Abstract:
A magnetic head of the present invention includes a magnetic pole that faces a surface of a recording medium, moves relative to the surface in a direction along the surface, and produces a line of magnetic force intersecting the surface of the recording medium; and a coil that excites the magnetic pole, wherein the magnetic pole includes a laminate with a coercivity of 800 A/m or less, the laminate including two or more layers stacked in a direction along the movement relative to the surface of the recording medium, the two or more layers including a first magnetic layer located at a frontmost position of the movement, and a second magnetic layer located at a rearmost position of the movement, the second magnetic layer having a saturation magnetic flux density higher than a saturation magnetic flux density of the first magnetic layer.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a magnetic head that applies a magnetic field to a recording medium, and an information storage device that accesses a recording medium for information storage and retrieval using a magnetic field. 
         [0003]    2. Description of the Related Art 
         [0004]    With the development of the information society, the amount of information has been increasing. For responding to this information amount increase, the development of information recording systems and information storage devices with a dramatically-high recording density has been awaited. In particular, magnetic disks in which information is accessed via a magnetic field has gained attention as an information-rewritable, high-density recording medium, and has been actively studied and developed for further enhancing their recording densities. 
         [0005]    As a magnetic recording system for recording information on a magnetic disk, in-plane recording in which a recording medium is magnetized in a direction along its surface (in-plane direction) has been widely used, but in recent years, perpendicular recording in which a recording medium is magnetized in a direction perpendicular to its surface has been actively developed. The perpendicular recording provides advantages in that the recording density (line recording density) in the circumferential direction of the tracks can be enhanced and the failures of recorded information being destructed due to heat fluctuation can be reduced, and it is expected that this perpendicular recording will be widely employed instead of the conventional in-plane recording. 
         [0006]      FIG. 1  is a diagram for explaining the operation principle of perpendicular recording. 
         [0007]    A magnetic head  10 , which is shown in  FIG. 1 , includes thin-film coils  13  that generate a magnetic field according to information, a main magnetic pole  11  that produces a magnetic flux according to the magnetic field generated by the thin-film coils  13 , and an auxiliary magnetic pole  12  that picks up the magnetic flux produced by the main magnetic pole  11  and feeds it back to the thin-film coils  13  and the main magnetic pole  11 , and further includes a reproducing head  14  that detects a magnetic field by means of a reproducing element  14   a  to read information recorded on a magnetic disk  1 . 
         [0008]    Also, the magnetic disk  1  has a recording layer  1 A in which information is recorded, and a soft magnetic layer  1 B formed of a soft magnetic material, which are deposited on a substrate  1 C. Upon this magnetic disk  1  being rotationally driven in an arrow R direction, the magnetic head  10  relatively moves on the magnetic disk  1  in an arrow R′ direction, which is opposite to the arrow R direction. 
         [0009]    During information recording, an electric recording signal is input to the thin-film coils  13 , and thereby a magnetic field in a direction according to information is generated from the thin-film coils  13 . The generated magnetic field is supplied to the main magnetic pole  11 , and a magnetic flux according to the magnetic pole is generated from the main magnetic pole  11 . This magnetic flux is applied to the magnetic disk  1 , thereby passing through a soft magnetic layer  1 B of the magnetic disk  1 , and then the magnetic flux is diffused and returns to the auxiliary magnetic pole  12 , and is supplied to the thin-film coils  13  and the main magnetic pole  11 . The flow of the magnetic flux collected in a letter U-like magnetic path via the soft magnetic layer  1 B forms a recording magnetic field and the recording layer  1 A is magnetized in a direction perpendicular to its surface, thereby information being recorded. 
         [0010]    Known problems relating to the perpendicular recording magnetic head  10  shown in  FIG. 1  are, for example, pole erasure in which remanent magnetization remaining in the main magnetic pole  11  leaks and is thereby applied to the magnetic disk  1 , resulting in erasure of information previously recorded in the magnetic disk  1 , and side erasure in which a skew of the magnetic head destructs information recorded in an adjacent track. Since the magnetic head  10  relatively moves on the magnetic disk  1  in the arrow R′ direction, upon the occurrence of pole erasure or side erasure, a broad range of information recorded in the magnetic disk  1  may be erased, and even servo information, which indicates, for example, the positions on the magnetic disk  1 , may be erased, making it impossible to control the position of the magnetic head  10 . 
         [0011]    On this issue, a method in which the main magnetic pole of a magnetic head is made from, for example, an FeNi alloy, which exhibits a pole erasure preventing effect, is known. However, this FeNi alloy has a lower saturation magnetic flux density compared to, for example, an FeCo alloy, which has conventionally been used as a material for a main magnetic pole, causing a problem in that the recording density may be lowered. 
         [0012]    As techniques for preventing pole erasure and achieving a high recording density, Japanese Patent Application Publication No. 2004-281023 discloses a technique that employs a main magnetic pole formed of plural ferromagnetic materials and non-magnetic materials alternately deposited in a magnetic head movement direction R′, and Japanese Patent Application Publication No. 2003-242608 discloses a technique for forming a main magnetic pole having a surface facing a magnetic disk, the width of the surface becoming narrower toward the inflow side of the magnetic disk (the front of the magnetic head moving direction R′), and becoming wider toward the outflow side of the magnetic disk (the rear of the magnetic head movement direction R′). According to the technique disclosed in Japanese Patent Application Publication No. 2004-281023, two ferromagnetic layers formed of ferromagnetic materials faces via a non-magnetic layer formed of a non-magnetic material, and their magnetization directions are opposite to each other, thereby making it possible to reduce remanent magnetization, and according to the technique disclosed in Japanese Patent Application Publication No. 2003-242608, a magnetic flux can efficiently be concentrated at the tip of the main magnetic pole, thereby making it possible to enhance the recording density. Accordingly, when the techniques described in Japanese Patent Application Publication No. 2004-281023 and 2003-242608 are employed in combination, it can be considered possible to achieve both pole erasure prevention and a high recording density. 
         [0013]    However, the technique disclosed in Japanese Patent Application Publication No. 2004-281023 provides only a fairly limited number of combinations of ferromagnetic materials (e.g., FeCo) and non-magnetic materials (e.g., Ru) constituting a main magnetic pole. For example, when a main magnetic pole is formed of a combination of FeCo and Ru, plating, favorable in cost efficiency and mass productivity, cannot be used for a method for depositing these, and the method will substantially be limited to sputtering, causing a problem in that the manufacture costs will rise. Also, even when the techniques disclosed in Japanese Patent Application Publication Nos. 2004-281023 and 2003-242608, there is a problem in that side erasure cannot sufficiently be prevented. 
