Abstract:
A magnetic field vector sensor includes a substrate parallel to a plane, a support mobile relative to it and rotatable about a vertical rotation axis perpendicular to it, a magnetic field source generating a field having a moment in a non-perpendicular direction, the source being fixed to the support with no degree-of-freedom to exert torque on the support when a field to be measured is present, the field being non-collinear with the moment, a transducer to convert torque exerted on the support into a field amplitude of a component of the field along a measurement axis in the plane, wherein the source comprises a magnetostrictive permanent magnet for generating the field having a moment whose direction varies with stress on the magnet, and wherein the sensor further comprises a controllable device to reversibly modify the moment direction, and a stress generator to vary stress and hence moment direction.

Description:
RELATED APPLICATIONS 
       [0001]    Under 35 USC 119, this application claims the benefit of the priority date of French Patent Application 1160325, filed Nov. 14, 2011, the contents of which are herein incorporated by reference. 
       FIELD OF DISCLOSURE 
       [0002]    The invention concerns a magnetic field vector sensor and a method of measuring a magnetic field using that sensor. 
       BACKGROUND 
       [0003]    A vector sensor is a sensor that enables measurement of the components of a magnetic field to be measured in a frame of reference with two or three dimensions. Here, in a frame of reference with two dimensions, these components are denoted c X  and c Y . In a frame of reference with three dimensions, these components are denoted c X , c Y  and c Z . A vector sensor thus enables measurement of the direction, and preferably also of the amplitude, of the magnetic field. The components c X , c Y  and c Z  correspond to the orthogonal projection of the magnetic field to be measured onto the axes X, Y and Z, respectively, of the frame of reference. 
         [0004]    Known magnetic field vector sensors include:
   a fixed substrate extending essentially parallel to a plane called the “substrate plane”,   a support mobile relative to the substrate rotatable about at least a vertical rotation axis Z perpendicular to the substrate plane,   a magnetic field source having a magnetic moment in a direction that is not perpendicular to the substrate plane, this source being fixed to the mobile support with no degree of freedom so as to exert a mechanical torque on the support about the vertical rotation axis in the presence of a magnetic field to be measured that is not colinear with the direction of this magnetic moment,   at least one transducer adapted to convert the torque exerted on the mobile support about the rotation axis into an electrical signal representative of the amplitude of a component of the magnetic field to be measured along a measurement axis contained in the substrate plane.   
 
         [0009]    When the magnetic field source is immersed in the magnetic field to be measured, a torque Γ appears that is a function of the magnetic moment of the source, the amplitude of the magnetic field to be measured and the angle between the magnetic moment of the source and the direction of the magnetic field to be measured. This torque Γ tends to cause the mobile support to turn so that the direction of the magnetic moment of the source is aligned with the direction of the magnetic field to be measured. A torque is therefore exerted on the mobile support. This torque, or a magnitude representative of this torque, such as an angular displacement, is measured by the transducers. 
         [0010]    However, to determine unambiguously the components c X  and c Y  of the magnetic field to be measured contained in the substrate plane of the sensor, at least two magnetic field sources are necessarily required the magnetic moments of which are aligned in two different directions. Consequently, the known sensors include at least one first mobile support and one second mobile support each carrying a respective one of these magnetic field sources. 
         [0011]    The presence of a plurality of mobile supports and a plurality of magnetic field sources gives rise to the following problems in particular: 
         [0012]    it increases the overall size of the sensor, 
         [0013]    it can cause measurement errors from one sensor to another because it is difficult to position the magnetic field sources in each sensor and to orient them precisely with respect to each other in exactly the same manner, and 
         [0014]    magnetic coupling may exist between the different magnetic field sources, which introduces a skew into the measurement. 
         [0015]    The prior art also includes US2011/0140693A1 and the paper by Dirk Ettelt et al, “A novel Microfabricated High Precision Vector Magnetometer”, 2011 IEEE Sensors Proceedings: Limerick, Ireland, 28-31 Oct. 2011, pages 2010-2013. 
       SUMMARY 
       [0016]    The invention aims to remedy at least one of these drawbacks. It therefore consists in a magnetic field vector sensor according to claim  1 . 
