Patent Publication Number: US-2006014053-A1

Title: Magnetic disk and magnetic disk device provided with the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2004-210462, filed Jul. 16, 2004, the entire contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention relates to a magnetic disk and a magnetic disk device provided with the same.  
      2. Description of the Related Art  
      In recent years, magnetic disk devices have been widely used as external recording devices of computers and image recording devices. In general, a magnetic disk device comprises a case in the form of a rectangular box. The case contains a magnetic disk for use as a magnetic recording medium, a spindle motor that supports and rotates the disk, magnetic heads for writing and reading information to and from the disk, and a head actuator that supports the heads for movement with respect to the disk. The case further contains a voice coil motor that rotates and positions the head actuator, a board unit that has a head IC and the like, etc. A printed circuit board for controlling the respective operations of the spindle motor, voice coil motor, and magnetic heads through the board unit is screwed to the outer surface of the case.  
      Further miniaturization of magnetic disk devices has recently been advanced so that they can be used as recording devices for a wider variety of electronic apparatuses, or smaller-sized electronic apparatuses in particular. Accordingly, magnetic disks are expected to be further reduced in size and enhanced in recording density. Proposed in Jpn. Pat. Appln. KOKAI Publication No. 2003-22634, for example, is a magnetic disk of the so-called discrete-track-recording (DTR) type, as a magnetic disk that is small-sized and ensures high-density recording. This DTR magnetic disk has rugged surfaces, and a magnetic material that can record data is formed on its projections. The surfaces of the magnetic disk are rugged and previously formed having patterned regions, including a servo region to which servo data are recorded and a data region to which a user can record data. A large number of projections or magnetic tracks are formed on the data region.  
      According to the DTR magnetic disk described above, the adjacent magnetic tracks are divided by recesses, so that crosstalk between the magnetic tracks can be prevented to ensure high-density recording. In the DTR magnetic disk, the magnetic tracks are distributed at a high density such that their pitch is not lower than the visible light wavelength. Therefore, rainbows such as interference fringes cannot be seen, so that a recording surface of the magnetic disk cannot be recognized visually. Thus, in the case of a single-sided disk, the recording surface cannot be identified. In incorporating the magnetic disk into a magnetic disk drive or the like, therefore, it is hard accurately to set its position relative to the magnetic head.  
      In increasing the recording capacity, the recording layer should preferably be provided on each side of the magnetic disk. For the same reason as aforesaid, however, the side, obverse or reverse, of the magnetic disk cannot be discriminated with ease. Also in this case, it is hard appropriately to orient the magnetic disk when it is incorporated into a magnetic disk device.  
     BRIEF SUMMARY OF THE INVENTION  
      According to an aspect of the invention, there is provided a magnetic disk comprising a disk-shaped substrate having a center hole; and recording regions provided individually on obverse and reverse surfaces of the substrate, the recording regions having patterned magnetic material shapes, the respective pattern shapes of the recording regions on the obverse and reverse sides being different.  
      According to another aspect of the invention, there is provided a magnetic disk device comprising:  
      a magnetic disk which comprises a disk-shaped substrate having a center hole, and recording regions provided individually on obverse and reverse surfaces of the substrate, the recording regions having patterned magnetic material shapes, the respective pattern shapes of the recording regions on the obverse and reverse sides being different;  
      a drive unit which supports and rotates the magnetic disk at a constant speed;  
      a head which performs information processing for the magnetic disk; and  
      a head actuator which radially moves the head with respect to the magnetic disk, the magnetic disk being located in a direction such that each of the servo region patterns and a movement path of the head on the magnetic disk are in line with each other. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       FIG. 1A  is a plan view showing a surface pattern of a magnetic disk according to an embodiment of the invention;  
       FIG. 1B  is a plan view showing a reverse pattern of the magnetic disk;  
       FIG. 2  is an enlarged perspective view, partially in section, showing a data region pattern of the magnetic disk;  
       FIG. 3  is a diagram typically showing a servo region pattern of the magnetic disk;  
       FIG. 4  is a diagram schematically showing optical reflection factors of a data region pattern and a servo region pattern of the magnetic disk;  
       FIG. 5  is an exploded perspective view showing an HDD according to the embodiment of the invention;  
       FIG. 6  is a block diagram schematically showing a configuration of the HDD;  
       FIG. 7  is a diagram illustrating head positioning control in the HDD; and  
       FIG. 8  is a diagram illustrating address detection processing in a channel of the HDD. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      A magnetic disk according to an embodiment of this invention will now be described in detail with reference to the accompanying drawings.  
