Abstract:
According to an aspect of an embodiment, a magnetic device comprises a head for writing data into or reading data from a medium, the head having an actuator for changing a flying height of the head over the medium, a storage for storing characteristic information of areas of the medium and a controller for controlling the actuator on the basis of the characteristic information of the areas of the medium when writing data into or reading data from the areas of the medium.

Description:
BACKGROUND 
       [0001]    1. Field 
         [0002]    The present technique relates to a method for controlling a levitation value of a head with respect to a storage medium. 
         [0003]    2. Description of the Related Art 
         [0004]    Examples of the related art pertaining to the technique of controlling a levitation value of a head include Japanese Unexamined Patent Application Publication Nos. 05-20635 and 2006-24289. 
       SUMMARY 
       [0005]    According to an aspect of an embodiment, a magnetic device comprises a head for writing data into or reading data from a medium, the head having an actuator for changing a flying height of the head over the medium, a storage for storing characteristic information of areas of the medium and a controller for controlling the actuator on the basis of the characteristic information of the areas of the medium when writing data into or reading data from the areas of the medium. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0006]      FIG. 1  is a block diagram showing a basic structure of one example of a storage unit of embodiments; 
           [0007]      FIG. 2  is a diagram showing an RDC and a pre-amp IC together with an internal structure of a magnetic head; 
           [0008]      FIG. 3  is a section view of the magnetic head; 
           [0009]      FIG. 4  is a graph showing a relationship of heater electric current and heater electric power when a resistance value of a heater is 100Ω; 
           [0010]      FIG. 5  is a graph showing a relationship of the heater electric current and an expansion of the magnetic head; 
           [0011]      FIG. 6  is a graph showing a relationship between a levitation value of the magnetic head and a SN ratio; 
           [0012]      FIG. 7  is a graph showing a relationship of the SN ratio and an error rate; 
           [0013]      FIG. 8  is a chart showing dispersion of coercive force of a magnetic storage medium; 
           [0014]      FIG. 9  is an enlarge view of part of the magnetic head and the magnetic disk; 
           [0015]      FIG. 10  is a (first) flowchart of a process for preparing a table showing a relationship of each sector in a circumferential direction and the heater electric current; 
           [0016]      FIG. 11  is a (first) graph showing information of correspondence between each sector in the circumferential direction and the error rate; 
           [0017]      FIG. 12  is a (first) detailed flowchart of a process for calculating a required heater electric current in each sector in a circumferential direction; 
           [0018]      FIG. 13  is a (first) table showing the relationship between each sector in a circumferential direction and the heater electric current; 
           [0019]      FIG. 14  is a flowchart explaining operations of the exemplary embodiment; 
           [0020]      FIG. 15  is a (second) flowchart of a process for preparing a table showing a relationship of each sector in a circumferential direction and the heater electric current; 
           [0021]      FIG. 16  is a graph showing information of correspondence between each sector in a circumferential direction and an overwrite characteristics; 
           [0022]      FIG. 17  is a (second) detailed flowchart of a process for calculating a required heater electric current in each sector in the circumferential direction; 
           [0023]      FIG. 18  is a graph showing a relationship between a levitation value of the magnetic head and the overwrite characteristics; 
           [0024]      FIG. 19  is a (second) table showing the relationship between each sector in a circumferential direction and the heater electric current; 
           [0025]      FIG. 20  is a (second) graph showing information of correspondence between each sector in the circumferential direction and the error rate; and 
           [0026]      FIG. 21  is a (third) table showing the relationship between each sector in a circumferential direction and the heater electric current. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    A magnetic disk unit (HDD: Hard Disk Drive) is mounted in various products such as desktop personal computers, notebook type personal computers, servers, navigation apparatuses and AV (Audio Visual) machines. With a demand on increase of a storage capacity of the HDD, it has been required to increase recording density of a magnetic disk. It is necessary to narrow gaps between bits of the magnetic disk to increase signals that can be recorded in order to increase the recording density. 
         [0028]    When the bit density (BPI) of the magnetic disk is increased, however, it becomes necessary to decrease a flying height of the magnetic head so that the head approaches more to the magnetic disk to write or read information. 