         [0017]    Being able to modify the direction of the magnetic moment of the magnetic field source enables the use of only one magnetic field source to measure the components c X  and c Y . The overall size of this sensor is therefore limited because only one magnetic field generator is used to measure the two components c X  and c Y . Moreover, the problems with positioning the different magnetic field sources relative to each other are eliminated. The risk of magnetic coupling between these different magnetic field sources is also eliminated because only one magnetic field source is used. 
         [0018]    Embodiments of this sensor may have one or more of the features of the dependent claims. 
         [0019]    These embodiments of the sensor also have the following advantages: 
         [0020]    using a permanent magnet, constituted of a one-piece material or a stack of antiferromagnetic/ferromagnetic layers with a non-zero overall remnant magnetization enables the electrical power consumption of the sensor to be reduced, 
         [0021]    orienting the direction of the magnetic moment of the magnetic field source parallel or perpendicular to the stretching direction according to the value of the magnetrostriction coefficient λ s  enables the sensitivity of the sensor to be increased, 
         [0022]    using a beam the ends at least of which are of piezoelectric material enables a uniaxial stress to be applied to the permanent magnet in a simple and controlled manner, 
         [0023]    using as the magnetic field source first and second electrical conductors extending in respective directions enables simple control of the intensity of the magnetic moment generated by the source as well as its direction, 
         [0024]    using a frame connected to the support by means of a set of beams and connected to the substrate by means of another set of beams deformable in flexing and in torsion enables simple implementation of a sensor capable of measuring the components c X , c Y  and c Z , 
         [0025]    alternately aligning the magnetic moment of the source with the axes X and Y of the frame of reference enables simple measurement of the components c X  and c Y  of the magnetic field to be measured. 
         [0026]    The invention also consists in a method according to claim  8  of measuring a magnetic field using the above sensor. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0027]    The invention will be better understood after reading the following description, provided by way of nonlimiting example only and given with reference to the drawings, in which: 
           [0028]      FIG. 1  is a diagrammatic illustration in vertical section of a vector sensor, 
           [0029]      FIG. 2  is a diagrammatic illustration in plan view of the sensor from  FIG. 1 , 
           [0030]      FIG. 3  is a diagrammatic illustration in vertical section of a first embodiment of a stress generator used in the sensor from  FIG. 1 , 
           [0031]      FIG. 4  is a diagrammatic illustration in plan view of the stress generator from  FIG. 3 , 
           [0032]      FIG. 5  is a flowchart of a method of fabricating the sensor from  FIG. 1 , 
           [0033]      FIGS. 6 to 17  are diagrammatic illustrations in vertical section of different steps in the fabrication of the sensor from  FIG. 1 , 
           [0034]      FIG. 18  is a flowchart of a method of measuring a magnetic field using the sensor from  FIG. 1 , 
           [0035]      FIG. 19  is a diagrammatic illustration of a second embodiment of a stress generator that may be used in the sensor from  FIG. 1 , 
           [0036]      FIG. 20  is a diagrammatic illustration of another embodiment of a magnetic field source that may be used in the sensor from  FIG. 1 , 
           [0037]      FIG. 21  is a diagrammatic illustration in vertical section of a third embodiment of a stress generator that may be used in the sensor from  FIG. 1 , and 
           [0038]      FIG. 22  is a diagrammatic illustration of another embodiment of a magnetic field source that may be used in the sensor from  FIG. 1 . 
       
    
    
       [0039]    In these figures, the same references are used to designate the same elements. 
       DETAILED DESCRIPTION 
       [0040]    In the remainder of this description, features and functions well known to a person skilled in the art are not described in detail. 
         [0041]      FIGS. 1 and 2  represent a magnetic field vector sensor  2 . This sensor  2  is adapted to measure the orthogonal projection of the magnetic field to be measured onto three orthogonal axes  10 ,  12  and  14 . Here these axes are parallel to the directions X, Y and Z, respectively, of an orthogonal frame of reference. The directions X and Y are horizontal directions while the direction Z is a vertical direction. Hereinafter, terms such as “above”, “below”, “upper” and “lower” are defined relative to the direction Z. In  FIG. 1 , the axis  14  is perpendicular to the plane of the sheet. To limit the overall size of  FIG. 2 , it has been squeezed in one direction so that the axes  12  and  14  do not appear as being perpendicular to each other. 
