Patent Publication Number: US-8531794-B2

Title: Patterned media with an alternating series of concentric servo zones and overlap zones

Description:
FIELD OF THE INVENTION 
     The invention relates to the field of disk drive systems and, in particular, to servo patterns imprinted on a patterned magnetic disk to align a slider with data on the magnetic disk. 
     BACKGROUND 
     Many computing systems use disk drive system systems for mass storage of information. Magnetic disk drives typically include one or more sliders that include a read head and a write head. An actuator/suspension arm holds the slider above a magnetic disk. The magnetic disk includes data regions and servo sectors. A voice coil motor (“VCM”) moves the actuator/suspension arm to position the slider over selected magnetically written data with the feedback of servo data. Electronics on the disk drive system include a write driver, a read signal preamplifier, a read-write channel, a controller, and firmware. The controller typically is an assortment of circuit chips connected on a printed circuit board. The controller includes one or more microprocessors, memory, servo control circuits, hard disk control circuits, spindle motor drivers, and VCM drivers. The read-write channel can include analog to digital conversion circuits, data clocks, servo clocks, and phase locked loops. 
     Both the data regions and servo sectors can include information that is magnetically written by the write head onto the magnetic disk and also read back by the read head from the magnetic disk. The data regions include data tracks that are available to store end-user files and disk drive system parameter data (or maintenance data). The data is written typically in 512 byte or 4 kilobyte data blocks. Each data block usually has a data sync field, the actual data (typically encoded and possibly encrypted), and error correction data. The end-user is free to store new data and later modify the data. 
     The servo sectors include servo data that is used to position the slider. Servo data is typically only written at the manufacturing facility and cannot be changed by the end-user. There are various techniques for writing servo data on a magnetic disk; in a typical method called self-servo writing the servo data is step-wise propagated from the inner diameter to the outer diameter using the write head to write servo data that is later used for servo track following to assist the writing of additional servo data. One complicating factor for self-servo writing (as well as normal data writing) is the radial read-write offset length (“RWO”) between the write head and the read head. The RWO changes with the slider&#39;s angular position over the magnetic disk based on the location of the read head and write head on the slider and the arc made by the actuator over the magnetic disk. Often in self-servo writing, the read head is offset toward the inner diameter from the write head and the servo data is written from the inner diameter to the outer diameter. 
       FIGS. 18A-18C  illustrate a close-up view of a slider&#39;s  122  variable angular position over a magnetic disk. As mentioned, the slider  122  is configured with a read head  130  and a write head  140  that are operable to read from and write to the magnetic disk. When the slider  122  is positioned towards the inner diameter of the magnetic disk, the RWO  900  between the read head  130  and the write head  140  is at its smallest dimension relative to the magnetic disk. As the slider  122  moves to the center of the magnetic disk, the RWO increases (RWO  901  of  FIG. 18B ), reaching its maximum RWO ( 902 ) at the outer diameter of the magnetic disk in  FIG. 18C . 
     Servo data may include a synchronization field (servo sync), a sector address mark (“SAM”), sector-ID, track-ID (sometimes called the cylinder-ID), a servo burst, a repeatable runout (“RRO”) value, and a pad. Data tracks are usually identified by a combination of the track-ID, servo burst, and/or RRO value. 
     The servo sync is typically the first servo data read by the read head as it passes through a servo sector. The servo sync can be used by the read-write channel to establish servo frequency and servo clock phase. Portions of the servo sync can also be used for automatic gain control in the disk drive system electronics. The servo sync can be written with either a single magnetic polarity or with an alternating polarity as demonstrated in U.S. Patent Application Pub. No. 2006/0279871A1. The servo sync is sometimes referred to as a preamble. 
     The servo frequency in conventional disk drive systems is constant from the inner diameter to the outer diameter of the magnetic disk. As a result of this constant frequency, the servo sectors increase in circumferential length proportional to radial location. For instance, the circumferential length of the servo sector at the outer diameter of the magnetic disk may be about twice the length of the servo sector at the inner diameter. 
     If a zoned servo architecture is employed, the servo frequency increases between servo zones from the inner diameter of the magnetic disk to the outer diameter. The servo frequency changes between the zones roughly with the mean radius of each servo zone. The servo frequency within each zone usually remains constant. Because the servo sector is broken into shorter radial servo zones, the circumferential length of the servo sectors does not vary as much as in the conventional servo design. The reduced variance in circumferential length when using zoned servo provides an advantage when using patterned media as the servo patterns for zoned servo can be more uniform in circumferential length from the inner diameter of the magnetic disk to the outer diameter. See U.S. patent application Ser. No. 12/699,581 (“the &#39;581”) and the description below of Dry Planarization Design Rules #1 and #2. 
     Examples of zoned servo can be found in U.S. Pat. No. 6,178,056 FIGS. 2B and 2C; U.S. Pat. No. 7,012,773 FIGS. 10, 15, 20, 28 and column 11 (“the &#39;773”); and U.S. Pat. No. 7,715,138 FIG. 2A. The &#39;773 FIG. 10 demonstrates a design with a series of concentric zones making up an alternating series of normal servo zones that are single frequency (“servo zones”) and overlap zones that are dual frequency (“dual frequency zones”). In the dual frequency zones, half of the servo sectors use a first servo frequency that is the same as the bordering lower frequency servo zone while the remaining servo sectors use a second servo frequency that is the same as the bordering higher frequency servo zone. In the &#39;773 FIG. 10 design, the servo zones and dual frequency zones are arranged in continuous radial servo sectors. FIGS. 15, 20, and 25 of the &#39;773 demonstrate other possible zoned servo arrangements in which the servo sectors are not radially continuous. 
     The SAM (also called a servo address mark, start of servo mark, and servo sync byte) acts as a starting point from which to locate other servo data. For instance, the track-ID, sector-ID, and servo burst can be positioned a predefined distance from the SAM in a predefined order. The SAM is typically a unique magnetic shape so that it is more easily distinguishable by the disk drive system electronics from other magnetic information written on the magnetic disk. The SAM may not follow the same rules or constraints as other data that is written on the magnetic disk. For instance, the SAM may be written at a different frequency or otherwise differ in width and/or spacing from the other servo data. 
