Abstract:
A servo system for writing servo information on a recording medium of a hard disk drive and for determining head position based on the servo information with a tracking accuracy, the servo system including a first magnetic head and a servo writer. The servo writer is configured to write the servo information on the recording medium by forming, using the first magnetic head, a first servo pattern comprising a servo burst pattern on a segment of a first track of a first layer of the recording medium; and by forming a second servo pattern comprising addressing information on a segment of a second track of a second layer of the recording medium. The second track is arranged half a width of the first track from the first track to increase the tracking accuracy of the servo system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/378,778 (now U.S. Pat. No. 9,111,562), filed Aug. 14, 2014, which is a 371 of International Application No. PCT/SG2013/000065, filed Feb. 18, 2013, which claims the benefit of priority of Singapore Patent Application No. 201201136-7, filed 17 Feb. 2012. The entire disclosures of the above applications are incorporated herein by reference. 
    
    
     FIELD 
     Various aspects of this disclosure relate to recording media and methods of writing servo information on the same. 
     BACKGROUND 
     In conventional hard disks having a rotating magnetic medium, there is only one magnetic layer to record data. As such, the servo pattern information is recorded in servo wedges. This way of storing servo information is also known as embedded servo.  FIG. 1  is a schematic  100  showing the arrangement of servo pattern information  102  along a track on the magnetic layer. Servo information  102  is interspersed with data information  104 . 
     The position of the read/write head is demodulated from the servo wedges. Thus, the sampling frequency of the servo control system is thus limited by the number of servo wedges in one revolution, and the rotating speed of disk. To improve the servo performance, such as tracking accuracy for high track density drive, a higher sampling rate is needed, which in turn requires more servo wedges to be placed in the magnetic layer. However, this will cause less data sector to be available for recording the user data. This is undesirable. 
     SUMMARY 
     In various embodiments, a method of writing servo information to a recording medium may be provided. The method include forming a first servo pattern (including a servo burst pattern) on a segment of a first track of a first layer of the recording medium, the first track having a track width. The method may further include forming a second servo pattern (including addressing information) on a segment of a second track of a second layer of the recording medium. The second track is arranged from the first track by half the track width. 
     In various embodiments, a recording medium may be provided. The recording medium may include a first servo pattern including a servo burst pattern on a segment of a first track of a first layer of the recording medium. The method may further include a second servo pattern including addressing information on a segment of a second track of a second layer of the recording medium. The second track may be arranged from the first track by half the track width. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which: 
         FIG. 1  shows the arrangement of servo pattern information along a track on the magnetic layer. 
         FIG. 2  shows a top planar view of a recording medium. 
         FIG. 3A  shows the cross sectional side view of a portion of a data recording medium along a plane parallel to the circumference of the data recording medium according to various embodiments;  FIG. 3B  shows the cross-sectional side view across three data tracks and two servo tracks according to various embodiments;  FIG. 3C  shows the top view of a data track and two servo tracks according to various embodiments; and  FIG. 3D  shows types of address information according to various embodiments. 
         FIG. 4  shows a method of writing servo information according to various embodiments on a recording medium. 
         FIG. 5A  shows a cross sectional side view of a data recording medium along a plane parallel to the circumference of the data recording medium with servo information stored according to various embodiments;  FIG. 5B  shows the cross-sectional side view across a data track and a servo track with servo information stored according to various embodiments; and  FIG. 5C  shows the top view of a data track and a servo track with servo information stored according to various embodiments. 
         FIG. 6  shows the top view of portions of data tracks and two servo tracks with servo information stored according to various embodiments. 
         FIG. 7  shows the side view of portion along a servo track in the first layer and a data track along the second layer with data and servo information stored according to various embodiments. 
         FIG. 8 , which include  8 A to G, shows a method to arrange the servo information shown in  FIG. 7  according to various embodiments; wherein  FIG. 8A  shows a top view of servo tracks in a first layer after a writing pass according to various embodiments; wherein  FIG. 8B  shows the side view of a portion along a servo track in the first layer and the overlying second layer after a writing pass according to various embodiments; wherein  FIG. 8C  shows the cross section of the first track on the first layer as well as overlying tracks and on the second layer; wherein  FIG. 8D  shows a wide head according to various embodiments; wherein  FIG. 8E  shows a wide head in operation according to various embodiments; wherein  FIG. 8F  shows a top view of servo tracks a first layer as well as portions of data tracks in a second layer after a subsequent writing pass according to various embodiments; and wherein  FIG. 8G  shows the side view of a portion along a servo track in the first layer and the overlying second layer after a subsequent writing pass according to various embodiments. 
         FIG. 9A  shows a signal map of a conventional single layer recording medium;  FIG. 9B  shows another signal map of a conventional single layer recording medium;  FIG. 9C  shows a signal map of a multi-layer recording medium according to various embodiments;  FIG. 9D  shows the signal map of the multi-layer recording medium after a writing pass is carried out according to various embodiments;  FIG. 9E  shows the signal map of the multi-layer recording medium after a subsequent writing pass is carried out according to various embodiments. 
         FIG. 10  shows the side view of portion along a servo track in the first layer and a data track along the second layer with data and servo information stored according to various embodiments. 
         FIG. 11 , which includes  FIGS. 11A  to H, shows a method to arrange the servo information shown in  FIG. 10  according to various embodiments; wherein  FIG. 11A  shows a top view of portions of a plurality of tracks configured to store servo information after a first step; wherein  FIG. 11B  shows a side view of  FIG. 11A ; wherein  FIG. 11C  shows a top view of portions of a plurality of tracks configured to store servo information after a second step; wherein  FIG. 11D  shows a side view of  FIG. 11C ; wherein  FIG. 11E  shows a top view of portions of a plurality of tracks configured to store servo information after a third step; wherein  FIG. 11F  shows a side view of  FIG. 11E ; wherein  FIG. 11G  shows a top view of portions of a plurality of tracks configured to store servo information after a fourth step; and wherein  FIG. 11H  shows a side view of  FIG. 11G . 
         FIG. 12  shows the signal map of a multi-layer recording medium after a writing pass is carried out according to various embodiments. 
