Patent Publication Number: US-2012044595-A1

Title: Perpendicular magnetic recording medium (pmrm) and magnetic storage systems using the same

Description:
FIELD OF THE INVENTION 
     The present invention relates to data storage systems, and more particularly, this invention relates to a perpendicular magnetic recording medium (PMRM), and magnetic storage apparatuses using PMRM. 
     BACKGROUND OF THE INVENTION 
     The heart of a computer is a magnetic disk drive which typically includes a rotating disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and/or write heads over selected circular tracks on the rotating disk. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to, and reading magnetic signal fields from, the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions. 
     In typical systems, the disk is made of a magnetic recording medium composed of crystal grains, which form into groups called clusters. Storage capacity is determined by the composition of the magnetic recording medium, which should robustly tolerate heat and interference from external magnetic fields, while minimizing medium noise, such that it provides a good medium with which to write data to. Current approaches for optimizing performance generally involve reducing the size of crystal grains within the magnetic medium. Conventional methods for reducing crystal grain size produce smaller crystal grains, but these smaller crystal grains also exhibit deteriorated crystal orientation and reduced magnetic isolation. This in turn leads to increased interaction between the smaller crystal grains, which results in an increase in the overall cluster size distribution (e.g., the average cluster size increases, even with smaller crystal grains) and limits improvements to the recording and reproducing characteristics of the medium. Therefore, a method and/or system of overcoming the current limitations of reducing cluster size which can be used in recording and reproducing data with magnetic media would be very beneficial. 
     SUMMARY OF THE INVENTION 
     In one embodiment, a perpendicular magnetic recording medium includes a first interlayer comprising Ru or a Ru alloy, a second interlayer above the first interlayer comprising Ru or a Ru alloy, and a third interlayer formed between the first interlayer and the second interlayer that reduces an average cluster size of the second interlayer. 
     In another embodiment, a perpendicular magnetic recording medium includes a first interlayer comprising Ru or a Ru alloy, a second interlayer above the first interlayer comprising Ru or a Ru alloy, and a third interlayer formed between the first interlayer and the second interlayer that reduces an average cluster size of the second interlayer. The third interlayer has a thickness of between about 1.0 nm and about 3.0 nm and has a structure selected from a group consisting of: BCC, B2, C11b, L21, and D03. 
     In yet another embodiment, a method for forming a perpendicular magnetic recording medium includes forming a multilayer interlayer, comprising forming a first interlayer above a substrate, forming a second interlayer above the first interlayer, and forming a third interlayer between the first interlayer and the second interlayer, and forming a perpendicular magnetic recording layer above the multilayer interlayer. 
     Any of these embodiments may be implemented in a magnetic data storage system such as a disk drive system, which may include a magnetic head, a drive mechanism for passing a magnetic medium (e.g., hard disk) over the magnetic head, and a controller electrically coupled to the magnetic head. 
     Other aspects and advantages of the present invention will become apparent from the following detailed description, which, when taken in conjunction with the drawings, illustrate by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a fuller understanding of the nature and advantages of the present invention, as well as the preferred mode of use, reference should be made to the following detailed description read in conjunction with the accompanying drawings. 
         FIG. 1  is a simplified drawing of a magnetic recording disk drive system. 
         FIG. 2A  is a schematic representation in section of a recording medium utilizing a longitudinal recording format. 
         FIG. 2B  is a schematic representation of a conventional magnetic recording head and recording medium combination for longitudinal recording as in  FIG. 2A . 
         FIG. 2C  is a magnetic recording medium utilizing a perpendicular recording format. 
         FIG. 2D  is a schematic representation of a recording head and recording medium combination for perpendicular recording on one side. 
         FIG. 2E  is a schematic representation of a recording apparatus adapted for recording separately on both sides of the medium. 
         FIG. 3A  is a cross-sectional view of one particular embodiment of a perpendicular magnetic head with helical coils. 
         FIG. 3B  is a cross-sectional view of one particular embodiment of a piggyback magnetic head with helical coils. 
         FIG. 4A  is a cross-sectional view of one particular embodiment of a perpendicular magnetic head with looped coils. 
         FIG. 4B  is a cross-sectional view of one particular embodiment of a piggyback magnetic head with looped coils. 
         FIG. 5  is a cross-sectional view of one particular embodiment of a perpendicular magnetic recording medium (PMRM) utilizing a third interspersed layer of magnetic crystal grains. 