      As shown in  FIGS. 1A, 1B  and  2 , a magnetic disk  50  according to the present embodiment comprises a substrate  54  in the form of a flat disk having a center hole  52  and recording layers  56  formed on at least one surface of the substrate (obverse and reverse surfaces of the substrate in this case). Each of the recording layers  56 , which constitutes a recording region, has the form of a ring that coaxially covers all the area of the substrate  54  except its inner and outer peripheral edge portions. Each recording layer  56  is formed of a ferromagnetic material, e.g., CoCrPt, and is patterned. Those regions of the layer which have no magnetic material are filled with a nonmagnetic material, e.g., SiO 2 . Thus, the resulting magnetic disk has a leveled surface and serves for perpendicular magnetic recording.  
      The magnetic disk  50  is formed as a DTR medium.  FIG. 1A  shows a pattern of the recording layer  56  on the obverse side of the disk  50 .  FIG. 1B  shows a pattern of the layer  56  on the reverse side of the disk  50 . Roughly speaking, each pattern of the recording layer  56  includes a data region pattern  58  and a plurality of servo region patterns  60 .  
      As shown in  FIG. 2 , the substrate  54  is formed of glass, for example, and has a base layer (SUL)  66  on each of its obverse and reverse surfaces. The substrate  54  may be formed of aluminum in place of glass. The data region pattern  58  and the servo region patterns  60  are formed on each base layer  66 .  
      The data region pattern  58  forms a recording region where user data are recorded and reproduced by a head of a magnetic disk device (mentioned later), and is composed of projections of a magnetic material on the surface of the substrate  54 . More specifically, the data region pattern  58  has a plurality of circular ring-shaped magnetic tracks  62  that serve as perpendicular recording layers of a ferromagnetic material (CoCrPt). These magnetic tracks  62  are arranged substantially coaxially with the center hole  52  and side by side at predetermined periods or track pitches Tp in the radial direction of the substrate  54 .  
      The magnetic tracks  62  that adjoin in the radial direction of the substrate  54  are divided by nonmagnetic guard belt portions  64  in the form of recesses to which data cannot be recorded. According to the present embodiment, SiO 2  is implanted in the nonmagnetic guard belt portions  64  in order to level the disk surface. Further, a thin carbon protective film is formed on the magnetic disk surface, and it is coated with a lubricant. A protective layer may be formed directly on the irregular surface without embedding the guard belt portions  64  in the surface.  
      A radial width Tw of each magnetic track  62  that extends in the radial direction of the substrate  54  is larger than a width TN of each nonmagnetic guard belt portion  64 . In the present embodiment, the ratio of the radial width of each magnetic track  62  to that of each nonmagnetic guard belt portion  64  is 2:1, and the data region pattern  58  has a magnetic occupancy of 67%. Since the data region pattern  58  has a high track density exceeding 120 kTPI, for example, the radial pattern period (track pitch) Tp is shorter than a visible light wavelength. Thus, a rainbow pattern that is formed by light diffraction by the magnetic tracks  62  cannot be visually recognized in the magnetic disk  50 .  
      As shown in  FIGS. 1A and 1B , the ring-shaped magnetic tracks  62  that constitute the data region pattern  58  are sectored in the circumferential direction of the substrate  54  by the servo region patterns  60 . In these drawings, the data region pattern  58  is shown to be divided in fifteen sectors. Actually, however, the data region pattern  58  is divided in 100 servo sectors or more.  
      Each servo region pattern  60  is a prebid region in which necessary information for positioning the head of the magnetic disk device is implanted in a magnetic or nonmagnetic manner. Each servo region pattern  60  has an arcuate shape that coincides with a movement path of the head. Further, each servo region pattern  60  is a circumferentially extended pattern such that its circumferential length along the circumference of the substrate  54  increases in proportion to the radial position on the substrate, that is, a region on the outer peripheral side of the substrate is longer. The servo region patterns  60  of the obverse-side recording layer  56  of the substrate  54  and the servo region patterns  60  of the reverse-side recording layer  56  are arranged in different orders in the circumferential direction. For example, the patterns on the obverse side are arranged in the counterclockwise direction, and those on the reverse side in the clockwise direction. Thus, the recording regions of the magnetic disk  50  have patterned magnetic material shapes, one on the obverse side and another on the reverse.  
      One of the servo region patterns  60  will now be described in detail with reference to  FIG. 3 .  
