Abstract:
A laminated film structure is disclosed comprising multiple ferromagnetic layers achieving improved data recording performance. A non-magnetic spacer layer is disposed between an upper ferromagnetic layer and an antiferromagnetic coupled (AFC) structure. The AFC structure is comprised of a ferromagnetic layer and an antiferromagnetic slave layer. The ferromagnetic layer in the AFC structure, referred to as lower ferromagnetic layer, may contain tantalum to promote chromium segregation at the grain boundaries to achieve magnetic decoupling of the grains with relatively thin boundaries, improving medium signal-to-noise ratio while maintaining good thermal stability of the medium. In some embodiments, the interlayer is a five-element alloy such as a CoCrPtBTa alloy.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to laminated magnetic thin films for magnetic recording and more particularly to magnetic thin films having multiple de-coupled ferromagnetic layers. 
     2. Description of the Related Art 
     A typical head and disk system  100  illustrated in  FIG. 1  may include a magnetic transducer  102  supported by a suspension  104  as it flies above the disk  106 . The magnetic transducer  102 , usually called a “read/write head” or “slider,” may include elements that perform the task of writing magnetic transitions (the write head  108 ) and reading the magnetic transitions (the read head  110 ). The electrical signals to and from the read and write heads  110 ,  108  travel along conductive paths (leads)  112  which are attached to or embedded in the suspension  104 . The magnetic transducer  102  is positioned over points at varying radial distances from the center of the disk  106  to read and write circular tracks (not shown). The disk  106  is attached to a spindle  114  that is driven by a spindle motor  116  to rotate the disk  106 . The disk  106  comprises a substrate  118  on which a laminate  120  having multiple layers is deposited. The laminate  120  typically includes ferromagnetic layers in which the write head  108  records the magnetic transitions in which information is encoded. 
     Extremely small regions, or bits, on the ferromagnetic layers are selectively magnetized in chosen directions in order to store data on the disk  106 . To increase the amount of data that can be stored on the disks  106  the number of bits per unit area, storage density, must be increased. 
     As the storage density of magnetic recording disks has increased, the product of the remanent magnetization Mr (the magnetic moment per unit volume of ferromagnetic material) and the magnetic layer thickness t has decreased. Similarly, the coercive field or coercivity (H c ) of the magnetic layer has increased. This has led to a decrease in the ratio Mrt/H c . To achieve the reduction in Mrt, the thickness t of the magnetic layer can be reduced, but only to a limit because the layer will exhibit increasing magnetic decay, which has been attributed to thermal activation of small magnetic grains (the superparamagnetic effect). The thermal stability of a magnetic grain is to a large extent determined by K u V, where K u  is the magnetic anisotropy constant of the layer and V is the volume of the magnetic grain. As the layer thickness is decreased, V decreases. If the layer thickness is too thin, the stored magnetic information will no longer be stable at normal disk drive operating conditions. 
     One approach to the solution of this problem is to move to a higher anisotropy material (higher K u ). However, the increase in K u  is limited by the point where the coercivity H c , which is approximately equal to K u /Mr, becomes too great to be written by a conventional recording head. A similar approach is to reduce the Mr of the magnetic layer for a fixed layer thickness, but this is also limited by the coercivity that can be written. Another solution is to increase the intergranular exchange, so that the effective magnetic volume V of the magnetic grains is increased. However, this approach has been shown to be deleterious to the intrinsic signal-to-noise ratio (SNR) of the magnetic layer. 
