and 6). In consideration of accuracy during the process of formation, it is desirable to set the thickness MY 1 , MY 2  at more than or equal to 0.2 μm and to be equal to or less than the width MX 1 , MX 2 . 
   In sensing with the current sensor  1 B, first, a sense current is allowed to flow through the electrode films  43  and  44  to the element patterns  21 A and  21 B. Then, a current Im is supplied via the electrode films  41  and  42  to the thin film coils  31  and  32  such that the element patterns  21 A and  21 B detect a magnetic field Hm 1  generated from the turn portions  31 A and  31 B and a magnetic field Hm 2  generated from the turn portions  32 A and  32 B, respectively. For example, when a current Im is allowed to flow from the end  31 S of the thin film coil  31  to the end  31 E, and subsequently from the end  32 S of the thin film coil  32  to the end  32 E, as shown in  FIG. 13 , the current Im will flow in the −X direction (from near side to far side as seen in  FIG. 13 ) at the turn portions  31 A and  31 B. As a result, a magnetic field Hm 1  is generated which winds (in a clockwise as seen in  FIG. 13 ) the surroundings of the turn portions  31 A and  31 B, respectively, according to corkscrew rule. On the other hand, the current Im will flow in the +X direction (from far side to near side as seen in  FIG. 13 ) at the turn portions  32 A and  32 B. As a result, a magnetic field Hm 2  is generated which winds (in a counterclockwise as seen in  FIG. 13 ) the surroundings of the turn portions  32 A and  32 B, respectively, according to corkscrew rule. Hence, a composite magnetic field of the magnetic fields Hm 1  and Hm 2  toward the −Y direction is supplied to the element patterns  21 A and  21 B, respectively. Therefore, strength of the magnetic field applied on the element patterns  21 A and  21 B can be increased, resulting in greater variations in resistance value than the case of applying only the magnetic field Hm 1 . Here, in the same manner as in the first embodiment, the magnitude of the current Im can be estimated by detecting a variation in voltage drop (a variation in resistance value) between the electrode patterns  4  and  5 . 
   Thus, since the current sensor  1 B of this embodiment includes the thin film coil  32  configured so as to apply the magnetic field Hm 2 , which is the same in direction as the magnetic field Hm 1 , on each of the element patterns  21 A and  21 B, a composite magnetic field of the magnetic fields Hm 1  and Hm 2  can be applied on the element patterns  21 A and  21 B. This further increases the absolute value of the variation in the resistance value at the first magnetoresistive element  21 , permitting higher-accuracy measurement of a current Im. 
   Especially, like the first embodiment, each of the distance D 1  between the turn portions  31 A,  31 B and the element patterns  21 A,  21 B, and the distance D 2  between the turn portions  32 A,  32 B and the element patterns  21 A,  21 B is in the range of 0.4 μm to 1.0 μm. Also, each of the cross-sectional area S 1  of the turn portions  31 A and  31 B, and the cross-sectional area S 2  of the turn portions  32 A and  32 B is in the range of 0.4 μm 2  to 3.0 m 2 . With this configuration, the composite magnetic field of the magnetic fields Hm 1  and Hm 2  can be detected efficiently with reduction of the influence of heat generated from the thin film coils  31  and  32 . This permits high-accuracy measurement of a relatively weak current Im flowing through the thin film coils  31  and  32 , which is from 10 μmA to 50 mA. 
   Modification 2 
   Although the above embodiment describes the case of measuring the current Im which is in the range of 10 mA to 50 mA, it is possible to configure a current sensor so as to measure a weaker current Im, for example, in the range of 3 mA to 50 mA. In this case, each of the distances D 1  and D 2  is in the range of 0.2 μm to 0.4 μm (Conditional expressions 3 and 7), and each of the cross-sectional areas S 1  and S 2  is in the range of 0.4 μm 2  to 2.5 μm 2  (Conditional expressions 4 and 8). 
   Thus, the current sensor  1 B as a modification, which is configured to satisfy the conditional expressions 3, 4, 7, and 8, enables the composite magnetic field of the magnetic fields Hm 1  and Hm 2  to be detected efficiently with reduction of the influence of heat generated from the thin film coils  31  and  32 . This permits high-accuracy measurement of a current Im flowing through the thin film coils  31  and  32 , which is in the range of 3 mA to 50 mA. 
   Third Embodiment 