         [0014]    The present invention has been made in view of the above circumstances and provides a magnetic head and an information storage device capable of achieving both pole erasure and side erasure prevention, and a high recording density, while curbing a rise in manufacture costs. 
       SUMMARY OF THE INVENTION 
       [0015]    A basic feature of a magnetic head according to one aspect of the present invention includes: a magnetic pole that faces a surface of a recording medium, moves relative to the surface in a direction along the surface, and produces a line of magnetic force intersecting the surface of the recording medium; and a coil that excites the magnetic pole, wherein the magnetic pole includes a laminate with a coercivity of 800 A/m or less, the laminate including two or more layers stacked in a direction along the movement relative to the surface of the recording medium, the two or more layers including a first magnetic layer located at a frontmost position of the movement, and a second magnetic layer located at a rearmost position of the movement, the second magnetic layer having a saturation magnetic flux density higher than a saturation magnetic flux density of the first magnetic layer. 
         [0016]    It is known that pole erasure is highly correlated with the coercivity of a magnetic pole, and in order to prevent pole erasure, it is necessary to restrain the coercivity of the magnetic pole to around no more than 800 A/m. Meanwhile, in order to enhance the recording density of a magnetic head, it is necessary that a magnetic pole have a high saturation magnetic flux density. 
         [0017]    According to this basic feature of the magnetic head, the coercivity of the magnetic pole is 800 A/m or less, ensuring reliable prevention of pole erasure. Also, the frontmost part of the magnetic pole where side erasure easily occurs and there is only a small impact on the O/W (overwrite) performance is formed of the first magnetic layer with a low saturation magnetic flux density, and the rearmost part of the magnetic pole where there is a large impact on the O/W (overwrite) property is formed of the second magnetic layer with a high saturation magnetic flux density, so side erasure can effectively be prevented and the saturation magnetic flux density of the magnetic pole can be enhanced. 
         [0018]    Furthermore, an additional feature of the magnetic head, in which the first magnetic layer has a composition of Ni 100-x Fe x  (15≦x wt %), and the second magnetic layer has a composition of Fe x Co 100-x  (65≦x≦75 wt %), is preferable. 
         [0019]    It is known that vertical anisotropy occurs in a material having a composition range of Ni 100-x Fe x  (15&gt;x wt %). A magnetic layer having vertical anisotropy has a larger coercivity compared to a magnetic layer having in-plane anisotropy, and in addition, the coercivity may increase due to thermal stress during processing, causing a problem in that pole erasure easily occurs. According to this preferable embodiment, since the first magnetic layer having a composition of Ni 100-x Fe x  (15≦x wt %) has in-plane anisotropy, the coercivity can sufficiently be lowered, making it possible to prevent pole erasure. Also, since the second magnetic layer has a composition of Fe x Co 100-x  (65≦x≦75 wt %), the saturation magnetic flux density of the magnetic pole can be maintained to be sufficiently high (around 2.3 T or more). 
         [0020]    An additional feature of the aforementioned magnetic head, in which the first magnetic layer has a composition of Ni 100-x Fe x  (18≦x≦70 wt %), and the second magnetic layer has a composition of Fe x Co 100-x  (65≦x≦75 wt %), is further preferable. 
         [0021]    As a result of employing the first magnetic layer having a composition range of Ni 100-x Fe x  (18≦x≦70 wt %), the coercivity can efficiently be lowered and reliable pole erasure prevention can be ensured. 
         [0022]    Also, in an NiFe alloy, when the percentage of Fe is decreased, the saturation magnetic flux density value also decrease in line with that, and accordingly, if the first magnetic layer has a composition range of Ni 100-x Fe x  (18≦x≦70 wt %), the saturation magnetic flux density of the entire magnetic pole is also lowered, which may cause O/W performance deterioration. However, at this time, it has been understood that although the coercivity becomes lower as the percentage of the first magnetic layer relative to the magnetic pole is increased, when the percentage of the first magnetic layer exceeds a predetermined value (approximately 40%), the coercivity levels off. Accordingly, both pole erasure prevention and O/W performance enhancement can be achieved by restraining the percentage of the first magnetic layer having a composition range of Ni 100-x Fe x  (18≦x≦70 wt %), and enhancing the percentage of the second magnetic layer having a large impact on the saturation magnetic flux density of the entire magnetic pole. 
         [0023]    Furthermore, an additional feature of the aforementioned magnetic head, in which the first magnetic layer has a composition of Co x Ni y Fe z  (x+y+z=100, 0&lt;y≦10, 0&lt;x≦33 wt %), and the second magnetic layer has a composition of Fe x Co 100-x  (65≦x≦75 wt %), is preferable. 
         [0024]    As a result of the first magnetic layer having a composition of CO x Ni y Fe z  (x+y+z=100, 0&lt;y≦  10 ,  0 &lt;x≦33 wt %), the coercivity of the magnetic pole can be restrained to 800 A/m or less and pole erasure can reliably be prevented. 
         [0025]    An additional feature of the aforementioned magnetic head, in which the first magnetic layer has a composition of Fe x Co 100-x  (75≦x wt %), and the second magnetic layer has a composition of Fe x Co 100-x  (65≦x≦75 wt %), is preferable. 
         [0026]    As a result of the first magnetic layer having a composition of Fe x Co 100-x  (75≦x wt %), the magnetostriction can be restrained, and the coercivity can be lowered, thereby preventing pole erasure. 
         [0027]    Furthermore, an additional feature of the aforementioned magnetic head, in which the first magnetic layer has a composition of Co x Ni y Fe z  (x+y+z=100, 64≦x≦68, 15≦z≦20 wt %), and the second magnetic layer has a composition of Fe x Co 100-x  (65≦x≦75 wt %), is preferable. 
         [0028]    When the first magnetic layer has a composition of Co x Ni y Fe z  (x+y+z=100, 60≦x≦80, 10≦z≦20 wt %), pole erasure can also efficiently be prevented. 
         [0029]    Furthermore, an additional feature of the aforementioned magnetic head, in which the first magnetic layer and the second magnetic layer of the magnetic pole are formed by plating, is preferable. 
         [0030]    In the basic feature of the magnetic head, as materials for forming the first magnetic layer and the second magnetic layer, a combination of magnetic materials with saturation magnetic flux densities that are different from each other may be employed, and both of them may also be ferromagnetic materials. Accordingly, the range of material selection becomes wider and a combination of materials that can be deposited by plating, which is favorable in cost efficiency and mass productivity, can be employed. 
         [0031]    Furthermore, an additional feature of the magnetic head, in which the magnetic pole has a cross-sectional surface along the surface of the recording medium, the cross-sectional surface being narrow at a front part of the movement and being wide at a rear part of the movement, is preferable. 