     The sector-ID is used to identify the particular servo sector as the slider circles a track. A track may have 250 or more sequential servo sectors. The sector-ID provides the controller with the circumferential position of the slider. The sector-ID is typically substantially identical in each track of an individual servo sector as it propagates radially from inner diameter to outer diameter. The sector-ID may be a unique digital number identifying the specific servo sector, such as a sector-ID between one and 250 if there are 250 servo sectors in one track. The sector-ID may be split between several servo sectors to reduce the circumferential length of each servo sector; in this case, several servo sectors need to be read to determine the complete sector-ID. In some designs, the magnetic disk has a start of track mark and the controller includes a counter; in this case, a start of track mark resets the counter and the counter is incremented each time new a SAM is encountered by the read head to provide a running count for the complete sector-ID. In this specification, the term sector-ID is meant to include each of these possible designs. 
     The track-IDs are used to identify the particular radial position as the slider moves radially from the inner diameter to outer diameter. The track-ID is often written in a gray code digital format; there are many gray code formats and some formats encrypt the track-ID and/or provide error-correction redundancy. The track-ID can also be written using a plurality of phase patterns (e.g., chevron patterns), as demonstrated in FIGS. 4A, 4B, 8, and 10 of U.S. patent application Ser. No. 12/634,240 (“the &#39;240”). The track-ID provides the controller with the radial position of the slider. The track-IDs can ascend in numerical value within a specific servo sector from inner diameter to outer diameter; the track-ID can be substantially identical within the sequential servo sectors of a specific track. The track-ID may be a unique digital number identifying the specific radial position, such as a number between one and 100,000 if there are 100,000 unique gray code numbers in the servo sector from the inner diameter to the outer diameter. There is usually not a one to one correspondence between magnetically written data tracks and gray code track-IDs. The track-ID may also be split between several servo sectors to reduce the circumferential length of the track-ID in each servo sector; in this case, several servo sectors need to be read to determine the complete track-ID. In this specification, the term track-ID is meant to include each of these possible designs. 
     Servo bursts are used to center the slider on the magnetically written data tracks. The servo bursts are used to create a position error signal used by the controller to make fine adjustments to the slider position and center it over a track. The servo burst can be: (i) an ABCD servo burst as demonstrated in U.S. Pat. No. 6,490,111 FIG. 4; (ii) a checkerboard servo burst as demonstrated in U.S. Pat. No. 6,643,082 FIG. 10 and U.S. Pat. No. 7,706,092 FIGS. 6 and 7; or (iii) a phase servo burst as demonstrated in the &#39;581 FIG. 9 item 804. The &#39;581 is incorporated herein by reference. The servo burst can be written with either a single magnetic polarity or with an alternating polarity as demonstrated in the &#39;871. The read back signal of a servo burst will show a repeating series of isolated pulses generated from each magnetic transition. Checkerboard servo bursts with alternating polarity are often called DC-free null servo burst. Unlike the ABCD servo burst and checkerboard servo bursts, the phase servo bursts are configured with a slope. There is often not a one to one correspondence between the radial dimensions of track-IDs and the servo burst. The signal magnitude of a servo burst read back is typically used by the disk drive system electronics to identify a fraction of track-ID&#39;s width. Data tracks are usually identified by a combination of the servo data taken from a read back of the track-ID, servo burst strength, and/or RRO value. There is often not a one to one correspondence between the radial dimensions of a servo burst and a data track. 
     RRO values are determined usually during manufacturing and stored within the disk drive system for use during operation. If the RRO values are stored within the servo sectors, they are often stored as bits of information located after the servo burst. 
     Often there is a pad before and/or after the servo data. The pad does not necessarily include any specific data. The pad is used to accommodate read-to-write and write-to-read transition timing of the write driver, read signal preamplifier, and read-write channel. 
     Patterned magnetic disk designs have emerged recently to enhance the recording density by providing better track and/or bit isolation. For example, nano scale non-magnetic grooves may be patterned on the magnetic disk by removing magnetic material and leaving behind discrete tracks or bit “islands” of magnetic material. There are two common forms of patterned magnetic disk: Discrete Track Media (“DTM”) and Bit Patterned Media (“BPM”). In DTM, discrete tracks are patterned into the magnetic disk and data bits are magnetically written thereto. In BPM, individual bits may be patterned via track grooves and crossing bit grooves, creating islands of magnetic material. Both BPM and DTM establish data patterns where data may be magnetically written. Read back of pattern media will show magnetic transitions between the magnetized magnetic islands and non-magnetic grooves, such as in BPM; read back of pattern media will also show magnetic transitions occurring within a single magnetic island, such as in DTM. (Note that, unlike DTM or BPM, conventional non-patterned media has layers of magnetic material sputtered onto the entire front and back surfaces of the magnetic disk and there are typically no pre-formatted patterns). 
     In both BPM and DTM the disk patterning process can be used to create unique magnetic islands in the shape of various portions of the servo data. In U.S. Pat. No. 6,490,111 (“the &#39;111”) FIG. 4, for example, the pattern imprint includes magnetic islands in the shape of all the intended final servo data, including the gray code track-ID. With the &#39;111 design, the servo data is readable by the read head after bulk Direct Current (“DC”) magnetization (e.g., single magnetic polarity) of the magnetic islands because of the read back signal contrast between the presence and absence of magnetic material. The problem with this servo data writing approach, however, is that many of the available planarization constraints have difficulty dealing with the widely varying sizes and shapes of the gray code track-ID formats and sector-ID formats. Certain planarization constraints impose design rules on patterned magnetic disk. For liquid-based planarization, all non-magnetic grooves should be configured at or below a specified width that allows for the liquid to planarize the grooves through capillary forces. For dry planarization, such as vacuum deposit/etchback planarization, the ratio of magnetic island widths to non-magnetic groove widths needs to be constant everywhere (“Dry Planarization Design Rule #1”). It is also advantageous to ensure that magnetic island and non-magnetic groove widths are constant everywhere (“Dry Planarization Design Rule #2”). Servo patterns that comply with these planarization constraints are sometimes called planarization compatible servo (“PCS”) or planarization-compatible servo pattern (“PSP”). 
     An alternative approach to bulk DC magnetization of pre-patterned gray code track-ID, is to hard pattern only a portion of the servo data on the magnetic disk and fill in the remaining servo data by magnetically writing with the write head the desired servo data into the remaining portions of the servo pattern. This process has been called assisted servo track write for patterned media. In the &#39;581, for instance, the servo pattern includes a single servo write assist pattern and a plurality of checkerboard sub-patterns. The servo write assist pattern is comprised of radial magnetic islands and radial non-magnetic grooves. The servo write assist pattern can also, as demonstrated in FIG. 6 of the &#39;111, be comprised of circumferential magnetic rows and circumferential non-magnetic grooves. After assembly of the patterned magnetic disk into a disk drive system, the write head is used to magnetically write the track-ID in the servo write assist patterns. The writing of the track-ID by the write head does not change the shape of the magnetic islands and non-magnetic grooves of the servo write assist patterns. 
