Abstract:
A magnetic detector which includes a first thin-layer element and a second thin-layer element made of magnetic material with magnetic anisotropy in the plane possessing, in this plane, two easy axes of magnetization. A coercive field of one of the first and second thin-layer elements has a value different from that of the other thin-layer element. The two thin-layer elements have elongated and mutually parallel shapes perpendicular to their direction of easy magnetization in the absence of a magnetic field. The width of these thin-layer elements is such that it obliges at least one of the thin-layer elements to have its magnetization oriented along the length of the thin-layer element when there is no external magnetic field. Such a magnetic field sensor may find particular application to the measurement of magnetic fields.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a magnetoresistive sensor using the magnetoresistance of ferromagnetic materials. A sensor of this kind is designed to detect magnetic fields. In the field of low frequencies (continuous frequencies at 100 Hz typically), the two noise sources that limit the resolution of this type of sensor are the resistance fluctuation noise or Johnson noise and the thermal drift noise. Furthermore, in this type of sensor, two different magnetoresistive effects can be used: anisotropic magnetoresistance and giant magnetoresistance. 
     2. Discussion of the Background 
     The anisotropic magnetoresistance effect results from the dependence of the resistivity of a ferromagnetic material on the angle between its magnetization and the direction of the current. Owing to the anisotropic character of this physical effect, two measurement geometries can be considered: longitudinal geometry and the transversal geometry (planar Hall effect). The French patents 92 15551, 95 05659 and 96 08385 describe planar Hall effect sensors, especially with respect to the reduction of thermal drift noise. The total exploitable effect is in the range of 1% of the resistance of the active zone. The Johnson noise is associated with the same resistance which is in the range of the resistance per square unit of the film in terms of planar Hall effect geometry. 
     The effect of giant magnetoresistance was discovered in 1988 (see M. Baibich et al., Physic Review Letters, 61, 2472 (1988)) and results from the spin dependence of the resistance of a magnetic structure that can have an arrangement of magnetizations that differs according to the external magnetic field. The total exploitable effect may be in the range of 10% of the resistance of the active zone. Furthermore, since the geometry of measurement of this effect is necessarily longitudinal, the active zone can be sized in such a way that its resistance attains several tens of times the resistance per square unit of the film, the voltage equivalent of the Johnson noise increasing as the square root of the resistance. It can therefore be seen that the use of the giant magnetoresistance effect, especially with respect to sensors using the planar Hall effect, has two main advantages: an increase in the amplitude of the useful signal that is certainly capable of attaining one order of magnitude as well as an increase in the signal-to-noise ratio which too can attain a factor of ten. 
     SUMMARY OF THE INVENTION 
     The invention proposes to use the technique of giant magnetoresistance to obtain a linearizing of the signal and reduce the noise of thermal drift. 
     The invention therefore relates to a magnetic detector comprising a first thin-layer element made of magnetic material with magnetic anisotropy in the plane of the thin layer possessing, in this plane, an easy magnetization axis, characterized in that it comprises a second thin-layer element parallel to the first element, this second element being made of magnetic material with magnetic anisotropy in the plane and having, in this plane, an easy magnetization axis parallel to that of the first element, the coercive field of the second element having a value different from that of the first element, the two elements having elongated and mutually parallel shapes perpendicular to their direction of easy magnetization in the absence of a magnetic field and the width of these elements being such that it obliges at least one of the elements to have its magnetization, when there is no external magnetic field, oriented along the length of the element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various objects and characteristics of the invention shall appear in the following description given by way of an example and in the appended figures, of which: 
     FIGS. 1 a  to  1   c  show an exemplary embodiment of a device according to the invention; 
     FIGS. 2 a ,  2   b  show a standard type of giant magnetoresistance sensor; 
     FIGS. 3 a ,  3   b  show the sensor according to the invention; 
     FIG. 4 shows a Wheatstone bridge assembly of sensors according to the invention; 
     FIGS. 5 and 6 show embodiments of the Wheatstone bridge of FIG. 4; 
     FIGS. 7 a  to  7   c  show a device according to the invention comprising a current conducting line. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 a  and  1   b  describe a basic sensor according to the invention and its method of manufacture. On a substrate S, two superimposed layers of crystalline magnetoresistive materials are made, both having a magnetic anisotropy in the plane. The two layers have their easy magnetization axes (A 1 -B 1 , A 2 -B 2 ) in the absence of parallel magnetic field. The coercive fields of the two layers have different values. In other words, one of the layers (the layer  1  for example) is a hard magnetic material and the other layer is a soft magnetic material (the layer  2 ). Preferably, the two layers are separated by a non-magnetic layer  3 . 
     When these layers have been made, an element with an elongated shape, having the form of a strip, is made in these layers. This strip is oriented along an axis OX perpendicular to the direction of magnetization of the layers when there is no external magnetic field. The width of the strip is such it forces the magnetization of only one of the layers (the layer of soft magnetic material) to get oriented parallel to the axis OX. When there is no magnetic field, there is therefore, in the element obtained, one layer whose magnetization is perpendicular to the axis OX and the other layer whose magnetization is parallel to the axis OX. 
     For example, one of the layers is made of iron-nickel and the other is made of cobalt. 
     The making of a strip in the layers  1  and  2  can be done either by etching the strip in the layers  1  and  2  or by any method that destroys the magnetism and conductive nature on either side of the strip. 
     At both ends A and B of the sensor obtained, electrical contacts are then made to connect devices (not shown) to measure the resistance of the element. 
     The sensor thus described is the active component of a magnetic field detector using the giant magnetoresistance of a magnetic structure in which a single-axis magnetic anisotropy is generated in each of the magnetic elements forming the structure. An active material of this kind may be for example a spin valve type structure (for example Co/Cu/FeNi) deposited on a surface with modulated topology (it is possible for example to use the properties of faceting of stepped silicon surfaces) as shown in FIG. 1 (see document by A. Encinas et al. in Applied Physics Letters, 71, December 1997). In order to manufacture a magnetic field sensor, it is thus possible to exploit the possibility of controlling a single-axis magnetic anisotropy in each of the layers, which is counterbalanced by a shape anisotropy related to the shape of the structure. The approach of the invention leads to a significant simplification of the technology as compared with known techniques such as the one described in J. Daughton et al., IEEE Trans. Magn., 30, 4608 (1994). 
     FIGS. 2 a  and  2   b  respectively show a standard giant magnetoresistance structure and its response curve. FIG. 2 a  shows a parallel orientation of magnetizations in zero fields. FIG. 2 a  shows a discontinuous response (resistance/magnetization) of the sensor wherein the response signal is constant in a weak field. 
     FIGS. 3 a  and  3   b  respectively show the structure according to the invention and its field of response to the magnetic field. The curve of FIG. 3 b  shows a linear response zone. 
     The response signal can be expressed by the formula: 
     