       FIG. 3  shows the servo region pattern  60  that is provided on the obverse side of the magnetic disk  50 . This servo region pattern  60  is a pattern in a position where the head passes from left to right of  FIG. 3  in a passing direction X when the magnetic disk  50  is set in a drive. If the pattern  60  is represented by an arcuate servo region pattern shape, circular arcs on the outer and inner peripheral sides are situated on the left- and right-hand sides, respectively, of  FIG. 3 . The data region pattern  58  is located on either side of the servo region pattern  60 .  
      Roughly speaking, the servo region pattern  60  has a preamble portion  70 , an address portion  72 , and a burst portion  74  for deviation detection. Like the data region pattern  58 , it is composed of magnetic patterns formed of ferromagnetic projections and nonmagnetic patterns formed of recesses between the magnetic patterns.  
      The preamble portion  70  is provided to perform PLL processing and AGC processing. In the PLL processing, clocks for servo signal reproduction are synchronized with time delays that are caused by rotation eccentricity or the like of the magnetic disk  50 . The AGC processing serves to maintain an appropriate signal reproduction amplitude. The preamble portion  70  is formed as a repetitive pattern region that is substantially radially continuous at least in the radial direction of the substrate  54  and includes magnetic and nonmagnetic portions arranged alternately in the circumferential direction of the substrate  54 . The magnetic-nonmagnetic ratio of the preamble portion  70  is substantially 1:1, that is, its magnetic occupancy is about 50%. The circumferential repetition period, which varies in proportion to the radial distance, is not longer than the visible light wavelength even in an outermost peripheral portion of the substrate  54 . As in the case of the data region pattern, it is hard to identify the servo region pattern by light diffraction.  
      In the address portion  72 , a servo signal recognition code called a servo mark, sector information, cylinder information, etc. are formed in Manchester codes that are arranged at the same pitches as the circumferential pitches of the preamble portion  70 . The cylinder information has a pattern such that it changes with every servo track. In order to lessen the influence of a mistake in address reading during head seek operation, therefore, the information is Manchester-encoded and recorded after code conversion is performed such that variations from adjacent tracks called Gray codes are minimal. The magnetic occupancy of the address portion  72  is about 50%.  
      The burst portion  74  is an off-track detection region for detecting an off-track deviation from an on-track state of a cylinder address. It is formed with four marks or bursts A, B, C and D whose pattern phases are shifted in the radial direction. Each burst has a plurality of marks that are arranged at the same pitch periods as the preamble portion in the circumferential direction. A radial period is proportional to the change period of an address pattern, that is, to a servo track period. In the present embodiment, each burst is formed for  10  periods in the circumferential direction. In the radial direction, its patterns are repeated with a period twice as long as the servo track period. The magnetic occupancy of A, B, C and D burst patterns is about 75%.  
      Basically, each mark is designed for a rectangle, or more strictly, a parallelogram based on a skew angle at the time of head access. Depending on the machining performance, such as the stamper working accuracy, transfer formation, etc., however, the marks are somewhat rounded. Further, the marks are formed as nonmagnetic portions.  
      A detailed description of the principle of position detection based on the burst portion  74  is omitted. The off-track deviation is calculated by arithmetically processing an average amplitude value of reproduction signals for the burst portions A, B, C and D. Although the A, B, C and D burst patterns are used in the present embodiment, they may be replaced with conventional phase difference servo patterns or the like that are arranged as off-track detecting means. However, the magnetic occupancy of the phase difference servo patterns is about 50%.  
      In the case of a magnetic disk that has a low-density pattern with a track pitch of 400 nm or more, optical diffraction is caused by irregular track patterns if the substrate is roughened so that whole surface of a magnetic layer is irregular. Thus, reflected light from the data region pattern can be visually recognized as a rainbow-like diffracted light. In this case, the arcuate servo region pattern shape can be visually recognized with ease.  
      In the case of a magnetic disk that has a track pitch shorter enough than the visible light wave-length, optical diffraction never occurs, so that it is hard to recognize a rainbow pattern. If the whole surface of the magnetic layer is made irregular, therefore, it is difficult visually to recognize the servo and data regions.  
      If the recording layers have magnetic and nonmagnetic patterns, as in the magnetic disk  50  according to the present embodiment, on the other hand, the lower the magnetic occupancy of the patterns, the lower the intensity of reflected light is. This is because magnetic and nonmagnetic portions have somewhat different reflection factors. Also, this characteristic is attributable to influences of multi-path reflection from the embedded nonmagnetic portion and absorbance.  
