Patent Publication Number: US-7221530-B2

Title: Magnetic head with high reliability of the data protection, magnetic disk apparatus including the magnetic head and the method of recording information on the magnetic disk apparatus without miserasing the previously recorded data

Description:
This is a continuation of application Ser. No. 10/407,208, filed Apr. 7, 2003 now abandoned, which is a divisional of application Ser. No. 09/805,806 filed Mar. 13, 2001 now U.S. Pat. No. 6,940,690, the content of which is incorporated herein by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to magnetic recording apparatuses, particularly magnetic disk apparatuses and magnetic heads. 
   2. Description of the Related Prior Art 
   Magnetic disk apparatuses are classified into a longitudinal magnetic recording system and a perpendicular magnetic recording system, according to the direction of magnetization of a magnetic film. The perpendicular magnetic recording system can easily realize higher recording density than the longitudinal magnetic recording system, and are expected to be put to practical use in the future. An example of such magnetic heads is seen, for example, in the Journal of Magnetic Society of Japan, Vol. 19,Supplement No. S2,pp. 122–125. Further, time dependency of recorded magnetization in such magnetic recording films and effects of external magnetic field on the characteristics are described in, for example, IEEE Transaction on Magnetics Vol. 35,NO. 5,pp. 2652 (1999) “Activation Volume and Correlation of Media Noise In High-Density Longitudinal Recording Media” or IEEE Transaction on Magnetics Vol. 35,No. 5,pp. 2655 (1999) “Rigid Disk Medium for 20 Gb/in 2  Recording Demonstration”. 
   In both perpendicular magnetic recording and longitudinal magnetic recording systems mentioned above, magnetic heads having the same basic structures are used. A representative example of them will be described below referring to the part pertaining to the present invention. 
   A general view of a rotary actuator with a magnetic head mounted thereon is shown in  FIG. 1 , and an enlarged view of the magnetic head  1  is shown in  FIG. 2 .  FIG. 2  is a plan view as seen from above of the magnetic head  1 . For positioning of the magnetic head  1  in a magnetic disk apparatus, the rotary actuator  2  is used. The angle formed by the rotating direction (circumferential direction)  4  of the magnetic disk  3  and the thickness direction  91  of magnetic pole is defined as skew angle S. In the magnetic disk apparatus having the rotary actuator  2 , the skew angle varies according to the head position in the radial direction. Ordinarily, the skew angle S varies in the range of about 20 degrees, according to the radial head position. 
   As shown in  FIG. 2 , the magnetic head  1  comprises a first magnetic pole  15 , a recording magnetic pole  8  as a second magnetic pole, and a reproducing head portion  6 . The example shown in the figure is an example of ring head. The recording magnetic pole  8  has a film thickness  9  of the magnetic pole and a write track width  7 . In the case of a recording and reproducing head such as this example, a reproducing head  6  is disposed in stack with the recording head. Layout of a signal processing system and the like will be described later. In the following figures, the rotating direction (circumferential direction) of the magnetic disk is denoted by symbol  4 , and the skew angle is denoted by symbol S. 
   Attendant on the increase in the density of tracks in magnetic storage devices, the track width of magnetic heads has been made smaller and smaller. As the track width  7  of the recording head is reduced, it becomes difficult for a magnetic flux generated in the head to pass through the recording magnetic pole. Therefore, the recording magnetic pole of the head tends to be saturated easily, and leakage flux to the recording medium is reduced. Accordingly, in the case of recording information on a recording medium by use of a recording head having a small track width, it is required to enlarge the magnetic pole thickness  8  of the recording head or to enhance the saturation flux density of the magnetic pole material for the recording head, in order to generate a sufficient magnetic field from the recording magnetic pole. 
   SUMMARY OF THE INVENTION 
   The present invention provides a magnetic disk apparatus capable of securing a sufficiently stable recorded state even in magnetic recording with high record density. More particularly, the present invention solves the following difficulties. In the magnetic disk apparatus, at the time of recording information in an area where the above-mentioned skew angle is large, there is generated a condition: when the information is recorded to the target track, the magnetic field from a writing pole is applied to the adjacent tracks. By this condition, thermal decay of the recorded information on the medium at the tracks is accelerated. Over-writing the same track repeatedly results in further acceleration of the thermal decay of the recorded information. The present invention provides a magnetic head and a magnetic disk apparatus in which thermal relaxation of information recorded in adjacent tracks is not accelerated even in a region where the skew angle is large. 
   Furthermore, the present invention proposes an information recording system such that thermal decay is not easily accelerated. 
   According to an aspect of the present invention, there is provided a magnetic disk apparatus comprising at least a magnetic head for recording, a rotated magnetic disk, and a means for positioning the recording magnetic head relative to the rotated magnetic disk, wherein the recording magnetic head comprises a first magnetic pole disposed on the upstream side and a second magnetic pole disposed on the downstream side with respect to the rotating direction of the magnetic disk, the first and second magnetic poles have faced portions, and the second magnetic pole is so shaped that the length of projection of the second magnetic pole onto the surface of the magnetic disk measured along the radial direction of the magnetic disk is not more than the track pitch of the magnetic disk. 
   According to another aspect of the invention, there is provided a magnetic disk apparatus comprising at least a magnetic head for recording, a rotated magnetic disk, and a means for positioning the recording magnetic head relative to the rotated magnetic disk, wherein the recording magnetic head comprises a first magnetic pole disposed on the upstream side, and a second magnetic pole disposed on the downstream side with respect to the rotating direction of the magnetic head, the first and second magnetic poles are faced to each other, a write gap is formed between the faced sides of the first and second magnetic poles, and the shape of the second magnetic pole has a third side between a first side intersecting the write gap and one of second sides faced to the write gap. 
   The third side is provided on the side of the second magnetic pole nearer to the write gap in one mode, and is provided on the side of the second magnetic pole farther from the write gap in another mode. 
