Patent Publication Number: US-6212023-B1

Title: Quickly written servo-patterns for magnetic media including depositing after writing

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a divisional of application Ser. No. 08/757,908 filed Nov. 27, 1996, which issued as U.S. Pat. No. 5,991,104 on Nov. 23, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to magnetic media for recording information, and, more particularly, to disc drives with magnetic head assemblies which record information in tracks on thin film discs. 
     The computer industry continually seeks to reduce size of computer components and to increase the speed at which computer components operate. To this end, it is desired to reduce the size required to magnetically record bits of information. It is concomitantly important to maintain the integrity of the information as size is decreased, and magnetic storage of information must be virtually 100% error free. Moreover, the methods used to reduce size, increase speed and maintain information integrity in computer components must be very reproducible in a manufacturing setting and must not be overly costly. The present invention seeks to address these goals in a disc drive. 
     Disc drives which magnetically record, store and retrieve information on disc-shaped media are widely used in the computer industry. A write transducer is used to record information on the disc, and a read transducer is used to retrieve information from the disc. The reading and writing processes may be performed by a single structure, i.e., a read-write transducer, or alternatively may be performed by separate structures. In either case, the read transducer and the write transducer are generally both located on a single magnetic head assembly. The magnetic head assembly may include an air bearing slider which suspends the magnetic head assembly relative to the rotating disc by “flying” off air on the disc surface. 
     The magnetic head assembly is mounted on the end of a support or actuator arm, which positions the head radially on the disc surface. If the actuator arm is held stationary, the magnetic head assembly will pass over a circular path on the disc known as a track, and information can be read from or written to that track. Each concentric track has a unique radius, and reading and writing information from or to a specific track requires the magnetic head to be located above the track. By moving the actuator arm, the magnetic head assembly is moved radially on the disc surface between tracks. 
     The disc drive must be able to differentiate between tracks on the disc and to center the magnetic head over any particular track. Most disc drives use embedded “servo patterns” of magnetically recorded information on the disc. The servo patterns are read by the magnetic head assembly to inform the disc drive of track location. Tracks typically include both data sectors and servo patterns. Each data sector contains a header followed by a data section. The header may include synchronization information to synchronize various timers in the disc drive to the speed of disc rotation, while the data section is used for recording data. 
     Each servo pattern typically includes a “gray code” and a “servo burst”. The gray code indexes the radial position of the track such as through a track number, and may also provide a circumferential index such as a sector number. The servo burst is a centering pattern to precisely position the head over the center of the track. Each servo burst includes magnetic transitions on the inside of the track interleaved with magnetic transitions on the outside of the track. If the magnetic head is centered over the track, the signal read from the inside transitions will be equal and opposite to the signal read from the outside transitions. If the magnetic head is toward the inside of the track, the signal from the inside transitions will predominate, and vice versa. By comparing portions of the servo burst signal, the disc drive can iteratively adjust the head location until a zeroed position error signal is returned from the servo bursts indicating that the head is properly centered with respect to the track. 
     Servo patterns are usually written on the disc during manufacture of the disc drive, after the drive is assembled and operational. The servo pattern information, and particularly the track spacing and centering information, needs to be located very precisely on the disc. However, at the time the servo patterns are written, there are no reference locations on the disc surface which can be perceived by the disc drive. Accordingly, a highly specialized device known as a “servo-writer” is used during writing of the servo-patterns. Largely because of the locational precision needed, servo-writers are fairly expensive, and servo-writing is a time consuming process. 
     Most servo-writers operate using the disc drive&#39;s own magnetic head. The servo-writer takes precise positional references to properly position the heads in the disc drive for the writing of the servo patterns, and to properly space the tracks with respect to one another on the disc surface. For instance, the servo writer may have a physical position sensor which takes a positional reference from the axis of the drive spindle, and may have an optical position sensor which determines the location of the magnetic heads with respect to the axis of the drive spindle. With precise positioning of the magnetic head known, the magnetic head of the disc drive is used to write the servo pattern on the disc. The servo writer may also include a magnetic head which writes a clock track at an outer radius of the disc. Once written, servo patterns serve as the positional references on the disc surface used by the disc drive during the entire life of the disc drive. The servo patterns are used to properly center the head over the desired track prior to reading or writing any data information from or to that track. 
     One approach to avoid traditional servo-writing has been to injection mold or stamp servo patterns on a plastic substrate disc. The magnetic material layer is then applied at a consistent thickness over the entire disc surface, including the depressions and protrusions in the servo patterns. After the disc is mechanically fabricated (i.e., after all the layers are applied), a magnetic bias is recorded on the servo patterns. For instance, a first magnetic field may magnetically initialize the entire disc at a one setting. Then a second magnetic field, localized at the surface of the disc and perhaps provided by the magnetic head of the drive, is used to magnetize the protruding portions of the servo patterns relative to the depressions. Because the protrusions are closer than the depressions to the magnetic initialization, the magnetization carried by the protrusions may be different than the magnetization carried by the depressions. When read, the resulting disc servo patterns show magnetic transitions between the depressions and the protrusions. This approach, referred to as a PERM disc, is being pursued by the Sony Corp. 
