Abstract:
The present invention provides perpendicular recording media having a soft magnetic underlayer and magnetic regions which generate an external magnetic field in the soft magnetic underlayer. The soft magnetic underlayer is brought into a substantially single-domain state by the magnetic field, thereby reducing or eliminating unwanted noise in the soft underlayer. In a preferred embodiment, the recording medium includes a ring-shaped soft magnetic underlayer positioned between concentric ring-shaped magnetic regions.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/168,147 filed Nov. 29 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to perpendicular magnetic recording media, and more particularly relates to media designed to suppress soft magnetic underlayer noise. 
     BACKGROUND INFORMATION 
     Perpendicular magnetic recording systems have been developed for use in computer hard disk drives. Some examples of perpendicular magnetic recording heads for use in such systems are described in U.S. Pat. No. 4,438,471 to Oshiki et al., U.S. Pat. No. 4,541,026 to Bonin et al., U.S. Pat. No. 4,546,398 to Toda et al., U.S. Pat. No. 4,575,777 to Hosokawa, U.S. Pat. No. 4,613,918 to Kania et al., U.S. Pat. No. 4,649,449 to Sawada et al, U.S. Pat. No. 4,731,157 to Lazzari, and U.S. Pat. No. 4,974,110 to Kanamine et al. 
     Some examples of perpendicular magnetic recording media are described in U.S. Pat. No. 4,410,603 to Yamamori et al., U.S. Pat. No. 4,629,660 to Sagoi et al., U.S. Pat. No. 5,738,927 to Nakamura et al., and U.S. Pat. No. 5,942,342 to Hikosaka et al. 
     One of the challenges to implement perpendicular recording is to resolve the problem of soft underlayer noise. The noise is caused by fringing fields generated by magnetic domains in the soft underlayer that can be sensed by the reader. For the write process to be efficient, high moment materials, e.g., B S &gt;20 kG, may be used for the soft underlayer. If the domain distribution of such materials is not carefully controlled, very large fringing fields can introduce substantial amounts of noise in the read element. Not only can the reader sense the steady-state distribution of magnetization in the soft underlayer, but it can also affect the distribution of magnetization in the soft underlayer, thus generating time-dependent noise. Both types of noise should be minimized. 
     The present invention has been developed in view of the foregoing, and to address other deficiencies of the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention provides perpendicular recording media having a soft magnetic underlayer and magnetic regions which generate an external magnetic field in the soft magnetic underlayer. The soft magnetic underlayer is brought into a substantially single-domain state by the magnetic field. Reducing or eliminating multiple domains addresses the noise problem noted above. In a preferred embodiment, the magnetization in such a single-domain state is aligned radially without local domain walls. It is noted that a “single-domain” state is an approximation, which applies to materials without any magnetic defects. In actual magnetic films, the film will be magnetically saturated in accordance with the present invention in order to sufficiently reduce the number of domain walls, thus suppressing soft underlayer noise. 
     An aspect of the present invention is to provide a perpendicular magnetic recording medium including a soft magnetic underlayer and means for generating a magnetic field in the soft magnetic underlayer. 
     Another aspect of the present invention is to provide a perpendicular magnetic recording medium which includes a soft magnetic underlayer, a hard magnetic recording layer over the soft magnetic underlayer, and at least one magnetic region which generates a magnetic field in the soft magnetic underlayer. 
     A further aspect of the present invention is to provide a method of making a perpendicular magnetic recording medium. The method includes the steps of providing at least one magnetic region on a substrate disk, and providing a soft magnetic underlayer and a hard magnetic recording layer on the substrate disk in proximity to the at least one magnetic region. The magnetic region generates a magnetic field in the soft magnetic underlayer. 
     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 partially schematic side view of a perpendicular recording head and a perpendicular recording medium which may incorporate a reduced-noise soft magnetic underlayer in accordance with an embodiment of the present invention. 
     FIG. 2 is a partially schematic top view of a perpendicular recording medium illustrating a soft magnetic underlayer and hard magnetic regions which generate a radial magnetic field in the soft underlayer in accordance with an embodiment of the present invention. 
     FIG. 3 is a partially schematic radial section view of a perpendicular recording medium including hard magnetic regions which generate a radial magnetic field in the soft underlayer in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a partially schematic side sectional view of a perpendicular magnetic recording medium  10 . The medium  10  includes a substrate  12 , which may be made of any suitable material such as ceramic glass, amorphous glass or NiP plated AlMg. A magnetically soft underlayer  14  is deposited on the substrate  12 . Suitable soft magnetic materials for the underlayer  14  include CoFe and alloys thereof, FeAlN, NiFe, CoZrNb and FeTaN, with CoFe and FeAlN being preferred soft materials. A magnetically hard recording layer  16  is deposited on the soft underlayer  14 . Suitable hard magnetic materials for the recording layer  16  include multilayers of Co/Pd or Co/Pt, L10 phases of CoPt, FePt, CoPd and FePd and hcp Co alloys, with such multilayers and L10 phases being preferred hard materials. A protective overcoat  18  such as diamond-like carbon may be applied over the recording layer  16 . 
     FIG. 1 also illustrates a perpendicular recording head  20  positioned above the magnetic recording medium  10 . The recording head  20  includes a main pole  22  and an opposing pole  24 . During recording operations, magnetic flux is directed from the main pole  22  perpendicularly through the recording layer  16 , then in the plane of the soft underlayer  14  back to the opposing pole  24 . 
