Production of magnetic recording medium

Abstract: A magnetic recording medium comprising a substrate and, as a recording layer, a ferromagnetic metal thin layer formed on the substrate by deposition, in which the ferro­ magnetic metal thin layer contains oxygen atom in such a manner that concentrations of oxygen atom in a surface layer and an interfacial layer adjacent to the substrate are higher than that in an intermediate layer between the surface and interfacial layers, which has improved coercive force and smaller magnetic domain.

Description: Field of the Invention The present invention relates to the production of a magnetic recording medium with improved magnetic properties comprising a substrate and, as a recording layer, a ferromagnetic metal thin layer containing oxygen atoms. Background of the invention Generally, a magnetic recording medium comprising a ferromagnetic metal thin layer as a recording layer is produced by vacuum deposition of ferromagnetic metal or an alloy containing it on a substrate such as a plastic film with travelling it around a peripheral surface of a cylindrical can installed in a vacuum deposition apparatus. To produce the magnetic recording medium with improved magnetic properties, the vaporized ferromagnetic metal is obliquely impinged on the substrate and simultaneously oxydizing gas is jetted into a stream of the vaporized metal near a position at which the vaporized metal is impinged on the substrate at the maximum impinging angle (cf. Japanese Patent Kokai Publication (unexamined) Nos. 41442/1983 and 83328/1983) or at the minimum impinging angle (cf. Japanese Patent Kokai Publication (unexamined) Nos. 41443/1983 and 83327/1983). DE-A-32 41 775 discloses a magnetic recording medium with a comparable oxygen distribution in the ferromagnetic thin film layer. In the former method, however, the magnetic properties are not satisfactorily improved. Particularly, improvement of coercive force of the recording medium greatly depends on the deposition rate. As the deposition rate increases, a concentration of oxygen atom contained in the ferromagnetic metal thin layer increases in an interfacial layer adjacent to the substrate and decreases from the interfacial layer to the surface of the thin layer. Therefore, the effect of the contained oxygen atom is not effectively exerted and it is difficult to produce the magnetic recording medium with the improved magnetic properties at such a high deposition rate as is suitable for mass production. Japanese Patent Kokai Publication (unexamined) No. 83327/1983 describes that the latter method provides a magnetic recording medium in which the concentration of oxygen atom contained in the ferromagnetic metal thin layer gradually increases from the interfacial layer to the surface. According to the study by the present inventors, in the magnetic recording medium produced by said method, the concentration of oxygen atom contained in the ferromagnetic metal thin layer decreases from the interfacial layer to the surface. Since the degree of decrease of the oxygen atom concentration is smaller than the former method, although the effect of the contained oxygen atom is not so seriously deteriorated as in the former method, still improvement of the magnetic properties is not satisfactory. Particularly, it is also difficult to produce the magnetic recording medium with the improved magnetic properties at such a high deposition rate as is suitable for mass production. Japanese Patent Kokai Publication (unexamined) No. 37843/1983 a method to improve the efficiency of diagonal vapor deposition and to obtain a magnetic recording medium of improved coercive force by passing gaseous oxygen in larger amount, in a region of a smaller incident angle than in a region of a larger incident angle in the vapor depositing region where a vapor depositing material is blown to a substrate. In a vacuum vessel, a non-magnetic substrate is taken up from a roller onto a take-up roller via guiding roller. A vapor deposition material is disposed in the lower part of the vessel, and is heated to evaporate by a heating means, and is vapor-deposited at a prescribed incident angle on the surface of the substrate, whereby a ferromagnetic thin film is formed. A pipe provided with a nozzle is provided in the vapor depositing region, and oxygen flow is passed from the part where the incident angle of vapor deposition particles is small to the part where said angle is large. By this oxygen flow the evaporating particles are diffused from the region of a