Abstract:
A giant magnetoresistive (GMR) stack configured to operate in a current-perpendicular-to-plane (CPP) mode includes a ferromagnetic free layer, at least one synthetic antiferromagnet, at least one nonmagnetic spacer layer, and at least one antiferromagnetic pinning layer. The ferromagnetic free layer has a rotatable magnetic moment. The synthetic antiferromagnet includes a ferromagnetic reference layer having a fixed magnetic moment, a ferromagnetic pinned layer having a fixed magnetic moment, and a coupling layer positioned between the reference layer and the pinned layer, wherein the coupling layer is selected from the group consisting of Cu, Ag and CuAg. The nonmagnetic spacer layer is positioned between the free layer and the synthetic antiferromagnet. The antiferromagnetic pinning layer is positioned adjacent to the synthetic antiferromagnet.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S)  
       [0001]    This application claims priority from Provisional Application No. 60/288,344, filed May 3, 2001 entitled “Current-Perpendicular-To-Plane (CPP) Spin Valve Readers with Cu SAF and Geometrical Stabilization” by 0. Heinonen and M. Seigler. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates generally to a giant magnetoresistive (GMR) read sensor for use in a magnetic read head. In particular, the present invention relates to a current-perpendicular-to-plane (CPP) read sensor having an enhanced giant magnetoresistive response.  
           [0003]    GMR read sensors are used in magnetic data storage systems to detect magnetically-encoded information stored on a magnetic data storage medium such as a magnetic disc. A time-dependent magnetic field from a magnetic medium directly modulates the resistivity of the GMR read sensor. A change in resistance of the GMR read sensor can be detected by passing a sense current through the GMR read sensor and measuring the voltage across the GMR read sensor. The resulting signal can be used to recover the encoded information from the magnetic medium.  
           [0004]    A typical GMR read sensor configuration is the GMR spin valve, in which the GMR read sensor is a multi-layered structure formed of a nonmagnetic spacer layer positioned between a synthetic antiferromagnet (SAF) and a ferromagnetic free layer. The magnetization of the SAF is fixed, typically normal to an air bearing surface of the GMR read sensor, while the magnetization of the free layerrotates freelyin response to an externalmagnetic field. The SAF includes a reference layer and a pinned layer which are magnetically coupled by a coupling layer such that the magnetization direction of the reference layer is opposite to the magnetization of the pinned layer. The resistance of the GMR read sensor varies as a function of an angle formed between the magnetization direction of the free layer and the magnetization direction of the reference layer. This multi-layered spin valve configuration allows for a more pronounced magnetoresistive effect, i.e. greater sensitivity and higher total change in resistance, than is possible with anisotropic magnetoresistive (AMR) read sensors, which generally consist of a single ferromagnetic layer.  
           [0005]    A pinning layer is typically exchange coupled to the pinned layer of the SAF to fix the magnetization of the pinned layer in a predetermined direction. The pinning layer is typically formed of an antiferromagnetic material. In antiferromagnetic materials, the magnetic moments of adjacent atoms point in opposite directions and, thus, there is no net magnetic moment in the material.  
           [0006]    An underlayer is typically used to promote the texture of the layers (including the pinning layer) consequently grown on top of it. The underlayer is chosen such that its atomic structure, or arrangement, corresponds with a desired crystallographic direction.  
           [0007]    A seed layer is typically used to enhance the grain growth of the layers (including the underlayer) consequently grown on top of it. In particular, the seed layer provides a desired grain structure and size.  
           [0008]    One principal concern in the performance of GMR read sensors is the ΔR (the maximum absolute change in resistance of the GMR read sensor), which directly affects the GMR ratio. The GMR ratio (the maximum absolute change in resistance of the GMR read sensor divided by the resistance of the GMR read sensor multiplied by 100%) determines the magnetoresistive effect of the GMR read sensor. Ultimately, a higher GMR ratio yields a GMR read sensor with a greater magnetoresistive effect which is capable of detecting information from a magnetic medium with a higher linear density of data.  
