Abstract:
A magnetoresistive Z-axis gradient sensor chip, which is used to detect the gradient in the XY plane of a Z-axis magnetic field component generated by a magnetic medium; the sensor chip comprises a Si substrate, a collection of two or two groups of flux guide devices separated a distance Lg and an arrangement of electrically interconnected magnetoresistive sensor units. The magnetoresistive sensor units are located on the Si substrate and located above or below the edge of the flux guide devices as well; the flux guide devices convert the component of the Z-axis magnetic field into the direction parallel to the surface of the Si substrate along the sensing axis direction of the magnetoresistive sensing units. The magnetoresistive sensor units are electrically interconnected into a half bridge or a full bridge gradiometer arrangement, wherein the opposite bridge arms are separated by distance Lg. This sensor chip can be utilized with a PCB or in combination with a PCB plus back-bias magnet with casing. The sensor measures the Z-axis magnetic field gradient by using magnetoresistive sensors with in-plane sensing axes. This sensor chip has several advantages relative to a Hall Effect sensor device, including smaller size, lower power consumption, and higher magnetic field sensitivity.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to the field of magnetic sensors, and in particular, to a magneto-resistive Z-axis gradient sensor chip. 
       BACKGROUND ART 
       [0002]    Magnetoresistive gradient sensors are widely applied to gear tooth sensing and magnetic image sensors used in as POS machine magnetic heads and currency detector magnetic heads. Under normal circumstances, magnetoresistive sensors, for example, GMR and TMR sensors, have in-plane sensing axes, whereby the magnetic field sensing direction is parallel to the surface of the chip in which the sensors are located, while a Hall Effect sensor has a Z-axis sensing direction perpendicular to the surface of the chip. At present, when used in a magnetic image sensor, a Hall Effect sensor head usually has a Hall Effect sensor made of Murata&#39;s InSb thin film material, which measures a Z-axis magnetic field component. However, both the Hall sensor that measures a Z-axis magnetic field component and a GMR or TMR sensor that measures an in-plane magnetic field component have some problems: 
         [0003]    1) As InSb is not a standard material and a non-standard semiconductor manufacturing process is employed, the manufacturing process thereof is more complex with respect to the standard semiconductor manufacturing process of sensors such as TMR and GMR. 
         [0004]    2) At present, all GMR-based and TMR-based magnetic image sensors are mainly based on the in-plane magnetic field sensing direction, and when they are applied to currency detector magnetic heads, it is necessary to use a back-bias magnet with a deep groove cut on a surface, whose shape is relatively complex. Therefore, magnetic fields generated on the surface are not evenly distributed, while a back-bias magnetic block magnetized in the Z-axis direction can be used as an InSb-based Z-axis sensor back-bias magnet, which has a relatively simple structure. 
         [0005]    3) Compared with the GMR and TMR sensors, the Hall effect sensor has relatively low magnetic field sensitivity, poor stability (sensitivity, offset and resistance), and poor repeatability (it is difficult to control the change in the offset and the sensitivity). 
       SUMMARY OF THE INVENTION 
       [0006]    With respect to the existing problems above, the present invention proposes a magnetoresistive Z-axis gradiometer chip, which, in combination with a standard manufacturing process of GMR and TMR sensors and advantages of a Z-axis sensor back-bias magnet, successfully solves the shortcomings of the above problems, enabling the measurement of the gradient of the Z-axis magnetic field by using magnetoresistive sensors with in-plane sensing axes, and has several advantages relative to the Hall sensor, including smaller size, lower power consumption, and higher magnetic field sensitivity. 
         [0007]    The magnetoresistive Z-axis gradient sensor chip proposed in the present invention is used to detect the gradient in the XY plane of a component of a Z-axis magnetic field generated by a magnetic medium; the magnetoresistive Z-axis gradient sensor chip includes a Si substrate, magnetoresistive sensor units located on the Si substrate and electrically interconnected into a full bridge gradiometer or half bridge gradiometer, and two or two groups of flux guide devices located on the Si substrate; the magnetoresistive sensor units are located above or below the flux guide devices, and have a sensing direction parallel to the surface of the Si substrate, and the flux guide devices are used to convert the component of the Z-axis magnetic field generated by the magnetic medium into a sensing direction along the magnetoresistive sensing units; and 
         [0008]    each group of flux guide devices includes at least two flux guide devices, the two or two groups of flux guide devices are spaced apart by a distance Lg, and opposite bridge arms in the full bridge gradiometer or half bridge gradiometer are spaced apart by a distance Lg. 
         [0009]    Preferably, the magnetoresistive sensor units are GMR and/or TMR sensor units. 
         [0010]    Preferably, the flux guide devices are a soft magnetic alloy consisting of one or more elements selected from Co, Fe and Ni. 
