Abstract:
A spin valve structure, and method for manufacturing it, are described. The valve is subject to only small bias point shifts by sense current fields while at the same time has good GMR characteristics. This is achieved by introducing a layer of about 15 Angstroms of ruthenium between the seed layer and the free layer. This acts as an effective bias control layer with the added benefit of providing interfaces (to both the seed and the free layer) that are highly favorable to specular reflection of the conduction electrons. The HCP crystal structure of this ruthenium layer also improves the crystalline quality of the free layer thereby improving its performance with respect to the GMR ratio.

Description:
FIELD OF THE INVENTION 
     The invention relates to the general field of magnetic disk systems with particular reference to GMR based read heads and its bias control layer. 
     BACKGROUND OF THE INVENTION 
     An example of a read-write head for a magnetic disk system 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 a decrease in resistivity when the material is magnetized in a direction perpendicular to the easy axis, said decrease 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. 
     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 others 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 present invention is particularly directed to minimizing certain noise effects associated with multiple sensings of a magnetized layer. As first shown by Barkhausen in 1919, magnetization in iron can be irregular because of reversible breaking of magnetic domain walls, leading to the phenomenon of Barkhausen noise (BN). Long MR stripes are known to reduce this significantly for wide track recording but, for narrow-track (disk) recording, the problems of electrical signal spikes and instabilities can become serious concerns. Here, instability refers to the repeatability of output waveforms during repeated write/read cycles. The solution to this problem is to provide operating conditions conducive to single-domain films for MR sensor and to ensure that the domain configuration remains unperturbed after processing and fabrication steps. 
     One of two methods is usually used to reduce or eliminate BN. The first is to increase the effective magnetic length of the MR stripe. Long stripes exhibit reduced demagnetization at their ends and hence tend to retain single magnetic domain in the stripe. In the second method the presence of a small magnetic field in the easy axis direction of the stripe maintains a single domain and thus reduces RN. This is accomplished by depositing a longitudinal exchange bias layer directly in contact with the spin valve structure. However, a field of this sort tends to reduce the sensitivity of the sensor to a transverse field, resulting in a lower signal output. One solution to this problem that is commonly used in the prior art is to apply the longitudinal bias only in the two edge regions, as shown in FIG.  3 . The longitudinally biased edge regions  38  apply a small field to sensor  30 , eliminating BN while at the same time retaining the signal sensitivity of the central region. To provide this selective bias at the edges, a layer of exchange material like MnFe is deposited on the top edges of the MR-stripe. The selective longitudinal field ensures a single domain in the edge region thereby maintaining a single domain in the central section Also seen in the figure are contact leads  39 . 
     A routine search of the prior art was conducted. While several references to various laminated structures within spin valves were encountered, none of these disclosed the exact structure or process of the present invention. Several of the references found were, however, of interest. For example, U.S. Pat. No. 5,920,446(Gill) discloses a GMR structure made up of two free layers separated by copper. These free layers are themselves laminates of two ferromagnetic layers separated by a layer of ruthenium. The latter serves as an APC (anti-parallel coupling) layer. This arrangement allows the structure to function without a pinned layer, thereby reducing its thickness. 
     In U.S. Pat. No. 5,998,016, Sasaki et al. incorporate an anti-diffusion layer in their spin valve structure. The preferred material for this is tantalum but ruthenium is listed as one of the alternatives. Gill et al. (U.S. Pat. No. 5,701,222) and Fontana, Jr, et al. (U.S. Pat. No. 5,701,223) both describe variations on the basic spin valve structure. 
     SUMMARY OF THE INVENTION 
     It has been an object of the present invention is to provide a synthetic antiferromagnet spin valve with controllable bias point and high output performance. 
     Another object of the invention has been that it should be compatible with a range of free layer compositions and thicknesses. 
     A further object has been that the total thickness of said spin valve be less than about 400Å. 
     These objects have been achieved by introducing a layer of about 15 Angstroms of ruthenium between the seed layer and the free layer. This acts as an effective bias control layer with the added benefit of providing interfaces (to both the seed and the free layer) that are highly favorable to specular reflection of the conduction electrons. The HCP crystal structure of this ruthenium layer also improves the crystalline quality of the free layer thereby improving its performance with respect to the GMR ratio. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates the basic parts of a read-write head for a magnetic disk system. 
     FIG. 2 shows the layers that make up a typical spin valve structure. 
     FIG. 3 shows a bias control layer of the prior art. 