     A hybrid servo writing approach is to combine of small number of bootstrap patterns (which are operable after DC magnetization) and predominant servo write assist patterns (which require magnetic writing by the write head). The bootstrap patterns may include pre-patterned SAM patterns, gray code track-ID patterns, sector-ID patterns, and burst patterns that do not comply with the planarization constraints. The bootstrap patterns may be designed to comply with planarization constraints by using phase patterns (e.g., chevrons), such as shown in FIGS. 4A, 4B, 8, and 10 of the &#39;240. With either pre-patterned gray code or phase patterns, the bootstrap patterns are operational after bulk DC magnetization of the magnetic disk. The bootstrap patterns are typically located at the inner diameter of the magnetic disk and used for track following during the servo track writing of an initial set of servo write assist patterns by the write head. After the initial set of servo write assist patterns have been written by the write head using the bootstrap patterns for track following, additional servo write assist patterns can be written by the write head by track following on this initial set. The servo write assist patterns comply with the planarization constraints. See, for example: U.S. patent application Ser. No. 12/800,300 FIGS. 4 and 5; and the &#39;581 FIGS. 3, 5, 8, and 9. These references, however, do not address how best to write servo data across servo zone boundaries. Accordingly, there exists a need to provide a zoned servo architecture that enables robust servo writing beyond the writing of initial servo data, particularly addressing the need to write servo data across servo zone boundaries. 
     SUMMARY 
     Embodiments described herein provide for patterned media concentric zones with an alternating series of concentric servo zones and overlap zones. The overlap zones facilitate the writing of servo data between servo zones of different servo frequency. In one embodiment, the overlap zones are dual frequency zones. The dual frequency zones have a first set of overlap patterns with the substantially identical pattern as the bordering lower frequency servo zone and a second set of overlap patterns with the substantially identical pattern as the bordering higher frequency servo zone. A bootstrap zone can be included near the inner diameter to assist initial servo writing. In another embodiment, the overlap zones are bootstrap zones. All bootstrap zones have both bootstrap patterns and overlap patterns, the overlap patterns have the substantially identical pattern as a bordering servo zone. Bootstrap patterns only require DC magnetization for servo operability. If the bootstrap patterns do not comply with planarization constraints, their short radial and circumferential length prevents significant disruption to flight of the slider over the magnetic disk. The bootstrap patterns may also be designed to comply with planarization constraints by using phase patterns; multiple phase patterns can provide a substitute track-ID pattern. The overlap patterns require the writing of servo data within the overlap patterns using the write head for servo operability, but overlap patterns comply with the planarization constraints. The radial overlap length of the overlap zones are set to be greater than the RWO. The overlap zones can include extended sync patterns that are paired with data sync patterns in a bordering servo zone. 
     For the dual frequency zone embodiment, the servo patterns and overlap patterns may be arranged in servo sectors that propagate from the inner diameter of the magnetic disk to the outer diameter (e.g., either in an arcuate or generally straight fashion). In the overlap zones, these servo sectors can be alternating frequencies. The odd servo sectors, for instance, can include lower frequency overlap patterns while the even servo sectors can include higher frequency overlap patterns. The overlap patterns can be the substantially identical pattern as the bordering servo zones and/or the substantially identical servo frequency. 
     The radial overlap length of the overlap zones can accommodate the RWO of the slider. As discussed, the RWO changes with the slider&#39;s angular position over the magnetic disk based on the location of the read head and write head on the slider and the arc made by the actuator over the magnetic disk. The radial overlap length of the overlap zones can be adaptable such that the radial overlap length of each overlap zone is long enough to accommodate the maximum RWO in each overlap zone. The radial overlap length can also be some multiple of the maximum RWO for each overlap zone to accommodate manufacturing tolerances. To simplify the magnetic disk pattern design, the radial overlap length of the overlap zones can be a fixed length that is greater than the maximum RWO for all the overlap zones of the entire magnetic disk. 
     The overlap zones can also include extended sync patterns. The extended sync patterns can be read by the read head when the write head is passing over data patterns located outside the overlap zones. The read head signal establish data clock synchronization that is used to establish the correct write head signal frequency. The extended sync patterns can immediately follow the overlap patterns. These extended sync patterns therefore add further data capacity to the magnetic disk. The servo zones bordering the overlap zones may be subdivided into sub-zones with different data frequencies. E.g., each servo zone may have multiple sub-zones with each sub-zone having its own data frequency. 
     The maximum radial length of each servo zone is a function of the planarization constraints and the geometry of the magnetic disk. E.g., the circumference changes less with each additional centimeter of radius at the outer diameter of the magnetic disk when compared to the change in circumference near the inner diameter. 
     Each of the above embodiments may be implemented with a disk drive system and used to facilitate servo writing to the magnetic disk. In some embodiments, methods also provide for servo writing based on the patterned magnetic disk embodiments. Other exemplary embodiments may be described below. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Some embodiments of the present invention are now described, by way of example only, and with reference to the accompanying drawings. The same reference number represents the same element or the same type of element on all drawings. 
         FIG. 1  illustrates a disk drive system in an exemplary embodiment. 
         FIG. 2  illustrates a magnetic disk with a bootstrap zone and an alternating series of concentric servo zones and overlap zones in an exemplary embodiment. 
         FIG. 3  illustrates the detailed view of the boundary area between patterns with different frequencies in an exemplary embodiment. 
         FIG. 4  illustrates a block diagram of two servo zones bordering an overlap zone in an exemplary embodiment. 
         FIG. 5  illustrates a detailed view of the boundary area between patterns with different frequencies in a second exemplary embodiment. 
         FIG. 6  illustrates alternating servo zones and dual frequency zones along with a bootstrap zone at the inner diameter of the magnetic disk in an exemplary embodiment. 
         FIGS. 7A and 7B  illustrate a close-up view of a slider and its relative position over a bootstrap zone patterned at an outer diameter of the magnetic disk in an exemplary embodiment. 
         FIGS. 8A and 8B  illustrate a close-up view of the slider and its relative position over a bootstrap zone patterned at a middle diameter of the magnetic disk in an exemplary embodiment. 
         FIGS. 9A and 9B  illustrate a close-up view of the slider and its relative position over a bootstrap zone patterned at an inner diameter of the magnetic disk in an exemplary embodiment. 