     While servo patterns in PERM discs do not require much of the specialized servo-writing equipment otherwise necessary, other problems have arisen. The depressions in the disc surface have a detrimental effect on the flyability of the air bearing slider. Additionally, in traditional servo-patterns, the magnitude of the position error signal from the servo pattern is based on transitions from magnetism in one direction to magnetism in the opposite direction. Because the depressions in the PERM servo patterns make no significant contribution ot the output signal, the resultant position error signal of the servo patterns is half that of a traditional servo pattern. In practice, perhaps due to imperfect saturation of the magnetic medium in the depressions, the resultant position error signal of PERM servo patterns has an even lower signal to noise ratio, typically around one-third that of the traditional servo signal. Other methods to reduce the cost of servo-writing without the drawbacks of the PERM disc are desired. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a method and structure to create servo-patterns on magnetic discs without the use of servowriters. In one embodiment, a master servo-writing medium is brought into close proximity with the product “slave” disc, and the two are subjected to an external magnetic field which assists in transferring magnetic servo-patterns to the slave disc in a print-through process. The preferred external magnetic field alternates and rotates with respect to the master/slave combination. With a demagnetized product/slave disc, an assist field strength which is of a magnitude greater than the coercivity of the slave disc but lower than the coercivity of the master media can be used to transfer a magnetic servo-pattern onto the slave disc. In a different embodiment, any of several methods can be used to magnetically alter the magnetic layer in a servo-pattern configuration. For instance, portions of the magnetic layer may be photolithographically removed to leave only the signal generating portions of the servo-patterns. Alternative to removal of portions of the magnetic layer, those portions may have their crystal structure altered to render them non-magnetic, thus similarly leaving only the signal generating portions of the servo-patterns intact. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plan view of a computer disc drive. 
     FIG. 2 is a side view of the computer disc drive of FIG.  1 . 
     FIG. 3 is a greatly enlarged, cross-sectional perspective view of a portion of the thin film magnetic disc of FIG. 1, schematically showing magnetic flux. 
     FIG. 4 is a greatly enlarged top plan view of a servo pattern portion of the thin film magnetic disc of FIG. 1, schematically showing magnetic flux. 
     FIG. 4 a  is a greatly enlarged top plan view of an alternative servo pattern portion of the thin film magnetic disc. 
     FIG. 5 is a perspective schematic view of a slave disc being demagnetized in a magnetic field. 
     FIG. 6 is a graph of current versus time for the electromagnet of FIG. 5 during the magnetic print-through process. 
     FIG. 7 is a perspective view of a slave disc and master disc in alignment on a spindle for placement in the magnetic assist field of FIG.  5 . 
     FIG. 8 is a greatly enlarged, cross-sectional view of a portion of the slave disc and master disc of FIG. 7 during print through, schematically showing magnetic flux. 
     FIG. 9 is a graph of the circumferential component of the write field versus time during the magnetic print-through process. 
     FIG. 10 shows a magnetization curve for the master medium superimposed with a magnetization curve for the slave medium during alignment between the maximum electromagnet field and the demagnetizing field of the master medium. 
     FIG. 11 shows an alternative embodiment of the electromagnet of FIG.  5 . 
     FIG. 12 is a greatly enlarged top plan view of the magnetic medium of an alternative embodiment of the present invention, schematically showing magnetic flux. 
     FIG. 13 is a greatly enlarged top plan view of the magnetic medium of another alternative embodiment of the present invention, schematically showing magnetic flux. 
     FIG. 14 is a cross-sectional view of a portion the magnetic medium of either of FIGS. 12 or  13 , taken along line  14 — 14 . 
     FIG. 15 is a cross-sectional view of the magnetic medium of either of FIGS. 12 or  13 , alternative to FIG.  14 . 
    
    
     While the above-identified drawing figures set forth preferred embodiments, other embodiments of the present invention are also contemplated, some of which are noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 represent a disc drive structure  10 . Disc drive assembly  10  includes disc pack  12  and E-block assembly  14 . Disc pack  12  includes discs  16  stacked on drive spindle  18 . During use of the disc drive  10 , drive spindle  18  rotates discs  16  about axis  20 . Polar coordinates  21  are established based on the geometry of disc  16 , with the perpendicular distance from axis  20  to any location on disc  16  being a radius r, the circumferential dimension being Θ, and the axial dimension being z. 
     E-block assembly  14  includes servo spindle  22  and a plurality of actuator arms  24 . Each actuator arm  24  carries one or two flexure arms or suspension arms  26 . Each suspension arm  26  supports an air bearing magnetic head assembly  28  adjacent a surface of a disc  16 . As disc  16  rotates about drive spindle  18  at a high speed (such as 10 m/s or higher) relative to magnetic head assembly  28 , the aerodynamic properties of magnetic head assembly  28  cause assembly  28  to “fly” above the surface of disc  16 . The flying height of magnetic head assembly  28  above disc  16  is a function of the speed of rotation of disc  16 , the aerodynamic lift of the slider of magnetic head assembly  28 , and the spring tension in suspension arm  26 . 
     E-block assembly  14  is pivotable about pivot axis  30 . As E-block assembly  14  pivots, each magnetic head assembly  28  mounted at the tip of its suspension arm  26  swings through arc  32 . As each disc  16  rotates beneath its respective magnetic head assembly  28 , this pivoting motion allows the magnetic head assembly  28  to change track positions on its disc  16 . Each disc  16  has a landing zone  34  where the magnetic head assembly  28  lands, rests while the disc drive  10  is off, and takes off from when the disc drive  10  is started up. Each disc  16  has a data zone  36  where the magnetic head assembly  28  flies over the disc  16  and magnetically stores data. 
     To record information on the disc  16 , the write transducer on magnetic head assembly  28  creates a highly concentrated magnetic field. During writing, the strength of the concentrated magnetic field directly under the write transducer is greater than the coercivity of the recording medium (known as “saturating” the medium), and grains of the recording medium at that location are magnetized with a direction which matches the direction of the applied magnetic field. The grains of the recording medium retain their magnetization after the saturating magnetic field is removed. As the disc  16  rotates, the direction of the writing magnetic field is alternated based on bits of the information being stored, thereby recording a magnetic pattern on the track directly under the write transducer. 