   Furthermore, the magnetic pole is so shaped that, at a radial position where the angle formed-between the radial direction of the recording medium and the track width direction of the magnetic pole is maximum, the length of the projection of the magnetic pole onto the surface of the magnetic disk measured along the radial direction of the magnetic disk is not more than the track pitch of the magnetic disk. 
   According to a further aspect of the invention, there is provided a magnetic disk apparatus comprising at least a magnetic head for recording, a rotated magnetic recording medium, and a means for positioning the recording magnetic head relative to the rotated magnetic disk, wherein the recording magnetic head comprises a first magnetic pole disposed on the upstream side, and a second magnetic pole disposed on the downstream side with respect to the rotating direction of the magnetic disk, and a coil between the first and second magnetic poles, the first and second magnetic poles are faced to each other, a write gap is formed between the faced sides of the first and second magnetic poles, and the shape of the second magnetic pole comprises a pair of first sides intersecting the write gap, a pair of second sides substantially faced to the write gap, and a third side disposed between one of the first sides and one of the pair of second sides substantially faced to the write gap. 
   The third side is disposed on the side of the second magnetic pole nearer to the write gap in one mode, and is disposed on the side of the second magnetic pole farther from the write gap in another mode. 
   Furthermore, the second magnetic pole is so shaped that, at a position where the skew angle is maximum, the length of the projection of the second magnetic pole onto the surface of the magnetic disk measured along the radial direction of the magnetic disk is not more than the track pitch of the magnetic disk. 
   As has been mentioned above, magnetic disk apparatuses are classified into a longitudinal magnetic recording system and a perpendicular magnetic recording system, and the present invention can basically be applied to both systems. This point is the same in the modes or embodiments of the present invention illustrated below. 
   Besides, magnetic heads are classified into single-pole heads, ring heads and the like, and the present invention can be applied to both types of magnetic heads. This point is also the same in the modes or embodiments of the present invention illustrated below. 
   According to a still further aspect of the present invention, there is provided a magnetic disk apparatus comprising at least a magnetic head for recording, a rotated magnetic recording medium, and a means for positioning the recording magnetic head relative to the rotated magnetic recording medium, wherein the recording magnetic head comprises a magnetic pole and a coil, and, at a position where the skew angle S is maximum, the sum of P× sin(S) and W×cos(S) (where W is the width of the magnetic pole, and P is the pole thickness of the magnetic pole) is not more than the track pitch of the magnetic disk. 
   According to a still further aspect of the present invention, there is provided a magnetic disk apparatus comprising at least a magnetic head for recording, a rotated magnetic recording medium, and a means for positioning the recording magnetic head relative to the rotated magnetic recording medium, wherein the recording magnetic head comprises a first magnetic pole disposed on the upstream side, and a second magnetic pole disposed on the downstream side with respect to the rotating direction of the magnetic recording medium, the first and second magnetic poles have faced portions, and the second magnetic pole is so shaped that, at a position where the skew angle S is maximum, the sum of P×sin(S) and W×cos(S) (where W is the width of the second magnetic pole, and P is the pole thickness of the second magnetic pole) is not more than the track pitch of the magnetic recording medium. 
   As these two modes of the invention of magnetic head as mentioned above, the following particular examples may be given. In the case of a ring type magnetic head, there may be used a method of adjusting the pole thickness of the second magnetic pole, and a method of shaping the second magnetic pole to reduce the projected length as compared with the case of the conventional magnetic pole thickness. As for the shaping of the second magnetic pole, there may be used a method of shaping a corner portion of the second magnetic pole nearer to the write gap, and a method of shaping a corner portion of the second magnetic pole farther from the write gap. 
   Now, representative forms of a magnetic head according to the invention will be enumerated. 
   A first form is a magnetic head comprising a first magnetic pole, a second magnetic pole faced to the first magnetic pole, and a coil, wherein a write gap is formed between faced sides of the first and second magnetic poles, and the second magnetic pole has a third side between a first side intersecting the write gap and one of a pair of second sides substantially faced to the write gap. 
   A second form is a magnetic head comprising a first magnetic pole, a second magnetic pole faced to the first magnetic pole, and a coil, wherein a write gap is formed between faced sides of the first and second magnetic poles, and the shape of the second magnetic pole comprises a pair of first sides intersecting the write gap, and a pair of second sides faced to the write gap, and further comprises a third side between one of the first sides and one of the pair of second sides faced to the write gap. 
   The third side is provided on the side of the second magnetic pole nearer to the write gap in one mode, and is provided on the side of the second magnetic pole farther from the write gap. Of these modes, the former mode in which the third side is provided on the side nearer to the write gap is more useful in the case of a perpendicular magnetic recording system. The latter mode in which the third side is provided on the side farther from the write gap is more useful in the case of a longitudinal magnetic recording system. The difference between the two modes comes from the position of a recording point of magnetic inversion in the two systems. 
   Ordinarily, the shape of the first and second magnetic pole is basically a rectangle, and tetrangles are often used. Naturally, in view of machining accuracy, a trapezoid and somewhat irregular tetrangles or shapes with minute modifications may be used; in the present specification, the geometrical shapes including these various modified shapes will be described as rectangle. In addition, the second side substantially faced to the write gap includes a side farther from the write gap and a side nearer to the write gap, and the present invention can be embodied using any one of the two sides. 
   By use of a magnetic head having such a shape, the magnetic disk apparatus pertaining to the present application can be so constituted that, at a radial position where the skew angle is maximum, the length of the projection of the second magnetic pole onto the surface of the magnetic disk measured along the radial direction of the magnetic disk is not more than the track pitch of the magnetic disk. 
