Patent Publication Number: US-2005129985-A1

Title: Perpendicular magnetic recording media

Description:
BACKGROUND OF THE INVENTION  
      This application claims the priority of Korean Patent Application No. 2003-89364, filed on Dec. 10, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
      1. Field of the Invention  
      The present invention relates to a perpendicular magnetic recording medium, and more particularly, to a perpendicular magnetic recording medium for improving signal-to-noise ratio (SNR).  
      2. Description of the Related Art  
      Hard disk drives (HDDs), which are representative magnetic information storage media and which lead a rapid increase in recording density, are currently adopting longitudinal magnetic recording methods involving ring type head and longitudinal magnetic recording media. Conventional longitudinal magnetic recording methods, however, are limited in increasing recording density because of thermal instability of a recording medium, and a new recording method, the perpendicular magnetic recording method, is currently being actively developed.  
      The recording density of a present day longitudinal magnetic recording type HDD product is about 90-100 Gb/in 2 . It is expected that the recording density of a perpendicular magnetic recording type HDD could be higher than 200 Gb/in 2  and up to 500 Gb/in 2 .  
      A perpendicular magnetic recording method increases recording density by arranging the magnetic direction of unit bits, which are recorded on a medium, in a perpendicular direction. When such a perpendicular magnetic recording method is applied, data stability improves in spite of the decrease in bit size.  
      The perpendicular magnetic recording method uses a perpendicular magnetic recording medium having a double magnetic layer structure. In other words, a soft underlayer is added under a recording layer in a perpendicular magnetic recording medium in order to carry out perpendicular magnetic recording.  
      Referring to  FIG. 1 , a conventional perpendicular magnetic recording medium  10  includes a substrate  11 , a perpendicular magnetic recording layer  17  on which magnetic data is recorded by a writing head, and a perpendicular alignment underlayer  15 , which is formed before depositing the perpendicular magnetic recording layer  17  to improve the crystalline alignment and the magnetic characteristic of the perpendicular magnetic recording layer  17 . In addition, the perpendicular magnetic recording medium  10  includes a soft underlayer  13  formed under the perpendicular alignment underlayer  15  in order to increase the strength and spatial change rate of a magnetic field, which is generated from a pole type writing head, in a magnetic recording mode. The conventional perpendicular magnetic recording medium  10  is formed by sequentially stacking the soft underlayer  13 , the perpendicular alignment underlayer  15 , the perpendicular magnetic recording layer  17 , and a protection layer  19 , on the substrate  11 .  
      Here, the perpendicular alignment underlayer  15  may be referred to as an intermediate layer.  
      In the perpendicular magnetic recording medium  10  having a double magnetic layer structure, the soft underlayer  13  is important for performing high density recording.  
       FIG. 2  is a sectional view illustrating a perpendicular magnetic recording system using a conventional perpendicular magnetic recording medium  10 . A magnetic head  30  for writing and reading information on and from the perpendicular magnetic recording medium  10  includes a writing head  31  having a writing pole  33  and a return pole  35  for writing magnetic information on a recording layer  17 , and a reading head  37 , in other words, a magnetic resistance head for reading magnetic information recorded on the recording layer  17 . The structure of the magnetic head  30  for the perpendicular magnetic recording medium  10  is widely known, thus a further detailed description thereof will be omitted.  
      When a soft underlayer  13  is formed under the recording layer  17 , a virtual image head corresponding to the pole structure of the writing head  31  is formed in the soft underlayer  13 . Thus, a strong and sharp recording magnetic field is obtained compared to the case where the soft underlayer  13  is absent. The field strength is about doubled and a field gradient is increased by three to four times by forming the soft underlayer  13 .  
      Due to use of the soft underlayer  13 , a recording operation can be performed even when the recording layer  17  is formed of a material having high anisotropy magnetic field and coercive force. Accordingly, recording density is largely improved.  
      As described above, the soft underlayer  13  is essential for realizing the merits of the perpendicular magnetic recording method.  
      However, the soft underlayer  13  is formed of a magnetic substance, for example, a ferromagnetic substance. Thus, magnetic field leaking from the surface of the soft underlayer  13  is detected by the reading head  37 . The magnetic field thus operates as a noise source to deteriorate the SNR.  