         FIG. 6A  is a simplified drawing of one particular embodiment of seven adjacent in-phase crystal grains forming a magnetic cluster. 
         FIG. 6B  is a simplified drawing of one particular embodiment of seven adjacent crystal grains, where three of the adjacent crystal grains are in-phase and form a magnetic cluster. 
         FIG. 6C  is a simplified drawing of one particular embodiment of seven adjacent crystal grains, where two of the adjacent crystal grains are in-phase and form a magnetic cluster. 
         FIG. 7  is a plot showing one effect of smaller cluster size of the third interlayer, according to one embodiment. 
         FIG. 8  is a table showing comparisons between two exemplary embodiments and a comparative example. 
         FIG. 9  is a cross-sectional view of a perpendicular magnetic recording medium (PMRM) utilizing two or three interlayers, according to one embodiment and a comparative example. 
         FIG. 10  is a flowchart of a method for forming a perpendicular magnetic recording medium (PMRM), according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is made for the purpose of illustrating the general principles of the present invention and is not meant to limit the inventive concepts claimed herein. Further, particular features described herein can be used in combination with other described features in each of the various possible combinations and permutations. 
     Unless otherwise specifically defined herein, all terms are to be given their broadest possible interpretation including meanings implied from the specification as well as meanings understood by those skilled in the art and/or as defined in dictionaries, treatises, etc. 
     It must also be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless otherwise specified. 
     The following description discloses several preferred embodiments of disk-based storage systems and/or related systems and methods, as well as operation and/or component parts thereof. 
     In one general embodiment, a perpendicular magnetic recording medium includes a first interlayer comprising Ru or a Ru alloy, a second interlayer above the first interlayer comprising Ru or a Ru alloy, and a third interlayer formed between the first interlayer and the second interlayer that reduces an average cluster size of the second interlayer. 
     In another general embodiment, a perpendicular magnetic recording medium includes a first interlayer comprising Ru or a Ru alloy, a second interlayer above the first interlayer comprising Ru or a Ru alloy, and a third interlayer formed between the first interlayer and the second interlayer that reduces an average cluster size of the second interlayer. The third interlayer has a thickness of between about 1.0 nm and about 3.0 nm and has a structure selected from a group consisting of: BCC, B2, C11b, L21, and D03. 
     In yet another general embodiment, a method for forming a perpendicular magnetic recording medium includes forming a multilayer interlayer, comprising forming a first interlayer above a substrate, forming a second interlayer above the first interlayer, and forming a third interlayer between the first interlayer and the second interlayer, and forming a perpendicular magnetic recording layer above the multilayer interlayer. 
     Referring now to  FIG. 1 , there is shown a disk drive  100  in accordance with one embodiment of the present invention. As shown in  FIG. 1 , at least one rotatable magnetic disk  112  is supported on a spindle  114  and rotated by a disk drive motor  118 . The magnetic recording on each disk is typically in the form of an annular pattern of concentric data tracks (not shown) on the disk  112 . 
     At least one slider  113  is positioned near the disk  112 , each slider  113  supporting one or more magnetic read/write heads  121 . As the disk rotates, slider  113  is moved radially in and out over disk surface  122  so that heads  121  may access different tracks of the disk where desired data are recorded and/or to be written. Each slider  113  is attached to an actuator arm  119  by means of a suspension  115 . The suspension  115  provides a slight spring force which biases slider  113  against the disk surface  122 . Each actuator arm  119  is attached to an actuator  127 . The actuator  127  as shown in  FIG. 1  may be a voice coil motor (VCM). The VCM comprises a coil movable within a fixed magnetic field, the direction and speed of the coil movements being controlled by the motor current signals supplied by controller  129 . 
     During operation of the disk storage system, the rotation of disk  112  generates an air bearing between slider  113  and disk surface  122  which exerts an upward force or lift on the slider  113 . The air bearing thus counter-balances the slight spring force of suspension  115  and supports slider  113  off and slightly above the disk surface by a small, substantially constant spacing during normal operation. Note that in some embodiments, the slider  113  may slide along the disk surface  122 . 
     The various components of the disk storage system are controlled in operation by control signals generated by control unit  129 , such as access control signals and internal clock signals. Typically, control unit  129  comprises logic control circuits, storage (e.g., memory), and a microprocessor. The control unit  129  generates control signals to control various system operations such as drive motor control signals on line  123  and head position and seek control signals on line  128 . The control signals on line  128  provide the desired current profiles to optimally move and position slider  113  to the desired data track on disk  112 . Read and write signals are communicated to and from read/write heads  121  by way of recording channel  125 . 