     
    
    
     DESCRIPTION 
     The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. 
     The word “exemplary” is used herein to mean “serving as an example, instance, or illustration”. Any embodiment or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments or designs. 
     The word “over” used with regards to a deposited material formed “over” a side or surface, may be used herein to mean that the deposited material may be formed “directly on”, e.g. in direct contact with, the implied side or surface. The word “over” used with regards to a deposited material formed “over” a side or surface, may be used herein to mean that the deposited material may be formed “indirectly on” the implied side or surface with one or more additional layers being arranged between the implied side or surface and the deposited material. 
     The product can be a consumer electronic device which can be operable in various orientations, and thus it should be understood that the terms “top”, “bottom”, “base”, “down”, “sideways”, “downwards” etc., when used in the following description are used for convenience and to aid understanding of relative positions or directions, and not intended to limit the orientation of the recording medium or the product incorporating the recording medium. 
     Various aspects may provide a recording medium and a method of writing servo information on the same that is able to address at least partially the abovementioned challenges. 
       FIG. 2  is a schematic  200  showing a top planar view of a recording medium. The recording medium may have tracks  202  running in parallel to the circumference of the recording medium. The recording medium may also be divided into data sectors  204  with addressing information (eg. GrayCode  206  and other addressing information  208 ) interspersed between the data sectors. 
     In various embodiments, the recording medium may be in the shape of a disk. The disc may have a first main surface and a second main surface substantially parallel to the first main surface. The disc may also have a lateral side joining the first main surface and the second main surface. The lateral side may be substantially perpendicular to the first main surface and the second main surface. 
     The medium may be a magnetic medium. The first layer or servo layer may be a magnetic layer or more specifically a ferromagnetic layer. The second layer or data recording layer (alternatively called the data layer) may be a magnetic layer or more specifically a ferromagnetic layer. The first magnetic layer and the second magnetic layer may form a single monolithic structure. Further, the first magnetic layer and the second magnetic layer may be put on the same side of a disk platter and they may be read and/or processed together. 
     In other words, a magnetic medium including two layers may be provided to store information. One of the layer is stacked on top of the other layer. The second layer (which may alternatively be called the data recording layer) may be configured to store user data. References to “data” usually refer to “user data”, unless it is otherwise clear from the context. A portion of the servo information is stored in the first layer (which may alternatively be called the servo layer). 
       FIG. 3A  is a schematic  300   a  showing the cross sectional side view of a portion of a data recording medium along a plane parallel to the circumference of the data recording medium according to various embodiments. In various embodiments, the recording medium may include two layers, i.e. a first layer  302  and a second layer  304 . One layer may be stacked over another layer to form the disk (i.e. the two layers form a stacked arrangement). In various embodiments, the second layer  304  may be arranged over the first layer  302 . In various embodiments, the first layer  302  may be configured to record at least a portion of servo pattern information and is called the servo layer. In various embodiments, the second layer  304  may be configured to record data and is called the data recording layer (or data layer). The second layer  304  may be further configured to record addressing information of servo pattern information. The first layer  302  may be separated from the second layer  304  by one or more intermediate layers  306 . Alternatively, the second layer  304  may be on the first layer  302 . 
     The recording medium may have a plurality of tracks. The plurality of tracks (for instance, a first track and a second track) may run parallel to the circumference of the recording medium. As such, the first track may be parallel to the second track. The plurality of tracks may form a plurality of concentric rings on the recording medium. The plurality of concentric rings have a common centre coinciding with an axis of the disk running from the center of the first main surface of the disk to the second main surface of the disk. In operation, the recording medium may be rotated about the axis of the disk. Each track may include a plurality of segments along the length of the track. 
       FIG. 3B  is a schematic  300   b  showing the cross-sectional side view across three data tracks  304   a ,  304   b ,  3404   c  and two servo tracks  302   a ,  302   b  according to various embodiments.  FIG. 3B  may correspond to the view in a direction indicated by arrow  308  in  FIG. 3A . The data tracks  304   a ,  304   b ,  304   c  may be configured to record data. The data tracks  304   a ,  304   b ,  304   c  may be further configured to record addressing information of servo pattern information. The second layer or data layer  304  may include the data tracks  304   a ,  304   b ,  304   c . The servo tracks  302   a ,  302   b  may be configured to record at least a portion of servo information. For instance, the servo tracks  302   a ,  302   b  may be configured to record servo bursts. The first layer or servo layer  302  may include the servo tracks  302   a ,  302   b . A reader head  310  may be positioned over the tracks. 
       FIG. 3C  is a schematic  300   c  showing the top view of a data track  304   b  and two servo tracks  302   a ,  302   b  according to various embodiments.  FIG. 3C  may correspond to the view indicated by arrow  312  in  FIG. 3A . 
     In various embodiments, servo information may include a first servo pattern. The first servo pattern may include a servo burst pattern. Servo information may also include a second servo pattern. The second servo pattern may include addressing information. In various embodiments, the first servo pattern may be stored in the first layer or servo layer  302 . The second servo pattern (including addressing information) may be stored in the second layer or data recording layer  304 . Additionally, data may be stored in the second layer or data layer  304 . In various embodiments, storing the first servo pattern in the servo layer  302  may free up space in the data recording layer  304  and may improve surface utilization rate for the data recording layer  304 . 
       FIG. 3D  is a schematic  300   d  showing the types of address information according to various embodiments. Addressing information may include automatic gain control (AGC)  314 . Additionally, addressing information may include sector address mark (SAM)  316 . Addressing information may also include GrayCode  318 . AGC  314  may occupy 80 bits, while SAM  316  may occupy 12 bits and GrayCode  318  may occupy 24 bits. 
       FIG. 4  is a schematic  400  that shows a method of writing servo information according to various embodiments on a recording medium. The method may provide, in  402 , forming a first servo pattern (including a servo burst pattern) on a segment of a first track of a first layer of the recording medium, the first track having a track width. The method may further provide, in  404 , forming a second servo pattern (including addressing information) on a segment of a second track of a second layer of the recording medium. The second track may be arranged from the first track by half the track width. 
