Abstract:
An apparatus for reorienting the magnetic anisotropy of the soft underlay of a magnetic recording disc having a diameter less than that of the pallet, which operates by first heating the disc, then using a heat transfer plate to cool the disc in the presence of a magnetic field having a radial direction emanating from the center of the magnetic recording disc.

Description:
RELATED APPLICATIONS 
     None. 
     BACKGROUND 
     None. 
     SUMMARY 
     An apparatus for reorienting the magnetic anisotropy of the soft underlay of a magnetic recording disc having a diameter less than that of the pallet, which operates by first heating the disc, then using a heat transfer plate to cool the disc in the presence of a magnetic field having a radial direction emanating from the center of the magnetic recording disc. 
     Embodiments of the invention relate to an apparatus and method for reorienting the magnetic anisotropy of a magnetic recording disc, including a pallet holding a plurality of magnetic recording discs, in which at least a portion of the plurality of magnetic recording discs is smaller than the pallet; a plurality of heat transfer plates having a contacting surface; and a plurality of magnetic sources adjacent to at least a portion of the plurality of heat transfer plates, in which the magnetic sources individually have a magnetic center and provide a magnetic field having a radial direction emanating from the magnetic center, in which the heat transfer plate is configured to produce a radial magnetic pattern in a soft underlayer of a magnetic recording disc having a diameter less than that of the pallet. 
     As will be realized, this invention is capable of other and different embodiments, and its details are capable of modifications in various obvious respects, all without departing from this invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not as restrictive. These and various other features and advantages will be apparent from a reading of the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view of a magnetic disk drive of the related art. 
         FIG. 2  is a schematic representation of the film structure in accordance with a magnetic recording medium of the related art. 
         FIG. 3  is perspective view of a magnetic head and a magnetic disk of the related art. 
         FIG. 4  is a schematic view of a magnet array and a disc cross-sectional view of an embodiment of this invention. 
         FIG. 5  is a view of the calculated magnetic field at each disk. 
         FIG. 6  is a calculated view of the magnetic field magnitude at the surface of a disc. 
         FIG. 7  shows the magnetic array and resultant magnetic field. 
         FIG. 8  shows a pallet, which is used as a holding apparatus for the small form-factor discs. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the invention relate to perpendicular recording media, such as thin film magnetic recording disks having perpendicular recording, and to a method of manufacturing the media. Embodiments of the invention have particular applicability to high areal density magnetic recording media exhibiting low noise. 
     Embodiments of the invention describe an apparatus for making a perpendicular magnetic recording medium having a substrate and a magnetic underlayer on the substrate, the magnetic underlayer having an easy axis of magnetization substantially directed in a radial or transverse direction, and a process for manufacturing the perpendicular magnetic recording medium having such an axis of magnetization. 
     One embodiment of the apparatus uses the magnetic field produced by an array of permanent magnets to align the magnetic field of the soft underlayer to the desired magnetic orientation. Another embodiment uses electromagnets to align the magnetic field of the soft underlayer to the desired magnetic orientation. 
     The increasing demands for higher areal recording density impose increasingly greater demands on thin film magnetic recording media in terms of remanent coercivity (Hr), magnetic remanence (Mr), coercivity squareness (S*), medium noise, i.e., signal-to-medium noise ratio (SMNR), and narrow track recording performance. It is extremely difficult to produce a magnetic recording medium satisfying such demanding requirements. 
     The linear recording density can be increased by increasing the Hr of the magnetic recording medium, and by decreasing the medium noise, as by maintaining very fine magnetically non-coupled grains. Medium noise in thin films is a dominant factor restricting increased recording density of high-density magnetic hard disk drives, and is attributed primarily to inhomogeneous grain size and intergranular exchange coupling. Accordingly, in order to increase linear density, medium noise must be minimized by suitable microstructure control. 