     The above description of a typical magnetic disk storage system, and the accompanying illustration of  FIG. 1  is for representation purposes only. It should be apparent that disk storage systems may contain a large number of disks and actuators, and each actuator may support a number of sliders. 
     An interface may also be provided for communication between the disk drive and a host (integral or external) to send and receive the data and for controlling the operation of the disk drive and communicating the status of the disk drive to the host, all as will be understood by those of skill in the art. 
     In a typical head, an inductive write head includes a coil layer embedded in one or more insulation layers (insulation stack), the insulation stack being located between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head. The pole piece layers may be connected at a back gap. Currents are conducted through the coil layer, which produce magnetic fields in the pole pieces. The magnetic fields fringe across the gap at the ABS for the purpose of writing bits of magnetic field information in tracks on moving media, such as in circular tracks on a rotating magnetic disk. 
     The second pole piece layer has a pole tip portion which extends from the ABS to a flare point and a yoke portion which extends from the flare point to the back gap. The flare point is where the second pole piece begins to widen (flare) to form the yoke. The placement of the flare point directly affects the magnitude of the magnetic field produced to write information on the recording medium. 
       FIG. 2A  illustrates, schematically, a conventional recording medium such as used with magnetic disc recording systems, such as that shown in  FIG. 1 . This medium is utilized for recording magnetic impulses in or parallel to the plane of the medium itself. The recording medium, a recording disc in this instance, comprises basically a supporting substrate  200  of a suitable non-magnetic material such as glass, with an overlying coating  202  of a suitable and conventional magnetic layer. 
       FIG. 2B  shows the operative relationship between a conventional recording/playback head  204 , which may preferably be a thin film head, and a conventional recording medium, such as that of  FIG. 2A . 
       FIG. 2C  illustrates, schematically, the orientation of magnetic impulses substantially perpendicular to the surface of a recording medium as used with magnetic disc recording systems, such as that shown in  FIG. 1 . For such perpendicular recording the medium typically includes an under layer  212  of a material having a high magnetic permeability. This under layer  212  is then provided with an overlying coating  214  of magnetic material preferably having a high coercivity relative to the under layer  212 . 
       FIG. 2D  illustrates the operative relationship between a perpendicular head  218  and a recording medium. The recording medium illustrated in  FIG. 2D  includes both the high permeability under layer  212  and the overlying coating  214  of magnetic material described with respect to  FIG. 2C  above. However, both of these layers  212  and  214  are shown applied to a suitable substrate  216 . Typically there is also an additional layer (not shown) called an “exchange-break” layer or “interlayer” between layers  212  and  214 . 
     In this structure, the magnetic lines of flux extending between the poles of the perpendicular head  218  loop into and out of the overlying coating  214  of the recording medium with the high permeability under layer  212  of the recording medium causing the lines of flux to pass through the overlying coating  214  in a direction generally perpendicular to the surface of the medium to record information in the overlying coating  214  of magnetic material preferably having a high coercivity relative to the under layer  212  in the form of magnetic impulses having their axes of magnetization substantially perpendicular to the surface of the medium. The flux is channeled by the soft underlying coating  212  back to the return layer (P 1 ) of the head  218 . 
       FIG. 2E  illustrates a similar structure in which the substrate  216  carries the layers  212  and  214  on each of its two opposed sides, with suitable recording heads  218  positioned adjacent the outer surface of the magnetic coating  214  on each side of the medium, allowing for recording on each side of the medium. 
       FIG. 3A  is a cross-sectional view of a perpendicular magnetic head. In  FIG. 3A , helical coils  310  and  312  are used to create magnetic flux in the stitch pole  308 , which then delivers that flux to the main pole  306 . Coils  310  indicate coils extending out from the page, while coils  312  indicate coils extending into the page. Stitch pole  308  may be recessed from the ABS  318 . Insulation  316  surrounds the coils and may provide support for some of the elements. The direction of the media travel, as indicated by the arrow to the right of the structure, moves the media past the lower return pole  314  first, then past the stitch pole  308 , main pole  306 , trailing shield  304  which may be connected to the wrap around shield (not shown), and finally past the upper return pole  302 . Each of these components may have a portion in contact with the ABS  318 . The ABS  318  is indicated across the right side of the structure. 