smaller incident angle to the region of a larger incident angle. By such method, the magnetic recording medium having improved coercive force is obtained. Summary of the invention One object of the invention is to provide a magnetic recording medium with improved magnetic properties. Another object of the invention is to provide a magnetic recording medium comprising a ferromagnetic metal thin layer as a recording layer with high coercive force and small magnetic domain. Further object of the invention is to provide a magnetic recording medium with improved distribution of oxygen atom concentration in the ferromagnetic metal thin layer. Still another object of the invention is to provide an improved method for producing a magnetic recording medium comprising a ferromagnetic metal thin layer as a recording layer with a high coercive force and small magnetic domain, which method comprises obliquely impinging a vaporized ferromagnetic metal on a substrate and simultaneously jetting oxydizing gas on the substrate. According to the present invention, there is provided a magnetic recording medium comprising a substrate and, as a recording layer, a ferromagnetic metal thin layer formed on the substrate by deposition, in which the ferro-magnetic metal thin layer contains oxygen atoms in such a manner that concentrations of oxygen atom in a surface layer and an interfacial layer adjacent to the substrate are higher than that in an intermediate layer between the surface and interfacial layers, whereby the magnetic domain is made extremely small and the coercive force is greatly increased. Further, there is provided a method for producing a magnetic recording medium of the invention, which comprises impinging a ferromagnetic metal material at an oblique impinging angle on a substrate with travelling it around a peripheral surface of a cylindrical can from a point at which the stream of the vaporized metal material is impinged at a high impinging angle to a point at which the stream is impinged at a low impinging angle, and simultaneously jetting an oxydizing gas directly onto the substrate, wherein an angle between the stream of the oxydizing gas and that of the vaporized ferromagnetic metal material is not greater than 30° and a central angle about the longitudinal axis of the cylindrical can of the directly jetted arc area of the substrate starting from the minimum impinging angle point toward a point at which the impinging angle is greater is not greater than 10°, characterized in that the oxygen gas is supplied to the substrate at the impinging angle point (A) and further jetting an oxydizing gas directly into the substrate at least at the maximum impinging angle point from a direction which does not intersect the stream of the vaporized ferromagnetic metal and a central angle about the longitudinal axis of the cylindrical can of the directly jetted arc area of the substrate starting from the maximum impinging angle point toward a point at which the impinging angle is not greater than 20°. Brief description of the drawings Fig. 1 is a schematic sectional view of a vacuum deposition apparatus suitable for practicing the method of the present invention, Fig. 2 is an enlarged sectional view of the main part of the apparatus of Fig. 1, Fig. 3 is a schematic sectional view of another vacuum deposition apparatus suitable for practicing the modified method of the present invention, Fig. 4 is an enlarged sectional view of the main part of the apparatus of Fig. 3, Figs. 5 and 6 are each a schematic view of a vacuum deposition apparatus for producing a conventional magnetic recording medium, Fig. 7 shows the distributions of the oxygen atom concentration in the ferromagnetic metal thin layers of the magnetic recording mediums produced in Examples and Comparative Examples, Fig. 8 shows relationships between the coercive force and the average oxygen atom concentration of the magnetic recording mediums produced in Examples and Comparative Examples, and Fig. 9 shows relationships between the size of magnetic domain and the average oxygen atom concentration of the magnetic recording mediums produced in Examples and Comparative Examples. Detailed description of the invention The present invention is based on the study of the relationship between the distribution of the oxygen atom concentration through the thickness of the ferromagnetic metal thin layer and magnetic properties of the recording medium. In the conventional ferromagnetic metal thin layer in which the oxygen atom concentration decreases from the