Abstract:
A magnetic reproducing apparatus has an MR head for each recording surface of a recording medium. The apparatus is capable of supplying a proper sense current to each MR head even if a control board of the apparatus is replaced, thereby improving the durability and reliability of the MR heads. Each MR head reads information from a recording surface of a recording medium in response to a sense current supplied to the MR head. The read information is decoded through a decoder. Whenever a power source of the apparatus is turned on, a resistance value of a magnetoresistive element of each MR head is measured. The measured resistance value is converted into a proper sense current value with the use of a conversion table. Based on the proper sense current value, a sense current is supplied to the MR head until the power source of the apparatus is turned off. As a result, the sense current supplied to the MR head is correct even if the control board is replaced.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a magnetic reproducing apparatus having MR (magnetoresistive) heads, and particularly, to a magnetic reproducing apparatus capable of supplying an optimum sense current to each MR head for reproducing information recorded on a magnetic recording medium. 
     2. Description of the Related Art 
     Magnetic recording-reproducing apparatuses such as magnetic disk units and magnetic tape units employ combined heads each consisting of a wound thin-film magnetic head (inductive head) for recording information on a magnetic recording medium and an MR head for reproducing the recorded information. 
     The MR head has a magnetoresistive element whose resistance changes in response to an external magnetic field, to detect a leakage magnetic field due to magnetization reversal on a recording medium where information is recorded. When using the magnetoresistive element, a sense current is supplied thereto. Compared with the inductive head, the MR head is capable of providing a relatively large output that is proportional to a flux quantity from a recording medium, independently of a relative speed between the MR head and the recording medium. The MR head, therefore, is suitable for reproducing information from high-density recording tracks. If an optimum sense current is supplied, the MR head provides a large reproduced signal irrespective of a relative speed between the MR head and a recording medium such as a disk. The MR head is widely used because it satisfies requirements for increasing the capacity of high-density magnetic disk units used as external storage units of computers. 
     A magnetic reproducing apparatus having MR heads generally consists of a casing and a control board. The casing accommodates magnetic disks, a driving system for the magnetic disks, an actuator for writing and reading information to and from the magnetic disks through combined heads, and a head driving IC. The control board is attached to the outside of the casing and usually has a servo system for positioning the heads on tracks of the magnetic disks, a read-write circuit for decoding information read from the magnetic disks and writing information to the magnetic disks, a memory such as a ROM for storing a control program, a RAM for temporarily storing data, and a control circuit for carrying out various control tasks. 
     When a power source is turned on, the apparatus reads sense current values to be supplied to the MR heads from the memory on the control board and supplies sense currents accordingly to the MR heads to read various pieces of data from the magnetic disks. The sense current values stored in the memory are optimum values found through tests carried out in advance. 
     The sense current values stored in the memory will not be optimum for the MR heads if the control board is replaced due to a repair. If sense current values stored in the replaced control board are extremely high for the MR heads, they will shorten the service lives of the MR heads and deteriorate the reliability thereof. 
     To cope with this problem, there is a technique not to use the sense current values stored in the replacement control board. Instead, the technique unconditionally supplies small sense currents to the MR heads to read surface analysis (SA) data from the magnetic disks. The SA data contains sense current values to be supplied to the MR heads. 
     The small currents to read the SA data, however, are not optimum for the MR heads, and therefore, raise a problem of incorrectly reading the SA data from the magnetic disks. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a magnetic reproducing apparatus capable of solving the problems of the prior art, always supplying optimum sense current to MR heads even if a control board is replaced, and improving the durability and reliability of the MR heads. 
     In order to accomplish the object, a first aspect of the present invention provides a magnetic reproducing apparatus having at least one recording medium for magnetically recording information, an MR head arranged for each recording surface of the recording media, a current supply circuit for supplying sense currents to the MR heads, respectively, so that each MR head may read information from the recording medium, and a decoder for decoding the read information. The magnetic reproducing apparatus consists of a resistance measuring circuit for measuring a resistance value of a magnetoresistive element of each MR head whenever a power source of the magnetic reproducing apparatus is turned on, a memory for storing a conversion table containing resistance values of magnetoresistive elements measured in advance and sense current values corresponding to the resistance values, and a sense current setter for reading, from the conversion table, sense current values corresponding to the resistance values of the MR heads measured by the resistance measuring circuit and setting the read sense current values in the current supply circuit, so that the current supply circuit keeps the set sense current values until the magnetic reproducing apparatus is turned off and supplies sense currents to the MR heads based on the set sense current values. 