         [0032]    As a result of employing the magnetic pole with a cross-sectional surface along the surface of the recording medium, the cross-sectional surface being narrowed at a front part of the movement and being widened at a rear part of the movement, the failure of the front side of the magnetic pole running over to an adjacent track upon occurrence of a skew angle during the driving of the head can be prevented, thereby preventing side erasure which erases recorded information. 
         [0033]    Furthermore, an additional feature of the magnetic head, in which the magnetic pole meets a relationship of S2/(S1+S2)&gt;0.35 where the area of a surface of the first magnetic layer facing the surface of the recording medium is S1, and the area of a surface of the second magnetic layer facing the surface of the recording medium is S2, is preferable. 
         [0034]    As a result of providing S2/(S1+S2)&gt;0.35, the write core width can be narrowed while the O/W performance being maintained, making it possible to reliably preventing pole erasure. 
         [0035]    Furthermore, an additional feature of the magnetic head, in which the magnetic pole has a saturation magnetic flux density greater than 2.1 T and a coercivity lower than 800 A/m for the entire layers included in the magnetic pole, is preferable. 
         [0036]    As a result of employing the magnetic pole having a saturation magnetic flux density greater than 2.1 T and a coercivity lower than 800 A/m, both pole erasure prevention and a high recording density can reliably be achieved. 
         [0037]    Furthermore, an additional feature of the magnetic head, in which the magnetic pole has an alloy film having a composition different from the compositions of the first magnetic layer and the second magnetic layer between the underlayer, and the first magnetic layer and the second magnetic layer, is preferable. 
         [0038]    Furthermore, an additional feature in which the magnetic pole has a non-magnetic underlayer, and the first magnetic layer and the second magnetic layer are formed on the underlayer by plating, is preferable, and an additional feature in which the underlayer of the magnetic pole contains at least one kind of element from among Ru, Pd, Pt, Rh, Au, Cu, NiP, NiMo and NiCr, is further preferable. 
         [0039]    In perpendicular head processing, when an unwanted plated underlayer portion is removed by means of ion milling after a magnetic pole is formed by plating, the removed underlayer portion may reattach to the magnetic pole side wall. When the plating underlayer is formed of a magnetic material, the re-adhesion layer has magnetism and thus it also functions as a portion of the magnetic pole, causing a problem in that the magnetic pole width increases and the cross-sectional surface shape deforms. As a result of forming the plating underlayer from a non-magnetic conductive material, the re-adhesion layer becomes non-magnetic, thereby the magnetic pole width increase and magnetic pole cross-sectional surface shape deformation due to the re-adhesion layer can be prevented. Also, when the underlayer is non-magnetic, the underlayer does not function as a part of the magnetic pole, and accordingly, the trouble of removing only an unwanted portion of the underlayer with high accuracy to conform to the magnetic pole width and shape, which become thinner toward to its tip, can be saved. 
         [0040]    Furthermore, an additional feature of the magnetic head, in which the underlayer of the magnetic pole consists of Ru, is preferable. 
         [0041]    The crystal structure of a magnetic film with a high percentage of Fe and a high saturation magnetic flux density is a body-centered cubic lattice, and the coercivity of a magnetic film with a body-centered cubic lattice can be lowered by controlling the magnetic film to have a (110) orientation. As a result of using Ru as an underlayer, a magnetic film with a body-centered cubic lattice can be controlled to have a (110) orientation, making it possible to lower the coercivity. Also, a magnetic film having a crystal structure of a face-centered cubic lattice tends to have a lowered coercivity irrespective of the underlayer, and even when using Ru as the underlayer, a low coercivity can also be obtained. 
         [0042]    When using Ru as the underlayer, a volatile oxide easily is formed on the surface and a failure easily occurs during plating, and accordingly, it is preferable to form a thin conductive film such as NiFe on the Ru. 
         [0043]    Furthermore, an additional feature of the magnetic head, in which the magnetic pole has a magnetic underlayer, and the first magnetic layer and the second magnetic layer are formed on the underlayer by plating is preferable, and an additional feature in which the underlayer of the magnetic pole contains at least one kind of element from among Ni, Fe and Co is further preferable. 
         [0044]    A basic feature of an information storage device according to another aspect of the present invention, which accesses a recording medium for information storage and retrieval using a magnetic field, includes: a magnetic pole that faces a surface of the recording medium and produces a line of magnetic force intersecting the surface of the recording medium; a coil that excites the magnetic pole; and a movement mechanism that moves the magnetic pole relative to the surface of the recording medium in a direction along the surface, wherein the magnetic pole includes a laminate with a coercivity of 800 A/m or less, the laminate including two or more layers stacked in a direction along the movement relative to the surface of the recording medium, the two or more layers including a first magnetic layer located at a frontmost position of the movement, and a second magnetic layer located at a rearmost position of the movement, the second magnetic layer having a saturation magnetic flux density higher than a saturation magnetic flux density of the first magnetic layer. 
         [0045]    According to the information storage device basic feature, it is possible to prevent pole erasure and record information with a high recording density. 
         [0046]    Also, an additional feature of the aforementioned information storage device, in which the first magnetic layer has a composition of Ni 100-x Fe x  (15≦x wt %), and the second magnetic layer has a composition of Fe x Co 100-x  (65≦x≦75 wt %), is preferable. 
         [0047]    According to this additional feature of the preferable information storage device, since the first magnetic layer has in-plane anisotropy, it is possible to lower the coercivity, preventing pole erasure. Also, as a result of the second magnetic layer having a composition of Fe x Co 100-x  (65≦x≦75 wt %), a high saturation magnetic flux density can be achieved and the O/W performance can be enhanced. 
         [0048]    Furthermore, an additional feature of the aforementioned information storage device, in which the first magnetic layer has a composition of Co x Ni y Fe z  (x+y+z=100, 0&lt;y≦10, 0&lt;x≦33 wt %), and the second magnetic layer has a composition of Fe x Co 100-x  (65≦x≦75 wt %), is preferable. 
         [0049]    According to this additional feature of the preferable information storage device, the coercivity of the magnetic pole can be restrained to 800 A/m or less, ensuring reliable pole erasure prevention. 
         [0050]    Furthermore, an additional feature of the aforementioned information storage device, in which the first magnetic layer has a composition of Fe x Co 100-x  (75≦x wt %), and the second magnetic layer has a composition of Fe x Co 100-x  (65≦x≦75 wt %), is preferable. 
         [0051]    According to this additional feature of preferable information storage device, it is possible to restrain magnetostriction in the first magnetic layer, lowering the coercivity. 
         [0052]    For the information storage device, only basic features have been mentioned here, and the information storage device includes not only the above features, but also various features corresponding to the respective features of the magnetic head mentioned above. 
