Abstract:
A recording medium providing improved writeability in perpendicular recording applications includes a magnetic recording layer having an axis of magnetic anisotropy substantially perpendicular to the surface thereof, an exchange-spring layer ferromagnetically exchange coupled to the magnetic recording layer and having a coercivity less than the magnetic recording layer coercivity, and a coupling layer between the magnetic recording layer and the exchange-spring layer. The coupling layer regulates the ferromagnetic exchange coupling between the magnetic recording layer and the exchange-spring layer.

Description:
RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of, and claims priority to, co-pending application Ser. No. 11/051,536 filed on Feb. 4, 2005 and entitled Incoherently-Reversing Magnetic Laminate with Exchange-Coupled Ferromagnetic Layers. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1Field of the Invention  
         [0003]     This invention relates to perpendicular magnetic recording media and more particularly to apparatus and methods for improving the writeability of perpendicular magnetic recording media.  
         [0004]     2. Description of the Related Art  
         [0005]     One of the primary challenges to increasing areal densities of magnetic recording media is overcoming the constraints imposed by the superparamagnetic effect. The superparamagnetic effect becomes significant when microscopic magnetic grains on the recording media become so small that they lose their ability to maintain their magnetic orientations. This condition may result in “flipped bits,” a condition where the magnetization of the bits suddenly and spontaneously reverses from north to south. Such a condition corrupts the data stored on the media, rendering the data as well as the storage device it is stored on unreliable and unusable.  
         [0006]     In conventional longitudinal recording media, data bits are aligned horizontally, parallel to the surface of the disk. In perpendicular recording media, the data bits are aligned vertically, perpendicular to the disk. For example, referring to  FIG. 1 , a typical perpendicular recording device  100  may include a recording head  102  and a magnetic recording medium  104 . The recording head  102  may include a write element  106 , for writing magnetic signals to the recording medium  104 , and a read element  108 , to detect magnetic signals stored on the recording medium  104 .  
         [0007]     The magnetic recording medium  104  may include a magnetic recording layer  110 , to store data, and a soft underlayer  112  to provide a return path for magnetic write fields generated by the write element  106 . The magnetic recording layer  110  may comprise various magnetic grains each having a magnetic easy axis substantially perpendicular to the media surface, thereby allowing the grains to be vertically magnetized. When writing, the write element  106  generates a magnetic write field  116  that travels vertically through the magnetic recording layer  110  and returns to the write element  106  through the soft underlayer  112 . In this manner, the write element  106  magnetizes vertical regions  114 , or bits  114 , in the magnetic recording layer  110 . Because of the easy axis orientation, each of these bits  114  has a magnetization  118  that points in a direction substantially perpendicular to the media surface.  
         [0008]     Because of the ability to utilize a soft underlayer in the perpendicular geometry, write fields generated by the perpendicular write element  106  may be substantially larger than conventional longitudinal recording write fields. This allows use of media  104  having a higher coercivity (Hc) and anisotropy energy (Ku), which is more thermally stable. Furthermore, unlike longitudinal recording, where the magnetic fields between two adjacent bits have a destabilizing effect, the magnetic fields of magnetization  118  of bits in perpendicular recording media  104  stabilize each other, enhancing the overall stability of perpendicular magnetic recording media even further. This allows for closer bit packing.  
         [0009]     Referring to  FIG. 2 , as mentioned, one benefit of perpendicular recording is that the magnetic recording medium  104  is placed within the gap between the write element  106  and the soft underlayer  112 , thereby allowing significantly higher write fields. When the write element  106  is writing the magnetic recording layer  110 , the soft underlayer  112  reacts to the applied field of write element  106  in such a way that a mirror image  200  of the write element  106 , or a secondary write pole  200 , is generated in the soft underlayer  112 . The write element  106  and the secondary write pole  200  together produce an apparent recording head  106 ,  200 . In certain embodiments, the magnetic recording medium  104  may include an non-magnetic overcoat  202 , above the magnetic recording layer  110 , and an exchange break layer  204  to magnetically decouple the magnetic recording layer  110  from the soft underlayer  112 .  