         [0042]    The sensor  2  includes a rigid substrate  4  extending essentially in a horizontal plane called the “substrate plane”. The thickness in the vertical direction of this substrate  4  is typically greater than 10 μm, 50 μm or 100 μm. 
         [0043]    The sensor  2  also includes a mobile support  6  rotatable about the axes  10 ,  12  and  14 . Here this support has a rectangular or square horizontal section. The support  6  is mechanically connected to the substrate  4  by means of an articulated mechanism  16  ( FIG. 2 ) enabling the support to rotate about the axes  10 ,  12  and  14 . 
         [0044]    Here this mechanism  16  includes: 
         [0045]    a rigid frame  18  that is not deformed in response to the rotation of the support  6 , 
         [0046]    a set of two beams  20   a  and  20   b  adapted to hold the frame  16  suspended above the substrate  4 , and 
         [0047]    a set of two beams  22   a  and  22   b  adapted to hold the frame  6  suspended above the substrate  4 . 
         [0048]    Here the frame  18  is suspended over a bottom  24  ( FIG. 1 ) of a cavity  26  hollowed out in an upper face of the substrate  4 . This cavity  26  is also delimited by vertical walls  28 . By way of illustration only, the horizontal section of this cavity  26  is rectangular or square. 
         [0049]    The cross section of the frame  18  is also rectangular or square, for example. The frame  18  includes a rectangular or square central opening passing through it along the axis  10 . The support  6  is received inside this central opening. Here the horizontal section of the support  6  is also rectangular or square. 
         [0050]    The ends of the beams  20   a  and  20   b  are anchored with no degree of freedom, on one side, to the wall  28  and, on the opposite side, to the frame  18 . These beams  20   a  and  20   b  are disposed in diametrically opposite positions on respective opposite sides of the axis  10 . Here the beams  20   a  and  20   b  mechanically connect respective corners of the frame  18  and vertical walls  28  of the cavity  26 . These beams  20   a  and  20   b  extend along the axis  12 . 
         [0051]    The length L ext  along the axis  12  and the width l ext  perpendicular to that axis of the beams  20   a  and  20   b  are chosen so as: 
         [0052]    to enable deformation in flexing of the beam parallel to the horizontal plane so as to authorize rotation of the frame  16  about the axis  10 , and 
         [0053]    to enable deformation in torsion of the beam about the axis  12  so as to authorize rotation of the frame  16  about the axis  12 . 
         [0054]    To this end, the width l ext  is at least ten or twenty times less than the length L ext , for example. 
         [0055]    The thickness e ext  of the beams  20   a  and  20   b  in the vertical direction is sufficient to maintain the frame  18 , the beams  22   a  and  22   b,  and the support  6  suspended above the bottom  24 . Here the thickness e ext  is equal to the thickness of the frame  18  and the support  6 . For example, this thickness e ext  is greater than or equal to 10 μm. 
         [0056]    The ends of the beams  22   a  and  22   b  are anchored with no degree of freedom, on one side, to the frame  18  and, on the opposite side, to the support  6 . Here these beams  22   a  and  22   b  extend essentially along the axis  14 . To be more precise, the beams  22   a  and  22   b  are disposed diametrically opposite each other with respect to the axis  10 . Here they connect respective corners of the support  6  to facing corners of the frame  18 . 
         [0057]    The length L int  along the axis  14  and the width l int  perpendicular to that axis of the beams  22   a  and  22   b  are chosen so as: 
         [0058]    to enable deformation in flexing of the beam parallel to the to the horizontal plane so as to authorize rotation of the support  6  relative to the frame  18  about the axis  10 , and 
         [0059]    to enable deformation in torsion of the beam about the axis  14  so as to authorize rotation of the support  6  about the axis  14 . 
         [0060]    The thickness e int  of the beams  22   a  and  22   b  in the direction Z is sufficient to maintain the support  6  suspended above the bottom  24 . 
