Patent Publication Number: US-7582368-B2

Title: Perpendicular magnetic recording medium with an exchange-spring recording structure and a lateral coupling layer for increasing intergranular exchange coupling

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to perpendicular magnetic recording media, such as perpendicular magnetic recording disks for use in magnetic recording hard disk drives, and more particularly to a perpendicular magnetic recording medium with an “exchange-spring” recording layer structure. 
     2. Description of the Related Art 
     Horizontal or longitudinal magnetic recording media, wherein the recorded bits are oriented generally parallel to the surfaces of the disk substrate and the planar recording layer, has been the conventional media used in magnetic recording hard disk drives. Perpendicular magnetic recording media, wherein the recorded bits are stored in the recording layer in a generally perpendicular or out-of-plane orientation (i.e., other than parallel to the surfaces of the disk substrate and the recording layer), provides a promising path toward ultra-high recording densities in magnetic recording hard disk drives. A common type of perpendicular magnetic recording system is one that uses a “dual-layer” medium. This type of system is shown in  FIG. 1  with a single write pole type of recording head. The dual-layer medium includes a perpendicular magnetic data recording layer (RL) on a “soft” or relatively low-coercivity magnetically permeable underlayer (SUL) formed on the substrate. The RL is typically a granular ferromagnetic cobalt alloy, such as a CoPtCr alloy with a hexagonal-close-packed (hcp) crystalline structure having the c-axis oriented generally perpendicular to the RL. 
     The SUL serves as a flux return path for the field from the write pole to the return pole of the recording head. In  FIG. 1 , the RL is illustrated with perpendicularly recorded or magnetized regions, with adjacent regions having opposite magnetization directions, as represented by the arrows. The magnetic transitions between adjacent oppositely-directed magnetized regions are detectable by the read element or head as the recorded bits. 
       FIG. 2  is a schematic of a cross-section of a prior art perpendicular magnetic recording disk showing the write field H acting on the recording layer RL. The disk also includes the hard disk substrate that provides a generally planar surface for the subsequently deposited layers. The generally planar layers formed on the surface of the substrate also include a seed or onset layer (OL) for growth of the SUL, an exchange break layer (EBL) to break the magnetic exchange coupling between the magnetically permeable films of the SUL and the RL and to facilitate epitaxial growth of the RL, and a protective overcoat (OC). As shown in  FIG. 2 , the RL is located inside the gap of the “apparent” recording head (ARH), which allows for significantly higher write fields compared to longitudinal or in-plane recording. The ARH comprises the write pole ( FIG. 1 ) which is the real write head (RWH) above the disk, and a secondary write pole (SWP) beneath the RL. The SWP is facilitated by the SUL, which is decoupled from the RL by the EBL and produces a magnetic image of the RWH during the write process. This effectively brings the RL into the gap of the ARH and allows for a large write field H inside the RL. However, this geometry also results in the write field H inside the RL being oriented nearly normal to the surface of the substrate and the surface of the RL, i.e., along the perpendicular easy axis of the RL grains, as shown by typical grain  1  with easy axis  2 . The nearly parallel alignment of the write field H and the RL easy axis has the disadvantage that relatively high write fields are necessary to reverse the magnetization because minimal torque is exerted onto the grain magnetization. Also, a write-field/easy-axis alignment increases the magnetization reversal time of the RL grains, as described by M. Benakli et al.,  IEEE Trans. MAG  37, 1564 (2001). 
     For these reasons, “tilted” media have been theoretically proposed, as described by K.-Z. Gao et al.,  IEEE Trans. MAG  39, 704 (2003), in which the magnetic easy axis of the RL is tilted at an angle of up to about 45 degrees with respect to the surface normal, so that magnetization reversal can be accomplished with a lower write field and without an increase in the reversal time. However, there is no known fabrication process to make a high-quality recording medium with a RL having a tilted easy axis. 