         [0010]     One of the problem for conventional perpendicular media is that the magnetization  206 , or magnetic easy axis  206 , of the magnetic recording layer  110  is oriented nearly parallel to the magnetic field  116 . This geometry has the disadvantage that relatively high reversal fields are necessary to reverse the magnetization  206  of the grains  208  of the magnetic recording layer  100  because little or no torque is exerted onto the grain magnetization  206  by the magnetic write field  116 . Furthermore, such a nearly parallel alignment of field  116  and magnetization  206  has the additional disadvantage that the magnetization reversal time of the media grains  208  is increased. For these reasons, there have been proposals to produce magnetic media comprising magnetic grains having a magnetic easy axis that is tilted, or non-parallel, with respect to the surface normal. However, at the present time, apparatus and methods for producing high-quality recording media with a uniformly tilted easy axis do not exist.  
         [0011]     Accordingly, what are needed are apparatus and methods for improving the writeability of perpendicular magnetic recording media. Further needed are apparatus and methods for producing perpendicular magnetic recording media that behaves like media with a tilted easy axis, while still being fully compatible with currently used processes and structures for producing perpendicular recording media. Such apparatus and methods are disclosed herein.  
       SUMMARY OF THE INVENTION  
       [0012]     The present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available apparatus and methods. Accordingly, the present invention has been developed to provide apparatus and methods for improving the writeability of perpendicular magnetic recording media that overcome many or all of the above-discussed shortcomings in the art.  
         [0013]     In one embodiment in accordance with the invention, a recording medium for perpendicular recording applications includes a magnetic recording layer having an axis of magnetic anisotropy substantially perpendicular to the surface thereof, an exchange-spring layer ferromagnetically exchange coupled to the magnetic recording layer and having a coercivity less than the magnetic recording layer coercivity, and a coupling layer between the magnetic recording layer and the exchange-spring layer. The coupling layer regulates the ferromagnetic exchange coupling between the magnetic recording layer and the exchange-spring layer. Preferably, the coupling layer has a thickness less than the exchange-spring layer thickness. The magnetic recording layer and the exchange-spring layer comprise a cobalt alloy suitable to achieve an appropriate and sufficiently low level of inter-granular exchange coupling within each respective layer.  
         [0014]     In certain embodiments, the exchange-spring layer, the magnetic recording layer, or both, may comprise a material such as CoPt, CoPtCr, and may optionally include an oxide such as a Si, Ti, and Ta oxide. In selected embodiments, the coercivities of the magnetic recording layer and the exchange-spring layer are adjusted by among other process parameters modifying the amount of platinum therein. The coupling layer may comprise a material such as CoRu, CoCr, CoRuCr, and may optionally comprise an oxide such as a Si, Ti, and Ta oxide.  
         [0015]     In certain embodiments, the coupling layer has a thickness of less than two nanometers. More particularly, the coupling layer may have a thickness of between 0.2 nanometers and 1 nanometer. Likewise, in certain embodiments, the exchange-spring layer has a thickness of less than ten nanometers and, more particularly, may have a thickness of between two nanometers and six nanometers. In selected embodiments, the exchange-spring layer is thicker than the coupling layer. Furthermore, in certain embodiments, the inter-granular exchange coupling of the exchange-spring layer is greater than the inter-granular exchange coupling of the magnetic recording layer.  
         [0016]     In another embodiment in accordance with the invention, a recording device for perpendicular recording applications includes a recording head and a recording medium configured for perpendicular recording. The recording medium comprises a magnetic recording layer having an axis of magnetic anisotropy substantially perpendicular to the surface thereof, an exchange-spring layer between the magnetic recording layer and the recording head, the exchange-spring layer ferromagnetically exchange coupled to the magnetic recording layer and having a coercivity less than the magnetic recording layer coercivity, and a coupling layer between the magnetic recording layer and the exchange-spring layer. The coupling layer regulates the ferromagnetic exchange coupling between the magnetic recording layer and the exchange-spring layer. The magnetic recording layer and the exchange-spring layer preferably comprise a cobalt alloy.  
