Patent Publication Number: US-2010118432-A1

Title: Magnetic storage apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2008-290353, filed Nov. 12, 2008, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     One embodiment of the present invention relates to a magnetic storage medium that is incorporated in a magnetic storage apparatus, and has a servo pattern in which magnetic bodies magnetized to a south pole (S-pole) or a north pole (N-pole) are discretely arranged in a non-magnetic substance at least in a line direction of a recording track. 
     2. Description of the Related Art 
     Magnetic storage medium such as a bit-patterned media are widely known. In such magnetic storage media, when a servo pattern is set up, magnetic bodies are arranged in a non-magnetic substance with any pattern. The magnetic bodies are magnetized in a unidirectional magnetic field. In a servo pattern, magnetic pole of the magnetic bodies is adjusted to one of the magnetic poles (see, for example, Japanese Patent Application Publication (KOKAI) No. 2008-77772). 
     When adjacent magnetic bodies are magnetized in an opposite direction to each other, the magnetic field circulates. Therefore, the magnetization is less likely to be reversed. On the other hand, when adjacent magnetic bodies are magnetized in the same direction to each other, the magnetization is likely to be reversed. When the magnetization of the servo pattern is reversed, servo pattern cannot be read correctly in the tracking servo control. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       A general architecture that implements the various features of the invention will now be described with reference to the drawings. The drawings and the associated descriptions are provided to illustrate embodiments of the invention and not to limit the scope of the invention. 
         FIG. 1  is an exemplary plan view of an internal structure of a magnetic storage medium drive apparatus, i.e., a hard disk drive (HDD) apparatus, according to an embodiment of the invention; 
         FIG. 2  is an exemplary partially enlarged schematic of a surface structure of a magnetic disk in the embodiment; 
         FIG. 3  is an exemplary enlarged perspective view of the surface of the magnetic disk in the embodiment; 
         FIG. 4  is an exemplary vertical cross-sectional view taken along the line  4 - 4  in  FIG. 3  according to the embodiment; 
         FIG. 5  is an exemplary partially enlarged schematic of a servo sector area in the embodiment; 
         FIG. 6  is an exemplary block diagram of a tracking servo control system in the embodiment; 
         FIG. 7  is an exemplary schematic of a rectifier circuit in the embodiment; 
         FIG. 8  is another exemplary schematic of the rectifier circuit in the embodiment; 
         FIG. 9  is an exemplary schematic waveform of a reproduction waveform swinging from positive to negative and vice versa with respect to a reference voltage of 0 V in the embodiment; 
         FIG. 10  is an exemplary schematic waveform of a direct current offset and a low-frequency wave component extracted from the reproduction waveform in the embodiment; 
         FIG. 11  is an exemplary schematic reproduction waveform swinging from positive to negative and vice versa with respect to a reference voltage of 0 V, with a direct current offset and a wave component corrected in the embodiment; 
         FIG. 12  is an exemplary schematic unidirectional reproduction waveform after conversion to an absolute value in the embodiment;  FIG. 13  is an exemplary schematic of a preamplifier comprising the rectifier circuit in the embodiment; 
         FIG. 14  is another exemplary schematic of the preamplifier comprising the rectifier circuit in the embodiment; 
         FIG. 15  is an exemplary partially enlarged cross-sectional view of the magnetic disk having a magnetic film laminated on a non-magnetic intermediate layer during a manufacturing process of the magnetic disk in the embodiment; 
         FIG. 16  is an exemplary partially enlarged cross-sectional view of the magnetic disk having a resist film formed on a surface of the magnetic film during the manufacturing process of the magnetic disk in the embodiment; 
         FIG. 17  is an exemplary partially enlarged cross-sectional view of the magnetic disk having the patterned magnetic film during the manufacturing process of the magnetic disk in the embodiment; 
         FIG. 18  is an exemplary partially enlarged cross-sectional view of the magnetic disk having a planarized recording layer during the manufacturing process of the magnetic disk in the embodiment; and 
         FIG. 19  is an exemplary schematic waveform of a reproduction signal read from the servo sector area to which a high-frequency write signal has been applied. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments according to the invention will be described hereinafter with reference to the accompanying drawings. In general, according to one embodiment of the invention, a magnetic storage apparatus comprises a magnetic storage medium that comprises a servo pattern in which magnetic bodies magnetized to one of an S-pole and an N-pole are discretely arranged in a non-magnetic substance at least in a recording track line direction, an electromagnetic conversion element configured to output a reproduction signal according to a magnetic field leaking from the magnetic bodies, a rectifier circuit configured to receive the reproduction signal swinging from positive to negative and vice versa corresponding to a magnetic pole, and generate a reproduction signal swinging to either a positive or negative direction according to the reproduction signal, and a control circuit configured to cause the electromagnetic conversion element to be positioned to a single recording track on the magnetic storage medium according to the reproduction signal generated in the rectifier circuit. 