     A magnetic medium  38  for disc  16  is illustrated in more detail in FIG.  3 . Magnetic medium  38  has a substrate  40  and an underlayer  42  deposited over the substrate  40 . Substrate  40  is preferably a nickel-phosphorous plated aluminum disc. Substrate  40  is relatively thick, such as about 0.1 inches, and provides the structural integrity for magnetic medium  38 . Other materials, such as glass or manganese-oxide, may also be suitable for substrate  40 . 
     Underlayer  42  is formed of a non-magnetic material, such as chromium or nickel-aluminum. Underlayer  44  is preferably 200 to 500 Angstroms thick. Underlayer  42  sets up a seeding crystallographic structure for proper crystal development in magnetic layer  44 . Underlayer  42  may be applied over substrate  40  by sputtering, and various sputter chamber parameters may contribute to the effectiveness of underlayer  42 . Other materials such as Mo, W, Ti, NiP, CrV and Cr alloyed with other substitutional elements have also been tried for underlayers, and workers skilled in the art will appreciate that any one of these types of underlayers may be found equivalently beneficial in applying the process of the present invention. 
     Magnetic layer  44  of a magnetic material is applied over underlayer  42 . Magnetic layer  44  is preferably formed of a cobalt-based alloy, such as a cobalt-chromium-tantalum alloy. The preferred cobalt-based magnetic layer  44  has a hexagonal close pack (HCP) crystal structure. Workers skilled in the art will appreciate that other types of magnetic layers may be equivalently used in practicing the present invention. 
     Magnetic layer  44  is preferably 100 to 300 Angstroms thick. Magnetic layer  44  may be applied over underlayer  42  by sputtering, and various sputter chamber parameters may contribute to the effectiveness of magnetic layer  18 . 
     To enhance the durability of the disc  16 , overcoat  46  is deposited over magnetic layer  44 . Overcoat  46  helps reduce wear of magnetic media  36  due to contact with the magnetic read-write head assembly  28 . Overcoat  46  also aids in corrosion resistance for the magnetic media  38 . Overcoat  46  preferably is a layer of sputtered amorphous carbon. Other materials which may be suitable for overcoat  46  include sputtered ceramic zirconium oxide and amorphous films of silicon dioxide. Overcoat  46  can be about 100 to 150 Angstroms thick, with a preferred thickness of about 120 Angstroms. Any of the substrate  40 , the underlayer  42  or the overcoat  46  may be textured as desired for beneficially affecting the tribology of the particular disc drive system  10 . 
     A lubricant layer  48  overlies overcoat  46 . Lubricant layer  48  also reduces wear and corrosion of the magnetic media  38 . The lubricant  48  is preferably a perfluoropolyether-based (PFPE) lubricant having a thickness of 10 to 20 Angstroms. Overcoat  46  and lubricant  48 , while not performing a magnetic function, greatly affect the tribology and wear and corrosion resistance in the disc drive system  10 . 
     Magnetic layer  44 , as originally deposited, is homogeneous in both the radial and circumferential directions, and carries no magnetic charge. After deposition of magnetic layer  44 , information is magnetically written on magnetic layer  44  as represented by + and − magnetization signs  50 ,  52 . In FIG. 3, multiple + and − magnetization signs  50 ,  52  and multiple magnetic flux arrows  54  are shown to indicate the direction of aligned magnetic domains and to indicate that numerous aligned domains contribute to each magnetic transition. The writing of the magnetic information occurs after disc  16  is fully fabricated including deposition of overcoat  46  and lubricant  48 . The magnetization is believed to be made up of numerous aligned magnetic domains in the structure of magnetic layer  44 . Data is then read from magnetic medium  38  by sensing the alternating direction of magnetization, that is, transition locations where the direction of aligned magnetic domains reverses. 
     FIG. 4 schematically shows an areal portion of servo-pattern information  56  magnetically recorded on disc  16 . Magnetization signs  50 ,  52  indicate the direction of magnetization from the aligned magnetic domains. In FIG. 4, transition boundaries  57  between areas of opposite magnetic domain alignment are shown in solid lines. The boundaries  61  of each track  58  are shown in small dashed lines, and a center line  59  of each track  58  is shown in larger dashed lines. The boundaries  61  of each track  58  and the center lines  59  are not recognizable by any physical properties of the magnetic medium  38 , but are shown for conceptual purposes only. During use of the disc drive  10 , the magnetic head assembly  28  is intended to be centered over a track  58  so the magnetic head assembly will accurately write information to and read information from that track  58 . In contrast to track boundaries  61  and center lines  59 , each transition boundary  57  is magnetically sensed by the magnetic head assembly  28  when it passes over the transition boundary  57 . 
     In the servo-patterns  56 , substantially all of the magnetic domains in magnetic medium  38  are aligned in one direction or the other. While transition boundaries  57  are shown in FIG. 4 as sharply defined areas, the true magnetic pattern may not have sharp transitions between opposite directions of magnetization The sharpness of the transition boundaries on a recording medium is one of the basic parameters in determining the density of the information which can be stored on the recording medium. 
     Servo pattern information  56  is magnetically written on magnetic medium  38  during manufacture of the disc drive  10 . Each servo pattern includes gray code information  60  and a servo burst  62 . Gray code information  60  contains indexing information to index each track  58  of the disc  16 . Each servo burst  62  includes a plurality of inside transitions  64 . Each servo burst  62  also includes a plurality of outside transitions  66 . Inside transitions  64  and outside transitions  66  are precisely located on the disc  16  in the radial direction to define the centerline  59  of each track  58 , and to maintain very consistent spacing between tracks  58 . 