   As the mode of the present inventior the following mode can also be adopted, which are excellent in the characteristics of thermal decay as compared with the magnetic disk apparatuses according to the prior art. 
   According to a primary point of the mode of the present invention, there is provided a magnetic disk apparatus comprising at least a magnetic head for recording, a rotated magnetic disk, and a means for positioning the recording magnetic head relative to the rotated magnetic disk, wherein the recording magnetic head comprises a first magnetic pole on the upstream side, and a second magnetic pole on the downstream side with respect to the rotating direction of the magnetic disk, the first and second magnetic poles have faced sides, and the second magnetic pole is so shaped that, at a position where the skew angle is maximum, the width of an overlapped area of the projection of the second magnetic pole onto the surface of the magnetic disk and the track width of the magnetic disk is not more than 10% of the track width. Further, the width of the overlapped area is preferably not more than 5% of the track width. 
   An important point in the above modes is that the length of the overlapped area of the projection of the magnetic pole onto the surface of the magnetic disk and the track width of the magnetic disk as measured along the track width direction is not more than 10% of the track width. Further, the width of the overlapped area is preferably not more than 5% of the track width. 
   Naturally, as mentioned above, absence of the overlapped area of the projection of the magnetic pole onto the surface of the magnetic disk and the track width of the magnetic disk is the most excellent in view of thermal decay. 
   Next, an information recording system in which thermal decay is not easily accelerated will be referred to. The method can suppress the acceleration of thermal decay attendant on appending of information onto a magnetic disk apparatus. Therefore, by using the method with the magnetic disk apparatus according to the present invention, the thermal decay at the time of recording on the magnetic disk apparatus can be suppressed, and the thermal decay on the magnetic disk apparatus attendant on repetition of recording can be effectively suppressed. 
   The method comprises storing modified or appended information in a sector different from a sector with data recorded therein, without overwriting on a part or entire body of the sector with data recorded therein, at the time of modifying or appending information by use of the magnetic disk apparatus. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partial general view of a magnetic disk apparatus; 
       FIG. 2  is an enlarged plan view of a magnetic head portion; 
       FIG. 3  is a plan view showing general constitution of a magnetic disk apparatus; 
       FIG. 4  is a sectional view showing general constitution of a magnetic disk apparatus; 
       FIG. 5  is a perspective view of a recording/reproducing portion; 
       FIG. 6A  is a bottom view as viewed from an air bearing surface of the recording/reproducing portion of a magnetic head; 
       FIG. 6B  is a vertical sectional view of a recording/reproducing portion of a magnetic head; 
       FIG. 7  is a plan view showing the positional relationship between a magnetic head and write tracks according to a first embodiment of the invention; 
       FIG. 8  is a plan view showing the positional relationship between a magnetic head and write tracks illustrating the problem in the prior art; 
       FIG. 9  is a diagram illustrating an example of magnetic field distribution at the center of track in a ring type magnetic head; 
       FIG. 10  is a diagram illustrating an example of magnetic field distribution at the track edge in the ring type magnetic head; 
       FIG. 11  is a plan view showing the positional relationship between a magnetic head and write tracks according to a second embodiment of the invention; 
       FIG. 12  is a plan view showing the positional relationship between a magnetic head and write tracks according to a third embodiment of the invention; 
       FIG. 13  is a plan view showing the positional relationship between a magnetic head and write tracks according to a fourth embodiment of the invention; 
       FIG. 14  is a sectional view showing an example of stack of a magnetic recording medium; 
       FIG. 15  is a diagram showing an example of magnetic field distribution in the case of using a longitudinal magnetic recording medium with a ring type magnetic head in the case of using a bi-layer perpendicular magnetic recording medium with a ring type magnetic head; 
       FIG. 16A  is a plan view of a magnetic pole before machining formed by a prior-art method; 
       FIG. 16B  is a plan view of an example of a magnetic pole after machining; 
       FIG. 17A  is a plan view of a magnetic pole before machining formed by a prior-art method; 
       FIG. 17B  is a plan view of an example of a magnetic pole after machining; 
       FIG. 18A  is a plan view of a magnetic pole before machining formed by a prior-art method; 
       FIG. 18B  is a plan view of an example of a magnetic; pole after machining; 
       FIG. 19A  is a plan view of a magnetic pole before machining formed by a prior-art method; 
       FIG. 19B  is a plan view of an example of a magnetic pole after machining; 
       FIG. 20  is a diagram illustrating a data modifying/appending system for a magnetic disk apparatus according to the invention; and 
       FIG. 21  is a plan view showing the positional relationship between a magnetic head and write tracks according to a further embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   First, representative examples of constitution of a magnetic disk apparatus and a magnetic head according to the present application will be described. 
     FIG. 3  is a schematic diagram showing the plan view of the magnetic disk-apparatus, and  FIG. 4  is a schematic diagram showing a sectional view of the apparatus. 
   The magnetic disk and the relationship between the magnetic disk and a rotary actuator are basically the same as those shown in  FIG. 1  and  FIG. 2 . 
   This example of magnetic disk apparatus comprises a magnetic disk  3 , and a magnetic head  1  for parrying out recording and/or reproduction on the magnetic recording medium. The magnetic head  1  is positioned and mounted through the rotary actuator  2  and a suspension arm  50 . The rotary actuator  2  performs rotational motion with a support point  0  as a center. The example shown in  FIG. 1  is an example in which a recording portion and a reproducing portion are mounted on one magnetic head. An inductive type head is used for the recording portion, and, for example, an MR head or the like is used for the reproducing portion. Ordinarily, the recording portion and the reproducing portion are provided as a laminate. 