         [0017]     In another embodiment in accordance with the invention, a method for improving the writeability of perpendicular recording media includes forming a magnetic recording layer having an axis of anisotropy substantially perpendicular to the surface thereof, forming an exchange-spring layer comprising substantially magnetically separated grains, the exchange-spring layer ferromagnetically exchange coupled to the magnetic recording layer and having a coercivity less than the magnetic recording layer coercivity, and disposing a coupling layer between the magnetic recording layer and the exchange-spring layer, the coupling layer regulating the ferromagnetic exchange coupling between the magnetic recording layer and the exchange-spring layer. The exchange-spring layer preferably comprises a cobalt alloy.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     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 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:  
         [0019]      FIG. 1  is a schematic diagram illustrating one embodiment of a conventional perpendicular recording device;  
         [0020]      FIG. 2  is a cross-sectional view of one embodiment of conventional perpendicular recording media;  
         [0021]      FIG. 3  is a cross-sectional view of one embodiment of perpendicular recording media using an exchange-spring structure in accordance with the invention;  
         [0022]      FIG. 4  is a schematic diagram illustrating the magnetization reversal of perpendicular recording media using an exchange-spring structure in accordance with the invention;  
         [0023]      FIG. 5A  is a graph illustrating one embodiment of a magnetic hysteresis loop for the magnetic recording layer by itself;  
         [0024]      FIG. 5B  is a graph illustrating one embodiment of a magnetic hysteresis loop for the exchange-spring layer by itself;  
         [0025]      FIG. 5C  is a graph illustrating one embodiment of a magnetic hysteresis loop for an exchange-spring structure in accordance with the invention;  
         [0026]      FIG. 6A  is a graph illustrating the low frequency signal amplitude versus the write current for an exchange-spring structure having an exchange-spring layer thickness of three nanometers and various coupling layer thicknesses, the graph compares exchange-spring structures to a reference media without an exchange spring structure;  
         [0027]      FIG. 6B  is a graph illustrating the normalized low frequency signal amplitude versus the write current for an exchange-spring structure having an exchange-spring layer thickness of three nanometers and various coupling layer thicknesses, the graph compares exchange-spring structures to a reference media without an exchange spring structure;  
         [0028]      FIG. 7A  is a graph illustrating the signal-to-noise ratio versus the recording density for an exchange-spring structure having an exchange-spring layer thickness of three nanometers and various coupling layer thicknesses, the graph compares exchange-spring structures to a reference media without an exchange spring structure;  
         [0029]      FIG. 7B  is a graph illustrating the signal-to-noise ratio versus the coupling layer thickness for an exchange-spring structure having an exchange-spring layer thickness of three nanometers and for a target bit length ( 1 T) and twice the target bit length ( 2 T), the graph compares exchange-spring structures to a reference media without an exchange spring structure; and  
         [0030]      FIG. 8  is a graph illustrating the bit error rate versus the coupling layer thickness for an exchange-spring structure having an exchange-spring layer thickness of three nanometers, the graph compares exchange-spring structures to a reference media without an exchange spring structure.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]     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.  
         [0032]     Furthermore, the described features, structures, or characteristics of the invention may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are disclosed to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention may be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.  
         [0033]     For the purposes of this description, the phrase “axis of magnetic anisotropy” is used to mean the magnetic easy axis of a magnetic material. The phrase X-alloy is used to mean an alloy including X, where X is an element from the periodic chart. An alloy includes at least one metal and may include non-metals as well.  
         [0034]     Referring to  FIG. 3 , a perpendicular magnetic recording medium  300  incorporating an exchange-spring structure  301  in accordance with the invention may comprise a magnetic recording layer  302  ferromagnetically coupled to an exchange-spring layer  304 . The magnetic recording layer  302  and the exchange-spring layer  304  are preferably layers of a cobalt alloy with a hexagonal close packed (hcp) crystalline structure that exhibit perpendicular magnetic anisotropy, such as a CoPt or CoPtCr alloys, with or without an oxide, such as oxides of Si, Ti and Ta. However, while the magnetic recording layer  302  may be relatively hard magnetically (e.g., Hk&gt;10 kOe), the exchange spring layer  304  may be considerably softer (e.g., Hk&lt;6 kOe). In certain embodiments, the magnetic recording layer  302  and the exchange-spring layer  304  are of a same or similar material and the coercivity of each is adjusted along with other processing parameters by modifying the amount of platinum contained therein. A higher proportion, or concentration, of platinum per volume of the magnetic recording layer  302  relative to the exchange spring layer  304  will increase the magnetic hardness of the recording layer  302  relative to the exchange spring layer  304 .  