     In various embodiments, the first track may be servo track  302   a  as shown in  FIGS. 3B and 3C  the second track may be data track  304   b  as shown in  FIGS. 3B and 3C . The first track may have a track width. The second track may also have a track width, which may be substantially equal to the track width of the first track. In fact, in various embodiments, the track width of each track in the recording medium, including each servo track and each data track, may be substantially equal to one another. 
     The second track  304   b  may be arranged from the first track  302   a  by half a track width. In other words, the second track  304   b  may be arranged from the first track  302   a  such that the projection of the second track  304   b  on a plane parallel to the first main surface of the recording medium (or the second main surface of the recording medium) is half a track width from the projection of the first track  302   a  on the plane. 
       FIG. 5A  is a schematic  500   a  showing a cross sectional side view of a data recording medium along a plane parallel to the circumference of the data recording medium with servo information stored according to various embodiments. The recording medium may include a first layer  502  and a second layer  504  on the first layer  502 .  FIG. 5B  is a schematic  500   b  showing the cross-sectional side view across a data track  504   b  and a servo track  502   a  with servo information stored according to various embodiments.  FIG. 5B  may correspond to the view in a direction indicated by arrow  508  in  FIG. 5A .  FIG. 5C  is a schematic  500   c  showing the top view of a data track  504   b  and a servo track  502   a  with servo information stored according to various embodiments.  FIG. 5C  may correspond to the view indicated by arrow  512  in  FIG. 5A . 
     In various embodiments, a recording medium may be provided. The recording medium may include a first servo pattern  514  (including a servo burst pattern) on a segment of a first track  502   a  of a first layer  502  of the recording medium. The recording medium may also include a second servo pattern  516  (including addressing information) on a segment of a second track  504   b  of a second layer  504  of the recording medium. The second track  504   b  may be arranged from the first track  502   a  by half the track width. 
     In other words, servo information may be arranged on the recording medium such that a first portion of the servo information (i.e. the first servo pattern  514  including a servo burst pattern) is on a first layer  502 . A second portion of the servo information (i.e. a second servo pattern  516  including addressing information) is on a second layer  504 . The first servo pattern  514  may be stored on a segment of the first track  502   a  located in the first layer  502  of the recording medium. The second servo pattern  516  may be stored on a segment of the second track  504   b  located in the second layer  504  of the recording medium. The projection of the first track  502   a  on a plane parallel to the first main surface (or second main surface) of the recording medium may be half a track width from the projection of the second track  504   b  on the plane. 
     In order to demodulate the addressing information in real time, the addressing information may be required to be of a good quality. As the second layer (data recording layer) is positioned closer to a reader (or read head) during reading, it may be desirable for addressing information to be stored in the second layer (data recording layer). The addressing information may include automatic gain control (AGC). Additionally, addressing information may include sector address mark (SAM). Addressing information may also include GrayCode. 
     According to various embodiments, the automatic gain control (AGC) may also be used to establish a feedback clock signal for synchronization of media read and write operations to minimize the effect of spindle speed fluctuation. The AGC may also be used to normalize the amplitude of read-back signal for gray code detector and SAM detector. The SAM may provide the down-track information which tells the number of servo sectors that read head has crossed. The GrayCode may indicate the servo track number (track address) that read head is following. All servo sectors on a servo track have the same servo track number encoded. 
     The number of track address markings may be reduced to 100 while still maintaining performance of the track seeking process. While the addressing information such as the GrayCode is stored in the second layer (data recording layer), the total area of the data area for storing servo information may still less than 2%. As such, by storing the servo bursts in the first layer (servo layer) and storing the addressing information in the second layer (first layer), large amount of area may be devoted for storing data without reducing performance. 
       FIG. 6  is a schematic  600  showing the top view of a data track  604   b , portions of data tracks  604   a ,  604   c  and two servo tracks  602   a ,  602   b  with servo information stored according to various embodiments. Only portions of the data tracks  604   a ,  604   c  having servo information  616   a ,  616   c ,  616   d ,  616   f  are shown. The remaining portions of the data tracks  604   a ,  604   c  are indicated by dashed lines. The second layer (including data tracks  604   a ,  604   b ,  604   c ) may be arranged over the first layer (including servo tracks  602   a ,  602   b ). 
     The recording medium may include a first servo pattern  614   a ,  614   c  (including a servo burst pattern) on a segment of a first track  602   a  of a first layer of the recording medium. The recording medium may also include a second servo pattern  616   b ,  616   e  (including addressing information) on a segment of a second track  604   b  of a second layer of the recording medium. The recording medium may further include a third servo pattern  614   b ,  614   d  including a subsequent servo burst pattern on a segment of a third track  602   b  of the first layer of the recording medium. The third track  602   b  may be arranged from the second track  604   b  by half a track width. 
     The second track  604   b  may be between the first track  602   a  and the third track  602   b . “Between” may mean that the projection of the center line of the second track  604   b  on a plane parallel to the first main surface (or second main surface) of the recording medium is between the projection of the center line of the first track  602   a  on the plane and the projection of the center line of the third track  602   b  on the plane. The center line of a track runs along the track. 
     In various embodiments, the servo burst pattern may be of a first frequency (II) and the subsequent servo burst pattern may be of a second frequency (f 2 ). The servo burst patterns provide dual frequency signals to a read head. A positioning error signal (PES) may generated by finding the difference in signal amplitude between f 1  and f 2 . When a read head is positioned over the edge between the first track  602   a  and the third track  602   b  (i.e. over the entire width of the second track  604   b ), PES may be at a minimum. The read head may ideally be positioned over the edge during reading for good track following. 
     In various embodiments, the servo burst pattern may be of a single frequency or of multi-frequencies. For instance, the servo burst pattern may also be of triple frequencies. In various embodiments, the servo burst pattern and the subsequent servo burst pattern may be of one frequency. The servo burst pattern and the subsequent servo burst pattern may be direct current (DC). 