         [0011]    Preferably, the flux guide devices are elongated, and have a major axis along a Y-axis direction and a minor axis along an X-axis direction, and a length Ly thereof is greater than a width Lx and is also greater than a thickness Lz. 
         [0012]    Preferably, when a vertical distance from the magnetoresistive sensor unit to a Y-axis center line of the corresponding flux guide device is less than or equal to ⅓*Lx, a working range of the magnetic field of the magnetoresistive sensor unit can be increased. 
         [0013]    Preferably, the greater the vertical distance from the position of the magnetoresistive sensor unit to the Y-axis center line is, or the greater the thickness Lz of the flux guide device is, or the smaller the width Lx of the flux guide device is, the higher the magnetic field sensitivity of the magnetoresistive sensor unit is. 
         [0014]    Preferably, the magnetoresistive Z-axis gradient sensor chip includes two of the flux guide devices which are arranged into an array of two rows and one column, wherein the row direction thereof is the Y-axis direction, the column direction is the X-axis direction, and the row spacing Lg corresponds to a gradient feature distance. 
         [0015]    Preferably, two bridge arms in the half bridge gradiometer are corresponding to the two flux guide devices, respectively, the two bridge arms are located at the same position on the same side of the Y-axis center line of the corresponding flux guide device, and the magnetoresistive sensor units on the two bridge arms have the same sensing direction. 
         [0016]    Preferably, two half bridges in the full bridge gradiometer are corresponding to the two flux guide devices, respectively, two bridge arms of each of the half bridges are symmetrically distributed on two sides of the Y-axis center line of the corresponding flux guide device, two bridge arms connected to the same power supply electrode are located at the same position on the same side of the Y-axis center line of the corresponding flux guide device, and the magnetoresistive sensor units in the full bridge gradiometer have the same sensing direction. 
         [0017]    Preferably, two bridge arms in either of two half bridges of the full bridge gradiometer are located at the same position on the same side of the Y-axis center lines of two flux guide devices, respectively, two bridge arms connected to the same power supply electrode are corresponding to the same flux guide device and symmetrically distributed on two sides of the Y-axis center line of the flux guide device, and the magnetoresistive sensor units in the full bridge gradiometer have the same sensing direction. 
         [0018]    Preferably, each group of flux guide devices includes 2*N (N&gt;1) flux guide devices, the two groups of flux guide devices form an array of two rows and one column, wherein the row direction is along the Y-axis direction, and the column direction is along the X-axis direction; each group of flux guide devices forms an array of N rows and one column, wherein the row direction is along the Y-axis direction, and the column direction is along the X-axis direction; a row spacing Ls between flux guide devices in each group is much less than Lg. 
         [0019]    Preferably, magnetoresistive sensor units on two bridge arms of the half bridge gradiometer are corresponding to N flux guide devices in the two groups of flux guide devices, respectively, the magnetoresistive sensor units on the two bridge arms are located at the same position on the same side of the Y-axis center line of the corresponding flux guide device, and the magnetoresistive sensor units have the same sensing direction. 
         [0020]    Preferably, magnetoresistive sensor units of two half bridges in the full bridge gradiometer are corresponding to N flux guide devices in the two groups of flux guide devices, respectively, magnetoresistive sensor units of two bridge arms of each of the half bridges are symmetrically distributed on two sides of the Y-axis center line of the corresponding flux guide device, magnetoresistive sensor units of two bridge arms connected to the same power supply electrode are corresponding to N flux guide devices in the two groups of flux guide devices, respectively and are located at the same position on the same side of the Y-axis center line of the corresponding flux guide device, and the magnetoresistive sensor units in the full bridge gradiometer have the same sensing direction. 
         [0021]    Preferably, magnetoresistive sensor units of two bridge arms in either of two half bridges of the full bridge gradiometer are corresponding to N flux guide devices in the two groups of flux guide devices, respectively and are located at the same position on the same side of the Y-axis center line of the corresponding flux guide device, magnetoresistive sensor units of two bridge arms connected to the same power supply electrode are corresponding to N flux guide devices in the same group of flux guide devices and symmetrically distributed on two sides of the Y-axis center line of the corresponding flux guide device, and the magnetoresistive sensor units in the full bridge gradiometer have the same sensing direction. 
         [0022]    Preferably, each of the bridge arms has the same number of magnetoresistive sensor units, the magnetoresistive sensor units are electrically interconnected into a two-port structure by series connection, parallel connection or a combination of series connection and parallel connection, and the bridge arms all have the same electrical interconnection structure. 