         [0035]     The materials specified above maybe suitable to achieve an appropriate (i.e., low) level of inter-granular exchange coupling in the magnetic recording layer  302  and the exchange spring layer  304 , respectively. Although, the inter-granular exchange coupling of the exchange spring layer  304  may exceed that of the magnetic recording layer  302 , it is preferable that the inter-granular exchange coupling of the exchange spring layer  304  be sufficiently low to minimize or reduce negative effects, such as lower signal-to-noise ratio or the like, that a higher inter-granular exchange coupling might have on the magnetic recording layer  302 . In certain embodiments, the exchange-spring layer has a thickness of less than ten nanometers, and more preferably between about two nanometers and six nanometers.  
         [0036]     A coupling layer  306  is disposed between the magnetic recording layer  302  and the exchange-spring layer  304  to regulate or mediate the exchange coupling between the two layers  302 ,  304 . This aids the magnetization reversal process of the magnetic recording layer  302  by exerting an additional bias field and torque on the grains of the magnetic recording layer  302  upon applying a reverse magnetic field. The coupling layer  306  is preferably a weakly magnetic or non-magnetic granular alloy layer with an hcp crystalline structure, such as a CoRu, CoCr or CoRuCr alloy, with or without an oxide, such as oxides of Si, Ti, and Ta, which is suitable to mediate a ferromagnetic coupling of appropriate strength between the magnetic recording layer  302  and the exchange-spring layer  304 . Depending on the choice of material, and more particularly on the concentration of cobalt in the coupling layer  306 , the coupling layer  306  may have a thickness of less than two nanometers, and more preferably between about 0.2 nanometers and 1 nanometer. Although in certain embodiments, the thickness of the coupling layer  306  may exceed 1 nanometer. Because cobalt is highly magnetic, a higher concentration of cobalt in the coupling layer  306  may be offset by thickening the coupling layer  306  in order to achieve an optimal inter-layer exchange coupling between the magnetic recording layer  302  and the exchange-spring layer  304 .  
         [0037]     As will be discussed in more detail hereinafter, the inter-layer exchange coupling between the magnetic recording layer  302  and the exchange-spring layer  304  may be optimized, in part, by adjusting the materials and thickness of the coupling layer  306 . Preferably, the inter-layer exchange coupling is not so weak that the exchange-spring layer  304  and the magnetic recording layer  302  behave as independent entities. Likewise, the inter-layer exchange coupling is preferably not so strong that the magnetic behavior of the exchange-spring layer  304  and the magnetic recording layer  302  are rigidly bound together. The inter-layer exchange coupling should be adjusted such that the magnetization of the exchange-spring layer  304  reverses before that of the magnetic recording layer  302 , while exerting enough torque onto the grains of the magnetic recording layer  302  to aid in the magnetic reversal of the magnetic recording layer  302 .  
         [0038]     Furthermore, as mentioned, in preferred embodiments, the exchange-spring layer  304  is magnetically softer (lower coercivity) than the magnetic recording layer  302 . Also, the exchange-spring layer  304  may be characterized by an inter-granular exchange coupling that is greater than the inter-granular exchange coupling of the magnetic recording layer  302 . By adjusting the thickness and materials of the coupling layer  306  to optimize the inter-layer exchange coupling between the exchange-spring layer  304  and the magnetic recording layer  302 , negative effects caused by the exchange-spring layer&#39;s  304  higher inter-granular exchange coupling, such as lower signal-to-noise ratios or the like, may be at least partially isolated from the magnetic recording layer  302 .  
         [0039]     In certain embodiments, an overcoat  308  maybe physically and preferably not magnetically coupled to the exchange-spring layer  304 . Similarly, the magnetic recording layer  302  may be physically coupled to a soft underlayer  312  by way of an exchange-break layer  310 . The soft underlayer  312  may be a multi-layer structure that provides a mirror image  314  (i.e., a secondary write pole  314 ) of a real write head  316 , thereby allowing large write fields to pass through the media  300 . The exchange-break layer  310  may be used to magnetically decouple the magnetic recording layer  302  from the soft underlayer  312 .  