         [0053]    As described above, according to the basic features of the magnetic head and the information storage device of the present invention, it is possible to achieve both pole erasure and side erasure prevention, and a high recording density, while curbing a rise in manufacture costs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0054]      FIG. 1  is a diagram for explaining the operation principle of perpendicular recording; 
           [0055]      FIG. 2  is an appearance diagram of a hard disk device; 
           [0056]      FIG. 3  is a functional block diagram of the hard disk device; 
           [0057]      FIG. 4  is a schematic configuration diagram of a magnetic head; 
           [0058]      FIG. 5  is a schematic diagram of a tip portion of a main magnetic pole; 
           [0059]      FIG. 6  is a diagram of the main magnetic pole viewed from the magnetic disk side; 
           [0060]      FIGS. 7(A) and 7(B)  are diagrams each illustrating the main magnetic pole having a first layer and a second layer formed on an underlayer; 
           [0061]      FIG. 8  is a graph indicating the relationship between main magnetic pole coercivity Hc [A/m] and occurrence or non-occurrence of pole erasure; 
           [0062]      FIG. 9  is a graph indicating the saturation magnetic flux densities and coercivities of various magnetic materials that have conventionally and widely been used as materials for main magnetic poles, and a laminated film of plural magnetic materials; 
           [0063]      FIG. 10  is a diagram indicating the relationship between the percentage of Fe and saturation magnetic flux density Bs in an FeCo alloy, which is used as a second magnetic layer; 
           [0064]      FIG. 11  is a diagram indicating the percentage of Fe and saturation magnetic flux density Bs in an NiFe alloy, which is used as a first magnetic layer; 
           [0065]      FIG. 12  is a diagram indicating the relationship between the percentage of Fe and coercivity Hc in an NiFe alloy; 
           [0066]      FIG. 13  is a diagram indicating the relationship between the percentage of Ni and coercivity Hc in an NiFe alloy; 
           [0067]      FIG. 14  is a diagram indicating the B-H curve of an NiFe alloy having no vertical anisotropy; 
           [0068]      FIG. 15  is a diagram indicating the B-H curve of an NiFe alloy having vertical anisotropy; 
           [0069]      FIG. 16  is a diagram indicating the relationship between the thickness of a first magnetic layer relative to that of the entire main magnetic pole, and coercivity Hc; 
           [0070]      FIG. 17  is a diagram indicating the relationship between the thickness of a first magnetic layer relative to that of the entire main magnetic pole, and saturation magnetic flux density Bs; 
           [0071]      FIG. 18  is a diagram indicating the relationship between the respective percentages of Co, Ni and Fe in a CoNiFe alloy, and saturation magnetic flux density Bs; 
           [0072]      FIG. 19  is a diagram indicating the relationship between the respective percentages of Co, Ni and Fe in a CoNiFe alloy, and coercivity Hc; 
           [0073]      FIG. 20  is a diagram indicating the relationship between the percentage of Fe and magnetostriction λ in an FeCo alloy; 
           [0074]      FIG. 21  is a diagram indicating the relationship between magnetostriction λ and coercivity Hc in an FeCo alloy; 
           [0075]      FIG. 22  is a diagram indicating the relationship between the percentage of Fe and the coercivity Hc in an FeCo alloy by underlayer; and 
           [0076]      FIG. 23  is a diagram indicating the relationship between O/W performance and write core width. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0077]    Hereinafter, a specific embodiment of the basic features and additional features described above, with reference to the drawings. 
         [0078]      FIG. 2  is an appearance diagram of a hard disk device  100 , which is a specific embodiment of the aforementioned information storage device. 
         [0079]    The hard disk device  100  shown in  FIG. 2  is used by being connected to or incorporated in a host apparatus typified by, for example, a personal computer. 
         [0080]    As shown in  FIG. 2 , a housing  101  in the hard disk device  100  includes a magnetic disk  1  in which information is recorded, a spindle motor  102  that rotates the magnetic disk  1  in an arrow R direction, a floating head slider  104  provided near a surface of the magnetic disk  1  and facing the surface of the magnetic disk  1 , an arm axis  105 , a carriage arm  106  having the floating head slider  104  fixed to an tip thereof, the carriage arm  106  moving above the magnetic disk  1  along the surface thereof about the arm axis  105 , a voice coil motor  107  that drives the carriage arm  106 , and a control circuit  108  that controls the operation of the hard disk device  100 . The combination of the spindle motor  102  and the voice coil motor  107  corresponds to an example of the movement mechanism in the basic feature of the information storage device described in “Summary of the Invention.” 
         [0081]    A tip of the floating head slider  104  is provided with a magnetic head  109 , which is a specific feature of the aforementioned magnetic head and applies a magnetic field to the magnetic disk  1 , and the hard disk device  100  records information on the magnetic disk  1  or reads information recorded on the magnetic disk  1 , using this magnetic field. In a normal situation, the hard disk device  100  includes plural magnetic disks  1 , and is provided with the magnetic head  109  for each of the plural magnetic disks. However, for ease of description, the description of the present embodiment is given focusing on one magnetic disk  1  and one magnetic head  109  provided for the magnetic disk  1 . 
         [0082]      FIG. 3  is a functional block diagram of the hard disk device  100 , and  FIG. 4  is a schematic configuration diagram of the magnetic head  109 . 
         [0083]    As shown in  FIG. 3 , the hard disk device  100  includes the spindle motor  102 , the voice coil motor  107 , the control circuit  108  and the magnetic head  109 , etc, which are shown also in  FIG. 2 . The control circuit  108  includes a hard disk control section  111  that controls the entire hard disk device  100 , a servo control section  112  that controls the spindle motor  102  and the voice coil motor  107 , a voice coil motor drive section  113  that drives the voice coil motor  107 , a spindle motor drive section  114  that drives the spindle motor  102 , a formatter  115  that formats the magnetic disk  1 , a read/write channel  116  that generates write current that carries write information to be written in the magnetic disk  1 , and converts reproduction signals obtained by reading via the magnetic head  109  information recorded in the magnetic disk  1 , into digital data, a buffer  117  which is used as a cache in the hard disk control section  111 , and a RAM  118  which is used as a work area in the hard disk control section  111 . 
         [0084]      FIG. 4  shows a partial cross-sectional structure of the magnetic head  109 . This magnetic head  109  appears to move in an arrow R′ direction, which is opposite to the direction of the magnetic disk  1  rotation as a result of the magnetic disk  1  being rotated in the arrow R direction in a state in which the magnetic head  109  is positioned on the magnetic disk  1 . 