   Reference to  FIGS. 14 to 17 , a current sensor  1 C as a third embodiment of the invention will be described below.  FIG. 14  is a perspective view showing a perspective configuration of the current sensor  1 C.  FIG. 15  shows a cross-sectional configuration in the direction indicated by the arrows (−X direction) along the line XV-XV of the current sensor  1 C shown in  FIG. 14 . 
   The current sensor  1 C can be obtained by adding a second mangetoresitive element  22 , and a third thin film coil  33  (hereinafter referred to simply as a thin film coil  33 ) as a third conductor, to the current sensor  1 A of the first embodiment. The third embodiment will be explained specifically, provided the description overlapping with the first embodiment has been left out of the following. 
   In the current sensor  1 C, as shown in  FIG. 14 , the second magnetoresistive element  22  having element patterns  22 A and  22 B is disposed side by side in the Y-axis direction so as to be adjacent a first magnetoresistive element  21  in the second layer L 2 , and it is connected in series to the first magnetoresistive element  21 . The element patterns  22 A and  22 B extend in the X-axis direction, and are disposed adjacent each other in the Y-axis direction and connected in parallel to each other. Additionally, the current sensor  1 C is configured such that the thin film coil  33  winds in the first layer L 1  while including turn portions  33 A and  33 B that extend in the X-axis direction in correspondence with the element patterns  22 A and  22 B, respectively. In other words, as shown in  FIG. 15 , the current sensor  1 C has such a structure that the second layer L 2  including the first and second magnetoresistive elements  21  and  22 , and the first layer L 1  including the thin film coils  31  and  33  are stacked in the order listed, via an underlayer  3  made of Al 2 O 3  or the like, on a substrate  2  made of silicon or the like. In the cross section of  FIG. 15 , the first and second magnetoresistive elements  21 ,  22  and the thin film coils  31 ,  33  are covered with insulating films Z 1  and Z 2 , respectively, which are made of Al 2 O 3 , or the like, and they are electrically isolated from each other. Furthermore, a plurality of electrode films  41  to  47  (shown in  FIG. 14 ) are provided on the insulating film Z 2 . 
   The thin film coil  33  is a thin film pattern made of a high conductive metal material such as copper. One end  33 S of the thin film coil  33  is connected via a contact hole (not shown) to the electrode film  45 , and the other end  33 E is connected via a contact hole (not shown) to the electrode film  46 . It is set here such that a current Im flows from the end  33 S to the end  33 E. 
   The element patterns  22 A and  22 B are aimed at detecting a third magnetic field Hm 3  (hereinafter referred to simply as a magnetic field Hm 3 ) generated from a current Im and are disposed, in the stacking direction, at their respective corresponding areas with respect to the turn portions  33 A and  33 B of the thin film coil  33 , as shown in  FIG. 15 . The element patterns  22 A and  22 B are disposed such that they extend in the X-axis direction and are adjacent each other in the Y-axis direction, and are connected in parallel by electrode patterns  5  and  6 . Here, the electrode pattern  5  is connected via a contact hole (not shown) to the electrode film  44 , and the electrode pattern  6  is connected via a contact hole (not shown) to the electrode film  47 . When a read current is allowed to flow through the element patterns  22 A and  22 B, respectively, they undergo variations in resistance value according to a magnetic field Hm 3  generated from a current Im flowing through the thin film coils  33 A and  33 B. It is so configured that, the element patterns  22 A and  22 B undergo variations in the resistance value in the opposite direction to the variations in the element patterns  21 A and  21 B which can be generated by a magnetic field Hm 1 . For example, a bias magnetic field having strength corresponding to a bias point BP 1  (see  FIG. 8 ) is previously applied in the +Y direction to the element patterns  21 A,  21 B,  22 A, and  22 B in the magnetoresistive elements  21  and  22 . Thereafter, if a current Im is allowed to flow in the +X direction as shown in  FIG. 16 , the magnetic field Hm 1  in the +Y direction can be applied to the element patterns  21 A and  21 B, so that the resistance change rate increases (the resistance value increases), as apparent from  FIG. 8 . On the other hand, a magnetic field Hm 3  in the −Y direction can be applied to the element patterns  22 A and  22 B, so that the resistance change rate decreases (the resistance value decreases), as apparent from  FIG. 8 . 