     During use of the disc drive  10 , inside transitions  64  and outside transitions  66  are used to center the magnetic head  28  over a track  58 . The signal read from servo bursts  62  depends on the radial position of the magnetic head  28  with respect to the centerline  59  of a track  58 . If the magnetic head  28  is centered over the track  58 , the signal read from the inside transitions  64  will be equal to the signal read from the outside transitions  66 . If the magnetic head  28  is toward the inside of the track  58 , the signal from the inside transitions  64  will be stronger than the signal from the outside transitions  66 . If the magnetic head  28  is toward the outside of the track  58 , the signal from the outside transitions  66  will be stronger. By comparing portions of the servo burst  62  signal, the disc drive  10  can iteratively adjust the head  28  location until a zeroed position error signal is returned from the servo bursts  62 , indicating that the head  28  is properly centered with respect to the track  58 . 
     Traditionally, the servo patterns  56  are written on the magnetic medium  38  during manufacture with a servo writer. Writing of the magnetic signals requires two precisely positioned passes of the magnetic head  28  over each track  58 : one for the inside transitions  64  and one for the outside transitions  66 . The magnetic head  28  typically writes a signal which is around one track-width wide, considerably wider than either the inside transitions  64  or the outside transitions  66 . The only way the servo bursts  62  can be written with such a head  28  is by erasing on each pass part of what was written in the previous pass. The track-centered gray code information  60  is written by matching the magnetization direction during consecutive passes of the magnetic head  28 . This process of matching the magnetization of a previous pass to create a recorded magnetic transition which is wider than the width of the recording head is referred to as “stitching”. 
     FIG. 4 a  shows an alternative configuration for servo bursts  62 . This configuration is quite similar to the configuration of FIG. 4, but the inside transitions  64  are reversed with the outside transitions  66  in every other track  58   a ,  58   c ,  58   e . This servo burst configuration of FIG. 4 a  produces the strongest position error signal when the head is at a track boundary  61 . The position error signal decreases monotonically as the head  28  approaches the center line  59 , and becomes zeroed out when the head  28  is centered over the center line  59 . Writing of the magnetic signals shown in FIG. 4 a  still requires two precisely positioned passes of the magnetic head  28  over each track  58 : one for the inside transitions  64  and one for the outside transitions  66 . The servo burst configuration of FIG. 4 a  may be preferable to the servo burst configuration of FIG. 4 due to the resultant position error signal. Workers skilled in the art will appreciate that either configuration of FIG. 4 or FIG. 4 a  may work suitably. 
     The present invention relates to a better method of writing the servo pattern information, both less expensive and faster. In a first embodiment of the present invention, the servo patterns are written from a master disc  70  (shown in FIGS. 7 and 8) to a product “slave” disc  16  by magnetic proximity printing, or magnetic print-through. One master disc  70  is used to print consecutively onto a very large number of product discs  16 . The master disc  70  has a servo master pattern written thereon, and the magnetic field from the servo master pattern is used to magnetized the transitions of the servo pattern on each of the product slave discs  16 . As shown in FIG. 8, the preferred master disc  70  includes a magnetic layer  92  on top of an underlayer  94  and a substrate  96 . 
     For print-through or magnetic proximity printing to be effective, the coercivity of the master disc  70  (H cm ) should be higher than the coercivity of the “slave” medium  38  (H cs ). Preferably the master disc coercivity is 1.2 to 1.5 times as great as the slave disc coercivity, such as H cm ≧3,500 Oersteds and H cs =2,500 Oersteds. A high coercivity on the slave medium  38  is desired because high coercivity leads to sharper transitions and higher attainable storage densities in the resulting disc drive  10 . The even higher coercivity of the master medium  70  is necessary for the print-through process to be most effective while not destroying the magnetization recorded on the master medium  70 . 
     Obtaining a high coercivity master medium  70  is easier because there is no magnetic noise requirements on the master medium  70 . The master disc  70  can use a wide choice of substrate materials, including nickel-phosphored aluminum, silicon or plastic. The master medium  70  should also have a high product of remanent flux density and thickness B r t. For instance, a B r t≧1.0 Tesla·μin may be necessary to produce the preferred write fields. Preferably the master disc  70  has a B r t≧1.8 Tesla·μin. 
     The magnetic pattern on the master disc  70  can be fabricated by one-of-a-kind master writer intended only for writing of the master disc  70 . The one-of-a-kind master writer should be able to write the servo master pattern without any need for stitching. The servo master writer should also be able to provide a very high write field gradient, by the head flying very low and having a very small gap and a low throat height, and by the head having a high moment material. In this way, a very high intensity servo master pattern is written onto the master disc  70  with essentially conventional recording techniques. The preferred master disc  70  has a minimum transition spacing b of approximately b=40 μin (1000 Angstroms). 
     After writing of the master disc  70 , the magnetic proximity printing by the master disc  70  on a product “slave” disc  16  is preferably achieved as follows. First, the slave disc  16  is demagnetized using equipment as schematically shown in FIG.  5 . The slave disc  16  is placed on a spindle  72  which rotates about axis  74 . The rotating slave disc  16  is submersed in a large, powerful magnetic field  84  produced by electromagnet  76 . Electromagnet  76  includes a large magnetic yoke  78  and pole pieces  80  which are magnetized with a magnetizing current through conductor  82 . The pole pieces  80  and the yoke  78  of electromagnet  76  are made of a material with high magnetic permeability, high saturation magnetization, low remanance and low coercivity. For example, the material for pole pieces  80  and yoke  78  could be made of permalloy, mu-metal, or similar materials. Electromagnet  76  is oriented with respect to disc  16  such that the magnetic field  84  produced is in-plane relative to disc  16 . 
     The amount and direction of electric current through conductor  82  can be modulated as necessary for the desired timewise adjustment of electromagnet field  84 . FIG. 6 represents the preferred current through conductor  82  as a function of time, and thus the magnitude of the preferred magnetic field  84  produced by electromagnet  76  as a function of time. During demagnetization  86 , the current i and electromagnet field  84  H e  start at a very high value. The initially high magnitude of the electromagnet field  84  H e  should be higher than the coercivity of the slave medium  38  H cs . The direction of the current and electromagnet field  84  is alternated at a frequency which is high compared to the frequency of rotation of the slave disc  16 . The amplitude of the current and electromagnet field  84  is gradually reduced to zero. 
