Abstract:
A perpendicular magnetic recording media with a patterned soft magnetic underlayer is disclosed. The recording media may be a disk including a substrate, a patterned underlayer, and a magnetically hard recording layer. The underlayer may be provided in the form of concentric bands having widths less than the track widths of the recording layer. The patterned underlayer reduces or eliminates adjacent track interference.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application Serial No. 60/221,500 filed Jul. 27, 2000. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to magnetic recording media, and more particularly relates to perpendicular magnetic recording media having a patterned soft magnetic underlayer that reduces intersymbol and adjacent track interference. 
     BACKGROUND INFORMATION 
     Perpendicular magnetic recording systems have been developed for use in computer hard disk drives. A typical perpendicular recording head includes a trailing write pole, a leading return or opposing pole magnetically coupled to the write pole, and an electrically conductive magnetizing coil surrounding the yoke of the write pole. The bottom of the opposing pole has a surface area greatly exceeding the surface area of the tip of the write pole. 
     Conventional perpendicular recording media typically include a hard magnetic recording layer and a soft magnetic underlayer which provide a flux path from the trailing write pole to the leading opposing pole of the writer. To write to the magnetic recording media, the recording head is separated from the magnetic recording media by a distance known as the flying height. The magnetic recording media is moved past the recording head so that the recording head follows the tracks of the magnetic recording media, with the magnetic recording media first passing under the opposing pole and then passing under the write pole. Current is passed through the coil to create magnetic flux within the write pole. The magnetic flux passes from the write pole tip, through the hard magnetic recording track, into the soft underlayer, and across to the opposing pole. 
     Perpendicular recording designs have the potential to support much higher linear densities than conventional longitudinal designs, especially when a bilayer perpendicular media with a soft magnetic underlayer is used. Magnetization transitions on the bilayer recording disk are recorded by a trailing edge of the trailing pole and reproduce the shape of the trailing pole projection on the media plane. However, due to the change in skew angle as the recording head travels in an arc across the disk and possible misalignment of the write pole, this can result in unwanted side writing when a rectangular shaped write pole is used. 
     The underlayer used in perpendicular magnetic recording causes relative long-range intersymbol interference, both along the track and from track to track. The intersymbol interference effects are caused by a data dependent changes in the length of the magnetic return path from the reader through the underlayer and back through a much larger return pole area. For example, if the vertical domain orientation of the previously recorded data bits and the data bits on the adjacent tracks are all in the same direction as the data bit being read, then the surrounding domains may impede the flow of flux through the larger return pole. In this case, the sensed field by the reader would be lower in amplitude. This intersymbol and adjacent track interference may be a problem with traditional high density PRML recording codes and hence should be controlled. 
     One way to minimize the return path effect is to use a DC balanced code in which the number of up-pointing domains and the number of down-pointing domains are equal over some short distance such as 32 bits. This type of code is customarily used in CD recording. An example is the eight of fourteen modulation (EFM) code which uses 14 bits to encode a byte of data and another 3 bits to cause the code to be DC balanced. Such a code would make the magnetic data DC balanced over a 1 micron circle if the channel bit length were say 60 nm. The problem with DC balanced codes is that they result in a lower effective bit density as compared with PRML codes. 
     Unfortunately, DC balancing in the radial direction cannot adequately be addressed. The encoding can be DC balanced within a data record. However, the adjacent track spacing, the location of the gaps between data records, and the old information between tracks and in the gaps are not well controlled. For example, there is little side erasing in perpendicular recording. Hence, if the 6-sigma TMR is 30 percent of the track pitch, there will be large areas of old information that will cause the readback signals to be distorted even though the read back head does not pass directly over this data. 
     If the recording films are removed or made magnetically dead, e.g., by ion implantation between the tracks, the old information between tracks will be reduced. Further, if the recording films are removed or made dead where the inter-record gaps will occur, the old information interference from the gap will be reduced. The problem with removing the films is that one is left with a surface topology that may affect the ability of the recording head to fly on the surface. These known options require processing of an otherwise finished disk in order to apply the pattern. Therefore, there is a risk that the magnetic properties of the recording films or the flyability of the surface will be adversely affected. 
