Abstract:
A specular spin valve structure that is more robust than currently available specular spin valves is described. The improved stability is achieved by a using a modified pinned layer that is a laminate of three layers—a layer nickel-chromium, between about 3 and 4 Angstroms thick, sandwiched between two layers of cobalt-iron. A key requirement is that the cobalt-iron layer closest to the copper separation layer must be about twice as thick as the other cobalt-iron layer. A process for manufacturing this structure is also disclosed.

Description:
FIELD OF THE INVENTION 
     The invention relates to the general field of magnetic disk systems with particular reference to GMR based read heads. 
     BACKGROUND OF THE INVENTION 
     Read-write heads for magnetic disk systems have undergone substantial development during the last few years. In particular, older systems in which a single device was used for both reading and writing, have given way to configurations in which the two functions are performed by different structures. An example of such a read-write head is schematically illustrated in FIG.  1 . The magnetic field that ‘writes’ a bit at the surface of recording medium  15  is generated by a flat coil, two of whose windings  14  can be seen in the figure. The magnetic flux generated by the flat coil is concentrated within pole pieces  12  and  13  which, while being connected at a point beyond the top edge of the figure, are separated by small gap  16 . Thus, most of the magnetic flux generated by the flat coil passes across this gap with fringing fields extending out for a short distance where the field is still powerful enough to magnetize a small portion of recoding medium  15 . 
     The present invention is directed towards the design of read element  20  which can be seen to be a thin slice of material located between magnetic shields  11  and  12  ( 12  doing double duty as a pole piece, as just discussed). The principle governing the operation of read sensor  20  is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance). In particular, most magnetic materials exhibit anisotropic behavior in that they have a preferred direction along which they are most easily magnetized (known as the easy axis). The magneto-resistance effect manifests itself as an increase in resistivity when the material is magnetized in a direction perpendicular to the easy axis, said increase being reduced to zero when magnetization is along the easy axis. Thus, any magnetic field that changes the direction of magnetization in a magneto-resistive material can be detected as a change in resistance. 
     It is now known that the magneto-resistance effect can be significantly increased by means of a structure known as a spin valve. The resulting increase (known as Giant magneto-resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of the solid as a whole. 
     The key elements of a spin valve structure are shown in FIG.  2 . In addition to a seed layer  22  on a substrate  21  and a topmost cap layer  27 , these key elements are two magnetic layers  23  and  25 , separated by a non-magnetic layer  24 . The thickness of layer  24  is chosen so that layers  23  and  25  are sufficiently far apart for exchange effects to be negligible (the layers do not influence each other&#39;s magnetic behavior at the atomic level) but are close enough to be within the mean free path of conduction electrons in the material. If, now, layers  23  and  25  are magnetized in opposite directions and a current is passed though them along the direction of magnetization (such as direction  28  in the figure), half the electrons in each layer will be subject to increased scattering while half will be unaffected (to a first approximation). Furthermore, only the unaffected electrons will have mean free paths long enough for them to have a high probability of crossing over from  23  to  25  (or vice versa). However, once these electron ‘switch sides’, they are immediately subject to increased scattering, thereby becoming unlikely to return to their original side, the overall result being a significant increase in the resistance of the entire structure. 
     In order to make use of the GMR effect, the direction of magnetization of one the layers  23  and  25  is permanently fixed, or pinned. In FIG. 2 it is layer  25  that is pinned. Pinning is achieved by first magnetizing the layer (by depositing and/or annealing it in the presence of a magnetic field) and then permanently maintaining the magnetization by over coating with a layer of antiferromagnetic material, or AFM, (layer  26  in the figure). Layer  23 , by contrast, is a “free layer” whose direction of magnetization can be readily changed by an external field (such as that associated with a bit at the surface  15  of a magnetic disk). 
     The structure shown in FIG. 2 is referred to as a top spin valve because the pinned layer is at the top. It is also possible to form a ‘bottom spin valve’ structure where the pinned layer is deposited first (immediately after the seed and pinning layers). In that case the cap layer would, of course, be over the free layer. The present invention is further directed to a type of spin valve that we refer to as a “specular spin valve” because the conduction electrons are specularly reflected at the NiCr/NiFe interface. We have found, however, that specular spin valves as known to the prior art, for example: 
     NiCr55/NiFe65/CoFe10/Cu24/CoFe20/MnPt200/NiCr50 (where the numbers represent layer thicknesses in Angstroms and CoFe20 is the pinned layer) 
     while exhibiting almost twice the GMR effect of non-specular spin valve structures, had lower production yields due to ESD (electrostatic discharge) damage as well as higher scatter in signal amplitude and greater sensitivity to temperature. Thus there exists a need for a specular spin valve that is more robust than those currently known to the art. 
     A routine search of the prior art was conducted. While several references to various laminated structures within spin valves were encountered, none of these deal with specular spin valves and how to make them more robust. Several of the references found were, however, of interest. For example, Fontana, Jr. et al. (U.S. Pat. No. 5,701,223) forms a pinned layer by strongly coupling two ferromagnetic films in an antiferromagnetic configuration i.e. the two films have a relative antiparallel orientation. The magnetic moments of the two ferromagnetic layers are required to be almost the same. Under these conditions, a pinning layer of nickel oxide may be used and this has the advantage that a cap layer is no longer needed. 