     According to another embodiment of the invention, a manufacturing method of a magnetic storage medium comprises magnetizing, in a servo pattern of the magnetic storage medium, magnetic bodies that are discretely arranged in a non-magnetic substance at least in a recording track line direction with a high-frequency write signal. 
       FIG. 1  is a schematic of an internal structure of an embodiment of a magnetic storage apparatus according to the present invention, that is, a hard disk drive (HDD)  11 . The HDD  11  comprises a casing, that is, a housing  12 . The housing  12  comprises a box-shaped base  13  and a cover (not depicted). The base  13  partitions an interior space having, for example, a flat rectangular cuboid shape, that is, a housing space. The base  13  may be formed, for example, by casting metallic material such as aluminum (Al). The cover is connected to an opening of the base  13 . The cover and the base  13  seal the housing space. The cover maybe formed, for example, by pressing a piece of plate material. 
     In the housing space, one or more magnetic disks  14  are arranged. The magnetic disk  14  is an example of a magnetic storage medium. The magnetic disk  14  is mounted on a spindle hub of a spindle motor  15 . The spindle motor  15  can rotate the magnetic disks  14  at a high speed of, for example, 5400 rpm, 7200 rpm, 10000 rpm, or 15000 rpm. The individual magnetic disks 14 are recognized as so-called bit patterned media, which will be described later. 
     A carriage  16  is also housed in the housing space. The carriage  16  comprises a head stack assembly  17 . The head stack assembly  17  is rotatably connected to a spindle  18  that extends vertically from a bottom plate of the base  13 . A plurality of carriage arms  19  that horizontally extends from the spindle  18  is partitioned in the head stack assembly  17 . The head stack assembly  17  may be formed by, for example, extruding aluminum (Al) . 
     A head suspension  21  is mounted on a tip of each of the carriage arms  19 . The head suspension  21  extends in the forward direction from the tip of the carriage arm  19 . A flexure is attached to a tip of the head suspension  21 . The flexure supports a floating head slider  22 . The floating head slider  22  can change its position with respect to the head suspension  21  by using the flexure. The floating head slider  22  has thereon a head element, i.e., an electromagnetic conversion element (not depicted). 
     The electromagnetic conversion element comprises a write head element and a read head element. The write head element has a so-called single-pole-type head. The single-pole-type head generates a magnetic field with its thin film coil pattern. The magnetic field is applied to the magnetic disk  14  from the vertical direction orthogonal to the surface of the magnetic disk  14  according to the effect of the main magnetic pole. This magnetic field enables to write information to the magnetic disk  14 . On the other hand, the read head element is a giant magneto resistive (GMR) element or a tunneling magneto resistive (TMR) element. With the GMR element or the TMR element, a resistance change of a spin-valve film or a tunnel junction film occurs depending on a direction of the magnetic field from the magnetic disk  14 . With such resistance change, information can be read out from the magnetic disk  14 . 
     When the magnetic disk  14  is rotated, air current is generated on the surface of the magnetic disk  14 . Then, due to the air current, a positive pressure, that is an ascending force, and a negative pressure are applied on the floating head slider  22 . The ascending force and the negative pressure balance with a pressing force of the head suspension  21 , and thus, the floating head slider  22  can keep floating at a relatively high rigidity while the magnetic disk  14  is rotated. 
     To the head stack assembly  17 , a voice coil motor (VOM)  23  is linked. The voice coil motor  23  allows the head stack assembly  17  to rotate about the spindle  18 . This rotation of the head stack assembly  17  enables the carriage arms  19  and the head suspension  21  to swing. While the floating head slider  22  floats, when the carriage arm  19  swings about the spindle  18 , the floating head slider  22  can move along a radius line of the magnetic disk  14 . As a result, the electromagnetic conversion element mounted on the floating head slider  22  can traverse the concentric recording track between the innermost recording track and the outermost recording track. Thus, the electromagnetic conversion element can be positioned on a desired recording track according to the movement of the floating head slider  22 . 
     At the tip of the head suspension  21 , a load tub  24  extending forward therefrom is partitioned. The load tub  24  can move in the radial direction of the magnetic disk  14  by the swinging of the carriage arm  19 . On the moving path of the load tub  24 , a ramp member  25  is disposed outside the magnetic disk  14 . The ramp member  25  is secured on the base  13  and receives the load tub  24 . The ramp member  25  may be formed from a hard plastic material, for example. 