   As the magnetic recording medium in the case of longitudinal magnetic recording system, there are used, for example, cobalt-chromium based alloys such as cobalt-chromium alloys, cobalt-platinum alloys, cobalt-chromium-tantalum alloys, cobalt-chromium-platinum alloys, cobalt-chromium-tantalum-platinum alloys or samarium-cobalt based alloys. As the recording medium in the case of perpendicular magnetic recording, there are used cobalt-chromium based alloys, terbium-iron-cobalt alloys and the like. 
   A signal processing system is chiefly shown in  FIG. 4 . An electric signal sent to the magnetic head  1  through a preamplifier  15  is converted into a magnetic information and recorded in the magnetic recording medium mounted on a rotated magnetic disk  3  by use of the magnetic head  1 . At the time of reproduction, conversely, magnetic information recorded in the magnetic recording medium is converted into an electric signal by the magnetic head  1 , and the converted information is transmitted to the preamplifier  15 . The recording or reproduced signal is sent through the preamplifier  15  to a package board  16  on which a signal processing circuit and a controller are mounted. Positioning of the magnetic head  1  to a target track is carried out by the rotary actuator  2  driven by a voice coil motor  17 . In  FIG. 3 , the upper side of the broken line is the enclosure side, and the lower side is the package board side. In the example shown, the magnetic disk  3  includes a first disk ( 3 - 1 ), a second disk ( 3 - 2 ) and a third disk ( 3 - 3 ). Symbol  4  in  FIG. 3  and  FIG. 4  denotes the rotating direction (circumferential direction) of the magnetic disk. 
     FIG. 5  is an enlarged view of an information recording/reproducing portion,  FIG. 6(A)  is a bottom view as viewed from an air bearing surface of the magnetic head, and  FIG. 6(B)  is a vertical sectional view of the magnetic head. Referring to the figures, concrete structure of the example of magnetic head will be described. As mentioned above, the magnetic head  1  comprises as one body a recording head  5  having only a recording function, and a reproducing head  6  having only a reproducing function. 
   As shown in  FIG. 5 , the recording head  5  comprises a recording magnetic pole  8  and an upper shield film  19 , with a coil  18  therebetween. The upper shield film  19  functions also as a first magnetic pole. Naturally, the shield and the first magnetic pole may be provided as separate bodies. 
   By generating an alternating recording current in the coil  18 , an alternating magnetic field is generated between the write pole  8  and the upper shield film  19 , whereby magnetic information is recorded on the magnetic recording medium. In  FIG. 6(B) , the magnetic field  51  is shown schematically. 
   The reproducing head  6  is sandwiched between the upper shield  19  and a lower shield film  20 , whereby leakage flux from the neighborhood is shielded, and only the information directly beneath the target position can be reproduced easily. A layer  23  is the shield gap ( FIG. 6(A)  and  FIG. 6(B) ). The form of this example is now widely used for magnetic heads. 
   An insulation film  22 , for example, an alumina layer is provided between a first magnetic pole (functioning also as upper shield)  19  and a second magnetic pole (upper shield)  19 ,the insulation film  2 ,and the second magnetic pole coil (recording pole)  8  are laminated, and one side of the laminate constitutes an air bearing surface. As shown in FIG  6 (B), the magnetic head  1  is faced to the magnetic disk  3 . therefore, the portion of the insulation film  22  constitutes a write gap. The magnetic disk  3  comprises a recording medium layer  25  laminated on a substrate  24 . 
   Although the present invention is naturally useful for magnetic disk apparatuses in general, it is particularly useful for a magnetic disk apparatus designed for a high record density of not less than 10 Gbit/in 2 . Naturally, this is due to the fact that the track width is smaller than the conventional values. Generally, the pole thickness of the second magnetic pole may be several micrometers, and, usually, 3 to 4 μm. The thickness of the first magnetic pole also is about several micrometers. As the track width, a value of about 50 to 80% of the track pitch is adopted. Naturally, these specific values are determined according to required specifications of the magnetic disk. 
   Next, the positional relationship of the magnetic head and recording tracks according to the invention will be described.  FIG. 7 ,  FIG. 8  and  FIGS. 11 to 13  show the positional relationship of magnetic pole, recording track (data track) and adjacent tracks, as viewed from above. In addition, rotating direction (circumferential direction) and radial direction of the disk are also shown. In these figures, the same components or members are denoted by the same symbol; accordingly, individual descriptions may be omitted for each of the figures. Furthermore,  FIG. 9  and  FIG. 10  show examples of the recording magnetic field distribution at a central. 
     FIG. 7  shows the positioning relationship between a magnetic head and recording tracks according to a first embodiment of the invention. The figure shows the positional relationship, as viewed from above, of a data track  10  and adjacent tracks  11 ,  12  in the case where the skew angle is S. The track pitch is denoted by numeral  13 . As the magnetic head, only a first (lower) pole  15  and a second (upper) pole  9  are illustrated. The areas of symbols  10 ′,  11 ′ and  12 ′ in the figure are the areas of inversion of magnetization corresponding to recording tracks in the recording medium layer. This applies also in the similar figures. 
   In the present example, the projected length  14  of the upper pole  9  in the radial direction of disk is set to be not more than the track pitch. For this purpose, as shown in  FIG. 7 , a corner of the upper pole  9  is cut away so that the magnetic pole of the recording head does not overlap on the adjacent track portion. 
   More concrete description will be given below. The magnetic head for recording in this example comprises a first magnetic pole  15  disposed on the upstream side, and a second magnetic pole  9  disposed on downstream side with respect to the rotating direction of the magnetic disk. The first and second magnetic poles are faced to each other, and the portion between the faced sides  30 ,  31  of the first pole  15  and the second pole  9  constitutes a write gap  36 . The second magnetic pole  9  has a third side  34  between a first side  35  intersecting the write gap  36  and one  33  of second sides ( 31 ,  33 ) faced to the write gap  36 . Thus, the second pole  9  is so shaped that, at a position where the skew angle (S) is maximum, the length  14  of the projection of the second pole  9  onto the surface of the magnetic disk measured along the radial direction of the magnetic disk (namely, projected length) is not more than the track pitch  13  of the magnetic disk. 
   According to this arrangement, acceleration of thermal decay due to head field does not occur. To ensure that, at a position where the skew angle is maximum, the length of the projection of the magnetic pole onto the surface of the magnetic disk as measured along the radial direction of the magnetic disk is not more than the track pitch of the magnetic disk, a corner of the upper pole  16  is cut away, and the degree of cutting away is set in consideration of the field intensity required for recording. 