      Thus, even in the case of a high-density pattern from which optical diffraction cannot be expected, the arcuate traces of the servo region patterns  60  can be optically discriminated by a difference in reflected light intensity. This can be done in a manner such that a certain or greater difference in magnetic occupancy is provided between the data region pattern  58  and the servo region patterns  60 .  
      If there is a difference of about 10% in optical reflection factor, the patterns can be discriminated satisfactorily. In the present embodiment, the magnetic occupancy of the data region pattern  58  is about 67%, while the respective magnetic occupancies of the preamble portion  70  and the address portion  72  of each servo region pattern  60  are 50%. Thus, the difference in reflection factor from the data region pattern is great enough for the optical recognition of the servo region patterns.  
       FIG. 4  shows an optical microscope image near the servo region pattern  60 . The magnetic tracks  62 , fine patterns, etc. are invisible. The preamble portion  70  and the address portion  72  of the servo region pattern  60  can be optically recognized even if they are darker and denser than the data region pattern  58 . Arcuate servo patterns can be discriminated more clearly through a polarizing filter, for example.  
      A preferable line width that can be visually recognized is 10 μm or more. Preferably, therefore, the length of the combination of the preamble portion  70  and the address portion  72  of the innermost peripheral servo sector should be 0.01 mm or more. The line width of 10 μm is a visible limit and cannot be regarded as easily identifiable by eyes. However, the circumferential lengths of the servo region patterns  60  increase with distance from the inner periphery, depending on the radial position on the substrate, and line widths of the inner and outer peripheral portions are about 10 μm and 20 μm, respectively. The servo region patterns  60  can be easily visually observed by being enlarged at a low magnification through a magnifying glass. Thus, the length of the combination of the preamble portion and the address portion of the innermost peripheral servo sector is adjusted to 0.01 mm or more. In the present embodiment, the repetition frequency and circumferential pitch of the preamble portion  70  are adjusted so that the line width is 50 μm or more that can be directly visually recognized with ease without using any magnifying microscope or the like.  
      As mentioned before, each servo region pattern  60  is substantially in the shape of a circular arc. This servo region pattern shape is effective in discriminating the obverse and reverse of the magnetic disk  50 . If the servo region pattern is perfectly radial, it is symmetrical. Although the servo region patterns on each disk surface can be discriminated, therefore, the side, obverse or reverse, on which the patterns are formed cannot be identified. Since the servo region patterns  60  are formed in the head passing direction X, as shown in  FIG. 3 , servo information cannot be easily identified if the side of the magnetic disk is mistaken. In an assembly process in which the magnetic disk  50  having the servo region patterns  60  previously formed thereon is incorporated in the magnetic disk device as the drive, it is essential to set the disk  50  without mistaking its side. Thus, it is effective to form the arcuate servo region patterns by which the side, obverse or reverse, of the magnetic disk  50  can be recognized with ease.  
      Besides, the movement path of the head of the magnetic disk device is an arcuate path around a rotary drive mechanism, which will be mentioned later. Preferably, therefore, the servo region patterns  60  of the magnetic disk  50  should be arcuate patterns that are substantially coincident with the head movement path.  
      The following is a brief description of a method of manufacturing the magnetic disk  50  described above. Manufacturing processes include a transfer process, a magnetic processing process, and a finishing process. First, a method of manufacturing a stamper that constitutes a base of a pattern used in the transfer process will be described.  
      A method of manufacturing a stamper can be divided into steps of drawing, development, electro-forming, and finishing. In the pattern drawing, a part of the magnetic disk to be demagnetized is exposed for drawing from its inner periphery to outer periphery on a resist-coated matrix by using an electron beam exposure unit of a matrix-rotation type. The resulting structure is subjected to development, RIE, etc. to form a matrix with irregular patterns. After this matrix is treated for electrical conductibility, its surface is electroformed with nickel. Subsequently, the nickel is separated from the matrix, and a disk-shaped stamper of nickel is formed by punching for inside and outside diameters. The stamper has projections on those parts which are to be demagnetized. Stampers for the obverse and reverse surfaces of the magnetic disk are formed individually.  
      In the transfer process, the irregularities of the stamper are transferred to the magnetic disk by the imprint lithography using an imprinter of a synchronous double-sided transfer type. More specifically, base layers are first formed individually on the opposite sides of the substrate  54  that is formed of glass or silicon, and magnetic layers of a ferromagnetic material are further formed overlapping the base layers.  