     Gill (U.S. Pat. No. 5,898,549) forms a pinned layer from three separate pinned layers. The first of these is formed on the pinning layer and, together with the second pinned layer is formed of a high resistivity material such as NiFeCr. They are separated by an anti-parallel coupling layer. The third pinned layer is of low resistivity material such as cobalt. 
     In U.S. Pat. No. 5,920,446, Gill describes a laminated free layer formed from two ferromagnetic layers separated by a non-magnetic, conducting spacer layer. A key feature is that the two outer layers of the laminate are coupled in an anti-parallel configuration. This arrangement allows the device to operate without a pinned (or pinning) layer. 
     SUMMARY OF THE INVENTION 
     It has been an object of the present invention to provide a specular spin valve structure that has greater thermal stability of the pinned layer at high reverse fields than specular spin valves of the prior art. 
     Another object of the invention has been to improve production yields by reducing losses due to ESD damage. 
     A further object of the invention has been that said spin valve structures exhibit lower scatter in signal amplitudes than specular spin valves of the prior art. 
     A still further object of the invention has been to provide a process for manufacturing said spin valve structure. 
     These objects have been achieved by a using a modified pinned layer that is a laminate of three layers—a layer nickel-chromium, between about 3 and 4 Angstroms thick, sandwiched between two layers of cobalt-iron. A key requirement is that the cobalt-iron layer closest to the copper separation layer must be about twice as thick as the other cobalt-iron layer. A process for manufacturing this structure is also disclosed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross-section of a read-write head for a magnetic disk system. 
     FIG. 2 is a cross-section of a spin valve structure formed according to the teachings of the prior art. 
     FIG. 3 is a cross-section of a spin valve structure formed according to the teachings of the present invention. 
     FIG. 4 is a flowchart summary of the process of the present invention. 
     FIG. 5 compares stability of the pinned field as a function of annealing temperature for spin valve structures of the prior art with spin valves of the present invention. 
     FIG. 6 compares the GMR ratio as a function of the “against pinning initialization field” for spin valve structures of the prior art with spin valves of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 3, we show there a schematic cross-section of a top spin valve structure that embodies the present invention. A comparison with FIG. 2 of the prior art will reveal that the key novel feature of the invention is the insertion of an additional layer within the pinned layer. What had been pinned layer  25  in the structure shown in FIG. 2 has been replaced by pinned layer  35  which is a laminate of layers  31 ,  32 , and  33 . A key feature of the invention is that, for the pinned layer ( 35 ), the center layer ( 32 ) should be nickel-chromium between about  3  and  4  Angstroms thick, the lower layer ( 31 ) should be cobalt-iron, between about 10 and 15 Angstroms thick, and the top layer ( 33 ) should also be cobalt iron, between about 5 and 7 Angstroms thick. Additionally, for the structure to function properly, the thickness of layer  31  should be about twice the thickness of layer  33 . 
     We now provide a description of a process for manufacturing the spin valve of the present invention which begins with the provision of suitable substrate  21  (as seen in FIG.  3 ). Examples of suitable substrate materials include aluminum oxide and silicon oxide. 
     Next, seed layer  22  is deposited onto substrate  21 . This is a magneto-resistance enhancing layer. It consists of a layer of nickel-chromium or nickel-iron-chromium and has a thickness between about 40 and 60 Angstroms. Layer  22 , as well as all subsequent layers, was deposited by means of DC-magnetron sputtering. The use of this material (instead of, for example, tantalum) is essential if the structure is to exhibit specular reflection at the interfaces. 
     The next layer to be laid down is free layer  23 . This is depicted in FIG. 3 as a single layer but, in actuality, it is made up of a layer of nickel-iron, between about 30 and 75 Angstroms thick, in contact with layer  22 , followed by a layer of cobalt-iron which is between about 3 and 10 Angstroms thick. 
     Copper layer  24 , between about 18 and 25 Angstroms thick, is then deposited over free layer  23 . 
     This is followed by the key step of forming laminated pinned layer  35 . First, layer  31  of cobalt-iron, between about 10 and 15 Angstroms thick, is deposited on the copper layer  24 . Then, a layer of nickel-chromium  32 , between about 3 and 4 Angstroms thick, is deposited onto layer  31 . Finally, layer  33 , also of cobalt iron and having a thickness between about 5 and 7 Angstroms, is deposited on layer  32 . Within the ranges just given, the relative thicknesses of layers  31  and  33  must be about 2:1. 
     The structure is completed by depositing pinning layer  26  (which is a layer of manganese-platinum between about 150 and 250 Angstroms thick) on laminated pinned layer  35 . This is followed by the deposition of cap layer  27  which is nickel-chromium having a thickness between about 30 and 50 Angstroms. The cap layer is needed to keep the CoFe from oxidizing during annealing. 
     The process of the present invention ends with said annealing step. The structure of FIG. 3 is heated to a temperature between about 250 and 300° C. for between about 3 and 5 hours in the presence of a magnetic field whose strength is between about 1,500 and 2,500 oe. A summary of the process of the invention in flow chart form is given in FIG.  4 . 
     Note that the compositions of the various alloys mentioned in the above description are (in atomic) %: 
     nickel-iron 80% nickel; nickel-chromium 60% nickel; nickel-iron-chromium 48% nickel &amp; 40% chromium; cobalt-iron 90% cobalt; and manganese-platinum 55% manganese. 
     Experimental confirmation of the effectiveness of the present invention was obtained through the formation of several spin valve structures according to the teachings of the present invention. The results are summarized in TABLE I below: 
     