   Here, the turn portions  31 A,  31 B and the element pattern  21 A,  21 B are disposed apart a distance D 1  in the Z-axis direction, and the turn portions  32 A,  32 B and the element patterns  22 A,  22 B are disposed apart a distance D 3  in the Z-axis direction. Each of the distances D 1  and D 3  is in the range of 0.4 μm to 1.0 μm (Conditional expressions 1 and 11). Each of the turn portions  31 A and  31 B has a rectangle cross section defined by a width MX 1  and a thickness MY 1 , and has a cross-sectional area S 1  (=MX 1 *MY 1 ). Similarly, the turn portions  32 A and  32 B are shaped as a rectangle defined by a width MX 3  and a thickness MY 3 , and have a cross-sectional area S 3  (=MX 3 *MY 3 ). Specifically, it is arranged such that each of the widths MX 1  and MX 2  is less than or equal to 3.0 μm, and each of the cross-sectional areas S 1  and S 3  is in the range of 0.4 μm 2  to 3.0 μm 2  (Conditional expressions 2 and 12). In consideration of accuracy during the process of formation, it is desirable to set the thickness MY 1 , MY 3  more than or equal to 0.2 μm and less than or equal to the width MX 1 , MX 3 . Thus, the dimension in a YZ cross section of the thin film coil  31  is made equal to that of the thin film coil  33 , and further the distance D 1  in the stacking direction (the Z-axis direction) between the thin film coil  31  and the first magnetoresistive element  21  is made equal to the distance D 3  between the thin film coil  33  and the second magnetoresistive element  22 . Accordingly, the absolute value of the magnetic field Hm 1  applied on the element patterns  21 A and  21 B is equal to that of the magnetic field Hm 3  applied on the element patterns  22 A and  22 B.  FIG. 16  shows in enlarged dimension part of  FIG. 15 , and it is an explanatory diagram for explaining the actions of the magnetic fields Hm 1  and Hm 3  on the element patterns  21 A,  21 B and the element patterns  22 A,  22 B, respectively. 
     FIG. 17  is a schematic diagram illustrating a circuit configuration of an ammeter including the current sensor  1 C shown in  FIGS. 14 and 15 . In  FIG. 17 , a portion surrounded by a broken line corresponds to the current sensor  1 C. As shown in  FIG. 17 , the first magnetoresistive element  21  and the thin film coil  31  are disposed adjacent each other, and the second magnetoresistive element  22  and the thin film coil  33  are disposed adjacent each other. Here, each of the first and second magnetoresistive elements  21  and  22  is illustrated as a resistor that can be formed by connecting in parallel a plurality of element patterns. The first and second magnetoresistive elements  21  and  22  are commonly coupled at a first connection point P 1  (the electrode pattern  5 ), and are finally grounded via the electrode film  44 . Constant current sources  51  and  52  coupled to each other at a second connection point P 2  are disposed on the side opposite to the first connection point P 1  in the first and second magnetoresistive elements  21  and  22 . Specifically, the end on the side opposite to the first connection point P 1  in the first magnetoresistive elements  21  is connected to the constant current source  51  via the electrode film  43  functioning as a third connection point P 3 . The end on the side opposite to the first connection point P 1  in the second magnetoresistive elements  22  is connected to the constant current source  52  via the electrode film  47  functioning as a fourth connection point P 4 . The constant current sources  51  and  52  supply equal constant current I 0  to each element pattern of the first and second magnetoresistive elements  21  and  22 . The thin film coils  31  and  33  are connected to each other with a conductor connecting the electrode films  42  and  45 , thereby functioning as a single conductor line. 
   In the current sensor  1 C so configured, the magnitudes of the magnetic fields Hm 1  and Hm 3  can be obtained on the basis of a potential difference V 0  between the third and fourth connection points P 3  and P 4  (a difference between a couple of voltage drops generated in the first and second magnetoresistive elements  21  and  22 ) when a voltage is applied between the first and second connection points P 1  and P 2 . This permits estimation of the magnitude of a current Im. 
   In  FIG. 17 , I 0  is the constant currents from the constant current sources  51  and  52  when a predetermined voltage is applied between the first and second connection points P 1  and P 2 , and R 1  and R 2  are the entire resistance value of the magnetoresistive elements  21  and  22 , respectively. When no magnetic fields Hm 1 , Hm 3  is being applied, a potential V 1  at the third connection point P 3  (the electrode film  43 ) can be expressed as follows.
 