         [0061]    For example, the dimensions L int , l int  and e int  are chosen like the dimensions L ext , l ext  and e ext , respectively. 
         [0062]    Respective transducers are provided for measuring a physical magnitude representative of the torque exerted on the support  6  about the axes  10 ,  12  and  14 . Here the measured physical magnitude is an angular displacement. To this end, the sensor  2  includes capacitive transducers  30  to  33  ( FIG. 2 ) used to measure the angular displacement θ Z  of the support  6  about the axis  10  and capacitive transducers  36  and  38  used to measure the angular displacements θ X  and θ Y , respectively, about the axes  12  and  14 , respectively. 
         [0063]    Each transducer  30  to  33  is formed of:
   a mobile electrode  40  mechanically connected with no degree of freedom to the support  6  by means of a rigid arm  42 , and   a fixed electrode  44  fixed with no degree of freedom to the substrate  4 .   
 
         [0066]    Here the electrodes  40  and  44  are interdigitated to increase the measurement sensitivity. 
         [0067]    Rotation of the support  6  about the axis  10  leads to displacement of the electrode  40 , which modifies the distance between the electrodes  40  and  44  and therefore the capacitance of the transducer. Thus the angle θ Z  can be measured. 
         [0068]    The transducer  36  includes two electrodes  46  and  47  fixed with no degree of freedom to the bottom  24  below the support  6 . The electrodes  46  and  47  are disposed on either side of the axis  10  along the axis  14 . Here these electrodes  46 ,  47  are symmetrical to each other with respect to an axis corresponding to the projection of the axis  12  on the bottom  24 . These electrodes  46 ,  47  are represented in dashed line in  FIG. 2 . Accordingly, if the support  6  turns about the axis  12 , on one side of the axis  12 , the support moves toward one of the electrodes  46  or  47  and, on the other side of the axis  12 , the support  6  moves away from the other of these electrodes  46 ,  47 . This therefore modifies the capacitor formed by the electrodes  46  and  47  with the bottom of the support  6  and thus enables the angle θ X  to be measured. 
         [0069]    The transducer  38  is produced with the aid of two electrodes  50  and  51 . These electrodes  50  and  51  are disposed as described for the transducer  36 , for example, but with that description transposed for the measurement of rotation about the axis  14 . 
         [0070]    A magnetic field source  54  is fixed with no degree of freedom to the support  6 . This source  54  generates a magnetic field having a magnetic moment. The direction  55  of this magnetic moment is represented by an arrow in  FIG. 2 . 
         [0071]    In this particular embodiment, this source is a magnetostrictive permanent magnet with a non-zero overall remanent magnetization. In the remainder of this description, this magnet also carries the reference  54 . To be more precise, it is the inverse magnetostrictive properties of this magnet that are used here, i.e. the possibility of modifying the magnetic properties of the magnet by applying mechanical stresses to it. The magnetic property of the magnet that is modified by the application of stress is the direction  55  of the magnetic moment. 
         [0072]    In this embodiment, the permanent magnet  54  is a stack of alternating layers of ferromagnetic material and layers of antiferromagnetic material deposited directly onto the layer of ferromagnetic material. The antiferromagnetic layers serve to trap the direction of the magnetic moment of the ferromagnetic layers. The stack preferably comprises a plurality of ferromagnetic layers and a plurality of antiferromagnetic layers. Such stacks are well known for producing the trapped layer or the reference layer used in spintronics, for example, for the production of radio-frequency oscillators. 
         [0073]    In the absence of an exterior magnetic field, this stack has a preferential direction of magnetization close to saturation, also known as the “trapping direction”. For example, in the absence of any mechanical stress, this direction is aligned with the axis  12 . 
         [0074]    The ferromagnetic layer is typically produced with the aid of:
   a polycrystalline alloy containing iron or iron and cobalt, possibly with one or more substitution elements such as gallium or terbium, or   an amorphous composition including iron or iron and cobalt with one or more amorphization components such as boron, nitrogen or carbon and with one or more substitution elements such as gallium or terbium (Tb).   
 
         [0077]    The antiferromagnetic layer may be produced in a manganese alloy such as a PtMn, IrMn, FeMn or NiMn alloy. 