       FIG. 7  is a schematic  700  showing the side view of portion along a servo track in the first layer  702  and a data track along the second layer  704  with data and servo information stored according to various embodiments.  FIG. 7  shows a recording medium including a first servo pattern  714  (including a servo burst pattern) on a segment of a first track of a first layer  702  of the recording medium according to various embodiments.  FIG. 7  further shows a second servo pattern  716  (including addressing information) on a segment of a second track of a second layer  704  of the recording medium according to various embodiments. The second track may be arranged from the first track by half the track width. The addressing information may include auto gain control (AGC), sector address mark (SAM) and GrayCode. 
     The portion of the second layer (data recording layer)  718  corresponding to the first servo pattern  714  in the first layer may be configured to store data. 
     The portion of the first layer (servo layer)  720  corresponding to the second servo pattern  716  in the second layer may be configured to be left unused. 
     In various embodiments, a portion of the second layer corresponding to a segment of the first layer means portions of the data track(s) adjacent to the segment of the first layer or portions of the data track(s) separated from the segment of the first layer by one or more intermediate layers within the recording medium. Conversely, a portion of the first layer corresponding to a segment of the second layer means portions of the servo track(s) adjacent to the segment of the second layer or portions of the servo track(s) separated from the segment of the second layer by one or more intermediate layers within the recording medium. The projection of the segment of the first layer on a plane parallel to the first main surface (or second main surface) of the recording medium may overlap entirely or substantially with the projection of the corresponding portion of the second layer on the plane. Conversely, the projection of the segment of the second layer on a plane parallel to the first main surface (or second main surface) of the recording medium may overlap entirely or substantially with the projection of the corresponding portion of the first layer on the plane. In various embodiments wherein the second layer is on or over the first layer, the corresponding portion of the second layer to a segment on the first layer may directly on or above the segment on the first layer. Additionally, the corresponding portion of the first layer to a segment on the second layer may be directly below the segment on the second layer. 
     In various embodiments, the method may include using shingled writing scheme. The method may include forming the servo information in a writing pass, followed by rewriting some of the servo information in a subsequent writing pass. 
       FIG. 8A  to G illustrate a method to arrange the servo information shown in  FIG. 7  according to various embodiments.  FIG. 8A  is a schematic  800   a  showing the top view of servo tracks  802   a ,  802   b ,  802   c ,  802   d  in a first layer  802  after a writing pass according to various embodiments.  FIG. 8B  is a schematic  800   b  showing the side view of a portion along a servo track  802   a  in the first layer  802  and the overlying second layer  804  after a writing pass according to various embodiments.  FIG. 8B  may correspond to the view in a direction indicated by arrow  812  in  FIG. 8A . 
     In various embodiments, the method may include forming an intermediate servo pattern  816   a  (including the addressing information) on a subsequent segment of the first track  802   a  of the first layer  802  of the recording medium. In other words, the method may include forming the intermediate servo pattern  816   a  on an adjacent segment on the same layer and on the same track when forming the first servo pattern  814   a.    
     In various embodiments, forming the first servo pattern  814   a  (including the servo burst pattern) on the segment of the first track  802   a  of the first layer  802  of the recording medium may be carried out during a writing pass of a magnetic head. Forming the intermediate servo pattern  816   a  (including the addressing information) on the subsequent segment of the first track  802   a  of the first layer  802  of the recording medium may be carried out during the same writing pass of the magnetic head. In other words, the intermediate servo pattern  816   a  may be formed together with the first servo pattern  814   a  on a writing pass. 
     In various embodiments, the first servo pattern  814   a  (including the servo burst pattern) on the segment of the first track  802   a  of the first layer  802  of the recording medium may also be formed on the portion of the second layer  804  corresponding to the first servo pattern  814   a  on the segment of the first track  802   a  of the first layer  802 . The first servo pattern  814   a  on the segment of the first track  802   a  of the first layer  802  of the recording medium may also be formed on at least a part of a segment of the second track  804   a  of the second layer  804  of the recording medium. The part of the segment may include about half of the second track  804   a  directly on or above the segment of the first track  802   a  of the first layer  802  of the recording medium. The first servo pattern  814   a  on the segment of the first track  802   a  of the first layer  802  of the recording medium may also be formed on at least a part of a segment of the track  804   e  of the second layer  804  of the recording medium. The part of the segment may also include about half the track  804   e  directly on or above the segment of the first track  802   a  of the first layer  802  of the recording medium. 
       FIG. 8C  shows a schematic  800   c  showing the cross section of the first track  802   a  on the first layer as well as overlying tracks  804   a  and  804   e  on the second layer  804 .  FIG. 8C  may corresponds to  FIG. 8B  when viewed in the direction indicated by arrow  808  in  FIG. 8B . In various embodiments, the intermediate servo pattern  816   a  (including the addressing information) may be formed on the portion  830  of the second layer  804  corresponding to the intermediate servo pattern  816  on the first track  802   a  of the first layer  802 . The intermediate servo pattern  816   a  formed on the subsequent segment of the first track  802   a  of the first layer  802  of the recording medium may also be formed on at least a part of a segment  830   a  of the second track  804   a  of the second layer  804  of the recording medium. The part of the segment  830   a  may include about half of the second track  804   a  directly on or above the segment of the first track  802   a  of the first layer  802  of the recording medium. The first servo pattern  814   a  on the segment of the first track  802   a  of the first layer  802  of the recording medium may also be formed on at least a part of a segment  830   b  of the track  804   e  of the second layer  804  of the recording medium. The part of the segment  830   b  may also include about half the track  804   e  directly on or above the segment of the first track  802   a  of the first layer  802  of the recording medium. 
     The first servo pattern  814   a  and the intermediate servo pattern  816   a  on the second layer may not be necessary. The first servo pattern  814   a  may be replaced during the subsequent writing pass. The intermediate servo pattern  816   a  may be replaced by the user data in the data recording. 