     The ramp member  25  has a ramp  25   a  extending along the moving path of the load tub  24 . The ramp  25   a,  when moving away from the rotation axis of the magnetic disk  14 , moves away from a virtual plane comprising the surface of the magnetic disk  14 . Accordingly, when the carriage arm  19  rotates about the spindle  18  to move away from the rotation axis of the magnetic disk  14 , the load tub  24  moves upward on the ramp  25   a.  Then, the floating head slider  22  is removed from the surface of the magnetic disk  14  to move outside the magnetic disk  14  and rest. On the other hand, when the carriage arm  19  swings about the spindle  18  to move toward the rotation axis of the magnetic disk  14 , the load tub  24  moves downward on the ramp  25   a.  Then, the ascending force due to the rotation of the magnetic disk  14  is applied to the floating head slider  22 . The ramp member  25  and the load tub  24  cooperate together to establish a so-called load/unload mechanism. 
     As illustrated in  FIG. 2 , servo sector areas  28  as plural curved lines (e.g., 200 lines) extending along the radial direction of the magnetic disk  14  are defined on both surfaces of the magnetic disk  14 . The servo sector areas  28  are spaced at regular intervals in the circumferential direction. In the servo sector area  28 , a servo pattern is set up. Magnetic information written to the servo pattern is read by using the electromagnetic conversion element on the floating head slider  22 . The floating head slider  22  is positioned in the radial direction of the magnetic disk  14  according to the information read from the servo pattern. A circular recording track is defined according to the position of the floating head slider  22 . The floating head slider  22  moves in the radial direction to define concentric recording tracks. The curved shape of the servo sector areas  28  are set based on the moving path of the electromagnetic conversion element. 
     Data areas  29  are formed between adjacent servo sector areas  28 . In the data areas  29 , the electromagnetic conversion element is positioned according to the servo pattern and travels on the recording tracks. Along the recording tracks, the write head element of the electromagnetic conversion element writes magnetic information, while the read head element of the electromagnetic conversion element reads magnetic information therealong. 
     As illustrated in  FIG. 3 , magnetic dots  31  in plural lines are concentrically arranged on the surface of the magnetic disk  14 . The individual magnetic dots  31  are cylinders, that is, magnetic pillars each having a central axis orthogonal to the surface of the magnetic disk  14 . A diameter of the magnetic pillar is exemplarily set to about 20 nano millimeters. An interval of the central axes is exemplarily set to about 22 to 23 nano millimeters. The magnetic pillars are separated by a non-magnetic substance  32 . In  FIG. 3 , as an example, three lines of the magnetic pillars form a recording track  33 . That is, adjacent recording tracks  33  are magnetically separated by the non-magnetic substance  32 . In the individual lines, the magnetic pillars are separated by the non-magnetic substance  32 . 
       FIG. 4  is a cross-sectional structure of the magnetic disk  14 . The magnetic disk  14  comprises a base material, i.e., a substrate  34 . The substrate  34  may be formed with a disk-shaped Si base  34   a,  and an amorphous SiO 2  film  34   b  that extends on both surfaces of the Si base  34   a,  for example. In the figure, only the front surface of the Si base  34   a  is illustrated. The substrate  34  may be a glass substrate or an aluminum substrate. 
     An underlayer  35  extends on the front surface of the substrate  34 . The underlayer  35  may be formed with a soft magnetic substance, such as an iron-cobalt-tantalum (FeCoTa) film or a nickel-iron (NiFe) film. The underlayer  35  has therein an easily-magnetizable axis in an in-plane direction parallel to the surface of the substrate  34 . 
     A non-magnetic intermediate layer  36  extends on the front surface of the underlayer  35 . The non-magnetic intermediate layer  36  may be formed with a tantalum (Ta) adhesion layer laminated on the front surface of the underlayer  35  and a ruthenium (Ru) layer laminated on the front surface of the tantalum adhesion layer, for example. 
     A recording layer  37  is formed on the front surface of the non-magnetic intermediate layer  36 . The recording layer  37  comprises the magnetic dots  31  disposed on the front surface of the non-magnetic intermediate layer  36 . The magnetic dots  31  are formed of a cobalt-iron (CoFe) alloy. Each of the magnetic dots  31  has therein an easily-magnetizable axis in the vertical direction orthogonal to the surface of the substrate  34 . The magnetic dots  31  each have a downward magnetization toward the surface of the substrate  34  and an upward magnetization away from the surface of the substrate  34  so as to record binary information. A space between the magnetic dots  31  is filled with the non-magnetic substance  32 . The non-magnetic substance  32  is formed of silicon dioxide (SiO 2 ) , for example. The magnetic dots  31  and the non-magnetic substance  32  form a flat surface. The flat surface, i.e., the front surface of the recording layer  37  is coated with a protective film  38  such as a diamond-like carbon (DLC) film, and a lubricating film  39  such as a perfluoropolyether (PFPE) film. Similarly, on the back surface of the substrate  34 , the underlayer  35 , the non-magnetic intermediate layer  36 , the recording layer  37 , the protective film  38 , and the lubricating film  39  are laminated in this order. 