         [0029]    When the levitation of the head is decreased so that the head approaches to the magnetic disk, dispersion of magnetic characteristic of the magnetic disk caused substantially in a circumferential direction affects more to performances for writing and reading information. 
         [0030]    Specifically, the dispersion of the magnetic characteristic occurs substantially in the circumferential direction by influence and distribution of thickness of a texture formed on a surface of a substrate of the magnetic disk substantially in the circumferential direction to give a magnetic anisotropy to a magnetic layer. 
         [0031]    Noticing on the problem caused in the magnetic disk, i.e., on the dispersion of the magnetic characteristic, the present exemplary embodiment improves the writing and reading performances by accurately controlling the levitation of the magnetic head and improves the storage capacity by increasing the density more. 
         [0032]    The exemplary embodiment will be explained below with reference to the drawings. 
       First Embodiment 
     Drawing of Hardware Structure of HDD: 
       [0033]      FIG. 1  is a block diagram briefly showing one exemplary hardware structure of the HDD of the present embodiment. As shown in  FIG. 1 , the HDD  100  is composed of a printed circuit assembly (PCA)  11  for controlling the entire HDD  100  and transmission and receiving of signals with a host unit (not shown) via a host interface and a disk enclosure (DE)  12 . 
         [0034]    The PCA  11  has a hard disk controller (HDC)  111 , a micro controller unit (MCU)  112 , a read channel (RDC)  113 , a random access memory (RAM)  114 , a read only memory (ROM)  115  and a servo combo chip (SVC)  116 . The HDC  111  makes controls such as interface protocol control, data buffer control, disk format control and the like. The MCU  112  controls the HDC  111 , the RDC  113  and the SVC  116  and manages memory within the HDD  100  such as the RAM  114  and the ROM  115  by carrying out arithmetic operations. The RDC  113  carries out coding and decoding that are processes for writing or reading data to/from a magnetic disk  125 , i.e., a storage medium. The HDC  111 , the MCU  112  and the RDC  113  compose a control section  110 . The RAM  114  stores various data including intermediate data of the arithmetic operation carried out by the MCU  112 . The ROM  115  stores programs and data executed by the MCU  112 . The SVC  116  makes control of driving current for a voice coil motor (VCM)  122  and a spindle motor (SPM)  124  within the DE  12  on the basis of instructions from the MCU  112 . 
         [0035]    The DE  12  includes a pre-amplifier IC  121 , the VCM  122 , an actuator  123 , the SPM  124 , a magnetic disk, a magnetic head  126  and temperature sensor nodes (TSNS)  127 . Although  FIG. 1  shows a case provided with two magnetic disks  125  and a pair of magnetic heads  126  for each of the magnetic disk  125 , the number of the magnetic disk  125  and the magnetic head  126  is not limited to the case of  FIG. 1 . The pre-amplifier IC  121  has a write driver  121 W for amplifying a write signal to supply to the magnetic head  126 , a read driver  121 R for amplifying a read signal from the magnetic head  126  and a heater driver  121 H for driving a heater (not shown) within the magnetic head  126  by a number N of channels corresponding to a number N of the magnetic head  126  and selectively switch their operation/non-operation. The heater is an actuator. The VCM  122  drives the actuator  123  supporting the magnetic head  126  substantially in a radial direction of the magnetic disk. The SPM  124  rotates the magnetic disk  125  by a predetermined number of revolutions. The magnetic head  126  has a write head for recording write signals to the corresponding magnetic disk  125 , a read head for reading read signals from the corresponding magnetic disk  125  and a heater. The TSBS  127  is a sensor for detecting temperature within the DE  12 , i.e., environmental temperature of the HDD  100 , and is a thermister for example. 