      A resist is applied to both surfaces of the perpendicular-recording magnetic disk by spin coating. After the disk is baked, it is chucked by its center hole  52 . For example, liquid SiO 2  (SOG) is used as the resist. In this state, the opposite sides of the magnetic disk are sandwiched between two types of stampers that are provided for the reverse and obverse surfaces, individually, whereby the whole surfaces are pressed uniformly. Thus, the irregular patterns of the stampers are transferred to the resist surface. By this transfer process, the parts to be demagnetized are formed as recesses in the resist.  
      Then, in the magnetic processing process, the magnetic layer surface of the parts to be demagnetized is exposed after the residual resist at the respective bottoms of the recesses of the resist is removed. At that part where the magnetic layer is to be left, the resist is formed as projections. Then, only those parts of the magnetic layer which are situated corresponding to the recesses are removed by ion milling using the resist as a guard layer, whereby the magnetic material is worked into a desired pattern.  
      Subsequently, SiO 2  films are formed individually to an adequate thickness on the opposite surfaces of the magnetic disk by, for example, sputtering, thereby eliminating the irregularities of the disk surfaces. By removing the SiO 2  films to the depth of the magnetic layer surfaces by reverse sputtering, the flat pattern magnetic disk can be obtained having the recesses filled with the nonmagnetic material.  
      In the final finishing process, the disk surfaces are polished further to improve the levelness, and the carbon protective film is formed thereafter. The magnetic disk according to the present embodiment is completed by further application of the lubricant.  
      The following is a description of a hard disk drive (HDD) as the magnetic disk device that is provided with the magnetic disk  50  described above.  
      As shown in  FIGS. 5 and 6 , a magnetic disk device  10  comprises a flat, rectangular disk enclosure  13 . The enclosure  13  has a box-shaped base  12  and a top cover  11  that hermetically closes a top opening of the base  12 .  
      The disk enclosure  13  contains the magnetic disk  50 , a spindle motor  15 , magnetic heads  33 , and a head actuator  14 . The spindle motor  15  supports and rotates the disk. The magnetic heads  33  are used to record and reproduce information to and from the disk. The head actuator  14  supports the magnetic heads for movement with respect to the magnetic disk  50 . The enclosure  13  further contains a voice coil motor (hereinafter, referred to as a VCM)  16 , a ramp load mechanism  18 , an inertia latch mechanism  20 , and a flexible printed circuit board unit (hereinafter, referred to as an FPC unit)  17 . The VCM  16  rotates and positions the head actuator  14 . The ramp load mechanism  18  holds the magnetic heads  33  in a position off the magnetic disk  50  when the heads are moved to the outermost periphery of the disk. The inertia latch mechanism  20  holds the head actuator  14  in a shunt position. The FPC unit  17  is mounted with circuit components, such as a preamplifier. The base  12  has a bottom wall, and the spindle motor  15 , head actuator  14 , VCM  16 , etc. are arranged on the inner surface of the bottom wall.  
      As mentioned before, the magnetic disk  50  is a small-diameter patterned medium with a perpendicularly magnetized dual-film structure, both surfaces of which are processed for DTR. More specifically, the disk  50  has recording layers  56  on its obverse and reverse surfaces. It is formed having a diameter of 1.8 or 0.85 inch. The magnetic disk  50  is coaxially fitted on a hub (not shown) of the spindle motor  15  and fixed to the hub by a clamp spring  21 . The magnetic disk  50  is supported and rotated at a given speed by the spindle motor  15  as a driver unit.  
      The head actuator  14  has a bearing portion  24  fixed on the bottom wall of the base  12 , two arms  27  attached to the bearing portion, and suspensions  30  extending individually from the arms. The magnetic heads  33  are supported individually on the respective extended ends of the suspensions  30 . The arms  27 , suspensions  30 , and heads  33  are supported for rotating motion around the bearing portion  24 . The heads  33  include a down-head that faces the obverse-side recording layer of the magnetic disk  50  and an up-head that faces the reverse-side recording layer of the disk. In each magnetic head  33 , a slider for use as a head body is mounted with a magnetic head element that includes a read element (GMR element) and a write element.  
      The VCM  16  has a voice coil  22  attached to the head actuator  14 , a pair of yokes  38  fixed to the base  12  and opposed to the voice coil, and a magnet (not shown) fixed to one of the yokes. The VCM  16  generates a rotational torque around the bearing portion  24  in the arms  27  and moves the magnetic heads  33  in the radial direction of the magnetic disk  50 .  
      The FPC unit  17  has a rectangular board body  34  that is fixed on the bottom wall of the base  12 . Electronic components, connectors, etc. are mounted on the board body. The FPC unit  17  has a belt-shaped main flexible printed circuit board  36  that electrically connects the board body  34  and the head actuator  14 . The magnetic heads  33  that are supported by the head actuator  14  are connected electrically to the FPC unit  17  through a relay FPC (not shown) and the main flexible printed circuit board  36 .  
