THIN FILM VERTICAL MAGNETIC RECORDING MEDIUM WITH A SUBSTRATE, AN INTERMEDIATE LAYER AND A MAGNETISABLE LAYER

Abstract: The present invention relates to a magnetic recording medium for vertical recording of the type comprising a substrate (1), an intermediate layer (2) formed on the substrate and a magnetisable layer (3) formed on the intermediate layer. A medium according to the invention is characterised in that the intermediate layer (2) comprises a first constituent of an element selected from the group consisting of tungsten (W), molybdenum (Mo), niobium (Nb), titanium (Ti), vanadium (V), iridium (Ir) and zirconium (Zr), and a second constituent consisting of either cobalt (Co), chromium (Cr), or a solid substitutional solution alloy of CoCr, wherein the amounts of the first and second constituents are present in a fixed stoichometric ratio so as to form an intermetallic compound with a hexagonal crystalline structure, and the magnetisable layer (3) comprises a ternary alloy of the form (Co 100-y Cr y )X, wherein y is approximately in the range of 14 to 22 and wherein X is selected from the group consisting of W, Mo, Nb, Ti and V, and has an hexagonal crystalline structure with its C-axis oriented generally perpendicular to said substrate (1). The hexagonal cell constants of the intermediate layer (2) and the magnetisable layer (3) are related by approximately integral multiples whereby the hexagonal crystalline orientation of the magnetisable layer (3) is enhanced.

Description: This invention relates to a thin film metal alloy medium for vertical magnetic recording, and in particular to such a medium with improved perpendicular magnetic anisotropy. A thin film metal alloy medium for vertical magnetic recording typically comprises a substrate, a nonmagnetisable intermediate layer formed on the substrate and a magnetisable layer having an ordered crystalline formation with a generally perpendicular magnetic anisotropy formed on the nonmagnetisable intermediate layer. One type of material used as the magnetisable layer is a cobalt-chromium (CoCr) alloy which is sputter-deposited to form a film having a hexagonal close packed (HCP) crystalline structure with the [00.2] axis (the C-axis) oriented perpendicular to the medium substrate. An important factor in the production of a vertical magnetic recording medium is the ability to control the orientation of the HCP CoCr or cobalt-chromium-X (CoCrX) alloy magnetisable layer. The degree to which the C-axis of the HCP structure can be aligned perpendicular to the substrate strongly influences the magnetic properties, such as the perpendicular (or vertical) and horizontal coercivities. The magnetic properties of a thin film alloy medium made with various substrates, a titanium (Ti) intermediate layer and a CoCr perpendicular magnetisable layer are described by Kobayashi, et al. in "High Density Perpendicular Magnetic Recording on Rigid Disks", Fujitsu Scientific and Technical Journal , Vol. l9, No. 1, pp. 99-126, March, l983. This article discloses that substantially improved perpendicular magnetic anisotropy of the CoCr film is achieved with the use of a Ti intermediate layer, and suggests that the C-axis orientation of the CoCr film is improved because the Ti intermediate layer isolates the influence of the substrate from the CoCr film. A thin film alloy medium having a nonmagnetisable cobalt-tantalum (CoTa) alloy as an intermediate layer and a CoCr perpendicular magnetisable layer is disclosed in Japanese published unexamined patent application JA-A-59-77620. It is also known to use ternary alloys, such as cobalt-chromium-tantalum (CoCrTa), as the material for the magnetisable layer. EP-A-93838 discloses a medium for vertical magnetic recording having a soft magnetisable intermediate layer of an amorphous CoTa alloy and a magnetisable layer of CoCrTa with perpendicular magnetic anisotropy. The magnetic properties of a vertical recording medium with an 8000 Angstroms (8×10⁻⁷m) thick CoCrTa magnetisable layer deposited on a 1000 Angstroms (1×10⁻⁷m) thick Cr intermediate layer are described by Langland and Albert in "Recording on Perpendicular Anisotropy Media with Ring Heads", IEEE Transactions on Magnetics , Vol. MAG17, No. 6, November, l981, pp. 2547-2549. The object of the present invention is to provide an improved magnetic recording medium for vertical recording. The present invention relates to a magnetic recording medium for vertical recording of the type comprising a substrate, an intermediate layer formed on the substrate and a magnetisable layer formed on the intermediate layer. A medium according to the invention is characterised in that the intermediate layer comprises a first constituent of an element selected from the group consisting of tungsten (W), molybdenum (Mo), niobium (Nb), titanium (Ti), vanadium (V), iridium (Ir) and zirconium (Zr), and a second constituent consisting of either cobalt (Co), chromium (Cr), or a solid substitutional solution alloy of CoCr, wherein the amounts of the first and second constituents are present in a fixed stoichometric ratio so as to form an intermetallic compound with a hexagonal crystalline structure, and the magnetisable layer comprises a ternary alloy of the form (Co 100-y Cr y )X, wherein y is approximately in the range of 14 to 22 and wherein X is selected from the group consisting of W, Mo, Nb, Ti and V, and has an hexagonal crystalline structure with its C-axis oriented generally perpendicular to said substrate (1). The hexagonal cell constants of the intermediate layer and the magnetisable layer are related by approximately integral multiples whereby the hexagonal crystalline orientation of the magnetisable layer is enhanced. The resulting vertical recording medium has substantially improved perpendicular magnetic anisotropy because there is minimal C-axis dispersion of the magnetisable layer. This is believed to be the result of the close lattice matching of the hexagonal cell constants of the intermediate layer and the magnetisable layer. In order that the invention may be more readily understood an embodiment will now be described with reference to the accompanying drawings, in which: Fig. 1 is a side view in section of a vertical magnetic recording medium according to the present invention including a substrate, an intermediate layer and a magnetisable layer, Fig. 2 is a B-H hysteresis loop for a (Co₈₀Cr₂₀)₉₀W₁₀ magnetisable layer formed on a (Co₈₀Cr₂₀)₃W intermediate layer in the recording medium illustrated in Fig. 1, Fig. 3 is a graph of perpendicular and horizontal coercivities of the medium of Fig. 1 as a function of heater power for a (Co₈₀Cr₂₀)₉₀W₁₀ magnetisable layer formed on a (Co₈₀Cr₂₀)₃W intermediate layer, Fig. 4 is an X-ray diffraction curve illustrating a peak intensity for Co₃W and corresponding points where diffraction peaks would occur for Co₈₂Cr₁₈ and (Co₈₀Cr₂₀)₉₀W₁₀, and Fig. 5 is a graph of perpendicular and horizontal coercivities of the medium as a function of the percentage of W in the (Co₈₀Cr₂₀)W magnetisable layer. A vertical magnetic recording medium in accordance with the present invention comprises a substrate 1, an intermediate layer 2 deposited on the substrate and a magnetisable layer 3 deposited on the intermediate layer 2. The intermediate layer comprises either a binary or ternary intermetallic compound. As used herein the term "intermetallic compound" refers to those chemical compositions which are more than a simple mixture in the form of an alloy, but in which the constituents are present in a fixed stoichometric ratio so that the composition can be essentially represented by a chemical formula. An intermetallic binary compound of two elements, such as cobalt and tungsten, is an intermediate phase which exists only at the discrete stoichometric ratio of three atoms of cobalt to one atom of tungsten. The Co₃W intermetallic compound is indicated on the published phase diagrams for cobalt and tungsten, such as in Constitution of Binary Alloys , McGraw Hill, l958, p. 519. In a ternary intermetallic compound, the first