       FIG. 5  is an example of the servo sector areas  28 . Each of the servo sector areas  28  comprises a preamble  41 , a servo mark address  42 , an amplitude/burst  43 , and a recording/reproducing timing mark  44 , in this order from an upstream side. The preamble  41 , the servo mark address  42 , the amplitude/burst  43 , and the recording/reproducing timing mark  44  together forma servo pattern. In the preamble  41 , magnetic bodies  45  in plural lines are arranged in a non-magnetic substance  46 . The individual magnetic bodies  45  extend in the radial direction of the magnetic disk  14 , for example. The magnetic bodies  45  are each magnetized to an N-pole or an S-pole. According to an arrangement of the magnetic bodies  45 , a specific magnetic pattern is set up on the single recording track  33 . The magnetic bodies  45  are spaced at regular intervals in the circumferential direction of the magnetic disk  14 , for example. A size of the magnetic bodies  45  is determined based on a magnitude of the magnetic field produced by the write head element of the electromagnetic conversion element or material characteristics of the magnetic bodies  45 . The magnetization direction of a single magnetic body  45  is uniformed. Therefore, the preamble  41  ensures that signals read from the read head element of the electromagnetic conversion element are synchronized. At the same time, gain is adjusted according to the signals read from the read head element of the electromagnetic conversion element. The “upstream side” and the “downstream side” are specified by the traveling direction of the floating head slider  22  determined while the magnetic disk  14  rotates. 
     Similarly, in the servo mark address  42 , magnetic bodies  47  in plural lines are arranged in the non-magnetic substance  46 . The individual magnetic bodies  47  extend in the radial direction of the magnetic disk  14 . The magnetic bodies  47  are each magnetized to an N-pole or an S-pole. A size of the magnetic bodies  47  is determined based on a magnitude of the magnetic field produced by the write head element of the electromagnetic conversion element or material characteristics of the magnetic bodies  47 . The magnetization direction of each single magnetic body  47  is uniformed. With this arrangement of the magnetic bodies  47 , a specific magnetic pattern is set up on the single recording track  33 . Magnetic patterns are different from one recording track  33  to another, and reflect track numbers. At the same time, a specific magnetic pattern common to the recording tracks  33  is set up. This magnetic pattern reflects sector numbers. 
     Similarly, in the amplitude/burst  43 , magnetic bodies  48  in plural lines are arranged in the non-magnetic substance  46 . The individual magnetic bodies  48  extend in the radial direction of the magnetic disk  14 . The magnetic bodies  48  are sectioned into a track width of the recording track, i.e., a track pitch Tp, in the radial direction of the magnetic disk  14 . A prescribed number of the magnetic bodies  48  form a single burst group  48   a.  In a first area  49   a  of the uppermost stream, two adjacent burst groups  48   a  have an interval therebetween of a track pitch Tp in the radial direction of the magnetic disk  14 . Similarly, in a second area  49   b  that is downstream of the first area  49   a  and adjacent thereto, two adjacent burst groups  48   a  have an interval therebetween of a track pitch Tp in the radial direction of the magnetic disk  14 . The burst groups  48   a  in the second area  49   b  and the burst groups  48   a  in the first area  49   a  are displaced by one track pitch Tp in the radial direction of the magnetic disk  14 . Similarly, in a third area  49   c  that is downstream of the second area  49   b  and adjacent thereto, two adjacent burst groups  48   a  have an interval therebetween of a track pitch Tp in the radial direction of the magnetic disk  14 . The burst groups  48   a  in the third area  49   c  and the burst groups  48   a  in the second area  49   b  are displaced by a half track pitch Tp in the radial direction of the magnetic disk  14 . Similarly, in a fourth area  49   d  that is downstream of the third area  49   c  and adjacent thereto, two adjacent burst groups  48   a  have an interval therebetween of a track pitch Tp in the radial direction of the magnetic disk  14 . The burst groups  48   a  in the fourth area  49   d  and the burst groups  48   a  in the third area  49   c  are displaced by one track pitch Tp in the radial direction of the magnetic disk  14 . The electromagnetic conversion element moving along the central line of the recording track  33  passes the burst groups  48   a  in the first area  49   a  and the second area  49   b  sequentially. Then, in both areas, the magnetic fields at the same level of strength are detected, and reproduction signals at the same level of intensity are sequentially output. When the electromagnetic conversion element deviates from the central line of the recording track  33 , magnetic field of the first area  49   a  or the second area  49   b  increases in strength. The other magnetic field of the first area  49   a  and the second area  49   b  than the one increased in strength decreases in strength. A difference occurs between the levels of reproduction signals subsequently output based on the amount of difference between the strengths. This difference is used to perform tracking control of the electromagnetic conversion element. 