     Perpendicular writing is achieved by forcing flux through the stitch pole  308  into the main pole  306  and then to the surface of the disk positioned towards the ABS  318 . 
       FIG. 3B  illustrates a piggyback magnetic head having similar features to the head of  FIG. 3A . Two shields  304 ,  314  flank the stitch pole  308  and main pole  306 . Also sensor shields  322 ,  324  are shown. The sensor  326  is typically positioned between the sensor shields  322 ,  324 . 
       FIG. 4A  is a schematic diagram of one embodiment which uses looped coils  410 , sometimes referred to as a pancake configuration, to provide flux to the stitch pole  408 . The stitch pole then provides this flux to the main pole  406 . In this orientation, the lower return pole is optional. Insulation  416  surrounds the coils  410 , and may provide support for the stitch pole  408  and main pole  406 . The stitch pole may be recessed from the ABS  418 . The direction of the media travel, as indicated by the arrow to the right of the structure, moves the media past the stitch pole  408 , main pole  406 , trailing shield  404  which may be connected to the wrap around shield (not shown), and finally past the upper return pole  402  (all of which may or may not have a portion in contact with the ABS  418 ). The ABS  418  is indicated across the right side of the structure. The trailing shield  404  may be in contact with the main pole  406  in some embodiments. 
       FIG. 4B  illustrates another type of piggyback magnetic head having similar features to the head of  FIG. 4A  including a looped coil  410 , which wraps around to form a pancake coil. Also, sensor shields  422 ,  424  are shown. The sensor  426  is typically positioned between the sensor shields  422 ,  424 . 
     In  FIGS. 3B and 4B , an optional heater is shown near the non-ABS side of the magnetic head, e.g., to induce thermal protrusion, thereby reducing flying height of the head relative to the disk. A heater (Heater) may also be included in the magnetic heads shown in  FIGS. 3A and 4A . The position of this heater may vary based on design parameters such as where the protrusion is desired, coefficients of thermal expansion of the surrounding layers, etc. 
     In conventional magnetic medium, cluster sizes which comprise the magnetic medium affect the performance of the magnetic medium. The larger the magnetic clusters, the less amount of data may be stored to the magnetic medium. Put another way, by reducing the cluster size increased recording density may be achieved, according to preferred embodiments. This reduced cluster size may be achieved in several ways, according to various embodiments. In a first embodiment, the physical size of crystal grains may be reduced. In another embodiment, magnetic decoupling between neighboring crystal grains may be enhanced. According to another embodiment, size distribution may be narrowed, while avoiding degradation of the magnetic medium. In yet another embodiment, crystallographic texture may be improved while suppressing degradation of the magnetic medium to as great an extent as possible. 
       FIG. 5  illustrates a cross-sectional view depicting each layer of a perpendicular magnetic recording medium (PMRM)  500  according to one embodiment. A substrate layer  502  provides a foundation for subsequent layers, and may be comprised of any material known to one of skill in the art, such as glass, silicon, etc. Above the substrate layer  502 , a soft magnetic layer  504  is positioned to return magnetic flux from a magnetic head. Above the soft magnetic layer  504 , a crystalline seed layer  506  is positioned. The crystalline seed layer  506  has good crystallographic texture, which provides adequate crystal grain size for subsequent layers. This crystalline seed layer  506  is positioned below a series of interlayers comprised of a single metal, a metal alloy, combinations of metals, etc. The first interlayer  508  and second interlayer  512  may comprise Ru, a Ru alloy, etc., according to some embodiments. Positioned between the first and second interlayers  508 ,  512  is a third interlayer  510  having a body-centered cubic crystal (BCC) structure, B2 structure, C11b structure, L21 structure, D03 structure, etc. 
     When the third interlayer  510  utilizes a BCC structure, it may comprise Cr, V, etc., and preferably may have a thickness of between about 1.0 nm and about 3.0 nm. When the third interlayer  510  has any other structure, such as a B2, C11b, L21, D03, etc., structure, it preferably may be comprised of an intermetallic material or compound. For example, the intermetallic compound may include at least two elements selected from Al, Si, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Ru, Ta, and Re. Layered immediately above the second interlayer  512  is a perpendicular magnetic recording layer  514 , in some approaches. The perpendicular magnetic recording layer  514  has good crystallographic texture, according to one embodiment, due to at least one of several characteristics, including: reduced crystal grain size, narrower size distribution due to crystal rotation, and further enhancement of magnetic decoupling due to crystal rotation. 