     This demagnetization process  86  renders the magnetic domains of the slave medium  38  in a relatively uniform non-aligned magnetization state so the magnetic domains can be commonly magnetized with a servo writing field of lesser strength than if demagnetization  86  had not been performed. For instance, disc drives  10  normally have a write field, H w , which is about 3 times the coercivity of the medium  38  so that the write field will reliably overwrite or erase old data. The write field is localized in the magnetic medium  38  of the product disc  16  directly adjacent the magnetic head  28 . To write a servo pattern into an ideally demagnetized slave medium  38 , a localized write field which is only slightly greater than the coercivity of the magnetic layer  44  of slave disc  16  may be sufficient. The preferred localized write field for the present invention is approximately 1.3 times the slave coercivity (H w ≧1.3H cs ). 
     After demagnetization  86 , a master disc  70  is mounted on spindle  72  in close proximity to slave disc  16  as shown in FIGS. 7 and 8, such that the servo master patterns on master disc  70  are in alignment with the desired locations of servo patterns on slave disc  16 . This alignment may occur by using a conventional contact mask aligning station. 
     The horizontal time axis in FIG. 6 is broken to indicate the alignment step  88  between the slave disc  16  and the master disc  70 . After alignment  88 , spindle  72  is rotated so master disc  70  and slave disc  16  rotate together relative to the magnetic field  90  produced by electromagnet  76 . Electromagnet  76  is oriented with respect to the master/slave combination such that the magnetic field  90  produced is in-plane relative to medium  38 . The large (i.e., nonlocalized) magnetic field  84  produced by electromagnet  76  assists in transferring the magnetic pattern of master disc  70  onto slave medium  38 . During servo-writing  90  (FIG.  6 ), the current i and electromagnet field  84  of electromagnet  76  H e  preferably start at zero, gradually increase to a maximum value, and then gradually decrease to zero. The direction of the current i, and hence the direction of electromagnet field  84  H e , is alternated at a frequency which is high compared to the frequency of rotation of the slave/master disc combination. For the preferred magnetic parameters of master medium  70  and slave medium  38  given previously, a maximum magnetic assist field  84  during servo-writing  90  of H e =2780 Oersteds is appropriate. 
     The master disc  70  and slave disc  16  are shown in FIG. 7 as being the same shape, with the master disc  70  having a slightly greater diameter and a slightly greater thickness than the slave disc  16 . However, workers skilled in the art will appreciate that there are no size, shape or thickness requirements for the master medium  70 . In fact the master medium  70  can take on any shape provided that it bears a magnetic servo master pattern which will areally correspond to the desired servo pattern on the slave disc  16 . 
     Because the proximity printing operation occurs in a manufacturing setting and without using an air bearing magnetic head  28 , there are no tribology requirements for either the master disc  70  or the slave disc  16  at the time of print-through. It is not necessary for the overcoat  46  and the lubricant  48  to yet be applied to either the surface of the master disc  70  or the slave disc  16 . Accordingly, master medium  70  and slave medium  38  are depicted in FIG. 8 without an overcoat layer  46  or a lubricant layer  48 . The separation d between the magnetic layer  92  of the master medium  70  and the magnetic layer  44  of the slave disc  16  is dependent upon the smoothness of the respective surfaces and the flatness of the discs  16 ,  70 . Workers skilled in the art will appreciate that, if print-through is performed prior to deposition of the overcoat layer  46  and lubricant layer  48 , master medium  70  and slave medium  38  should be maintained in a non-corrosive environment throughout the print-through process. Alternatively, overcoat layers  46  and lubricant layers  48  may be used on both the master medium and the slave medium  38 . A practically achievable separation d between the magnetic layer  92  of the master medium  70  and the magnetic layer  44  of the slave medium  38  is about d=4 μin (1000 Angstroms). 
     If the original magnetic transitions within the master medium  70  are very sharp, the sharpness of the magnetic transition will be affected by the electromagnet assist field  84 . In the limit, for high values of the electromagnet assist field  84  relative to the coercivity of the master medium  70 , a series of originally sharp transitions on the master medium  70  will change into an approximately sinusoidal magnetization pattern. In addition to being able to survive a large rotating field, the sinusoidal distribution in the master medium  70  also provides a larger stray field for writing into the slave medium  38 . Accordingly, the preferred method of practicing the present invention involves use of a master medium  70  with sinusoidal magnetic transitions, particularly for servo bursts  62 . 
     During the print-through operation, the electromagnet assist field  90  enhances the local magnetic field produced by the master medium  70 . The peak writing field can be expressed as H w =H e +H dm (d), where H dm (d) is the demagnetizing field produced by the magnetic transitions in the master medium  70  at a separation height d away from the magnetic layer of the master medium  70  and centered between magnetic transitions. For the preferred values of B r t=1.8 Tesla·μin, b=40 μin and d=4 μin given previously, H dm (d)=527 Oersteds, so peak write field H w =2780+527=about 3300 Oersteds. This provides the H w /H cs =3300/2500=1.32 for writing the servo pattern into the demagnetized slave medium  38 . When the direction of the electromagnet assist field  90  is reversed (i.e., against the direction of the demagnetizing field H dm (d)), H w =2780−527=about 2250 Oersteds=0.9 H cs  (as given earlier, H cs =2500 Oersteds). Accordingly, the electromagnet assist field  90  does not reverse magnetism of the magnetically aligned domains after they have been written, regardless of subsequent reversal of the direction of electromagnet assist field  90 . 