         [0078]    Here the ferromagnetic layer is a CoFeB alloy and the antiferromagnetic layer is a PtMn alloy. 
         [0079]    Such a stack has a positive magnetrostriction coefficient λ s  typically between 50 and 120 ppm and a coercitive magnetic field between 1 and 20 Oe (one Oersted is approximately equal to 79.57 A.m −1 ). 
         [0080]    Because of the action of a uniaxial tension mechanical stress applied to the magnet  54  perpendicularly to its preferential magnetization direction, the direction  55  of its magnetic moment turns. For example, to cause the direction of the magnetic moment of the magnet  54  to turn 90°, a mechanical stress greater than 100 MPa, and preferably between 100 MPa and 1 GPa, must be applied. Moreover, all directions of the magnetic moment of this magnet  54  between 0° and 90° are stable directions, i.e. can be maintained indefinitely for as long as the corresponding mechanical stress is applied. 
         [0081]    To modify the direction  55  of the magnetic moment, the sensor  2  includes a device  56  ( FIG. 1 ). Here this device  56  is a mechanical stress generator. In the remainder of this description, this generator carries the same numerical reference as the device  56 . The generator  56  preferably generates only a uniaxial stress perpendicular to the preferential magnetization direction of the magnet  54 . 
         [0082]    Finally, the sensor  2  includes a processing unit  58  adapted to control the generator  56  and to process the measurements from the transducers  30  to  33 ,  36  and  38 . 
         [0083]      FIGS. 3 and 4  show the generator  56  in more detail. Here the generator  56  is identical to that described in French patent application FR 2 905 793 published 14 Mar. 2008. Accordingly, only an outline description of this generator is given here. For more details, the reader may consult the French patent application FR 2 905 793. 
         [0084]    The generator  56  includes a beam  60  each end  62 ,  63  of which is anchored with no degree of freedom to an upper face of the support  6 . Between these ends  62 ,  63 , the beam  60  forms a central branch  64  suspended above the upper face of the support  6 . 
         [0085]    The ends  62 ,  63  and preferably the central branch  64  are produced in a piezoelectric material such as PZT (lead zirconate titanate). 
         [0086]    To actuate this piezoelectric material and to generate a uniaxial stress in the longitudinal direction of the beam, each end  62  and  63  is disposed between lower and upper control electrodes  66 ,  67 . 
         [0087]    The permanent magnet  54  is fixed with no degree of freedom, for example directly, to the central branch  64  so that its preferential magnetization direction is perpendicular to the longitudinal axis of the beam  60 . Here the longitudinal axis of the beam  60  is parallel to the direction Y. 
         [0088]    The uniaxial mechanical stress exerted on the magnet  54  typically increases from 1 MPa/V and preferably from at least 10 MPa/V as a function of the potential difference between the electrodes  66  and  67 . 
         [0089]    The method of fabricating the sensor  2  will now be described with reference to the method of  FIG. 5  and with the aid of  FIGS. 6 to 17 . 
         [0090]    During a step  70 , the substrate  4  is procured and a layer  72  of electrical insulation is deposited on its upper face ( FIG. 6 ). The substrate  4  is typically a silicon substrate. 
         [0091]    During a step  74 , the electrodes  46 ,  47 ,  50  and  51  are deposited and etched on the surface of the layer  72  ( FIG. 7 ). In  FIG. 7  and the subsequent figures only the electrodes  50  and  51  can be seen. 
         [0092]    During a step  76 , a sacrificial layer  78  is deposited on top of the electrodes  46 ,  47 ,  50  and  51  and then polished ( FIG. 8 ). The sacrificial layer  78  is a silicon oxide layer, for example. 
         [0093]    During a step  80 , a silicon layer  82  is deposited on the layer  78  by an epitaxial growth process ( FIG. 9 ). This silicon layer  82  typically has a vertical thickness greater than 10 μm. 
         [0094]    During a step  84 , a cavity  86  is hollowed out substantially at the centre of the upper face of the layer  82  by photolithography and etching ( FIG. 10 ). This cavity  86  is intended to be located under the central branch  64  of the beam  60 . 