       Internal Structure of Magnetic Head: 
       [0036]      FIG. 2  is a diagram showing the RDC  113  and the pre-amp IC  121  together with an internal structure of the magnetic head  126 . As shown in  FIG. 2 , a heater control circuit  121 A is provided within the pre amplifier IC  121  and the read head  126 R, the write head  126 W and the heater  126 H composed of a coil are provided within the magnetic head  126 . The read signal read by the read head  126 R from the magnetic disk  125  is amplified by the read amplifier  121  and is supplied to the RDC  113 . The write head  126 W receives the write signal from the RDC  113  via the write driver  121 W and writes into the magnetic disk  125 . A calorific value of the heater  126 H is controlled by a heater control circuit  121 A via a pre amplifier IC  121 H. It is noted that there is a merit that the heater may be manufactured in a process of thin film magnetic head by constructing the heater  126 H by the coil. The coil also has a merit that it takes only a short time until when it generates heat after flowing an electric current and that its response characteristic is good. 
       Section View of Magnetic Head: 
       [0037]      FIG. 3  is a section view showing a main part of the magnetic head  126 . The  126 W shown in  FIG. 2  has a structure in which a coil  1263  is wound around an upper magnetic pole  1261  and a lower magnetic pole  1262  as shown in  FIG. 3  for example. When electric current is supplied to the coil  1263 , a magnetic field is generated in a write gap and the write signal is written to the magnetic disk  125 . The  126 R has a structure in which an upper shield-cum-electrode  1265  and a lower shield-cum-electrode  1266  are formed within an insulating layer using aluminum oxide Al 2 O 3  known as alumina and a read element  1267  is disposed at position of a read gap of a face opposing to a medium  1268 . The upper shield-cum-electrode  1265  and the lower shield-cum-electrode  1266  absorb magnetic flux other than those to be flown into the read element  1267 . A resistance value of the read element  1267  changes on the basis of the magnetic flux flowing thereto. The read head  126 H reads signals by utilizing the changes of the resistance value. The calorific value of the heater  126 H is controlled by a heater electric current supplied thereto and corresponding to the calorific value, each section of the magnetic head  126  including a magnetic disk resin section  1264  made of an insulating material such as a ceramic material around the heater thermally expands like an expansion  1264  indicated by a dotted line in  FIG. 3 . This thermal expansion occurs on a levitating face of the magnetic head  126 , i.e., in a direction facing to the magnetic disk  125 . A value of the thermally expanded portion is called as magnetic disk expansion value (projection value). Normally, the levitation of the magnetic head  126  is kept in F 1 . The thermal expansion corresponding to a heater electric power occurs as shown by the dotted line in  FIG. 3  by supplying the heater electric current and the expansion value PQ changes corresponding to the heater electric power. Accordingly, a spacing between an edge portion  1260  of the magnetic head  126  on the side of the medium and the medium decreases by the magnetic disk expansion value PQ and becomes F 2  as shown in  FIG. 3 . It is noted that the spacing will be described later by using FIG.  9 . 
       Graphs Showing Corresponding Relationship: 
       [0038]    Graphs showing various corresponding relationships will be explained below. Each figure is used in a process of preparing a corresponding relationship between each sector of the magnetic disk described later and divided into a predetermined number in a circumferential direction of a track of the magnetic disk and the heater electric current in each sector. 
         [0039]      FIG. 4  is a graph showing the corresponding relationship of the heater electric current and the heater electric power when a resistance value of the heater  126 H is 100Ω. 
         [0040]      FIG. 5  is a graph showing a relationship of the heater electric current and the magnetic disk expansion value. Points plotted by squares indicate the relationship between the heater electric power and the magnetic head expansion value when a read operation is carried out in the magnetic disk. Meanwhile, points plotted by triangles indicate the relationship between the heater electric power and the magnetic head expansion value when a write operation is carried out in the magnetic disk. A reason why the corresponding relationship of the write operation is different from that when the read operation is carried out is because electric current is supplied to the write coil when the write operation is carried out and because the magnetic disk expands by the both heats generated by the write coil and generated by the heat. 
         [0041]      FIG. 6  is a graph showing a relationship between a levitation value of the magnetic head and a SN ratio (signal to noise ratio) of the read signal read from the read head  126 R when the magnetic head levitation value changes. As it is apparent from  FIG. 6 , the higher the magnetic head levitation value, the lower the S/N ratio becomes and the lower the magnetic head levitation value, the higher the S/N ratio, improving the signal quality. 