     A need therefore exists to pattern the substrate before the films are added in such a way that neither the magnetic properties of the recording films or the flyability of the surface are adversely affected. The present invention has been developed in view of the foregoing. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention is to provide a perpendicular magnetic recording media including means for reducing intersymbol interference between adjacent tracks of a magnetically hard recording layer of the recording media. 
     Another aspect of the present invention is to provide a perpendicular magnetic recording media comprising a substrate, a patterned magnetically soft underlayer on the substrate, and a magnetically hard recording layer over the patterned underlayer, wherein the patterned underlayer comprises discontinuities between adjacent tracks of the recording layer. 
     A further aspect of the present invention is to provide a method of making a perpendicular magnetic recording media. The method comprises the steps of providing a substrate, providing a patterned magnetically soft underlayer on the substrate, and depositing a magnetically hard recording layer over the patterned underlayer, wherein the patterned underlayer comprises discontinuities between adjacent tracks of the recording layer. 
     These and other aspects of the present invention will be more apparent from the following description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top view of a perpendicular magnetic recording disk which includes a patterned soft magnetic underlayer in accordance with an embodiment of the present invention. 
     FIG. 2 is a partially schematic side sectional view of the perpendicular magnetic recording disk taken through section  2 — 2  of FIG.  1 . 
     FIGS. 3 a - 3   h  are side sectional views of a portion of a magnetic recording disk, illustrating fabrication steps for making a patterned soft magnetic underlayer in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a top view and FIG. 2 is a side sectional view of a magnetic recording disk  10  in accordance with an embodiment of the present invention. For purposes of illustration, the components of the magnetic recording disk  10  shown in FIG. 2 are not drawn to scale. The magnetic recording disk  10  includes a substrate  12  which may be made of any suitable material such as ceramic glass, amorphous glass, aluminum or NiP plated AlMg. 
     As shown in FIG. 2, patterned magnetically soft underlayer  14  is deposited in grooves on the surface of the substrate  12 , thereby forming concentric bands of the underlayer material. The bands may be continuous or discontinuous. Suitable soft magnetic materials for the patterned underlayer  14  include CoFe and alloys thereof, FeAlN, NiFe, CoZrNb and FeTaN, with CoFe and FeAlN being typical soft materials. A magnetically hard recording layer  16  is deposited over the patterned soft underlayer  14 . Suitable hard magnetic materials for the recording layer  16  include multi-layers of Co/Pd or Co/Pt, L 10  phases of CoPt, FePt, CoPd and FePd and hcp Co alloys, with such multi-layers and L 10  phases being typical hard materials. The recording layer  16  and soft underlayer  14  may have any desired thickness, e.g., from 5 to 50 nanometers. A thin protective overcoat  18  such as diamond-like carbon may be applied over the recording layer  16 . 
     FIGS. 3 a - 3   h  are side sectional views illustrating various fabrication steps for making a perpendicular magnetic recording media with a patterned soft underlayer in accordance with an embodiment of the present invention. 
     As shown in FIG. 3 a , a standard resist layer  20  is deposited over the substrate  12 . A stamper  22  is then pressed into the surface of the resist layer  20  in order to transfer a format pattern from the surface of the stamper  20  into the surface of the resist layer  20 , as illustrated in FIG. 3 b . When the stamper  22  is removed, portions of the resist layer are compressed  24 . 
     As shown in FIG. 3 c , the resist layer  20  is then etched using standard techniques until the compressed portions  24  of the resist layer  20  are etched away and the underlying substrate  12  is exposed. The etching process continues and portions of the substrate  12  are removed until a desired etch depth is reached in order to form grooves  26  in the substrate  12 . It is important to make sure there is sufficient amount of resist  20  on the surface of the substrate  12 , so that only the portions of the substrate  12  that should be etched are etched. The etch depth is determined by the desired thickness of the patterned soft underlayer. In this exemplary embodiment, an etch depth of 5 to 50 nanometers is typically used. 