     The sum of the rotating assist field  90  of electromagnet  76  and the maximum demagnetizing field of the master medium  70  must not exceed the coercivity of the master medium  70 —otherwise the master servo pattern on the master medium  70  would be altered by the print-through process. With the preferred values given, the demagnetizing field within the master magnetic layer (i.e., at d=0) H dm =720 Oersteds, so the write field within the master medium  70  does not exceed the master medium coercivity H cm . 
     Rotation of the electromagnet assist field  84  relative to the slave disc  16  enhances the local magnetic field produced by the master medium  70 . The magnetic transitions in the master medium  70  occur in the circumferential direction Θ. The peak writing field occurs in the slave medium  38  when the magnetic assist field  90  points in the same direction as the circumferential magnetic transitions from the master medium  70 , i.e., along line  98  in FIG. 7 where the magnetic assist field  84  is tangential to the slave disc  16 . Rotation of the electromagnet assist field  84  relative to the aligned slave/master combination assures that the direction of the electromagnet assist field  84  and the circumferential direction of the master transitions will align for all areas of the disc  16  because all areas of the disc  16  pass through line  98 . 
     FIG. 9 shows the write field in the circumferential direction H wΘ  which is sensed by the slave medium  38  at a location centered between magnetized transitions on the magnetic master medium  70  (i.e., x=b/2). Superimposed with dashed line is the outline of the electromagnet assist field  84  from FIG.  6 . For simplicity, the high frequency switching of the direction of current and electromagnet field has not been shown, but rather the sensed field is shown continuously in both the positive and the negative directions. 
     The write field in the circumferential direction is characterized by a number of large sinusoidal variations  100  in magnitude with a period  102 . These sinusoidal variations  100  are due to the rotation of the master/slave disc combination (i.e., with polar coordinate orientation) within the stationary electromagnet field  84  (i.e., with cartesian coordinate orientation). Period  102  reflects the rate of rotation of disc  16  relative to electromagnet field  84 . 
     During demagnetization  86 , the write field is equally centered about the horizontal axis in both + and − directions. During servo-writing print-through  90 , the write field is centered about a positive value, with the shift being caused by the demagnetizing field H dm (d) of the master disc  70 . The write field is thus greater than the electromagnet assist field  84  in a positive direction, corresponding with a write field which exceeds the coercivity of the slave medium  38  sufficient to magnetically write on the slave medium  38 . The magnitude of the write field is less than the electromagnet assist field  86  in the negative direction, corresponding with a write field which is less than the coercivity of the slave medium  38  sufficient to avoid reversal of the aligned magnetic domains. 
     The positive direction of the offset of the write field during servo-writing  90  is due to the positive direction of the demagnetizing field of the master disc  70 . In locations where the master disc  70  is magnetized in the opposite direction, the write field during servo-writing will be offset in the negative direction. The servo-writing print through process thus magnetizes a servo-pattern into the product slave disc  16  having full transitions from magnetism in one direction to magnetism in the opposite direction, resulting in a strong position error signal. 
     FIG. 10 shows a magnetization curve  104  for the master medium  70  superimposed with a magnetization curve  106  for the slave medium  38  during alignment between the maximum electromagnet assist field  84  and the demagnetizing field of the master medium  70 . The dashed line  108  represents the magnetization path of the master medium  70  due to alternation of the direction of magnetism in the electromagnet field  84 . The dashed line  110  represents the magnetization path of the slave medium  38  due to alternation of the direction of magnetism in the electromagnet field  84 . The relationship between the demagnetization field, the electromagnet assist field  84  and the slave coercivity causes the slave medium  38  to take on a residual magnetization corresponding to the magnetization of the servo-master pattern. At the same time, the higher coercivity of the master medium  70  keeps the master medium  70  from losing magnetization due to the electromagnet field  84 . Workers skilled in the art will appreciate that the magnetization curve  106  of the slave medium  38  and the magnetization curve  104  of the master medium  70  and the magnitude of the electromagnet assist field  84  must interact to achieve the beneficial results of the present invention. 
     Workers skilled in the art will recognize that there may be other feasible methods for aligning the electromagnet assist field  84  with the circumferential direction of the magnetic transitions on the slave disc  16 . As an alternative embodiment, shown in FIG. 11, the master/slave disc combination may be placed between two electromagnets  112 ,  114  at 90° angles to each other. The first electromagnet  112  is driven with a current 90° out of phase with respect to the current driven through the second electromagnet  114 . The direction of the magnetic assist field is in-plane relative to the master/slave combination. In this case, no physical rotation of the discs  16 ,  70  may be required, but the design of the electromagnets  112 ,  114  becomes more difficult. 
     As a second alternative embodiment, a middle ground could be chosen between the large, non-localized electromagnet field  84  described above and the highly localized, point magnetic filed of a traditional recording head. An electromagnet could be constructed with a linear gap extending immediately adjacent the master/slave combination from axis  74  (shown in FIG. 7) radially outward. The linear gap should be “ultrawide” (say, for example, 40,000 tracks wide), as compared to the about 1 track width of a traditional recording head. The magnetic assist field created by the ultrawide head should be highly concentrated at a distance from the electromagnet which roughly corresponds with the distance to the master/slave interface, running the entire radius of the slave disc  16 . The direction of the magnetic assist field at the master/slave interface should be in-plane and circumferential. For instance, the pole tips of the ultrawide head could be approximately 1-5 mm apart, extending a full 2½ inch radius of the slave disc  16 , with pairs of opposite pole tips arranged both above and below the master/slave interface(s). Having ultrawide heads both above and below the master/slave interface is helpful in concentrating the magnetic field created thereby closer to the master/slave interface. As compared to a distance of 100-200 Angstroms from the head poles to the magnetic medium in tradiation recording, the target magnetic layer of the slave disc in magnetic print-through printing is at least the thickness of the master disk away from one of the ultrawide heads, and at least the thickness of the slave disc away from the other of the ultrawide heads. The master/slave combination is rotated with respect to the linear magnetic assist field, so the linear magnetic assist field sweeps across the entire surface of the master/slave interface in one rotation of the master/slave combination. The linear magnetic assist field is used to magnetize all tracks  58  on the disc  16  equally and at the same time. 