         [0023]    Preferably, the magnetoresistive Z-axis gradient sensor chip is electrically connected to a PCB by wire bonding. 
         [0024]    Preferably, the magnetoresistive Z-axis gradient sensor chip is electrically connected to a PCB by Through Silicon Vias (TSV). 
         [0025]    Preferably, the magnetoresistive Z-axis gradient sensor chip is mounted onto a PCB, for detecting a component of a Z-axis magnetic field generated by a permanent magnet magnetic medium, the magnetoresistive Z-axis gradient sensor chip is designed to have high magnetic field sensitivity and a saturated magnetic field higher than the Z-axis magnetic field generated by the permanent magnet magnetic medium. 
         [0026]    Preferably, the magnetoresistive Z-axis gradient sensor chip is mounted onto a PCB, the back of the PCB is provided with a permanent magnet block to generate a magnetic field perpendicular to the magnetoresistive Z-axis gradient sensor chip, and the magnetoresistive Z-axis gradient sensor chip is designed to have a saturated magnetic field greater than a magnetic field generated by the permanent magnet. 
         [0027]    Preferably, the magnetoresistive Z-axis gradient sensor chip is mounted onto a PCB, the back of the PCB is provided with a permanent magnet for generating a magnetic field perpendicular to the magnetoresistive Z-axis gradient sensor chip, the PCB is placed in a casing, the casing includes a mounting bracket and a pin electrode located at the back, the PCB and the magnetoresistive Z-axis gradient sensor chip are located in the mounting bracket, and the magnetoresistive Z-axis gradient sensor chip is designed to have a saturated magnetic field higher than a magnetic field of the permanent magnet and have high magnetic field sensitivity. 
         [0028]    Preferably, the sensing direction of the magnetoresistive sensor units is the X-axis direction. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    In order to describe the technical solutions in technologies of embodiments of the present invention more clearly, the accompanying drawings to be used in the descriptions about the technologies of the embodiments are briefly introduced in the following. Apparently, the accompanying drawings in the following descriptions are merely some embodiments of the present invention, and those of ordinary skill in the art can also derive other accompanying drawings from these accompanying drawings without making creative efforts. 
           [0030]      FIG. 1  is a diagram of a magnetoresistive Z-axis gradient sensor chip and the principle of measurement of a Z magnetic field thereof; 
           [0031]      FIG. 2  is a diagram of positions of flux guide devices and magnetoresistive units; 
           [0032]      FIG. 3  is a schematic structural diagram of a half-bridge-structure magnetoresistive Z-axis gradient sensor chip with dual flux guide devices; 
           [0033]      FIG. 4  is a schematic diagram of electrical connections between magnetoresistive units of the half-bridge-structure magnetoresistive Z-axis gradient sensor chip with dual flux guide devices; 
           [0034]      FIG. 5  is a schematic structural diagram of a full-bridge-structure magnetoresistive Z-axis gradient sensor chip with dual flux guide devices; 
           [0035]      FIG. 6  is a first schematic diagram of electrical connections between magnetoresistive units of the full-bridge-structure magnetoresistive Z-axis gradient sensor chip with dual flux guide devices; 
           [0036]      FIG. 7  is a second schematic diagram of electrical connections between magnetoresistive units of the full-bridge-structure magnetoresistive Z-axis gradient sensor chip with dual flux guide devices; 
           [0037]      FIG. 8  is a schematic structural diagram of a half-bridge-structure magnetoresistive Z-axis gradient sensor chip with multiple flux guide devices; 
           [0038]      FIG. 9  is a schematic diagram of electrical connections between magnetoresistive units of the half-bridge-structure magnetoresistive Z-axis gradient sensor chip with multiple flux guide devices; 
           [0039]      FIG. 10  is a schematic structural diagram of a full-bridge-structure magnetoresistive Z-axis gradient sensor chip with multiple flux guide devices; 
           [0040]      FIG. 11  is a first schematic structural diagram of electrical connections between magnetoresistive units of the full-bridge-structure magnetoresistive Z-axis gradient sensor chip with multiple flux guide devices; 
           [0041]      FIG. 12  is a second schematic structural diagram of electrical connections between magnetoresistive units of the full-bridge-structure magnetoresistive Z-axis gradient sensor chip with multiple flux guide devices; 
           [0042]      FIG. 13  is a diagram of connections between magnetoresistive units on a bridge arm in the full-bridge-structure magnetoresistive Z-axis gradient sensor chip with multiple flux guide devices, where a represents series connection; and b represents parallel connection; 
           [0043]      FIG. 14  is a schematic diagram showing that a magnetoresistive Z-axis gradient sensor chip is applied to detection of a hard magnetic image; 
           [0044]      FIG. 15  is a schematic diagram showing that a magnetoresistive Z-axis gradient sensor chip is applied to a magnetic head of a soft magnetic image; and 
           [0045]      FIG. 16  is a schematic diagram of a magnetoresistive Z-axis gradient sensor chip with a mounting structure. 
       