     In the recording/reproducing timing mark  44 , magnetic bodies  51  in plural lines are arranged in the non-magnetic substance  46 . The individual magnetic bodies  51  extend in the radial direction of the magnetic disk  14 , for example. The magnetic bodies  51  are each magnetized to an N-pole or an S-pole. According to an arrangement of the magnetic bodies  51 , a specific magnetic pattern is set up on the single recording track  33 . A size of the magnetic bodies  51  is determined based on a magnitude of the magnetic field produced by the write head element of the electromagnetic conversion element or material characteristics of the magnetic bodies  51 . The magnetization direction of a single magnetic body  51  is uniformed. Thus, the recording/reproducing timing mark  44  ensures the timing of the read and write operations by the electromagnetic conversion element. 
     As illustrated in  FIG. 6 , a motor driver circuit  54  is connected to the voice coil motor  23 . The motor driver circuit  54  supplies driving current to the voice coil motor  23 . The voice coil motor  23  is driven by the supplied driving current with the displacement amount determined by a rotation amount (rotation angle) of the head stack assembly  17 . 
     To a write head element  55  and a read head element  56  of the electromagnetic conversion element, a preamplifier  57  is connected. To the preamplifier  57 , a read/write channel circuit  58  is connected. The read/write channel circuit  58  modulates and demodulates a signal according to a predetermined modulation/demodulation method. When the electromagnetic conversion element passes the data area  29 , which is out of the servo sector area  28 , a modulated signal, i.e., a write signal is supplied to the preamplifier  57 . The preamplifier  57  converts the write signal to the write current signal. The converted write current signal is supplied to the write head element  55 . Similarly, when the electromagnetic conversion element passes the data area  29 , a read signal output from the read head element  56  is amplified by the preamplifier  57  to supply the signal to the read/write channel circuit  58 . The read/write channel circuit  58  demodulates the read signal. 
     To the motor driver circuit  54  and the read/write channel circuit  58 , a hard disk controller (HDC)  59  is connected. The HDC  59  supplies a control signal to the motor driver circuit  54  so as to control the output, i.e., the driving current, of the motor driver circuit  54 . Similarly, the HDC  59  transmits an unmodulated write signal to the read/write channel circuit  58 , while receiving a demodulated read signal from the read/write channel circuit  58 . An unmodulated write signal may be generated with the HDC  59  based on data transmitted from a host computer, for example. Such data may be transmitted to the HDC  59  via a connector  61 . To the connector  61 , a control signal cable or a power cable (both are not depicted) from a main board of the host computer may be connected, for example. Moreover, the HDC  59  reproduces data based on the demodulated read signal. The reproduced data may be output from the connector  61  to the host computer. The HDC  59 , when exchanging data, can use a buffer memory  62 , for example. The buffer memory  62  temporarily stores data therein. The buffer memory  62  may comprise a synchronous dynamic random access memory (SDRAM), for example. 
     To the HDC  59 , a microprocessor unit (MPU)  63  is connected. The MPU  63  has a central processing unit (CPU)  65  that runs a computer program stored in a read only memory (ROM)  64 , for example. The computer program is a tracking servo control program according to an embodiment. The tracking servo control program may be provided as so-called firmware. The CPU  65  can, for example, obtain data from a flash ROM  66  upon operating. Such a computer program and data can be temporarily stored in a random access memory (RAM)  67 . The ROM  64 , the flash ROM  66 , and the RAM  67  may be directly connected to the CPU  65 . 
     The write head element  55  of the electromagnetic conversion element, when writing data, faces the data area  29  in the magnetic disk  14 . The electromagnetic conversion element is positioned in a radial direction of the magnetic disk  14  according to the tracking servo control. Details of the tracking servo control will be described later. At the same time, the recording/reproducing timing mark  44  specifies the write operation timing according to the rotation of the magnetic disk  14 . The HDC  59  generates a write signal based on data supplied from the host computer, for example. The write signal is transmitted to the read/write channel circuit  58 . The read/write channel circuit  58  modulates the write signal according to a predetermined modulation method. The modulated write signal is converted by the preamplifier  57 . The converted write current signal is supplied to the write head element  55 . The write head element  55  performs a write operation. The magnetic disk  14  rotates at a constant speed according to the servo control, for example. 
     Similarly, the read head element  56  of the electromagnetic conversion element, when reading data, faces the data area  29  in the magnetic disk  14 . The electromagnetic conversion element is positioned in a radial direction of the magnetic disk  14  according to the tracking servo control. The recording/reproducing timing mark  44  specifies the read operation timing according to the rotation of the magnetic disk  14 . The read/write channel circuit  58  supplies a sense current to the read head element  56 . A voltage change according to the magnetization direction of the data area  29  is monitored with the sense current. The voltage change is amplified by the preamplifier  57 . To the preamplifier  57 , a direct current bias is applied through a coupling capacitance. As a result, a positive voltage is output from the preamplifier  57 , depending on one of an N-pole and an S-pole. On the other hand, a negative voltage is output from the preamplifier  57 , depending on the other pole. That is, the preamplifier  57  outputs a reproduction signal with a voltage change swinging from positive to negative and vice versa. The read/write channel circuit  58  demodulates the reproduction signal. The HDC  59  reproduces data from the demodulated reproduction signal. The reproduced data is output from the connector  61  to the host computer. 
