A MAGNETIC RECORDING MEDIUM

Abstract: In a magnetic recording medium for horizontal recording, an underlayer of a body-centred-cubic (BCC) chromium-based alloy with a lattice cell constant greater than chromium (Cr), such as chromium-vanadium (CrV) or chromium-iron (CrFe), is formed between the substrate and a CoPt or CoPtCr magnetic layer.

Description: This invention relates to a magnetic recording medium. Alloys of cobalt and platinum containing various percentages of platinum have been used as the magnetic material in thin film magnetic recording disks for horizontal recording. In such disks, the hexagonal close packed (HCP) crystalline structure of the cobalt-platinum (CoPt) alloy is formed on the substrate, or on an intermediate underlayer, so that the C-axis, i.e. the [002] axis, of the CoPt film is either in the plane of the film or has a component in the plane of the film. The coercivity (H c ) of CoPt films is dependent upon the composition of the alloy, with the maximum H c occurring at approximately 20 atomic percent (at.%) platinum. See J. A. Aboaf, et al., "Magnetic Properties and Structure of Co-Pt Thin Films", IEEE Trans on Magnetics , MAG-19, 1514 (1983), and M. Kitada, et al., "Magnetic Properties of Sputtered Co-Pt Thin Films", J. Appl. Phys. 54 (12), December 1983, pp. 7089-7094. The coercivity and other properties of cobalt-platinum films have been reported by Opfer, et al. in an article entitled "Thin-Film Memory Disc Development," Hewlett-Packard Journal , November 1985, pp. 4-10. A CoPt thin film magnetic recording medium with between 10 and 30 at.% Pt is described in U.S. -A- 4,438,066. A thin film disk with a cobalt-platinum-chromium (CoPtCr) magnetic layer, wherein chromium (Cr) is added to improve the corrosion resistance of the magnetic layer, is described in Japanese patent application 198568, published May 22, 1984. The CoPtCr magnetic layer is deposited onto a nickel-phosphorus (NiP) film formed on a suitable substrate. In order to improve the magnetic properties of the CoPt magnetic layer in certain types of disks, various types of underlayers may be formed between the substrate and the CoPt magnetic layer. The use of a Cr underlayer in a CoPt thin film disk is described in the above referenced article by Opfer, et al. and in EP -A- 145157. EP -A- 140513 describes various magnetic recording structures for horizontal recording in which a Co-based HCP magnetic layer is deposited onto a Co-based primarily body-centred-cubic (BCC) non-magnetic layer. One of the possible structures suggested in the EP -A- 140513 is a CoPt magnetic layer, with Pt between 1 and 35 at.%, formed on a XCo layer, where X is either Cr or vanadium (V) with X between 60 and 99 at.%. It is suggested that the in-plane orientation of the C-axis of the HCP magnetic layer is enhanced because of "pseudo lattice" matching of the (002) plane of the BCC underlayer with the (110) plane of the HCP magnetic layer. The present invention seeks to provide an improved CoPt or CoPtCr thin film magnetic recording medium for horizontal recording. A magnetic recording medium comprising a layer of a cobalt-platinum magnetic alloy formed on a non-magnetic underlayer on a substrate, is characterised, according to the invention, by the underlayer consisting of a body-centred cubic alloy of chromium and either vanadium or iron. Preferably, the underlayer consists of a film at least 20 nm thick of an alloy of chromium and vanadium. How the invention can be carried out will now be described by way of example, with reference to the accompanying drawings, in which:- Fig. 1 is a graph of squareness M r /M s as a function of CoPt magnetic layer thickness for different underlayers; Figs. 2A and 2B depict lattice structures for BCC and HCP crystals and the respective planes to be lattice matched; and Fig. 3 is a graph of the lattice misfit between the HCP phase of CoPt and various BCC Cr alloys. In order to demonstrate the improved magnetic properties of a CoPt disk embodying the present invention, CoPt record disks were first made with Cr and chromium-cobalt (CrCo) underlayers between the substrate and the CoPt magnetic layer. In all the following experimental examples described herein the substrate was semiconductor grade single-crystal silicon. In the first example, a Cr underlayer 50 nm thick was deposited by DC magnetron sputtering onto a silicon substrate at an Argon pressure of 0.266 Pa (2 x 10⁻³ Torr) and a substrate temperature of 128°C. Thereafter, a 25 nm thick cobalt-platinum alloy film with 20 at.