     It is known that substantially improved SNR can be achieved by the use of a laminated magnetic layer of two (or more) separate magnetic layers that are spaced apart by a nonmagnetic spacer layer. This discovery was made by S. E. Lambert, et al., “Reduction of Media Noise in Thin Film Metal Media by Lamination”, IEEE Transactions on Magnetics, Vol. 26, No. 5, September 1990, pp. 2706-2709, and subsequently patented in IBM&#39;s U.S. Pat. No. 5,051,288. The reduction in intrinsic media noise by lamination is believed due to a decoupling of the magnetic interaction or exchange coupling between the magnetic layers in the laminate. The use of lamination for noise reduction has been extensively studied to find the favorable spacer layer materials, including Cr, CrV, Mo and Ru, and spacer thicknesses, from 5 to 400 angstrom, that result in the best decoupling of the magnetic layers, and thus the lowest media noise. This work has been reported in papers by E. S. Murdock, et al., “Noise Properties of Multilayered Co-Alloy Magnetic Recording Media”, IEEE Transactions on Magnetics, Vol. 26, No. 5, September 1990, pp. 2700-2705; A. Murayama, et al., “Interlayer Exchange Coupling in Co/Cr/Co Double-Layered Recording Films Studied by Spin-Wave Brillouin Scattering”, IEEE Transactions on Magnetics, Vol. 27, No. 6, November 1991, pp. 5064-5066; and S. E. Lambert, et al., “Laminated Media Noise for High Density Recording”, IEEE Transactions on Magnetics, Vol. 29, No. 1, January 1993, pp. 223-229. U.S. Pat. No. 5,462,796 and the related paper by E. Teng et al., “Flash Chromium Interlayer for High Performance Disks with Superior Noise and Coercivity Squareness”, IEEE Transactions on Magnetics, Vol. 29, No. 6, November 1993, pp. 3679-3681, describe a laminated low-noise disk that uses a discontinuous Cr film that is thick enough to reduce the exchange coupling between the two magnetic layers in the laminate but is so thin that the two magnetic layers are not physically separated. 
     Increased storage density while maintaining good thermal stability may be achieved by two ferromagnetic films antiferromagnetically coupled together across a nonferromagnetic spacer film. Some laminates may include two ferromagnetic films decoupled from one another and a third ferromagnetic film antiferromagnetically coupled to one of the ferromagnetic films. The third film is typically referred to as the antiferromagnetic slave layer. Because the magnetic moments of the two antiferromagnetically-coupled films are oriented antiparallel, the net remnant Mrt of the ferromagnetic layers is reduced by the Mrt of the antiferromagnetic slave layer. This reduction in Mrt is accomplished without a reduction in the thermal stability of the recording medium because the volumes of the grains in the antiferromagnetically-coupled films add constructively. The medium also enables much sharper magnetic transitions to be achieved with reduced demagnetization fields, resulting in a higher linear bit density for the medium. 
     In view of the foregoing it is clear that laminated magnetic thin films for magnetic recording must have a high signal-to-noise ratio. Accordingly, it would be advancement in the art to provide a laminated magnetic thin film with increased the signal-to-noise ratio compared to currently available media having multiple ferromagnetic layers with or without antiferromagnetically coupled layers. 
     SUMMARY OF THE INVENTION 
     The present invention has been developed in response to the present state of the art, and in particular improves the signal-to-noise ratio (SNR) of multiple ferromagnetic layer thin film laminates both with and without antiferromagnetic coupling. In some embodiments, a laminate may include an upper ferromagnetic layer located closest to the magnetic transducer, a lower ferromagnetic layer located beneath the upper layer, and a antiferromagnetic slave layer beneath the lower ferromagnetic layer. A spacer layer may be disposed between the upper and lower ferromagnetic layers and serve to decouple the upper and lower ferromagnetic layers. An antiferromagnetic coupling layer may be disposed between the lower ferromagnetic layer and the antiferromagnetic slave layer and serve to antiferromagnetically couple the lower ferromagnetic layer and the antiferromagnetic slave layer. 
     The lower ferromagnetic layer may comprise an alloy having magnetically decoupled grains. In one embodiment decoupling may be achieved by using an alloy having chromium enriched boundary regions. The presence of chromium in the boundary regions is known to decouple grains. Other elements, such as boron, are known to decouple grains but result in amorphous nonmagnetic boundary regions, which effectively reduce the size of the grains resulting in reduced thermal stability. In some embodiments, the lower ferromagnetic alloy may contain amounts of tantalum which causes chromium to come out of solid state solution inside grains and collect at grain boundaries. In one embodiment the alloy used for the lower ferromagnetic layer is a five-element alloy having tantalum as one of its constituents. In the illustrated embodiment, the five-element alloy is CoPt 13 Cr 20 B 5 Ta 1  (i.e., 13 atomic percent (13 at. %) Pt, 20 at. % Cr, 5 at. % B, 1 at. % Ta, with the balance being Co). Experiments conducted by the inventors have shown use of this alloy significantly increases SNR without decreasing thermal stability. 