 V 1= I 0* R 1
 
   A potential V 2  at the fourth connection point P 4  (the electrode film  47 ) can be 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  can be expressed as follows: 
   
     
       
         
           
             
               
                 
                   
                     
                       
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                 b 
                 ) 
               
             
           
         
       
     
   
   As described above, the first magnetoresistive element  21  (the element patterns  21 A and  21 B) and the second magnetoresistive element  22  (the element patterns  22 A and  22 B) are arranged such that their respective resistance values R 1 , R 2  vary in opposite directions each other in response to the magnetic fields Hm 1 , Hm 3 . This arrangement allows sign of the variations ΔR 1  and ΔR 2  to differ from each other. Therefore, in Equation (b), the resistance values R 1  and R 2  before the application of the magnetic fields Hm 1  and Hm 3  are cancelled, while the variations ΔR 1  and ΔR 2  remain. 
   If the first and second magnetoresistive elements  21  and  22  have the same characteristics, that is:
 
R1=R2=R, and
 
ΔR1=−ΔR2=ΔR
 
   Equation (b) can be transformed 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 
                           
                           ) 
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 c 
                 ) 
               
             
           
         
       
     
   
   Therefore, the first and second magnetoresistive elements  21  and  22 , where the relationship between an applied magnetic field and a resistance variation is known, makes it possible to find the magnitudes of the magnetic fields Hm 1  and Hm 3 . This permits estimation of the magnitudes of a current Im. Since both of the first and second magnetoresistive elements  21  and  22  are used to perform sensing, it is capable of sensing resistance variations two times that in sensing with only one of the first and second magnetoresistive elements  21  and  22 . This is advantageous for high-accuracy measured value. 
   Thus, the current sensor  1 C of the third embodiment further includes: the second magnetoresistive element  22  formed in the second layer L 2  and including the element patterns  22 A and  22 B connected in parallel; and the thin film coil  33  which is formed so as to wind in the first layer L 1  while including the turn portions  32 A and  32 B in correspondence to the element patterns  22 A and  22 B, respectively, and which is configured to apply the magnetic field Hm 3  on the element patterns  22 A and  22 B. This enables, besides the effects in the first embodiment, the current sensor  1 C to measure a current with higher accuracy using both of the first and second magnetoresistive elements  21  and  22 . It is configured here such that, upon the application of the magnetic field Hm 3  on the element patterns  22 A and  22 B, the resistance value R 2  varies in the opposite direction of the variations in the resistance value R 1  of the element patterns  21 A and  21 B which can be caused by the magnetic field Hm 1 . This permits higher-accuracy measurement of a current Im on the basis of the difference V 0  in voltage drop which can be caused when the same constant current I 0  is allowed to flow through the first and second magnetoresistive elements  21  and  22 . 
   Additionally, in the current sensor  1 C of the third embodiment, each of the distances D 1  and D 3  is in the range of 0.4 μm to 1.0 μm, and each of the cross-sectional areas S 1  and S 2  is in the range of 0.4 μm 2  to 3.0 μm 2 . Thereby, the magnetic fields Hm 1  and Hm 3  can be detected efficiently with reduction of the influence of the heat generated from the thin film coils  31  and  33 . This permits high-accuracy measurement of a relatively weak current Im flowing through the thin film coils  31  and  33 , which is in the range of 10 mA to 50 mA. 
   Modification 3 
   Although the third embodiment describes the case of measuring a current Im which is in the range of 10 mA to 50 mA, it is possible to configure a current sensor so as to measure a weaker current Im, for example, in the range of 3 mA to 50 mA. In this case, each of the distances D 1  and D 3  is in the range of 0.2 μm to 0.4 μm (Conditional expressions 3 and 9), and each of the cross-sectional areas S 1  and S 3  is in the range of 0.4 μm 2  to 2.5 μm 2  (Conditional expressions 4 and 10). 
   Thus, the current sensor  1 C as a modification, which is configured to satisfy the conditional expressions 3, 4, 9, and 10, enables high-accuracy measurement of a current Im flowing through the thin film coils  31  and  33 , which is in the range of 3 mA to 50 mA. 
   Fourth Embodiment 
   Reference to  FIGS. 18 and 19 , a current sensor  1 D as a fourth embodiment of the invention will be described below. 
   The current sensor  1 D can be obtained by adding a second thin film coil  32  as a second conductor, and a fourth thin film coil  34  as a fourth conductor (hereinafter referred to simply as a thin film coil  34 ) to the current sensor  1 C of the third embodiment. The thin film coil  32  has the same configuration as that described in the second embodiment. The current sensor  1 D of the fourth embodiment will be explained specifically, provided the descriptions overlapping with the first to third embodiments have been left out of the following. 