         [0042]      FIG. 7  is a graph showing a relationship between the SN ratio and an error rate. The error rate is a rate of a number of times when test data is not correctly read with respect to a number of written times when the test data is read after writing the test data to the magnetic disk. As it is apparent from  FIG. 7 , the error rate decreases when the S/N ratio is high, i.e., the magnetic head levitation value decreases. As a result, the error rate of the read signal drops, improving the signal quality. The error rate increases when the S/N ratio drops, i.e., the magnetic head levitation value increases on the other hand. As a result, the error rate of the read signal increases, lowering the signal quality. 
         [0043]      FIG. 8  is a chart showing dispersion of coercive force of a magnetic storage medium. It shows values of equal coercive force by a pattern of contour lines. A dotted chain line or a dotted line shows distribution of values of equal coercive force. When the coercive force thus disperses, it affects the S/N ratio on the same circumference. The dispersion of the coercive force is also caused by dispersion of thickness in fabricating the magnetic storage medium. The magnetic storage medium is constructed by sequentially laminating a base film, a magnetic film (recording film), a protection film and a lubricant film on a substrate such as glass and aluminum. The dispersion of the thickness of the magnetic disk becomes significant when the levitation value of the magnetic head decreases and it largely depends on the thickness of the protection and lubricant films in particular. The thickness of the protection film is 4.0 nm for example and that of the lubricant film is 1.0 nm for example. The dispersion of the thickness of the lubricant and protection films changes a gap between the recording film of the magnetic storage medium and the magnetic head and affects the S/N ratio described above. When the thickness of the protection film fluctuates in a range of ±0.5 nm for example, the S/N ratio fluctuates in a range of ±0.3 dB. 
         [0044]      FIG. 9  is an enlarge view of part of the magnetic head  126  and the magnetic disk  125 . As shown in  FIG. 9 , the magnetic disk  125  has a structure in which the protection film  125   b  is laminated on a surface of the recording film  125   a  composed of a single layer or a multiple layer formed by overlapping on the base film on the substrate made of textured aluminum or the like and the lubricant film  125   c  is laminated on the surface of the protection film  125   b . Because the substrate is textured, boundaries between the respective layers are not also completely smooth and very small irregularities are seen also on the surface of the magnetic disk  125  as shown in  FIG. 9 . Here, while the distance F 2  between the edge portion  1260  and the surface of the magnetic disk  125  explained in  FIG. 3  has been defined as the spacing, a distance PQ′ between the edge portion  1260  and the recording film  125   a  will be defined as a magnetic spacing. As explained in  FIG. 8 , the texture of the substrate formed substantially in the circumferential direction is thought to be affecting the magnetic characteristics such as the coercive force why it disperses approximately in the circumferential direction. 
       Overall Flow of Process for Preparing Table: 
       [0045]    A process for preparing a table that correlates the sector of the track and the heater electric current will be explained below by using  FIG. 10 . The table is prepared for cases when internal temperature of the HDD is low (0° C.), normal (40° C.) and high (60° C.). It is because the characteristics of the magnetic head is influenced by an environment in which the HDD is used. It is noted that the table is prepared in unit of each magnetic head and the track of the storage medium corresponding to each magnetic head and each magnetic head. This process for preparing the table is carried out in a fabrication process for example. 
         [0046]    In Step S 001 , it is judged whether or not all of the magnetic heads have been measured. When all of the magnetic heads have not been measured, the process shifts to Step S 200 . 
         [0047]    In Step S 002 , a magnetic head to be measured is selected. The process then shifts to Step S 003 . 
         [0048]    In Step S 003 , it is judged whether or not the table has been prepared in all of the tracks on the magnetic disk  125 . When the table has not been prepared in all of the tracks, the process shifts to Step S 004 . 
         [0049]    In Step S 004 , a track to be measured is selected. The process then shifts to Step S 005 . 
         [0050]    In Step S 005 , it is judged whether or not the read check has been carried out on all of the sectors of the track selected in Step S 004 . When the read check has been carried out on all of the sectors, the process shifts to Step S 007 . When the read check has not been carried on all of the sectors on the other hand, the process shifts to Step S 006 . 