         [0085]    The magnetic head  109  is provided with a main magnetic pole  210  that produces a magnetic flux, coils  250  that generate a magnetic field, an auxiliary magnetic pole  230  that picks up the magnetic flux produced by the main magnetic pole  210  and feeds it back to the coils  250  and the main magnetic pole  210 , and a reproducing head  240  that reads information recorded in the magnetic disk  1  in this order from the rear side of the movement direction R′, and it also includes a yoke  220  connecting the main magnetic pole  210  and the auxiliary magnetic pole  230 . The main magnetic pole  210  corresponds to an example of the magnetic pole in the aforementioned the information storage device and the basic features of the magnetic head, and a coil  250  corresponds to an example of the coil in the aforementioned information storage device and basic features of the magnetic head. 
         [0086]    Also, the magnetic disk  1  has a recording layer  1 A in which information is recorded, and a soft magnetic layer  1 B formed of a soft magnetic material deposited on a substrate  1 C. The magnetic disk  1  corresponds to an example of the recording medium in the aforementioned information storage device and basic features of the magnetic head. 
         [0087]    Hereinafter, a method for accessing the magnetic disk  1  will be described using  FIGS. 3 and 4 . 
         [0088]    When writing information on the magnetic disk  1 , write information to be recorded in the magnetic disk  1  and a logical address for the write position is sent from the host apparatus  200 , which is shown in  FIG. 3 , to the hard disk device  100 . The hard disk control section  111  converts the logical address to a physical address and conveys the physical address to the servo control section  112 . 
         [0089]    The servo control section  112  conveys an instruction to rotate the spindle motor  102 , to the spindle motor drive section  114 , and conveys an instruction to move the carriage arm  106  (see  FIG. 2 ), to the voice coil motor drive section  113 . The spindle motor drive section  114  drives the spindle motor  102  to rotate the magnetic disk  1 , and the voice coil motor drive section  113  drives the voice coil motor  107  to move the carriage arm  106 . As a result, the magnetic head  109  is positioned on the magnetic disk  1 . 
         [0090]    When the magnetic head  109  is positioned, the hard disk control section  111  conveys write signals to the read/write channel  116 , and the read/write channel  116  applies write current carrying write information to the magnetic head  109 . 
         [0091]    In the magnetic head  109 , the write signals are input to the coils  250 , which are shown in  FIG. 4 , and the coils  250  generate a magnetic field having a direction according to the write signals. In the main magnetic pole  210 , a magnetic flux according to the magnetic field that has been generated by the coils  250 , is released toward the magnetic disk  1 , and as a result, magnetization having a direction according to the information is formed on the recording layer  1 A of the magnetic disk  1 , thereby the information being recorded on the magnetic disk  1 . The magnetic flux that has formed the magnetization on the recording layer  1 A is collected by the auxiliary magnetic pole  230  via the soft magnetic layer  1 B, and fed back to the main magnetic pole  210  via the yoke  220 . 
         [0092]    Also, when reading information recorded in the magnetic disk  1 , a logical address for the recording position where the information is recorded is sent from the host apparatus  200  shown in  FIG. 3  to the hard disk device  100 . Subsequently, as in information writing, in the hard disk control section  111 , the logical address is converted to a physical address, the spindle motor  102  is rotationally driven to rotate the magnetic disk  1 , and the voice coil motor  107  is driven to move the carriage arm  106 , thereby the magnetic head  109  being positioned on the magnetic disk  1 . 
         [0093]    A reproducing element  240   a  that provides a resistance value according to the magnetic field generated from magnetization is incorporated in the magnetic head  109  shown in  FIG. 4 , and as a result of making current flow in the reproducing element  240   a , reproduction signals are generated according to the magnetization status. In the present embodiment, the specific type of the reproducing element  240   a  is not specifically limited, but for this reproducing element  240   a , for example, a GMR (giant magnetoresistance) element or a TMR (tunnel magnetoresistance) element can be employed. 
         [0094]    The reproduction signals, after being converted to digital data in the read/write channel  116  shown in  FIG. 3 , are sent to the host apparatus  200  via the hard disk control section  111 . 
         [0095]    Basically, the magnetic disk  1  is accessed for information storage and retrieval in such a manner as described above. 
         [0096]    Next, a further detailed description will be given of the magnetic head  109 . 
         [0097]      FIG. 5  is a schematic diagram of a tip portion of the main magnetic pole  210 , and  FIG. 6  is a diagram of the main magnetic pole  210  viewed from the magnetic disk  1  side. 
         [0098]    As shown in  FIG. 5 , the main magnetic pole  210  is formed so that the width of its surface facing the magnetic disk  1  becomes narrower toward the front of the magnetic disk  1  movement direction R′ and becomes wider toward the rear of the movement direction R′. As a result of the main magnetic pole  210  having a shape tapered from the rear to the front side of the movement direction R′, a failure that the front side of the main magnetic pole  210  runs over to an adjacent track when a skew angle occurs during the driving of the head can be prevented, making it possible to prevent side erasure, which erase recorded information. 
         [0099]    Also, as shown in  FIG. 6 , the main magnetic pole  210  has first layers  211 A with a relatively small saturation magnetic flux density (e.g., FeNi: saturation magnetic flux density Bs is 2.1 [T]), and second layers  211 B with a relatively large saturation magnetic flux density (e.g., FeCo: saturation magnetic flux density Bs is not less than 2.3 [T]) alternately deposited in four layers in total along the magnetic disk  1  movement direction R′, and as a result, the saturation magnetic flux density Bs of the entire main magnetic pole  210  is not less than 2.1 [T], and the coercivity Hc is restrained to 800 [A/m] or less. The first layer  211 A corresponds to an example of the first magnetic layer in the aforementioned information storage device and basic features of the magnetic head, and the second layer  211 B corresponds to an example of the second magnetic layer in the aforementioned information storage device and basic features of the magnetic head. In the present embodiment, since the coercivity Hc of the main magnetic pole  210  is 800 [A/m] or less, it is possible to reliably prevent pole erasure. Also since the first layers  211 A and the second layers  211 B are alternately deposited so that the first layer  211 A with a small saturation magnetic flux density Bs is disposed at the front of the movement direction R′ where side erasure easily occurs, and the second magnetic layer  211 B with a large saturation magnetic flux density Bs is disposed at the rear side of the movement direction R′ where side erasure is difficult to occur, it is possible to efficiently prevent side erasure and enhance the O/W performance. 
         [0100]    Also, for the materials for forming the first layers  211 A and the second layers  211 B, a combination of magnetic materials with saturation magnetic flux densities different from each other can be used, and a combination of materials that can be deposited by plating, which is favorable in cost efficiency and mass productivity, can also be used. In the present embodiment, a thin conductive film (e.g., NiFe) is formed on an underlayer of Ru, which is a non-magnetic material, and the first layers  211 A and the second layers  211 B are further formed thereon by plating. 