     A perpendicular recording medium that emulates a tilted medium and is compatible with conventional fabrication processes has been proposed. This type of medium uses an “exchange-spring” structure in the RL to achieve a magnetic behavior that emulates the behavior of a tilted medium. In an exchange-spring perpendicular recording medium, the RL structure is a composite of a magnetically “hard” layer (higher coercivity) and a magnetically “soft” layer (lower coercivity) that are ferromagnetically exchange-coupled. An intermediate coupling layer may be located between the hard and soft magnetic layers to reduce the strength of the interlayer exchange coupling. The two magnetic layers typically have different anisotropy fields (H k ). (The anisotropy field H k  of a ferromagnetic layer with uniaxial magnetic anisotropy K u  is the magnetic field that would need to be applied along the easy axis to switch the magnetization direction.) In the presence of a uniform write field H the magnetization of the lower-H k  layer will rotate first and assist in the reversal of the magnetization of the higher-H k  layer, a behavior that is sometimes called the “exchange-spring” behavior. Exchange-spring perpendicular recording media are described by R. H. Victora et al., “Composite Media for Perpendicular Magnetic Recording”,  IEEE Trans MAG  41 (2), 537-542, February 2005; and J. P. Wang et al., “Composite media (dynamic tilted media) for magnetic recording”,  Appl. Phys. Lett.  86 (14) Art. No. 142504, Apr. 4, 2005. Pending application Ser. No. 11/231,516, filed Sep. 21, 2005 and assigned to the same assignee as this application, describes a perpendicular magnetic recording medium with an exchange-spring RL structure formed of a lower high-H k  ferromagnetic layer, an upper low-H k  ferromagnetic layer and an intermediate coupling layer between the two ferromagnetic layers. 
     The problem of thermal decay exists for perpendicular recording media with conventional RLs and for media with exchange-spring RL structures. As the thickness of the RL structure decreases, the magnetic grains become more susceptible to magnetic decay, i.e., magnetized regions spontaneously lose their magnetization, resulting in loss of data. This is 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. Thus a RL with a high K u  is important for thermal stability. However, in a medium with an exchange-spring RL structure, one of the magnetic layers has very low K u , so that this layer can not contribute to the thermal stability of the RL. 
     To address the problem of thermal decay in exchange-spring media, pending application Ser. No. 11/372,295, filed Mar. 9, 2006 and assigned to the same assignee as this application, describes a perpendicular recording medium with an exchange-spring RL structure formed of two ferromagnetic layers with substantially similar anisotropy fields H k  that are ferromagnetically exchange-coupled by an intermediate nonmagnetic or weakly ferromagnetic coupling layer. Because the write head produces a larger magnetic field and larger field gradient at the upper portion of the RL, while the field strength decreases further inside the RL, the upper ferromagnetic layer can have a high anisotropy field. The high field and field gradient near the top of the RL, where the upper ferromagnetic layer is located, reverses the magnetization of the upper ferromagnetic layer, which then assists in the magnetization reversal of the lower ferromagnetic layer. Because both ferromagnetic layers in this exchange-spring type RL have a high anisotropy field and are sufficiently exchange coupled, the thermal stability of the medium is not compromised. 
     Both horizontal and perpendicular magnetic recording media that use recording layers of granular ferromagnetic cobalt alloys exhibit increasing intrinsic media noise with increasing linear recording density. Media noise arises from irregularities in the recorded magnetic transitions and results in random shifts of the readback signal peaks. High media noise leads to a high bit error rate (BER). Thus to obtain higher areal recording densities it is necessary to decrease the intrinsic media noise, i.e., increase the signal-to-noise ratio (SNR), of the recording media. The granular cobalt alloys in the RL structure should thus have a well-isolated fine-grain structure to reduce intergranular exchange coupling, which is responsible for high intrinsic media noise. Enhancement of grain segregation in the cobalt alloy RL can be achieved by the addition of segregants, such as oxides of Si, Ta, Ti, Nb, Cr, V, and B. These oxides tend to precipitate to the grain boundaries, and together with the elements of the cobalt alloy, form nonmagnetic intergranular material. 
     However, unlike horizontal recording media, where the complete absence of intergranular exchange coupling provides the best SNR, in perpendicular recording media the best SNR is achieved at some intermediate level of intergranular exchange coupling. Also, intergranular exchange coupling improves the thermal stability of the magnetization states in the media grains. Thus in perpendicular recording media, some level of intergranular exchange coupling is advantageous. 