      In addition, when an unstable domain wall exists in the soft underlayer  13 , such a domain wall interacts with a bit transition area recorded on the recording layer  17 . This results in an increase in transition noise as one type of noise generated from the recording layer  17 .  
     SUMMARY OF THE INVENTION  
      The present invention provides a perpendicular magnetic recording medium for obtaining an improved signal-to-noise ratio (SNR) by changing the composition of a soft underlayer.  
      According to a first aspect, the present invention provides a perpendicular magnetic recording medium comprising a perpendicular magnetic recording layer on a substrate and a soft underlayer formed between the substrate and the perpendicular magnetic recording layer, wherein the soft underlayer includes a plurality of soft underlayers having different saturation magnetizations, and at least one of the soft underlayers has a magnetization easy axis in a radial direction.  
      The soft underlayer may include a first soft underlayer and a second soft underlayer closer to the perpendicular magnetic recording layer than the first soft underlayer, the second soft underlayer having a larger saturation magnetization than that of the first soft underlayer.  
      The soft underlayer may include a first soft underlayer and a second soft underlayer closer to the perpendicular magnetic recording layer than the first soft underlayer, the second soft underlayer having a smaller saturation magnetization than that of the first soft underlayer.  
      The thickness of the second soft underlayer may be less than the thickness of the first soft underlayer.  
      The thickness of the second soft underlayer may be 1 nm or more and 50 nm or less.  
      The entire thickness of the soft underlayer may be 200 nm or less, and the thickness of the second soft underlayer closer to the perpendicular magnetic recording layer may be 50 nm or less.  
      According to another aspect, the present invention provides a perpendicular magnetic recording medium comprising a perpendicular magnetic recording layer on a substrate and a soft underlayer formed between the substrate and the perpendicular magnetic recording layer, wherein the soft underlayer includes a plurality of soft underlayers having different saturation magnetizations, and the entire thickness of the soft underlayers is 200 nm or less while the thickness of the soft underlayer closer to the perpendicular magnetic recording layer is 50 nm or less.  
      At least one of the soft underlayers may have a magnetization easy axis in a radial direction.  
      The soft underlayer may be formed of a ferromagnetic substance or the combination of an antiferromagnetic substance and a ferromagnetic substance.  
      The soft underlayer may include one or more alloys selected from the group consisting of a NiFe-based alloy, an Fe-based alloy and a Co-based alloy.  
      The soft underlayer may include an alloy selected from the group consisting of NiFe, NiFeNb, NiFeCr, and a ternary or quaternary alloy thereof, FeAlSi, FeTaC, FeTaN, and a quaternary alloy thereof, and CoFe, CoZrNb, CoZrTa, and a ternary or quaternary alloy thereof.  
      The perpendicular magnetic recording medium may further comprise a perpendicular alignment underlayer between the soft underlayer and the perpendicular magnetic recording layer to improve the crystalline alignment of the perpendicular magnetic recording layer.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other features and advantages of the present invention will become more apparent by describing in detail an exemplary embodiment thereof with reference to the attached drawings in which:  
       FIG. 1  is a sectional view illustrating the structure of a conventional perpendicular magnetic recording medium;  
       FIG. 2  is a sectional view illustrating a perpendicular magnetic recording system using a perpendicular magnetic recording medium;  
       FIG. 3  is a sectional view illustrating the structure of a perpendicular magnetic recording medium according to a first embodiment of the present invention;  
       FIG. 4  is a plane view illustrating a magnetization easy axis of a soft underlayer of the perpendicular magnetic recording medium of  FIG. 3 ;  
       FIG. 5  is a perspective view illustrating a soft underlayer and a perpendicular magnetic recording layer used in simulations;  
       FIG. 6A  is a sectional view illustrating a first example of a soft underlayer, which is formed of a single layer having a small magnetization saturation of 600 emu/cm 3 ;  
       FIG. 6B  is a sectional view illustrating a second example of a soft underlayer, which is formed of a first soft underlayer having a magnetization saturation of 1,000 emu/cm 3  and a second soft underlayer having a saturation magnetization of 600 emu/cm 3 , namely, the saturation magnetization of the second soft underlayer closer to a perpendicular magnetic recording layer is smaller than that of the first soft underlayer;  
       FIG. 6C  is a sectional view illustrating a third example of a soft underlayer, which is formed of a single layer having a large saturation magnetization of 1,000 emu/cm 3 ;  
       FIG. 6D  is a sectional view illustrating a fourth example of a soft underlayer, which is formed of a first soft underlayer having a saturation magnetization of 600 emu/cm 3  and a second soft underlayer having a saturation magnetization of 1,000 emu/cm 3 , namely, the saturation magnetization of the second soft underlayer closer to a perpendicular magnetic recording layer is larger than that of the first soft underlayer;  
       FIG. 7  is a graph illustrating signal-to-noise ratios (SNR) of the first through fourth examples of  FIGS. 6A through 6D  predicted by micromagnetic simulation; and  
       FIGS. 8A through 8D  are graphs illustrating changes in the SNRs of a perpendicular magnetic recording layer only (RL), an uppermost underlayer only (Top SUL), both first and second soft underlayers (SUL(sum)), and both a perpendicular magnetic recording layer and first and second soft underlayers (Total), in the first through fourth examples of  FIGS. 6A through 6D , respectively. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention will now be described more fully with reference to the accompanying drawings, in which an exemplary embodiment of the invention is shown. However, the present invention should not be construed as being limited thereto.  