      As mentioned before, the magnetic disk  50  has the obverse and reverse sides and is set in the base  12  with the obverse and reverse sides aligned so that the head movement path of the magnetic disk device is substantially coincident with the arcuate shape of the servo region patterns  60  of the magnetic disk. The specifications of the magnetic disk  50  fulfill outside and inside diameters, recording and reproducing characteristics, etc. that are adaptive to the magnetic disk device. Each arcuate servo region pattern  60  has its center of circular arc on the circumference of a circle that is concentric with the magnetic disk and has its radius equivalent to the distance from the rotation center of the magnetic disk to the center of the bearing portion  24  of the head actuator  14 . The radius of the circular arc is equivalent to the distance from the bearing portion  24  to each magnetic head  33 . In other words, each servo region pattern  60  has the shape of a circular arc that is always substantially coincident with the head movement path even when the magnetic rotates. The radius of the circular arc of each servo region pattern  60  is equivalent to the distance from the bearing portion  24  to each magnetic head  33 . The center of the circular arc moves along a circular path that is concentric with the magnetic disk and varies in synchronism with the angle phase on the disk on which the patterns are formed. The radius of the path of the center of the circular arc is equivalent to the distance from the center of the spindle motor  15  to the center of the bearing portion  24 .  
      A printed circuit board (hereinafter, referred to as a PCB)  40  for controlling the respective operations of the spindle motor  15 , VCM  16 , and magnetic heads through the FPC unit  17  is fixed to the outer surface of the bottom wall of the base  12 , and faces the base bottom wall.  
      As shown in  FIG. 6 , a large number of electronic components are mounted on the PCB  40 . These electronic components mainly include four system LSI&#39;s, a hard disk controller (hereinafter, referred to as a HDC)  41 , a read/write channel IC  42 , an MPU  43 , and a motor driver IC  44 . Further, the PCB  40  is mounted with a connector that can be connected to a connector on the side of the FPC unit  17  and a main connector for connecting the HDD to an electronic apparatus such as a personal computer.  
      The MPU  43  is a controller of a drive operating system and includes a ROM, RAM, CPU, and logic processor, which realize a positioning control system according to the present embodiment. The logic processor is an arithmetic processor composed of a hardware circuit and is used for high-speed arithmetic processing. Further, operating software (FW) is saved in the ROM, and the MPU controls the drive in accordance with this FW.  
      The HDC  41  is an interface section in the HDD. It exchanges information with an interface between the disk drive and a host system, e.g., a personal computer, the MPU  43 , the read/write channel IC  42 , and the motor driver IC  44 , thereby managing the whole HDD.  
      The read/write channel IC  42  is a head signal processor associated with read/write operation. It is composed of a circuit that switches channels of a head amplifier IC and processes recording and reproducing signals, such as read/write signals. The motor driver IC  44  is a drive unit for the VCM  16  and the spindle motor  15 . It drivingly controls the spindle motor for constant rotation and applies a VCM manipulated variable from the MPU  43  as a current value to the VCM, thereby driving the head actuator  14 .  
      A configuration of a head positioning controller will now be described in brief with reference to  FIG. 7 .  
       FIG. 7  is a block diagram of the head positioning controller. In  FIG. 7 , symbols C, F, P and S individually designate transfer functions of the system. Specifically, a control object P is equivalent to the head actuator  14  that includes the VCM  16 , while a signal processor S is an element that is realized by a channel IC and an MPU (part of off-track detecting means).  
      A control processor includes a feedback controller C (first controller) and a synchronous suppression/compensation section (second controller), and specifically, is realized by an MPU.  
      The operation of the control processor will be described in detail later. The signal processor S generates track current position (TP) information on the magnetic disk  50  in accordance with a reproducing signal including address information from the servo region patterns  60  right under a head position (HP). Based on a target track position (RP) on the magnetic disk  50  and a position error (E) between the target track position and a current position (TP) of each magnetic head  33  on the magnetic disk  50 , the first controller C outputs an FB control value U 1  in a direction to lessen the position error.  
      The second controller F is an FF compensation section for correcting the shape of the magnetic track on the magnetic disk  50 , vibration that is synchronous with the disk rotation, etc. It saves a previously calibrated rotation synchronous compensation value in a memory table. Normally, the second controller F never uses the position error (E), and outputs an FF control value U 2  based on servo sector information (not shown) from the signal processor S with reference to the table. The control processor adds up the respective outputs U 1  and U 2  of the first and second controllers C and F, and supplies the resulting value as a control value U to the VCM  16  through the HDC  41 , thereby driving the magnetic heads  33 .  