     What is needed is a perpendicular magnetic recording medium with an exchange-spring RL structure that has optimal intergranular exchange coupling to produce high SNR, and high thermal stability, as well as superior writability. 
     SUMMARY OF THE INVENTION 
     The invention is a perpendicular magnetic recording medium with a RL structure that includes an exchange-spring structure and a ferromagnetic lateral coupling layer (LCL) that mediates intergranular exchange coupling in the exchange-spring structure. The exchange-spring structure is made up of two ferromagnetically exchange-coupled magnetic layers (MAG 1  and MAG 2 ), each with perpendicular magnetic anisotropy. MAG 1  and MAG 2  are either in direct contact with one another or have a coupling layer (CL) located between them. The CL provides the appropriate ferromagnetic coupling strength MAG 1  and MAG 2 . The LCL is located on top of and in contact with MAG 2  or below MAG 2  with MAG 2  on top of and in contact with the LCL. 
     The LCL may be formed of Co, or ferromagnetic Co alloys, such as CoCr alloys. The Co alloys may include one or both of Pt and B. The ferromagnetic alloy in the LCL has significantly greater intergranular exchange coupling than the ferromagnetic alloy in MAG 2 , which typically will include segregants such as oxides. The LCL alloy should preferably not include any oxides or other segregants, which would tend to reduce intergranular exchange coupling in the LCL. Because the LCL grain boundaries overlay the boundaries of the generally segregated and decoupled grains of MAG 2  with which it is in contact, and the LCL and MAG 2  grains are strongly coupled perpendicularly, the LCL introduces an effective intergranular exchange coupling in the MAG 2 , or more precisely it enables a combined LCL+MAG 2  system with a tunable level of intergranular exchange. 
     The invention is also a perpendicular magnetic recording system that includes the above-described medium and a magnetic recording write head. 
     For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a schematic of a prior art perpendicular magnetic recording system. 
         FIG. 2  is a schematic of a cross-section of a prior art perpendicular magnetic recording disk showing the write field H acting on the recording layer (RL). 
         FIG. 3A  is a schematic of a cross-section of a perpendicular magnetic recording disk with an exchange-spring recording layer (RL) made up of two ferromagnetically exchange-coupled magnetic layers (MAG 1  and MAG 2 ). 
         FIG. 3B  is a schematic of a cross-section of a perpendicular magnetic recording disk with an exchange-spring recording layer (RL) made up of two magnetic layers (MAG 1  and MAG 2 ) separated by a ferromagnetic coupling layer (CL), and the fields H 1  and H 2  acting on MAG 1  and MAG 2 , respectively. 
         FIG. 4A  is a schematic showing one implementation of the invention wherein the lateral coupling layer (LCL) is deposited directly on MAG 2 . 
         FIG. 4B  is a schematic showing another implementation of the invention wherein the LCL is deposited on the CL and MAG 2  is deposited directly on the LCL. 
         FIGS. 5A-5B  illustrate schematically the grains and magnetizations in MAG 2  without the LCL ( FIG. 5A ) and with the LCL ( FIG. 5B ). 
         FIG. 6  is a graph of bit error rate (BER) as a function of the recording head write current for two reference structures and the structure according to this invention. 
         FIG. 7A  is a schematic of a modeling test structure and a description of the test parameters for testing jitter and T 50  as a function of intergranular exchange coupling. 
         FIG. 7B  is a graph of jitter as a function of intergranular exchange coupling for the modeling parameters of  FIG. 7A . 
         FIG. 7C  is a graph of T 50  as a function of intergranular exchange coupling for the modeling parameters of  FIG. 7A . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 3A  is a schematic of a cross-section of a perpendicular magnetic recording disk according to the prior art with an exchange-spring recording layer (RL) made up of two ferromagnetically exchange-coupled magnetic layers (MAG 1  and MAG 2 ). MAG 1  and MAG 2  each has perpendicular magnetic anisotropy. However, MAG 1  and MAG 2  have different magnetic properties, so that they respond differently to the applied write field. For example, one of MAG 1  and MAG 2  can be magnetically soft and the other magnetically hard. The magnetic grains in the soft layer are exchange-decoupled from one another, meaning that there is very low intergranular exchange coupling in the soft layer. With a proper interlayer exchange coupling between the grains in MAG 1  and MAG 2 , the soft grains will rotate first under the applied write field, while at the same time providing an exchange field to the hard grains to effectively tilt their easy axis, thus assisting in the magnetization reversal of the grains in the hard layer. 
