MAGNETO-OPTICAL DISK

Abstract: A magneto-optical disk which has superposed on a transparent substrate three films, i.e. a film of magneto-optical recording medium, an interference film formed on one side of the film, and a protective film formed on the other side of tne film, is enabled to acquire notably high magneto-optical recording and reproducing properties and long durability by forming the interference film with a composition of AOx, LiAOy, or BizDOu (wherein A stands for at least one member selected from the group consisting of Ta and Nb, D for at least one member selected from the group consisting of Ge, Si, Ti, Pb, Zn, Al, and B, and x, y, z, and u stand for numerical values respectively satisfying the formulas 1.5 </= x < 2.5, 1.5 </= y < 3, 11 </= z </= 13, and 16 </= u </= 23).

Description: DESCRIPTION MAGNETO-OPTICAL DISK Technical Field This invention relates to a magneto-optical disk and more particularly to a novel magneto-optical disk possessing a dielectric film. Background Art The magneto-optical disk which records information in a layer of magneto-optical recording medium and reproduces or erases the recorded information by the projection of a beam of light on the layer is expected to find extensive utility in various applications because it is capable of recording replaceable information in high density. In the εtratal structures heretofore proposed for magneto- optical disks of this class, the structure which has superposed on a transparent substrate formed of glass or such a transparejit macromolecular substance as polycarbonate, acrylate, epoxy, or polymethyl penteήe, as the first layer, a dielectric film such as of SiOp, SiO, or CeOp serving the purpose of protecting a layer of magneto-optical recording medium of the ϊb-Fe type, Tb-Ee-Co type, or Gd-Tb-Fe type having a thickness exceeding 300 A, and, at the same time, allowing the portion of the light for reading information which is reflected within the aforementioned layer of magneto-optical recording medium to undergo multiple reflection (interference) therein thereby increasing the apparent Kerr's angle of rotation of the reflected light for reading and enhancing the reproduction C/N ratio (enhancement effect), as the second layer, the aforementioned layer of magneto-optical recording medium and, as the third layer, a dielectric film such as of SiOp, or SiO, or CeO or a film of such a metal as Al, Bi, or Pb for the protection of the aforementioned layer of magneto- optical recording medium (the specification of Japanese Patent Application Disclosure SHO 59(^98-4-)-110,0_36 and the specification of Japanese Patent Application Disclosure SHO 59(1984-)-121,J68) and the structure which has superposed on the same transparent substrate as mentioned above, as the first layer, a dielectric film for the protection of a layer of magneto-optical recording medium having a thickness of not o more than _O0 A, as the second layer, the aforementioned layer of magneto-optical recording medium, as the third layer, a dielectric film serving the purpose of protecting the afore¬ mentioned layer of magneto-optical recording medium and, at the same time, allowing the light for reading information which passes through the film to undergo multiple reflection (interference) between the film and a metal film thereby increasing the apparent Faraday's angle of rotation of the reflected light for reading and enhancing the reproduction C/N ratio (enhancement effect) and, as the fourth layer, the aforementioned metal film (the specification of Japanese Patent Application Disclosure SHO 60(1985)-129,95 an the specification of Japanese Patent Application Disclosure SHO 60( 985)-209,9 7), for example, have found recognition as the most promising versions from the viewpoint of practica¬ bility of magneto-optical disks. In the magneto-optical disks of the foregoing structures, however, the prevention of dropout of the recorded information and the enhancement of the reproduction C/N ratio have not be fully attained. An object of this invention is to provide a magneto- optical disk which solves these problems, excels in both long durability and magneto-optical recording and reproducing properties, and completely deserves practical adoption. Disclosure of Invention The magneto-optical disk according to the present invention is provided with an interference film of a composition repre¬ sented by AOx, LiAOy, or BizDOu (wherein A stands for at least one element selected from the group consisting of Ta and Nb, D stands for at least one element selected from the group consisting of Ge, Si, Ti, Pb, Zn, Al, and B, and x, y, z, and u stand for numerical values respectively satisfying 1.5 - x<2.5, 1-5 ^3<3, 11 ? z > 13, and 16 ≤ u 23) and a protective film other than the interference film. The present invention requires the composition of the aforementioned interference film to be represented by AO , LiAOy.' or BizDOu (wherein A stands for at least on element selected from the group consisting of Ta and Nb, D stands for at least one element selected from the group consisting of Ge, Si, Ti, Pb, Zn, Al, and B, and x, y, z, and u stand for numerical values respectively satisfying 1.5 _> x<2.5, 1.5 * _. y < 3, 11 * ≥ z * _. 