Abstract:
Magnetic recording media having a biased soft magnetic underlayer are disclosed. The soft magnetic underlayer includes at least two soft magnetic layers separated by a thin non-magnetic layer. Magnetostatic coupling between the soft magnetic layers causes the magnetizations in the layer to be anti-parallel. The biased soft magnetic underlayer is brought into a substantially single-domain state, thereby reducing or eliminating unwanted noise in the soft underlayer. In a preferred embodiment, the recording media includes a perpendicular magnetic recording disk.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/180,415 filed Feb. 4, 2000 and No. 60/249,072 filed Nov. 14, 2000. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to perpendicular magnetic recording media, and more particularly relates to media in which noise is suppressed through the use of a laminated soft magnetic underlayer. The lamination controls the magnetic domain structure of the soft underlayer. 
     BACKGROUND INFORMATION 
     Perpendicular magnetic recording systems have been developed for use in computer hard disk drives. An approach to perpendicular magnetic recording requires the use of recording media with a magnetically soft underlayer which provides a flux path from the trailing write pole to the leading return pole of the writer. The soft underlayer enables substantially stronger fields than can be generated with a ring head in conventional longitudinal recording systems. The soft underlayer also provides sharper field gradients which enable writing on high coercivity media. In addition, the soft underlayer helps during the read operation. During the read back process, the soft underlayer produces the image of magnetic charges in the magnetically hard layer, effectively increasing the magnetic flux coming from the media. This provides a higher playback signal. 
     One of the challenges of implementing perpendicular recording is to resolve the problem of soft underlayer noise. The noise is caused by fringing fields generated by magnetic domains, or uncompensated magnetic charges, 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 magnetic 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 magnetic recording media having a laminated soft magnetic underlayer which includes multiple layers that generate opposed magnetic fields in the soft magnetic underlayer. The laminated soft magnetic underlayer may be brought into a substantially single-domain state by the magnetic fields. Reducing or eliminating multiple domains addresses the noise problem noted above. In one embodiment, the magnetization is aligned radially in a direction substantially perpendicular to the recording tracks of a disk in order to reduce or eliminate local domain walls in the soft underlayer. 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 may 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 hard magnetic recording layer and a soft magnetic underlayer. The underlayer comprises means for generating a noise-reducing magnetic field in the soft magnetic underlayer. 
     Another aspect of the present invention is to provide a laminated soft magnetic underlayer of a perpendicular magnetic recording medium. The laminated soft magnetic underlayer comprises first and second layers of magnetically soft material separated by a non-magnetic spacer layer. 
     A further aspect of the present invention is to provide a method of making a magnetically soft underlayer of a perpendicular magnetic recording disk. The method includes the steps of depositing a first magnetically soft layer on a substrate disk, depositing a non-magnetic spacer layer on the first magnetically soft layer, and depositing a second magnetically soft layer on the non-magnetic spacer layer. The arrangement of the magnetically soft layers and the non-magnetic spacer layer generates opposed magnetic fields 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 sectional perspective view of a laminated soft magnetic underlayer of a perpendicular recording disk, illustrating opposed radial magnetic fields that are generated through the magnetically soft layers of the underlayer in accordance with an embodiment of the present invention. 
     FIG. 3 is a partially schematic top view of a laminated magnetically soft underlayer of a perpendicular recording disk, illustrating concentric rings of magnetically soft material separated by non-magnetic bands in accordance with an embodiment of the present invention. 
     FIG. 4 is a partially schematic side sectional view of a laminated magnetically soft underlayer of a perpendicular recording disk, illustrating concentric rings of magnetically soft layers separated by non-magnetic bands in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention provides a laminated soft magnetic underlayer of a magnetic recording media. The laminated soft magnetic underlayer is characterized by an opposed magnetic field which effectively reduces or eliminates noise from the underlayer. For example, the laminated soft magnetic underlayer of a perpendicular magnetic recording disk may be radially biased. In one embodiment, two thin films of a soft magnetic material are separated by a thin non-magnetic spacer layer. If the thickness of the non-magnetic spacer layer is sufficient to exchange decouple the two magnetic layers due to magnetostatic coupling, the magnetizations in the two films will be aligned anti-parallel to minimize the magnetostatic energy. The soft underlayer may be deposited, for example, on a disk substrate as two soft magnetic layers separated by a thin non-magnetic spacer layer. To minimize the magnetostatic energy, the magnetization in the top and in the bottom layers will have opposite directions, i.e., the soft underlayer will be biased radially. 
     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 . 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, L 10  phases of CoPt, FePt, CoPd and FePd and hcp Co alloys, with such multilayers and L 10  phases being typical 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 . An electrically conductive magnetization coil  25  surrounds a yoke of the main pole  22 . During recording operations, current applied to the magnetization coil  25  induces a magnetic flux which 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 . 
     FIG. 2 is a partially schematic sectional perspective view of a laminated soft magnetic underlayer  14  of a perpendicular magnetic recording disk. For purposes of clarity, the thicknesses of the layers of the magnetic underlayer  14  are not drawn to scale. The laminated soft magnetic underlayer  14  includes a first magnetically soft layer  30  and a second magnetically soft layer  32 . A thin non-magnetic spacer layer  34  separates the first and second magnetically soft layers  30  and  32 . The non-magnetic spacer layer  34  exchange decouples the first and second soft magnetic layers  30  and  32 . As a result of magnetostatic coupling, magnetizations M 1  and M 2  in the first and second magnetically soft layers  30  and  32  are aligned anti-parallel. Although two soft magnetic layers  30  and  32  are shown in FIG. 2, additional soft magnetic layers separated by additional non-magnetic spacer layers may be used. Thus, from two to twenty or more soft magnetic layers may be provided in the laminated soft underlayer. 
     Suitable soft magnetic materials for the first and second magnetically soft layers  30  and  32  of the underlayer  14  include CoFe and alloys thereof, FeAlN, NiFe, CoZrNb, CoNiFe, FeTaN and alloys thereof, with FeAlN, NiFe, CoFe and alloys thereof being typical soft materials. The layers  30  and  32  may be made of the same or different materials. The thickness of each layer  30  and  32  may range from about 5 to 100 nm, typically from about 10 to 50 nm. 
     Suitable materials for the non-magnetic spacer layer  34  include oxides such as SiO 2  and MgO, nitrides such as BN and CN, carbides such as B 4 C and SiC, and metals and alloys such as Ta, Cu, Au and the like. The thickness of the non-magnetic spacer layer  34  may range from about 0.5 to 20 nm, typically from about 1 to 10 nm. 
     Since the aspect ratio of the disk radial size to the film thicknesses is very high, dividing the soft underlayer into a series of concentric bands may improve the effectiveness of the biasing. A partially schematic top view of a laminated soft magnetic underlayer  14  of a perpendicular magnetic recording disk in accordance with an embodiment of the present invention is shown in FIG. 3. A partially schematic side sectional view of the laminated soft underlayer  14  is shown in FIG.  4 . The second magnetically soft layer includes multiple concentric rings  32   a ,  32   b  and  32   c . As shown most clearly in FIG. 4, the first magnetically soft layer includes multiple concentric rings  30   a ,  30   b  and  30   c . The non-magnetic spacer layer  34  is also divided into concentric rings. Concentric non-magnetic bands  36   a  and  36   b  separate the rings of the first and second magnetically soft layers. Thus, the non-magnetic band  36   a  separates the rings  30   a  and  30   b  of the first magnetically soft layer, and also separates the rings  32   a  and  32   b  of the second magnetically soft layer. The non-magnetic band  36   b  separates the rings  30   b  and  30   c  of the first magnetically soft layer, and also separates the rings  32   b  and  32   c  of the second magnetically soft layer. 
     As shown in FIG. 4, the non-magnetic separator layer  34  is applied on, and may be coextensive with, the rings  30   a ,  30   b  and  30   c  of the first magnetically soft layer. Similarly, the rings  32   a ,  32   b  and  32   c  of the second magnetically soft layer are deposited on, and may be coextensive with, the non-magnetic layer  34 . 
     As shown in FIG. 4, each of the non-magnetic bands  36   a  and  36   b  has a radial width T B , while the non-magnetic spacer layer  34  has a thickness T S  measured in the axial direction of the soft magnetic underlayer  14 . The radial width T B  of the bands  36   a  and  36   b  may be substantially larger than the thickness T S  of the non-magnetic separator layer  34 . For example, the ratio of T B :T S  may be greater than 2:1, typically greater than 10:1. 
     As shown in FIG. 4, the magnetization M l  of the concentric rings  30   a ,  30   b  and  30   c  of the first magnetically soft layer is anti-parallel to the magnetization M 2  of the concentric rings  32   a ,  32   b  and  32   c  of the second magnetically soft layer. Thus, an opposed radial magnetic field is generated in the laminated soft magnetic underlayer  14 . In accordance with an embodiment of the present invention, magnetic fringing fields M F  may be generated between the adjacent concentric rings  32   a ,  32   b  and  32   c  due to the presence of the non-magnetic bands  36   a  and  36   b . The fringing fields M F  may optionally be used for servo tracking purposes during recording operations. For example, such a subdivision of the soft underlayer can be utilized in servo designs where the read-element may sense the narrow regions that separate the soft underlayer bands, e.g., by sensing the fringing fields M F . 
     The size, shape and magnetic characteristics of the concentric magnetic rings  30   a-c  and  32   a-c  and non-magnetic bands  36   a  and  36   b  shown in FIGS. 3 and 4 may be selected as necessary in order to provide a sufficient noise-reducing magnetic field radially through the soft underlayer. For example, the radial width of each magnetically soft ring  30   a-c  and  32   a-c  may typically range from several microns to several millimeters. The radial width T B  of each non-magnetic band  36   a  and  36   b  typically ranges from about 0.5 to about 10 micron. 
     Although continuous concentric circular magnetically soft rings  30   a-c  and  32   a-c  and non-magnetic bands  36   a  and  36   b  are shown in FIGS. 3 and 4, other geometries may be used as long as a sufficient magnetic field is generated in the soft underlayer. For example, the rings could be broken into sectors, or other types of patterned media may be provided. Although three sets of concentric magnetically soft rings  30   a-c  and  32   a-c  are shown in FIGS. 3 and 4, any suitable number of rings may be used, e.g., one, two, three, four, etc. 
     The first and second magnetically soft underlayers  30   a-c  and  32   a-c  may be anisotropic, for example, with their easy axes aligned along the radius of the disk. The magnetostatic coupling fields M 1 , M 2  generated through each layer  30   a-c  and  32   a-c  may be in excess of 5 Oe, for example, in excess of 50 Oe. 
     To achieve radial anisotropy in each magnetically soft layer  30   a-c  and  32   a-c , 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 layer films are deposited on an appropriate underlayer that would induce radially aligned stress in the soft layer films. Post-deposition annealing of the layers in a radially aligned magnetic field may also be used. 
     In accordance with an embodiment of the present invention, radially aligned magnetization 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 faster and less noisy. 
     The present recording media may be manufactured using conventional media tools. The structures of the disk may be of macroscopic sizes and may not require complicated lithography. Deposition of the soft magnetic underlayer features on a disk substrate may be accomplished by first depositing the non-magnetic bands by, e.g., standard lithography techniques, followed by sequential sputter deposition of the layers  30   a-c ,  34  and  32   a-c , and then chemical mechanical polishing. Alternatively, the layers  30   a-c ,  34  and  32   a-c  may be deposited sequentially by, e.g., sputtering, followed by removal of the band areas. The band areas may optionally be filled with non-magnetic material by sputtering or the like. Standard techniques for making patterned media, such as stamping, may also be used. 
     The following example is intended to illustrate various aspects of the present invention, and is not intended to limit the scope of the invention. 
     EXAMPLE 
     Perpendicular media were fabricated with identical Co/Pd multilayer recording layers with either a single layer FeAlN soft underlayer or a laminated soft underlayer comprising two soft magnetic layers separated by thin non-magnetic layer (FeAlN/Ta/NiFe). Auto-correlation signal to noise ratio (SNR) using pseudo-random sequence of bits covering a spectral range from up to 200 kfci was measured for both media with laminated and single layer soft underlayer. An increase in SNR of 4 dB was observed in going from the single layer soft underlayer media (˜12 dB) to the FeAlN/Ta/NiFe laminated soft underlayer media (˜16 dB). Since the recording layers in both cases were identical and the soft underlayer noise usually manifests itself at lower recording densities (up to ˜100 kfci), the boost in SNR is attributed to the reduction of soft underlayer noise by the laminated underlayer. 
     The present invention substantially suppresses soft underlayer noise, thus resolving one of the major problems in implementing perpendicular magnetic recording systems. In addition, the dynamic properties of the soft underlayer are improved. The biasing scheme is based on spontaneous alignment of the magnetization in the soft underlayer due to magnetostatic interaction between the magnetic layers separated by the thin non-magnetic spacer layer. In addition, the multi-band feature of the suggested soft underlayer design can be utilized in servo systems. 
     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.