       FIG. 3B  illustrates an exchange-spring medium like that described in the previously-cited pending application Ser. No. 11/372,295 wherein a coupling layer (CL) is located between MAG 1  and MAG 2 . The composite RL has at least two ferromagnetically exchange-coupled magnetic layers (MAG 1  and MAG 2 ), each with generally perpendicular magnetic anisotropy and with substantially similar anisotropy fields H k  , that are separated by the CL. The CL provides the appropriate ferromagnetic coupling strength between the magnetic layers. The composite RL structure takes advantage of the depth-dependent write field H. i.e., in general a write head produces a larger magnetic field and larger field gradient near the surface of the RL, while the field strength decreases further inside the RL. The high field and field gradient near the top of the RL, where MAG 2  is located, enables MAG 2  to be formed of a high-H k  material. As the magnetization of MAG 2  is reversed by the write field it assists in the magnetization reversal of the lower magnetic layer MAG 1 . In this non-coherent reversal of the magnetizations of MAG 1  and MAG 2 , MAG 2  changes its magnetization orientation in response to a write field and in turn amplifies the “torque,” or reverse field, exerted on MAG 1 , causing MAG 1  to change its magnetization direction in response to a weaker write field than would otherwise be required in the absence of MAG 2 . Although the write field acting on MAG 1  can be significantly less than the write field acting on MAG 2 , MAG 1  can have substantially the same H k  because of the torque created by the magnetization reversal of MAG 2 . MAG 1  and MAG 2  can thus have substantially the same material composition and hus substantially similar anisotropy fields Hk. 
     The medium in the form of a disk is shown in sectional view in  FIG. 3B  with the write field H. As shown in the expanded portion of  FIG. 3B , a typical grain  10  in MAG 2  has a generally perpendicular or out-of-plane magnetization along an easy axis  12 , and is acted upon by a write field H 2 . A typical grain  20  in MAG 1  below the MAG 2  grain  10  also has a perpendicular magnetization along an easy axis  22 , and is acted upon by a write field H 1  less than H 2 . In the presence of the applied write field H 2 , the MAG 2  acts as a write assist layer by exerting a magnetic torque onto MAG 1  that assists in reversing the magnetization of MAG 1 . 
     In this invention a multilayer RL structure with an additional layer in the exchange-spring RL has the improved writability of exchange-spring RLs as well as the noise reduction and thermal stability improvement found in RL structures that have an elevated level of intergranular exchange coupling. As shown in one implementation in  FIG. 4A , an additional layer, called a lateral coupling layer (LCL), is located on top of and in contact with MAG 2  in the exchange-spring structure. As shown in another implementation in  FIG. 4B , the LCL is located below MAG 2  with MAG 2  on top of and in contact with the LCL. The LCL mediates the intergranular exchange coupling in the exchange-spring structure. While the LCL is depicted in  FIGS. 4A and 4B  as being implemented with an exchange-spring structure that includes a CL, like that shown in  FIG. 3B , the LCL is also fully applicable to an exchange-spring structure without a CL, like that shown in  FIG. 3A . 
     A representative disk structure for the invention shown in  FIGS. 4A-4B  will now be described. The hard disk substrate may be any commercially available glass substrate, but may also be a conventional aluminum alloy with a NiP surface coating, or an alternative substrate, such as silicon, canasite or silicon-carbide. 
     The adhesion layer or OL for the growth of the SUL may be an AlTi alloy or a similar material with a thickness of about 2-5 nm. The SUL may be formed of magnetically permeable materials such as alloys of CoNiFe, FeCoB, CoCuFe, NiFe, FeAlSi, FeTaN, FeN, FeTaC, CoTaZr, CoFeTaZr, CoFeB, and CoZrNb. The SUL may also be a laminated or multilayered SUL formed of multiple soft magnetic films separated by nonmagnetic films, such as electrically conductive films of Al or CoCr. The SUL may also be a laminated or multilayered SUL formed of multiple soft magnetic films separated by interlayer films that mediate an antiferromagnetic coupling, such as Ru, Ir, or Cr or alloys thereof. 