         [0051]    In Step S 006 , the read check is carried out. The read check is carried out by writing test data to the magnetic disk by the write head of the magnetic head  126  and by reading the written test data by the read head of the magnetic head  126 . This read check is carried out in each sector in each track. When the read check has been carried out on all of the sectors, the process shifts to Step S 007  or corresponding information of a sector and the error rate in a certain track is prepared.  FIG. 11  shows the corresponding information of the error rate and the sector in the certain track. The process then shifts to Step S 008 . 
         [0052]    In Step S 008 , a table indicating the sector and the heater electric power in the certain track of the certain magnetic head is prepared based on the corresponding information prepared in Step S 007 . The process in Step S 008  will be explained in detail by using  FIG. 12 . 
       First Detailed Flow of Process for Preparing Table: 
       [0053]    In Step SA 01 , a minimum value of the error rate is found from the corresponding information created in Step S 007 . It is noted that the levitation value of the magnetic head when the error rate is minimum is a reference levitation value. Then, the process shifts to Step SA 02 . 
         [0054]    In Step SA 02 , it is judged whether or not a differential value between the value of error rate and the minimum value of the error rate found in Step SA 01  has been calculated in all of the sectors. When the differential value has been calculated in all of the sectors, the process shifts to Step S 003  in  FIG. 10 . When the differential value has not been carried out for all of the sectors on the other hand, the process shifts to Step SA 03 . 
         [0055]    In Step SA 03 , a differential value between the value of error rate of a certain sector and the minimum value of the error rate found in Step SA 01  is calculated. It is noted that the calculation of the differential value may be carried out in order from a sector whose number is small. Then, the process shifts to Step SA 04 . 
         [0056]    In Step SA 04 , a required S/N ratio in the certain sector is found from the differential value of the error rate calculated in Step SA 01  and the corresponding relationship between the error rate and the S/N ratio explained by using  FIG. 7 . The minimum value of the error rate is “3.2” in  FIG. 11  for example. Assume now to calculate a difference with a sector whose error rate is “3.4”. The S/N ratios corresponding to error rates of “3.2” and “3.4” are “16.5” and “13.7”, respectively, in  FIG. 7 . Then, the process shifts to Step SA 06 . 
         [0057]    In Step SA 05 , a required levitation value is found from the required S/N ratio found in Step SA 04  and the corresponding relationship of the S/N ratio and the magnetic head levitation value explained by using  FIG. 6 . Then, the required S/N ratio is “2.8”. The levitation values corresponding to the S/N ratios of “16.5” and “13.7” are “7.2” and “9.2” respectively in  FIG. 6  and the levitation value of the magnetic head may be decreased by “2.0” further. Then, the process shifts to Step SA 06 . 
         [0058]    In Step SA 06 , a required heater electric power may be found from the difference of the required levitation value found in Step SA 05  and the corresponding relationship of the expansion value of the magnetic head and the heater electric power explained by using  FIG. 5 . Here, the expansion value of the magnetic head to be expanded is “2.0”, so that the heater electric power necessary for expanding the magnetic head by “2.0” may be found to be 30 mW from  FIG. 5 . It is noted that a required heater electric power in a case when the write current is supplied to the magnetic head is also found in Step SA 06  as explained in  FIG. 5 . The process then shifts to Step SA 07 . 
         [0059]    In Step SA 07 , a required heater electric current is found from the required heater electric power found in Step SA 06  and the corresponding relationship of the heater electric power and the heater electric current explained by using  FIG. 4 . Because the necessary heater electric power is 30 mW, the heater electric current is 0.65 mA. The process shifts to Step SA 08 . 
         [0060]    In Step SA 08 , a table correlating the heater electric current found in Step SA 07  and the sector is prepared.  FIG. 13  shows the table correlating the heater electric current and the sector. It is found from the table that in the sector  2  of the track  2 , electric current I ( 2 ,  2 ) may be supplied to the heater to set the power W ( 2 ,  2 ). The process then returns to Step SA 02 . Thus, the table correlating each sector in a certain track with the heater electric power in each sector may be prepared. It is noted that the table is prepared for the both cases of reading and writing as explained in Step SA 05 . When a table correlating a sector and an error rate is prepared for a certain track, the process returns to Step S 003  to prepare a table for the next track. 