% platinum (Co₈₀Pt₂₀) was deposited onto the Cr underlayer without breaking vacuum in the sputtering chamber. This film had a coercivity H c of 1875 Oersteds (Oe) and a squareness S of 0.90. In a second example a thin film disk was fabricated under similar conditions with a 50 nm Cr₈₀Co₂₀ (20 at.% Co) underlayer and a 25 nm thick Co₈₀Pt₂₀ magnetic layer. This film had a coercivity H c of 1500 Oe and a squareness S of 0.88. By comparison with these two examples, a thin film disk with a 50 nm thick Cr₈₀V₂₀ (20 at.% V) underlayer and a 25 nm thick Co₈₀Pt₂₀ magnetic layer had a coercivity H c of 1775 and a squareness S of 0.96. The results of these three examples are listed in Table 1. While the disks with the Cr and CrV underlayers showed similar coercivities, the disk made with the CrV underlayer exhibited a higher squareness. Illustrated in Fig. 1 are the measured squareness values, i.e. the remanence magnetisation to saturization magnetisation (M r /M s ), for disks with Cr and CrV underlayers for various thicknesses of the Co₈₀Pt₂₀ magnetic layer. All of the disks from which the data of Fig. 1 was derived were formed at the same substrate deposition temperature, T s , of 128°C. The data points shown in solid on Fig. 1 are for disks in which a 25 nm (250 A) thick protective carbon overcoat was sputter deposited over the Co₈₀Pt₂₀ magnetic layer. The Cr₈₀V₂₀ alloy underlayer was also used in the fabrication of CoPtCr thin film alloy disks. Table 2 illustrates a comparison of the magnetic properties of two (Co₈₅Pt₁₅) 93.5 Cr 6.5 disks, one with a 40 nm Cr underlayer and one with a 40 nm Cr₈₀V₂₀ alloy underlayer; the other properties of the disks being identical. The CrV alloy underlayer resulted in a disk with slight improvement in coercivity H c , squareness S, and coercivity spareness S*. A 40 nm Co₈₀Pt₂₀ film deposited on a 50 nm Cr₈₀V₂₀ underlayer was examined by X-ray diffraction analysis. The results showed a mixture of crystallites of HCP and face-centre-cubic (FCC) phases. A Co₈₀Pt₂₀ film of the same thickness deposited on a 50 nm Cr underlayer, on the other hand, exhibited only an HCP phase, without any observable FCC phase. When the Cr₈₀V₂₀ underlayer was decreased to 20 nm thickness, the coercivity of a 50 nm Co₈₀Pt₂₀ film decreased to 400 Oe and the dominant phase in the Co₈₀Pt₂₀ film was FCC. In an article by Chen, et al. in IEEE Transaction Magnetics , MAG-17, 3187 (1981), wherein a study of FCC phase formation in cobalt-rhenium (CoRe) alloy films was investigated, the experimental results indicated that grain size of the crystallites decreased as a result of the transformation of HCP to FCC phase. The results by Chen, et al. indicated that a reduction in coercivity of the CoRe films was affected primarily by the reduction of grain size as a result of the polymorphic phase transformation, rather than the presence of the FCC crystallites themselves. However, the high coercivity values (approximately 1550 to 1630 Oe) exhibited by the 40 nm Co₈₀Pt₂₀ films on the 50 nm Cr₈₀V₂₀ sublayers indicate that the two-phase structure does not appreciably affect the coercivity mechanism. Thus the high coercivity and high squareness of a 40 nm Co₈₀Pt₂₀ film over a 50 nm Cr₈₀V₂₀ underlayer are consistent with the formation of a two-phase microstructure in the Co₈₀Pt₂₀ film without a reduction in grain size. It is believed that the improved squareness of CoPt films caused by the CrV underlayer is a result of the close lattice matching between the BCC CrV alloy and the HCP phase in the Co₈₀Pt₂₀ film. A BCC crystal