     The EBL is located on top of the SUL. It acts to break the magnetic exchange coupling between the magnetically permeable films of the SUL and the RL and also serves to facilitate epitaxial growth of the RL. The EBL may not be necessary, but if used it can be a nonmagnetic titanium (Ti) layer; a non-electrically-conducting material such as Si, Ge and SiGe alloys; a metal such as Cr, Ru, W, Zr, Nb, Mo, V and Al; a metal alloy such as amorphous CrTi and NiP; an amorphous carbon such as CN x , CH x  and C; or oxides, nitrides or carbides of an element selected from the group consisting of Si, Al, Zr, Ti, and B. If an EBL is used, a seed layer may be used on top of the SUL before deposition of the EBL. For example, if Ru is used as the EBL, a 1-8 nm thick NiFe or NiW seed layer may be deposited on top of the SUL, followed by a 3-30 nm thick Ru EBL. The EBL may also be a multilayered EBL. 
     The MAG 1  and MAG 2  layers may be formed of any of the known amorphous or crystalline materials and structures that exhibit perpendicular magnetic anisotropy. Thus, the MAG 1  and MAG 2  may each be a layer of granular polycrystalline cobalt alloy, such as a CoPt or CoPtCr alloy, with a suitable segregant such as oxides of Si, Ta, Ti, Nb, Cr, V and B. Also, MAG 1  and MAG 2  may each be composed of multilayers with perpendicular magnetic anisotropy, such as Co/Pt, Co/Pd, Fe/Pt and Fe/Pd multilayers, containing a suitable segregant such as the materials mentioned above. In addition, perpendicular magnetic layers containing rare earth elements are useable for MAG 1  and MAG 2 , such as CoSm, TbFe, TbFeCo, GdFe alloys. MAG 1  and MAG 2  may have substantially different magnetic properties, such as different anisotropy fields (H k ), to assure that they respond differently to the applied write field and thereby exhibit the exchange-spring behavior to improve writability. MAG 1  and MAG 2  may also have substantially the same anisotropy field H k , meaning that the H k  value for the layer with the lower H k  is at least 70% (and up to at least 90%) of the H k  value for the layer with the higher H k , and still exhibit the exchange-spring behavior as described above for the medium shown in  FIG. 3B . 
     The CL may be a hexagonal-close-packed (hcp) material, which can mediate a weak ferromagnetic coupling and also provide a good template for the growth of MAG 2 . Because the CL must enable an appropriate interlayer exchange coupling strength, it should be either nonmagnetic or weakly ferromagnetic. Thus the CL may be formed of RuCo and RuCoCr alloys with low Co content (&lt;about 60 atomic percent), or CoCr and CoCrB alloys with high Cr and/or B content (Cr+B&gt;about 30 atomic percent). Si-oxide or other oxides like oxides of Ta, Ti, Nb, Cr, V and B may be added to these alloys. The CL may also be formed of face-centered-cubic (fcc) materials, such as Pt or Pd or alloys based on Pt or Pd, because these materials enable a ferromagnetic coupling between magnetic layers of tunable strength (i.e., they reduce the coupling by increasing the thickness) and are compatible with media growth. 
     Depending on the choice of material for CL, and more particularly on the concentration of cobalt (Co) in the CL, the CL may have a thickness of less than 3.0 nm, and more preferably between about 0.2 nm and 2.5 nm. Because Co is highly magnetic, a higher concentration of Co in the CL may be offset by thickening the CL to achieve an optimal interlayer exchange coupling between MAG 1  and MAG 2 . The interlayer exchange coupling between MAG 1  and MAG 2  may be optimized, in part, by adjusting the materials and thickness of the CL. The CL should provide a coupling strength sufficient to have a considerable effect on the switching field (and the switching field distribution), but small enough to not couple the MAG 1  and MAG 2  layers rigidly together. 