     The first aspect is capable of setting proper sense currents for the MR heads according to the resistance values of the MR heads even if a control board attached to the magnetic reproducing apparatus is replaced. 
     In addition to the arrangement of the first aspect, a second aspect of the present invention employs a positioning mechanism for reading servo data from the recording media through the MR heads that receive the sense currents set by the sense current setter and positioning each MR head on a predetermined track on the recording medium according to the servo data a fetching circuit for reading surface analysis data from the predetermined track, fetching sense current data for each MR head from the surface analysis data, and storing the fetched sense current data, and a sense current resetter for resetting the sense current values in the current supply circuit to optimum ones for the MR heads based on the fetched sense current data. 
     The second aspect resets sense currents to be supplied to the MR heads according to surface analysis data that has been written in the recording media in advance, thereby ensuring the correctness of the sense currents to the MR heads even if the control board is replaced. 
     In addition to the arrangement of the second aspect, a third aspect of the present invention employs a comparator for comparing a casing number of the magnetic reproducing apparatus contained in the surface analysis data with a casing number of the magnetic reproducing apparatus stored in a nonvolatile memory arranged on the control board, and a prohibition circuit for prohibiting the sense current resetter from resetting the sense current values in the current supply circuit if a result of the comparison shows disagreement. 
     A fourth aspect of the present invention provides a magnetic reproducing apparatus having at least one recording medium for magnetically recording information, an MR head arranged for each recording surface of the recording media, a current supply circuit for supplying sense currents to the MR heads, respectively, so that each MR head may read information from the recording medium, and a decoder for decoding the read information. The magnetic reproducing apparatus consists of a nonvolatile memory arranged in a casing of the magnetic reproducing apparatus, for storing values of sense currents to be supplied to the MR heads, and a sense current setter for reading the sense current values from the nonvolatile memory when the magnetic reproducing apparatus is driven and setting the read values in the current supply circuit so that the current supply circuit supplies sense currents to the MR heads based on the set values. 
     The fourth aspect sets sense current values to be supplied to the MR heads according to sense current values stored in the nonvolatile memory arranged in the casing of the apparatus, thereby optimizing the sense currents to the MR heads even if a control board attached to the magnetic reproducing apparatus is replaced. 
     In addition to the arrangement of the fourth aspect, a fifth aspect of the present invention stores, in a nonvolatile memory, a program for reading surface analysis data from a predetermined track on a recording medium through an MR head. 
     In addition to the arrangement of the fifth aspect, a sixth aspect of the present invention employs a positioning mechanism for reading servo data from the recording media through the MR heads that receive the sense currents from the current supply circuit and positioning each MR head on a predetermined track on the recording medium according to the servo data, a fetching circuit for reading surface analysis data from the predetermined track, fetching sense current data for each MR head from the surface analysis data, and storing the fetched sense current data, and a sense current resetter for resetting the sense current values in the current supply circuit to optimum ones for the MR heads based on the fetched sense current data. 
     The sixth aspect resets sense currents to be supplied to the MR heads according to surface analysis data that has been written in the recording media in advance, thereby ensuring the correctness of the sense currents to the MR heads even if the control board is replaced. 
     A seventh aspect of the present invention is based on the second aspect and employs, as the sense current data for the MR heads contained in the surface analysis data, resistance values of the MR heads. According to the seventh aspect, the sense current resetter has a conversion table containing resistance values of MR heads and optimum sense current values corresponding to the resistance values, to convert the resistance values fetched from the surface analysis data into sense current values and reset the sense current values in the current supply circuit to the converted sense current values. 
     In addition to the arrangement of any one of the sixth and seventh aspects, an eighth aspect of the present invention employs a comparator for comparing a casing number of the magnetic reproducing apparatus contained in the surface analysis data with a casing number of the magnetic reproducing apparatus stored in a nonvolatile memory arranged on the control board, and a prohibition circuit for prohibiting the sense current resetter from resetting the sense current values in the current supply circuit if a result of the comparison shows disagreement. 
     In addition to the arrangement of the first aspect, a ninth aspect of the present invention employs a positioning mechanism for reading servo data from the recording media through the MR heads that receive the sense currents set by the sense current setter and positioning each MR head on a predetermined track on the recording medium according to the servo data, a nonvolatile memory arranged on the control board, for storing a conversion table containing MR head compositions and optimum sense current values corresponding to the compositions, a fetching circuit for reading surface analysis data from the predetermined track, fetching composition data for each MR head from the surface analysis data, and storing the fetched composition data, and a sense current resetter for reading optimum sense current values for the MR heads from the conversion table according to the fetched composition data and resetting the sense current values in the current supply circuit to the read optimum sense current values. 