         [0095]    During a step  88 , a sacrificial layer  90  is deposited to fill the cavity  86  ( FIG. 11 ). This layer  90  is then polished to render the upper face plane. The sacrificial layer is of polycrystalline silicon, for example. 
         [0096]    During a step  92 , a layer  94  of electrical insulation is deposited on the sacrificial layer  90  ( FIG. 12 ). During this same step, the lower electrodes  66  of the stress generator  56  are deposited and then etched on the layer  94 . These electrodes are in platinum, for example. 
         [0097]    During a step  95 , the piezoelectric material beam  60  is deposited and etched on the layer  94  ( FIG. 13 ). During this step, the upper electrodes  67  of the generator  56  are also deposited and etched. 
         [0098]    During a step  96 , the permanent magnet  54  is deposited and then etched ( FIG. 14 ). During this step  96 , gold electrodes  98  are also deposited and etched on the upper electrodes  67  to enable an electrical contact to be formed on these upper electrodes. 
         [0099]    During a step  100 , if necessary, a protection layer  102  is deposited and then etched to protect the magnet  54  during subsequent attacks by HF (hydrofluoric acid) vapour etching ( FIG. 15 ). 
         [0100]    During a step  104 , the support  6 , the frame  18  and the beams  20   a,    20   b,    22   a  and  22   b  are formed in the layer  82  by photolithography followed by Deep Reactive Ion Etching (DRIE) ( FIG. 16 ). 
         [0101]    Finally, during a step  106 , the sacrificial layers  90  and the sacrificial layer  78  situated under the support  6  are eliminated ( FIG. 17 ) to free the mechanism  16 , the support  6  and the beam  60 . These sacrificial layers are typically eliminated by an HF vapour etching step. 
         [0102]    The operation of the sensor  2  will now be described with reference to the  FIG. 18  method. 
         [0103]    Initially, during a step  110 , the stress generator  56  does not apply any stress to the magnet  54 . The magnet  54  then generates a magnetic field the direction  55  of the magnetic moment of which is aligned with the axis  12 . 
         [0104]    During a step  112 , the transducers  30  to  33  and  38  respectively measure the angles θ Z  and θ Y  of rotation about the axes  10  and  14 , respectively. These angles θ Z  and θ Y  are functions of the torque exerted by the magnetic field to be measured on the magnet  54 . 
         [0105]    Then, during a step  114 , the processing unit  58  samples the electrical signals generated by the transducers and then deduces from them the measured values of the angles θ Z  and θ Y . 
         [0106]    Then, during a step  116 , the processing unit  58  controls the generator  56  in such a manner as to align the direction  55  of the magnetic moment of the magnet  54  with the axis  14 . 
         [0107]    There follows a step  118  of measuring the angles θ Z  and θ X  with the aid of the transducers  30  to  33  and  36 . This step is similar to the step  112  except that the transducer  36  is used in place of the transducer  38 . 
         [0108]    During a step  120 , the processing unit  58  samples the electrical signals generated by the transducers during the step  118  and deduces from them the measured values of the angles θ Z  and θ X . Then, on the basis of the values of the angle θ Z  measured during steps  112  and  118 , respectively, the unit  58  establishes the value of the components c Y  and c X , respectively. The unit  58  also deduces the value of the component c Z  of the magnetic field to be measured from the measured values of the angles θ X  and θ Y  produced during the steps  112  and  118 . 
         [0109]    Numerous other embodiments of the sensor  2  are possible. For example, the device for modifying the direction of the magnetic moment of the magnet  54  may include a plurality of mechanical stress generators.  FIG. 19  represents one such device including the generator  56  and an additional magnetic stress generator  130 . The generator  130  is designed to exert a uniaxial mechanical stress on the magnet  54  in a direction different from that exerted by the generator  56 . For example, the generator  130  is identical to the generator  56  except that it is offset angularly relative to it, for example by 90°. The magnet  54  is located at the intersection of the beams  60  of the generators  56  and  130 . This intersection typically forms with the beams  60  of the generators  56  and  130  a single block of material. 
