Abstract:
A magnetoresistive layer system, in an environment of a magnetoresistive layer stack that works particularly on the basis of the GMR effect or the AMR effect, a layer array being provided which generates a magnetic field which acts upon the magnetoresistive layer stack, and the layer array having at least one hard magnetic layer and at least one soft magnetic layer. Furthermore, a sensor element, particularly for the detection of magnetic fields with respect to their strength and/or direction, having such a layer system.

Description:
BACKGROUND INFORMATION  
       [0001]     Magnetoresistive layer systems or sensor elements are known from the related art, whose operating point, for example, for use in motor vehicles, is shifted by auxiliary magnetic fields generated in various ways. In particular, the generation of such auxiliary magnetic fields by mounted macroscopic hard magnets or by field coils having current flowing through them is known. Besides that, German Patent Application No. DE 101 28 135.8 explains a concept in which a hard magnetic layer is deposited in the vicinity of a magnetoresistive layer stack, especially on or under the layer stack, which, above all, because of its stray field, couples into the actual sensitive layers. In this context, a coercivity that is as high as possible is foremost as target parameter and, on the other hand, the remanent magnetic field is foremost as limiting parameter. Furthermore, such a hard magnetic layer, in a vertical integration, effects an electrical short circuit of the adjacent sensitive layers of the magnetoresistive layer system, which limits a desired GMR effect or AMR effect, as well as the sensitivity of the layer system with respect to outer magnetic fields.  
         [0002]     It is an object of the present invention to provide a possibility of cost-effectively and simply generating a magnetic bias field or auxiliary magnetic field which acts upon a magnetoresistive layer stack, so as thereby being able to produce magnetoresistive sensor elements, especially for use in motor vehicles, in an inexpensive and yet reliable manner.  
       SUMMARY OF THE INVENTION  
       [0003]     The magnetoresistive layer system according to the present invention has the advantage, compared to the related art, that an increased magnetic stray field is made available at a simultaneously increased coercivity or coercive field strength, via the layer positioning provided in an environment of the magnetoresistive layer stack working particularly on the basis of the GMR effect or the AMR effect, the layer equipment being simply and cost-effectively generated or integrated into the layer system. In particular, the layer system has a very thin type of construction, above all with respect to the thickness of the hard magnetic layer.  
         [0004]     Besides that, it is of advantage that the layer system offers the possibility, within a certain scope, of varying the strength of the stray field generated by the hard magnetic and the soft magnetic layer, and that the particularly thin soft magnetic layer, which is coupled into the hard magnetic layer or is positioned adjacent to it, prevents the demagnetization of the hard magnetic layer in response to the application of an outer magnetic alternating field by domain stray fields (so-called creeping), as is described in Phys. Rev. Lett., 84, (2000), page 1816 and page 3462.  
         [0005]     By the way, a system made of a hard magnetic and a soft magnetic state generally has a higher magnetization as compared to a purely hard magnetic layer, i.e. a higher magnetic moment per volume. Because of that, at the same total thickness, the field strength of the magnetic stray field of a layer system increases with a hard magnetic and a soft magnetic layer, which are, in particular, ferromagnetically exchange coupled, compared to the field strength of only a hard magnetic layer.  
         [0006]     Thus, in the case of a ferromagnetic exchange coupled layer system having at least one soft magnetic and at least one hard magnetic layer, upon the application of an outer magnetic field having an orientation deviating from the direction of magnetization, the soft magnetic layer advantageously demonstrates a chiral magnetization which returns to an alignment parallel to the hard magnetic magnetization when the outer field is switched off, as is described in IEEE Trans. Magn., 27, (1991), page 3588. In particular, the magnetization of the soft magnetic layer is coherently rotated, and is not demagnetized by domain nucleation. Consequently, stray fields of other or additional soft magnetic layers (domain stray fields) are not able to demagnetize the hard magnetic layer at a small distance.  
         [0007]     Moreover, the concept of the construction of the magnetoresistive layer system may be adapted without problem to existing magnetoresistive sensor elements or layer systems having GMR multilayers, magnetoresistive sensor elements or layer systems according to the spin valve principle, AMR sensor elements or even sensor elements based on granular magnetoresistors or magnetoresistances brought about by structural changes of material properties, or may be integrated into appropriate production processes. The deposition of the individual layer of the layer system is not critical, in this context, compared to known influential factors.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  shows magnetization curves of variously constructed layer arrays in comparison.  
         [0009]      FIG. 2  shows a section through a magnetoresistive layer system on a substrate. 
     
    
     DETAILED DESCRIPTION  
       [0010]      FIG. 2  shows a substrate  10 , made, for example, of silicon or silicon oxide, upon which, above an optionally present buffer layer  11 , made, for example, of Cr, W or Mo, there is a hard magnetic layer  12 , and on the hard magnetic layer  12  there is a soft magnetic layer  13 . These two layers  12 ,  13  form a layer system  15 .  