       FIG. 3  is a sectional view illustrating the structure of a perpendicular magnetic recording medium  50  according to a first embodiment of the present invention.  
      Referring to  FIG. 3 , a perpendicular magnetic recording medium  50  includes a perpendicular magnetic recording layer  57  formed on a substrate  51 , and a soft underlayer  53  formed between the substrate  51  and the perpendicular magnetic recording layer  57 . In addition, the perpendicular magnetic recording medium  50  according to the first embodiment of the present invention may further include a perpendicular alignment underlayer  55  between the soft underlayer  53  and the perpendicular magnetic recording layer  57 . A protection layer  59  for protecting the perpendicular magnetic recording layer  57  from the outside may be formed on the perpendicular magnetic recording layer  57 . In addition, a lubrication layer (not shown) for reducing abrasion of a magnetic head  30  of  FIG. 2  and the protection layer  59  caused by the contact with the magnetic head  30  of  FIG. 2  may be further formed on the protection layer  59 .  
      Information is recorded on the perpendicular magnetic recording layer  57  by arranging the magnetization direction of unit bits, which are recorded by the operation of a writing head  31  of a magnetic head  30  of  FIG. 2 , in a perpendicular direction. Here, the perpendicular magnetic recording layer  57  is formed of a Co-based and/or Fe-based alloy ferromagnetic substance having excellent perpendicular magnetic anisotropy, for example, CoCrPtX (X═Nb, B, Ta, SiOx, O) or an ordered L10 type FePt alloy.  
      The perpendicular alignment underlayer  55 , in other words, an intermediate layer, is formed to improve the crystalline alignment and the magnetic characteristic of the perpendicular magnetic recording layer  57 . The perpendicular alignment underlayer  55  provides magnetic severance from the soft underlayer  53 . The perpendicular alignment underlayer  55  is formed to be as thin as possible.  
      The soft underlayer  53  includes a plurality of soft underlayers having different saturation magnetizations, for example, first and second soft underlayers  53   a  and  53   b.    
      At least one of the first and second soft underlayers  53   a  and  53   b  is formed to have a magnetization easy axis A in a radial direction as shown in  FIG. 4 . In addition, the second soft underlayer  53   b , which is closer to the perpendicular magnetic recording layer  57  than the first soft underlayer  53   a , may be formed to have a smaller thickness than the first soft underlayer  53   a . The soft underlayer  53  including the first and second soft underlayers  53   a  and  53   b  may be formed of a ferromagnetic substance. In another case, the soft underlayer  53  may be formed of the combination of an antiferromagnetic substance and a ferromagnetic substance. That is, the first and second soft underlayers  53   a  and  53   b  may be formed of a ferromagnetic substance on an antiferromagnetic substance, such as FeMn, IrMn, or PtMn.  
      When the first and second soft underlayers  53   a  and  53   b  are formed in a state in which a magnetic field is generated in a radial direction, the first and second soft underlayers  53   a  and  53   b  having an easy axis in the radial direction are obtained. Since the perpendicular magnetic recording medium  53  is manufactured in a circular shape and used in an HDD, the soft underlayer  53  of the perpendicular magnetic recording medium  50  is shown in a circular shape, in  FIG. 4 . Here, the radial direction denotes a central axis direction or an outer diameter direction of the disk shaped perpendicular magnetic recording medium  50 .  
