Magnetic data storage disk with low intrinsic media noise

Abstract: An improved CoPt based or CoNi based alloy magnetic data storage disk has a magnetic recording layer which is a laminated structure of relatively thin magnetic alloy films separated by relatively thin non-magnetic spacer films. The resulting laminated disk structure has substantially decreased intrinsic media noise at high linear recording densities.

Description: Field of the Invention This invention relates to magnetic data storage disks having low intrinsic media noise at high linear recording density. Background of the Invention One of the problems with thin film metal alloy media used in magnetic data storage disks, including CoPt alloy and CoNi alloy media, is that the intrinsic media noise increases with increasing linear recording density. Media noise arises from irregularities in the magnetic transitions and results in random shifts of the readback signal peaks. These random shifts are referred to as "peak jitter" or "time jitter". Thus, the higher the media noise, the higher the bit error rate. It is therefore desirable to develop a thin film metal alloy media which generates noise below a maximum acceptable level in order that data can be recorded at maximum linear density. The effect of intrinsic media noise, as measured by peak jitter and media signal-to-noise ratio (SNR), on the bit error rate in magnetic recording systems is described by Katz, et al., in "Effect of Bitshift Distribution on Error Rate in Magnetic Recording", IEEE Trans. on Magnetics , Vol. MAG-15, pp. 1050-1053, 1979. The measurement of media SNR is described by Belk, et al., in "Measurement of the Intrinsic Signal-to-Noise Ratio for High Performance Rigid Recording Media", J. Appl. Physics , 59(2), January 15, 1986, pp. 557-562. Assignee's U.S. Patent 4,789,598 describes a CoPtCr metal alloy magnetic recording disk wherein the high Cr concentration reduces the intrinsic media noise at high recording density. EPA-0243860 describes a data storage disk for vertical recording, having a multi-layered structure in order to provide increased coercivity. JP-A-59-142738 describes a data storage disk having two magnetic layers, one for recording servo information, one for recording data on. The intervening layer is described as preferably between 0.2 and 0.8 micrometers thick. Accordingly, the invention provides a magnetic data storage disk for horizontal recording comprising: a substrate and a laminated magnetic layer formed over the substrate, the laminated magnetic layer further comprising alternating films of a magnetic film of a cobalt-based alloy having platinum or nickel present in the alloy and a non-magnetic spacer film, the laminated layer having two of said magnetic films and one of said spacer films, and characterised in that said spacer film has a thickness less than approximately 8nm (80 Angstroms). The invention also provides a magnetic data storage disk for horizontal recording comprising: a substrate; and a laminated magnetic layer formed over the substrate, the laminated magnetic layer further comprising alternating films of a magnetic film of a cobalt-based alloy having platinum or nickel present in the alloy and a non-magnetic space film, and characterised in that the laminated layer has at least three of said magnetic films and two of said spacer films. The resulting laminated disk structure has substantially decreased intrinsic media noise at high linear recording densities. The benefits of improved signal-to-noise ratio (SNR) are best achieved when the non-magnetic spacer film is maintained as thin as possible, i.e. a thickness just sufficient to effectively interrupt the continuous nature of the subsequently deposited magnetic film. It is preferred that the magnetic data storage disk further comprises a non-magnetic underlayer between the substrate and the magnetic layer, the underlayer being formed of either chromium, molybdenum, or an alloy comprising chromium and vanadium. It is further preferred that the magnetic film alloy comprises cobalt, chromium and platinum, or includes molybdenum or chromium and the non-magnetic spacer film alloy is formed of chromium, molybdenum, or an alloy comprising chromium and vanadium, with each of the magnetic films having a thickness less than approximately 25nm (250 Angstroms). Brief Description of the Drawings Fig. 1 is a graph of SNR as a function of linear density for a single layer and a two-layer CoPtCr disk structure; Fig. 2 is a plot of SNR as a function of linear density for different laminated CoPtCr disk structures which illustrates the reduction in noise as the number of films increases; Fig. 3 is a bar chart illustrating measured signal and noise as a function of the number of films in the laminated disk structure; Fig. 4 is a plot of SNR as a function of linear density for a CoPtMo disk illustrating the general reduction in media noise with increasing number of films; Fig. 5 is a plot of the ratio of media noise to signal amplitude as a function of magnetic film thickness; and Fig. 6 is a plot of coercivity squareness as a function of magnetic film thickness. Detailed Description In order to illustrate the improved SNR for CoPt alloy laminated disks, and the dependence of that improvement on the number and thickness of the films, various types of CoPt alloy disks were fabricated with various types of non-magnetic spacer films. The disks were prepared in an RF-DC magnetron sputtering system (S-gun). The Ar sputtering pressure was approximately 1.333 x 10⁻⁵ Pa (1 x 10⁻⁷ Torr) before deposition and was maintained at 0.427 Pa (3.2m Torr) for each deposition. All depositions were conducted without substrate heat. The substrates were 5 1/4″ AlMg/NiP disks (133.35 mm). The surfaces of the substrates were RF discharge cleaned and the Mo, Cr, or CrV underlayers (30-40 nm, 300-400 Angstroms thick) were sputter deposited on the substrates before lamination of the magnetic layer. The magnetic layer was then sputter deposited by alternating the sputtering sources (magnetic film and spacer) to form the laminated structure. Table 1 depicts the magnetic properties of a single film (55nm, 550 Angstroms) Co₇₄Pt₉Cr₁₇ layer compared to a two-film (2 x 25nm, 2 x 250 Angstroms) layer separated by an 8nm (80 Angstroms) film of non-magnetic Cr₈₀V₂₀. Fig. 1 is a plot of the SNR (dB) versus linear recording density for the two horizontal recording disks whose data are given in Table 1, where SNR is the ratio of base-to-peak signal amplitude (in microvolts) to RMS media noise (in microvolts). Note that a two-film structure of CoPtCr increases the SNR by about 8-9dB at 3,000 flux reversals per millimeter (fr/mm). The 8nm (80 Angstroms) Cr₈₀V₂₀ layer which separated the two coPtrCr films had no effect on the isolated readback pulses, thus indicating that the lamination did not adversely affect the recording properties. Fig. 2 depicts the effect of laminating CoPtCr with up to 6 films (6 x 9nm, 6 x 90 Angstroms) where adjacent films are separated by Cr₈₀V₂₀ non-magnetic spacer films (4nm, 40 Angstroms). A single film structure yielded only about a 5 dB SNR at 3,000 fr/mm, compared to approximately a 20 dB SNR for the 6-film laminated structure. An overall comparison of the effect of lamination on signal and noise is shown in Fig. 3. As is apparent, the effect of lamination on signal is minimal but the reduction in noise is dramatic as the number of films in the lamination increases. Fig. 4 depicts SNR data for a Co₆₈Pt₁₂Mo₂₀ disk, laminated with either 8 nm (80 Angstroms) or 4 nm (40 Angstroms) of non-magnetic Mo, as a function of linear density. Note that lamination improves the SNR at 3,000 fr/mm in a manner similar to that for CoPtCr for one and two-film structures (Fig. 1). However, the 3-film structure (3 x 13.4 nm, 3 x 134 Angstroms) with 8 nm (80 Angstroms) Mo spacer films, was not as good as the 2-film structure. When the Mo spacer thickness was reduced from 8nm (80 Angstroms) to 4 nm (40 Angstroms), a SNR improvement was achieved. This indicates it is desirable to have the non-magnetic spacer films as thin as possible, i.e. the minimum thickness sufficient to permit the growth of a magnetic film. Similar results of improved SNR were obtained for disks where the magnetic films of CoPtCr were also alloyed with Mo and separated by non-magnetic spacer films of Mo (4 nm 40 Angstroms). Improved results in SNR were also observed for disks made according to the present invention when the spacer films were either Cr, Mo or CrV having thickness values between approximately 4 nm and 8 nm (40 and 80 Angstroms). The low-noise laminated thin film CoPt alloy disk is based upon the finding that a single thinner CoPt alloy film exhibits superior low-noise characteristics than a thicker film (see Fig. 5, where S o is zero-to-peak signal amplitude of an isolated transition). It has also been experimentally determined that the improved SNR with a reduction in CoPt alloy film thickness is accompanied by a reduction in coercivity squareness, S* (see Fig. 6). It is believed that the low S* value is indicative of a lesser degree of exchange coupling between grains in the polycrystalline CoPt alloy film. Thus, according to the present invention, but contrary to conventional thin film CoPt alloy disks, it is desirable to have a lamination of multiple films, where each film has a relatively low value of S*, since this results in low intrinsic media noise. It is apparent, by reference to Figs. 5 and 6, that this phenomenon is also applicable to thin CoNi alloy films. It is therefore believed that improved low noise media can also be achieved by the lamination of thin CoNi alloy films with thin non-magnetic spacer films. The above description relates only to the inventive structure which forms a part of the recording media, and not to the conventional well-known portions of the media and the media fabrication processes. For example, in the fabrication of thin film metal alloy disks, it is known to provide a protective overcoat, such as a sputtered carbon film, over the magnetic film. In the present invention the protective overcoat is formed over the uppermost film in the laminated film structure.