         FIG. 10  illustrates alternating servo zones and bootstrap zones in an exemplary embodiment. 
         FIG. 11  illustrates a servo pattern in the servo zones of the magnetic disk in an exemplary embodiment. 
         FIGS. 12-15  illustrate exemplary embodiments of bootstrap patterns. 
         FIG. 16  illustrates the slider&#39;s RWO and servo writing while track following on a DC-magnetized bootstrap pattern in an exemplary embodiment. 
         FIG. 17  illustrates the slider&#39;s RWO and servo writing while track following on the servo data written into an odd servo sector to write servo data into an even servo sector. 
         FIGS. 18A-18C  illustrate a close-up view of a slider&#39;s variable angular position with its associated changes to RWO in an exemplary embodiment. 
         FIGS. 19 ,  20 A, and  20 B illustrate the variables and calculations used to determine optimal radial dimensions of servo zones in an exemplary embodiment. 
         FIG. 21  illustrates a servo zone boundary between higher and lower frequency servo zones with extended sync patterns that provide data clock synchronization in an exemplary embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The figures and the following description illustrate specific exemplary embodiments of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the invention and are included within the scope of the invention. Furthermore, any examples described herein are intended to aid in understanding the principles of the invention, and are to be construed as being without limitation to such specifically recited examples and conditions. As a result, the invention is not limited to the specific embodiments or examples described below, but by the claims and their equivalents. 
       FIG. 1  illustrates a simplified overhead view of a typical disk drive system  100 , which is suitable to include a magnetic disk  110 , as exemplarily described herein. In the disk drive system  100 , the magnetic disk  110  is rotatably mounted upon a motorized spindle  120 . A slider  122 , having a read head  130  and a write head  140  fabricated thereon, is mounted upon an actuator  150  to “fly” above the surface of the rotating magnetic disk  110 . The disk drive system  100  may also include a controller  170  that controls and drives a positional voltage to a VCM  108  to control the position of the actuator  150 . The disk drive system  100  may also include an inner diameter crash stop  160  to hold the read head  130  and the write head  140  still at a fixed radius relative to the center of the magnetic disk  110 . For example, the actuator  150  pivots about the pivot point  175  against the crash stop  160  to prevent the read head  130  and the write head  140  from traveling past a certain point at the inner diameter. The disk drive system  100  may include other components (e.g., a spindle motor used to rotate the magnetic disk  110 ) that are not shown for the sake of brevity. Additionally, certain components within the disk drive system  100  may be implemented as hardware, software, firmware, or various combinations thereof. 
     In conventional servo writing, a circular track pattern is created by pushing the actuator  150  of the disk drive system  100  against the crash stop  160  and then writing a single track pattern or a group of concentric track patterns at increasing radii. Once enough concentric tracks have been written, the read head  130  may sense previously written servo data and allow propagation of new servo tracks (e.g., both servo bursts and track-ID) across the surface of the magnetic disk  110 . That is, the read head  130  may track follow over previously written servo data while the offset write head  140  is used to write new servo data over tracks that have not yet been servo written. 
     With the advent of patterned media, servo writing is presented with new challenges. For example, track trajectory is generally not concentric with the center of rotation of the spindle  120  because it is difficult if not impossible to concentrically align data patterns with the spindle  120 . 
     Zoned Servo 
       FIG. 2  illustrates one example of the magnetic disk  110  employing a bootstrap zone  400  and an alternating series of servo zones  230  and overlap zones  270 . Within each of the servo zones  230 , the servo frequency can be constant or can be within a range of frequencies. A constant servo frequency in an individual servo zone  230  enhances predictability of the servo frequency, but the circumferential width of the servo patterns generally must increase proportionally to the radius. If the servo frequency is a range of frequencies, then the circumferential width of the servo patterns can be constant within the servo zones  230 . The servo frequency is generally different in each servo zone  230  and the servo zones  230  are patterned from the inner diameter of the magnetic disk  110  to the outer diameter according to increasing servo frequency. 
     The servo zones  230  include both servo patterns and data patterns. Usually, the servo patterns are placed into predictably located servo sectors and the data patterns are placed in data regions between the servo sectors. The data patterns can be arranged into one or more concentric sub-zones within the servo zones  230 . The data frequency within each sub-zone can be is constant or can be within a range of frequencies. In this manner, the data frequency can be optimized with more sub-zones while maintaining a simpler servo zone layout. The data frequency within each sub-zone scales roughly with mean radius. The two sub-zones bordering an overlap zone can be the same frequency. 
       FIG. 4  illustrates the dual frequency zone embodiment. A higher frequency servo zone  230 - 1  and a lower frequency servo zone  230 - 2  border the overlap zone  270 - 1 . In the odd servo sectors  220 - 1  and  220 - 3 , the overlap zone has overlap patterns that are substantially identical to (or substantial identical servo frequency as) the servo patterns of the lower frequency servo zone  230 - 2 . In the even servo sectors  220 - 2  and  220 - 4 , the overlap zone has overlap patterns that are substantially identical to the servo patterns of the higher frequency servo zone  230 - 1 . To propagate the writing of servo data into the dual frequency servo zone  270 - 1 , the read head can track follow on the lower frequency servo zone  230 - 2 . Once the servo data is written into the overlap patterns of the overlap zone  270 - 1 , the overlap patterns&#39; servo data can be used to propagate the servo data into the higher frequency servo zone  230 - 1 . The servo data can be propagated in the opposite radial direction by track following on the higher frequency servo zone  230 - 1  to write servo data in the overlap zone  270 - 1  then track follow using the overlap zone  270 - 1  servo data to write servo data into the lower frequency servo zone  230 - 2 . 
       FIG. 3  illustrates the detailed view of the boundary area between patterns  300  with different frequencies in an exemplary embodiment. Note the width differences of the radial magnetic columns  301  and the non magnetic grooves  302  in the boundary area between patterns  300 , which results in different servo frequencies of the two patterns. The term “radial” with respect to the columns is intended to designate an orientation of the columns in a relatively straight line from the inner diameter of the magnetic disk  110  to the outer diameter. The actual dimensions of the radial columns may vary but generally conform to the planarization constraints discussed above. The patterns also include burst patterns  320  (shown here as a plurality of checkboard sub-patterns).  FIG. 3  shows an embodiment in which the two patterns commence circumferentially together at the leading edge of a servo sector  220 . 