      When the first and second soft underlayers  53   a  and  53   b  are formed to align the easy axis A in the radial direction with sufficient anisotropy field Hk, a domain wall is absent in the first and second soft underlayers  53   a  and  53   b . Thus, a transition noise problem due to the domain wall does not occur.  
      The thickness of the soft underlayer 53 is 200 nm or less, and the thickness of the soft underlayer closer to the perpendicular magnetic recording layer  57 , that is, the second soft underlayer  53   b , is 50 nm or less. The thickness of the second soft underlayer  53   b  is 1 nm or more and 50 nm or less, for example, 10 nm or more and 50 nm or less, and must be less than the thickness of the first soft underlayer  53   a.    
      The soft underlayer  53  may include one or more alloys selected from the group consisting of a NiFe-based alloy, an Fe-based alloy and a Co-based alloy. More specifically, the soft underlayer  53  may include an alloy selected from the group consisting of NiFe, NiFeNb, NiFeCr, and a ternary or quaternary alloy thereof, FeAlSi, FeTaC, FeTaN, and a quaternary alloy thereof, and CoFe, CoZrNb, CoZrTa, and a ternary or quaternary alloy thereof.  
      On the other hand, the second soft underlayer  53   b  may have a larger saturation magnetization than the first soft underlayer  53   a.    
      As described in the following examples, when the saturation magnetization of the second soft underlayer  53   b  is larger than the first soft underlayer  53   a , the signal-to-noise ratio (SNR) is improved. Thus, the perpendicular magnetic recording medium  50  according to the present invention may include a second soft underlayer  53   b  having a larger saturation magnetization than the first soft underlayer  53   a.    
      Even when the saturation magnetization of the second soft underlayer  53   b  is less than that of the first soft underlayer  53   a , the SNR is superior to the case of a single layer soft underlayer  13  of a conventional perpendicular magnetic recording medium  10  of  FIG. 1 . Thus, the perpendicular magnetic recording medium  50  according to the present invention may include a second soft underlayer  53   b  having a lower saturation magnetization than the first soft underlayer  53   a.    
      The perpendicular magnetic recording medium  50  according to the present invention having a plurality of soft underlayers  53   a  and  53   b  having different saturation magnetizations can provide a SNR higher than that of a conventional perpendicular magnetic recording medium.  
       FIG. 5  is a perspective view illustrating a soft underlayer  153  and a perpendicular magnetic recording layer  157  used in simulations. The simulations are performed to examine the effect of the soft underlayer  153  on SNR. Here, the existence of a perpendicular alignment underlayer is ignored.  
      In the simulations, the perpendicular magnetic recording layer  157  is formed of a CoCrPtX material to a thickness of 10 nm and the soft underlayer  153  is formed to a thickness of 90 nm to have a saturation magnetization Ms of 600 and/or 1,000 emu/cm 3 . In addition, a bit pattern B having a width of 100 nm and a length of 30 nm is formed on the perpendicular magnetic recording layer  157 . When the length of a bit is 30 nm, a linear recording density of the bit is 800 kfci (kilo flux reversal per inch).  
      The formation conditions of the perpendicular magnetic recording layer  157  are a saturation magnetization Ms of 550 emu/cm 3 , an axis magnetic anisotropy Ku of 3.5×106 erg/cm 3 , an exchange coupling A* of 0 erg/cm, Δθ of 10°, and α of 0.05.  
      Here, the exchange coupling A* is a constant denoting the interaction among grains in the perpendicular magnetic recording layer  157 , and a smaller exchange coupling value is better.  
      Δθ denotes the tilt amount of the aligned direction of the grains, and a smaller Δθ value is better.  
      α denotes a magnetic damping constant. When a magnetic field is applied, a spin-up or spin-down is carried out through precession. As the α value is reduced, the spin-up or spin-down is carried out at a high speed.  
      The formation conditions of the soft underlayer  153  are a saturation magnetization Ms of 600 and/or 1,000 emu/cm 3 , Hk of 10 Oe, Hex of 0, an easy axis of the Y-axis in  FIG. 5 , and α of 0.05.  