     The ninth aspect sets an optimum sense current for each MR head according to the composition of the MR head. 
     The magnetic reproducing apparatus with MR heads of the present invention provides the following advantages: 
     (1) When configured to measure the resistance value of each MR head whenever turned on, read sense current values from a conversion table according to the measured resistance values, and set sense currents to the MR heads based on the read values, the apparatus is capable of supplying correct sense currents to the MR heads even if a control board attached to the apparatus is replaced. 
     (2) When configured to measure the resistance value of each MR head whenever turned on, read sense current values from a conversion table according to the measured resistance values, set sense currents to the MR heads based on the read values, fetch surface analysis data from recording media through the MR heads, and reset the sense currents to the MR heads according to sense current data contained in the surface analysis data, the apparatus is capable of supplying more correct sense currents to the MR heads even if the control board is replaced. 
     (3) When configured to employ a nonvolatile memory installed in the casing of the apparatus for storing sense current values for the MR heads, read the sense current values from the memory, and set sense currents to the MR heads based on the read values, the apparatus is capable of always supplying optimum sense currents to the MR heads even if the control board is replaced. 
     (4) When configured to employ a nonvolatile memory installed on the control board for storing a conversion table containing MR head compositions and sense currents corresponding to the compositions and read optimum sense currents from the table according to the compositions of the MR heads, the apparatus is capable of supplying sense currents that are optimum for the compositions of the MR heads. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more clearly understood from the description as set forth below with reference to the accompanying drawings, wherein: 
     FIG. 1 is a general plan view showing a magnetic reproducing apparatus according to the present invention; 
     FIG. 2A is a vertical section showing the apparatus of FIG. 1; 
     FIG. 2B is an enlarged section showing an MR head of the apparatus of FIG. 2A; 
     FIG. 3 is a circuit diagram showing an example of a control board of the apparatus of FIG. 2A; 
     FIG. 4 is a circuit diagram showing essential parts for determining sense currents to MR heads according to the present invention; and 
     FIGS. 5 to  11  are flowcharts showing sequences of setting sense current values for MR heads according to first to seventh embodiments of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments of the present invention will be explained in detail with reference to the drawings. 
     FIG. 1 shows a magnetic reproducing apparatus  1  according to the present invention. In this embodiment, the apparatus  1  is a magnetic disk unit having MR heads  2 . 
     The magnetic disk unit  1  has a casing  10  in which at least one magnetic disk  3  serving as a recording medium is fixed to a rotation shaft  4 . The shaft  4  is rotated by a spindle motor  5 . Each magnetic disk  3  has recording surfaces each of which faces an MR head  2  for reading information magnetically recorded on the magnetic disk  3 . The MR head  2  is attached to the tip of a carriage  6 . The carriage  6  is driven by a voice coil motor (VCM)  7 . Data read from the disk  3  through the MR head  2  is passed through a flexible cable  8  to a head IC arranged in the casing  10 . 
     FIG. 2A shows the apparatus of FIG. 1 having three magnetic disks  3 . The same numerals as those of FIG. 1 represent the same parts. The casing  10  has a bathtub shape covered with a top cover  11 . The bottom of the casing  10  is provided with a control board  9  having a circuit for positioning the MR heads  2 , a circuit for decoding data read through the MR heads  2 , etc. Circuits arranged inside the casing  10  and the control board  9  are connected to each other through flexible cables (not shown). 
     FIG. 2B is an enlarged view showing a typical structure of any one of the MR heads  2 . The MR heads  2  are arranged to face the corresponding magnetic disks  3 . Each MR head  2  consists of an MR film  21 , a soft magnetic film  22 , a nonmagnetic isolation film  23 , a conductive lead layer  24 , magnetic shields  25   a  and  25   b , and a nonmagnetic insulation layer  26 . The MR film  21  detects a flux change as a current change. The MR film  21  is made of ferromagnetic material NiFe. The soft magnetic film  22  is made of CoZr whose MR effect is small. The nonmagnetic isolation film  23  is, for example, a Ti film serving as a conductive intermediate film. The film  23  may be made of a conductive material or an insulating material. The three films  21  to  23  are laminated one upon another and are electrically joined together. The conductive lead layer  24  is made of Au, and the magnetic shields  25   a  and  25   b  are made of, for example, NiFe films. 