     FIG. 4 shows the layers that make up the spin valve structure of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An ultra-thin free layer, combined with a high GMR ratio, is an effective means to form a high output spin valve head capable of over 30 Gb/in 2  magnetic recording. However, as the free layer thickness decreases, it becomes difficult to control the bias point provided by the bias compensation layer (BCL), while also achieving a high GMR ratio and good softness. It is, however, known that an increase in magneto-conductance can be achieved (beyond that of NiFe or Co alone) by replacing most of the NiFe (or Co) in the free layer with a non-magnetic metal such as Cu. A structure of this type is referred to as a spin-filter spin valve (SFSV). 
     For a SFSV, the free layer is placed between the Cu spacer and an additional high-conductance-layer (HCL). The SFSV reduces sense current fields in the free layer by shifting the sense current center toward the free layer, resulting in a small bias point shift by the sense current fields. A high GMR ratio is maintained even in the ultra-thin free layer because the HCL improves the mean free path of spin-up electrons while maintaining the mean free path difference between spin-up and spin-down electrons. 
     In a recent application (HT99-028 application Ser. No. 09/483,937 dated Jan. 18, 2000), Horng et al. disclosed a low resistance NiCr/Ru/NiCr conductor lead structure, in which specular reflection of the conduction electrons at the NiCr/Ru interface is responsible for the improved conductivity. Since ruthenium has a HCP (hexagonal close packed) crystal structure, which is the same as CoFe, it was reasoned that NiCr/Ru could also serve as an effective BCL for use in specular CoFe based spin valves. 
     We will disclose the present invention by describing the process used to manufacture it. In the course of doing so, the structure of the present invention will also become evident. Referring now FIG. 4, the process begins with the provision of suitable dielectric underlayer  41  such as aluminum oxide or silicon oxide on whose surface is deposited a seed layer  42 . The preferred material for the seed layer is nickel-chromium having a thickness of about 55Å. 
     Normal (prior art) practice would now be to form, on the seed layer, a NiFe/CoFe free layer. In a significant departure from prior art practice, the material used for BCL  48  is ruthenium. This layer should have a hexagonal close-packed crystal structure. Our preferred deposition method has been DC-magnetron sputtering at a deposition power of about 20 watts, a gas pressure of about 2 millitorr, and a target/substrate spacing of about 2 inches, resulting in a deposition rate of about 0.3 Angstroms/second. Other, similar deposition methods could also have been used. Ruthenium layer  48  is not patterned but covers the entire seed. The preferred thickness for the ruthenium layer is about 15 Å. The ruthenium-seed layer interface will be discussed further below and is itself a key feature of the invention. 
     Next, free layer  43  is deposited on ruthenium layer  48 . The invention is such that it will operate effectively for a wide range of free layer compositions and thicknesses. In a first embodiment, the free layer is cobalt-iron about 20 Å thick. In a second embodiment, the free layer is cobalt about 20 Å thick, while in a third embodiment, the free layer is a laminate of 3 Å of cobalt-iron on about 32 Å of nickel-iron. The ruthenium-free layer interface will also be discussed further below since it is also a key feature of the invention. 
     Then, non-magnetic copper spacer layer  44  is deposited on free layer  43 . The preferred thickness for the copper layer is about 20 Å. This is followed by the deposition of synthetic antiferromagnetic layer  45  on copper spacer layer  44 . Although shown in the figure as a single layer, the synthetic antiferromagnetic layer is usually a laminate of several layers. Our preferred laminate composition has been (starting immediately above the copper spacer layer): a cobalt-iron layer about 19 Å thick; a ruthenium layer about 6-9 Å thick, with 7.5 Å being preferred; and a second cobalt-iron layer about 21 Å thick. On the latter is a manganese-platinum layer between about 100 and 200 Å thick with about 120 Å being preferred. The structure is completed by depositing cap layer  46  (about 50 Å of nickel-chrome) on synthetic antiferromagnetic layer  45 . 
     The final step in the process of the present invention is a two stage annealing treatment of the completed spin valve head. In the first stage the structure is heated in a magnetic field of between about 5,000 and 10,000 Oersted at a temperature between about 250° and 280° C. for between about 120 and 300 minutes. This allows synthetic antiferromagnetic layer  45  to acquire a permanent pinned layer direction. For the second annealing stage, heating is performed in a magnetic field of between about 50 and 100 Oersted at a temperature between about 280 and 290° C. for between about 60 and 120 minutes. This resets the sensor layer direction of layer  43 . 
     Confirmation of the effectiveness of the general approach taken by the present invention, as well as of the improved performance of devices made according to the teachings of the present invention, was obtained from the following experimental data: 
     Using the CVC GMR sputtering system, spin valves having the following structure were formed: 55 Å NiCr seed layer/BCU 20 Å CoFe/(20-21)Å Cu/19 Å CoFe/7.5 Å Ru/21Å CoFe/120 Å MnPt/50 Å NiCr. BCLs of Cu, CuNi, and Ru, within this structure, were then compared. The magnetic properties of the annealed “free layer” structures were measured by a B-H looper and Phase Matrix tester. The results are summarized in TABLE I below: 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                 Structure 80.9% NiFe/MnPt 43% 
                 Bs 
                 Hc 
                 Hk 
                 Rs 
                 Field to close HA 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 NiCr55/Ru15/CoFe20/Cu20/NiCr50 
                 0.31 
                 6.13 
                 8.57 
                 31.90 
                 3 Oe 
               