     According to the domain theory, a magnetic material is composed of a number of submicroscopic regions called domains. Each domain contains parallel atomic moments and is always magnetized to saturation, but the directions of magnetization of different domains are not necessarily parallel. In the absence of an applied magnetic field, adjacent domains may be oriented randomly in any number of several directions, called the directions of easy magnetization, which depend on the geometry of the crystal. The resultant effect of all these various directions of magnetization may be zero, as is the case with an unmagnetized specimen. When a magnetic filed is applied, the domains most nearly parallel to the direction of the applied field grow in size at the expense of the others. This is called boundary displacement of the domains or the domain growth. A further increase in magnetic field causes more domains to rotate and align parallel to the applied field. When the material reaches the point of saturation magnetization, no further domain growth would take place on increasing the strength of the magnetic field. 
     A magnetic material is said to possess a uniaxial anisotropy when all domains are oriented in the same direction in the material. On the other extreme, a magnetic material is said to be isotropic when all domains are oriented randomly. 
     The ease of magnetization or demagnetization of a magnetic material depends on the crystal structure, grain orientation, the state of strain, and the direction and strength of the magnetic field. The magnetization is most easily obtained along the easy axis of magnetization but most difficult along the hard axis of magnetization. 
     Magnetic quenching to achieve a desired magnetic orientation may be achieved using various apparatuses and methods. 
     “Anisotropy energy” is the difference in energy of magnetization for these two extreme directions, namely, the easy axis of magnetization and the hard axis of magnetization. For example, a single crystal of iron, which is made up of a cubic array of iron atoms, tends to magnetize in the directions of the cube edges along which lie the easy axes of magnetization. A single crystal of iron requires about 1.4×10 5  ergs/cm 3  (at room temperature) to move magnetization into the hard axis of magnetization, which is along a cubic body diagonal. 
     The anisotropy energy U A  could be expressed in an ascending power series of the direction cosines between the magnetization and the crystal axes. For cubic crystals, the lowest-order terms take the form of Equation (1),
 
 U   A   =K   1 (α 1   2 α 2   2 +α 2   2 α 3   2 +α 3   2 α 1   2 )+ K   2 (α 1   2 α 2   2 α 3   2 )  (1)
 
     where α 1 , α 2  and α 3  are direction cosines with respect to the cube, and K 1  and K 2  are temperature-dependent parameters characteristic of the material, called anisotropy constants. 
     Anisotropy constants can be determined from (1) analysis of magnetization curves, (2) the torque on single crystals in a large applied field, and (3) single crystal magnetic resonance. 
     The total energy of a magnetic substance depends upon the state of strain in the magnetic material and the direction of magnetization through three contributions. The first two consist of the crystalline anisotropy energy of the unstrained lattice plus a correction that takes into account the dependence of the anisotropy energy on the state of strain. The third contribution is that of the elastic energy, which is independent of magnetization direction and is a minimum in the unstrained state. The state of strain of the crystal will be that which makes the sum of the three contributions of the energy a minimum. The result is that, when magnetized, the lattice is always distorted from the unstrained state, unless there is no anisotropy. 
     “Magnetostriction” refers to the changes in dimension of a magnetic material when it is placed in magnetic field. It is caused by the rotation of domains of a magnetic material under the action of magnetic field. The rotation of domains gives rise to internal strains in the material, causing its contraction or expansion. 
     The requirements for high areal density impose increasingly greater requirements on magnetic recording media in terms of coercivity, remanent squareness, low medium noise and narrow track recording performance. It is extremely difficult to produce a magnetic recording medium satisfying such demanding requirements, particularly a high-density magnetic rigid disk medium for longitudinal and perpendicular recording. The magnetic anisotropy of longitudinal and perpendicular recording media makes the easily magnetized direction of the media located in the film plane and perpendicular to the film plane, respectively. The remanent magnetic moment of the magnetic media after magnetic recording or writing of longitudinal and perpendicular media is located in the film plane and perpendicular to the film plane, respectively. 
     A substrate material conventionally employed in producing magnetic recording rigid disks comprises an aluminum-magnesium (Al—Mg) alloy. Such Al—Mg alloys are typically electrolessly plated with a layer of NiP at a thickness of about 15 microns to increase the hardness of the substrates, thereby providing a suitable surface for polishing to provide the requisite surface roughness or texture. 
     Other substrate materials have been employed, such as glass, e.g., an amorphous glass, glass-ceramic material which comprises a mixture of amorphous and crystalline materials, and ceramic materials. Glass-ceramic materials do not normally exhibit a crystalline surface. Glasses and glass-ceramics generally exhibit high resistance to shocks. 
     Almost all the manufacturing of a disk media takes place in clean rooms where the amount of dust in the atmosphere is kept very low, and is strictly controlled and monitored. After one or more cleaning processes on a non-magnetic substrate, the substrate has an ultra-clean surface and is ready for the deposition of layers of magnetic media on the substrate. The apparatus for depositing all the layers needed for such media could be a static sputter system or a pass-by system, where all the layers except the lubricant are deposited sequentially inside a suitable vacuum environment. 
       FIG. 1  shows the schematic arrangement of a magnetic disk drive  10  using a rotary actuator. A disk or medium  11  is mounted on a spindle  12  and rotated at a predetermined speed. The rotary actuator comprises an arm  15  to which is coupled a suspension  14 . A magnetic head  13  is mounted at the distal end of the suspension  14 . The magnetic head  13  is brought into contact with the recording/reproduction surface of the disk  11 . The rotary actuator could have several suspensions and multiple magnetic heads to allow for simultaneous recording and reproduction on and from both surfaces of each medium. 
     An electromagnetic converting portion (not shown) for recording/reproducing information is mounted on the magnetic head  13 . The arm  15  has a bobbin portion for holding a driving coil (not shown). A voice coil motor  19  as a kind of linear motor is provided to the other end of the arm  15 . The voice motor  19  has the driving coil wound on the bobbin portion of the arm  15  and a magnetic circuit (not shown). The magnetic circuit comprises a permanent magnet and a counter yoke. The magnetic circuit opposes the driving coil to sandwich it. The arm  15  is swingably supported by ball bearings (not shown) provided at the upper and lower portions of a pivot portion  17 . The ball bearings provided around the pivot portion  17  are held by a carriage portion (not shown). 
     A magnetic head support mechanism is controlled by a positioning servo driving system. The positioning servo driving system comprises a feedback control circuit having a head position detection sensor (not shown), a power supply (not shown), and a controller (not shown). When a signal is supplied from the controller to the respective power supplies based on the detection result of the position of the magnetic head  13 , the driving coil of the voice coil motor  19  and the piezoelectric element (not shown) of the head portion are driven. 
     A cross sectional view of a conventional longitudinal recording disk medium is depicted in  FIG. 2 . A longitudinal recording medium typically comprises a non-magnetic substrate  20  having sequentially deposited on each side thereof an underlayer  21 ,  21 ′, such as chromium (Cr) or Cr-alloy, a magnetic layer  22 ,  22 ′, typically comprising a cobalt (Co)-base alloy, and a protective overcoat  23 ,  23 ′, typically containing carbon. Conventional practices also comprise bonding a lubricant topcoat (not shown) to the protective overcoat. Underlayer  21 ,  21 ′, magnetic layer  22 ,  22 ′, and protective overcoat  23 ,  23 ′, are typically deposited by sputtering techniques. The Co-base alloy magnetic layer deposited by conventional techniques normally comprises polycrystallites epitaxially grown on the polycrystal Cr or Cr-alloy underlayer. 
     A conventional perpendicular recording disk medium, shown in  FIG. 3 , is similar to the longitudinal recording medium depicted in  FIG. 2 , but with the following differences. First, a conventional perpendicular recording disk medium has soft magnetic underlayer  31  of an alloy such as Permalloy instead of a Cr-containing underlayer. Second, as shown in  FIG. 3 , magnetic layer  32  of the perpendicular recording disk medium comprises domains oriented in a direction perpendicular to the plane of the substrate  30 . Also, shown in  FIG. 3  are the following: (a) read-write head  33  located on the recording medium, (b) traveling direction  34  of head  33  and (c) transverse direction  35  with respect to the traveling direction  34 . 