      The rotation synchronous compensation value table is calibrated in an initial stage of operation. If the position error (E) becomes larger than a preset value, the table starts to be calibrated again, whereupon the synchronous compensation value is updated.  
      An operation for detecting the position error by the reproducing signal will now be described in brief with reference to  FIG. 7 .  
      The magnetic disk  50  is rotated at a fixed rotational speed by the spindle motor  15 . The magnetic heads  33  are elastically supported by gimbals that are attached to the suspensions  30 . They are designed to fly with a fine gap above the magnetic disk surface, balanced by an air pressure that is generated as the disk rotates. Thus, a head reproducing element can detect a magnetic flux leakage from the disk magnetic layer with a given magnetic gap above the disk surface.  
      As the magnetic disk  50  rotates, its servo region patterns  60  pass right under the magnetic heads  33  in a given period. Fixed-period servo processing can be executed by detecting track position information from reproducing signals for the servo region patterns.  
      Once the HDC  41  recognizes one of servo region pattern identification flags called servo marks in the servo region patterns  60 , the timing for the arrival of each servo region pattern can be anticipated, since the servo marks are arranged at predetermined intervals. Accordingly, the HDC  41  urges the channel to start servo processing when the preamble portion  70  comes right under the magnetic head.  
      The following is a description of an address reproduction processing configuration in the channel. As shown in  FIG. 8 , an output signal from a head amplifier IC (HIC) that is connected to the magnetic heads  33  is read by the channel IC. After it is subjected to longitudinal signal equalization by an analog filter as an equalizer  45 , the signal is sampled as a digital value by an ADC  46 .  
      A magnetic field leakage from the magnetic disk  50  is perpendicular magnetization and is a magnetic/nonmagnetic pattern. However, DC offset components are thoroughly removed by the high-pass characteristic of the HIC and equalizer processing of a front-stage portion of the channel IC for longitudinal equalization. Thus, an analog filter post-output from the preamble portion  70  is substantially a false sine wave. A difference from a conventional perpendicular magnetic medium lies in that the signal amplitude is halved.  
      The magnetic disk according to the present embodiment is not limited to a patterned medium. However, selection of the direction of the magnetic flux leakage of the servo region patterns may cause misidentification of 1 or 0, and hence, failure in code detection in the channel. Thus, the magnetic disk polarity can be properly set according to the magnetic flux leakage of the patterns.  
      In the channel IC, the processing is switched depending on its reproducing signal phase. A reproducing signal clocks are synchronized with medium pattern periods in pull-in processing. Sector cylinder information is read in address reading processing. Burst portion processing is carried out as necessary information for off-track detection.  
      A detailed description of the pull-in processing is omitted. In this processing, the timing for sampling the ADC is synchronized with a sine-wave reproducing signal, and AGC processing is performed to adjust the signal amplitudes of digital sample values to a certain level. Periods  1  and  0  of a disk pattern are sampled at four points.  
      Then, in reproducing the address information, noises of the sample valued are lowered by a FIR filter  47 . The sample values are converted into sector information and track information through Viterbi decoding processing based on maximum likelihood estimation by a Viterbi decoder  48  or gray code reverse conversion by a gray processor  49 . Thus, servo track information of the magnetic heads  33  can be obtained.  
      Subsequently, in the burst portion  74 , the channel proceeds to off-track detection processing. The signal amplitudes are subjected to sample-hold integral processing in the order of the burst signal patterns A, B, C and D, and a voltage value equivalent to an average amplitude is outputted to the MPU  43 , whereby a servo processing interrupt is issued to the MPU. On receiving this interrupt, the MPU  43  reads the burst signals in the time series by an internal ADC, and converts them into off-track values by DSP. Based on these off-track values and the servo track information, the servo track positions of the magnetic heads  33  are detected precisely.  
      According to the magnetic disk  50  and the HDD constructed in this manner, the side, obverse or reverse, of the magnetic disk can be visually recognized, and the assembly of the disk device can be easily managed without failing to be aware of the side by the supplied medium. Further, each servo region pattern is formed in the shape of a circular arc corresponding to the head movement path. This is advantageous to the seek performance and the prevention of lowering of SN ratios at the inner and outer peripheries of the disk, so that the performance of the magnetic disk device can be improved.  