         [0061]    When it is judged that the tables have been prepared in all of the tracks on the magnetic disk  125  in Step S 003 , the process returns to Step S 001  to carry out the process described above for the remaining magnetic head to prepare tables. 
         [0062]    The tables correlating the heater electric current and the sector are prepared for all of the tracks of the magnetic disk for each head of the magnetic disk unit as described above. Then, these tables are stored in the storage sections such as the ROM and the magnetic disk. 
       Overall Flow of Process for Controlling Levitation Value: 
       [0063]    A process for controlling the levitation value of the magnetic head to the magnetic disk based on the control values of the tables prepared in the abovementioned processes will be explained below by using  FIG. 14 . A sector is used as a unit of correcting the levitation value of the magnetic head in the present embodiment. 
         [0064]    In Step S 101 , the control section  110  judges whether or not there has been a request of write or read from a host unit via the host interface. It is noted that when there is a request from the host unit, the control section  110  stores the request in the RAM  114 . Where there is the request from the host unit, the process shifts to Step S 102 . 
         [0065]    In Step S 102 , the control section  110  judges whether the request from the host unit is a read request or a write request. Because the magnetic head expands when the request is a write request by supplying the current to the coil as described above, the table in writing is selected in Step S 106  described later. Then, the process shifts to Step S 103 . 
         [0066]    In Step S 103 , the control section  110  obtains temperature within the magnetic disk unit via the TSBS  127 . It is because the relationship between the heater electric power and the thermal expansion value differs depending on the temperature within the magnetic disk unit. The process shifts to Step S 104 . 
         [0067]    In Step S 104 , the control section  110  selects a magnetic head based on the request from the host unit. Then, the control section  110  passes information of the selected magnetic head to the SVC  116 . The SVC  116  controls the actuator  123  based on the received information of the magnetic head. The process shifts to Step S 105 . 
         [0068]    In Step S 105 , the control section  110  selects a track based on the request from the host unit. Then, the control section  110  passes information of the selected track to the SVC  116 . Then, the SVC  116  controls the actuator  123  and the SPM  124  based on the received information of the track. The process shifts to Step S 106 . 
         [0069]    In Step S 106 , the control section  110  selects a table from the RAM  114  based on the processes from Step S 102  through Step S 105 . The table is stored in the magnetic disk and the control section  110  reads it to the RAM  114  when the magnetic disk unit is activated. The process shifts to Step S 107 . 
         [0070]    In Step S 107 , the control section  110  judges whether or not the magnetic head has arrived to a sector before a certain number of sectors from a target sector. It takes time until when the magnetic head expands after supplying current to the heater. Therefore, the current is supplied to the heater when the magnetic head arrives at the sector before the certain number of sectors from the target sector to which the read or write operation should be carried out. The certain number of sectors is stored in the magnetic disk as a parameter and the control section  110  reads it to the RAM  114  when the magnetic disk unit is activated. It is noted that the control section  110  judges whether the magnetic head has arrived at the sector before the certain number of sectors from the target sector by obtaining information on position of the magnetic head from the SVC  116 . Then, the process shifts to Step S 108 . 
         [0071]    In Step S 108 , the control section  110  supplies the current to the heater  126 H of the magnetic head based on the table. Specifically, the control section  110  passes information of the current to be supplied to the heater control circuit  121 A at first. Then, based on the information, the heater control circuit  121 A supplies the current to the heater  126 H via the heater driver  121 H. The process then shifts to Step S 109 . 
         [0072]    In Step S 109 , the control section  110  judges whether or not the magnetic head has arrived at the target sector by comparing information related to the request from the host unit stored in the RAM  114  and the information on the position of the magnetic head obtained from the SVC  116 . When the magnetic head has arrived at the target sector, the process shifts to Step S 110 . 
         [0073]    In Step S 110 , the control section  110  executes the read or write operation based on the information on the request from the host unit stored in the RAM  114 . Then, the process ends. 
         [0074]    Thus, it is possible to expand the magnetic head in a sector whose error rate is high on the same track. Therefore, it is possible to lower the levitation value of the magnetic head and to improve the S/N ratio in the sector whose error rate is high. 