   In this example, the third side  34  is provided on the side of the magnetic head farther from the write gap. 
   Naturally, the present invention can be applied to various recording heads and magnetic disk apparatuses other than the illustrated here. 
   The ring type magnetic head can be used for both longitudinal magnetic recording and perpendicular magnetic recording, and spacing of the write gap and other characteristics are selected according to the differences in the recording systems. A comparatively narrower write gap is used for longitudinal magnetic recording, and a comparatively wider write gap is used for perpendicular magnetic recording. In the perpendicular magnetic recording, the perpendicular component of the magnetic flux to the recording medium surface is utilized. Therefore, designing of the magnetic head, particularly the write gap, is so conducted that the perpendicular component of the head field is principally generated on the surface of the recording medium. On the other hand, in the longitudinal magnetic recording, the in-plane component of the magnetic flux relative to the recording medium surface is utilized. Therefore, designing of the magnetic head, particularly the write gap, is so conducted that the in-plane component of the head field is principally generated on the surface of the recording medium. Accordingly, the magnetic heads according to the invention particularly suited to both recording systems are designed differently depending on the system. As shown in  FIG. 15 , a large longitudinal component of head field is present at the write gap portion in the case of longitudinal magnetic recording, whereas a large perpendicular component of head field is present throughout the magnetic pole in the case of perpendicular magnetic recording. Therefore, finally determined recorded state on the medium is located at the write gap portion in the case of longitudinal magnetic recording, whereas it is located at an end portion on the trailing side of the upper pole or main pole in the case of perpendicular magnetic recording. 
   The characteristic feature pertaining to the gist of the invention is that, at a radial position where the skew angle is maximum, the length of the projection of the second pole onto the surface of the magnetic disk measured along the radial direction of the magnetic disk is not more than the track pitch of the magnetic recording metidum, and an important point or basic portion in realizing the characteristic feature in view of the above-described background is as follows. The portion to be paid attention to is a portion of the second pole farther from the write gap in the case of longitudinal magnetic recording, and a portion of the second pole nearer to the write gap in the case of perpendicular magnetic recording. From this point of view,  FIG. 7  shows an embodiment particularly suitable for longitudinal magnetic recording, while  FIG. 12  shows an embodiment particularly suitable for perpendicular magnetic recording. 
   Next, the effects of the present invention will be described. 
     FIG. 8  is similar to  FIG. 7  but differs from  FIG. 7  in that the thought of the present invention is not applied to the upper pole  9 . In this example, the projection of the second pole on the track surface overlaps on the adjacent track adjacent to the data track. Namely, where the pole thickness of the recording head is large, a portion of a tip of the write pole  9  has an overlap  17  on the position of the adjacent track  12  when the skew angle is large. For example, in the case of a rectangular write pole where the track pitch  13  is 1.3 micrometers, the write track width  7  is 1 micrometers and the pole thickness is 3 micrometers, a portion of the upper pole  9  overlaps on the position of the adjacent track  12  in an area where the skew angle is not less than 5.8 degrees. 
   Calculation results of distribution of write field generated from a ring type recording head are shown in  FIGS. 9 and 10 .  FIG. 9  and  FIG. 10  shows calculation results of write field distribution at a track center portion and a track edge, respectively. Here, x-axis is the circumferential direction  4  of the magnetic disk, and y-axis is the track width direction. Besides, x-component and y-component of write field are defined as Hx and Hy, respectively. In each of  FIGS. 9 and 10 , layout of lower pole, write gap and upper pole is shown at the upper portion, the distribution of magnetic field (Hx) in x-direction is shown in the intermediate graph, and the distribution of magnetic field (Hy) in y-direction is shown in the lower graph. 
   As seen from  FIG. 9 , at the track center, Hx is maximum at the position directly beneath the gap  13 , and has an intensity distribution along the x-axis direction; the intensity decreases rapidly as the distance from the gap increases. The value of Hy is substantially 0 at any position. As seen from  FIG. 10 , at the track edge also, Hy is maximum at the position directly beneath the gap  13 , and decreases rapidly as the distance from the gap increases. On the other hand, as contrasted to the case of the track center, Hy gradually trails and has a value of about several hundred oersteds until the tip portion of the upper pole is reached. As a result, in the area where the skew angle is large, a magnetic field at the time of recording information is applied also to the data previously recorded in the adjacent track. For example, at the track end portion, there is a magnetic field of 230 Oe at the end point A of the upper pole. Where one half of the upper pole overlaps on the adjacent track, it is seen from  FIG. 10  that the magnetic field at the point of (A+B)/2, namely a field of 810 Oe at maximum is applied to the adjacent track. Where ⅓ of the upper pole overlaps on the adjacent track, the magnetic field at the point of ( 2 A+B)/3 in  FIG. 10 , namely a field of 620 Oe at maximum is applied to the adjacent track. 
   In magnetic disks, the phenomenon of relaxation of recorded state, and a reduction of reproduction amplitude with time, under the effect of the phenomenon of thermal relaxation of recorded data (hereinafter referred to as thermal decay) has been reported, as mentioned above. 
   This is due to change of the recorded state by thermal energy at an ambient temperature for a recording medium. The presence of an external field gives energy to individual crystal grains, like heat, and therefore accelerates the thermal decay. 
   Magnetization recorded on a magnetic disk suffers thermal relaxation, and the remnant magnetization decreases with time. It has been described that thermal relaxation is accelerated by the presence of an externally applied magnetic field. A simulation of magnetic recording has been done by use of the Landau-Lifschitz equation, and applied field dependency of remnant magnetization after 10 years has been calculated. The results are shown in Table 1. 
   