         [0101]    As a result of using Ru as the underlayer, a magnetic layer with a body-centered cubic lattice in which the percentage of Fe is high and the saturation magnetic flux density is also high can be controlled to have a (110) orientation, making it possible to reduce the coercivity Hc. Also, as a result of forming a thin conductive film on Ru, a failure in plating can be reduced. 
         [0102]      FIGS. 7(A) and 7(B)  are diagrams each illustrating a main magnetic pole with a first layer and a second layer formed on an underlayer. 
         [0103]    As a shown in  FIG. 7(A) , when an unwanted plating underlayer portion is removed by means of ion milling after a main magnetic pole  210 ′ is formed by plating, the removed portion of the underlayer  211 C may reattach to the main magnetic pole  210 ′. When the underlayer  211 C is formed of a magnetic material, this underlayer  211 C portion also functions as a portion of the main magnetic pole  210 ′, and accordingly, as shown in  FIG. 7(B) , it is necessary to cut the underlayer  211 C so as to become thinner toward its tip. In the present embodiment, since the underlayer  211 C is formed of a non-magnetic material, the trouble of cutting the underlayer  211 C with high accuracy to conform to the width of the main magnetic pole can be saved. Also, increased width of a magnetic pole and deformation of a magnetic pole cross-sectional surface due to the reattaching layer can be prevented. Also, it is preferable to use a material containing at least one kind of element from among Ru, Pd, Pt, Rh, Au, Cu, NiP, NiMo and NiCr for the non-magnetic underlayer  211 C. Also, for a magnetic underlayer, a material containing at least one kind of element from among Ni, Fe and Co can be employed. In particular, when NiFe is used for the underlayer, as in the Ru underlayer, a magnetic layer with a body-centered cubic lattice where the percentage of Fe is high and the saturation magnetic flux density is also high can be controlled to have a (110) orientation, making it possible to reduce the coercivity Hc. 
         [0104]    As described above, according to the present embodiment, it is possible to achieve both pole erasure and side erasure prevention and a high recording density, while curbing a rise in manufacture costs. 
         [0105]    Although the above description has been given for an example of a main magnetic pole with first layers and second layers alternately deposited in four layers in total, the magnetic pole in the magnetic head and information storage device described in the “SUMMARY OF THE INVENTION” may have a first magnetic layer and a second magnetic layer deposited in two layers. It may also have first magnetic layers and second magnetic layers deposited in four layers or more, or may also have a third layer, which is different from the first magnetic layers and the second magnetic layers. This third layer may be a non-magnetic material if it is formed of a material having conductivity. If this third layer is a magnetic material, it is preferable that its coercivity is as low as possible from the viewpoint of pole erasure and side erasure prevention. 
         [0106]    Also, when a first magnetic layer and a second magnetic layers are deposited, the saturation magnetic flux density Bs of the entire magnetic head is a sum of the saturation magnetic flux densities of the individual layers, but the coercivity Hc of the entire magnetic head cannot be determined simply from the coercivities of the individual layers because the coercivities vary depending on their crystallinity, etc. Accordingly, it is preferable that the saturation magnetic flux density of the entire magnetic head is enhanced by making the layer thickness of the second magnetic layer with a high saturation magnetic flux density to be as thick as possible (S2/(S1+S2)&gt;0.35 where the area of the surface of the first magnetic layer facing a surface of the recording medium is S1, and the area of the surface of the second magnetic layer facing a surface of the recording medium is S2). 
         [0107]    Also, for the second magnetic layer in the magnetic head and information storage device basic features, FeCo (60&lt;Fe&lt;80 wt %) or FeCoNi (55&lt;Fe&lt;80 at %, 20&lt;Co&lt;45 wt % and 0&lt;Ni&lt;20 wt %), etc., can be employed, and for the first magnetic layer in the magnetic head and information storage device basic features, it is preferable to use an FeNi alloy (Fe&gt;75 wt %) or an FeCo alloy (Fe&gt;75 wt %), a CoNiFe alloy (60≦Co≦80, 10≦Fe≦20 wt %), FeCoNi (55&lt;Fe&lt;80 at %, 20&lt;Co&lt;45 wt % and 0&lt;Ni&lt;20 wt %) etc. Furthermore, if a third layer is deposited between the first magnetic layer and the second magnetic layer, for the third layer, a permalloy, 50%-nickel permalloy, NiP, NiFeMo, NiMo, Ru, Pd, Pt, Rh or Cu, etc., can be used. 
       EXAMPLES 
       [0108]    Hereinafter, examples of the present invention will be described. 
         [0109]    First, a first example will be described. 
         [0110]      FIG. 8  is a graph indicating the relationship between main magnetic pole coercivity Hc [A/m] and occurrence or non-occurrence of pole erasure. 
         [0111]    In  FIG. 8 , for each of (1) a first main magnetic pole formed of an NiFe alloy alone, (2) a second main magnetic pole formed of an FeNi alloy and an FeCo alloy, (3) a third main magnetic pole formed of an FeNi alloy and an FeCo alloy, (4) a fourth main magnetic pole formed of an FeCo alloy alone, (5) a fifth main magnetic pole formed of an FeNi alloy and an FeCo alloy, and (6) a sixth main magnetic pole formed of an FeCo alloy alone, the coercivity Hc [A/m] in the hard axis direction is shown with a white bar, and the coercivity Hc [A/m] in the easy axis direction is shown with a black bar, and the result of confirmation of pole erasure occurrence is also shown. 
         [0112]    As shown in  FIG. 8 , pole erasure occurs only in the sixth main magnetic pole having an easy axis direction coercivity Hc in the axis direction greater than 800 [A/m]. Accordingly, it can be understood that pole erasure can be prevented by adjusting the coercivity Hc of the entire main magnetic pole to be 800 [A/m] or less. 
         [0113]      FIG. 9  is a graph indicating the saturation magnetic flux densities and coercivities of various magnetic materials that have conventionally and widely been used as materials for main magnetic poles, and a lamianated film of plural magnetic materials. 
         [0114]    In  FIG. 9 , the abscissa axis corresponds to saturation magnetic flux density Bs [T], and the ordinate axis corresponds to coercivity Hc [A/m], and CoNiFe-based magnetic materials are plotted with triangles, NiFe-based materials are plotted with squares, FeCo-based materials are plotted with diamonds, laminated films of FeNi and FeCo, and laminated films of CoNiFe and FeCo are plotted with circles. 