         [0011]     On soft magnetic layer  13 , a magnetoresistive layer stack  14  is provided that is known per se, and preferably works on the basis of the GMR effect (“giant magnetoresistance”) or the AMR effect (“anisotropic magnetoresistance”). Preferably layer stack  14  has a plurality of individual layers which work on the principle of coupled multilayers or the spin valve principle. Layer stack  14  and layer system  15  are thus vertically integrated, and together form a magnetoresistive layer system  5 . Furthermore, magnetoresistive layer stack  14  may also be made up of a CMR material (“colossal magnetoresistance”), such as La 0.67 Ca 0.33 MnO 3 . In this case, magnetoresistive layer stack  14  has a material in which a structural change (“Jahn-Teller effect”) may be induced by a magnetic field or also a temperature change, which effects an electrical transition of the material from a conductor or metal to an insulator. Thereby changes in the electrical resistance of more than 100% may appear. Moreover, by such a CMR material, there is also understood powder magnetoresistance (“PMR”) in which a magnetoresistance between individual granular magnetic parts having different magnetizations is generated.  
         [0012]     Preferably, a ferromagnetic exchange coupled, thin, soft magnetic layer  13  is deposited on hard magnetic layer  12 . In this context, one makes use of the fact that, in a certain layer thickness range, soft magnetic layer  13  ensures both an increased coercivity and an increased amount of the magnetic stray field of layer system  15 . In particular, in relation to a comparable layer thickness of a purely hard magnetic layer, soft magnetic layer  13  increases the amount of the stray field overproportionately corresponding to the high saturation magnetization of soft magnetic layer  13 .  
         [0013]     This permits designing layer system  15  to have a layer  11 ,  12  that is in each case ferromagnetic, but on one hand soft magnetic and on the other hand hard magnetic, at the same stray field to be generated and the same or greater coercivity, to be thinner than a purely hard magnetic layer would be designed, using corresponding parameters. This decreased thickness increases the electrical resistance of layer system  15 , and therewith the GMR effect or the AMR effect in magnetoresistive layer stack  14 , which also leads to an improved sensitivity of layer system  5  in response to a measurement of the magnetic fields acting upon it from the outside.  
         [0014]     By the way, the comparatively expensive hard magnetic materials of hard magnetic layer  12  are a relevant cost factor compared to the comparatively inexpensive soft magnetic materials of soft magnetic layer  13 , i.e. the production costs for layer system  15  are reduced by the use of soft magnetic layer  13 . In addition, soft magnetic layer  13  prevents demagnetization of hard magnetic layer  12  when there is an outer magnetic alternating field.  
         [0015]     Preferably, according to  FIG. 2 , a soft magnetic layer  13  made of a CoFe alloy such as Co 90 Fe 10 , Co, Fe, Ni, an FeNi alloy such as Fe 18 Ni 81  as well as magnetic alloys containing these materials is deposited on or below hard magnetic layer  12 , to have a thickness between 1 nm and 50 nm, via which, as was explained, properties of layer system  15  are adjustable. Preferably soft magnetic layer  13  has a thickness of 1 nm to 10 nm. The hard magnetic layer is preferably made of a CoCrPt alloy such as Co 75 Cr 13 Pt 12 , a CoSm alloy such as Co 80 Sm 20 , a CoCr alloy such as Co 80 Cr 20 , a CoCrTa alloy such as Co 84 Cr 13 Ta 3 , a CoPt alloy such as Co 50 Pt 50  or an FePt alloy such as Fe 50 Pt 50 . The thickness of hard magnetic layer  12  is preferably between 20 nm and 100 nm.  
         [0016]     Preferably, soft magnetic layer  13  is located between magnetoresistive layer stack  14  and hard magnetic layer  12 .  
         [0017]     Alternatively to the example explained with the aid of  FIG. 2 , a plurality of especially differently composed soft magnetic layers  13  and/or soft magnetic layers  13  of different thickness may be provided, which are located under, or preferably, according to  FIG. 2 , on hard magnetic layer  12 , and which preferably each have a thickness between 1 nm and 50 nm, particularly 1 nm to 10 nm, and are made of the aforementioned materials. Furthermore, layer system  15  may also be constructed of multilayers of a plurality of soft magnetic layers  13  and hard magnetic layers  12 , having layer pairs corresponding to  FIG. 2 .  
         [0018]     What these variants have in common is that the ferromagnetic coupled soft magnetic and hard magnetic layers  12 ,  13  are always deposited as double layers or multilayers in the vicinity of magnetosensitive layer stack  14 .  
         [0019]     What is technologically advantageous, because it is particularly simple to implement, is the already mentioned deposition of layer system  15  underneath or above layer stack  14 . However, layer system  15  may alternatively also be situated on one side or on both sides next to layer stack  14 , or even be integrated into layer stack  14 .  
         [0020]      FIG. 1  shows a first magnetization curve  1 , i.e. the strength of magnetization as a function of a magnetic field, for an exclusively hard magnetic layer, a second magnetization curve  2  for this hard magnetic layer having a thin soft magnetic layer applied to it, and a third magnetization curve  3  for this hard magnetic layer having soft magnetic layer applied to it that is thicker compared to curve  2 . In this context, the magnetization is the sum of the magnetic moments, i.e. an increased magnetization also means an increased field strength of the stray field.  
         [0021]     One may infer from  FIG. 1  that layer system  15 , depending on the choice of layer thickness of the soft magnetic layer, has an increased coercivity compared to purely hard magnetic layer  12 , and an increased remanent magnetization. This is based on the fact that soft magnetic layer  13  generates a comparatively great stray field, based on the great magnetic moment of the materials forming it, and that the coupling of soft magnetic layer  13  into hard magnetic layer  12  aligns this great magnetic moment in the direction of the magnetization of hard magnetic layer  12 . Thereby is yielded overall a high field strength of the stray field.