     FIG. 18  is a perspective view showing a perspective configuration of the current sensor  1 D.  FIG. 19  shows a cross-sectional configuration in the direction indicated by the arrows (−X direction) along the line XIX-XIX in the current sensor  1 D of  FIG. 18 . 
   In the current sensor  1 D, a second magnetoresistive element  22  is formed in an area other than the area where a first magnetoresistive element  21  is formed in the second layer L 2 . The second magnetoresistive element  22  includes element pattern  22 A and  22 B extending in the X-axis direction and being adjacent in the Y-axis direction and connected in parallel, and it is connected in series to the first magnetoresistive element  21 . The thin film coil  34  is configured so as to wind in an area other than the area where the thin film coil  32  is formed at a third layer L 3 , on the side opposite to the thin film coil  33  with the second magnetoresistive element  22  interposed therebetween. In other words, as shown in  FIG. 19 , the current sensor ID has such a structure that the third layer L 3  including the thin film coils  32  and  34 , the second layer L 2  including the first and second magnetoresistive elements  21  and  22 , and the first layer L 1  including the thin film coils  31  and  33  are stacked in the order listed, via an underlayer  3  made of Al 2 O 3  or the like, on a substrate  2  made of silicon or the like. In the cross section of  FIG. 19 , the thin film coils  32 ,  34 , the first and second magnetoresistive elements  21 ,  22 , and the thin film coils  31 ,  33  are covered with insulating films Z 3 , Z 1 , and Z 2 , respectively, which are made of Al 2 O 3  or the like, and they are electrically isolated from each other. Furthermore, a plurality of electrode films  41  to  47  (shown in  FIG. 18 ) are provided on the insulating film Z 2 . 
   The thin film coil  33  is configured so as to apply the magnetic field Hm 3  on the element patterns  22 A and  22 B under the supply of a current Im. One end  33 S of the thin film coil  33  is connected via a contact hole (not shown) to an electrode film  45 , and the other end  33 E is connected via a contact hole (not shown) to an end  34 S of the thin film coil  34  (see  FIG. 18 ). The thin film coil  34  includes turn portions  34 A and  34 B that extend in the X-axis direction so as to correspond to the element patterns  22 A and  22 B, respectively. The thin film coil  34  is configured such that it applies a fourth magnetic field Hm 4  that is the same in direction as the magnetic field Hm 3  (hereinafter referred to simply as a magnetic field Hm 4 ), on the element patterns  22 A and  22 B under the supply of a current Im. One end  34 S of the thin film coil  34  is connected via a contact hole (not shown) to the end  33 E, and the other end  34 E is connected via a contact hole (not shown) to an electrode film  46  (see  FIG. 18 ). Therefore, the thin film coils  33  and  34  configure a single conductor line in the circuit configuration. In an alternative, electrode films  42  and  45  may be connected to each other with an external conductor (not shown) such that the thin film coils  31  to  34  substantially function as a single conductor line. This allows a current Im to flow through the thin film coils  31 ,  32 ,  33 , and  34  in the order listed or in the reverse order. Like the thin film coils  31  to  33 , the thin film coil  34  is a thin film pattern made of a high conductive metal material such as copper. 
   In the current sensor  1 D so configured, a current Im flowing through the thin film coils  31  and  32  causes the two magnetic fields Hm 1  and Hm 2  to act on the element patterns  21 A and  21 B, as shown in  FIG. 20 . At the same time, a current Im flowing through the thin film coils  33  and  34  causes the two magnetic fields Hm 3  and Hm 4  to act on the element patterns  22 A and  22 B.  FIG. 20  shows in enlarged dimension part of  FIG. 19 , and it is an explanatory diagram for explaining the actions of the magnetic fields Hm 1  and Hm 2  on the element patterns  21 A and  21 B, respectively, and the actions of the magnetic fields Hm 3  and Hm 4  on the element patterns  22 A and  22 B, respectively. Here, the turn portions  32 A,  32 B and the element pattern  21 A,  21 B are disposed apart a distance D 2  in the Z-axis direction, and the turn portions  34 A,  34 B and the element patterns  22 A,  22 B are disposed apart a distance D 4  in the Z-axis direction. Each of the distances D 2  and D 4  is in the range of 0.4 μm to 1.0 μm (Conditional expressions 17 and 18). Consequently, all the distances D 1  to D 4  are in the same range. Each of the turn portions  32 A and  32 B has a rectangle cross section defined by a width MX 2  and a thickness MY 2 , and has a cross-sectional area S 2  (=MX 2 *MY 2 ). Each of the turn portions  34 A and  34 B has a rectangle cross section defined by a width MX 4  and a thickness MY 4 , and has a cross-sectional area S 4  (=MX 4 *MY 4 ). Specifically, like the cross-sectional areas S 1  and S 3 , it is arranged such that each of the widths MX 2  and MX 4  is less than or equal to 3.0 μm, and each of the cross-sectional areas S 2  and S 4  is in the range of 0.4 μm 2  to 3.0 μm 2  (Conditional expressions 19 and 20). In consideration of accuracy during the process of formation, it is desirable to set the thickness MY 2 , MY 4  at more than or equal to 0.2 μm and to be equal to or less than the width MX 2 , MX 4 . 