interfacial layer adjacent to the substrate toward the surface layer, although the oxygen atom concentration is high in the interfacial layer in which the magnetic interaction is weak due to large distance between the magnetic particles caused by a shadow effect of the oblique impingement, the oxygen atom concentration is low in the surface layer in which the magnetic interaction is strong due to narrow distance between the subsequently grown magnetic particles. Therefore, the effect of the contained oxygen atom is not effectively exerted so that the high coercive force and good magnetic properties are not achieved. However, the oxygen atom concentration in the interfacial layer of the ferromagnetic metal thin layer is controlled by supplying an enough amount of oxygen when nuclei of the magnetic particles are formed at the maximum impinging angle point and thereby the size of the slantingly grown particle in a curved pillar shape can be reduced. In addition, if the oxygen atom concentration in the surface layer in which the magnetic interaction is strong is made higher than that in an intermediate layer between the interfacial and surface layers, non-magnetic oxides are formed in the surface layer and the magnetic interaction is suppressed. Therefore, the effect of the contained oxygen atom is effectively exterted so that the magnetic domain size further minimized, and the magnetic properties are much improved. In the specification, the surface layer of the ferromagnetic metal thin layer is intended to mean a surface layer which contains oxygen atoms which are supplied by jetting the oxydizing gas and chemically bound to the ferromagnetic metal and to exclude a contaminated layer. The oxygen atom concentration in the ferromagnetic metal thin layer is lowest in its intermediate layer. That in the surface layer is 1.5 to 6.0 times that in the intermediate layer, and that in the interfacial layer is 1.2 to 3.0 times that in the intermediate layer. The oxygen atom concentration in the ferromagnetic metal thin layer is lowest in its intermediate layer. That in the surface layer is 1.5 to 6.0 times that in the intermediate layer, and that in the interfacial layer is 1.2 to 3.0 times that in the intermediate layer. The oxygen atom concentration in the intermediate layer is from 5 to 15% of the total number of all component atoms contained in the intermediate layer, and that in the whole ferromagnetic metal thin layer is from 10 to 30% of the total number of the component atoms contained in the whole layer. When the oxygen atom concentrations are in these ranges, the magnetic recording medium has the coercive force not lower than 63.6 kA/m (800 oersted) and the magnetic domain not greater than 0.3 µm. If the oxygen atom concentration in the interfacial layer is less than the above range, is is dificult to sufficiently reduce the size of the ferromagnetic metal particle which forms a nucleus. If the oxygen atom concentration in the surface layer is less than the above range, the non-magnetic oxide around the metal particles are not developed enough to obtain high coercive force and to reduce the size of the magnetic domain. If the oxygen atom concentration either in the interfacial or surface layer is higher than the above range, the magnetic properties may change for the worse. In the specification, the whole ferromagnetic metal thin layer is intended to mean a layer which contains oxygen atom which is supplied by jetting the oxydizing gas and bound with the ferromagnetic metal and to exclude a skin layer contaminated by oxygen liberated from organic material or migrated from air. The magnetic recording medium is produced by a method according to claims 1 and 2. In the present invention, specific examples of the substrate are a plastic film made of a generally used polymer such as polyesters (eg. polyethylene terephthalate, etc.), polimides, polyamides, etc. and a metallic film made of a non-magnetic metal such as copper. Specific examples of the ferromagnetic metal material are a ferromagnetic metal such as Co, Ni, Fe, etc., their oxides, an alloy containing at least one of them and their compounds such as Co-P, Co-Ni-P, etc. The present invention will be illustrated by reference to the accompanied drawings by way of example. Fig. 1 is a schematic sectional view of an embodiment of a vacuum deposition apparatus suitable for practicing the method of the invention in which the oxydizing gas is jetted on the substrate at or near the minimum impinging angle position. A vacuum chamber 1 is kept in vacuo