    
    
     DETAILED DESCRIPTION 
       [0046]    The present invention is described below in detail with reference to the accompanying drawings and in combination with embodiments. 
       First Embodiment 
       [0047]      FIG. 1  is a schematic diagram of a magnetoresistive Z-axis gradiometer chip, including a Si substrate  1  and a magnetoresistive Z-axis sensor  20 . The magnetoresistive Z-axis sensor  20  is located on the Si substrate  1 . The magnetoresistive Z-axis sensor includes two or two groups of multiple flux guide devices  2  and magnetoresistive units  3  (which include  3 ( 1 ) or  3 ( 2 ) in the figure). The magnetoresistive units  3  are located above or below the flux guide devices  2 . For simplification, this figure only shows the situation where the magnetoresistive units  3  are located below the flux guide devices  2 , and actually the situation where the magnetoresistive units  3  are located above the flux guide devices  2  is also included. The magnetoresistive units  3  are electrically connected into a half bridge or full bridge structure gradiometer.  FIG. 1  further shows the principle of measurement of a Z-axis magnetic field. After the Z-axis magnetic field  4  passes through the flux guide devices  2 , as the flux guide devices  2  are made of a high-permeability soft magnetic alloy material, including soft magnetic alloy materials consisting of Ni, Fe, Co and other elements, a magnetic field  5  ( 1 ) or  5  ( 2 ) deflects at a position near the magnetoresistive units  3  ( 1 ) or  3  ( 2 ) close to an upper surface or a lower surface of the flux guide devices  2 , and magnetic field components  5  ( 3 ) and  5  ( 4 ) parallel to the substrate emerge, which can thus be detected by the magnetoresistive units  3  ( 1 ) or  3  ( 2 ) having in-plane magnetic field sensitivity. 
         [0048]      FIG. 2  is a diagram of positions of the magnetoresistive units  3  ( 1 ) or  3  ( 2 ) and the upper surface or lower surface of the flux guide devices  2 . The flux guide devices  2  are elongated, and have a major axis long a Y-axis direction and a minor axis along an X-axis direction, the magnetoresistive units are located at a position Y 1  or Y 2  between a Y-axis edge  7  ( 1 ) or  7  ( 2 ) and a Y-axis center line  6 , and Y 1  and Y 2  are symmetric about the Y-axis center line  6 . It can also be seen from  FIG. 1  that, after the Z-axis magnetic field  4  passes through the flux guide devices  2 , components  5  ( 4 ) and  5  ( 3 ) of the X-axis magnetic field at the positions Y 1  and Y 2  are in opposite directions. 
         [0049]    The magnetoresistive units  3  are GMR and/or TMR sensor units, the magnetic field sensing direction thereof is the X-axis direction, and the magnetoresistive units  3  are electrically connected into a full bridge or half bridge gradiometer. It should be noted that the magnetoresistive units being located above or below the flux guide devices in the present invention refers to that the magnetoresistive units are located vertically above or below a range within respective edges of the flux guide devices. 
       Second Embodiment 
       [0050]      FIG. 3  and  FIG. 4  are diagrams of another type of magnetoresistive Z-axis half bridge gradient sensor chip and electrical connections thereof, in which magnetoresistive units  3  ( 5 ) and  3  ( 6 ) on two bridge arms of a half bridge are located in the flux guide devices A 1  and B 1 , respectively and occupy the same position: magnetoresistive units  3  ( 5 ) and  3  ( 6 ) are located at Y 1  or Y 2 . For ease of description,  FIG. 3  and  FIG. 4  only show the situation where they are located at Y 1 , and they have the same magnetic field sensing direction. In this case, a half bridge output signal may be calculated in the following manner: 
         [0051]    After a Z-axis magnetic field HZ passes through the flux guide devices, an X-axis magnetic field HX is obtained, and a correlation between HZ and HX is: 
         [0000]        HX 1 =HZ 1 *SXZ   (1)
 