         [0110]    This device is capable of generating a uniaxial mechanical stress in all directions parallel to the substrate plane  4  included within an angular sector from 0 to 90°. This can prove useful for finer adjustment of the direction  55  of the magnetic moment. 
         [0111]    The magnetic field source can equally include a permanent magnet the direction of the magnetic moment of which cannot be modified. One such embodiment is shown in  FIG. 20 . For example, the support  6  from  FIG. 20  includes the permanent magnet  54 , the mechanical stress generator  56  and an additional permanent magnet  134  deposited directly onto the substrate  6 . 
         [0112]    In  FIG. 20 , in the absence of any mechanical stress, the magnetic moments of the magnets  54  and  134  are parallel to the direction X. If the generator  56  modifies the direction  55  of the magnetic moment of the magnet  54  to align it with the direction Y, the magnetic moment of the source is then at an angle of 45° to the direction X, for example. The direction of the magnetic moment of the magnetic field source is in this case the result of combining the directions of the magnetic moments of the magnets  54  and  134 . Except for this difference, this variant functions as described above. 
         [0113]    Other embodiments of the mechanical stress generator are possible. For example, the generator can be actuated thermally and not with the aid of piezoelectric material.  FIG. 21  represents such an embodiment. In this figure, the permanent magnet  54  is deposited on a suspended beam  138  the geometry of which is identical to that of the beam  60 , for example. Ends  139  and  140  of this beam are fixed with no degree of freedom to the upper face of the support  6  by means of rigid blocks  144  and  145 . The beam  150  is produced in a material having a coefficient of thermal expansion identical to or different from that of the material used to produce the support  6 . 
         [0114]    A heat source  148  is provided for heating the substrate  6  more than the beam  138 . Under these conditions, either the beam  138  or the substrate  6  expands more than the other, which leads to the appearance of a uniaxial mechanical stress in the magnet  54  fixed to a central branch  150  of the beam  138  suspended above the upper face of the support  6 . 
         [0115]    The magnetic field source is not necessarily a permanent magnet. For example, it can also consist of conductive wires carrying a direct current.  FIG. 22  represents one such magnetic field source  160 . Here the source  160  includes two electrical conductors  162  and  164  fixed with no degree of freedom to the upper face of the support  6 . In this embodiment, the magnet  54  and the stress generator  56  are omitted. 
         [0116]    The conductor  162  extends parallel to the axis  14 . The conductor  164  extends in a different direction. For example, the conductor  164  here traces out a Z on the upper surface of the support  6 , the transverse bar of this Z extending over the majority of the diagonal of the support  6  that is parallel to the axis  12 . The horizontal bars of this Z, and only these horizontal bars, are preferably disposed between metal layers electrically connected to ground to form a magnetic screen. 
         [0117]    The conductors  162  and  164  are electrically insulated from each other so as to be adapted to be supplied with power alternately. Here the end of each conductor  162 ,  164  is connected to ground and its opposite other end is connected to a direct current voltage source V + . 
         [0118]    The device  56  is replaced by a device  166  adapted to cause a current to flow alternately in the conductors  162  and  164 . To this end, the device  166  includes two switches  168 ,  170 . To modify the direction of the magnetic moment of the source  160 , the device  166  opens one of the switches  168 ,  170  and closes the other one. The remainder of the operation of the sensor is identical to that described above. 
         [0119]    Accordingly, in this embodiment, the magnetic field source includes first and second conductors electrically insulated from each other and fixed with no degree of freedom to the mobile support, these electrical conductors extending in respective different directions parallel to the substrate plane, and the device for modifying the direction of the magnetic moment includes switches that can be operated to supply power to the first and second electrical conductors alternately. 
         [0120]    The magnetic moment of the magnetic field source does not necessarily turn through exactly 90°. The direction of the magnetic moment can turn through an angular value different from 90° but must not turn through a multiple of 180°. In particular, it is not necessary for the direction of the magnetic moment to be parallel to the rotation axes  12  and  14  of the support  6 . A simple calculation, such as a change of frame of reference, makes it possible to establish the components c X  and c Y  of the magnetic field to be measured with a source magnetic moment that is not aligned either with the axis  12  or with the axis  14 . 