     In various embodiments, the magnetic head may write an intermediate servo pattern  816   a  (including addressing information AGSCO) and the first servo pattern  814   a  (including servo burst of a first frequency f 1 ) in the first track  802   a . The magnetic head may then move one track width to write a subsequent intermediate servo pattern  816   b  (including addressing information AGSC 1 ) and the third servo pattern  814   b  (including subsequent servo burst of a second frequency f 2 ) in the third track  802   b . The magnetic head may then repeat the previous steps by moving to subsequent servo tracks (such as  802   c ,  802   d  etc.) to write the subsequent intermediate servo patterns such as  816   c ,  816   d  (including addressing information AGSC 2 , AGSC 3  etc.) and servo patterns such as  814   c  and  814   d  (including subsequent servo bursts). 
     In various embodiments, the magnetic head may be a wide head.  FIG. 8D  is a schematic  800   d  showing a wide head  822  according to various embodiments.  FIG. 8E  is a schematic  800   e  showing the wide head  822  in operation according to various embodiments. The wide head may have a width w and a length p. The width w may be more than 300 nm. The wide head  822  may be wide enough to provide enough magnetic field to saturate the servo layer  802  for writing servo information. 
     The wide head  822  may be configured to confine the magnetic field such that prior information that has been written on preceding tracks are not overwritten. The wide head  822  may, for instance, have a shielding component to shield the magnetic field to confine the magnetic field. The wide head  822  may write one track at a time. 
     For each of the servo track, the addressing information (including the AGC, the SAM and the GrayCode) may be written in one go of a full track writing. The wide head  822  may be used to write the intermediate servo pattern  816   a  (including addressing information) and the first servo pattern  814   a  (including servo burst of a first frequency f 1 ) in the first track  802   a . The wide head  822  may then move by one track width and writes the subsequent intermediate servo pattern  816   b  (including addressing information) and the third servo pattern  814   b  (including subsequent servo burst of a second frequency f 2 ) in the third track. This may be repeated until the wide head  822  has completed writing the servo information over the entire recording medium. 
       FIG. 8F  is a schematic  800   f  showing the top view of servo tracks  802   a ,  802   b ,  802   c ,  802   d  in a first layer  802  as well as portions of data tracks  804   a ,  804   b ,  804   c  in a second layer  804  after a subsequent writing pass according to various embodiments.  FIG. 8G  is a schematic  800   g  showing the side view of a portion along a servo track  802   a  in the first layer  802  and the overlying second layer  804  after a subsequent writing pass according to various embodiments.  FIG. 8G  may correspond to the view in a direction indicated by arrow  824  in  FIG. 8G . 
     In various embodiments, forming the second servo pattern  820   a  (including addressing information) on a segment of the second track  804   a  of the second layer  804  of the recording medium may be carried out during a subsequent writing pass of a subsequent magnetic head. The subsequent writing pass may be conducted at drive level during drive self-test. 
     In various embodiments, the intermediate servo pattern  816   a  formed on the subsequent segment of the first track  802   a  of the first layer  802  of the recording medium during the first pass may be used as a reference when forming the second servo pattern  820   a  on the segment of the second track  804   a  of the second layer  804  during the second pass. Similarly, the subsequent intermediate servo patterns such as  816   b ,  816   c  formed on respective subsequent segments of the subsequent first tracks such as  802   b ,  802   c  etc. of the first layer  802  of the recording medium during the first pass may be used as a reference when forming the subsequent second servo patterns such as  820   b ,  820   c  on respective segments of the subsequent second tracks such as  804   b ,  804   c  etc. of the second layer  804  during the second pass. In others words, the addressing information written on the recording medium during the initial writing pass may be used as a reference when writing the addressing information during the subsequent writing pass. 
     In various embodiments, the subsequent intermediate servo pattern  816   b  formed on the subsequent segment of the third track  802   b  of the first layer  802  of the recording medium during the first pass may be used as a reference when forming the second servo pattern  820   a  on the segment of the second track  804   a  of the second layer  804  during the second pass. In various embodiments, both the intermediate servo pattern  816   a  and the subsequent intermediate servo pattern  816   b  may be used as a reference when forming the second servo pattern  820   a.    
     The subsequent magnetic head may be a product head. 
     In various embodiments, the intermediate servo pattern  816   a  formed on the subsequent segment of the first track  802   a  of the first layer  802  of the recording medium during the first pass may be distorted when forming the second servo pattern  820   a  on the segment of the second track  804   a  of the second layer  804  during the second pass. Similarly, the subsequent intermediate servo patterns such as  816   b ,  816   c  formed on the respective subsequent segments of the subsequent first tracks such as  802   b ,  802   c ,  802   d  etc. of the first layer  802  of the recording medium during the first pass may be distorted when forming the second servo pattern such as  820   b ,  820   c  on the respective segments of the subsequent second tracks  804   b ,  804   c  etc. of the second layer  804  during the second pass. 
     In other words, the addressing information written during the initial writing pass may be distorted. There may be residual patterns or no patterns in the subsequent segment of tracks such as  802   a ,  802   b ,  802   c ,  802   d  etc. of the first layer  802  of the recording medium after the second writing pass. The subsequent segment may be configured to be unused over the lifetime of the recording medium. 
     In various embodiments, the subsequent intermediate servo pattern  816   b  formed on the subsequent segment of the third track  802   b  of the first layer  802  of the recording medium during the first pass may be distorted when forming the second servo pattern  820   a  on the segment of the second track  804   a  of the second layer  804  during the second pass. 
     In various embodiments, the first servo pattern  814   a ,  814   b  etc. (including the servo bursts) on the segment of the first track  802   a ,  802   b  etc. of the first layer  814  of the recording medium may remain over the lifetime of the recording medium. 
     In various embodiments, additional servo patterns may be formed during the writing pass or subsequent writing pass on the corresponding segments of the second track  804   a ,  804   b ,  804   c  etc. of the second layer  804  to first servo patterns  814   a ,  814   b  etc. The corresponding segments may be overwritten with data  818  during self-test phase or the actual user data recording. 
     Various embodiments provide a recording medium wherein the number of steps required may be double or more than double the number of servo tracks. Various embodiments may provide a method in which half of the number of steps for writing servo information is transferred to the drive level during the subsequent writing pass. Various embodiments may provide a method with low manufacturing costs as writing during the initial writing pass (at disk media level) may be more expensive than the subsequent writing pass (at drive level). 