       
         
           R 
           1 
           =R 
           0 
           +SH 
         
       
     
     where: 
     R 0  designates the offset, 
     S designates the slope of the response curve, 
     H designates the external magnetic field. 
     Starting from a structure which already has a fixed axis of anisotropy, the invention, by cutting up the strips perpendicularly to this axis, gives a configuration where the zero field magnetizations of the two layers are perpendicular. The anisotropy induced by the cutting up runs counter to the initial anisotropy and, for each of the layers, there is a threshold bandwidth below which the axis of easy magnetization becomes parallel to the strip and therefore rotates by 90° with respect to the initial situation. The magnetic configuration aimed at is obtained when only one of the two layers has its anisotropy controlled by the cutting up. 
     Depending on the thickness of the layers, our estimates indicate that the strips should be cut up with widths of typically 1 and 10 μm making the method accessible by the techniques of optical lithography. 
     FIG. 4 shows a Wheatstone bridge assembly with four sensors used to overcome thermal drifts. Two sensors C 1  and C′ 1  of the two opposite arms of the bridge are of the type according to the invention and the other two sensors C 2  and C′ 2  are of the type shown in FIGS. 2 a  and  2   b . In the latter sensors, the magnetoresistance signal is constant R 2 =R 0  on a range of magnetic field corresponding to the coercivity of the softest magnetic material. 
     The assembly of FIG. 4 subtracts the resistance R 0 , which is the main source of thermal drift. Indeed, the signal at the output of the bridge can be written as follows: 
     