     The MR head  2  having such a structure is used by supplying a sense current to the conductive lead layer  24 . When the sense current is optimum, the MR head  2  provides a large reproduced signal irrespective of a relative speed between the MR head  2  and the disk  3 , and therefore, it is important to supply a proper sense current to each MR head  2 . 
     FIG. 3 shows an example of the control board  9  of FIG.  2 A. The control board  9  includes a drive controller  30  and a SCSI (small computer system interface) controller  40 . The drive controller  30  controls the driving of the spindle motor  5  for revolving the magnetic disks  3 , the VCM  7  for positioning the MR heads  2 , and the head IC  12 . The casing  10  may have a nonvolatile memory  13 . The drive controller  30  has a user logic  31 , a DSP (digital signal processor)  32 , a serve driver  33 , a servo demodulator  34 , a read-write circuit  35 , etc. The SCSI controller  40  has an MCU  41 , a flash memory  42 , a program memory  43 , a hard-disk controller  44 , a data buffer  45 , etc. The operations of these elements are known and, therefore, are not explained. Only the parts characteristic to the present invention to determine the sense currents to be supplied to the MR heads  2  will be explained. 
     FIG. 4 shows the characteristic parts of the present invention for determining sense currents to be supplied to the MR heads  2 . The rotation shaft  4  rotated by the spindle motor  5  holds the magnetic disks  3 . Each MR head  2  is arranged to face a corresponding recording surface of the magnetic disks  3 . Each MR head  2  is connected to an ON-OFF switch  14  in series. Pairs of the MR head  2  and switch  14  are connected in parallel with one another and are connected to a sense current supply circuit  15 . The circuit  15  receives a current from a power source  16  so that the circuit  15  may supply proper sense currents to the MR heads  2 , respectively. 
     A sense current setter  20  sets sense current values for the MR heads  2  in the sense current supply circuit  15 . The setter  20  has a controller  25 , a memory  26  for storing a conversion table containing resistance values of MR elements and sense currents corresponding to the resistance values, a main memory  27  that includes the memory  26 , and a resistance measuring unit  28 . A decoder  29  is related to the operation of the setter  20 . The switches  14  are directly connected to the MR heads  2 , respectively, and are turned on and off in response to signals from the controller  25 . A resistance measuring switch  17  and a decoding switch  18  are commonly connected to the MR heads  2 . When turned on, the switch  17  passes the outputs of the MR heads  2  to the resistance measuring unit  28 , and the switch  18  passes the outputs of the MR heads  2  to the decoder  29 . The switches  17  and  18  are turned on and off in response to the output of the controller  25 . 
     FIG. 5 is a flowchart showing a sequence of setting a proper sense current value for each MR head  2  in the sense current supply circuit  15  by the controller  25  according to the first embodiment of the present invention. The details of the flowchart will be explained with reference to FIG.  4 . 
     A power source switch  19  is turned on to activate the sense current setter  200 . The setter  20  turns on the switch  17  so that the output of each MR head  2  is transferred to the resistance measuring unit  28 . Then, the sequence of FIG. 5 is carried out. In the following explanation, an “i”th MR head  2  is expressed as an MR head #i. When the switch  19  is turned on, “i” is initialized to “0.” 
     Step  501  increments the MR head number #i by one. Since “i” is 0, it is incremented to 1. In step  502 , the controller  25  turns on the switch  14  of the MR head # 1  so that the sense current supply circuit  15  supplies a predetermined resistance measuring current to the MR head # 1 . In step  503 , the resistance measuring unit  28  measures a resistance value of the MR head # 1 . The measured value is transferred to the controller  25 . 
     In step  504 , the controller  25  refers to the resistance-sense current conversion table in the memory  26  and reads therefrom a sense current value corresponding to the measured resistance value. In step  505 , the controller  25  sets the read sense current value in the current supply circuit  15  as a sense current value for the MR head # 1 . 
     Step  506  checks to see if the MR head number #i is a maximum value #max. Namely, step  506  determines whether or not resistance values of all MR heads  2  have been measured. If #i is not #max, the flow returns to step  501 , which increments the MR head number #i by one and repeats steps up to step  506 . Repetitions of steps  501  to  506  are terminated if step  506  determines that resistance values of all MR heads  2  have been measured. Note that steps  504  and  505  can be carried out after the resistance values of all MR heads  2  have been measured, instead of being carried out between steps  503  and  506 . 