       FIG. 9A  is an image  900   a  showing the signal map of a conventional single layer recording medium. Red spots  902  may represent one logic state while blue spots  904  may represent another logic state.  FIG. 9B  is another image  900   b  showing the signal map of a conventional single layer recording medium. Red spots  906  may represent one logic state while blue spots  908  may represent another logic state.  FIG. 9C  is an image  900   c  showing the signal map of a multi-layer recording medium according to various embodiments. Red spots  910  may represent one logic state while blue spots  912  may represent another logic state. The region  914  may be a portion in which the addressing information is stored while the region  916  may be a portion in which the servo bursts is stored.  FIGS. 9D and 9E  show the signals maps obtained from a multi-layer recording medium in which servo information is written according to a method illustrated in  FIGS. 8A  to F according to various embodiments.  FIG. 9D  is an image  900   d  showing the signal map of the multi-layer recording medium after a writing pass is carried out according to various embodiments. Image  900   d  shows that the addressing information  918  are aligned to the servo bursts  920  in the servo tracks  922 .  FIG. 9E  is an image  900   e  showing the signal map of the multi-layer recording medium after a subsequent writing pass is carried out according to various embodiments. Image  900   e  shows that the addressing information  924  in the data tracks  926  are aligned by half a track width from the servo bursts  928  in the servo tracks  930 . 
       FIG. 10  is a schematic  1000  showing the side view of portion along a servo track in the first layer  1002  and a data track along the second layer  1004  with data and servo information stored according to various embodiments.  FIG. 10  shows a recording medium including a first servo pattern  1014  (including a servo burst pattern) on a segment of a first track of a first layer of the recording medium according to various embodiments.  FIG. 10  further shows a second servo pattern  1016  including addressing information on a segment of a second track of a second layer  1004  of the recording medium according to various embodiments. The data recording medium may further include a second servo pattern  1020  (including addressing information) on a portion of the first layer  1002 , the portion corresponding to the segment (for storing second servo pattern  1016 ) of the second track of the second layer  1004 . In other words, a portion of the first layer  1002  corresponding to servo pattern  1016  may be configured to store a second servo pattern  1020  (including addressing information). The addressing information may include auto gain control (AGC), sector address mark (SAM) and GrayCode. 
     The portion of the second layer (data recording layer)  1018  corresponding to the first servo pattern  1014  in the first layer may be configured to store data. 
     In various embodiments, the second layer  1004  may be over the first layer  1002 . The second servo pattern  1020  (including addressing information) maybe on a portion of the first layer  1002 , the portion of the first layer  1002  below the segment of the second track of the second layer  1004  (for storing the second servo pattern  1016 ). In various embodiments, the second servo pattern  1020  may be on a portion of the first layer  1002 , the portion of the first layer  1002  corresponding to the segment of the second track of the second layer  1004 . 
     In various embodiments, a portion of the second layer corresponding to a segment of the first layer means portions of the data track(s) adjacent to the segment of the first layer or portions of the data track(s) separated from the segment of the first layer by one or more intermediate layers within the recording medium. Conversely, a portion of the first layer corresponding to a segment of the second layer means portions of the servo track(s) adjacent to the segment of the second layer or portions of the servo track(s) separated from the segment of the second layer by one or more intermediate layers within the recording medium. The projection of the segment of the first layer on a plane parallel to the first main surface (or second main surface) of the recording medium may overlap entirely or substantially with the projection of the corresponding portion of the second layer on the plane. Conversely, the projection of the segment of the second layer on a plane parallel to the first main surface (or second main surface) of the recording medium may overlap entirely or substantially with the projection of the corresponding portion of the first layer on the plane. In various embodiments wherein the second layer is on or over the first layer, the corresponding portion of the second layer to a segment on the first layer may directly on or above the segment on the first layer. Additionally, the corresponding portion of the first layer to a segment on the second layer may be directly below the segment on the second layer. 
       FIGS. 11A  to H illustrate a method to arrange the servo information shown in  FIG. 10  according to various embodiments.  FIG. 11A  is a schematic  1100   a  showing a top view of portions of a plurality of tracks configured to store servo information after a first step.  FIG. 11B  is a schematic  1100   b  showing a side view of  FIG. 11A  when viewed from the direction indicated by  1112   a  in  FIG. 11A .  FIG. 11A  shows that the first step includes writing a first servo pattern  1114   a  including servo bursts to a segment along a first track  1102   a  in the first layer  1102 . The servo bursts may be of a first frequency. The first servo pattern  1114   a  may also be written to a portion of the second layer  1104  corresponding to the first servo pattern  1114   a  in the first layer  1102 . The portion of the second layer  1104  corresponding to the first servo pattern  1114   a  in the first layer  1102  may be configured to store data. As such, the first servo pattern  1114   a  on the portion of second layer  1104  (corresponding to the first servo pattern  1114   a  in the first layer  1102 ) may be overwritten with data. 
       FIG. 11C  is a schematic  1100   c  showing a top view of portions of a plurality of tracks configured to store servo information after a second step.  FIG. 11D  is a schematic  1100   d  showing a side view of  FIG. 11C  when viewed from the direction indicated by  1112   b  in  FIG. 11C .  FIG. 11C  shows that the second step includes writing a second servo pattern  1116   a  (including addressing information) to a segment along a second track  1104   a  in the second layer  1104 . The magnetic head may move half a track width after completion of the first step to carry out writing of the second track  1104   a  in the second step. The method may include writing the second servo pattern  1116   a  on the segment of the second track  1104   a  in the second layer  1104 . In various embodiments, the method may further include writing the second servo pattern  1120   a  (including addressing information) on a portion of the first layer  1102 , the portion of the first layer  1102  below the segment of the second track  1104   a  of the second layer  1104  (for writing the second servo pattern  1116   a ). The second servo pattern  1120   a  on the portion of the first layer  1102  may be written together with the second servo pattern  1116   a  on a segment of the second track  1104   a  in the second layer  1104 . The portion of the first layer  1102  may include about half of the first track  1102   a . The about half of the first track  1102   b  may be directly below the second servo pattern  1116   a . The portion of the first layer  1102  may include about half of the third track  1102   b . The about half of the first track  1102   b  may be directly below the second servo pattern  1116   a.    