     
       
         
             
             
             
             
           
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               Effective field 
               Theoretical value of 
                 
             
             
                 
               applied to 
               remnant magnetization 
             
             
                 
               adjacent tracks 
               after 10 years 
             
             
                 
               (Oe) 
               (relative value) 
               Note 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
          
             
                 
               0 
               0.97 
               Upper pole does 
             
             
                 
                 
                 
               not overlap on 
             
             
                 
                 
                 
               adjacent track 
             
             
                 
               200 
               0.955 
             
             
                 
               220 
               0.95 
             
             
                 
               620 
               0.90 
               ⅓ of upper pole 
             
             
                 
                 
                 
               overlaps on 
             
             
                 
                 
                 
               adjacent track 
             
             
                 
               810 
               0.86 
               ½ of upper pole 
             
             
                 
                 
                 
               overlaps on 
             
             
                 
                 
                 
               adjacent track 
             
             
                 
                 
             
          
         
       
     
   
   The magnetic field applied to the adjacent track is 230 Oe when the end point A of the upper pole is in contact with the adjacent track, 620 Oe when ⅓ of the upper pole overlaps on the adjacent track, and 810 Oe when ½ of the upper pole overlaps on the adjacent track. In view of Table 1 showing the relative value of remnant magnetization after 10 years, the remnant magnetization is reduced to 0.86 when one half of the upper pole overlaps on the adjacent track, and is reduced to 0.90 when ⅓ of the upper pole overlaps on the adjacent track. 
   In order to prevent a floating magnetic field from being applied to the adjacent track at a radial position where the skew angle is maximum, it suffices that the projected length  14  of the upper pole in the radial direction of the disk as shown in  FIG. 8  is not more than the track pitch  13 . Therefore, when the end portion  17  of the upper pole is cut away as described above so that the magnetic pole of the recording head does not overlap on the adjacent track, acceleration of thermal decay by head field will not occur. 
     FIG. 11  shows a second embodiment of the present invention. This embodiment shows another example of preventing the projection  14  of the magnetic pole  9  of the recording head from overlapping on the adjacent track. For ensuring that the projected length of the upper pole is not more than the track pitch, in the same manner as in the first embodiment, it suffices that the shape of the ordinary rectangular upper pole satisfies the following formula:
   TP≧P× sin(S)+ W× cos(S) 
where TP is the track pitch, P is the pole thickness of the upper pole, W is the track width of the recording head, and S is the skew angle of the magnetic head.
 