constituent is a single element and the second constituent is an alloy in which one of the elements in the alloy is present in solid substitutional solution. For example, the ternary intermetallic compound (Co₈₂Cr₁₈)₃W is identical in structure to Co₃W with the exception that certain Co atoms are replaced by Cr atoms in substitutional sites. Table 1 lists binary and ternary intermetallic compounds capable of use as intermediate layer in the recording medium of Figure 1. Each of the intermetallic compounds in Table 1 has a hexagonal crystalline structure. The lattice or cell constants for the binary intermetallic compounds are also indicated in Table 1. Each of the ternary intermetallic compounds in Table 1 has a hexagonal cell constant a₀ nearly identical to that of its corresponding binary intermetallic compound. This is because the Cr is present in solid substitutional form in the ternary alloy and because Co and Cr have nearly identical atomic radii. An example of a medium made according to the present invention comprises a silicon substrate, a ternary intermetallic compound of (Co₈₀Cr₂₀)₃W as an intermediate nucleating layer and a magnetisable layer of (Co₈₀Cr₂₀)W. Fig. 1 is a representation of such a disk in sectional view. The (Co₈₀Cr₂₀)₃W intermediate nucleating layer was formed by sputter deposition on the silicon substrate in a DC magnetron sputtering chamber with an argon pressure of 2 × 10⁻³ Torr and for various substrate deposition temperatures. Separate targets of Co₈₀Cr₂₀ and W were used to form an intermediate layer of thickness 500 Angstrom (5×10⁻⁸m). The power to the Co₈₀Cr₂₀ and W targets was controlled to give the 3 to 1 stoichometric ratio. A magnetisable layer of (Co₈₀Cr₂₀)₉₀W₁₀ of thickness 5000 Angstrom (5×10⁻⁷m) was then sputter deposited on the intermediate layer without breaking vacuum in the sputtering chamber but by altering the power to the respective targets. Fig 2 is a B-H hysteresis loop for the disk shown in Fig. 1. Such a medium has a vertical coercivity (perpendicular to the plane of the magnetisable layer) of 1000 Oersteds (Oe) and a horizontal coercivity (parallel to the plane of the magnetisable layer) of 75 Oe. As shown in Fig. 3, the formation of the medium at various substrate deposition temperatures, which is directly related to heater power, shows that the vertical or perpendicular coercivity increases generally linearly with higher power, whereas the in-plane or horizontal coercivity does not change appreciably. The ability to increase the perpendicular coercivity independently of the horizontal coercivity is a distinct advantage in the fabrication of a vertical magnetic recording medium, and is believed to be caused by lattice matching of the hexagonal cell constants between the intermediate nucleating layer and the magnetisable layer. For example, as shown in Table 1, the (Co₈₀Cr₂₀)₃W intermediate layer has a hexagonal cell constant approximately the same as Co₃W, i.e. a = 5.13 Angstroms (5.13×10⁻¹⁰m), which is approximately twice that of the hexagonal cell constant for the (Co₈₀Cr₂₀)₉₀W₁₀ magnetisable layer. The relationship between the hexagonal cell constants (a₀) for the intermediate layer and the magnetisable layer can be understood by reference to Table 1 and to Fig. 4, which is an X-ray diffraction curve for a 100 Angstroms (1×10⁻⁸m) thick layer of a binary intermetallic compound of Co₃W deposited on a silicon substrate. This film shows a peak X-ray intensity at approximately 43.5°. The hexagonal cell constant for a Co₃W layer can be determined from the X-ray diffraction curve (which yields the peak diffraction angle 2ϑ) and the relationship λ = 2D Sin ϑ, where λ is the wavelength of the X-ray and D is the Bragg plane spacing. The Bragg plane spacing D is directly related to the hexagonal cell constant a₀. Co₈₀Cr₂₀ is hexagonal with a cell constant a₀ = 2.52 Angstroms (2.52×10⁻¹⁰m). As shown in Table 1, intermediate nucleation layers of Co₃W, Co₃Mo, Co₂Nb, Co₃V, Cr₂Nb and Cr₂Zr exhibit a value of a₀ which is approximately twice that for