       FIG. 5  shows an alternate embodiment where the two patterns are aligned to end circumferentially together. The  FIG. 5  embodiment makes possible the pairing of extended sync pattern and data sync patterns. The extended sync patterns commence circumferentially after the burst pattern  320  and extends as a set radially from top to bottom of the overlap zone  270 - 1 . Data sync patterns in the higher frequency servo zone  230 - 1  are paired with the extended sync patterns of the overlap zone  270 - 1  and create continuous radial columns. The extended sync pattern will be discussed in more detail below. 
     Overlap Zones can be Dual Frequency Zones 
       FIG. 6  illustrates alternating servo zones and dual frequency zones along with a bootstrap zone  400  at the inner diameter of the magnetic disk  110  in an exemplary embodiment. E.g., the overlap zones are dual frequency zones. As with  FIG. 4 , the dual frequency zones  272 - 1 - 3  are between servo zones  230 - 1 - 4 , the servo patterns and overlap patterns are located within servo sectors  220 - 1 - 3  on the magnetic disk  110 , the overlap patterns in the odd servo sectors  220 - 1  and  220 - 3  are substantially identical to (or of the substantially identical servo frequency) as the bordering lower frequency servo zone, and the overlap patterns in the even servo sector  220 - 2  are substantially identical to (or of the substantially identical servo frequency) as the bordering higher frequency servo zone. In  FIG. 6 , a bootstrap zone  400  is located near the inner diameter of the magnetic disk  110 . The bootstrap zone  400  includes bootstrap patterns interspersed between the servo sectors  220 - 1 - 3 . The bootstrap patterns may also be interspersed within a subset of the servo sectors, such as all the odd servo sectors  220 - 1  and  220 - 3 . The bootstrap zone  400  also includes overlap patterns that are substantially identical to (or of the substantially identical servo frequency) as the bordering servo zone  230 - 4 . Within the servo zones  230 - 1 - 4 , the servo frequency increases from the inner diameter of the magnetic disk  110  to the outer diameter. In this embodiment, the outer diameter radial direction of the magnetic disk  110  is towards the servo zone  230 - 1 . Thus, the servo zone  230 - 1  has the highest servo frequency in  FIG. 6 . The overlap patterns in the dual frequency zones  272 - 1 - 3 , the overlap patterns in the bootstrap zone  400 , and the servo patterns in the servo zones  230 - 1 - 4  comply with planarization constraints. The region between the servo sectors can include data patterns for a data region. 
     The bootstrap patterns in the bootstrap zone  400  may include pre-patterned SAM patterns, gray code track-ID patterns, sector-ID patterns, and burst patterns that do not comply with the planarization constraints. With this pre-patterning, the bootstrap patterns are operational after bulk DC magnetization of the magnetic disk. If the bootstrap patterns do not comply with planarization constraints, their short radial and circumferential length prevents significant disruption to flight of the slider over the magnetic disk. The bootstrap patterns may also be designed to comply with planarization constraints by using phase patterns; multiple phase patterns can provide a substitute track-ID pattern, as demonstrated in FIGS. 4A, 4B, 8, and 10 of the &#39;240. The bootstrap patterns provide a starting point for the servo write process. The read head  130  can track follow on the bootstrap patterns to write servo data into the overlap patterns first in the bootstrap zone  400  and then into the beginning of the first servo zone  230 - 4 . To propagate the writing of servo data into the dual frequency servo zone  270 - 3 , the read head can track follow on the lower frequency servo zone  230 - 4 . Once the servo data is written into the overlap patterns of the dual frequency zone  272 - 3 , the servo data (written within the overlap patterns) can be used to track follow while writing new servo data into the higher frequency servo zone  230 - 3 . The servo data can be propagated in the opposite radial direction by placing the bootstrap zone  400  at the outer diameter of the magnetic disk  110  and following the same process outlined above (though in the opposite radial direction). The servo data can also be propagated by placing the bootstrap zone somewhere between the inner and outer diameter of the magnetic disk  110  and starting the servo writing process from this bootstrap zone. Examples of available bootstrap zone locations are shown and described in  FIGS. 7-9 . Examples of bootstrap patterns are shown and described in  FIGS. 11-15 . Although shown and described with respect to a certain number of servo sectors  220 , associated servo zones  230 - 1 - 4 , dual frequency zones  272 - 1 - 3 , and the bootstrap zone  400 , the invention is not intended to be so limited as these may be established as a matter of design choice. 
       FIGS. 7A and 7B  illustrate a close-up view of the slider  122  and its relative position over a bootstrap zone  400  patterned at an outer diameter of the magnetic disk  110  in an exemplary embodiment.  FIGS. 8A and 8B  illustrate a similar view of the slider  122  and its relative position over a bootstrap zone  400  patterned at a middle diameter of the magnetic disk  110  while  FIGS. 9A and 9B  illustrate a bootstrap zone  400  patterned at an inner diameter of the magnetic disk  110 . Generally, the location of the bootstrap zone  400  depends on the configuration of read head  130  and the write head  140  of the slider  122 . For example, a slider  122  configuration with the read head  130  located more toward the outer diameter of the magnetic disk  110  generally requires that the bootstrap zone  400  be located at the outer diameter, as illustrated in  FIGS. 7A and 7B . A slider  122  configuration with the read head  130  located more toward the inner diameter of the magnetic disk  110  generally requires that the bootstrap zone  400  be located at the inner diameter, as illustrated in  FIGS. 9A and 9B . Bootstrap zone location in the middle diameter of the magnetic disk  110 , as illustrated in  FIGS. 8A and 8B  are likely less effective as the RWO near the middle diameter must be minimal and alignment tolerances would be tight. 
     Overlap Zones can be Bootstrap Zones 
       FIG. 10  illustrates alternating servo zones and bootstrap in an exemplary embodiment. E.g., the overlap zones are bootstrap zones. As with  FIG. 6 , the bootstrap zones  274 - 1 - 3  are between servo zones  230 - 1 - 4 , the servo patterns and overlap patterns are located within servo sectors  220 - 1 - 3  on the magnetic disk  110 , the overlap patterns in the odd servo sectors  220 - 1  and  220 - 3  are substantially identical to (or of the substantially identical servo frequency) as the bordering lower frequency servo zone, and the overlap patterns in the even servo sector  220 - 2  are substantially identical to (or of the substantially identical servo frequency) as the bordering higher frequency servo zone. In  FIG. 10 , an additional bootstrap zone  400  is located near the inner diameter of the magnetic disk  110 . The bootstrap zone  400  includes bootstrap patterns interspersed between the servo sectors  220 - 1 - 3 . (The bootstrap patterns may also be interspersed within a subset of the servo sectors, such as all the odd servo sectors). The bootstrap zones  400  include also overlap patterns that are substantially identical to (or of the substantially identical servo frequency) as a bordering servo zone  230 . Within the servo zones  230 - 1 - 4 , the servo frequency increases from the inner diameter of the magnetic disk  110  to the outer diameter. In this embodiment, the outer diameter radial direction of the magnetic disk  110  is towards the servo zone  230 - 1 . Thus, the servo zone  230 - 1  has the highest servo frequency in  FIG. 10 . The overlap patterns in the bootstrap zones  274 - 1 - 3 , the overlap patterns in the bootstrap zone  400 , and the servo patterns in the servo zones  230 - 1 - 4  comply with planarization constraints. The region between the servo sectors can include data patterns. 