      A DTR system is a magnetic recording system in which error rates in data regions can be improved and the surface recording density can be increased. The increased recording density leads to an increase in recording capacity. Since the servo information, along with data tracks, is formed by implantation, the medium never requires servo information recording (STW: servo track write), which is an advantage of the use of the patterned medium to the HDD.  
      More specifically, the magnetic disk  50  has the arcuate servo region patterns  60  that depend on the configuration of the HDD, and its obverse and reverse are oriented as it is incorporated in the HDD. Accordingly, the magnetic disk  50  can produce the following functions and effects.  
      First, the magnetic disk  50  can ensure high seek performance. As mentioned before, the HDC  41  requests the channel to start serve processing at a timing when any of the servo region patterns  60  comes right under the magnetic head  33 . If the servo region patterns are arranged at equal spaces and if the magnetic heads  33  are fixed in the radial direction, the resulting timing error is within an allowable range and negligible despite some fluctuation of a servo region pattern crossing period that is attributable to eccentric mounting of the magnetic disk. However, the magnetic heads  33  move in a circular arc as they move at high speed in the radial direction of the magnetic disk  50  during seek operation, for example. Thus, the magnetic heads move in the circumferential direction as well as in the radial direction and arouse a problem.  
      If the servo region patterns are formed perfectly radially, for example, they are situated in fixed angle phases without depending on the radial position. Since the magnetic heads  33  also move in the circumferential direction, however, the angle phases vary with respect to the rotation center of the spindle motor  15 . Thus, a servo starting phase (distance from a servo region starting position in which a reproducing head is situated when a servo gate is booted) as viewed from the magnetic head side changes. This phase difference is settled depending on the seek speed, error in the magnetic head path, and control period. If the phase difference exceeds an allowable range, it is hard to fetch servo signals at the preamble portion  70 . Possibly, therefore, the servo mark (SAM) at the head of the address portion  72  may fail to be detected, thus resulting in a servo loss error.  
      The occurrence of the servo loss error can be prevent even during high-speed seek operation by estimating a timing error time from the seek speed and the cylinder information and correcting a servo gate rise time. In this case, however, the servo characteristic is changed by a fluctuation of the control period, so that the seek performance lowers inevitably. High-speed seek can be effectively enabled by forming the servo region patterns in a circular arc after the head movement path.  
      Secondly, the difference in the servo information detection SN between the inner and outer peripheries of the magnetic disk  50  can be reduced. The servo information detection SN at the inner periphery of the disk  50  is inevitably lowered due to a high linear recording density even though the servo region patterns  60  are arranged along the magnetic head movement path. If the servo region patterns are perfectly radial, however, the SN ratio on the inner peripheral side of the magnetic disk lowers drastically. A simulation indicates that the SN ratio at the outer peripheral portion of the disk also lowers. This is attributable to the skew angle of the magnetic heads. More specifically, the servo signals are applied with a skew to the magnetic heads, so that the build-up of the servo signals is degraded and entails a reduction of the amplitude.  
      In the case of a small-diameter magnetic disk, in particular, servo signal clocks are enhanced to a maximum in order to increase the format efficiency. Accordingly, lowering of the SN ratio at the innermost periphery of the magnetic disk directly influences address reading, off-track detection accuracy, etc. As in the present embodiment, therefore, the shapes of the servo region pattern  60  that advance parallel to the magnetic heads  33  are essential. In the present embodiment, prebid-length signal clocks of the servo region patterns are set in accordance with the circumferential length of the visually recognizable patterns, the detection SN at the inner peripheral portion of the magnetic disk, and the rotational speed of the spindle motor.  
      This invention is not limited directly to the embodiment described above, and its components may be embodied in modified forms without departing from the scope or spirit of the invention. Further, various inventions may be made by suitably combining a plurality of components described in connection with the foregoing embodiment. For example, some of the components according to the foregoing embodiment may be omitted. Furthermore, components according to different embodiments may be combined as required.  
      In the foregoing embodiment, the optical reflection factor of the data region pattern of the magnetic disk is higher than that of the servo region patterns. However, it is necessary only that the respective optical reflection factors of these patterns be different. In the case of a patterned medium (one-dot, one-bit type) in a limited sense, the magnetic occupancy of the data region pattern is rather lowered to about 30%, and the quantity of reflected light from the data region pattern is smaller than that from the servo region patterns. Owing to the imprint manufacture, moreover, the marks of the burst portions are magnetic, and the magnetic occupancy of the burst regions is 25%.  
      Further, the number of magnetic disk(s) in the HDD is not limited to one but may be increased as required.