     These positive characteristics of the perpendicular magnetic recording layer  514  may be caused by the third interlayer  510 , which leads to smaller magnetic crystal clusters in the recording layer  514 , since it has good crystalline quality from the first interlayer  508  and seed layer  506 , such that crystallinity and crystallographic texture of the layers above the third interlayer  510 , such as the second interlayer  514 , have better crystalline quality, as compared to conventional techniques of magnetic medium formation. 
     Above the perpendicular magnetic recording layer  514  is a protective overcoat layer  516 , and above the protective overcoat layer  516 , in some embodiments, a lubricating layer may be formed. Typically, the lubricating layer may be applied onsite as the magnetic disk drive having the PMRM therein is used. Although each layer is depicted having the same thickness in  FIG. 5 , the invention is not so limited. Each layer may have a different shape, thickness, length, depth, etc., and the design thereof may be determined by the affect desired. 
       FIG. 6A  illustrates a magnetic cluster  600 , according to one embodiment. In  FIG. 6A , seven adjacent crystal grains are shown. Of course, in use, more crystal gains are present in a magnetic medium.  FIG. 6A  is meant to illustrate the interaction of the crystal grains, and should not be construed as being limiting on the embodiments disclosed herein. Each crystal grain  601 ,  602 ,  603 ,  604 ,  605 ,  606 , and  607  has substantially identical rotational phase, and each crystal grain  601 ,  602 ,  603 ,  604 ,  605 ,  606 , and  607  forms a magnetic coupling  608  with all in-phase neighbors, creating a magnetic cluster  600  of seven grains. There may be magnetic clusters with more or less crystal grains, according to various embodiments. This pattern may be repeated across all or some of a magnetic medium, according to some embodiments. 
       FIG. 6B  illustrates seven adjacent crystal grains, some of which form a magnetic cluster  610 , according to one embodiment. Crystal grains  611 ,  616 , and  617  have substantially identical rotational phase, while crystal grains  612 ,  613 ,  614  and  615  are out-of-phase with  611 ,  616 , and  617  and with each other. Each in-phase crystal grain  611 ,  616 , and  617  forms a magnetic coupling  618  with all in-phase neighbors, creating a magnetic cluster  610  of three grains. This pattern may be repeated across all or some of a magnetic medium, according to some embodiments. 
       FIG. 6C  illustrates seven adjacent crystal grains, some of which form a magnetic cluster  620 , according to one embodiment. Crystal grains  626  and  627  have substantially identical rotational phase, while crystal grains  621 ,  622 ,  623 ,  624  and  625  are out-of-phase with  626  and  627  and with each other. In-phase crystal grains  626  and  627  form a magnetic coupling  628 , creating a magnetic cluster  620  of two grains. This pattern may be repeated across all or some of a magnetic medium, according to some embodiments. 
     Now referring to  FIG. 7 , one effect of smaller cluster size of the third interlayer is shown by the crystal grain distribution of the second interlayer, according to one embodiment. As can be seen, line  702 , which is the crystal angle difference of neighboring grains formed using conventional magnetic medium formation techniques, has a narrow distribution around 0 degree rotation, indicating that most of the crystal grains have the same or similar crystallography. In contrast, line  704 , which is the crystal angle difference of neighboring grains formed using magnetic medium formation techniques disclosed herein, has a wide distribution around 0 degrees, indicating that the crystal grains have different crystallography due to crystal rotation. 