      Here, the Y-axis operating as the easy axis corresponds to a radial direction. In this case, the X-axis corresponds to a track direction. As described above, when the soft underlayer  153  is formed while applying the magnetic field in the radial direction, the easy axis is formed in the radial direction.  
      Hk denotes a field to be applied from the outside in order to align the spin in a magnetization hard axis. As the Hk value increases, a larger magnetic field is required to align the spin from the easy axis to the hard axis.  
      Hex denotes an exchange field, and zero Hex means that an antiferromagnetic substance is not used to form the soft underlayer  153 . The soft underlayer  153  may be formed by arranging a ferromagnetic substance on an antiferromagnetic substance. In this case, the antiferromagnetic substance leads the spin of the ferromagnetic substance in a predetermined direction.  
      The simulations are performed on the four cases shown in  FIGS. 6A through 6D .  
      Referring to  FIG. 6A  of a first example, a soft underlayer  253  is formed as a single layer having a small saturation magnetization Ms of 600 emu/cm 3 .  
      Referring to  FIG. 6B  of a second example, a soft underlayer  353  is formed of a first soft underlayer  353   a  having a saturation magnetization Ms of 1,000 emu/cm 3  and a second soft underlayer  353   b  having a saturation magnetization Ms of 600 emu/cm 3 . That is, the saturation magnetization of the second soft underlayer  353   b  closer to a perpendicular magnetic recording layer  157  is smaller than the saturation magnetization of the first soft underlayer  353   a.    
      Referring to  FIG. 6C  of a third example, a soft underlayer  453  is formed as a single layer having a large saturation magnetization Ms of 1,000 emu/cm 3 .  
      Referring to  FIG. 6D  of a fourth example, a soft underlayer  553  is formed of a first soft underlayer  553   a  having a saturation magnetization Ms of 600 emu/cm 3  and a second soft underlayer  553   b  having a saturation magnetization Ms of 1,000 emu/cm 3 . That is, the saturation magnetization of the second soft underlayer  553   b  closer to a perpendicular magnetic recording layer  157  is larger than the saturation magnetization of the first soft underlayer  553   a.    
       FIG. 7  is a graph illustrating SNRs of the first through fourth examples of  FIGS. 6A through 6D . In the graph of  FIG. 7 , the SNRs of the perpendicular magnetic recording layers only and the SNRs of both the perpendicular magnetic recording layer and the soft underlayers are shown.  
       FIGS. 8A through 8D  are graphs illustrating changes in the SNRs of a perpendicular magnetic recording layer only RL, first and second soft underlayers SUL(sum), and both a perpendicular magnetic recording layer and first and second soft underlayers Total, in the first through fourth examples of  FIGS. 6A through 6D , respectively. The X-axis in  FIGS. 8A through 8D  is the same axis as the X-axis of  FIG. 5 , which denotes a track direction of recording magnetic information. The Y-axis in  FIGS. 8A through 8D  denotes the signals generated from the bits, which are recorded by moving a reading head to the X-axis of  FIG. 5 . More specifically, the reading head has a fixed position, but the recording medium is rotated.  
      As shown in the graphs of  FIGS. 7 and 8 A through  8 D, the SNRs of the perpendicular magnetic recording layers are the same in the first through fourth examples. However, the SNRs of both the perpendicular magnetic recording layers and the soft underlayers vary among the first through fourth examples.  
      When the soft underlayer is formed as a single layer, the SNR may be deteriorated compared to the SNR of the perpendicular magnetic recording layer only, as in the cases of the first and third examples. However, when the soft underlayer is formed as a double-layer having different saturation magnetizations, the SNR is improved compared to the SNR of the perpendicular magnetic recording layer only, as in the cases of the second and fourth examples. More specifically, when the saturation magnetization of the second soft underlayer closer to the perpendicular magnetic recording layer is larger than the saturation magnetization of the first soft underlayer as in the case of the fourth example, the SNR is increasingly improved.  
      Thus, since a perpendicular magnetic recording layer according to the present invention includes a soft underlayer formed of a first and second soft underlayers having different saturation magnetizations, the SNR is improved.  
      In addition, the soft underlayer is formed to have an easy axis in a radial direction, thus transition noise is increasingly improved.  
      While the present invention has been particularly shown and described with reference to an exemplary embodiment thereof, it will be understood by those of ordinary skill in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.