     The underlayer and magnetic layer are conventionally sequentially sputter deposited on the substrate in an inert gas atmosphere, such as an atmosphere of pure argon. A conventional carbon overcoat is typically deposited in argon with nitrogen, hydrogen or ethylene. Conventional lubricant topcoats are typically about 20 Å thick. 
     It is recognized that the magnetic properties, such as Hr, Mr, S* and SMNR, which are critical to the performance of a magnetic alloy film, depend primarily upon the microstructure of the magnetic layer which, in turn, is influenced by one or more underlying layers on which it is deposited. It is also recognized that an underlayer made of soft magnetic films is useful in perpendicular recording media because a relatively thick (compared to magnetic layer) soft underlayer provides a return path for the read-write head and amplifies perpendicular component of the write field in the recording layer. However, Barkhausen noise caused by domain wall motions in the soft underlayer can be a significant noise source. Since the orientation of the domains can be controlled by the uniaxial anisotropy, introducing a uniaxial anisotropy in the soft underlayer would be one way to suppress Barkhausen noise. When the uniaxial anisotropy is sufficiently large, the domains would preferably orient themselves along the anisotropy axis. 
     The uniaxial anisotropy could be controlled in several ways in the soft magnetic thin film materials. The most frequently applied methods are post-deposition annealing while applying a magnetic field and applying a bias magnetic field during deposition. However, both methods can cause complications in the disk manufacturing process. 
     A “soft magnetic” material is material that is easily magnetized and demagnetized. As compared to a soft magnetic material, a “hard magnetic” material is one that neither magnetizes nor demagnetizes easily. The problem of making soft magnetic materials conventionally is that they usually have many crystalline boundaries and crystal grains oriented in many directions. In such metals, the magnetization process is accompanied by much irreversible Block wall motion and by much rotation against anisotropy, which is usually irreversible. The preferred soft material would be a material fabricated by some inexpensive technique that results in all crystal grains being oriented in the same or nearly the same direction. However, “all grains” oriented in the same direction would be very difficult to produce and would not be the “preferred soft material.” In fact, very high anisotropy is not desirable. 
     A single disk deposition system includes a sputtering chamber having a sputter target located outside of the outer diameter of the disc. Atoms arrive obliquely to the plane of the disk, producing thin films with columnar grains that are magnetically oriented towards the vapor source and at a non-zero angle with respect to normal (i.e., a radial residual anisotropy). However, in the soft underlayer (“SUL”) employed in perpendicular media, the preferred magnetic orientation is toward the rim of the disc. Magnetic field induced pair ordering can also give rise to anisotropy in soft magnetic alloys. For circular static sputter sources as mentioned above, this anisotropy tends to be radial as well, reinforcing the shape anisotropy arising from oblique deposition. 
     However, when the same cathodes are used to deposit media on multiple small form-factor discs ( FIG. 8 ) that are arranged about the center of the sputter cathode, neither the angle of the incident vapor nor the magnetic field in the plane of the disc is symmetrical with the axis of rotation of the discs. As a consequence, the media crystal orientation has little radial anisotropy. Thus a means is needed to orient the microcrystals in the radial direction. To mitigate this problem, the SUL layer is reoriented as described herein. Another potential benefit is that the thickness of the SUL layer can be reduced as the anisotropy within the SUL layer increases, thereby allowing a thinner SLTL layer to be utilized; a potential manufacturing cost saving. 
     Using an apparatus similar to that shown in  FIG. 8 , without a step of reorienting the SUL layer, would produce a difference in the anisotropy orientation unless the disks were annealed as described herein. 
     Embodiments of the invention provide an apparatus for reorienting the magnetic anisotropy in the soft underlayer of a magnetic recording media suitable for high areal recording density exhibiting high SMNR. Embodiments of the invention also disclose a method of using the disclosed apparatus to achieve the desired orientation of the magnetic anisotropy. The underlayer is “soft” because it made of a soft magnetic material and it is called an “underlayer” because it resides under a recording layer. 