         [0121]    The direction of the magnetic moment of the generated magnetic field is not necessarily parallel to the substrate plane. This direction alternatively includes at least one non-zero component in the substrate plane and possibly also a non-zero component in the vertical direction 
         [0122]    The magnetic field source may also be a thin layer of soft anisotropic magnetic material, i.e. one having a magnetic moment measurable only when it is exposed to an exterior magnetic field. These materials typically have a coercitive field less than 10 or 70 A.m −1 . This material is a ferromagnetic material, for example. The ferromagnetic material is typically an amorphous composition containing iron or iron and cobalt with one or more amorphization elements such as boron, nitrogen or carbon and one or more substitution elements such as gallium and terbium. In response to a mechanical tensile stress exerted perpendicularly to its preferential magnetization direction, the direction of the magnetic moment turns through 90°. In contrast to the permanent magnet  54  described above, such a ferromagnetic layer has only two stable positions for 0° and 90°. It is possible to go from one of these stable positions to the other by applying a stress greater than 50 MPa and preferably a mechanical stress between 50 and 100 MPa. 
         [0123]    The permanent magnet can equally be produced in a single block of material having a sufficient magnetostriction coefficient, typically greater than 10 −6  and advantageously greater than 50.10 −6 , rather than a stack of ferromagnetic and antiferromagnetic layers. The production of such a permanent magnet using MEMS (Micro Electromechanical System) technology is described for example in: D. P. Arnold et al., “ Permanent Magnets for MEMS ”, Journal of Microelectromechanical systems, volume 18, no 6, December 2009. 
         [0124]    The magnetic field source can also be a block of soft isotropic magnetic material. This ferromagnetic material will for example be Metglas® that has a positive or negative magnetostriction coefficient λ s  according to its composition. 
         [0125]    The magnetic field source can also be produced with the aid of a block of magnetostrictive material having a negative magnetostriction coefficient λ s . In this case, its preferential magnetization direction must be parallel to the direction of the mechanical tensile stress to cause the direction of its magnetic moment to turn. 
         [0126]    The mechanical stress generator can also be designed to exert a uniaxial compression mechanical stress instead of a uniaxial tension mechanical stress. In this case, the preferential magnetization directions described above must be turned through 90° parallel to the substrate plane. 
         [0127]    The mechanical stress generator can also be produced differently. For example, it can be a generator generating a mechanical stress with the aid of electrostatic deformation. 
         [0128]    The transducers for measuring the rotation about the axes  10 ,  12 , and  14  can be produced differently. For example, the transducers may be replaced by piezoelectric transducers. To this end, beams of piezoelectric material that extend parallel to the beams of the mechanism  16  are provided. Like the beams of the mechanism  16 , these beams are anchored by one of their ends to the frame and by the other end to the substrate, in order to measure the rotation of the frame about the axis  10 . The beams can equally be anchored by one of their ends to the frame and by the other end to the mobile support  6 , to measure the rotation of the support  6  about the axis  10 . One example of such piezoresistive beams is described for example in the following paper: D. Ettelt et al., “ A new low consumption  3 D compass using integrated magnets and piezoresistive nano - gauges ”, Transducers 2011 Conference, Beijing, China. 
         [0129]    Alternatively, an optical transducer is used to measure the rotation of the support  6 . 
         [0130]    Alternatively, the beams  20   a,    20   b  and  22   a,    22   b  are produced differently. For example, it is not necessary for these beams to connect the corners of the frame  18  to the corners of the cavity  26  or the corners of the support  6  to the corners of the frame  18 . The beams can be anchored between vertical and parallel faces of the support  6 , the frame  18  and the walls  28 . 
         [0131]    Other geometries of the frame  18  are possible. For example, the central opening can be circular. 
         [0132]    Alternatively, the control electrodes of the piezoelectric material beam are placed differently. For example, they can be interdigitated and distributed over the whole of the surface of the beam. 
         [0133]    If it is required only to measure the components c X  and c Y  of the magnetic field in the substrate plane, the degree of freedom in rotation about the horizontal axes may be omitted. For example, the beams  20   a,    20   b  and  22   a,    22   b  are then conformed so as not to be deformed in torsion.