     The LCL may be formed of Co, or ferromagnetic Co alloys, such as CoCr alloys. The Co alloys may include one or both of Pt and B. The LCL is deposited directly on MAG 2  in the  FIG. 4A  implementation, or the LCL is deposited on the CL and MAG 2  is deposited directly on the LCL in the  FIG. 4B  implementation. The ferromagnetic alloy in the LCL has significantly greater intergranular exchange coupling than the ferromagnetic alloy in MAG 2 . The LCL alloy should preferably not include any oxides or other segregants, which would tend to reduce intergranular exchange coupling in the LCL. Because the LCL grain boundaries overlay the boundaries of the generally segregated and decoupled grains of the MAG 2  with which it is in contact, and the LCL and MAG 2  grains are strongly coupled perpendicularly, the LCL introduces an effective intergranular exchange coupling in the MAG 2 , or more precisely it enables a combined LCL+MAG 2  system with a tunable level of intergranular exchange. This is depicted in  FIGS. 5A-5B , which illustrate schematically the grains and magnetizations in MAG 2  without the LCL ( FIG. 5A ) and with the LCL ( FIG. 5B ). The total LCL+MAG 2  thickness should be in the range of approximately 2-10 nm, preferably in the range of approximately 3-7 nm. The LCL portion of the total LCL+MAG 2  thickness should be between about 10-90%, with a preferred range of about 20-60%. The optimal LCL thickness can be determined experimentally by varying the thickness and measuring the performance of the disks to determine which thickness provides the most suitable level of intergranular exchange coupling for the combined LCL+MAG 2  system. 
     The OC formed on top of the RL may be an amorphous “diamond-like” carbon film or other known protective overcoats, such as Si-nitride. 
     The structure according to this invention has been tested experimentally by using a MAG 1 /CL/MAG 2  exchange spring structure of CoPtCr—Ta-oxide/RuCo/CoPtCr—Si-oxide onto which an LCL of Co (1−x) Cr x  (x=0.1 to 0.22) was deposited.  FIG. 6  shows the recording results of BER as a function of recording head write current for several specific structures: 
     (a) a first reference structure MAG 1 /MAG 2  dual MAG layer structure of CoPtCr—Ta-oxide/CoPtCrSi-oxide without the LCL; 
     (b) a second reference structure MAG 1 /CL/MAG 2  exchange spring structure of CoPtCr—Ta-oxide/RuCo/CoPtCr—Si-oxide (4 nm) without the LCL; and 
     (c) a MAG 1 /CL/MAG 2  exchange spring structure of CoPtCr—Ta-oxide/RuCo/CoPtCr—Si-oxide (3 nm) with a LCL of Co 9O Cr 10  (1.5 nm). 
     From a comparison with the reference structures, it is evident from  FIG. 6  that the recording performance of the structure with the LCL is improved even beyond the recording performance of the second reference structure, which by itself is much better than the Ta-oxide/Si-oxide dual layer first reference structure. The significant error rate improvement shown in  FIG. 6  was achieved for comparable signal levels and track width. Additionally, magnetic hysteresis loops of these structures revealed a significantly reduced saturation field (by approximately 10%) for the structure with the LCL at comparable nucleation fields, corroborating the higher level of intergranular exchange coupling being present in the overall structure due to the highly exchange-coupled grains in the CoCr LCL. 
     The improvements provided by the structure of this invention have also been established by a micromagnetic modeling study of the role of intergranular exchange coupling in a MAG 1 /CL/MAG 2  exchange-spring structure. For this purpose, key recording performance parameters of jitter (which is the positioning error for bit transitions, measured as the standard deviation of the zero crossings for the readback voltage) and T 50  (which is the width of an isolated transition, measured as the distance between the +50% and the −50% points of the signal) were calculated as a function of the intergranular exchange coupling (H ex ) in MAG 2 , which mimics the addition of the CoCr LCLs in our experiments. A description of the structure and parameters for this study is shown in  FIG. 7A , with the calculations shown in  FIGS. 7B-7C . In  FIG. 7A , Dc is the average grain diameter used in the modeling.  FIG. 7B  demonstrates that a noise reduction, due to an enhanced intergranular exchange coupling in MAG 2 , can be achieved. Also,  FIG. 7C  shows that the T 50  values can be reduced to a certain degree, further improving the overall recording performance. 
     While the present invention has been particularly shown and described with reference to the preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the invention. Accordingly, the disclosed invention is to be considered merely as illustrative and limited in scope only as specified in the appended claims.