       Second Embodiment 
       [0075]    The levitation value has been controlled based on the error rate calculated by writing the test data to the magnetic disk by the write head of the magnetic head  126  and by reading the written data by the read head of the magnetic head  126  in the first embodiment. Therefore, the calculated error rate is what generally evaluates the write and read performances. A case of controlling the levitation value based on an overwrite characteristic that evaluates the write performance in writing will be explained in a second embodiment. 
       Second Overall Flow of Process for Preparing Table: 
       [0076]    A process for preparing a table that correlates the sector of the track and the heater electric current will be explained below by using  FIG. 15 . 
         [0077]    In Step S 201 , it is judged whether or not all of the magnetic heads have been measured. When all of the magnetic heads have not been measured, the process shifts to Step S 202 . 
         [0078]    In Step S 202 , a magnetic head to be measured is selected. The process then shifts to Step S 203 . 
         [0079]    In Step S 203 , it is judged whether or not the table has been prepared in all of the tracks on the magnetic disk  125 . When the table has not been prepared in all of the tracks, the process shifts to Step S 204 . 
         [0080]    In Step S 204 , a track to be measured is selected. The process then shifts to Step S 205 . 
         [0081]    In Step S 205 , it is judged whether or not the write check has been carried out on all of the sectors of the track selected in Step S 204 . When the write check has been carried out on all of the sectors the process shifts to Step S 207 . When the write check has not been carried on all of the sectors on the other hand, the process shifts to Step S 206 . 
         [0082]    In Step S 206 , the write check is carried out. The write check is carried out by writing data of certain frequency fa to the magnetic disk by the write head of the magnetic head  126  at first. Then, a level Vfa of the data of frequency fa is obtained by a harmonic sensor of the RDC  113  for example. Further, data of different frequency fb is written from the state in which the data of frequency of fa has been written. Next, a level Vfa′ of the data of frequency fa is measured. Finally, a rate of Vfa and Vfa′ is calculated as the overwrite characteristic. The overwrite characteristic correlates with the levitation value of the magnetic head.  FIG. 18  shows the corresponding relationship of the overwrite characteristic and the levitation value. This write check is carried out to each sector of each track. When the write check has been carried out to all of the sectors, the process shifts to Step S 207  to prepare corresponding information of a sensor in a certain track and the overwrite characteristic.  FIG. 16  shows the corresponding information of the overwrite characteristic and the sector in the certain track. The process then shifts to Step S 208 . The process in Step S 208  will be explained below in detail by using  FIG. 17 . 
       Second Detailed Flow of Process for Preparing Table: 
       [0083]    In Step SB 01 , a minimum value of the overwrite characteristic is found from the corresponding information created in Step S 207 . It is noted that the levitation value of the magnetic head when the overwrite characteristic is minimum is a reference levitation value. Then, the process shifts to Step SB 02 . 
         [0084]    In Step SB 02 , it is judged whether or not a differential value between the value of overwrite characteristic and the minimum value of the overwrite characteristic found in Step SB 01  has been calculated in all of the sectors. When the differential value has been calculated in all of the sectors the process shifts to Step S 203  in  FIG. 15 . When the differential value has not been carried out for all of the sectors on the other hand, the process shifts to Step SB 03 . 
         [0085]    In Step SB 03 , a differential value between the value of overwrite characteristic of a certain sector and the minimum value of the overwrite characteristic found in Step SB 01  is calculated. It is noted that the calculation of the differential value may be carried out in order from a sector whose number is small. Then, the process shifts to Step SB 04 . 
         [0086]    In Step SB 04 , a required levitation value in the certain sector is found from the differential value of the overwrite characteristic calculated in Step SB 01  and the corresponding relationship between the overwrite characteristic and the levitation value shown in  FIG. 18 . The minimum value of the overwrite characteristic is “−33” in  FIG. 16  for example. Assume now to calculate a difference with a sector whose overwrite characteristic is “−30”. The levitation values corresponding to overwrite characteristics of “−33” and “−30” are “8.0” and “12.0”, respectively, in  FIG. 18 . The difference of required levitation value is “4.0”. Then, the process shifts to Step SB 05 . 