has a single cell constant, a BCC . The (200) plane of a BCC crystal is a square with dimensions (2 0.5 )(a BCC ) × (2 0.5 )(a BCC ). See Fig. 2A. An HCP crystal has two cell constants, c HCP which is parallel to the [002] or C-axis, and a HCP which is perpendicular to the C-axis and parallel to the [010] axis. The (110) plane of the HCP crystal is rectangular with dimensions (c HCP ) × (3 0.5 )(a HCP ). See Fig. 2B. The HCP CoPt phase is "lattice matched" to the BCC underlayer if the (110) plane of the CoPt has similar dimensions to the (200) plane of the BCC underlayer. Table 3 lists the lattice cell constants, as determined from X-ray diffraction analysis, for Co₈₀Pt₂₀ disks made with Cr and Cr alloy underlayers. The misfit of the cell constants of the HCP CoPt magnetic layer to the respective cell constant for various BCC Cr alloy underlayers is depicted graphically in Fig. 3. As shown by Fig. 2, Cr₈₀V₂₀ provides a closer lattice match than either Cr or CrCo. Another alloy which has an a BCC value greater than that of CrCo or Cr, and which would provide a good lattice matching with CoPt, is chromium-iron (CrFe). The misfit of HCP Co₈₀Pt₂₀, assuming a HCP = 0.262 nm and c HCP = 0.422 nm, with BCC Cr₈₀Fe₂₀, having an a BCC = 0.30 nm, is shown in Fig. 2. Thus, CrFe underlayers also result in a CoPt film with improved squareness over prior art films with CrCo and Cr underlayers. To summarise, both the BCC chromium-vanadium (CrV) or chromium-iron (CrFe) alloys have a lattice cell constant, denoted a BCC , greater than that of BCC CrCo or BCC Cr alone, and thus more closely match the lattice structure of the hexagonal-close-packed (HCP) CoPt or CoPtCr crystallites in the magnetic layer. When a silicon substrate is used, as in the above-described examples, an underlayer is required to improve the C-axis orientation of the HCP phase of the magnetic layer in the plane of the film. The CrV and CrFe underlayers serve this purpose. When the substrate is of some other material, such as a nickel-phosphorus (NiP) film formed on an aluminium alloy disk, however, an underlayer may not be absolutely necessary but only beneficial to improve the in-plane C-axis orientation. The underlayers of the present invention enhance the C-axis orientation and thereby improve the magnetic properties of the thin film disk when the substrate is a NiP film on an aluminium alloy disk. The above description relates only to the formation of the magnetic layer and underlayer on the substrate and not to the well-known aspects of the recording 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, essentially amorphous carbon film, over the magnetic layer and in certain instances to provide an adhesion layer, such as a sputtered film of titanium, between the overcoat and the magnetic layer.

Claims: A magnetic recording medium comprising a layer of a cobalt-platinum magnetic alloy formed on a non-magnetic underlayer on a substrate, characterised by the underlayer consisting of a body-centred cubic alloy of chromium and either vanadium or iron. A magnetic recording medium as claimed in claim 1, in which the underlayer consists of a film at least 20 nm thick of an alloy of chromium and vanadium. A magnetic recording medium as claimed in claim 1 or claim 2, in which the layer of magnetic alloy comprises a mixed phase of HCP and FCC crystallites. A magnetic recording medium as claimed in any preceding claim, in which the magnetic alloy also contains chromium. A magnetic recording medium as claimed in any preceding claim, in which the substrate is of silicon. A magnetic recording medium as claimed in any of claims 1 to 4, in which the substrate consists of an aluminium alloy having a nickel-phosphorus film formed thereon beneath the underlayer. A magnetic recording medium as claimed in any preceding claim, further comprising a protective overcoat formed over the layer of magnetic alloy.

Inventors: HOWARD, JAMES KENT
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1987
Main classification: G11B5
IPC codes: H01F10, G11B5, C22C19