           [0009]    A key determinant of the GMR ratio is the material used for the coupling layer in the SAF. The sense current that is passed through the GMR read sensor consists of majority spin electrons (spin is in the same direction of the magnetization) and minority spin electrons (spin is in the opposite direction of the magnetization). Majority spin electrons exhibit very little resistance and enhance the signal produced by the sense current, while minority spin electrons exhibit very high resistance and diminish the signal produced by the sense current. In current-in-plane (CIP) read sensors, the sense current is passed through in a direction parallel to the layers of the read sensor. In order to maximize the mean free path of the majority spin electrons and the signal produced by the sense current, the majority spin electrons should be confined to the reference layer, free layer, and the spacer layer. It is therefore desirable for the coupling layer in the SAF to reflect majority spin electrons back into the reference layer in order to prevent the majority spin electrons from passing through into the pinned layer and scattering as minority spin electrons. In CPP read sensors, however, the sense current is passed through in a direction perpendicular to the layers of the read sensor. The reflection of majority spin electrons at the reference layer/coupling layer interface acts to increase the resistance of the majority spin electrons, which has the effect of diminishing the signal produced by the sense current. It is therefore desirable for the coupling layer to allow majority spin electrons to pass through without any appreciable scattering in order to enhance the signal produced by the sense current, and ultimately increase the GMR ratio of the read sensor. It is important, however, to ensure that the magnetic coupling between the reference layer and the pinned layer is maintained in order for the read sensor to function properly.  
           [0010]    The present invention addresses these and other needs, and offers other advantages over current devices.  
         BRIEF SUMMARY OF THE INVENTION  
         [0011]    The present invention is a giant magnetoresistive (GMR) stack configured to operate in a current-perpendicular-to-plane (CPP) mode. The GMR stack includes a ferromagnetic free layer, at least one synthetic antiferromagnet (SAF), at least one nonmagnetic spacer layer, and at least one antiferromagnetic pinning layer. The ferromagnetic free layer has a rotatable magnetic moment. The SAF includes a ferromagnetic reference layer having a fixed magnetic moment, a ferromagnetic pinned layer having a fixed magnetic moment, and a coupling layer positioned between the reference layer and the pinned layer, wherein the coupling layer is selected from the group consisting of Cu, Ag and CuAg. The nonmagnetic spacer layer is positioned between the free layer and the SAF. The antiferromagnetic pinning layer is positioned adjacent to the SAF. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a layer diagram of a first embodiment of a spin valve of the present invention.  
         [0013]    [0013]FIG. 2 is a layer diagram of a second embodiment of a spin valve of the present invention.  
         [0014]    [0014]FIG. 3 is a graph of the Kerr rotation of a synthetic antiferromagnet portion of the present invention.  
         [0015]    [0015]FIG. 4 is a graph of the coupling field of a synthetic antiferromagnet portion of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0016]    [0016]FIG. 1 is a layer diagram of a first embodiment of a current-perpendicular-to-plane (CPP) spin valve  10  of the present invention. Spin valve  10  is configured as a dual spin valve and includes a seed layer  12 , an underlayer  14 , a first pinning layer  16 , a first synthetic antiferromagnet (SAF)  18 , a first spacer layer  20 , a free layer  22 , a second spacer layer  24 , a second SAF  26 , and second pinning layer  28 . Seed layer  12  is preferably NiFeCr or Ta. Underlayer  14  is preferably NiFe or CoFe, and is positioned adjacent to seed layer  12 . First pinning layer  16  is an antiferromagnetic material, preferably selected from the group consisting of PtMn, IrMn, NiMn, NiO and FeMn, and is positioned adjacent to underlayer  14 . First SAF  18  includes a ferromagnetic pinned layer  30 , a ferromagnetic reference layer  34 , and a coupling layer  32  positioned between pinned layer  30  and reference layer  34 , and is positioned such that pinned layer  30  is adjacent to first pinning layer  16 . Coupling layer  32  is preferably selected from the group consisting of Cu, Ag and CuAg, reference layer  34  is preferably CoFe, and pinned layer  30  is preferably CoFe. Free layer  22  is a ferromagnetic material, preferably CoFe or NiFe. First spacer layer  20  is a nonmagnetic material, preferably copper, and is positioned between first SAF  18  and free layer  22 . Second SAF  26  includes a ferromagnetic reference layer  36 , a ferromagnetic pinned layer  40 , and a coupling layer  38  positioned between reference layer  36  and pinned layer  40 . Reference layer  36  is preferably CoFe, coupling layer  38  is preferably selected from the group consisting of Cu, Ag and CuAg, and pinned layer  40  is preferably CoFe. Second pinning layer  28  is an antiferromagnetic material, preferably selected from the group consisting of PtMn, IrMn, NiMn, NiO and FeMn, and is positioned adjacent to pinned layer  40  of second SAF  26 . Second spacer layer  24  is a nonmagnetic material, preferably copper, and is positioned between free layer  22  and second SAF  26 .  