     In this way, the controller  25  sequentially turns on the switches  14  of the MR heads  2 , sequentially supplies the predetermined measuring current from the sense current supply circuit  15  to the MR heads  2 , measures the resistance of each MR head  2 , determines a sense current value for the MR head  2  according to the measured resistance value, and sets the determined sense current value in the supply circuit  15 . If sense current values for all MR heads  2  are set in the circuit  15 , the controller  25  turns off the switch  17  and turns on the decoding switch  18 . As a result, data detected by the MR heads  2  on the magnetic disks  3  is transferred to the decoder  29 , which decodes the data into a servo signal and data signal. These signals are transferred to the controller  25 . 
     FIG. 6 is a flowchart showing a sequence of setting a sense current value for each MR head  2  in the sense current supply circuit  15  according to the second embodiment of the present invention. When the power source switch  19  is turned on the controller  25  sequentially supplies a predetermined current to the MR heads  2 , measures a resistance value of each MR head  2 , retrieves a sense current value from the conversion table in the memory  26  according to the measured value, and sets the retrieved current value in the circuit  15 . These are carried out in steps  501  to  506  that are equal to those of the first embodiment of FIG. 5, and therefore, will not be explained again. 
     The second embodiment differs from the first embodiment in that it supplies sense currents to the MR heads  2  based on the sense current values set in the circuit  15  to position the MR heads  2  on the magnetic disks  3 , reads surface analysis data from the disks  3 , and resets the sense current values in the circuit  15  to optimum ones for the MR heads  2  according to the surface analysis data. 
     More precisely, after step  506  determines that sense current values for all MR heads  2  have been set according to measured resistance values, step  601  supplies a sense current to a “k”th MR head #k according to the sense current value set in step  505  for the MR head #k. Once step  506  provides “YES,” it is assumed that the controller  25  turns off the resistance measuring switch  17  and on the decoding switch  18 . 
     In step  602 , the MR head #k reads data from the magnetic disk  3 , the decoder  29  decodes the data into servo data, which is supplied to the controller  25 , and the controller  25  positions the MR head #k according to the servo data. When the MR head #k is positioned onto a predetermined track, the other MR heads are also positioned onto the same cylinder. Step  603  reads surface analysis data from the magnetic disk  3  through the MR head #k. The surface analysis data contains optimum current values for the MR heads  2 . 
     In step  604 , the controller  25  retrieves the sense current values for the MR heads  2  from the surface analysis data. In step  605 , the controller  25  resets the sense current values in the sense current supply circuit  15  to the retrieved sense current values. Thereafter, the circuit  15  supplies sense currents to the MR heads  2  according to the reset sense current values. 
     FIG. 7 is a flowchart showing a sequence of setting a sense current for each MR head  2  in the sense current supply circuit  15  according to the third embodiment of the present invention. The third embodiment is based on the second embodiment. The difference between them is that the third embodiment inserts steps  701  and  702  between steps  603  and  604  of the second embodiment. Explanation of steps the same as those of the second embodiment will be omitted. 
     According to the third embodiment, step  603  reads surface analysis data, step  701  detects a casing number of the apparatus  1  from the surface analysis data, and step  702  checks to see if the detected casing number agrees with a casing number of the apparatus  1  recorded in, for example, a nonvolatile memory arranged on the control board  9 . If they disagree with each other, steps  604  and  605  are carried out, as in the second embodiment, to fetch sense current data for each MR head from surface analysis data and reset the sense current values in the sense current supply circuit  15  to optimum ones according to the fetched sense current data. 
     If step  702  determines that the detected casing number agrees with the stored casing number, the routine ends. In this case, sense currents set in step  505  for the MR heads  2  are used as they are. 
     FIG. 8 is a flowchart showing a sequence of setting a sense current value for each MR head  2  in the sense current supply circuit  15  according to the fourth embodiment of the present invention. The fourth embodiment arranges a nonvolatile memory  13  in the casing  10  as shown in FIG.  3  and stores sense current values for the MR heads  2  in the memory  13 . When the power source switch  19  of FIG. 4 is turned on, the MR head number “i” is initialized to “0.” 
     Step  801  increments the MR head number #i by one. Since the initial value of the number “i” is 0, it is incremented to “1” at first. In step  802 , the controller  25  reads a sense.current value for the MR head # 1  from the memory  13 . In step  803 , the controller  25  sets the read sense current value in the sense current supply circuit  15  for the MR head # 1 . 