   In sensing with the current sensor  1 D, first, a sense current is allowed to flow through the electrode films  43  and  44  to the first magnetoresistive element  21  (the element patterns  21 A and  21 B) and to the second magnetoresistive element  22  (the element patterns  22 A and  22 B). Then, a current Im is supplied via the electrode films  41  and  42  to the thin film coils  31  and  32  such that the element patterns  21 A and  21 B detect a magnetic field Hm 1  generated from the turn portions  31 A and  31 B, and a magnetic field Hm 2  generated from the turn portions  32 A and  32 B, respectively. Similarly, a current Im is supplied via the electrode films  45  and  46  to the thin film coils  33  and  34  such that the element patterns  22 A and  22 B detect a magnetic field Hm 3  generated from the turn portions  33 A and  33 B, and a magnetic field Hm 4  generated from the turn portions  34 A and  34 B, respectively. 
   Consider now for example the case where, with the electrode films  42  and  45  connected to each other by an external conductor, a current Im that has flown through the thin film coils  31  and  32  in this order is allowed to flow from the end  33 S of the thin film coil  33  to the end  33 E, and then from the end  34 S of the thin film coil  34  to the end  34 E. In this case, as shown in  FIG. 20 , the current Im will flow in the −X direction (from near side to far side as seen in  FIG. 20 ) at the turn portions  33 A and  33 B. As a result, a magnetic field Hm 3  is generated which winds (in a clockwise as seen in  FIG. 20 ) the surroundings of the turn portions  33 A and  33 B, respectively, according to corkscrew rule. On the other hand, the current Im will flow in the +X direction (from far side to near side as seen in  FIG. 20 ) at the turn portions  34 A and  34 B. As a result, a magnetic field Hm 4  is generated which winds (in a counterclockwise as seen in  FIG. 20 ) the surroundings of the turn portions  34 A and  34 B, respectively, according to corkscrew rule. Hence, a composite magnetic field of the magnetic fields Hm 3  and Hm 4  toward the −Y direction is supplied to each of the element patterns  22 A and  22 B. Therefore, the magnetic field strength applied on the element patterns  22 A and  22 B can be increased, resulting in greater variations in the resistance value than the case where only the magnetic field Hm 3  is applied on the second magnetoresistive element  22 , as in the third embodiment. Here, the magnitude of the current Im can be estimated with higher accuracy by detecting a difference between a variation in voltage drop (a variation in resistance value) between the electrode patterns  4  and  5 , and that between the electrode patterns  5  and  6 . 
   Thus, the current sensor  1 D of the fourth embodiment is configured such that the first magnetoresistive element  21  detects the magnetic fields Hm 1  and Hm 2  which can be generated from a current Im, and such that the second magnetoresistive element  22  detects the magnetic fields Hm 3  and Hm 4  which can be generated from a current Im. With this configuration, the presence of the first and second magnetoresistive elements  21  and  22  permits higher-accuracy measurement of a current Im flowing through the thin film coils  31  to  34 , while maintaining a compact configuration. Especially, the element patterns  22 A and  22 B are configured such that, upon the application of the magnetic fields Hm 3  and Hm 4 , the resistance value varies in the opposite direction of the variations in the element patterns  21 A and  21 B of the first magnetoresistive element  21  which can be caused by the magnetic fields Hm 1  and Hm 2 . Moreover, the same constant current  10  is allowed to flow through the first and second magnetoresistive elements  21  and  22 . Therefore, based on a difference in voltage drop produced, a current Im can be measured with higher accuracy. 
   Additionally, in the current sensor  1 D of the fourth embodiment, each of the distances D 1  to D 4  is in the range of 0.4 μm to 1.0 μm, and each of the cross-sectional areas S 1  to S 4  is in the range of 0.4 μm 2  to 3.0 μm 2 . Thereby, the magnetic fields Hm 1  to Hm 4  can be detected efficiently with reduction of the influence of heat generated from the thin film coils  31  to  34 . This permits high-accuracy measurement of a relatively weak current Im flowing through the thin film coils  31  to  34 , which is in the range of 10 mA to 50 mA. 
   Modification 4 
   Although the fourth embodiment describes the case of measuring a current Im which is in the range of 10 mA to 50 mA, it is possible to configure a current sensor so as to measure a weaker current Im, for example, in the range of 3 mA to 50 mA. In this case, each of the distances D 1  to D 4  is in the range of 0.2 μm to 0.4 μm (Conditional expressions 3, 7, 13, and 14), and each of the cross-sectional areas S 1  to S 4  is in the range of 0.4 μm 2  to 2.5 μm 2  (Conditional expressions 4, 8, 15, and 16). 
   Thus, the current sensor  1 D as a modification, which is configured so as to satisfy the conditional expressions 3, 4, 7, 8, and 13 to 16, enables the magnetic fields Hm 1  to Hm 4  to be detected efficiency with reduction of the influence of heat generated from the thin film coils  31  to  34 . This permits high-accuracy measurement of a current Im flowing through the thin film coils  31  to  34 , which is in the range of 3 mA to 50 mA. 
   EXAMPLES 
   The following is a numerical example of the current sensor of the invention. This example relates to a current sensor having the configuration corresponding to the first embodiment and its modification. Specifically, the strength (in average) of the magnetic field Hm 1  exerted on the element pattern  21 A, and the temperature variation ΔT of the element pattern  21 A were simulated. Table 1 shows the results. 
   
     
       
         
             
             
             
             
             
             
           
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
                 
                 
                 
               Hm1 
                 
             
             
                 
               Im 
               D1 
               S1 
               [×10 3 /4π 
               ΔT 
             
             
                 
               [mA] 
               [μm] 
               [μm 2 ] 
               A/m] 
               [° C.] 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
             
          
             
               Com. Example 
               3 
               0.4 
               3.0 
               4 
               — 
             
             
               1-1 
                 
                 
                 
               (NG) 
             
             
               Example 1-1 
               3 
               0.4 
               2.5 
               5 
               &lt;0.01 
             
             
               Example 1-2 
               10 
               0.2 
               0.4 
               32 
               0.07 
             
             
               Example 1-3 
               10 
               0.2 
               0.1 
               50 
               0.34 
             
             
               Example 1-4 
               50 
               0.2 
               0.4 
               170 
               1.7 
             
             
               Com. Example 
               50 
               0.2 
               0.32 
               180 
               2.2 
             
             
               1-2 
                 
                 
                 
                 
               (NG) 
             
             
               Com. Example 
               10 
               1.0 
               4.0 
               &lt;5 
               — 
             
             
               2-1 
                 
                 
                 
               (NG) 
             
             
               Example 2-1 
               10 
               1.0 
               3.0 
               5 
               &lt;0.01 
             
             
               Example 2-2 
               10 
               0.6 
               3.0 
               11 
               &lt;0.01 
             
             
               Example 2-3 
               10 
               0.6 
               1.6 
               15 
               0.01 
             
             
               Example 2-4 
               10 
               0.4 
               1.6 
               18 
               0.01 
             
             
               Example 2-5 
               50 
               0.4 
               0.4 
               150 
               1.7 
             
             
               Com. Example 
               50 
               0.4 
               0.32 
               160 
               2.2 
             
             
               2-2 
                 
                 
                 
                 
               (NG) 
             
             
                 
             
          
         
       
     
   