       
         
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 ΔR/ΔR 
               
               
                 Structure 
                 B s   
                 H c   
                 H e   
                 H k   
                 R s   
                 ΔR/R 
                 ΔR 
                 (Ta) 
               
               
                   
               
             
             
               
                 Ta75/NiFe65/CoFe10/Cu30/CoFe20/MP200/Ta50 
                 0.67 
                 4.35 
                 8.0 
                 13.4 
                 12.7 
                 6.01 
                 0.77 
                 1.00 
               
               
                 NiCr55/NiFe60/CoFe3/Cu24/CoFe20/MP200/NiCr50 
                 0.55 
                 5.04 
                 5.7 
                  7.6 
                 12.6 
                 9.85 
                 1.24 
                 1.61 
               
               
                 NiCr55/NiFe60/CoFe3/Cu24/CoFe15/MP200/NiCr50 
                 0.55 
                 4.70 
                 5.0 
                  5.4 
                 13.6 
                 8.84 
                 1.20 
                 1.56 
               
               
                 NiCr55/NiFe60/CoFe3/Cu24/CoFe13/NiCr3/CoFe7/MP200/NiCr50 
                 0.53 
                 6.18 
                 7.2 
                  9.3 
                 13.4 
                 8.03 
                 1.07 
                 1.39 
               
               
                 NiCr55/NiFe60/CoFe3/Cu24/CoFe10/NiCr3/CoFe5/MP200/NiCr50 
                 0.53 
                 4.96 
                 4.9 
                  5.5 
                 13.6 
                 6.75 
                 0.92 
                 1.19 
               
               
                   
               
               
                 where B s  = magnetic moment, H c  = free layer coercivity (oe), H e  = inter-layer coupling field, H k  = anisotropy field (oe), R s  = sheet resistance (ohm/sq.), ΔR/R = GMR ratio, ΔR = resistance change due to GMR effect, and ΔR/ΔR(Ta) = resistance change relative to a non-specular structure.  
               
             
          
         
       
     
     In TABLE I, the first row is for a non-specular spin valve. The next two rows are for specular spin valve structures of the prior art, while the last two rows are examples of specular spin valves formed according to the teachings of the present invention. As can be seen, the substitution of a laminated pinned layer of the present invention for a conventional one did reduce AR somewhat relative to conventional specular spin valves. However, this loss in performance was more than made up by the improved robustness of the new structure as illustrated in FIGS. 5 and 6. 
     FIG. 5 plots Hex (exchange bias field strength in CoFe/AFM) in oersted as a function of annealing temperature (30 minutes in a field of 2,000 gauss). Curve  51  is for a non-specular structure, curves  52  and  53  are for specular structures of the prior art, while curve  54  is for a specular spin valve based on the present invention. As can be seen, the structures of the prior art show a more rapid drop in H ex  for temperatures greater than about 150° C. relative to the structure based on the present invention. Note that CZB=NiCr and CZN=NiFeCr. The blocking temperature for the structure of the present invention is around 275° C. (curve  54 ), about 50° C. higher than for the Ta based GMR structure. 
     FIG. 6 is a plot of ΔR/R (measured at 100 oe and expressed as %) as a function of the “against pinning initialization field” (in oersted). This term refers to the practice of applying a high initialization field of about 7,000 oe to reverse the pinned layer direction and then measuring ΔR/R as a function of a field applied in the opposite direction to this initialization field. Curve  61  is for a non-specular spin valve, curve  62  is a specular spin valve of the prior art, and curve  63  is for a spin valve formed according to the teachings of the present invention. While the non-specular and the disclosed structures both exhibit relatively little dependence on the against field, with the disclosed structure having a significantly better ΔR/R, the performance of the specular structure of the prior art is seen to degrade significantly when the against field exceeds about 2,000 showing that its associated pinned layer is less robust than that which is associated with the present invention. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.