     EXPERIMENTS 
     A PMRM  900  having a cross-sectional structure as shown in  FIG. 9  was produced using a sputtering apparatus. A soft magnetic underlayer  904 , a seed layer  906 , a first interlayer  908 , a second interlayer  910 , a perpendicular magnetic recording layer  912 , and a protective overcoat layer  914  were stacked in succession on a substrate  902  using DC magnetron sputtering, and a sample for evaluation was prepared (Comparative Example 1). A glass substrate of diameter 65 mm and thickness 0.635 mm was used for the substrate  902 . The substrate  902  was not heated. The soft magnetic underlayer  904  had a composite structure in which, under conditions of Ar gas pressure 0.7 Pa, an Fe-34 at % Co-10 at % Ta-5at % Zr alloy film of thickness 15 nm was formed, a Ru film of thickness 0.6 nm was stacked thereon, and another Fe-34at % Co-10 at % Ta-5 at % Zr alloy film of thickness 15 nm was stacked thereon. The seed layer  906  was an Ni-8 at % Cr-6 at % W alloy film of thickness 7 nm which was formed under conditions of Ar gas pressure 0.7 Pa. The first interlayer  908  was a Ru film of thickness 8 nm which was formed under conditions of Ar gas pressure 1 Pa. The second interlayer  910  was a Ru film of thickness 8 nm which was formed under conditions of Ar gas pressure 5 Pa. The perpendicular magnetic recording layer  912  was a Co-21 at % Cr-18 at % Pt-5 mol % SiO 2 -5 mol % TiO 2 -1.5 mol % Co 3 O 4  alloy film of thickness 13 nm which was formed under conditions of gas pressure of 5 Pa using a mixed gas comprising 1.5 vol % oxygen with Ar. The protective overcoat layer  914  was a carbon film of thickness 3.5 nm which was formed under conditions of 0.6 Pa using a mixed gas comprising 8 vol % nitrogen with Ar. This medium was used to evaluate microstructure and magnetic clusters, and the recording and reproduction characteristics were not evaluated, so no lubricant layer was provided. 
     The difference between Exemplary Embodiments 1 and 2, and Comparative Example 1 as shown in Table 1 in  FIG. 8  lies in the absence or presence of a third interlayer which is positioned between the first and second interlayers: the media in Exemplary Embodiments 1-2 have the third interlayer  916 , while this interlayer is not present in Comparative Example 1. 
     The crystal grain size of the media of Exemplary Embodiments 1 and 2, and Comparative Example 1 were measured using a thin-film X-ray diffraction apparatus. This process involved measuring the in-plane diffraction spectra, and the spectra obtained were analyzed, and the crystal grain size was obtained using the Scherrer method. As shown in Table 1, in  FIG. 8 , it is clear that the grain size of the second Ru interlayer and the perpendicular magnetic recording layer in the media of Exemplary Embodiments 1 and 2 was finer than that of Comparative Example 1. 
     The actual cluster size and distribution were then measured by a process involving analysis of the minor loop, using a Kerr effect magnetic characteristics evaluation apparatus. The saturation magnetization value Ms measured by means of a vibrating sample magnetometer was used for calibrating the absolute value of magnetization. As shown by the results in Table 1, in  FIG. 8 , it is clear that the media of Exemplary Embodiments 1 and 2 had a finer cluster size than the medium of Comparative Example 1 by around 11% to 15%, and the distribution was narrower by at least 10 points. This was consistent with the results from analysis of TEM images. 
     As described above, a preferred structure of the third interlayer is a BCC structure, and therefore it preferably comprises Cr and/or V, or an alloy in which one of Cr and V are a primary component. 
     A medium having the same structure as that of Exemplary Embodiment 1 was produced in which the third interlayer was a Cr—Ti alloy film of thickness 2.5 nm which was formed under conditions of Ar gas pressure 0.9 Pa (Exemplary Embodiment 3). In this exemplary embodiment, two targets, a Cr target and a Ti target, were sputtered at the same time, and the alloy composition was changed by varying the sputtering proportions. 
     A medium having the same structure as that of Exemplary Embodiment 1 was produced in which the third interlayer was replaced with a Cr—V alloy film of thickness 2.5 nm which was formed under conditions of Ar gas pressure 0.9 Pa (Exemplary Embodiment 4). In this exemplary embodiment, two targets, a Cr target and a V target, were sputtered at the same time, and the alloy composition was changed by varying the sputtering proportions. 
     A preferred compositional range of the third interlayer comprising a CrTi alloy or a CrV alloy is described using the media of Exemplary Embodiments 3 and 4. According to the results of testing on Exemplary Embodiments 3 and 4, the effect of refining the crystal grain size is greater when the Ti content is 15 at %-80 at %, more preferably 40 at %-60 at %, with respect to Cr in the case of a CrTi alloy, and when the V content is 30 at %-70 at %, more preferably 40 at %-60 at %, with respect to Cr in the case of a CrV alloy. However, if the added concentration of Ti exceeds 30 at % in the case of a CrTi alloy, the crystallinity markedly deteriorates, and the crystallinity of the second Ru or Ru alloy interlayer above, and also of the perpendicular magnetic recording layer is lost, and this is clearly undesirable. In an overall context, these results indicate that the Ti content is preferably 15 at %-25 at % with respect to Cr in the case of a CrTi alloy, and the V content is preferably 30 at %-70 at %, more preferably 40 at %-60 at %, with respect to Cr in the case of a CrV alloy. 