     The bootstrap patterns in the bootstrap zones  274 - 1 - 3  and  400  may include pre-patterned SAM patterns, gray code track-ID patterns, sector-ID patterns, and burst patterns that do not comply with the planarization constraints. With this pre-patterning, the bootstrap patterns are operational after bulk DC magnetization of the magnetic disk. If the bootstrap patterns do not comply with planarization constraints, but their short radial and circumferential length prevents significant disruption to flight of the slider  122  over the magnetic disk  110 . The bootstrap patterns may also be designed to comply with planarization constraints by using phase patterns; multiple phase patterns can provide a substitute track-ID pattern, as demonstrated in FIGS. 4A, 4B, 8, and 10 of the &#39;240. 
     The bootstrap patterns provide a starting point for the servo write process both at the beginning of the servo write process and between servo zones of different servo frequency. E.g., at the beginning of the servo write process, the read head  130  can track follow on the bootstrap patterns in the bootstrap zone  400  to write servo data into the overlap patterns first in the bootstrap zone  400  and then into the beginning of the first servo zone  230 - 4 . E.g., the read head  130  can track follow on the bootstrap patterns in the bootstrap zones  274 - 1 - 3  to write servo data into the overlap patterns first in the bootstrap zone  274 - 3  and then into the beginning of the second servo zone  230 - 3 . The servo data can be propagated in the opposite radial direction by placing a bootstrap zone at the outer diameter and following the same process outlined above (though in the opposite radial direction). The servo data can also be propagated by starting the servo write process at a bootstrap zone somewhere between the inner and outer diameter of the magnetic disk  110 . Examples of available bootstrap zone locations are shown and described in  FIGS. 7-9 . Examples of bootstrap patterns are shown and described in  FIGS. 11-15 . Although shown and described with respect to a certain number of servo sectors  220 , associated servo zones  230 - 1 - 4 , bootstrap zones  274 - 1 - 3  and  400 , the invention is not intended to be so limited as these may be established as a matter of design choice. 
       FIG. 11  illustrates a pattern  300  that can be used as a servo pattern in a servo zone or an overlap pattern in an overlap zone. The pattern  300  of this embodiment includes a plurality of radial magnetic columns  301  patterned on the magnetic disk  110  separated by radial non magnetic grooves  302  (e.g., via the removal of the magnetic material between the radial magnetic columns  301 ). The radial magnetic columns  301  and radial non magnetic grooves  302  define a servo write assist pattern  310  in which servo data may be written. 
     The pattern  300  also includes burst pattern  320  comprising a plurality of magnetic islands  341  separated by non magnetic depressions  342 , or “valleys”, (e.g., again via the removal of the magnetic material between the magnetic islands  341 ). The burst pattern  320 , in this embodiment, comprises a first and second checkerboard sub-pattern  321  and  322 , respectively, that are radially offset by some fraction of a magnetic island dimension (e.g., between a quarter to half an island width). The checkerboard sub-patterns  321  and  322  may be substantially symmetric. For example, each of the checkerboard sub-patterns  321  and  322  may comprise about the same size and number of magnetic islands  341 . The invention, however, is not intended be limited to the checkerboard sub-patterns  321  and  322  exemplary embodiment as other types of burst patterns may be employed. For example, an ABCD pattern can be configured with four sub-patterns, such as with the A &amp; B patterns at the location of the sub-pattern  321  and the C &amp; D patterns at the location of the sub-patterns  322 . Alternatively, the burst pattern  320  may be configured with three or four roughly equal sized checkerboard sub-patterns with each being radially offset to one another by some fraction of a magnetic island dimension. 
     The pattern  300  also includes a pad pattern  351  comprising a number of radial magnetic columns  301  and radial non magnetic grooves  302 . The pad pattern  351  may be used as a read-to-write timing delay buffer between the pattern  300  and subsequent data patterns. Alternatively or additionally, the pad pattern  351  may be used to write additional servo data such as RRO values. The physical dimensions and spacings of the radial magnetic columns  301  in the pad pattern  351  may be different from the magnetic column  301  dimensions elsewhere in the pattern  300 . 
       FIGS. 12-15  illustrate exemplary bootstrap patterns that may be patterned on the magnetic disk  110 . As mentioned, the bootstrap pattern has servo data pre-patterned into the magnetic disk  110  to facilitate subsequent servo writing. The bootstrap patterns may include pre-patterned servo sync patterns, SAM patterns, gray code track-ID patterns, sector-ID patterns, and burst patterns that do not all comply with the planarization constraints. With this pre-patterning, the bootstrap patterns are operational after bulk DC magnetization of the magnetic disk  110 . 
     In  FIG. 12 , much of the bootstrap pattern  550  does not comply with planarization constraints. The  552  area for instance can include SAM patterns, gray code track-ID patterns, and sector-ID patterns that are irregular in shape and do not comply with planarization constraints. The burst pattern  553  can be designed in a checkerboard pattern that is more likely to comply with planarization constraints. In  FIG. 12  there are four checkboard sub-patterns but two or three sub-patterns can also be used. The servo sync pattern  551  and pad pattern  554  (which include a plurality of radial magnetic columns  301  and radial non magnetic grooves  302 ) can more easily comply with planarization constraints due to their repetitive pattern. The bootstrap pattern  550  is operational after bulk DC magnetization of the magnetic disk  110 . 
       FIG. 13  illustrates another bootstrap pattern  600  with an ABCD pattern  607 . In this embodiment, a series of radial magnetic columns  301  and radial non magnetic grooves  302  provide a servo sync pattern  604 , a SAM pattern  605 , a gray code track-ID pattern  606 , and an ABCD pattern  607 . The bootstrap pattern  600  is operational after bulk DC magnetization of the magnetic disk. 
     Bootstrap patterns  650  and  700  (of  FIGS. 14 and 15 , respectively) are designed to better comply with the planarization constraints. In each of these embodiments, the bootstrap patterns  650  and  700  are configured with phase patterns.  FIG. 14  has sets of radial magnetic columns  654  bookending the phase patterns. The phase patterns have magnetic columns  651  and non magnetic grooves  652  which make acute angles to the radial direction and look like chevrons. The sets of radial magnetic columns  654  at the start the bootstrap pattern  650  can function as a SAM pattern. The phase patterns of  FIG. 14  can be used similar to other burst patterns (such as ABCD patterns or checkerboard sub-patterns) to determine fractions of a track width.  FIG. 15  is illustrated with five sets of phase patterns, which provide additional track-ID servo data, similar to the use of gray code track-ID patterns. See FIGS. 4A, 4B, 8, and 10 of the &#39;240 for alternative designs. 
       FIGS. 16 and 17  illustrate servo writing via a bootstrap pattern  550  in an exemplary embodiment. In this embodiment, the bootstrap pattern  550  has pre-patterned servo sync patterns and SAM patterns  559 , sector-ID and track-ID patterns  552 , checkboard sub-patterns  553 , and a pad pattern  554 . The SAM pattern, sector-ID pattern, and the track-ID pattern need not comply with planarization constraints. With this pre-patterning, the bootstrap patterns are operational after bulk DC magnetization of the magnetic disk  110 . The bootstrap pattern  550  may be patterned within a bootstrap zone  400  near the inner diameter of the magnetic disk  110 , as illustrated in  FIG. 6 . The bootstrap pattern  550  may also be patterned within any of the bootstrap zones illustrated in  FIG. 10  items  270 - 1 - 3  and  400 . In both  FIGS. 6 and 10 , the bootstrap zone  400  near the inner diameter of the magnetic disk  110  includes both bootstrap patterns and overlap patterns. During servo writing, the read head  130  on the slider  122  can track follow on the bootstrap patterns  550  (that have been DC magnetized) and write servo data into the patterns  300  of servo sectors  220  using the write head  140  on the slider  122 . As illustrated in  FIG. 17 , the read head  130  can track follow on the servo data previously written into the pattern  300  of an odd servo sector  220 - 1  to write servo data into the pattern  300  of an even servo sector  220 - 2  using the write head  140  on the slider  122 . The servo sectors  220  may alternate between “odd” and “even” as demonstrated in  FIGS. 6 and 10 . The drive electronics interpret the slider&#39;s  122  position from the read signal from the read head  130 , adjusts the position of the slider  122  over the magnetic disk  110 , and controls the write signal to the write head  140 . Note that the RWO between the read head  130  and the write head  140  results in the writing of servo data into a different track than the track that is read for track following. This RWO allows the controller  170  to stepwise propagate the servo data writing from the inner diameter of the magnetic disk  110  to the outer diameter by track following on previously written servo data read by the offset read head  130 . 
     Radial Overlap Lengths of Overlap Zones 
     It is likely desirable to not place user data within the overlap zones. Bootstrap patterns, for instance, are place between servo sectors (a region normally reserved for user-data) and would likely disrupt the normal data layout architecture. Dual frequencies, for instance, would create greater complexity for the servo control electronics. This increased complexity may only be manageable during the servo writing process at the manufacturing site where additional processing power and time can be allocated. If no data is placed into the overlap zones, less real estate is available on the magnetic disk  110  for data patterns. To avoid substantial losses to data capacity for the magnetic disk  110 , the radial overlap length of the overlap zones can be minimized A primary factor for the radial overlap length of the overlap zones is the RWO. 
     In the  FIG. 6  embodiment, the overlap zones are dual frequency zones. In the  FIG. 10  embodiment, the overlap zones are bootstrap zones. For both embodiments, a first set of servo data is read for track following to write a second set of servo data. In the dual frequency zones, servo data is first written to the first set of overlap patterns (such as the lower frequency overlap patterns of the odd servo sectors  220 - 1  and  220 - 3 ) then used for track following to write servo data into the second set of overlap patterns (such as the higher frequency overlap patterns of the even servo sectors  220 - 2 ). In the bootstrap zone embodiment of  FIG. 10 , the bootstrap patterns (after DC magnetization) are used for track following to write the overlap patterns. In either embodiment, aggressive decreases to the radial overlap length of the overlap zone may at some point result in reliability issues during the servo writing process. Sliders with larger RWO will likely require overlap zones with longer radial overlap lengths and sliders with smaller RWO will likely enable overlap zones with shorter radial overlap lengths. 
     In one embodiment, all the overlap zones  270  may be configured with a radial distance that is at least greater than or equal to the maximum RWO  902  of the slider  122  for all radial locations of the magnetic disk  110 . For the rough dimensions shown in  FIGS. 18A-C , the radial overlap length of every overlap zone would be greater than or equal to the RWO of  FIG. 18C . A substantially constant radial overlap length for every overlap zone can simplify the design of the magnetic disk  110 , but could reduce slightly the data capacity of the disk drive device  100 . A substantially constant radial length can be used on just one side of each magnetic disk  110  or both sides of each magnetic disk  110 . The substantially constant radial overlap length should be greater than the maximum RWO for any of the overlap zones  270  on each side of the magnetic disk  110  and/or on both sides of the magnetic disk  110 . In an alternative embodiment, the radial overlap length of each overlap zone  270  may be individually configured based on the RWO at the radius of each overlap zone. For example considering  FIGS. 18A-C , the overlap zones  270  closer to the inner diameter of the magnetic disk  110  may be configured with shorter radial overlap lengths than the overlap zones  270  that are nearer the outer diameter. 
     Radial Lengths of Servo Zones 
     Another consideration regarding the layout of servo zones  230  and overlap zones  270  involves the geometry of the magnetic disk  110 . Near the inner diameter, for instance, the circumference of a track will double when the radius increases from one centimeter to two centimeters. Near the middle diameter of the magnetic disk  110 , the circumference of a track will only increase 50% when the radius increases from two centimeters to three centimeters. Near the outer diameter of the magnetic disk  110 , the circumference of a track will only increase by 25% when the radius increases from four centimeters to five centimeters. If the planarization constraints allow a maximum 25% deviation within a servo zone  230 , then near the outer diameter of the magnetic disk  110  the maximum servo zone  230  radial length can be about one centimeter. However, near the inner diameter of the magnetic disk  110 , a maximum 25% deviation would limit the maximum servo zone  230  radial length to about 2.5 milimeters. Near the middle diameter of the magnetic disk  110 , a maximum 25% deviation would limit the maximum servo zone  230  radial length to about five millimeters. 