         [0017]    The magnetizations of first and second SAFs  18  and  26  are fixed while the magnetization of free layer  22  rotates freely in response to an external magnetic field emanating from a magnetic medium. Reference layer  34  and pinned layer  30  are magnetically coupled by coupling layer  32  such that the magnetization direction of reference layer  34  is opposite to the magnetization direction of pinned layer  30 . The magnetization of pinned layer  30  is pinned by exchange coupling first pinning layer  16  with pinned layer  30 . Underlayer  14  promotes the crystallographic texture of the layers (including first pinning layer  16 ) consequently grown on top of underlayer  14 . Seed layer  12  enhances the grain growth of the layers (including underlayer  14 ) consequently grown on top of seed layer  12 . Reference layer  36  and pinned layer  40  are magnetically coupled by coupling layer  38  such that the magnetization direction of reference layer  36  is opposite to the magnetization direction of pinned layer  40 . The magnetization of pinned layer  40  is pinned by exchange coupling second pinning layer  28  with pinned layer  40 . The resistance of spin valve  10  varies as a function of the angles that are formed between the magnetization of free layer  22  and the magnetizations of reference layers  34  and  36 .  
         [0018]    The GMR signal produced by spin valve  10  is generated by a sense current flowing perpendicularly through the layers of spin valve  10 . Coupling layer  32  of first SAF  18  and coupling layer  38  of second SAF  26  reduce the resistance of majority spin electrons flowing through spin valve  10  by allowing majority spin electrons to pass through coupling layers  32  and  38  without any appreciable scattering. Unlike prior art coupling layers formed of ruthenium which reflect majority spin electrons and transmit minority spin electrons, coupling layers  32  and  38  instead transmit majority spin electrons and reflect minority spin electrons. In this way, coupling layers  32  and  38  enhance the GMR signal produced by spin valve  10 .  
         [0019]    The thickness of coupling layer  32  of first SAF  18  is preferably in the range of about 6 Å to about 10 Å. Within this thickness range, the optimum coupling between pinned layer  30  and reference layer  34  is achieved. Similarly, the thickness of coupling layer  38  of second SAF  26  is preferably in the range of about 6 Å to about 10 Å. Within this thickness range, the optimum coupling between reference layer  36  and pinned layer  40  is achieved.  
         [0020]    The coupling between pinned layer  30  and reference layer  34  in first SAF  18  can be further enhanced by geometrical designs of the structure of first SAF  18 . By increasing the aspect ratios (the height perpendicular to a medium plane divided by the length along the medium plane) of pinned layer  30  and reference layer  34  (so that they are now greater than the aspect ratio of free layer  22 ), the magnetizations of pinned layer  30  and reference layer  34  will tend to align along the vertical plane (the height perpendicular to the medium plane). Because it is desirable for the magnetizations of pinned layer  30  and reference layer  34  to be aligned along the vertical plane in opposite directions, the increased aspect ratios enhance the coupling between pinned layer  30  and reference layer  34 . Preferably, the aspect ratios of pinned layer  30  and reference layer  34  are about 2. In the same way, the coupling between reference layer  36  and pinned layer  40  in second SAF  26  can be further enhanced by increasing the aspect ratios of reference layer  36  and pinned layer  40 .  
         [0021]    The following table shows the calculated resistance-area (RA) product and the GMR ratio for spin valve  10 , where coupling layers  32  and  38  are copper and have an 8 Å thickness. The following table also shows the calculated RA product and the GMR ratio for a prior art spin valve that is identical to spin valve  10  except coupling layers  32  and  38  are replaced with ruthenium coupling layers having an 8 Å thickness. Both spin valve  10  and the prior art spin valve include the following layers: a NiFeCr 55 Å seed layer, a NiFe 10 Å underlayer, a PtMn 150 Å first pinning layer, CoFe pinned and reference layers in the first SAF, a Cu 30 Å first spacer layer, a CoFe free layer, a Cu 30 Å second spacer layer, CoFe pinned and reference layers in the second SAF, and an IrMn 70 Å second pinning layer. For both spin valve  10  and the prior art spin valve, calculations were performed for two different thicknesses for the free layer and the pinned and reference layers in both SAFs: 30 Åand 50 Å.  