     A rectifier circuit  71  is connected between the preamplifier  57  and the read/write channel circuit  58 . The rectifier circuit  71 , according to a reproduction signal swinging from positive to negative and vice versa, generates a reproduction signal swinging only to either the positive or negative direction. That is, a reproduction signal swinging from positive to negative is turned into, for example, a positive reproduction signal regarding the absolute value. The rectifier circuit  71  supplies a rectified reproduction signal to the read/write channel circuit  58 . 
     An offset correction circuit  72  is connected at a preceding stage of the rectifier circuit  71 . The offset correction circuit  72  has an amplifier  73  that is connected between the rectifier circuit  71  and the preamplifier  57 . To the amplifier  73 , the reproduction signal is supplied by the preamplifier  57 . An integral circuit  74  is also connected to the amplifier  73 . A bias voltage is applied to the amplifier  73  with the integral circuit  74 . When the bias voltage is generated, the reproduction signal is supplied to an input terminal of the integral circuit  74  from the preamplifier  57 . With the integral circuit  74 , a direct current offset and a low-frequency wave component are extracted from the reproduction signal. With the amplifier  73 , the direct current offset is eliminated from the reproduction signal swinging from positive to negative and vice versa. As a result, symmetry with respect to the reference voltage of 0 volt (V) is improved. In the rectifier circuit  71 , the absolute value is generated according to the corrected reproduction signal. Accordingly, the amplitude fluctuation is eliminated so that amplitude of the output of the rectifier circuit  71  is moderate. 
       FIG. 7  is an example of the rectifier circuit  71 . The rectifier circuit  71  comprises a first transistor circuit  75  and a second transistor circuit  76 . The first and the second transistor circuits  75  and  76  each have a collector to which a voltage Vcc is commonly applied, and an emitter to which a resistance  77  is commonly connected. An output voltage Vout is derived from the emitters. The first transistor circuit  75  has a base to which a positive polarity output signal Vin 1  is supplied by the preamplifier  57 . The second transistor circuit  76  has a base to which a reverse polarity output signal Vin 2  is supplied by the preamplifier  57 . 
     As is evident from  FIG. 7 , when a positive voltage is supplied to the positive polarity output signal Vin 1 , a signal is fed to the first transistor circuit  75 . In this case, a negative voltage is supplied to the reverse polarity output signal Vin 2 . Accordingly, no voltage is fed to the second transistor circuit  76 . The output of the positive polarity output signal Vin 1  is reflected in the output voltage Vout. As illustrated in  FIG. 8 , when a negative voltage is supplied to the positive polarity output signal Vin 1 , no voltage is fed to the first transistor circuit  75 . On the other hand, a positive voltage is supplied to the reverse polarity output signal Vin 2  to feed a signal to the second transistor circuit  76 . The output of the reverse polarity output signal Vin 2  is reflected in the output voltage Vout. The positive polarity signal and the reverse polarity signal are supplied to the read/write channel circuit  58 . Therefore, two lines of the rectifier circuit  71  are connected to the read/write channel circuit  58 . One input of the rectifier circuit  71  is exchangeable with the other input of the rectifier circuit  71 . 
     Following is a scenario of the tracking servo control. The read head element  56  of the electromagnetic conversion element faces the servo sector area  28  in the magnetic disk  14 . When the magnetic disk  14  rotates, the read head element  56  passes through the preamble  41 , the servo mark address  42 , the amplitude/burst  43 , and the recording/reproducing timing mark  44 , in this order. A voltage change according to the magnetization direction of the magnetic body is monitored with the sense current. The voltage change is amplified by the preamplifier  57 . A reproduction waveform swinging from positive to negative and vice versa with respect to the reference voltage of 0 V, as illustrated in  FIG. 9 , is output through the coupling capacitance. This reproduction waveform is input to the amplifier. 
     The reproduction waveform swinging from positive to negative and vice versa is concurrently supplied to the integral circuit  74 . As illustrated in  FIG. 10 , with the integral circuit  74 , a direct current offset and a low-frequency wave component are extracted from the reproduction signal. A bias voltage is applied to the amplifier  73  with the integral circuit  74 . The bias voltage is subtracted from the reproduction waveform output by the preamplifier  57 . As a result, the direct current offset is subtracted from the reproduction waveform swinging from positive to negative and vice versa. As illustrated in  FIG. 11 , the amplifier  73  supplies the corrected reproduction waveform to the rectifier circuit  71 . Symmetry with respect to the reference voltage of 0 V is improved in the corrected reproduction waveform. 