Claims: A magnetic data storage disk for horizontal recording comprising: a substrate; and a laminated magnetic layer formed over the substrate, the laminated magnetic layer further comprising alternating films of a magnetic film of a cobalt-based alloy having platinum or nickel present in the alloy and a non-magnetic spacer film, characterised in that the laminated layer has at least two of said magnetic films and one of said spacer films, but excluding a disk having two magnetic films, and one spacer, where the spacer film has a thickness greater than approximately 8nm (80 Angstroms). A magnetic data storage disk as claimed in claim 1 wherein each of the spacer films has a thickness less than approximately 8nm (80 Angstroms). A magnetic data storage disk as claimed in any preceding claim, further comprising a non-magnetic underlayer between the substrate and the magnetic layer. A magnetic data storage disk as claimed in any preceding claim wherein the magnetic film alloy comprises cobalt, chromium and platinum. A magnetic data storage disk as claimed in claim 1 or 2 wherein the magnetic film alloy includes chromium or molybdenum. A magnetic data storage disk as claimed in any preceding claim wherein the non-magnetic spacer film is formed of chromium, molybdenum or an alloy comprising chromium and vanadium. A magnetic data storage disk as claimed in any preceding claim wherein each of the magnetic films has a thickness less than approximately 25nm (250 Angstroms). A magnetic data storage disk as claimed in any preceding claim wherein the laminated magnetic layer has a coercivity squareness S* in the range of approximately 0.6 to 0.8. A magnetic data storage disk as claimed in any preceding claim, wherein a protective overcoat is formed over the film furthest from the substrate. The use in a magnetic data storage disk for horizontal recording comprising a substrate and a magnetic layer formed over the substrate, of laminations in the magnetic layer comprising alternating films of a magnetic film of a cobalt-based alloy having platinum or nickel present in the alloy and a non-magnetic spacer film, there being at least two of said magnetic films and one of said spacer films, to reduce intrinsic media noise.

Inventors: AHLERT RICHARD HENRY, HOWARD JAMES KENT, LAMBERT STEVEN EUGENE, SANDERS IAN LEWIS, AHLERT, RICHARD HENRY, HOWARD, JAMES KENT, LAMBERT, STEVEN EUGENE, SANDERS, IAN LEWIS
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1990
Main classification: G11B5
IPC codes: G11B5