                                                                                 Coupling   PL, RL, FL   RA Product   GMR Ratio           Layer   Thickness (Å)   (Ωμm 2 )   (%)                                        Ru   50   0.0627   3           Ru   30   0.0666   0           Cu   50   0.0509   12           Cu   30   0.0554   8.2                      
 
         [0022]    [0022]FIG. 2 is a layer diagram of a second embodiment of a CPP spin valve  50  of the present invention. Spin valve  50  is configured as a bottom spin valve and includes a seed layer  52 , an underlayer  54 , a pinning layer  56 , a synthetic antiferromagnet (SAF)  58 , a spacer layer  60 , and a free layer  62 . Seed layer  52  is preferably NiFeCr or Ta. Underlayer  54  is preferably NiFe or CoFe, and is positioned adjacent to seed layer  52 . Pinning layer  56  is an antiferromagnetic material, preferably selected from the group consisting of PtMn, IrMn, NiMn, NiO and FeMn, and is positioned adjacent to underlayer  54 . SAF  58  includes a ferromagnetic pinned layer  64 , a ferromagnetic reference layer  68 , and a coupling layer  66  positioned between pinned layer  64  and reference layer  68 , and is positioned such that pinned layer  64  is adjacent to pinning layer  56 . Coupling layer  66  is preferably selected from the group consisting of Cu, Ag and CuAg, reference layer  68  is preferably CoFe, and pinned layer  64  is preferably CoFe. Free layer  62  is a ferromagnetic material, preferably CoFe or NiFe. Spacer layer  60  is a nonmagnetic material, preferably copper, and is positioned between SAF  58  and free layer  62 .  
         [0023]    The magnetization of SAF  58  is fixed while the magnetization of free layer  62  rotates freely in response to an external magnetic field emanating from a magnetic medium. Reference layer  68  and pinned layer  64  are magnetically coupled by coupling layer  66  such that the magnetization direction of reference layer  68  is opposite to the magnetization direction of pinned layer  64 . The magnetization of pinned layer  64  is pinned by exchange coupling pinning layer  56  with pinned layer  64 . Underlayer  54  promotes the crystallographic texture of the layers (including pinning layer  56 ) consequently grown on top of underlayer  54 . Seed layer  52  enhances the grain growth of the layers (including underlayer  54 ) consequently grown on top of seed layer  52 . The resistance of spin valve  50  varies as a function of an angle that is formed between the magnetization of free layer  62  and the magnetization of reference layer  68 .  
         [0024]    The GMR signal produced by spin valve  50  is generated by a sense current flowing perpendicularly through the layers of spin valve  50 . Coupling layer  66  of SAF  58  reduces the resistance of majority spin electrons flowing through spin valve  50  by allowing majority spin electrons to pass through coupling layer  50  without any appreciable scattering. Unlike prior art coupling layers formed of ruthenium which reflect majority spin electrons and transmit minority spin electrons, coupling layer  66  instead transmits majority spin electrons and reflects minority spin electrons. In this way, coupling layer  66  enhances the GMR signal produced by spin valve  50 .  
         [0025]    The thickness of coupling layer  66  of SAF  58  is preferably in the range of about 6 Åto about 10 Å. Within this thickness range, the optimum coupling between pinned layer  64  and reference layer  68  is achieved.  
         [0026]    The coupling between pinned layer  64  and reference layer  68  in SAF  58  can be further enhanced by geometrical designs of the structure of SAF  58 . By increasing the aspect ratios (the height perpendicular to a medium plane divided by the length along the medium plane) of pinned layer  64  and reference layer  68  (so that they are now greater than the aspect ratio of free layer  62 ), the magnetizations of pinned layer  64  and reference layer  68  will tend to align along the vertical plane (the height perpendicular to the medium plane). Because it is desirable for the magnetizations of pinned layer  64  and reference layer  68  to be aligned along the vertical plane in opposite directions, the increased aspect ratios enhance the coupling between pinned layer  64  and reference layer  68 . Preferably, the aspect ratios of pinned layer  64  and reference layer  68  are about 2.  
         [0027]    [0027]FIG. 3 is a graph of the Kerr rotation of a synthetic antiferromagnet (SAF) portion of the present invention. The SAF includes a CoFe 50 Å pinned layer, a Cu coupling layer, and a CoFe 50 Å reference layer. The graph shows the Kerr rotation (mdeg) for various coupling layer thicknesses as a function of an applied magnetic field (Oe). The optimum coupling is achieved when the coupling layer thickness is in the range of about 8 Å to about 9 Å.  
         [0028]    [0028]FIG. 4 is a graph of the coupling field of a synthetic antiferromagnet (SAF) portion of the present invention. The SAF includes a 30 Å pinned layer, a Cu or CuAg coupling layer, and a 30 Å reference layer. The graph shows the coupling field (Oe) for both a Cu coupling layer and a CuAg coupling layer as a function of coupling layer thickness (A). The optimum coupling is achieved when the coupling layer thickness is in the range of about 7 Å to about 10 Å.  
         [0029]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.