     As illustrated in  FIG. 12 , the absolute value of the reproduction waveform swinging from positive to negative and vice versa is generated with the rectifier circuit  71 . Then, the reproduction waveform swings only to the positive direction with respect to the reference voltage of 0 V. That is, a unidirectional reproduction waveform is obtained. As described above, because the symmetry with respect to the reference voltage of 0 V is improved in the corrected reproduction waveform, the amplitude fluctuation in the unidirectional reproduction waveform is maximally eliminated. Thereafter, the unidirectional reproduction waveform is supplied to the read/write channel circuit  58 . The read/write channel circuit  58  generates a position error signal based on the unidirectional reproduction waveform. By using the position error signal, the HDC  59  calculates a driving amount of the voice coil motor  23 . Then, the HDC  59  outputs a control signal to the motor driver circuit  54  based on the obtained driving amount. With the motor driver circuit  54 , the obtained driving amount is supplied to the voice coil motor  23 . Thus, the electromagnetic conversion element is positioned on the specified recording track. 
     As illustrated in  FIG. 13 , the rectifier circuit  71  may be incorporated in the preamplifier  57 . The preamplifier  57  comprises a first stage amplifier circuit  81  and a subsequent stage gain amplifier circuit  82 . A high pass filter  83  is inserted between the first stage amplifier circuit  81  and the gain amplifier circuit  82 . The high pass filter  83  removes a direct current component in the reproduction signal, i.e., the output of the first stage amplifier circuit  81 . The rectifier circuit  71  is inserted between the output of the first stage amplifier circuit  81  and the high pass filter  83  in parallel. A switching element  86  is interposed between the first stage amplifier circuit  81  and the high pass filter  83 . The switching element  86  switches a first path  84  that is directly connected to the high pass filter  83  from the output of the first stage amplifier circuit  81 , and a second path  85  connected to the high pass filter  83  from the output of the first stage amplifier circuit  81  through the rectifier circuit  71 . The reproduction signal from the data area  29  passes through the first path  84 . The reproduction signal from the servo sector area  28  passes through the second path  85 . The rectifier circuit  71 , from a reproduction signal swinging from positive to negative and vice versa, generates the unidirectional reproduction signal swinging only to the positive direction with respect to the reference voltage of 0 V, as described above. The switching operation interacts with a servo gate signal. When the reproduction signal swinging from positive to negative and vice versa passes through the high pass filter  83 , sag is generated in the reproduction signal on removal of the direct current component. The sag degrades the symmetry of the reproduction signal. The absolute value of the reproduction signal is generated before the reproduction signal passes through the high pass filter  83 , and then, distortion of the reproduction waveform is suppressed to reduce the error rate. This type of the preamplifier  57  can be integrated, for example, in a semiconductor element as one-chip. 
     Moreover, as illustrated in  FIG. 14 , a first stage high pass filter  87  can be inserted at the preceding stage of the rectifier circuit  71  in the preamplifier  57 . A cutoff frequency is set to low at the first stage high pass filter  87 , such as about 100 kilohertz (kHz). An attenuation factor is set to about −10 decibel (dB). The cutoff frequency at the subsequent stage high pass filter  83  is set to be higher than that of the first stage high pass filter  87 , such as about 1 millihertz (MHz). The attenuation factor is set to about −20 dB. The first stage high pass filter  87  removes a low-frequency wave component from the output of the first stage amplifier circuit  81 . As a result, the absolute value of the reproduction signal can be properly generated in the rectifier circuit  71 . 
     A manufacturing method of the magnetic disk  14  will now be simply explained. The substrate  34  is prepared first. The substrate  34  is mounted on a sputtering apparatus having a chamber in which a vacuum environment is established. In the chamber, a FeCoTa target is set, for example. The underlayer  35  is formed on the substrate  34 . The non-magnetic intermediate layer  36  is formed on the underlayer  35 . The sputtering apparatus is used to form the layers. In the sputtering apparatus, a tantalum target or a ruthenium target is similarly set. 
     Then, as illustrated in  FIG. 15 , a solid film of a magnetic film  91  is formed on the non-magnetic intermediate layer  36 . The magnetic film  91  is formed of a cobalt ferrite alloy, for example. The sputtering apparatus is used for the lamination, for example. A resist is applied to coat the magnetic film  91  by forming a resist film  92  on the magnetic film  91 . 