         [0087]    In Step SB 05 , a required heater electric power may be found from the required levitation value found in Step SB 04  and the corresponding relationship of the expansion value of the magnetic head and the heater electric power explained by using  FIG. 5 . Here, the difference of the required levitation value is “4.0”, so that the expansion value of the magnetic head to be expanded is found to be “4.0”. Furthermore, the heater electric power necessary for expanding the magnetic head in writing is found to be 25 mW from  FIG. 5 . The process then shifts to Step SB 06 . 
         [0088]    In Step SB 06 , a required heater electric current is found from the required heater electric power found in Step SB 05  and the corresponding relationship of the heater electric power and the heater electric current explained by using  FIG. 4 . Because the necessary heater electric power is 25 mW, the heater electric current is 0.45 mA. The process shifts to Step SB 07 . 
         [0089]    In Step SB 07 , a table correlating the heater electric current found in Step SB 06  and the sector is prepared.  FIG. 19  shows the table correlating the heater electric current and the sector. It is found from the table that in the sector m of the track  1  for example, electric current to be supplied to the heat is IW ( 1 , n) and the power is WW ( 1 , n) The process then returns to Step SB 02 . Thus, the table correlating each sector in a certain track with the heater electric current in each sector may be prepared. When a table correlating a sector and the overwrite characteristic is prepared for a certain track, the process returns to Step S 203  to prepare a table for the next track. 
         [0090]    When it is judged that the write request has been made from the host unit in the Step S 106  explained in the first embodiment by using  FIG. 14 , the current to be supplied to the heat is controlled based on the table prepared based on the overwrite characteristic. It enables one to control the levitation value corresponding to the information-writing characteristic of the magnetic head to the magnetic disk. 
       Third Embodiment 
       [0091]    A case of controlling the levitation value based on the error rate evaluating the read performance in reading will be explained in a third embodiment. 
         [0092]    While the read check explained in Step S 006  in  FIG. 10  in the first embodiment is different in the third embodiment, the other processes are the same, so that their explanation will be omitted here. 
         [0093]    The read check of the present embodiment is carried out by reading a servo frame written in advance to the magnetic disk by the read head of the magnetic head. This read check is carried out to a sector of each track to which the servo frame has been written. When the read check has been carried out for all of the sectors to which the servo frame had been written, corresponding information of the sector of a certain track to which the servo frame has been written with the error rate is prepared.  FIG. 20  shows the corresponding information of the sector to which the servo frame has been written and the error rate. As shown in  FIG. 20 , the error rate is plotted per 20 sectors because the servo frame is formed per 20 sectors for example. Then, the corresponding information of the sector in a certain track and the error rate is prepared by linearly approximating the error rates of the sectors between the servo frames. The table based on the error rate calculated by reading the servo frames may be prepared by carrying out the processes explained in  FIG. 10  based on the corresponding information thus prepared.  FIG. 21  shows the table. It is apparent from  FIG. 21  that in a sector  1  in a track N−1, the current to be supplied to the heater is IR (N−1,  1 ) and the power is WR(N−1,  1 ). 
         [0094]    When it is judged that a read request has been made by the host unit in Step S 106  explained in  FIG. 14  in the first embodiment, the current to be supplied to the heater is controlled based on the table prepared based on the error rates calculated by reading the servo frames. It enables one to control the levitation value corresponding to the information reading characteristics of the magnetic head to the magnetic disk in reading. 
         [0095]    The embodiments described above do not limit other modes. Accordingly, they may be modified within a scope not changing the subject matters. For example, although the heater has been used as the levitation value control section in the present embodiment, a piezoelectric element may be used. Furthermore, although the table correlating the heater electric current and the sector for all of the tracks of the magnetic disk has been prepared in the embodiments, it is possible to prepare a table correlating the heater electric current and the sector for a certain zone that is an aggregate of tracks. Still more, it is possible to prepare a table correlating the heater electric current with an arbitrary number of sectors by preparing a table correlating the heater electric current and three consecutive sectors for example. 
         [0096]    According to the present embodiments, it is possible to improve the writing and reading performances and to increase the density more to improve the storage capacity by controlling the levitation value of the head accurately by considering the magnetic characteristics of the magnetic storage medium.