   This example differs from the example of  FIG. 7 , and is not an example in which the shape of the second pole is defined by use of a third side. Namely, in this example, the shape of the second pole is a tetrangle and the pole thickness thereof is controlled. The same idea may be used also in the case of a single pole head. 
     FIG. 12  shows a third embodiment of the present invention. In the same manner as in the first embodiment, for ensuring that the projected length  14  of the upper pole  9  is not more than the track pitch  13 , an end portion of the upper pole is cut away. While in the first embodiment the end portion of the upper pole  9  on the downstream side with respect to the rotating direction of the disk was cut away, in this embodiment an end portion on the side of the gap  36  is cut away. In this case, the side  37  is the third side, also known as the third face  37 . The upper pole  9  includes a trailing face  60 , a leading face  62 , a first side-face  64  and a second side-face  66 . 
   As has been described above, at the time of recording a magnetization inversion by a magnetic head on a rotated disk, the position at which the magnetization inversion is finally determined is substantially directly beneath the gap in the longitudinal magnetic recording system, but it is near the faced line of the upper pole farther from the gap in the perpendicular magnetic recording system. This embodiment can be applied to not only the longitudinal magnetic recording system but also the perpendicular magnetic recording system. 
     FIG. 13  shows a fourth embodiment of the present invention. This embodiment differs from the second embodiment shown in  FIG. 11  only in that the recording point of magnetization inversion is shifted. 
   This embodiment can be applied to not only the longitudinal magnetic recording system but also the perpendicular magnetic recording system. 
   Now, perpendicular magnetic recording, particularly an example of perpendicular magnetic recording with a bi-layer medium will be described. 
   This case requires more the present invention, and this example is an example for which the effects of the present invention are displayed more effectively. 
   Magnetic recording with a bi-layer medium means that a recording medium is constituted of two layers of magnetic material. As shown in  FIG. 14 , a magnetic disk generally comprises on a disk substrate  40  a ground layer  41 , a chromium alloy layer  42 , a magnetic recording layer  43  and an upper protective film  44 . The constitution of the laminate may be improved or modified variously, but the basic constitution is as described above. For example, the chromium alloy layer  42  may be omitted. Examples of the material for the magnetic recording layer has been described above, and the bi-layer medium may comprise, for example, a soft magnetic film  46  or the like beneath a first magnetic recording layer  45 . Since this technology itself is well known, detailed description thereof is omitted. 
     FIG. 15  shows an example of distribution of recording field under the recording gap and magnetic poles in a ring head. Layout of the recording gap and magnetic poles is shown in an upper portion of the figure, longitudinal component of head field at the time of recording on a longitudinal magnetic recording medium is shown in an intermediate portion of the figure, and distribution of perpendicular field component at-the time of recording on a bi-layer perpendicular magnetic recording medium is shown in a lower portion of the figure. In this example, the distribution of longitudinal field component at the time of recording on the longitudinal magnetic recording medium is the same as the examples described above. On the other hand, the distribution of perpendicular field component at the time of recording on the bi-layer perpendicular magnetic recording medium is different from the above examples in that the area with a large perpendicular field component extends below the magnetic poles. 
   On the other hand, in the case of a general magnetic head, for example, the example of  FIG. 10 , the distribution of Hy has a maximum value at an end portion of the write gap, and the perpendicular field component gradually decreases as the end of the magnetic pole is approached. At the end portion of the magnetic pole, no large component exists. 
   Due to such a difference in magnetic field distribution, in this example of the bi-layer medium there is a stricter requirement as to the setting of distance between the recording pole and the adjacent track. Namely, in the perpendicular magnetic recording on a bi-layer medium, the influence of the magnetic pole on the adjacent track is greater than that in ordinary cases. Even a small degree of overlap of the magnetic pole on the adjacent track would causes great acceleration of thermal decay. Accordingly, the effect of applying the present invention is more conspicuous in such an example of bi-layer medium. 
   Next, an example of the method of producing the magnetic head according to the invention will be described. 
     FIGS. 16(A) to 19(B)  are plan views illustrating the major points of the method of producing the magnetic head according to the invention. In these figures, like the above examples, an example of a ring head is treated, and only the projections of a first magnetic pole  15  and a second magnetic pole  9  onto the disk surface are shown. 
   The important point of the method of producing a magnetic disk according to the invention relates to processing of the plan view shape of the magnetic pole, and only the steps of this process are illustrated. 