     Then, as illustrated  FIG. 16 , the resist film  92  is patterned using nanoimprint lithography. A mold  93  is pressed on the resist film  92  to cover areas corresponding to the magnetic dots  31 , and the magnetic bodies  45 ,  47 ,  48 , and  51 . The resist film  92 , when formed, is exposed after the mold-pressing. As illustrated in  FIG. 17 , an etching treatment is performed after the exposure. The magnetic film  91  is scraped to form the magnetic dots  31 , and the magnetic bodies  45 ,  47 ,  48 , and  51  therefrom. In other words, the magnetic dots  31 , and the magnetic bodies  45 ,  47 ,  48 , and  51  are formed of the magnetic film  91  remaining on the non-magnetic intermediate layer  36 . 
     After the magnetic dots  31 , and the magnetic bodies  45 ,  47 ,  48 , and  51  are formed, a filler is applied to coat the non-magnetic intermediate layer  36 . The filler comprises a silicon dioxide. A spin-coat method is employed for the coating. Once the filler is cured, a planarization polishing process is performed. As a result, as illustrated in  FIG. 18 , a space among the magnetic dots  31 , and the magnetic bodies  45 ,  47 ,  48 , and  51  is filled with the filler. The filler forms the non-magnetic substances  32  and  46 . Thus, the surface of the recording layer  37  is planarized. The protective film  38  is formed on the recording layer  37 . A chemical vapor deposition (CVD) method is employed on the formation of the layers. The lubricating film  39  is deposited to coat the protective film  38 . A so-called dipping method is employed for the coating. In the dipping method, the substrate  34  is dipped into a solution containing perfluoropolyether, for example. 
     Ion injection may be employed to form the magnetic dots  31  and the magnetic bodies  45 ,  47 ,  48 , and  51 . Once the ion is injected into the magnetic film  91 , the magnetic film  91  is converted to a soft magnetic substance. The ion nullifies the magnetic coercive force of the magnetic film  91 . Therefore, the non-magnetic substance  32  can be formed. This ion injection can improve the surface flatness of the recording layer  37 . 
     The servo sector area  28  is established in the magnetic disk  14 . When establishing the servo sector area  28 , the recording layer  37  of the magnetic disk  14  is exposed to a high-frequency write signal. The magnetic disk  14  may be mounted on a servo track writer (STW), or incorporated in the HDD  11 . The write head element  55  of the electromagnetic conversion element faces the magnetic disk  14 . In synchronization with the rotation of the magnetic disk  14 , a high-frequency signal is supplied to the write head element  55 . According to the high-frequency signal, the magnetic field to be applied to the write head element  55  is alternated between an N-pole and an S-pole at a predetermined period. As a result, the N-pole and the S-pole are randomly arranged on a recording track, as illustrated in  FIG. 19 . Because magnetic films used for the bit-patterned media have magnetic domains with a strong exchange coupling force therebetween, each of the magnetic bodies  45 ,  47 ,  48 , and  51  has unidirectional magnetization inevitably. Even if the writing magnetic field is applied to a part of each of the magnetic bodies  45 ,  47 ,  48 , and  51 , the reversal of the magnetization is induced at each of the magnetic bodies  45 ,  47 ,  48 , and  51  as a whole. 
     In the magnetic bodies, the high-frequency write signal is used to magnetize the servo sector area  28  in which the N-pole and the S-pole are randomly arranged. In the magnetic bodies  45 ,  47 ,  48 , and  51  with the N-pole and the S-pole adjacent to each other, the magnetization is stable, resulting in avoiding the magnetization reversal. In particular, when the intervals among the magnetic bodies  45 ,  47 ,  48 , and  51  and a half-cycle of the high-frequency write signal correspond to one another, the number of combinations of adjacent N-pole and the S-pole is reliably increased. Therefore, the possibility of the magnetization reversal is dramatically decreased. 
     In the conventional bit-patterned media, all the magnetic bodies  45 ,  47 ,  48 , and  51  are unidirectionally magnetized. Therefore, the servo sector area  28  with one pole is positioned in the non-magnetic substance  46 . Accordingly, only the unidirectional reproduction signal is supplied to the HDC  59  upon tracking servo control. As described-above, if a unidirectional reproduction signal is generated due to the rectifier circuit  71 , the HDC  59  can perform the signal processing as in the conventional one. Upon tracking servo control process, the HDC  59  can also perform the process same as that of the conventional HDC. Moreover, the tracking servo control process can be used for the conventional bit-patterned media. Even though the magnetization reversal is induced by heat fluctuation or aging deterioration, only the unidirectional reproduction signal is supplied to the HDC  59 . 
     As described above, a magnetic storage medium in an embodiment has a servo pattern with which magnetization is reliably maintained. 
     The various modules of the systems described herein can be implemented as software applications, hardware and/or software modules, or components on one or more computers, such as servers. While the various modules are illustrated separately, they may share some or all of the same underlying logic or code. 
     While certain embodiments of the inventions have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.