     In contrast to the magnetic assist field  84  produced in FIGS. 5 and 11, the magnetic assist field of the ultrawide head configuration maintains circumferential alignment with respect to the master/slave combination. While this servo-writing process may require more rotations of the master/slave combination as compared to the large magnetic assist field  84  produced by the electromagnet of FIGS. 5 or  11 , servo-writing is still achieved much more quickly than with traditional methods. 
     As a third alternative embodiment, a toroidal (doughnut) shaped electromagnet/conductor combination could be constructed. The toroidal combination should be constructed to produce an alternatable magnetic field which is circumferentially directed about axis  74 . The master/disc combination may then be positioned within this electromagnet during the print-through process. With the toroidal combination, the print-through magnetic assist field is circumferentially oriented at all locations on the disc. The aligned magnetic domains, which can be theoretically viewed as vectors, are thus commonly aligned or oriented in the circumferential direction without a radial component, similar in orientation to the direction of the magnetic domains produced in traditional servo-writing. Such an arrangement would be more complicated in terms of mechanical construction of the electromagnet for the assist field, but would tend to reduce any radial component to the direction of aligned magnetic domains. 
     Workers skilled in the art will appreciate that the transitions of the master medium  70  may differ significantly, both in sharpness and in areal extent, from the magnetic transitions  56  which are subsequently recorded on the slave medium  38 . For instance, a decrease in definition of the image transferred by the proximity printing can be compensated for by adjusting the areal overlap between bursts on the master medium  70 . With reference to FIG. 4, good definition along a track  58  of the slave disc  16  is not necessary as long as the magnetization in the recording medium  38  gets close to ±Br half way between transitions  56 . In reading the slave servo pattern and detecting areas of servo bursts  62 , a sinusoidal output wave shape may actually be preferable to a sequence of spikes separated by long stretches of base line. 
     The present print-through process has been described with reference to only one side of a disc  16 . Workers skilled in the art will appreciate that one-sided discs may be used in the disc drive product, or that two one-sided discs may be joined back-to-back to form a two-sided disc in the disc drive product. In the alternative, two master discs  70  can be used in the print through process, each aligned against a respective side of the product disc  16 . The magnetic flux patterns within the magnetic layers  44 , underlayers  42 , and substrate  40  and the relatively large distance between magnetic layers  44  on the two sides of the product disc  16  prevent any measurable magnetic transfer from top side to bottom side through the product disc  16 . 
     An alternative disc  120  with simplified writing of the servo pattern is schematically shown with reference to FIG.  12 . In FIG. 12, portions  122  of the recording medium  38  do not support a magnetic signal, i.e., the magnetic domains of portions  122  are not present or otherwise are not capable of magnetic alignment regardless of any processing by an external magnetic field. The boundaries of non-magnetic portions  122  are defined during fabrication of disc  16  and before any servo-writing takes place. The remaining portions  124  of the recording medium have a signal magnetically recorded thereon, as indicated by + and − magnetization signs  50 ,  52 . In magnetic signal portions  124 , due to the application during servo-writing of a magnetic field, the magnetic domains of the magnetic layer  44  are all oriented in the same direction. Because all magnetic signal portions  124  bear a common signal in the same direction, there are no transitions within each signal portions  124  within a track  58 . The transitions which can be sensed are between the aligned magnetic signal portions  124  and the non-magnetic portions  122 . Magnetic signal portions  124  are in a pattern which represents both gray code information  60  and servo bursts  62 . Workers skilled in the art will see that the alternative servo-burst pattern shown in FIG. 4 a  could similarly be readily modified by the present invention to consist of aligned magnetic signal portions and the non-magnetic portions. 
     In FIG. 12, product disc  120  has non-magnetic portions  122  which are essentially “islands” in a “sea” of commonly recorded magnetic signal portions  124 . FIG. 13 represents an alternative embodiment of FIG.  12 . In FIG. 13, product disc  128  has magnetic portions  124  which are “islands” in a “sea” of non-magnetic portions  122 . The transitions which can be sensed are again between the aligned magnetic signal portions  124  and the non-magnetic portions  122 . 
     Unlike traditional servo patterns, magnetic signal portions  124  of the servo patterns of FIGS. 12 and 13 are all aligned in the same, single direction. In reading the servo-patterns of FIGS. 12 and 13, the transitions sensed by the read head  28  are not between opposite directions of magnetic alignment. Rather, the sensed transitions are between the positive magnetic field provided over magnetic signal portions  124  and the absence of magnetic field or the weaker magnetic field signal when flying over a non-magnetic portion  122 . This provides a position error signal of the servo patterns which is theoretically half that of a traditional servo pattern. Workers skilled in the art will appreciate that the data portions (not shown) of tracks  58 , which have no non-magnetic portions  122 , are written during use of the disc drive by traditional methods including reversing the direction of magnetization to create full magnetic transitions. 
     Centering over the track  58  is accomplished by comparing portions of the signal read from the servo bursts  62 . If the magnetic head  28  is centered over the track  58 , the signal read from the inside transitions  64  will be equal to the signal read from the outside transitions  66 . If the magnetic head  28  is toward the inside of the track  58 , the signal from the inside transitions  64  will be stronger than the signal from the outside transitions  66 . If the magnetic head  28  is toward the outside of the track  58 , the signal from the outside transitions  66  will be stronger. By comparing portions of the servo burst  62  signal, the disc drive  10  can iteratively adjust the head  28  location until a zeroed position error signal is returned from the servo bursts  62 , indicating that the head  28  is properly centered with respect to the track  58 . 