     In some embodiments, the lower ferromagnetic layer may also have a lower coercivity than the upper ferromagnetic layer. The lower coercivity may conform to the lower intensity magnetic field that reaches the lower ferromagnetic layer from the magnetic transducer  102 . 
     Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment of the invention. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussion of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment. 
     Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize that the invention can be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention. 
     These features and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the advantages of the invention will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments that are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings, in which: 
         FIG. 1  is a schematic representation of one embodiment of a read/write head and recording medium of the prior art; 
         FIG. 2  is an illustration of one embodiment of the layer structure of a laminated magnetic thin film medium of the prior art invention; and 
         FIG. 3  is an illustration of one embodiment of a layer structure for ferromagnetic layers and an anti-ferromagnetic slave layer comprising a laminated magnetic thin film medium of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. 
     Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize that the invention can be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention. 
     Referring to  FIG. 1 , a typical prior art head and disk system  100  may include a magnetic transducer  102  supported by a suspension  104  as it flies above the disk  106 . The magnetic transducer  102 , usually called a “read/write head” or “slider,” may include elements that perform the task of writing magnetic transitions (the write head  108 ) and reading the magnetic transitions (the read head  110 ). The electrical signals to and from the read and write heads  110 ,  108  travel along conductive paths (leads)  112  which are attached to or embedded in the suspension  104 . The magnetic transducer  102  is positioned over points at varying radial distances from the center of the disk  106  to read and write circular tracks (not shown). The disk  106  is attached to a spindle  114  that is driven by a spindle motor  116  to rotate the disk  106 . The disk  106  comprises a substrate  118  on which a laminate  120  having multiple layers is deposited. The laminate  120  may include ferromagnetic layers in which the write head  108  records the magnetic transitions in which information is encoded. 
     Referring to  FIG. 2 , the laminate  120  may include a preseed layer  202 , a seed layer  204 , one or more under layers  206 , magnetic layers  208 , and an overcoat  210 . The preseed layer  202  and seed layer  204  may provide a crystalline growth template upon which underlayer  206  can grow in the correct ( 200 ) orientation with small grain size, which further provides a growth template for the ferromagnetic layers to growth in (11.0) orientation with small grain size. The overcoat  210  may be a diamond-like carbon or silicon nitride layer protecting the magnetic layers  208  from abrasion or corrosion. In the illustrated embodiment, the preseed layer  202  is embodied as CrTi 50  and the seed layers  204  is embodied as RuAl layer and the underlayer is embodied as CrTi 20 . 
     Referring to  FIG. 3 , the magnetic layer  208  may include one or more ferromagnetic layers  302   a ,  302   b . The ferromagnetic layers will typically be locally magnetized by the transducer  102  in order to write data to the disk  106 . The ferromagnetic layers  302   a ,  302   b  will typically have high coercivity (H c ) and thermal stability in order to improve retention of data written thereto. In some embodiments, the ferromagnetic layer  302   b  located beneath the ferromagnetic layer  302   a  may have a lower coercivity than the layer  302   a  in order to conform to the weaker magnetic field from the transducer  102  reaching the ferromagnetic layer  302   b  due to layer  302   b &#39;s increased distance from the transducer  102 . A more detailed description of ferromagnetic layers having varied magnetic anisotropies can be found in U.S. patent application Ser. No. 10/628,011, filed Jul. 23, 2004 and entitled MAGNETIC ANISOTROPY ADJUSTED LAMINATED MAGNETIC THIN FILMS FOR MAGNETIC RECORDING, which is incorporated herein by reference. 