by means of an evacuating system 2. A cylindrical can 3 is installed in the central part of the apparatus 1, and a substrate 4 such as a plastic film is unwound from a roll 5, guided by a guide roll 6 and travelled around the peripheral surface of the cylindrical can 3. Then, it is guided by another guide roll 7 and wound by a wind-up roll 8. Beneath the cylindrical can 3, a heating vessel 9 is placed, in which a ferromagnetic metal material 10 is contained and heated to vaporize. The vapor of the material 10 is obliquely impinged on the substrate 4 by means of a deposition-preventing plate 11 placed between the cylindrical can 3 and the vessel 9. Simultaneously with deposition of the material 10, an oxydizing gas is jetted on the substrate 4 at least at a minimum impinging angle point A from a gas inlet nozzle 12 placed between the can 3 and the plate 11. As shown in Fig. 2, the position ofthe nozzle 12 is so selected that the distance D from the supply opening 12a to the position A is not longer than 15 cm and the angle α between the stream of the oxydizing gas and the stream B of the vaporized metal material impinged by the minimum impinging angle ϑ is not greater than 30°. Further, the central angle β about the longitudinal axis O of the cylindrical can 3 of the directly jetted arc area E of the substrate starting from the minimum impinging angle point A toward a point at which the impinging angle is greater is not greater than 10°. When the oxydizing gas is jetted on the substrate at least at the minimum impinging angle point A under such conditions, the concentration of oxydizing gas is maximum near the point A and, in addition, oxygen filled in the chamber 1 is tend to be trapped in the ferromagnetic metal thin layer in a relatively high concentration near the maximum impinging angle point C since the deposition rate of the ferromagnetic metal material is lower near the point C. As the result, in one hand, when the nuclei of the particles of the ferromagnetic material are formed on the substrate 4 near the point C, particles having a sufficiently small size and containing comparatively large amount of oxygen atom are formed. On the other hand, particles are suitabley grown near the point A due to rich oxygen and the deposited ferromagnetic metal material contains a sufficient amount of oxygen atom at a part in which magnetic interaction increases as the particles grow to form a ferromagnetic metal thin layer having a small magnetic domain and high coercive force. In the thus formed ferromagnetic metal thin layer, since the oxygen atom concentration in the interfacial layer and that in the surface layer are high, magnetization tends to direct in a direction perpendicular to the substrate, which is suitable for high density recording, and the distribution of the oxygen atom concentration is idealized, that is, the oxygen atom concentration in the intermediate layer is lowest and that in the interfacial layer is higher than that in the intermediate layer and lower than that in the surface layer. Thereby, the magnetic recording medium having improved magnetic properties is produced. Since a large amount of the oxydizing gas is jetted at least at the minimum impinging angle point A, oxygen atoms are trapped by the ferromagnetic metal material in a higher concentration and the coercive force is more increased as the deposition site is increased. Therefore, the magnetic recording medium having sufficiently improved magnetic properties, for example, the magnetic domain of not larger than 0.3 micron can be produced under conditions suitable for mass production, that is, under the conditions in which the minimum impinging angle of the vaporized ferromagnetic metal material is not greater than 60° and the deposition rate is not lower than 100 nm/sec (1,000 Å/sec). If the minimum impinging angle of the vaporized material is greater than 60°, the efficiency of deposition is deteriorated and unsuitable for the mass production. In addition, any impinging disturbance between the oxydizing gas and the vaporized ferromagnetic metal material hardly occur, and the impinging angle is not fluctuated, which is often found in the conventional method comprising jetting the oxydizing gas into the stream of the vaporized ferromagnetic metal material. Fig. 3 shows another vacuum deposition apparatus suitable for practicing the improved method of the invention. This apparatus has substantially the same construction as that of Fig. 1 except that the apparatus of Fig 3 further comprises a gas inlet tube 13 near