         [0115]    As described above, in order to prevent pole erasure, it is necessary that the coercivity Hc of the main magnetic pole be no more than 800 [A/m], and furthermore, in order to achieve a high recording density, it is required that the saturation magnetic flux density Bs of the main magnetic pole is not less than 2.1 [T]. As shown in  FIG. 9 , each of the NiFe-based materials (materials plotted with squares) has a coercivity Hc of 800 [A/m] or less, but has a small saturation magnetic flux density Bs. Each of the CoNiFe-based materials (materials plotted with triangles) has an overly high coercivity Hc or an overly small saturation magnetic flux density Bs, so none of them meets both conditions. Only one of the FeCo-based materials (materials plotted with diamonds) meets both conditions, but the others have a problem in that their coercivities Hc are overly high. As described above, there are only a few materials that can reliably achieve both a high recording density and pole erasure prevention with one layer alone. Meanwhile, laminated films of FeNi and FeCo and laminated films of CoNiFe and FeCo (materials plotted with circles) meet both coercivity Hc and saturation magnetic flux density Bs conditions. Accordingly, it can be understood that the coercivity Hc of the entire main magnetic pole can be restrained while a high saturation magnetic flux density Bs being maintained, by forming the main magnetic pole of plural layers. 
         [0116]    Next, a second example will be described. 
         [0117]      FIG. 10  is a diagram indicating the relationship between the percentage of Fe and saturation magnetic flux density Bs in an FeCo alloy, which is used as the second magnetic layer. 
         [0118]    As shown in  FIG. 10 , when the percentage of Fe in an FeCo alloy increases, the saturation magnetic flux density Bs also become higher, and furthermore, when the percentage of Fe exceeds 75%, the saturation magnetic flux density Bs is gradually lowered. In order to provide a saturation magnetic flux density Bs of not less than 2.1 [T] in the entire main magnetic pole and to enhance O/W, it is sufficient if the FeCo alloy, which is the second magnetic layer, has a saturation magnetic flux density Bs exceeding 2.1 [T], but in order to provide a sharp magnetic field gradient and also improve the recording performance, it is desirable that the saturation magnetic flux density Bs is as high as possible, and a saturation magnetic flux density Bs of around 2.3 [T] is required. This condition is met if the percentage of Fe is from 65% to 75%, and the effectiveness of the present invention can thereby be proven. 
         [0119]      FIG. 11  is a diagram indicating the percentage of Fe and saturation magnetic flux density Bs in an NiFe alloy, which is used as the first magnetic layer. 
         [0120]    As shown in  FIG. 11 , when the percentage of Fe in an NiFe alloy increases, the saturation magnetic flux density Bs is also enhanced. However, since the first magnetic layer has only a small impact on the O/W performance, a low coercivity Hc is required rather than a high saturation magnetic flux density Bs. 
         [0121]      FIG. 12  is a diagram indicating the relationship between the percentage of Fe and coercivity Hc in an NiFe alloy, and  FIG. 13  is a diagram indicating the relationship between the percentage of Ni and coercivity Hc in an NiFe alloy. The percentage of Ni is referred to as the percentage of Ni around the Ni-content region corresponding to the low-Fe-content region in  FIG. 12  where Hc sharply deteriorates. The coercivities in the easy axis direction are plotted in black and the coercivities in the hard axis direction are plotted in white. 
         [0122]    As shown in  FIGS. 12 and 13 , in an NiFe alloy, from around the point where the percentage of Fe is less than 15% and the percentage of Ni exceeds 85%, the coercivity Hc sharply increase. This can be considered to be resulted from vertical anisotropy occurring in an NiFe alloy with the percentage of Fe percentage being less than 15% (that is, the percentage of Ni is more than 85%). 
         [0123]      FIG. 14  is a diagram indicating the B-H curve of an NiFe alloy having no vertical anisotropy, and  FIG. 15  is a diagram indicating the B-H curve of an NiFe alloy having vertical anisotropy. 
         [0124]      FIG. 14  shows the B-H curve of an NiFe alloy in which the percentage of Ni is 79.27%, and  FIG. 15  shows the B-H curve of an NiFe alloy in which the percentage of Ni is 88.6%. As shown in  FIG. 14 , in the NiFe alloy in which the percentage of Ni is 79.27%, no vertical anisotropy occurs and the coercivity Hc is restrained. However, as shown in  FIG. 15 , in the NiFe alloy in which the percentage of Ni is 88.6%, vertical anisotropy occurs and the coercivity Hc deteriorates. Accordingly, it is preferable that the percentage of Fe in an NiFe alloy is not less than 15%. 
         [0125]    Next, a third example will be described. 
         [0126]      FIG. 16  is a diagram indicating the relationship between the thickness of a first magnetic layer relative to that of the entire main magnetic pole, and coercivity Hc, and  FIG. 17  is a diagram indicating the relationship between the thickness of a first magnetic layer relative to that of the entire main magnetic pole, and saturation magnetic flux density Bs. 
         [0127]    In  FIGS. 16 and 17 , (1) the results of a first main magnetic pole with a first magnetic layer of an NiFe alloy (with an Ni content of 10%) and a second magnetic layer of an FeCo alloy deposited are plotted with diamonds, (2) the results of a second main magnetic pole with a first magnetic layer of an NiFe alloy (with an Ni content of 35%), and a second magnetic layer of an FeCo alloy deposited are plotted with squares, (3) the results of a third main magnetic pole with a first magnetic layer of an NiFe alloy (with an Ni content of 50%) and a second magnetic layer of an FeCo alloy deposited are plotted with triangles, (4) the results of a fourth main magnetic pole with a first magnetic layer of an NiFe alloy (with an Ni content of 80%) and a second magnetic layer of an FeCo alloy deposited are plotted with circles, and (5) the results of a fifth main magnetic pole with a first magnetic layer of a CoNiFe alloy and a second magnetic layer of an FeCo alloy deposited are plotted with crosses. 
         [0128]    As shown in  FIG. 16 , the first magnetic pole in which the percentage of Fe in the NiFe alloy is 90% has a smaller decrease of the coercivity Hc compared to the other magnetic poles. As shown in  FIG. 17 , the fourth magnetic pole in which the percentage of Fe in the NiFe alloy is 20% has a larger decrease of the saturation magnetic flux density Bs compared to the other magnetic poles. In view of these results, in the examples in  FIGS. 16 and 17 , the second magnetic pole in which the percentage of Fe is 65% and the third magnetic pole in which the percentage of Fe is 50% are preferable. At the portion where the thickness of the first magnetic layer relative to the main magnetic pole is thin, the fourth magnetic pole in which the percentage of Fe is 20% is also preferable in addition to the second magnetic pole in which the percentage of Fe is 65% and the third magnetic pole in which the percentage of Fe is 50%. According to the above, it can be understood that both conditions for the coercivity Hc and the saturation magnetic flux density Bs can be met if the percentage of Fe is from 18% to 70%. It can also be understood that both pole erasure prevention and O/W performance enhancement can be achieved by restraining the percentage of the thickness of an NiFe alloy relative to the entire main magnetic pole to around 17%, and enhancing the percentage of the FeCo alloy, which has a large impact on the saturation magnetic flux density of the entire magnetic pole. 