     A partially schematic top view of the magnetic recording medium  10  is shown in FIG.  2 . The ring-shaped soft magnetic underlayer  14  is positioned between an inner magnetic ring-shaped band  26  and an outer magnetic ring-shaped band  28 . The inner and outer magnetic bands  26  and  28  may be made of any suitable magnetic material such as hcp Co alloys (e.g., CoPt 12 Cr 13  with M S ˜600 emu/cc and 4πM S ˜7.5 kG), L10 phases of FePt, CoPt, FePd or CoPd (e.g., with M S ˜1,200 emu/cc and 4πM S ˜15 kG), L10 alloys such as FePtB, and rare earth magnetic materials such as NdFeB (e.g., with 4πM S ˜14 kG) and SmCo (e.g., with 4πM S ˜12 kG), with such hcp Co alloys L10 phases being preferred materials. The size, shape and magnetic characteristics of the magnetic bands  26  and  28  may be selected as necessary in order to provide a sufficient magnetic field radially through the soft underlayer  14 . For example, the radial width of each magnetic band  26  and  28  may typically range from about 0.1 to about 100 mm, and the thickness of each magnetic band  26  and  28  may typically range from about 0.1 to about 50 microns. The radial width of the soft underlayer  14  typically ranges from about 5 to about 100 mm. 
     Although continuous concentric circular magnetic bands  26  and  28  are shown in FIG. 2, other geometries may be used as long as a sufficient magnetic field is generated in the soft underlayer  14 . For example, discontinuous ring-shaped bands may be used, e.g., the bands may have gaps around their circumferences. Furthermore, non-circular bands may be used, e.g., square, octagonal, etc. Alternatively, multiple discrete magnetic elements may be arranged in a desired pattern. Although two concentric bands  26  and  28  are shown in FIG. 2, any suitable number of bands may be used, e.g., one, two, three, four, etc. The magnetic bands  26  and  28  may be deposited on a disk substrate in the presence of radial magnetic field. Deposition in a radial magnetic field causes net remanent magnetization in the magnetic bands to be aligned radially, which, in turn, creates a radially distributed magnetic field in the plane of the disk substrate between the bands. 
     FIG. 3 is a partially schematic side sectional view of the magnetic recording medium  10  in accordance with an embodiment of the invention. Although a single-sided disk is shown in FIG. 3, double-sided media may alternatively be used. The soft underlayer  14  is located between the magnetic bands  26  and  28 . Preferably, the soft underlayer  14  and at least a portion of the magnetic bands are located in the same plane, as shown in FIG.  3 . The thicknesses of the soft underlayer  14  and the magnetic bands  26  and  28  may be different, as shown in FIG. 3, or their thicknesses may be the same. 
     As shown in FIG.  3 . The magnetic recording layer  16  is applied on, and is preferably coextensive with, the soft underlayer  14 . The protective coating  18  is applied over the recording layer  16  and the magnetic bands  26  and  28 . 
     The soft underlayer  14  preferably has radial anisotropy with the easy axis aligned along the radius of the disk and a coercivity smaller than the minimum radial field induced by the magnetic bands  26  and  28 . The magnetic bands  26  and  28  typically generate fields in excess of 10 Oe, more preferably in excess of 50 or 60 Oe. 
     To make a soft underlayer with built-in radial anisotropy several approaches can be used. Deposition in an external radial magnetic field (field induced anisotropy) may be used. Magnetostriction may be used if the soft underlayer film is deposited on an appropriate underlayer that would induce radially aligned stress in the soft underlayer film. Post-deposition annealing of the soft underlayer in radially aligned magnetic field may also be used. 
     If the coercivity of the soft underlayer material  14  is smaller than the fields generated by the concentric magnetic bands  26  and  28 , the entire soft underlayer  14  will be saturated radially in the direction of the applied field. Radially aligned magnetization also improves dynamic properties of the soft underlayer and reduces Barkhausen noise since the magnetization switching during the write process inside the soft underlayer will follow magnetization rotation rather than domain wall motion, which is known to be a faster and less noisy process. 
     The present recording media may be manufactured using conventional media tools. All of the structures of the disk are of macroscopic sizes and do not require complicated lithography as, for example, patterned servo technologies or patterned media. Deposition of the magnetic features on a disk substrate can be done directly utilizing shadow masks placed in proximity of the substrate. For example, the magnetic bands  26  and  28  may first be deposited using a shadow mask. Next, another shadow mask may be used to deposit the soft underlayer  14  and the recording layer  16 . After the second shadow mask is removed, the protective overcoat  18  may be deposited over the recording layer  16  and magnetic bands  26  and  28 . 
     Both boundary element modeling and analytical calculations show that fields with the magnitudes of about 6 to 60 Oe or higher can be achieved in accordance with the present invention, for example, with band separation of 2 cm, hard magnetic material thicknesses of 1-10 μm, and 4πM S  of about 14 kG for NdFeB (4πM S ˜12 kG for SmCo). If stronger fields are necessary, thicker bands can be deposited. For example, if an hcp Co alloy is used, e.g., CoPtCr with M S ˜600 emu/cc, 4πM S ˜7.5 kG, the thickness of the bands may be increased (almost doubled in the case of CoPt 12 Cr 13  alloy) in order to achieve fields comparable to the fields generated with NdFeB or SmCo. 
     The magnetic field from a single band may be expressed as:          H   ∼     4                 π                     M   S     ·     δ        [       1   r     -     1     r   +   w         ]             ,                          
     where δ is the thickness of the magnetic band, w is the radial width of the band, and r is the radial distance away from the edge of the band. For a band made of NdFeB with 4πM S ˜14 kG, thickness α of 10 μm, r=1 cm, and w=0.3 cm, the field H is equal to about 32 Oe. Provided that there is a second magnetic band, e.g., as shown in FIG. 2, the magnitude of the achievable field doubles to approximately 64 Oe. 
     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.