       FIG. 11E  is a schematic  1100   e  showing a top view of portions of a plurality of tracks configured to store servo information after a third step.  FIG. 11F  is a schematic  1100   f  showing a side view of  FIG. 11E  when viewed from the direction indicated by  1112   c  in  FIG. 11E .  FIG. 11E  shows that the third step includes writing a third servo pattern  1114   b  (including a subsequent servo burst) to a segment along a third track  1102   b  in the first layer  1102 . The magnetic head may move half a track width after completion of the second step to carry out writing of the third track  1102   b  in the third step. The subsequent servo burst may be of a second frequency. The third servo pattern  1114   b  may also be written to a portion of the second layer  1104  corresponding to the third servo pattern  1114   b  in the first layer  1102 . The portion of the second layer  1104  corresponding to the third servo pattern  1114   b  in the first layer  1102  may be configured to store data. As such, the third servo pattern  1114   b  on the portion of second layer  1104  (corresponding to the third servo pattern  1114   b  in the first layer  1102 ) may be overwritten with data. 
       FIG. 11G  is a schematic  1100   g  showing a top view of portions of a plurality of tracks configured to store servo information after a fourth step.  FIG. 11H  is a schematic  1100   h  showing a side view of  FIG. 11G  when viewed from the direction indicated by  1112   d  in  FIG. 11G .  FIG. 11G  shows that the fourth step includes writing a fourth servo pattern  1116   b  (including addressing information) to a segment along a fourth track  1104   b  in the second layer  1104 . The magnetic head may move half a track width after completion of the third step to carry out writing of the fourth track  1104   b  in the fourth step. The method may include writing the fourth servo pattern  1120   b  (including addressing information) on a portion of the first layer  1102  corresponding the fourth servo pattern  1116   b  on the segment of the second track  1104   a  in the second layer  1104 . In various embodiments, the method may further include writing the fourth servo pattern  1120   b  (including addressing information) on a portion of the first layer  1102 , the portion of the first layer  1102  below the segment of the fourth track  1104   b  of the second layer  1104  (for writing the second servo pattern  1116   b ). The fourth servo pattern  1120   b  on the portion of the first layer  1102  may be written together with the fourth servo pattern  1116   b  on a segment of the second track  1104   a  in the second layer  1104 . 
     The method may include subsequent steps to write the subsequent servo pattern including servo bursts to the servo tracks or to write the subsequent servo pattern including addressing information to the data tracks. The steps may be repeated until the entire surface of the recording medium is written. 
     The first step and the second step may be carried out in a single pass. The first step, the second step, the third step, the fourth steps and the subsequent steps may be carried out in a single pass. 
     The magnetic head may be a wide head. 
     The first servo pattern (including the servo burst pattern) on the segment of the first track of the first layer, the second servo pattern including addressing information on the segment of the first track of the first layer and the second servo pattern (including addressing information) on the segment of the second track of the second layer may be formed in a single pass. 
     Various embodiments may include doubling the total track numbers as the magnetic head is moved half a track width for each step. 
     In various embodiments, the second layer (data recording layer) may include disk repeatable run out (RRO) compensation information. 
     In various other embodiments, the first step may include writing a first servo pattern  1114   a  including a servo burst pattern to a segment along a first track  1102   a  in the first layer  1102 . The second step may include moving the magnetic head by one track width. The second step may include writing a third servo pattern  1114   b  including a subsequent servo burst pattern to a segment along a third track  1102   b . The subsequent steps may include moving the magnetic head by one track width. The subsequent steps may include writing subsequent servo patterns including subsequent servo burst patterns until all the servo burst patterns have been written. The servo patterns including servo bursts may be written by a wide head. In other words, the magnetic head may be a wide head. The method may further include writing servo information  1116   a  including addressing information to a segment along the second track  1104   a  in the second layer  1102 . Writing the servo pattern  1116   a  including the addressing information may be carried out using a subsequent magnetic head. The method may further include moving the subsequent magnetic head by one track width. The method may further include writing a fourth servo burst pattern  1116   b  including addressing information to a segment along the fourth track  1104   b  in the second layer  1104  (using the subsequent magnetic head). The method may also include writing subsequent servo patterns including addressing information by moving the subsequent magnetic head by one track width at a time and writing the addressing information until all the addressing information has been written. The subsequent magnetic head may be a product head. 
       FIG. 12  is an image  1200  showing the signal map of a multi-layer recording medium after a writing pass is carried out according to various embodiments. Image  1200  shows that the addressing information  1224  in the data tracks  1226  are aligned by half a track width from the servo bursts  1228  in the servo tracks  1230 . 
     In various embodiments, a method of writing servo information may be provided. The method may include forming a first servo pattern (including a servo burst pattern) on a segment of a first track of a first layer of the recording medium, the first track having a track width. The method may further include forming a second servo pattern (including addressing information) on a segment of a second track of a second layer of the recording medium. The second track may be arranged from the first track by half the track width. 
     The method may further include forming a third servo pattern (including a subsequent servo burst pattern) on a segment of a third track of the first layer of the recording medium. The third track may be arranged from the second track by half a track width. The second track may be between the first track and the third track. 
     In various embodiments, the servo burst pattern may be of a first frequency and the subsequent servo burst pattern may be of a second frequency. Alternatively, the servo burst pattern and the subsequent servo burst pattern may be of a single frequency. The servo burst pattern and the subsequent servo burst pattern may be of direct current (DC). 
     The addressing information may include auto gain control (AGC), sector address mark (SAM) and GrayCode. 
     In various embodiments, the second layer may be over the first layer. 
     The method may further include forming an intermediate servo pattern (including the addressing information) on a subsequent segment of the first track of the first layer of the recording medium. 