         [0000]        HX 2 =HZ 2 *SXZ    (2)
 
         [0052]    where SXZ is a magnetic field conversion parameter of the flux guide devices, which is related to the geometric structure and material property of the flux guide devices, HZ1 and HZ2 are components of the Z-axis magnetic field corresponding to the flux guide devices A 1  and B 1 , respectively, and HX1 and HX2 are sensing-axis magnetic field components at the magnetoresistive sensor units after passing through the flux guide devices A 1  and B 1 , respectively. 
         [0053]    The final half bridge output signal is 
         [0000]        V out= HX 1 *S−HX 2 *S=SXZ*HZ 1 *S−SXZ*HZ 2 *S   
         [0000]      = SXZ*S* ( HZ 1 −HZ 2)   (3)
 
         [0054]    where S is sensitivity. 
         [0055]    The gradient of the Z-axis magnetic field measured by the Z-axis magnetoresistive gradient sensor chip is: 
         [0000]      Gradient  HZ =( HZ 1 −HZ 2)/ Lg=V out/( Lg*SXZ*S )  (4)
 
         [0056]    Thus, the gradient of the Z-axis magnetic field is directly proportional to the output signal of the magnetoresistive Z-axis half bridge gradient sensor chip. 
         [0057]    The Lg refers to a distance between two or two groups of flux guide devices, also referred to as a gradient feature distance. 
       Third Embodiment 
       [0058]      FIGS. 5-7  are a schematic structural diagram of a full-bridge-structure gradient sensor chip and diagrams of five types of electrical connections between magnetoresistive units corresponding thereto. In  FIG. 5 , the respective positions Y 1  and Y 2  corresponding to the flux guide devices A 1  and B 1  are occupied by magnetoresistive units  3 ( 7 )- 3 ( 10 ) corresponding to four bridge arms of the full bridge, respectively. In  FIG. 6 , two half bridges of the full bridge are corresponding to the flux guide devices A 1  and B 1 , respectively. The magnetoresistive units at the positions Y 1  and Y 2  of each flux guide device have the same magnetic field sensing direction, and magnetoresistive units of two bridge arms connected to an electrode have the same magnetic field sensing direction. The output signal of the full bridge structure gradient sensor chip shown in  FIG. 5  is as shown by the formulas (5)-(7), and the measured gradient of the Z-axis magnetic field is as shown by the formula (8): 
         [0000]        V−=HX 1 *S− (− HX 1)* S= 2 *HX 1 *S    (5)
 
         [0000]        V+=HX 2 *S− (− HX 2)* S= 2*HX2 *S    (6)
 
         [0000]        V out= V+−V−= 2 *SXZ*S *( HZ 1 −HZ 2)   (7)
 
         [0000]      Gradient  HZ =( HZ 1 −HZ 2)/ Lg=V out/(2* Lg*SXZ*S )   (8)
 
         [0059]    As shown in  FIG. 7 , two bridge arms corresponding to two half bridges of the full bridge are corresponding to the same position Y 1  or Y 2  in the two flux guide devices A 1  and B 1 , respectively and have the same magnetic field sensing direction, and magnetoresistive units of the two bridge arms connected to the same electrode are located at different positions in the same flux guide device. The output signal of the full bridge structure gradient sensor chip shown in  FIG. 7  is as shown by the formulas (9)-(11), and the measured gradient of the Z-axis magnetic field is as shown by the formula (12): 
         [0000]        V−=HX 1 *S−HX 2 *S    (9)
 
         [0000]        V+=−HX 1 *S −(− HX 2)* S    (10)
 
         [0000]        V out =V+−V−=− 2 *SXZ*S *( HZ 2 −HZ 1)   (11)
 
         [0000]      Gradient  HZ= ( HZ 1 −HZ 2)/ Lg=−V out/(2 *Lg*SXZ*S )   (12)
 
       Fourth Embodiment 
       [0060]      FIGS. 8-13  are diagrams of a Z-axis gradient sensor with multiple flux guide devices and electrical connections thereof. The number of the multiple flux guide devices is 2*N (N is an integer greater than 1); the multiple flux guide devices are classified as two groups, that is, group A and group B, are corresponding to  2  ( 11 ) and  2  ( 12 ), and are arranged into an array of two rows and one column; the row direction thereof is the Y-axis direction, the column direction is the X-axis direction, and a row spacing between the two groups is Lg. In addition, the group A and the group B each include N flux guide devices, in which the N flux guide devices of the group A are numbered  2 ( 11 )- 1  to  2 ( 11 )-N respectively, the N flux guide devices of the group B are numbered  2  ( 12 )- 1  to  2 ( 12 )-N respectively; each group is arranged into a structure of N rows and one column, and a distance between rows is a gradient feature group spacing Ls, wherein Ls is much less than Lg, and similarly, each flux guide device is corresponding to two positions Y 1  and Y 2 . 
         [0061]    Similar to the Z-axis gradient sensor chip of the two-flux guide device structure, structures of the Z-axis gradient sensor with multiple groups of flux guide devices may also be classified into two types of structures, i.e., half bridge and full bridge, which are one-to-one corresponding to those of the Z-axis gradient sensor chip with dual flux guide devices. The difference lies in that, in the Z-axis gradient sensor chip with dual flux guide devices, each bridge arm is corresponding to the position Y 1  or Y 2  of the flux guide device A or B, and for the Z-axis gradient sensor chip with multiple flux guide devices, each bridge arm is corresponding to the position Y 1  or Y 2  corresponding to N flux guide devices A 1 -AN or B 1 -BN in the group A or group B. 
         [0062]      FIGS. 8-9  are diagrams of a half-bridge-structure Z-axis gradient sensor with multiple flux guide devices and electrical connections thereof, in which magnetoresistive units corresponding to two bridge arms that form the half bridge are corresponding to the same position Y 1  or Y 2  in N flux guide devices of the group A and N flux guide devices of the group B, respectively, and have the same magnetic field sensing direction. 
         [0063]    Similarly, the Z-axis gradient sensor with multiple flux guide devices also has two types of full bridge structures. 
         [0064]      FIGS. 10-12  are diagrams of a full-bridge-structure Z-axis gradient sensor with multiple flux guide devices and a structure thereof. Four bridge arms forming a full bridge in  FIG. 10  are located at two respective positions Y 1  and Y 2  corresponding to N flux guide devices of the group A and N flux guide devices of the group B. 
         [0065]    The first type of connections between magnetoresistive units are as shown in  FIG. 11 , two half bridges of the full bridge are corresponding to the position Y 1 /Y 2  of N flux guide devices of the group A or the position Y 1 /Y 2  of N flux guide devices of the group B, respectively, the magnetoresistive units at the position Y 1  or Y 2  have the same magnetic field sensing directions, and two bridge arms connected to the same electrode are located at the same position Y 1  or Y 2  in different flux guide devices of A/B or B/A. 
         [0066]    The second type of connections between magnetoresistive units are as shown in  FIG. 12 , bridge arms corresponding to either of two half bridges of the full bridge are corresponding to the same position Y 1  or Y 2  of N flux guide devices in two groups of flux guide devices, respectively, and have the same magnetic field sensing direction, and two bridge arms connected to the same electrode are located at different positions in the same group of flux guide devices. 