the maximum impinging angle point C and the oxydizing gas is jetted from the nozzle 13 on the substrate near the point C. As shown in Fig. 4, the position of the nozzle 13 is so selected that the distance H from the supply opening 13a to the position C is not larger than 50 cm, and the central angle y about the longitudinal axis O of the cylindrical can 3 of the directly jetted arc area F of the substrate starting from the maximum impinging angle point C toward a point at which the impinging angle is smaller is not greater than 20°. In the apparatus of Fig. 3, since the oxydizing gas is jetted from the nozzle 13 on the substrate near the maximum impinging angle point C, the size of the particle nucleus of the ferromagnetic metal is more easily controlled to obtain smaller particle, and oxygen atom is trapped in the interfacial layer in a higher concentration. As the oxydizing gas, pure oxygen gas or a mixture of oxygen and diluent gas (eg. nitrogen, etc.) is preferably used. The present invention will be hereinafter explained further in detail by following Examples. Example 1 The vacuum deposition apparatus of Fig. 1 installed with the cylindrical can 3 of 60 cm in diameter was used. A base film 4 of polyethylene terephthalate was travelled around the peripheral surface of the rotating cylindical can 3 through the guide roll 6 and wound by the wind-up roll 8 through the guide roll 7. On the other hand, an alloy 10 of cobalt and nickel in a weight ratio of 8:2 was charged in the vessel 9. Then, the chamber 1 was evacuated by the system 2 to a pressure of about 6.66 mPa 5x10⁻⁵ torr). and the alloy was heated to evaporate and obliquely impinged on the substrate with the minimum impinging angle of 50° at the deposition rate of 80 nm/sec (800 Å/sec). Simultaneously, the oxygen gas was jetted from the nozzle 12 on the film 4 at the minimum impinging angle point A with varying the oxygen pressure in the chamber. The distance D between the opening 12a of the nozzle 12 and the point A was 5cm, and the angle and direction of the oxygen gas was so selected that the angle α was 20° and the angle β was 10°. In this manner, the ferromagnetic metal thin layer containing the Co-Ni alloy was formed on the base film 4 and cut in the form of plural magnetic recording tapes each having the predetermined width. Example 2 In place of the apparatus used in Example 1, the vacuum deposition apparatus of Fig. 3 was used, and the ferromagnetic metal thin layer was formed on the base film in the same manner as in Example 1 except that the oxygen gas was also jetted from the nozzle 13 on the base film 4 at the maximum impinging angle point C with varying the pressure of the jetted gas, and plural magnetic recording tapes were produced. The distance H between the opening 13a of the nozzle 13 and the point C was 15 cm, and the angle and direction of the oxygen gas was so selected that the angle γ was 20°. Comparative Example 1 In place of the apparatus used in Example 1, the apparatus of Fig. 5 was used, which has substantially the same construction as that of Fig. 1 except that the gas inlet nozzle 14 was placed between the cylindrical can 3 and the plate 11 in place of the nozzle 12 so that the gas was jetted in the direction shown by the arrow into the stream B of the vaporized ferromagnetic metal material but not directly on the base film 4. In this manner, the ferromagnetic metal thin layer was formed on the base film, and plural magnetic recording tapes were produced. Comparative Example 2 In place of the apparatus used in Example 1, the apparatus of Fig. 6 was used, which has substantially the same construction as that of Fig. 1 except that the gas inlet nozzle 15 was placed near the maximum impinging angle point C in place of the nozzle 12 so that the gas was jetted in the direction shown by the arrow into the stream B of the vaporized ferromagnetic metal material but not directly on the base film 4. In this manner, the ferromagnetic metal thin layer was formed on the base film and plural magnetic recording tapes were produced. The magnetic recording tapes produced in Examples and Comparative Examples under the constant oxygen pressure of 0.26 Pa (2×10⁻⁴ torr.) were examined for distribution of oxygen atom concentration with an Auger electron spectrometer. The results are shown in Fig. 7, in which Curves A, B, C and D correspond to the tapes produced in Example 1, Example 2, Comparative Example 1 and Comparative Example 2, respectively. A is clear from these results, while, in the recording tapes produced in