               
                 NiCr55/Ru20/CoFe20/Cu20/NiCr50 
                 0.27 
                 6.96 
                 9.15 
                 29.26 
                 3 Oe 
               
               
                 NiCr55/Ru15/NiFe34CoFe2/Cu20/NiCr50 
                 0.28 
                 6.42 
                 7.80 
                 27.12 
                 1.5 Oe   
               
               
                 NiCr55/Cu15/NiFe34—CoFe2/Cu20/NiCr50 
                 0.31 
                 8.04 
                 12.52 
                 24.86 
                 8 Oe 
               
               
                 NiCr55/Cu15/CoFe2o/Cu2o/NiCr50 
                 0.30 
                 10.31 
                 16.08 
                 28.01 
                 20 Oe  
               
               
                 NiCr55/CuNi15/CoFe20/Cu20/NiCr50 
                 0.28 
                 10.35 
                 16.55 
                 30.24 
                 20 Oe  
               
               
                   
               
               
                 where B s  = magnetic moment, H c  = free layer coercivity (oe), H k  = anisotropy field (oe), and R s  = sheet resistance (ohm/sq.), HA = hard axis.  
               
             
          
         
       
     
     It was found that, after anneal, CoFe or (NiFe—CoFe) formed on the Cu layer exhibited isotropic behavior, while these layers, when deposited on the Ru, showed anisotropy characteristics along with lower Hc and Hk. Hk for CoFe (or NiFe—CoFe) formed on the Ru and Cu (or CuNi) layers was 8-9 and &gt;13 Oe, respectively. The tangential field that was needed to close the CoFe (or NiFe—CoFe) free layer hard axis (HA) loop for the Ru-based structures was much smaller than for the Cu- or CuNi- based structures. 
     It was found that the low field (50 Oe), 290° C., 2 hours free layer reset anneal caused little pinned layer rotation (less than 1 degree). The performance of SyAF SV made with Ru and Cu BCL layers is compared in TABLE II below: 
     
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
               
                 Structure 80.9% NiFe/MnPt 43% 
                 Bs 
                 Hc 
                 He 
                 Hk 
                 Rs 
                 Dr/r 
                 Dr 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 CZB55/Ru15/CoFe20/Cu20/CeFe19/Ru7.5/CoFe21/MP120/NiCr50 
                 0.30 
                 5.69 
                 4.93 
                 6.27 
                 16.9 
                 12.0 
                 2.02 
               
               
                 NiCr55/Cu15/CoFe20/Cu20/CoFe19/Ru7.5/CoFe21/MP120/NiCr50 
                 0.28 
                 6.64 
                 −12.97 
                 11.71 
                 16.8 
                 10.9 
                 1.84 
               
               
                 NiCr55/Ru15/NiFe34CoFe2/Cu20/CoFe19/Ru7.5/CoFe21/MP120/NiCr50 
                 0.30 
                 4.26 
                 2.71 
                 2.55 
                 16.0 
                 9.8 
                 1.56 
               
               
                 NiCr55/Cu15/NiFe34—CoFe2/Cu20/CoFe19/Ru7.5/CoFe21/MP120/NiCr50 
                 0.30 
                 11.84 
                 −6.04 
                 7.42 
                 15.5 
                 10.2 
                 1.58 
               
               
                   
               
             
          
         
       
     
     Thus, for SVs made with a CoFe free layer, besides improving free layer anisotropy, SyAFSVs formed with a BCL made of ruthenium yield higher Drtr than those formed with Cu BCLs. Since H k  for the Ru based spin valve is much smaller than for the Cu based spin valve, the former device has a higher sensor sensitivity, resulting in higher output performance. 
     When compared with SyAF SVs made with a 34A NiFe-2ACoFe free layer, SyAF SVs having a CoFe free layer gained 30% in signal amplitude. A CoFe free layer could not, however, be used in this structure because of the poor free layer anisotropy when it is formed on a Cu BCL. 
     Device modeling for the Ru (BCL)-SyAF spin valve has also been done. This showed that two major specular reflection effects were playing an important role in determining the operating characteristics of the structure of the present invention. One originates at the Ru/CoFe (free layer) interface and contributes to enhancing the GMR ratio (Dr/r). The other originates at the NiCr/Ru interface and serves to shift the current center toward the free layer. This results in only a small bias point shift by the sense current fields. 
     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.