     Step  804  checks to see if the MR head number #i is a maximum number #max. Namely, step  804  determines if sense current values for all MR heads  2  have been read out of the memory  13 . If #i is not #max, the flow returns to step  801 , which increments the number #i by one and repeats steps up to step  804 . Repetitions of steps  801  to  804  are terminated when step  804  determines that sense currents for all MR heads  2  have been read out of the memory  13 . 
     In this way, the controller  25  sequentially reads sense current values for the MR heads  2  out of the memory  13  and sets the read sense current values in the sense current supply circuit  15 . When sense current values have been set for all MR heads  2  in the circuit  15 , the controller  25  turns on the decoding switch  18 . As a result, data detected on the magnetic disks  3  through the MR heads  2  is transferred to the decoder  29 , which decodes the data into a servo signal and data signal. These signals are transferred to the controller  25 . 
     FIG. 9 is a flowchart showing a sequence of setting a sense current value for each MR head  2  in the sense current supply circuit  15  according to the fifth embodiment of the present invention. The fifth embodiment reads sense current values for the MR heads  2  out of the memory  13  arranged in the casing  10  as in the fourth embodiment and carries out steps  601  to  605  as in the second embodiment. 
     More precisely, the fifth embodiment carries out steps  801  to  804  as in the fourth embodiment and, as in the second embodiment, positions an MR head #k on the magnetic disk  3  by using the sense current value set in the sense current supply circuit  15 , reads surface analysis data from the magnetic disk  3 , and resets the sense current values for the MR heads  2  in the supply circuit  15  to sense current values obtained from the is a combination of the second and fourth embodiments, and therefore, the detailed explanation of FIG. 9 is omitted because the procedure thereof has already been explained in FIGS. 6 and 8. 
     FIG. 10 is a flowchart showing a sequence of setting a sense current value for each MR head  2  in the sense current supply circuit  15  according to the sixth embodiment of the present invention. The sixth embodiment is based on the fifth embodiment and differs therefrom in that it inserts steps  701  and  702  of the third embodiment between steps  603  and  604  of the fifth embodiment. Namely, step  702  of the sixth embodiment checks to see if a casing number of the apparatus  1  detected from surface analysis data agrees with a casing number of the apparatus  1  stored in a nonvolatile memory arranged on the control board  9 , as in the third embodiment. If they agree with each other, the sixth embodiment carries out steps  604  and  605  as in the fifth embodiment. If they disagree with each other, the sixth embodiment does not carry out steps  604  and  605 . In FIG. 10, the same steps, as those of the third and fifth embodiments are represented by the same step numbers and their explanations are not repeated. 
     FIG. 11 is a flowchart showing a sequence of setting a sense current value for each MR head  2  in the sense current supply circuit  15  according to the seventh embodiment of the present invention. The seventh embodiment is based on the second embodiment and differs therefrom in that the seventh embodiment replaces steps  604  and  605  of the second embodiment with steps  1101  to  1103 . Steps  501  to  603  of the seventh embodiment are the same as those of the second embodiment, and therefore, are not explained again. 
     The seventh embodiment employs the nonvolatile memory  13  arranged in the casing  10  of FIG. 3 or the nonvolatile memory arranged on the control board  9 , to store a conversion table containing MR head compositions and optimum sense current values corresponding to the compositions. The magnetic disks contain surface analysis data that includes data related to the composition of each MR head  2 . 
     According to the seventh embodiment, step  603  reads the surface analysis data through an MR head #k. Step  1101  reads composition data for each MR head from the surface analysis data. In step  1102 , the controller  25  reads sense current values corresponding to the compositions of the MR heads  2  from the conversion table. In step  1103 , the controller  25  resets the sense current values for the MR heads  2  in the sense current supply circuit  15  to those read in step  1102 . 
     As a result, sense currents that are optimum for the compositions of the MR heads  2  are supplied to the MR heads  2 , to improve the reproduction efficiency of the MR heads  2 . 
     In any one of the embodiments, a control program for reading surface analysis data from a magnetic disk and retrieving sense current data for each MR head from the surface analysis data, a control program for retrieving a casing number from the surface analysis data, a control program for retrieving the composition of each MR head from the surface analysis data, etc., are stored in the program memory  43  of FIG. 3 or in any other memory. 
     Although the embodiments mentioned above relate to hard-disk units, the present invention is applicable to any other magnetic disk units that employ MR heads.