         [0040]     Referring to  FIG. 4 , absent a magnetic field and prior to reversal, the magnetization  400   a ,  402   a  of both the exchange-spring layer  304  and the magnetic recording layer  302  may point in either a north or south direction. Upon applying a reverse magnetic field  404   a , the magnetization  400   b  of the softer exchange-spring layer  304  may begin to reverse, thereby exerting a torque onto the magnetically harder magnetic recording layer  302 . As the magnetic field  404   b  increases, the magnetization  402   c  of the magnetic recording layer  302  begins to reverse and follow the magnetization  400   c  of the exchange-spring layer  304 . Finally, as the magnetic field  404   c  increases further, the magnetization  400   d ,  402   d  of both the exchange-spring layer  304  and the magnetic recording layer  302  reverses entirely. Advantageously, the exchange-spring media  300  exhibits a magnetization reversal behavior which is similar to a magnetic recording layer having a tilted magnetic easy axis, while still being fully compatible with conventional perpendicular media deposition and fabrication processes and structures.  
         [0041]     Referring to  FIGS. 5A through 5C , several hysteresis loops generated with a Kerr magnetometer are illustrated for the magnetic recording layer  302  by itself ( FIG. 5A ), the exchange-spring layer  304  by itself ( FIG. 5B ), and an exchange spring structure  301  comprising both the magnetic recording layer  302  and the exchange-spring layer  304  coupled together with a coupling layer  306  ( FIG. 5C ) in accordance with the present invention. In this example, the magnetic recording layer  302  is cobalt platinum chromium tantalum oxide (CoPtCrTaOx), the exchange-spring layer  304  is cobalt platinum chromium silicon oxide (CoPtCrSiOx), and the coupling layer  306  is cobalt ruthenium (CoRu).  
         [0042]     As illustrated by  FIGS. 5A and 5B , the narrower hysteresis loop of  FIG. 5B  compared to that of  FIG. 5A  shows that the coercivity of the CoPtCrSiOx exchange-spring layer  304  by itself is less than that of the CoPtCrTaOx magnetic recording layer  302  by itself. This is true even for a CoPtCrSiOx exchange-spring layer  304  that is sixteen nanometers thick. When the CoPtCrSiOx exchange-spring layer  304  is thinned (as in the exchange-spring structure  301  of  FIG. 3 ), the coercivity of the exchange-spring layer  304  is significantly less as shown by the steeper slope  500  of the hysteresis loop of  FIG. 5C . The subsequent magnetization of the magnetically harder magnetic recording layer  302  is shown by the slower approach to saturation  502 , as shown by the reduced slope  504  of the hysteresis loop.  
         [0043]     As illustrated by  FIG. 5C , when the CoPtCrTaOx magnetic recording layer  302  and CoPtCrSiOx exchange-spring layer  304  are combined into an exchange-spring structure  301  like that illustrated in  FIG. 3  with a CoRu layer as the coupling layer  306  the hysteresis loop of  FIG. 5C  closes for a magnetic field (H) of approximately 9 kOe, whereas the hysteresis loop of  FIG. 5A  closes for a magnetic field (H) of approximately 12 kOe, showing that all or most of the grains of the exchange-spring structure  301  of  FIG. 5C  may be switched with a magnetic field reduced by approximately twenty-five percent.  
         [0044]     Referring to  FIGS. 6A and 6B , the low-frequency signal amplitude (LFTAA) versus the write current (I(mA)) is illustrated for a thickness of 3 nm for the exchange-spring layer  304  and various thicknesses of the coupling layer  306 . In this example, the exchange-spring layer  304  is CoPtCrSiOx and the coupling layer  306  is CoRu. The reference layer is CoPtCrTaOx magnetic recording media without the exchange-spring layer  304  or the coupling layer  306 .  