       
           ΔV/I =( R   1   2   −R   2   2 )/(2 R   1 +2 R   2 )=1/2. S.H   (2) 
       
     
     The sensors C 1  and C′ 1  being defined by sectioning the magnetic structure and, furthermore, the resistance of such a sensor being basically related to its dimensions, it is necessary to choose two patterns in such a way that the second sensors have the same resistance. This balances the bridge. FIG. 5 shows an approach used to attain this goal. 
     In FIG. 5, the two types of sensors C 1 /C′ 1  and C 2 /C′ 2  consist of strips sectioned perpendicularly to the main axis of anisotropy with different bandwidths. 
     For C 1 /C′ 1  type sensors, the width of the strips w 1  is chosen to be low enough to obtain the 90° magnetic configuration described here above, thus enabling the linearization of the magnetoresistive signal. These sensors therefore show a response R 1 =R 0 +S.H. For the C 2 /C′ 2  type sensors, the width of the strips w 2  is chosen to be greater in such a way that the two magnetic layers keep their easy axis parallel to the initial axis of anisotropy. This response will therefore be R 2 =R 0 . The two types of sensors are organized as shown in FIG.  5 . 
     The number of parallel-connected strips serves to control the resistance level. Indeed, if i (i=1 or 2) type sensor includes n i  parallel-connected strips with a width w i , its resistance R i  is proportional to (n i .w i ) −1 . It is therefore enough to choose n 1 , n 2 , w 1 , w 2  in such a way that n 1 .w 1 =n 2 .w 2 . 
     In FIG. 6, the two types of sensors C 1 /C′ 1  and C 2 /C′ 2  are made with the same pattern. However, for the C 2 /C′ 2  type sensors, the pattern is oriented in parallel to the initial axis of anisotropy and no longer perpendicularly. Consequently, the shape anisotropy induced by the cutting up of the strips gets added to (and no longer opposes) the initial anisotropy in such a way that the magnetoresistance signal of the sensor keeps the initial form (FIG. 2 b ). The two types of sensors are organized as shown in FIG.  6 . 
     According to one alternative embodiment, the Wheatstone bridge assembly is eliminated. It is then planned to orient the magnetization of one of the two layers by means of an electrical control line. The sensor is therefore formed by a single magnetoresistive component subdivided so as to linearize the magnetoresistance signal. On top of this component C 4  or beneath it, there is an electrical control line LC (control strip LC) that generates a magnetic field in the component. The direction of one of the two magnetizations (for example M 2 ) of the component is controlled by applying a current having a controlled sign. FIG. 7 a  shows this device. A current generator G is connected by an inverter INV to the control line LC. 
     By applying a current +I in the control line, the magnetization M 2  due to the current has a given sense and, for a field to be detected parallel to the direction, the angle between the two magnetizations of the active structure is 90+ε (FIG. 7 b ). The response of the sensor is then equal to: R 0 +S.H. 
     By applying a current −I to the control line LC and, therefore, through a reversal of the magnetization M 2 , the angle becomes 90−ε (FIG. 7 c ) and the response of the sensor R 0 −S.H. The electronic difference between the two responses makes it possible to subtract the DC component R 0  from the element (responsible for the thermal drift) and to keep only the useful signal 2.S.H. It must be noted that, as compared with bridge assemblies, this approach provides a gain by a factor of 4 in the amplitude of the useful signal before any amplification.