     The beneficial result of the magnetic media of FIGS. 12 and 13 comes due to simplification of the servo-writing process. With the boundaries of non-magnetic portions  122  pre-defined, placing the magnetic pattern on servo patterns becomes much easier, less time-consuming, and less expensive. Because all of the magnetic domains which bear a signal are aligned in a single direction, they can be commonly written by an external magnetic field which is areally much larger than the highly localized magnetic field used to record traditional servo patterns and data on the disc  16 . For instance, the servo patterns of FIG. 12 can be written with a single pass of a magnetic head  28  over a track  58 , rather than two passes per track  58  and “stitching” used during traditional servo-writing. 
     The servo patterns of FIGS. 12 and 13 can also be written by a specialized ultrawide servo writer head that produces a linear magnetic field in the radial direction, and by sweeping the linear magnetic field over the entire surface of disc  16  in a single pass. The servo patterns of FIGS. 12 and 13 can alternatively be written by submersing the disc  16  into a large rotating or circumferential magnetic field  84 , similar to that described earlier with reference to FIGS. 5 and 11. 
     Workers skilled in the art will recognize that the present invention could be used for the servo burst  62  portion of the servo-pattern information, while leaving the gray code information  60  recorded by traditional methods. Provided the centering servo burst information  62  was written in accordance with this invention, gray code information  60  could be sequentially written by the magnetic head  28  of the disc drive system  10  at a later time, without any servo-writer equipment present. Writing of the gray code information  60  would merely involve a traversal of the magnetic head  28  outward on the disc  16  to write indexing information to each of tracks  58  as defined by the servo bursts  62 . The outward traversal of magnetic head  28  should be performed slowly and carefully to verify that complete gray code information  60  is provided to each of the tracks  58  without skipping any tracks  58 . 
     Numerous methods are contemplated for creating the pattern of non-magnetic portions  122  on the magnetic medium  38 . For instance, as shown in FIG. 14, the magnetic layer  44  may be removed from non-magnetic portions  122  prior to deposition of the overcoat layer  46 . The overcoat  46  and lubricant layer  48  extend over both magnetic portions  124  and non-magnetic portions  122 , and help with smoothing the surface for flyability and for providing corrosion protection. Because the magnetic layer  44  is absent from portion  122 , portion  122  does not support a magnetic charge. Magnetic flux  54  is sensed by the read transducer only when it is flying over magnetically charged portion  124 . 
     One method to remove the magnetic medium from non-magnetic portions  122  is with a photolithographic etching process during fabrication of the disc  16 . Photolithographic etching processes are well known in the semi-conductor art, and will only be summarized here. After deposition of magnetic layer  44  but prior to deposition of the overcoat  46 , a photoresist mask is applied over the magnetic layer  44 . The process linewidth that is required for the photolithographic etching process is approximately equal to the flux transition to flux transition spacing within the servo-pattern, typically about one micron or more, which is within state of the art linewidth resolution capabilities. The photoresist mask has a pattern which covers all the magnetic portions  124  of the servo-pattern, but leaves the non-magnetic portions  122  unprotected. The photoresist mask should also cover and protect the entirety of the data portions on the disc  16 . An acid or other chemical etching step may then be used to remove portions of the magnetic layer  44  which are not protected by the photoresist mask. After etching, the photoresist mask may be dissolved or otherwise completely removed. Further fabrication of the disc  16 , including deposition of the overcoat  46  and lubricant layers  48  over the surface of the disc  16 , may then be completed. 
     Workers skilled in the art will recognize that alignment of the disc axis with the photolithographic pattern is critical to the success of this type of servo-writing process. The servo-pattern information must be precisely located in the radial direction to assure that the servo-pattern information is concentric with the rotation of the disc  16  relative to the magnetic read head  28 . 
     An alternative method to remove the magnetic medium in non-magnetic portions  122  is to place a photolithographic lift-off pattern on the underlayer  42  of disc  16  prior to sputtering of the magnetic layer  44 . The lift-off pattern corresponds with the design and location of the non-magnetic portions  122 . After the magnetic layer  44  is applied over the entire surface of the disc  16 , the lift-off pattern and overlying magnetic layer  44  may be lifted off in the desired areas  122 . 
     Another alternative method to remove the magnetic medium in non-magnetic portion  122  is to use ion milling. The ion milling process is known in other arts as a way to selectively and accurately remove thin layers of material, and will not be specifically described here. The ion milling process must be accurately controlled to assure that magnetic layer  44  is removed only in the specific pattern of non-magnetic portions  122 . 
     Other alternative methods to create non-magnetic portions  122  include placing material  130  which is magnetically non-responsive within the magnetic layer  44 . This is shown with reference to FIG. 15, wherein non-magnetic portions  122  exist within magnetic layer  44 . In either the etching or lift-off processes, the surface irregularities created as described with reference to FIG. 14 could be filled with some hard material  130  such as tantalum. Filling the non-magnetic portions  122  to the same overall height as magnetic portions  124  is helpful in improving flyability and tribology of the disc  16 . 
     Another alternative method to create the non-magnetic areas is through an ion implantation or other process that destroys or alters the magnetic properties of the magnetic layer  44 . The magnetic altering process may be accomplished with a photoresist mask overlying and protecting the magnetic portions of the magnetic layer  44 . With ion implantation for instance, ions of a “kill” material that disrupts the crystallographic structure of the cobalt-based magnetic layer  44  may be implanted to create the non-magnetic portions  122  at the locations which are not covered by the photoresist mask. Various other types of processes can alternatively be used to reduce the magnetism of the magnetic layer  44  in locations corresponding to non-magnetic portions  122 . 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.