         [0045]     As illustrated in  FIG. 6A , for an exchange-spring structure  301  with an exchange-spring layer  304  having a thickness of three nanometers and a coupling layer  306  having a thickness of four or six angstroms, the signal amplitude saturates at lower write currents compared to the reference layer demonstrating that these structures are easier to write. These exchange-spring structures  301  also have higher signal amplitudes than the reference layer indicating that these structures add to the signal. If the CoRu layer thickness is increased to nine angstroms, however, the inter-layer exchange coupling between the exchange-spring layer  304  and the magnetic recording layer  302  is reduced and the writing improvements substantially disappear.  FIG. 6A  also illustrates that the signal amplitude and saturation for a nine angstrom coupling layer  306  either tracks, or is only marginally higher than the reference layer. In  FIG. 6B , which shows the normalized signal of the data shown in  FIG. 6A , the pure writability improvement becomes more evident because the writability improvement has been separated from the simultaneously occurring signal increase as the coupling layer  306  thickness is reduced.  
         [0046]     Referring to  FIGS. 7A and 7B , the signal-to-noise ratio (SNR) is illustrated versus the recording density ( FIG. 7A ) and the coupling layer thickness ( FIG. 7B ). As illustrated by  FIG. 7A , an exchange-spring structure  301  having a CoPtCrSiOx exchange-spring layer  304  that is three nanometers thick and a CoRu coupling layer  306  that is three or six angstroms thick has a better or higher signal-to-noise ratio than the CoPtCrTaOx recording media reference layer alone. An exchange-spring structure  301  having a CoPtCrSiOx exchange-spring layer  304  that is three nanometers thick and a CoRu coupling layer  306  that is nine angstroms thick, on the other hand, has a worse signal-to-noise ratio than the reference layer. Of course those of skill in the art will recognize that the specific thickness of the CoRu coupling layer  306  that provides satisfactory signal-to-noise ratios may be different when the coupling layer  306  comprises different materials or compositions along with, or in place of, Co and Ru.  
         [0047]      FIG. 7B  illustrates the signal-to-noise ratio for an exchange spring structure  301  having different thicknesses for the coupling layer  306 . The signal-to-noise ratio is illustrated for both a target bit length ( 1 T) and double the target bit length ( 2 T). As can be seen from  FIG. 7B , the signal-to-noise ratio for  1 T data has a lower signal-to-noise ratio than the  2 T data since the density is doubled. As further illustrated by  FIG. 7B , the signal-to-noise ratio improves for an exchange-spring structure  301  with a three nanometer CoPtCrSiOx exchange-spring layer  304  in an intermediate thickness range of the coupling layer  306  for both a  1 T (corresponding to a target bit length indicated by the solid square shapes  700 )) and a  2 T (corresponding to twice the target bit length indicated by the solid circle shapes  702 ) measurement, compared to a CoPtCrTaOx reference layer by itself (indicated by the open shapes  704 ). As is also illustrated by  FIG. 7B , once the coupling layer  306  reaches approximately eight angstroms, the signal-to-noise ratio falls below the signal-to-noise ratio of the reference layer. Thus,  FIGS. 7A and 7B  indicate that a coupling layer  306  in the range of four to seven angstrom provides a better signal-to-noise ratio than the reference layer alone, at least for a Co 20 Ru 80  coupling layer  306 . For coupling layers  306  comprising other materials or compositions, the optimal thickness may change.  
         [0048]     Referring to  FIG. 8 , the bit error rate versus the CoRu coupling layer  306  is illustrated for an exchange-spring structure  301  having a CoPtCrSiOx exchange-spring layer  304  that is three nanometers thick. As illustrated, the improved signal-to-noise ratio shown in  FIGS. 7A and 7B  is reflected by an improved error rate, where the error rate of the exchange-spring structure is improved significantly with respect to a CoPtCrTaOx reference layer (indicated by empty square shape  802 ) in the CoRu coupling layer  306  thickness range of four to seven angstrom.  
         [0049]     The higher signal-to-noise ratios and bit error rates for the exchange spring structure  301  as illustrated by  FIGS. 7A, 7B , and  8  illustrate that the unique materials and thicknesses used for the exchange spring layer  304  and the coupling layer  306 , as recited in the claims, do improve the signal-to-noise ratio and bit error rate of the reference layer, or the magnetic recording layer  302 , by itself. This is advantageous in comparison to conventional exchange spring structures, where the magnetically softer exchange spring layer often worsens the signal-to-noise ratio and bit error rate of the underlying magnetic recording layer.  
         [0050]     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.