         [0129]    Next, a fourth example will be described. 
         [0130]      FIG. 18  is a diagram indicating the relationship between the respective percentages of Co, Ni and Fe in a CoNiFe alloy, and saturation magnetic flux densities Bs, and  FIG. 19  is a diagram indicating the relationship between the respective percentages of Co, Ni and Fe in a CoNiFe alloy, and coercivities Hc. 
         [0131]    In  FIG. 18 , CoNiFe alloy compositions with a saturation magnetic flux density Bs of not less than 2.3 [T] are plotted with circles, CoNiFe alloy compositions with a saturation magnetic flux density Bs of not less than 2.2 [T] and less than 2.3 [T] are plotted with triangles, CoNiFe alloy compositions with a saturation magnetic flux density Bs of not less than 2.1 [T] and less than 2.2 [T] are plotted with squares, and CoNiFe alloy compositions with a saturation magnetic flux density Bs of not less than 2 [T] and less than 2.1 [T] are plotted with diamonds. As shown in  FIG. 18 , it can be understood that each CoNiFe alloy with a saturation magnetic flux density Bs of more than 2.1 [T] has a low percentage of Ni. 
         [0132]    Also, in  FIG. 19 , CoNiFe alloy compositions with a coercivity Hc of not less than 240 [A/m] and less than 480 [A/m] are plotted with circles, CoNiFe alloy compositions with a coercivity Hc of not less than 480 [A/m] and less than 720 [A/m] are plotted with triangles, and CoNiFe alloy compositions with a coercivity Hc of not less than 720 [A/m] and less than 960 [A/m] are plotted with squares. As shown in  FIG. 19 , it can be understood that in order to restrain the coercivity Hc to less than 720 [A/m], it is necessary to make the percentage of Co to be 33% or less and also to make the percentage of Ni to be 10% or less. 
         [0133]    Next, a fifth example will be described. 
         [0134]    In the fifth example, a first magnetic layer and a second magnetic layer are both made of FeCo alloys, but their percentages of Fe are different from each other. The first magnetic layer has a composition of Fe x Co 100-x  (75≦x wt %), and the second magnetic layer has a composition of Fe x Co 100-x  (65≦x≦75 wt %). As shown in  FIG. 10 , when the percentage of Fe exceeds 75%, the saturation magnetic flux density Bs is lowered, so plural magnetic layers with their saturation magnetic flux densities Bs different from each other can be deposited using FeCo alloys. 
         [0135]      FIG. 20  is a diagram indicating the relationship between the percentage of Fe and magnetostriction λ in an FeCo alloy, and  FIG. 21  is a diagram indicating the relationship between magnetostriction λ and coercivity Hc in an FeCo alloy. 
         [0136]    It can be understood that as shown in  FIG. 20 , the magnetostriction λ is restrained to less than “3.5” if the percentage of Fe in the FeCo alloy is not less than 75%, and as shown in  FIG. 21 , the coercivity Hc can be restrained to around 800 [A/m] if the magnetostriction λ is less than “3.5”. Accordingly, it can be understood that the coercivity can be decreased while restraining the magnetostriction by providing the first magnetic layer with a composition of Fe x Co 100-x  (75≦x wt %), thereby making it possible to prevent pole erasure. 
         [0137]    Next, a sixth example will be described. 
         [0138]    In  FIGS. 16 and 17 , the coercivity Hc values and the saturation magnetic flux density Bs values of a CoNiFe alloy having a composition of Co x Ni y Fe z  (x+y+z=100, 60≦x≦80, 10≦z≦20 wt %) are plotted with crosses. It can be understood that the coercivity Hc can be effectively decreased and a high saturation magnetic flux density Bs can be provided irrespective of the first magnetic layer thickness, by using a CoNiFe alloy with a composition of Co x Ni y Fe z  (x+y+z=100, 60≦x≦80, 10≦z≦20 wt %) as the first magnetic layer. 
         [0139]      FIG. 22  is a diagram indicating the relationship between the percentage of Fe and coercivity Hc in an FeCo alloy with various underlayers. 
         [0140]    In  FIG. 22 , the coercivities Hc of FeCo alloys with an underlayer of Ru are plotted with diamonds, the coercivities Hc of FeCo alloys with an underlayer of NiFe are plotted with crosses, and the coercivities Hc of FeCo alloys with an underlayer of a material other than Ru and NiFe are plotted with squares. As shown in  FIG. 22 , when NiFe, which is a magnetic material, or Ru, which is a non-magnetic material, is used for an underlayer and the percentage of Fe in the FeCo alloy is not less than 65%, the coercivity Hc can be restrained to 800 [A/m] or less. 
         [0141]      FIG. 23  is a diagram indicating the relationship between O/W performance and write core width. 
         [0142]    In  FIG. 23 , the results of NiFe alloys are plotted with squares, the results of laminated layers of an NiFe alloy, which is a first magnetic layer, and an FeCo alloy, which is a second magnetic layer (here, the percentage of the cross-sectional area of the FeCo alloy exceeds 35%) are plotted with small circles, the results of laminated layers of an NiFe alloy, which is a first magnetic layer, and an FeCo alloy, which is a second magnetic layer (here, the percentage of the cross-sectional area of the FeCo alloy is 35% or less) are plotted with large circles, and the results of FeCo alloys are plotted with diamonds. 
         [0143]    As shown in  FIG. 23 , it can be understood that the laminated layers the results of which are plotted with small circles (with the percentage of the cross-sectional area of the FeCo alloy exceeding 35%) has a high O/W performance, which is higher than those of the NiFe alloys and equal to those of FeCo alloys from large core width to narrow core width. Meanwhile, it can be understood that the laminated layers the results of which are plotted with large circles (the percentage of the cross-sectional area of the FeCo alloy is 35% or less) has an inferior O/W performance when compared with the FeCo alloys and the laminated layers (the percentage of the cross-sectional area of the FeCo alloy exceeds 35%) for the same core width, since many of their results are plotted above those of the FeCo alloys and the laminated layers. As describe above, it can be understood that both pole erasure prevention and a high recording density can be achieved by meeting a relationship of S2/(S1+S2)&gt;0.35 where the area of the surface of the first magnetic layer facing a surface of the recording medium is S1, and the area of the surface of the second magnetic layer facing a surface of the recording medium is S2.