         [0067]    The magnetoresistive Z-axis gradient sensor chip with dual flux guide devices or multiple flux guide devices, in the full bridge or half bridge structure, has multiple magnetoresistive units corresponding to the position Y 1  or Y 2  of the same flux guide device. The magnetoresistive units may form a series structure as in  FIG. 13( a )  and may also form a parallel structure as in  FIG. 13( b ) , or form a series and parallel combined structure. In the magnetoresistive Z-axis gradient sensor with dual flux guide devices, each bridge arm is corresponding to the position Y 1  or Y 2  of one flux guide device. Therefore, the magnetoresistive units finally form a two-port structure in the position Y 1  or Y 2  by series connection, parallel connection or a combination of series connection and parallel connection, to form a bridge arm. 
         [0068]    For a full bridge or half bridge structure with multiple flux guide devices, the bridge arms are corresponding to the same position X or Y of N flux guide devices in the group A or group B. Therefore, in addition to series connection, parallel connection or a combination of series connection and parallel connection in the position X or Y, series connection, parallel connection or a combination of series connection and parallel connection between the N flux guide devices is further included, to finally form a two-port structure and make up a bridge arm. 
         [0069]    In addition, for the Z-axis gradient sensor chip of a half bridge structure or a full bridge structure, each bridge arm has the same number of magnetoresistive units, and the series and parallel electrical connection structures thereof in the flux guide devices are also the same. 
       Fifth Embodiment 
       [0070]      FIG. 14  is a structural diagram showing that a magnetoresistive Z-axis sensor chip is applied to identification of a hard magnetic image, which includes a PCB  6  and a magnetoresistive Z-axis gradient sensor chip  100 . The magnetoresistive Z-axis gradient sensor chip  100  is located on the PCB  6 , and a hard magnetic image  7  is located on a magnetic image detection surface parallel to the surface of the chip, and a component of a Z-axis magnetic field of the hard magnetic image  7  is detected by the magnetoresistive Z-axis gradient sensor chip  100 , so as to identify the magnetic image  7 . In this case, the magnetoresistive Z-axis gradient sensor chip is required to have high magnetic field sensitivity, and the measuring range of the Z-axis magnetic field thereof is greater than that of the Z-axis magnetic field generated by the hard magnetic image  7 . 
       Sixth Embodiment 
       [0071]      FIG. 15  is a structural diagram showing that a magnetoresistive Z-axis gradient sensor chip is applied to identification of a soft magnetic image or applied to a gear sensor, which includes a PCB  6  and a back magnetic block  8 . The magnetoresistive Z-axis gradient sensor chip  100  is located on the PCB  6 , and the back-bias magnetic block  8  is located at the back of the PCB  6 . The magnetization direction of the back-bias magnetic block  8  is perpendicular to the PCB  6 , that is, along Z-axis direction. A soft magnetic image  7 ′ is located on a surface parallel to the magnetoresistive Z-axis sensor chip  100 . In this case, the magnetoresistive Z-axis gradient sensor chip is required to have high magnetic field sensitivity, and the measuring range of the Z-axis magnetic field thereof is greater than that of the Z-axis magnetic field generated by the back-bias magnetic block  8 . 
       Seventh Embodiment 
       [0072]      FIG. 16  is a schematic diagram of a mounting structure of a magnetoresistive Z-axis gradient sensor chip, which, when applied to a currency detector magnetic head, further includes a PCB  6 , a back-bias magnetic block  8  and a casing  200 . The magnetoresistive Z-axis gradient sensor chip  100  is mounted onto the PCB  6 , the back-bias magnetic block  8  is located at the back of the PCB  6 , the PCB  6  and the magnetoresistive Z-axis gradient sensor chip  100  are mounted in the casing  200 , the casing  200  includes a mounting bracket  9  and a pin electrode  12 , and the magnetoresistive Z-axis gradient sensor chip  100  is designed to have a saturated magnetic field higher than the Z-axis magnetic field generated by the back-bias magnetic block  8  and high magnetic field sensitivity. 
         [0073]    The above descriptions are merely preferred embodiments of the present invention, and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and changes. Any modification, equivalent replacement, improvement or the like made without departing from the spirit and principle of the present invention shall all fall within the protection scope of the present invention.