     Referring now to  FIG. 10 , a method  1000  for forming a perpendicular magnetic recording medium is shown according to one embodiment. The method may be performed in any desired environment, and may include any of the embodiments and/or approaches described herein. The method  1000  may include more or less steps than those described below. For example, in one embodiment, the method  1000  may include operations  1008 - 1010  only, not operations  1002 - 1006  and  1012 , etc. 
     For each of the operations described below, layers of a perpendicular magnetic recording medium are formed. Any formation method known in the art may be used to form these layers, such as sputtering, plating, electroplating, vapor deposition, plasma enhanced vapor deposition (PEVD), chemical vapor deposition (CVD), etc., and different formation methods may be used for all or some of the layers. 
     In operation  1002 , a substrate is formed. The substrate may comprise glass, silicon, or any other material as known in the art. 
     In operation  1004 , a soft magnetic layer is formed above the substrate and below a subsequent crystalline seed layer. The soft magnetic layer may be comprised of any material known in the art, such as FeCoTaZr, a FeCoTaZr alloy, Ru, a Ru alloy, combinations thereof, etc. In one approach, the soft magnetic layer may adhere the substrate to a crystalline seed layer formed subsequently in operation  1006 . 
     In operation  1006 , a crystalline seed layer is formed above the soft magnetic layer and below a subsequent multilayer interlayer. Any material may be used to form the seed layer as would be known to one of skill in the art, such as NiCrW, a NiCrW alloy, etc. The seed layer may have a thickness of about 2 nm to about 10 nm, such as about 7 nm. In one approach, the crystalline seed layer may have good crystallographic texture that provides adequate crystal grain size for subsequent layers, such as the multilayer interlayer and perpendicular magnetic recording layers formed in the next two operations. 
     In operation  1008 , a multilayer interlayer is formed above the soft magnetic layer. In one embodiment, the multilayer interlayer includes three layers, a first interlayer formed above the substrate, a second interlayer formed above the first interlayer, and a third interlayer formed between the first interlayer and the second interlayer. Of course, any number of interlayers may be used, including four, five, six, etc., as would enhance the properties of the layers formed subsequent to the interlayer. 
     According to one embodiment, the first interlayer and second interlayer may comprise Ru or a Ru alloy. In another approach, the first interlayer and the second interlayer may each have a thickness of between about 6 nm and about 10 nm, such as about 8 nm. 
     In another approach, the third interlayer may have a body-centered-cubic (BCC) structure, or a structure closely related to BCC, such as B2, C11b, L21, and D03. Additionally, for BCC structures, the third interlayer may comprise at least one of Cr, Ti, and V, such as CrTi having a Cr concentration of about 20 at %, CrV having a Cr concentration of about 50 at %, or alloys thereof. For B2, C11b, L21, and D03 structures, the third interlayer may comprise an intermetallic compound, such as at least two of Al, Si, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Ru, Ta, and Re. According to one embodiment, the third interlayer may have a thickness of between about 0.5 nm and about 3.0 nm, such as about 2.0 nm. 
     In operation  1010 , a perpendicular magnetic recording layer is formed above the multilayer interlayer. In one embodiment, the perpendicular magnetic recording layer may comprise CoCrPtSiO 2 TiO 2 Co 3 O 4  or an alloy thereof, or any other material known in the art. In some approaches, the perpendicular magnetic recording layer may have a thickness of about 7 nm to about 20 nm, such as about 16 nm. 
     In operation  1012 , a protective overcoat layer is formed above the perpendicular magnetic recording layer for protecting the perpendicular magnetic recording layer. The protective overcoat layer may comprise any material known in the art, such as alumina, carbon and carbon compounds, etc. In some embodiments, the protective overcoat layer may have a thickness of about 0.5 nm to about 2 nm, such as about 1 nm. 
     According to another embodiment, a system includes a perpendicular magnetic recording medium as described in any of the embodiments described above, at least one magnetic head for reading from and/or writing to the perpendicular magnetic recording medium, a magnetic head slider for supporting the magnetic head, and a control unit coupled to the magnetic head for controlling operation of the magnetic head. This embodiment may include any of the descriptions relating to  FIGS. 1-4B . 
     While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of an embodiment of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.