     As shown in FIG. 3 d , a layer of soft magnetic material  30  is deposited by standard deposition techniques on the substrate  12  and photoresist layer  20 , filling the grooves  26 . The photoresist layer  20  is then etched away from the substrate  12 , as shown in FIG. 3 e . During this etching process, the portion of the soft magnetic material  30  covering the photoresist layer  20  is also removed, leaving the portions of the soft magnetic material  30  filling the grooves  26 . 
     As shown in FIG. 3 f , the deposited soft magnetic material  30  has been polished down to the level of the substrate  12 . Standard techniques such as chemical mechanical polishing may be used. 
     In FIG. 3 g , the hard magnetic recording layer  16  is deposited using standard techniques over the substrate  12  and patterned soft underlayer  14 . The protective layer  18  is then deposited on the hard magnetic recording layer  16 . 
     As shown in FIG. 3 h , the hard magnetic recording layer  16  has a track width T W . Each band of the patterned underlayer  14  has a width U W . In accordance with the present invention, the track width T W  is greater than or equal to the underlayer band width U W . For example, the underlayer band width U W may be from about 70 to about 95 percent of the track width T W , typically from about 80 to about 90 percent. For many types of perpendicular magnetic recording media, the track width T W  typically ranges from about 30 to about 200 nanometers. 
     Alternatively, the structure shown in FIG. 3 c  may subsequently be processed as follows. The resist layer  20  is removed and the underlayer  14  is deposited over the etched substrate  12 . After chemical mechanical polishing, the recording layer  16  is deposited over the patterned underlayer  14  to form a structure as shown in FIG. 3 g.    
     In accordance with an embodiment of the present invention, the underlayer is located in the region in the middle of the track, with no underlayer in the region between tracks or in the gaps between the data records. A typical procedure is as follows: 
     1) Apply a thin, uniform coating of resist to the surface of the disk substrate. 
     2) Use either photo lithography or imprint lithography to make a pattern of pits or grooves in the resist. 
     3) Develop the photo resist or lightly etch the imprinted resist to open up holes where the substrate is to be etched. 
     4) Use reactive ion etching to make pits or grooves in the substrate that have the desired depth of the underlayer. In some cases the depth will be greater than the width of the pits or grooves. 
     5) Deposit the desired underlayer material on the substrate before the resist is removed such that the pits are filled or a little overfilled. 
     6) Remove the photo resist by a lift off process which will leave a relatively ragged surface. 
     7) Chemically mechanically polish the disk to remove the underlayer down to the level of the substrate. The result should be a flat substrate and underlayer that meets the required flatness specification. 
     There is an alternate process after step 4, namely remove the resist, deposit the underlayer, and then chemically mechanically polish down to the substrate. However, this process may not be preferred because it could take a long time to remove the unwanted underlayer material. 
     The embodiment shown in FIGS. 2 and 3 thus provides a patterned substrate which provides the pattern for the underlayer. Alternatively, elevated mesas of soft magnetic material may be provided on a substantially flat substrate surface in order to form the patterned underlayer. In either case, the patterned underlayer comprises discontinuities between adjacent tracks of the recording layer. 
     The initial magnetization of the films may be achieved as follows. The patterned underlayer will tend to locate the recorded bit so that they are directly over the underlayer because the recording head will be selectively imaged in the underlayer. If the coercivity of the recording layer were high enough, then the regions between the tracks would not be recorded. They would retain their previously magnetized state. 
     The region between tracks could be DC erased in order to minimize noise. However, the large domain between tracks may cause positive and negative data signal asymmetry when erased in the same direction. A solution to achieve symmetry would be to erase every other band between tracks up and down alternately. Then on the average the readback head would see a zero DC component from the track edges plus the data to be read. One would want to erase the gaps between data records in a similar manner so that the outer half of the gap is magnetized in the opposite direction as the inner half. The recording head may have to have two recording levels, a high level that is capable of erasure when there is no underlayer and a lower level that will not erase or record on the films without an underlayer. It is preferable that either the tip of the recording pole is trimmed or the long pole of the recording head is lined up with the track to prevent side writing of the adjacent track. 
     The intersymbol interference from reading will now be dominated by adjacent bits along the track. Filtering can compensate for this intersymbol interference. The filtering may be done prior to supplying the data to the read back channel. In this case, DC balanced codes may not have to be used. 
     Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims.