Magnetic recording media having CrMo underlayers

A magnetic recording media having a CrMo underlayer provides improved performance characteristics. In one embodiment, the recording media comprises a rigid substrate and an underlayer disposed over the substrate, in which the underlayer comprises CrMo. Preferably, the Mo crystals in the CrMo underlayer are at least about 140 .ANG. in the film growth direction for the 002 crystal plane. Advantageously, recording media with such a 002 crystal size, when used in conjunction with a magnetic layer, such as CoCrTaPtNi, exhibits significantly higher parametric qualities than underlayers using other materials. Typically, Mo in the range between about 7% and 16% in the CrMo alloy, more preferably between about 9% and 11%, will provide the recited 002 crystal size.

BACKGROUND OF THE INVENTION
 The present invention relates generally to magnetic recording media, and
 more particularly to high density magnetic recording disks having improved
 recording characteristics.
 Thin film magnetic recording disks generally comprise a disk substrate
 having a magnetic layer and a number of underlayers and overlayers
 deposited thereon. The nature and composition of each layer is selected to
 provide the desired magnetic recording characteristics, as generally
 recognized in the industry. An exemplary present day thin film disk is
 illustrated in FIG. 1 and comprises a non-magnetic disk substrate 10,
 typically composed of aluminum or an aluminum alloy. An amorphous
 nickel-phosphorus (NiP) layer 12 is formed over each surface of the disk
 substrate 10, typically by plating. The NiP layer is hard, and imparts
 rigidity to the aluminum substrate. A second underlayer in the form of a
 chromium ground layer 14 is formed over the NiP layer 12, typically by
 sputtering, and a magnetic layer 16 is formed over the ground layer 14.
 The magnetic layer 16 comprises a thin film of a ferromagnetic material,
 such as a magnetic oxide or magnetic alloy. Usually, a protective layer
 18, such as carbon film, is formed over the magnetic layer 16, and a
 lubricating layer 20 is formed over the protective layer.
 The presence of the NiP layer 12 and the chromium ground layer 14 have been
 found to improve the recording characteristics of the magnetic layer 16.
 In particular, a chromium ground layer formed over a NiP layer has been
 found to provide enhanced coercivity and reduced noise characteristics.
 Additionally, the NiP layer is often mechanically textured to create a
 roughened surface prior to formation of the chromium ground layer. This
 surface texturing has a substantial effect on the mechanical properties of
 the disk and its interaction with the recording transducer (read/write
 head), which typically "flies" over the disk surface on a cushion of air
 that is moved by the rotating disk. In particular, texturizing is highly
 beneficial to the magnetic recording system's ability to reliably
 withstand repeated starting and stopping of the disk, with its associated
 repeated contact between the read/write head and the disk's surface. The
 texturing may be circumferential, crosswise, or separated into start/stop
 and data zones, with the preferred geometry depending on the particular
 composition of the cobalt-containing magnetic layer, and on the specific
 disk drive design.
 Such magnetic recording constructions have been very successful and allow
 for relatively high recording densities. As with all successes, however,
 it is desired to provide magnetic recording disks having even higher
 recording densities. To increase recording densities beyond those of known
 practical magnetic recording media, it would be beneficial to promote
 certain types of crystal growth in the magnetic recording layer within the
 structure of the magnetic recording media.
 For this reason, it is desirable to have improved recording media having
 underlayers made of specific materials that promote improved performance
 in the magnetic layer of the recording media. It would be particularly
 desirable if such improved magnetic recording media were readily
 fabricated using existing thin film deposition and texturing equipment. It
 is also desirable if the underlayer further enhances the magnetic
 properties of recent cobalt-containing magnetic layers.
 SUMMARY OF THE INVENTION
 The present invention is directed to improved magnetic recording media. The
 underlayer used in the structure of magnetic recording media can effect
 the epitaxial crystal growth in the overlying magnetic recording layer,
 and the underlayer can be chosen to increase certain desired
 characteristics in the bulk magnetics and parametrics of the recording
 media. Specifically, certain materials in the underlayer promote a smaller
 grain size and a more equally spaced crystal distribution in the magnetic
 layer which generally increases the performance of the magnetic layer.
 The present invention provides magnetic recording media comprising a rigid
 substrate and an underlayer disposed over the substrate, in which the
 underlayer comprises a chromium molybdenum (CrMo) alloy. A magnetic layer
 is disposed over this underlayer, and is also disposed over a texturized
 surface. Generally, the substrate comprises aluminum, and an NiP layer is
 disposed over the substrate and below the underlayer. Preferably, the
 crystals in the CrMo underlayer are oriented with the (002) crystal plane
 parallel to the surface of the substrate. Additionally, the (002) crystal
 size in a film growth direction is at least about 140 .ANG.. The large
 crystal size is believed to facilitate epitaxial growth of the overlying
 magnetic layer. Advantageously, recording media with such a (002) crystal
 orientation, when used with a magnetic layer such as CoCrTaPtNi, exhibit
 significantly higher parametric qualities than underlayers using other
 materials. These improved parametric qualities will allow the recording
 media to have higher recording densities.
 Typically, Mo in the range between about 7% and 16% (atomic percent) in the
 CrMo alloy, more preferably between about 9% and 11%, will provide the
 recited (002) crystal orientation and preferred lattice constant for a
 better epitaxial growth with Co-alloy. Most preferably, underlayers will
 have about 10% Mo by atomic percent (Cr10Mo). It is believed that CrMo
 underlayer defines a surface that promotes evenly distributed epitaxial
 growth of Co crystals in the magnetic layer. The surface may also minimize
 the lateral crystal grain size of Co crystals in the magnetic layer.
 Preferably, the magnetic layer has Co crystals between about 60-150 .ANG.
 in lateral crystal size. These characteristics of the magnetic layer
 improve the performance characteristics of the resulting recording media.
 A further understanding of the nature and advantages of the invention may
 be realized by reference to the remaining portions of the specification
 and the drawings.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS
 Referring now to FIG. 2, magnetic recording media according to the present
 invention will usually be in the form of a magnetic recording disk 30
 comprising an aluminum alloy substrate 32, an NiP layer 34, an underlayer
 36 (generally comprising chromium, titanium, and preferably either copper
 or vanadium) a magnetic layer 38, a protective layer 40, and a lubricating
 layer 42. The various layers will be formed over at least one surface of
 the substrate 32, and preferably over both surfaces of the substrate,
 analogous to the structure illustrated in FIG. 1. A textured surface 44
 may be disposed below magnetic layer 38, typically being imposed on the
 NiP layer 34.
 Substrate 32 comprises a disk having a diameter and thickness suitable for
 formation of a conventional hard magnetic recording media. Typically,
 substrate 32 will be composed of aluminum or an aluminum alloy.
 Alternatively, nonmetallic substrates comprising glass, ceramic, carbon,
 glass-ceramic composites, glass-carbon composites, silicon, silicon
 carbide, and the like, may be used.
 NiP layer 34 will preferably be formed over the surface of the substrate
 using conventional plating techniques, ideally by means of electroless
 plating. The NiP layer will be deposited to a thickness generally in the
 range from about 500 .ANG. to 5000 .ANG.. Alternatively, though not
 necessarily preferably, the NiP layer 34 may be formed using more
 expensive vacuum deposition techniques.
 Once the NiP layer 34 has been formed over the substrate, the NiP layer can
 be mechanically textured in a conventional manner, often by means of tape
 or slurry abrasion. The type of texturing will depend, in part, on the
 nature of the magnetic alloy which is to be applied over NiP layer 34. For
 example, some magnetic alloys, such as cobalt chromium tantalum, benefit
 from circumferential texturing to achieve optimum coercivity and magnetic
 characteristics.
 Other magnetic alloys, including cobalt platinum chromium, are enhanced by
 alternative types of texturing. A particularly advantageous texture and a
 method for its production are described in co-pending U.S. patent
 application Ser. No. 08/503,785, now U.S. Pat. No. 5,798,164, the full
 disclosure of which is incorporated herein by reference. That exemplary
 texture comprises independently optimized texture zones for 1) data, and
 2) read/write recording head contact.
 An underlayer 36 is typically formed over NiP layer 34 to provide a surface
 for facilitating epitaxial growth of certain crystal structures in the
 magnetic layer 38. In a preferred embodiment, the underlayer 36 includes a
 chromium molybdenum (CrMo) alloy. With a balance comprising of chromium,
 the underlayer 36 has between about 6% and 20% of molybdenum by atomic
 percentage, preferably between about 7 and 16%, and most preferably
 between about 9 and 11%. Although not restricted in this manner, the
 underlayer 36 is usually between about 400 to 800 .ANG. thick.
 Referring to FIGS. 3A-3B, empirical evidence has shown that for underlayers
 36 having body-centered cubic (BCC) crystal structure, such as CrMo, it is
 desirable to maximize the growth of crystal size or grain size in the film
 growth direction as indicated by arrow 37 for the (002) crystal plane
 (FIG. 3A). For a body-center-cubic crystal lattice 40, the (002)
 orientation refers to the crystal plane 41 shown in FIG. 3B. Preferably,
 the 002 crystal plane is parallel to the surface of the substrate. The
 film growth direction is the direction in which the 002 crystal planes are
 stacked (i.e. film thickness grows), which in this embodiment is the
 vertical direction indicated by arrow 37.
 Referring back to FIG. 3A, the underlayer 36 having the larger (002)
 crystal size in the film growth direction 37, promotes desired epitaxial
 crystal growth in the overlying magnetic layer 38. FIG. 3A compares the
 size of crystals 42 in the Cr10Mo underlayer versus the crystals 44 in the
 Cr20V underlayer. As can be seen, the crystals 42 in the Cr10Mo underlayer
 are noticeably larger.
 Advantageously, large CrMo crystal size in the film growth direction 37 of
 the underlayer 36 promotes an in-plane orientation of the c-axis 46 of Co
 crystals in the overlying magnetic layer 38 (FIG. 3A). This is
 particularly desirable. Co crystals have a hexagonal-closed-packed (HCP)
 crystal structure and it is preferred that the 1120 plane be in the film
 growth direction (vertical for this embodiment). Positioning the c-axis 46
 in an in-plane orientation improves the parametric magnetic quality of the
 magnetic layer. The vertical crystal size 44 in underlayers such as Cr20V
 is significantly smaller (FIGS. 3A and 4).
 As the magnetic layer 38 generally mimics the underlayer, the crystal
 structure in the underlayer 36 influences the resulting magnetic layer.
 Referring to FIG. 4, underlayer 36 comprising chromium and molybdenum has
 a maximum (002) orientation crystal size in the film growth direction when
 the underlayer 36 has about 10% molybdenum by atomic percentage. As shown
 in the figure, when the amount of molybdenum is increased or decreased
 from the 10% mark, the film growth direction (002) crystal size is
 undesirably decreased. For the present invention, it is preferred that Mo
 content exists between about 7 and 16% molybdenum by atomic weight. This
 corresponds to a crystal size above about 140 .ANG. in the film growth
 direction 37.
 It is also particularly beneficial that the large vertical crystal size of
 Cr10Mo in the underlayer 36 also reduces the horizontal Co crystal size in
 the magnetic layer 38. Crystal size in the lateral direction is better
 when smaller. By reducing lateral crystal size in the magnetic layer,
 signal-to-noise ratio of the resulting recording media is improved. This
 is due in part to the reduced lateral crystal size and improved
 segregation between crystals. As shown in FIGS. 5A-5B, the Co crystals in
 the magnetic layer 38 over a Cr10Mo underlayer (FIG. 5A) have a horizontal
 or lateral crystal size substantially smaller than the same crystal
 orientation in Cr20V (FIG. 5B). Crystals in FIG. 5A range in size from
 about 60-150 .ANG. in lateral size. Preferably, the crystals are between
 about 60-100 .ANG. in lateral crystal size. The Co crystals in the
 magnetic layer 38 over the Cr10Mo underlayer are also noticeably more
 segregated.
 Smaller lateral crystal size creates a more distinct magnetic boundary or
 transition between areas as shown by FIG. 6A-6C. The larger crystals of
 region 100 has a more jagged boundary 102. The smaller crystals of region
 110 has a straighter, more defined boundary 112. Crystal grain-to-grain
 segregation also reduces crystal coupling which could cause an adjacent
 crystal 130 to change orientations unintentionally (FIG. 6C). This creates
 a less distinct transition and reduces signal quality. The sharper the
 transition, the better.
 The magnetic layer 38 is applied over the underlayer 36, again typically by
 sputtering in a conventional manner. These magnetic recording layers have
 magnetic recording characteristics which are generally advantageous for
 high density recording media. The magnetic layer may comprise CoCrPtB,
 CoPtCr, CoNiCr, or other cobalt-containing alloys. Preferably, magnetic
 layer 38 comprises CoCrNiTaPt. The layer is typically about 200 .ANG.
 thick between about 3%-6% Ni, 10%-18% Cr, 4%-6% Ta, and 3%-10% Pt, with
 the remainder Co. The Cr content may range between about 14%-18%, 14%-16%
 atomic percent. The Pt may range between about 3-8%, more preferably 3-6%
 atomic percent. In an exemplary embodiment, the magnetic layer has about
 3% Ni, 14% Cr, 6% Ta, and 3% Pt, with the remainder Co. The thickness of
 the magnetic layer 38 is not critical, typically being in the range from
 about 100 .ANG. to about 1000 .ANG.. Preferably, the crystals are evenly
 distributed and enhance the magnetic characteristics of the layer 38. The
 preferred crystal structure in the magnetic layer 38 has a c-axis 46 in
 the in-plane direction, uniform and small grain size, and grain-to-grain
 segregation. Large crystal size in the film growth direction (as noted by
 arrow 37 of FIG. 3A) provides a growth surface for facilitating epitaxial
 growth of the magnetic layer.
 A protective layer 40 is next formed over the magnetic layer 38, typically
 being composed of carbon sputtered to a thickness in the range from about
 50 .ANG. to about 200 .ANG.; a thickness of over 100 .ANG. being slightly
 preferred. The protective layer will usually be coated with a lubricant
 layer 42, for example, a fluorinated polyether or the like, typically
 having a thickness in the range from about 10 .ANG. to about 20 .ANG.. An
 alternative protective overcoat and method for its deposition are
 described in co-pending U.S. patent application Ser. No. 08/761,336, filed
 Dec. 10, 1996, now U.S. Pat. No. 5,858,477, the full disclosure of which
 is incorporated herein by reference.
 The following example is offered by way of illustration, not by way of
 limitation.
 EXPERIMENTAL
 Conventional NiP-coated aluminum disk substrates were prepared with either
 a full surface texture or a zone texture, as described above. The textured
 disks were 95 mm in diameter and 30.5 mil thick. A series of direct
 comparisons between media structures having a Cr10Mo underlayer with
 similar media structures having different percentages of Cr and Mo
 underlayer and a CrV underlayer were performed. The specific underlayers
 used in the tests were CrMo, in which Mo provided between 5.0-15.0% of the
 underlayer (all percentages being atomic percent), and CrV in which V
 provided about 20.0% of the underlayer, respectively.
 A sputtering machine was set up to apply heat of about 250-350.degree. C.
 to each side of the textured substrate. The underlayer was then sputtered
 at a pressure of between 2 and 12 mTorr, with a bias of between -100 and
 -250 volts. Sputtering of the underlayer was followed by sputtering of the
 magnetic layer at a pressure of 2 to 12 mTorr, also with a bias of between
 -100 and -250 volts. An initial carbon coating was sputtered over the
 magnetic layer in a 5.0-20.0% methane, H.sub.2, or N.sub.2 environment.
 The disks were then cooled and the remainder of the carbon layer was
 sputtered in a 10.0-30.0% N.sub.2 environment.
 As can be seen by examining FIGS. 5-13, the bulk magnetics and parametrics
 of the resulting recording media are noticeably better for the CrMo
 underlayers as compared to the conventional CrV underlayer. There is,
 specifically, an unexpected spike in performance for those underlayers 36
 containing about 10% Mo.
 For example, the effects of the differing underlayer materials in recording
 parametrizes such as high, medium and low frequency track average
 amplitude (LFTAA) are illustrated in FIGS. 5-7. As can be seen from this
 data, the high frequency track average amplitude (HFTAA) is significantly
 better for Cr5Mo and Cr10Mo as compared to the conventional Cr20V. The
 CrMo underlayers are also considerably better than conventional Cr20V
 underlayers for signal-to-noise ratio and non-linear transition shift
 performance (FIGS. 8-9), where it is understood that higher
 signal-to-noise ratios and lower non-linear transition shifts are
 desirable.
 Finally, it should be noted that for parametric qualities, such as
 resolution shown in FIG. 10 (where a higher value is more desirable, pulse
 width shown in FIG. 11 (where a smaller value indicating a sharper
 transition is desirable), overwrite shown in FIG. 12 (where a larger value
 is desirable), and media noise shown in FIG. 13 (where a smaller value is
 more desirable), recording media having CrMo underlayers generally perform
 better or at least equivalent to conventional Cr20V underlayer. A chromium
 underlayer with about 10% molybdenum (Cr10Mo) underlayer performed
 remarkably better in all categories as compared to the conventional CrV
 underlayer.
 As can be seen in FIG. 4, Cr10Mo has the largest vertical 002 orientation
 crystal size of about 144 .ANG.. Correspondingly, the best or nearly the
 best parametric performance in the resulting magnetic recording media
 occur with a Cr10Mo underlayer. The Cr10Mo underlayer also has a lattice
 constant essentially equivalent to the lattice constant for CrV (FIG. 14).
 Conventional theory would suggest that underlayers with similar lattice
 constants would result in magnetic layers of similar parametric qualities.
 It is believed, however, that the significantly larger (002) crystal size
 enables the Cr10Mo underlayer to facilitate the epitaxial growth of
 desired crystal structures and crystal structure distribution in the
 overlying magnetic layer. Furthermore, the large vertical crystal size
 also unexpectedly reduced lateral CO crystal size in the overlying
 magnetic layer 38, improving the performance of the recording media. Thus,
 the Cr10Mo underlayer has improved magnetic qualities as demonstrated in
 FIGS. 5-13.
 Although the foregoing invention has been described in some detail by way
 of illustration and example, for purposes of clarity and understanding, it
 will be obvious that certain changes and modifications may be practiced
 within the scope of the appended claims.