       FIG. 19  illustrates the magnetic disk  110  with radial dimension and zone frequency considerations of servo zones  230  that are used to compute servo zone radial lengths in an exemplary embodiment. For example, the design parameters used in constructing the servo patterns include the servo frequency f i  (i=1 . . . n, where n is simply used herein as an integer greater than 1, but not to designate actual frequency) of the servo zone  230  based on its radial dimension r i  (i=0 . . . n) on the magnetic disk  110 . 
     To conform to the various planarization constraints described above (e.g., the Dry Planarization Design Rules #1 and #2), the widths of data tracks and/or non magnetic grooves separating those tracks may increase (e.g., increased track pitch, T p ). A similar consideration is the changing “pitch” of the servo patterns within the servo zones  230 . In this regard, the maximum circumferential pitch difference may be calculated as (maximum C p −minimum C p ). Other factors to consider are the number of zones n on the magnetic disk  110 , the revolution N of the magnetic disk  110 , the innermost radius r 0  of the magnetic disk  110 , and the outermost radius r n  of the magnetic disk  110 . Generally, to comply with planarization constraints, the preamble pitch of a servo pattern is configured as C p =2πrN/f, where the minimum circumferential pitch C p  value is same for each servo zone  230  and the maximum circumferential pitch C p  value is same for each servo zone  230 . To conform to the land/groove dimension ratios of the Dry Planarization Design Rule #1 at data and servo zones, boundary conditions for the servo zones  230  are generally configured as the minimum C p =T P , which provides suitable fly high control of the slider  122 . In this regard, the maximum C p =T p  and [(maximum C p )−(minimum C p )]/2+(minimum C p )=T p . 
     With these concepts in mind, the following mathematical relationships may exist (e.g., without yet considering the overlap zones  270 ): 
     
       
         
           
             
               
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     When the overlap zones  270  of the servo zones  230  comprise a length 2l, boundary conditions are the minimum C P =T p , with the minimum C P  being same value for each zone servo zone  230 . From this, the following may be computed: 
     
       
         
           
             
               
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     The maximum C P  is also same value for each servo zone  230  and can be computed as follows: 
     
       
         
           
             
               
                 
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     Simple solutions to the sizing and pitch considerations of the servo zones  230 , the servo patterns therein, and the overlap zones  270  do not generally exist. However, these may be characterized in the following pattern  300  and graph  950  illustrated in  FIGS. 20A and 20B  respectively. The graph  950  illustrates the changing pitch and radial dimension of the pattern  300  based on its particular location on the magnetic disk  110  (e.g., within a particular servo zone  230 ). In  FIG. 20B , a bootstrap zone (Zone-B) and a series of alternating servo zones  230  (odd numbered zones) and overlap zones  270  (even numbered zones) are configured from the inner diameter of the magnetic disk  110  to the outer diameter according to increasing frequency. In this instance, Zone B is configured closest to the inner diameter the magnetic disk  110  followed by the servo zone  230  labeled “Zone 7”, having the lowest servo frequency in this example. In similar fashion, the servo zone  230  labeled “Zone 1” is configured closest to the outer diameter of the magnetic disk  110  and has the highest servo frequency in this example. Assuming that each of the servo zones is configured with the pattern  300  of  FIG. 20A , the graph  950  provides an insight as to how the pattern  300  may increase in radial dimension and pitch based on its location within higher frequency servo zones  230 . Similar insight can be gained from the graph  950  into the overlap zones  270 . While certain dimensions for a particular servo pattern on a magnetic disk are illustrated in the graph  950 , the invention is not intended to be limited to any particular dimension or servo pattern. The graph  950  is merely intended to provide one manner in which zoned servo may be sized and implemented. 
     Extended Sync Patterns 
       FIG. 21  illustrates a boundary area between patterns  300 - 1  and  300 - 2  wherein the overlap zone  270  has extended sync patterns. In the servo zone  230 - 1 , pattern  300 - 1  has a burst pattern  320 - 1 , a pad pattern  351 - 1 , and a data sync pattern (on the leftward portion of the data region  240 - 1   a ). In the servo zone  230 - 2 , pattern  300 - 2  has a burst pattern  320 - 2 , a pad pattern  351 - 2 , and a data sync pattern (on the leftward portion of the data region  240 - 2 ). Servo zone  230 - 1  has a higher servo frequency than the servo zone  230 - 2 . Between the servo zones  230 - 1  and  230 - 2  is an overlap zone  270  with an extended sync pattern (on the leftward portion of the data region  240 - 1   b ) that is substantially identical in frequency as the data sync pattern in data region  240 - 1   a . Note that, as in  FIG. 5 , the pad patterns  351 - 1  and  351 - 2  end circumferentially at trailing edge of the servo sectors  220 ; in this manner, pairs of the extended sync patterns of the overlap zone  270  and the data sync patterns in the higher frequency servo zone  230 - 1  commence circumferentially together along the trailing edge of the servo sectors  220  and align into a set of radial columns. Eg, the servo patterns and the overlap patterns are not aligned necessarily at the leading edge of the servo sectors  220  but are aligned at the trailing edge of the servo sector such that pairs of extended sync patterns and data sync patterns commence circumferentially together as a pair at the trailing edge of the servo sector. 
     The extended sync pattern enables the read head  130  (which has RWO from the write head  140 ) to acquire data clock synchronization for writing data in the data region  240 - 1   a . The overlap zone  270  also has overlap patterns that are substantially identical to the servo patterns in servo zone  230 - 2 . Data region  240 - 1  has a higher frequency than the data region  240 - 2 . As previously mentioned, the overlap zones  270  are generally not used for user data, so the data patterns would normally be limited to data regions  240 - 1   a  and  240 - 2  of  FIG. 21 . However, to increase data capacity of the magnetic disk  110 , the overlap zones  270  can include data patterns, such as in the data region  240 - 1   b  of  FIG. 21 . There may be pairs of extended sync patterns and data sync patterns between the servo sectors  220 , as many of the data blocks commence between the servo sectors in typical disk drive device data region layouts and/or are split in two by the servo sectors  220 . 
     Although specific embodiments were described herein, the scope of the invention is not limited to those specific embodiments. The scope of the invention is defined by the following claims and any equivalents thereof.