     In this apparatus, magnetic layers deposited on multiple small form-factor discs (e.g., 27 mm outer diameter) that lack the requisite radial anisotropy because they were deposited in a system designed for larger (e.g., 95 mm outer diameter) single discs, can be reoriented properly. This is done in a 2-step process where the discs are first heated in a standard heating station, then transferred to a cooling station where they are cooled in a magnetic field. Cooling the discs is equivalent to transferring heat out of the discs, therefore the heat transfer plates may also be referred to as cooling plates. A separate magnetic field is provided for each disc (see  FIG. 4 ). 
     The heating station may be the MDP-250, manufactured by Intevac, Inc., or its functional equivalent. The MDP-250 is a magnetic media deposition system which includes multiple process stations, including a heating station. The method described herein may be used with any heating station that provides substantially the same heating functionality as the MDP-250. 
     The cooling station may be the Seagate-designed PCS-3, or its functional equivalent. The method described herein may be used with any cooling station that provides substantially the same cooling functionality as the PCS-3. 
     In a preferred embodiment, each magnetic field is generated by small permanent magnetic arrays which are embedded in the faces of the cooling station heat transfer plates (see  FIG. 6 ). 
     In a second embodiment, each magnetic field is generated by electromagnets. However, this embodiment has the disadvantage of being more complicated and more expensive. 
     The magnets embedded in the faces of the cooling station heat transfer plates may comprise any combination of permanent magnets or electromagnets. 
     The cooling plates and magnets are then moved into position before injecting a gas that cools the discs. While cooling, a radial magnetic field ( FIGS. 3-5 ) is maintained at the surface of the disc. The magnetic field is reduced when the plates are separated prior to removing the discs from the chamber. 
     In another embodiment, the cooling function does not require heat transfer plates, but instead relies on convection, radiation, or any combination thereof while maintaining the appropriate magnetic field. Typically, the discs will cool more slowly if there is no conduction through the heat transfer plates, therefore this embodiment is not preferred. 
     The term “induced” is used because the magnetic field is an external factor that causes anisotropy. Unlike magnetic anisotropy caused by magnetocrystalline or shape anisotropy, anisotropy formed by a magnetic field is considered as induced anisotropy. 
     In accordance with embodiments of this invention, the substrates that may be used include glass, glass-ceramic, NiP/aluminum, metal alloys, plastic/polymer material, ceramic, glass-polymer, composite materials or other non-magnetic materials. 
     A soft underlayer should preferably be made of soft magnetic materials and the recording layer should preferably be made of hard magnetic materials. A soft underlayer is relatively thick compared to other layers. Any layers between the soft underlayer and the recording layer is called interlayer or intermediate layer. An interlayer can be made of more than one layer of non-magnetic materials. The purpose of the interlayer is to prevent an interaction between the soft magnetic underlayer and recording layer. An interlayer could also promote the desired properties of the recording layer. Conventional (longitudinal) media do not have a soft magnetic underlayer. Therefore, the layers named as “underlayer,” “seed layer,” “sub-seed layer,” or “buffer layer” of longitudinal media are somewhat equivalent to the intermediate layer(s) of perpendicular media. 
     The above description is presented to enable a person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the preferred embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Thus, this invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
     This application discloses several numerical range limitations. Persons skilled in the art would recognize that the numerical ranges disclosed inherently support any range within the disclosed numerical ranges even though a precise range limitation is not stated verbatim in the specification because this invention can be practiced throughout the disclosed numerical ranges. A holding to the contrary would “let form triumph over substance” and allow the written description requirement to eviscerate claims that might be narrowed during prosecution simply because the applicants broadly disclose in this application but then might narrow their claims during prosecution. Where the term “plurality” is used, that term shall be construed to include the quantity of one, unless otherwise stated. The entire disclosure of the patents and publications referred in this application are hereby incorporated herein by reference. Finally, the implementations described above and other implementations are within the scope of the following claims.