   In Table 1, Examples 1-1 to 1-4 correspond to the configuration of the modification in the first embodiment, and Examples 2-1 to 2-5 correspond to the configuration of the first embodiment. Comparative Examples 1-1, 1-2, 2-1, and 2-2 do not correspond to the present invention. The current Im [mA], distance D 1  [μm], cross-sectional area S 1  [μm 2 ], magnetic field Hm 1  [*10 3 /4π A/m], and temperature variation ΔT [° C.] of these samples are presented sequentially in left-to-right order. 
   Stable detecting operation in the current sensor of the embodiment requires more than or equal to 5 Oe (=5*10 3 /4π [A/m]) in the strength of a magnetic field Hm 1  applied on the element pattern. Therefore, in detecting the weakest current Im that is 3 [mA], it is required that the distance D 1  is 0.4 [μm] and the cross-sectional area S 1  is less than or equal to 2.5 [μm 2 ] (Example 1-1). When the cross-sectional area S 1  was 3.0 [μm 2 ], the strength of the magnetic field Hm 1  was insufficient, resulting in 4*10 3 /4π [A/m] (Comparative Example 1-1). On the other hand, it was found that, when the current Im was 10 mA, setting the cross-sectional area S 1  at more than or equal to 0.4 [μm 2 ] enabled the temperature variation ΔT to be held at 1.7° C. even for the minimum distance D 1  (D 1 =0.2 [μm]) (Example 1-4). It was also found that, if the cross-sectional area S 1  was set at 0.32 [μm 2 ], the temperature variation ΔT was 2.2° C., resulting in poor reliability of the current sensor (Comparative Example 1-2). 
   It was found that, when the distance D was 1.0 [μm] in detecting a current Im, namely 10 [mA], a magnetic field Hm 1  of more than or equal to 5 Oe (=5*10 3 /4π [A/m]) could be obtained by setting the cross-sectional area S 1  at less than or equal to 3.0 [μm 2 ] (Example 2-1). Under the same conditions, when the cross-sectional area S 1  was 4.0 [μm 2 ], the strength of the magnetic field Hm 1  was insufficient (Comparative Example 2-1). On the other hand, it was found that, when a current Im was the maximum, namely 50 mA, setting the cross-sectional area S 1  at more than or equal to 0.4 [μm 2 ] enabled the temperature variation ΔT to be held at 1.7° C. even for the minimum distance D 1  (D 1 =0.4 [μm]) (Example 2-5). It was also found that, if the cross-sectional area S 1  was set at 0.32 [μm 2 ], the temperature variation ΔT was 2.2° C., resulting in poor reliability of the current sensor (Comparative Example 2-2). 
   It was confirmed that excellent measurements were possible with the configuration to satisfy Conditional expressions 1 and 2 in measuring a current Im of in the range of 10 mA to 50 mA, as well as with the configuration to satisfy Conditional expressions 3 and 4 in measuring a current Im of in the range of 3 mA to 50 mA. 
   While the present invention has been shown in several forms, it is not so limited but is susceptible of various changes and modifications without departing from the spirit and scope of the claimed invention. For example, although the above embodiments employ the magnetoresistive element having the spin valve structure in which the magnetization direction of the pinned layer and the direction of the easy axis of magnetization of the free layer coincide with each other, the invention should not be limited to this. In an alternative, there may be used a magnetoresistive element having such a spin value structure that the magnetization direction of a pinned layer and the easy axis of magnetization of a free layer are orthogonal to each other. 
   Although in the foregoing embodiments each of the first and second magnetoresistive elements is configured by two element patterns, without limiting to this, three or more element patterns may form the first and second magnetoresistive elements, respectively. In an alternative, each of the first and second magnetoresistive elements may be formed by an element pattern extending in a first direction, and each of first to fourth conductors may be formed to have first to fourth extended portions extending in the first direction, respectively. Also in this case, the above-mentioned effects are obtainable by having each of the distances D 1  to D 4  between each of the first to fourth extended portions and each of the first to fourth magnetoresistive elements, respectively, and each of the cross-sectional areas S 1  to S 4  of the first to fourth extended portions, respectively, fall in their respective predetermined numerical ranges. 
   Although, in the above embodiments, the element patterns in the first and second magnetoresistive element are connected in parallel, they may be connected in series as in the current sensor  1 E shown in  FIG. 21 . In this case, without increasing the dimension in the first direction, the whole extension length of the element patterns functioning as a magnetosensitive part is longer to further increase the absolute value of the whole resistance value (impedance) in the first and second magnetoresistive elements. This permits high-accurate measurements of a weak current. 
   The current sensors of the invention can be used to measure a current value itself as described in the foregoing embodiments, and are also applicable to an eddy current inspection technique for inspecting a defective on printed wiring, and the like. For example, the current sensor may include a plurality of magnetoresistive elements disposed in line in order to sense an eddy current change as a magnetic flux change.