   Ordinarily, as shown in  FIGS. 6A and 6B , from a lower shield film  20  to a recording magnetic pole  8  are sequentially provided on a non-magnetic substrate. After the recording pole  8  is formed as a film, an end face on the rear end side of the recording pole  8  is flattened so as to be parallel to a reproducing head or the like. The flattening is carried out by lapping, FIB (Focused Ion Beam) processing or the like. According to the present invention, an ordinary rectangular upper pole is formed by such conventional method, and thereafter a desired portion gf the projection of which overlaps on the adjacent track is removed. For the removal, there may be used FIB, sputtering, ion milling and the like. The working conditions for this process may be as in the usual cases for the materials related. 
     FIGS. 16(A) ,  16 (B),  17 (A) and  17 (B) show an example of removing a corner portion of the second pole farther from the write gap, whereas  FIGS. 18(A) ,  18 (B),  19 (A) and  19 (B) show an example of removing a corner portion of the second pole nearer to the write gap. Numerals  34  and  37  denote a third side. 
     FIG. 16(A)  is a plan view of a magnetic pole formed by a prior-art method. The box-shaped area shown in  FIG. 16(A)  is trimmed by FIB from the head floating side, to produce the shape shown  FIG. 16(B) . 
     FIGS. 17(A) and 17(B)  show a second method of producing the magnetic head used in the first embodiment of the invention.  FIG. 17(A)  is a plan view of a magnetic pole formed by a prior-art method. As shown in  FIG. 17(A) , Ar sputtering or ion milling is carried out in a skew direction to produce the shape shown in  FIG. 17(B) . The sputtering or ion milling is preferably carried out in a skew direction of less than 90 degrees with respect to the track width direction. 
     FIGS. 18(A) and 18(B)  show another method of producing the magnetic head for use in the third embodiment of the invention.  FIG. 18(A)  is a plan view of a magnetic pole formed by a prior-art method. The box-shaped area shown in  FIG. 18(A)  is trimmed by FIB from the head floating side, to produce the shape shown in  FIG. 18(B) . 
     FIGS. 19(A) and 19(B)  show a further method of producing the magnetic head for use in the third embodiment of the invention.  FIG. 19  (A) is a plan view of a magnetic pole formed by a prior-art method. As shown in  FIG. 19  (A), Ar sputtering or ion milling is carried out in a skew direction to produce the shape shown in  FIG. 19(B) . The sputtering or ion milling is preferably carried out in a skew direction of less than 90 degrees with respect to the track width direction. 
   In this method, the lower pole may be a little machined away, depending on the sputtering or milling conditions. In this method, therefore, it is preferable to control, for example, power condition so that the lower pole would not greatly machined away. 
   Next, another example of the present application will be described.  FIG. 21 , like similar figures described above, shows the positional relationship of magnetic pole, data track and adjacent tracks in this example, as viewed from above. 
   The second magnetic pole is so shaped that, at a position where the skew angle is maximum, the overlapped portion  18  of the projection of the second pole  9  onto the surface of the magnetic disk and the track width  12 ′ of the magnetic disk is not more than 10% of the track width  12 ′. More preferably, the overlapped portion is in excess of 0% but not more than 10% of the track width, whereby the magnetic head obtained shows more excellent characteristics than the conventional magnetic heads. In this example, the third side  34  is used, and the projection of the second pole  9  onto the track surface overlaps on the track  12  adjacent to the recording track (data track)  10 . 
   The invention can be accomplished with single-pole magnetic heads or ring heads, irrespectively of the shapes of magnetic poles, by ensuring that the overlapped area of the projection of the magnetic pole onto the surface of the magnetic disk and the track width of the magnetic disk is set within a predetermined range. While these points of the present invention will not be illustrated more, the present invention can naturally be embodied with the magnetic poles having the shapes shown in  FIGS. 7 ,  11 ,  12  and  13 . Such magnetic poles can be produced in the same manner as the above-mentioned methods. 
   Next, a method of modifying or appending information on a magnetic disk apparatus will be described. 
     FIG. 20  shows a method of modifying or appending information on a magnetic disk apparatus according to the present invention. In an upper portion of  FIG. 20 , the recorded state before data are modified or appended is shown. The figure shows that the data are recorded on only a data block  1 . When a composition is modified or appended on a word processor, if recording or reproduction for repeated overwriting is repeated only on the data block  1  in which data are previously recorded, only the same portions of the adjacent tracks are repeatedly affected by a floating field of recording. In order to alleviate such a situation, at the time of overwriting, the entire data in the data block requiring overwriting are recorded on a data block (for example, data block  2 ) other than the previous data block. As a result of this process, the situation in which the floating field at the time of recording is repeatedly applied to the adjacent track at each overwriting can be obviated, and acceleration of thermal decay can be suppressed. 
   The present invention provides a magnetic disk apparatus capable of securing a sufficiently stable recorded state even in magnetic recording with high recording density. 
   From a technical point of view, the present invention provides a magnetic head and a magnetic disk apparatus such that thermal relaxation of recorded data in the adjacent tracks is not accelerated even in an area where the skew angle is large.