Co₈₀Cr₂₀, while intermediate nucleation layers of Co₃Ti and CrIr exhibit a value of a₀ which is approximately equal to that for Co₈₀Cr₂₀. Also shown on Fig. 4 are lines indicating where the peak X-ray intensities would occur for Co₈₂Cr₁₈ and (CoCr)₉₀W₁₀. Even though Fig. 4 indicates that Co₃W and Co₈₂Cr₁₈ have relatively close peak diffraction angles, 2ϑ, there is almost an identical match between Co₃W and (Co₈₀Cr₂₀)₉₀W₁₀ (the a₀ cell constant of Co₃W being almost exactly twice that for (Co₈₀Cr₂₀)₉₀W₁₀). Thus, the addition of W to the magnetisable layer changes the hexagonal cell constant to a closer match with the hexagonal cell constant for the intermediate nucleating layer. Fig. 5 shows the relationship between the perpendicular and horizontal coercivities of three media each with a (CoCr)W magnetisable layer deposited over a (CoCr)₃W intermediate nucleating layer for different percentages of W present in the magnetisable layer. All of the media show an excellent ratio of perpendicular to horizontal coercivities, with the minimum horizontal coercivity occurring at a W concentration of about 10 at.%. The thickness of the (CoCr)₃W intermediate nucleating layers for the media of Fig. 5 was 500 Angstroms (5×10⁻⁸m). However, other media were made with thicknesses of the intermediate nucleating layer of between 300 and 1000 Angstroms (3×10⁻⁸m and 1×10⁻⁷m) with no significant effect on the magnetic properties. The orientation of the (Co₈₀Cr₂₀)₃W/(Co₈₀Cr₂₀)₉₀W₁₀ film layer structure deposited at 95 Watts (Fig. 3) was established by X-ray diffraction to measure the dispersion of the C-axis. An absolute measurement of orientation was obtained by deposition of the film layer structure on a polyimide substrate and examination of the film layer structure in transmission and reflection. The X-ray diffraction spectra obtained in reflection showed that only the (0002) planes are parallel to the film layer surface. The X-ray diffraction spectra obtained in transmission, which is sensitive to planes with a normal component to the film surface, showed that only planes normal to the [00.2] axis are present. These results indicate that the (Co₈₀Cr₂₀)₃W intermediate nucleation layer is nearly perfectly lattice matched to the (Co₈₀Cr₂₀)₉₀W₁₀ magnetisable layer. In the magnetisable layers of the media made according to the present invention the Cr preferably comprises between approximately 14 and 22 at.% of the CoCr. This is the range of solid solution alloy of interest in vertical recording media, because, below approximately 14 at.%, the magnetisation is still principally in-plane and above approximately 22 at.% the magnetic moment has been appreciably diminished. The intermediate nucleating layer in a recording medium according to the present invention can be formed by sputter deposition from two separate targets, one of Co₈₀Cr₂₀ and one of W, by controlling the power to the respective targets so that the proper 3 to 1 stoichometric ratio can be achieved during the sputter deposition process. It is also possible to purchase commercially available targets of Co₃W or (CoCr)₃W so that a single target can be used to sputter deposit the intermediate nucleating layer. The above description and drawings relate only to the novel feature of a vertical recording medium according to the invention and not to the well known portions of the medium and the medium fabrication processes. For example, in the fabrication of a thin film metal alloy medium, it is known to provide a protective overcoat, such as a sputtered carbon film, over the magnetisable layer and in certain instances to provide an adhesion layer, such as a sputtered titanium film, between the overcoat and the magnetisable film. In addition, if the medium is to be used with vertical magnetic recording pole heads located on the same side of the medium, then it would be necessary to provide an additional soft magnetisable underlayer, such as nickel-iron (NiFe), between the intermediate nucleating layer and the substrate to provide a magnetic flux return path.