     In the illustrated embodiment, the ferromagnetic layers  302   a ,  302   b  are cobalt based ferromagnetic alloys. The upper ferromagnetic layer  302   a  may have the composition CoPt 13 Cr 19 B 7 . The lower ferromagnetic layer  302   b  may have the composition CoPt 13 Cr 20 B 5 Ta 1 . 
     A spacer layer  304  may be interposed between the ferromagnetic layers  302   a ,  302   b  in order to decouple the layers  302   a ,  302   b . Decoupling may be desirable to ensure that the magnetic grains forming the ferromagnetic layers  302   a ,  302   b  act independently. In order to store greater amounts of data, the number of grains per unit area, or grain density, may be increased. However, coupling between grains may result in grains acting collectively as one magnetic unit when changing the direction of magnetization. This coupling reduces effective magnetic grain density. Accordingly, decoupling the grains to ensure magnetic decorrelation may result in an increased magnetic grain density. 
     The spacer layer  304  may therefore comprise any material serving to decouple the ferromagnetic layers  302   a ,  302   b . In the illustrated embodiment, the spacer layer is ruthenium having a thickness resulting in decoupling of the ferromagnetic layers  302   a ,  302   b.    
     In some embodiments an antiferromagnetic slave layer  306  may be used to reduce the effective magnetization-thickness product (M r t) of the film. In the illustrated embodiment, the antiferromagnetic slave layer  306  has the composition CoCr 10 . An antiferromagnetic coupling layer  308  may be interposed between the lower ferromagnetic layer  302   b  and the antiferromagnetic slave layer  306 . In the illustrated embodiment, the antiferromagnetic coupling layer  308  is ruthenium having a thickness chosen to achieve antiferromagnetic coupling. 
     Reduction of grain size and decoupling of the grains provide a pathway to continuous improvement of the signal-to-noise ratio of magnetic media. However, accompanying these microstructural changes is a degradation of medium thermal stability. This is pronounced for laminated media since the upper ferromagnetic layer and the lower ferromagnetic layer are decoupled from each other, and therefore, need to be individually stable. To alleviate such degradation, an element selected to improve grain decoupling such as Ta may be included in the composition of the lower ferromagnetic layer. Ta is known to push Cr to the grain boundaries from inside the grains. With such Cr enrichment at the grain boundaries, the grains are well decoupled without significant increase in B content, which would result in significant increase of thickness of amorphous grain boundaries and refinement of grain size. As a result, medium signal-to-noise ratio can be improved without sacrificing thermal stability. 
     Table 1 compares signal-to-noise ratios at different linear recording densities (measured in kilo flux changes per inch or “kfci”) of two media, Medium 1 with CoPtCrBTa as the lower ferromagnetic layer according to this invention and Medium 2 with CoPtCrB as the lower ferromagnetic layer in prior art. Medium 1 has significantly higher SNR than Medium 2 by 0.3-0.5 dB across all densities in the measurement. The signal-to-noise ratio decay of the two media is equivalent as indicated in Table 2. 
     
       
         
               
             
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Signal to noise ratio v. recording density. 
               
             
          
           
               
                   
                 Recording Density (kfci) 
               
             
          
           
               
                   
                 0 
                 200 
                 400 
                 600 
                 800 
               
               
                   
                   
               
             
          
           
               
                   
                 CoCrPtBTa 
                 33.8 
                 31.7 
                 30.7 
                 30.2 
                 29.7 
               
               
                   
                 CoCrPtB 
                 33.5 
                 31.4 
                 30.4 
                 29.7 
                 29.3 
               
               
                   
                   
               
             
          
         
       
     
     Experiments conducted by the inventors have shown that the thermal stability of the CoCrPtBTa alloy is preserved, notwithstanding the decoupling of the grains. The CoCrPtBTa alloy has been found to have a signal-to-noise ratio decay of 1.7% per decade (a measure of thermal stability) which is equal to the decay of the CoCrPtB alloy. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.