     In various embodiments, forming the first servo pattern (including the servo burst pattern) on the segment of the first track of the first layer of the recording medium may be carried out during a writing pass of a magnetic head. In various embodiments, forming the intermediate servo pattern (including the addressing information) on the subsequent segment of the first track of the first layer of the recording medium may be carried out during the same writing pass of the magnetic head. In other words, the first servo information on the segment of the first track of the first layer and the intermediate servo information on the subsequent segment of the first track of the first layer may be formed during a single writing pass of the magnetic head. 
     In various embodiments, the magnetic head may be a wide head. 
     In various embodiments, forming the second servo pattern (including addressing information) on the segment of the second track of the second layer of the recording medium may be carried out during a subsequent writing pass of a subsequent magnetic head. 
     In various embodiments, the subsequent magnetic head may be a product head. 
     In various embodiments, the intermediate servo pattern formed on the subsequent segment of the first track of the first layer of the recording medium during the writing pass may be used as a reference when forming the second servo pattern on the segment of the second track of the second layer during the subsequent writing pass. 
     The intermediate servo pattern formed on the subsequent segment of the first track of the first layer of the recording medium during the writing pass may be distorted when forming the second servo pattern on the segment of the second track of the second layer during the subsequent writing pass. 
     In various embodiments, forming the second servo pattern (including addressing information) may further include forming the second servo pattern (including addressing information) on a portion of the first layer, the portion of the first layer corresponding to the segment of the second track of the second layer. 
     In various embodiments, the second layer may be over the first layer. Forming the second servo pattern (including addressing information) may further include forming the second servo pattern (including addressing information) on a portion the first layer, the portion of the first layer below the segment of the second track of the second layer. In other words, the second servo pattern (including addressing information) may be further formed on a portion of the first layer, the portion corresponding to (e.g. below) the segment of the second track of the second layer, in addition to forming the second servo pattern on the segment of the second track of the second layer. 
     In various embodiments, the first servo pattern (including the servo burst pattern on the segment of the first track of the first layer, the second servo pattern (including addressing information) on the segment of the first track of the first layer and the second servo pattern (including addressing information) on the segment of the second track of the second layer may be formed in a single writing pass. 
     The single writing pass may be carried out using a wide head. 
     In various embodiments, a recording medium may be provided. The recording medium may include a first servo pattern (including a servo burst pattern) on a segment of a first track of a first layer of the recording medium. 
     The recording medium may also include second servo pattern may include addressing information on a segment of a second track of a second layer of the recording medium. The second track may be arranged from the first track by half the track width. 
     In various embodiments, the recording medium may further include a third servo pattern (including a subsequent servo burst pattern) on a segment of a third track of the first layer of the recording medium. The third track may be arranged from the second track by half a track width. The second track may be between the first track and the third track. 
     In various embodiments, the servo burst pattern may be of a first frequency and the subsequent servo burst pattern may be of a second frequency. Alternatively, the servo burst pattern and the subsequent servo burst pattern may be of a single frequency. The servo burst pattern and the subsequent servo burst pattern may be of direct current (DC). 
     The addressing information may include auto gain control (AGC), sector address mark (SAM) and GrayCode. 
     In various embodiments, the second layer may be over the first layer. 
     In various embodiments, the recording medium may further include an intermediate servo pattern (including the addressing information) on a subsequent segment of the first track of the first layer of the recording medium. 
     The first servo pattern (including the servo burst pattern) on the segment of the first track of the first layer of the recording medium may be formed during a writing pass of the magnetic head. The intermediate servo pattern (including the addressing information) on the subsequent segment of the first track of the first layer of the recording medium may be formed during the same writing pass of the magnetic head. In other words, the first servo information on the segment of the first track of the first layer and the intermediate servo information on the subsequent segment of the first track of the first layer may be formed during a single writing pass of the magnetic head. 
     The magnetic head may be a wide head. 
     In various embodiments, the second servo pattern (including addressing information) on the segment of the second track of the second layer of the recording medium may be formed during a subsequent writing pass of a subsequent magnetic head. The subsequent magnetic head may be a product head. 
     In various embodiments, the intermediate servo pattern on the subsequent segment of the first track of the first layer of the recording medium formed during the writing pass may be used as a reference for the second servo pattern on the segment of the second track of the second layer formed during the subsequent writing pass. 
     In various embodiments, the intermediate servo pattern on the subsequent segment of the first track of the first layer of the recording medium formed during the writing pass may be distorted by the second servo pattern on the second track of the second layer formed during the subsequent writing pass. 
     In various embodiments, the second servo pattern (including addressing information) may be further include formed on a portion of the first layer, the portion of the first layer corresponding to the segment of the second track of the second layer. 
     In various embodiments, the second layer may be over the first layer. The second servo pattern (including addressing information) may be further formed on a portion of the first layer, the portion of the first layer below the segment of the second track of the second layer. In other words, the second servo pattern (including addressing information) may be further formed on a portion of the first layer, the portion corresponding to (e.g. below) the segment of the second track of the second layer, in addition to forming the second servo pattern on the segment of the second track of the second layer. 
     In various embodiments, the first servo pattern (including the servo burst pattern) on the segment of the first track of the first layer, the second servo pattern (including addressing information) on the segment of the first track of the first layer and the second servo pattern (including the addressing information) on the segment of the second track of the second layer may be formed in a single writing pass. 
     The single writing pass may be carried out using a wide head. 
     The data storage device may include a memory which is for example used in the processing carried out by the data storage device. A memory used in the embodiments may be a volatile memory, for example a DRAM (Dynamic Random Access Memory) or a non-volatile memory, for example a PROM (Programmable Read Only Memory), an EPROM (Erasable PROM), EEPROM (Electrically Erasable PROM), or a flash memory, e.g., a floating gate memory, a charge trapping memory, an MRAM (Magnetoresistive Random Access Memory) or a PCRAM (Phase Change Random Access Memory). 
     While several exemplary embodiments have been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist, including variations as to the choice of bearings. It will be understood by one skilled in the art that where fluid dynamic bearings are not used, the bias ring may be an optional feature. Also, the step of pre-assembly should be understood to be optional as the bias ring may be embedded or otherwise provided as an integral part of the stator, etc. 
     While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.