Claims: 1. A magnetic recording medium for vertical recording comprising a substrate (1), an intermediate layer (2) formed on said substrate and a magnetisable layer (3) formed on said intermediate layer, characterised in that said intermediate layer (2) comprises a first constituent of an element selected from the group consisting of tungsten (W), molybdenum (Mo), niobium (Nb), titanium (Ti), vanadium (V), iridium (Ir) and zirconium (Zr), and a second constituent consisting of either cobalt (Co), chromium (Cr), or a solid substitutional solution alloy of CoCr, wherein the amounts of the first and second constituents are present in a fixed stoichometric ratio so as to form an intermetallic compound with a hexagonal crystalline structure, and said magnetisable layer (3) comprises a ternary alloy of the form (Co100-yCry)X, wherein y is approximately in the range of 14 to 22 and wherein X is selected from the group consisting of W, Mo, Nb, Ti and V, and has an hexagonal crystalline structure with its C-axis oriented generally perpendicular to said substrate (1), the hexagonal cell constants of said intermediate layer (2) and said magnetisable layer (3) being related by approximately integral multiples whereby the hexagonal crystalline orientation of said magnetisable layer (3) is enhanced. 2. A magnetic recording medium as claimed in claim 1 characterised in that the first constituent of said intermediate layer (2) is either W, Mo, Nb, Ti or V, the second constituent of said intermediate layer (2) is Co or CoCr alloy, and the selected element X in said magnetisable layer (3) is the same as the first constituent in said intermediate layer (2). 3. A magnetic recording medium as claimed in claim 1 wherein the first constituent of said intermediate layer (2) is Ir, Nb or Zr, the second constituent of said intermediate layer (2) is Cr and the stoichometric ratio of the second constituent to the first constituent is 2 to 1. 4. A magnetic recording medium as claimed in claim 1 characterised in that said intermediate layer (2) comprises a mixture of Co and W, the amounts of Co and W being present in a stoichometric ratio of three to one so as to form the intermetallic compound Co₃W of cobalt and tungsten, the Co₃W having a hexagonal crystalline structure with its C-axis oriented generally perpendicular to said substrate (1), and said magnetisable layer (3) comprises a ternary alloy CoCrW of Co, Cr, and W having a hexagonal crystalline structure with its C-axis oriented generally perpendicular to said substrate (1), the amount of W present in the CoCrW alloy being such that the hexagonal cell constant of the CoCrW alloy is approximately half the hexagonal cell constant of the Co₃W intermetallic compound of said intermediate layer (2), whereby the hexagonal crystalline orientation of said magnetisable layer (3) is enhanced. 5. A magnetic recording medium as claimed in claim 1 characterised in that said intermediate layer (2) has a hexagonal crystalline structure comprising an intermetallic compound of W and the solid substitutional solution of Co and Cr, the amount of W and CoCr being present in a stoichometric ratio of 3 to 1 so as to form the intermetallic compound (CoCr)₃W, and said magnetisable layer (3) comprises a ternary alloy CoCrW of Co, Cr and W having a hexagonal crystalline structure with its C-axis oriented generally perpendicular to said substrate (1), the amount of W present in the CoCrW alloy being such that the hexagonal cell constant of the CoCrW alloy is approximately half the hexagonal cell constant of the (CoCr)₃W intermetallic compound of said intermediate layer (2), whereby the hexagonal crystalline orientation of said magnetisable layer (3) is enhanced. 6. A magnetic recording medium as claimed in any one of the preceding claims characterised in that it includes a protective overcoat formed over said magnetisable layer (3). 7. A magnetic recording medium as claimed in any one of the preceding claims characterised in that it includes a soft magnetisable underlayer between said intermediate layer (2) and said substrate (1) for providing a magnetic flux return path.

Inventors: HOWARD, JAMES KENT
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1986
Main classification: G11B5
IPC codes: G11B5