Comparative Examples, oxygen atom was contained in the ferromagnetic metal thin layer with gradually decreasing concentration from the interfacial layer to the surface layer, in the recording tapes produced in Examples, oxygen atom concentration was highest in the surface layer, next highest in the interfacial layer and lowest in the intermediate layer. The coercive force and the magnetic domain size were measured of the magnetic recording tapes produced in Examples and Comparative Examples. The results are shown in Figs. 8 and 9. Fig. 8 shows the relationship between the coercive force and the average oxygen atom concentration in the whole ferromagnetic metal thin layer. Curves A to D correspond to the tapes produced in Example 1, Example 2, Comparative Example 1 and Comparative Example 2, respectively. Fig. 9 shows the relationship between the size of the magnetic domain and the average oxygen atom concentration in the whole ferromagnetic metal thin layer. Curves A to D correspond to the tapes produced in Example 1, Example 2, Comparative Example 1 and Comparative Example 2, respectively. As is seen from the results shown in Figs. 8 and 9, the magnetic recording tapes of the invention produced in Examples had high coercive force and smaller magnetic domain than those produced in Comparative Examples. While the coercive force of the magnetic recording tape produced by the conventional methods is 55.6 kA/m (700 oersted) or less and its magnetic domain is 0.5 µm or larger, the coercive force of the magnetic recording tape of the invention is not lower than 63.6 kA/m (800 oersted), particularly up to 79.5 kA/m (1,000 oersted) and its magnetic domain is not larger than 0.3 µm.

Claims: A method for producing a magnetic recording medium comprising a substrate (4) and, as a recording layer, a ferromagnetic metal thin layer in which said thin ferromagnetic metal layer has an oxygen atom concentration in a surface layer of said metal layer and in an interfacial layer adjacent to said substrate higher than that in an intermediate area between said surface and said interfacial layer and wherein said oxygen atom concentration in said surface layer being from 1.5 to 6.0 times that of said intermediate layer and said oxygen atom concentration in said interfacial layer being from 1.2 to 3.0 times that of said intermediate layer, said oxygen atom concentration in said intermediate layer being from 5 to 15% of the total number of all component atoms contained in said intermediate layer, and the oxygen atom concentration in the whole of said ferromagnetic metal thin layer being from 10 to 30% of the total number of the component atoms contained in said whole layer, said thin ferromagnetic metal layer having a coercive force of not lower than 63.6 kA/m (800 Oe) and a magnetic domain of not greater than 0.3µm, which process comprises impinging a ferromagnetic metal material (10) at an oblique impinging angle on a substrate (4) with travelling it around a peripheral surface of a cylindrical can (3) from a point at which the stream of the vaporized metal material is impinged at a high impinging angle to a point at which the stream is impinged at a low impinging angle, and simultaneously jetting an oxidizing gas (12) directly into the substrate, wherein an angle (α) is not greater than 30° between the stream of the oxidizing gas and that of the vaporized ferromagnetic metal material impinged in the minimum angle impinging point (A), and a central angle (β) is not greater than 10° about the longitudinal axis of the cylindrical can (3) of the directly jetted arc area of the substrate (4) starting from the minimum impinging angle point (A) toward a point at which the impinging angle is greater, characterized in that the oxygen gas is supplied to the substrate at the minimum impinging angle point (Å). A method according to claim 1, which further comprises jetting an oxidizing gas directly into the substrate at least at the maximum impinging angle point (C) from a direction which does not intersect the stream of the vaporized ferromagnetic metal wherein a central angle (γ) is not greater than 20 ° about the longitudinal axis of the cylindrical can of the directly jetted arc area of the substrate starting from the maximum impinging angle point (C) toward a point at which the impinging angle is smaller.

Inventors: ISOE NOBORU, NIIMI HIDEAKI, WAKAI KUNIO, ISOE, NOBORU, NIIMI, HIDEAKI, WAKAI, KUNIO
Applicants: HITACHI MAXELL, HITACHI MAXELL LTD.
Application year: 1985
Main classification: G11B5
IPC codes: C23C14, G11B5