13, and 16 u 23). This is because these components are so stable in wide ranges of the variables x, y, and u that each of the compositions is capable of absor¬ bing the oxygen impinging thereon from the ambience and manifesting the effect thereof in preventing the layer of magneto-optical recording medium from oxidation to a great extent as compared with the conventional dielectric film. This interference film possesses the optimum index of refraction for efficient use of the enhancement effect which ensures retention of the reflectance necessary for reading (intensity of information) and improve the repro¬ duction C/N (contrast of information) and, therefore, manifests a marked effect of an interference film as compared with the conventional dielectric film. The conditions imposed on the variables x, y, and u are essential for the following reason. If the variables are less than the lower limits, a metal is formed within the dielectric film and this metal prevents formation of a homogenous transparent film. If they exceed the upper limits, there ensures presence of excess oxygen or nitrogen and the excess, oxygen or nitrogen enters the layer of magneto- optical recording medium and degrades the properties thereof. The conditions imposed on the variable z axe essential similarly. The position at which the dielectric film is disposed hinges on the thickness of the magneto-optical recording medium; the dielectric film is formed in an inner layer relative to the medium when the thickness exceeds 300 and in an outer layer relative thereto when the thickness is not more o than 300 A. The thickness of the dielectric film is desired o to fall in the range of 200 to 1,200 A. If this thickness o is less than 200 A, the heterogeneity of the film as due to occurrence of pinholes is aggravated and the layer of the magneto-optical recording medium is oxidized to the extent of inducing the phenomenon of dropout of information. Thus, the effect of the dielectric film in protecting the layer of the recording medium is not sufficient. If the thickness o exceeds 1,200 A, the dispersion of the thickness of the film increases so much as to mar the efficient use of the enhance¬ ment effect. As the composition of the protective film other than the interference film, when this protective film is formed (1) as the first layer on the transparent substrate, any of the compositions such as SiOp, SiO, and CeOp conventionally used for the dielectric film, any of the compositions here¬ tofore proposed by the present applicant such as E F._ 0 (wherein E stands for Zr or Cr, F for Hf or Zn, a for a numerical value satisfying the formula 0 * ≥ a ^ , and v for a numerical value satisfying the formula v > 0.5; Japanese Patent Application disclosure SHO 62(1987)-33,350), G fe Zr^ ,0 (wherein GT stands for I or Al, b for a numerical value satis¬ fying the ormula 0 < b < 1, and w or a numerical value satisfying the formula 0.6 w ≤ 1.9; Japanese Patent Application Disclosure SHO 62(1987)--3,851), JOc (wherein J stands for Gd, Tb, Dy, Ho, Yb, or Y and s for a numerical value satisfying the formula 0.6 ^ s ' .3; Japanese Patent Application Disclosure SHO 6 (1987)-4-7,8 7), and L H, H t (wherein L stands for Zr, B, or Cr, M for Hf, Al, or Ti, c for a numerical value satisfying the formula 0 $- c ^ 1, and t for a numerical value satisfying the formula 0.4- t ^0.9; Japanese Patent Application Disclosure SHO 60(1985)-208, 52), or any of the compositions used herein for the interference film such as A'O ,, LiA'O , , and x ' Bi ,D'0 , (wherein A' stands for at least one element selected z 1 u' from the group consisting of Ta and Nb, D' for at least one element selected from the group consisting of Ge, Si, Ti, Pb, Zn, Al, and B, x' for a numerical value satisfying the formula -1.5 l x' < 2.5-. y'for a numerical value satisfying the formula 1.5 = y' < 3, z' for a numerical value satisfying the formula 11 * _. z' _ 13, and u' for numerical value satisfying the formula 16 =- u = 23) can be used. When the protective film is formed (2) as the third layer, any of the compositions enumerated above in the case of (1) or any of such metallic elements as Al, Bi, and Pb conventionally used for the metal film can be used. This protective film is desired to be formed in a thickness of not less than 200 A. If the thickness of this o film is less than 200 A, the heterogeneity of the film as due to occurrence of pinholes is aggravated possibly to the extent of preventing the protective film from providing effective protection for the film of magneto-optical recording medium. When the protective film is formed as the third layer, a film of such metallic element as Al, Bi, or Pb may be formed as the fourth layer. The component films of the magneto-optical disk according to this invention may be formed by high-frequency sputtering, DC sputtering, vacuum evaporation, etc. for example. (Example) Now, working examples of the present invention will be cited below in conjunction with comparative experiments and an example of prior art. On grooved transparent substrate of polycarbonate 1.2 mm in thickness, a first layer was formed, then a second layer of a film of magneto-optical recording medium of the composition of Tb 0 p 7 ^ e n 6 -^° 0 08 ^^ un N° s - 1 - 9 1 ) or of Tb 0 .27 Fe 0.60 Cθ 0.13 (Run NθS* 92 " Λ 7 ^ was superposed in a thickness of 50 A, and a third layer was further superposed, invariably by the high-frequency sput¬ tering technique. In case of use of Run No.92 - 137 as the e second layer, an aluminum film of 1000 A was further super¬ posed as a fourth layer. The components, compositions, and thickness of the* first and third layer produced in the varying test runs are shown in the following table. On a varying magneto-optical disk produced consequently, information was recorded using a recording frequency of 2 MHz and a laser power of 4 mW. Then, the recorded information was reproduced using a reproducing laser power 1 mW. In the foil- lowing table, the initial reproduction C/N of a given magneto- optical disk is reported in the relative value A based on the value obtained with the specific sample using a first layer made of SiO (Run No. , as example of prior art) and the re¬ production C/N of the magneto-optical disk determined after 1,000 hours' exposure of the magneto-optical disk to an atmosphere at 60°C and 90% of relative humidity is reported in the relative value B based on the aforementioned initial reproduction C/N of the disk. - cont'd - (Note) Ex.* = Example C.E.** = Comparative Experiment - cont 'd - - cont'd - - cont'd - - cont ' d - - cont'd - It is clearly noted from the foregoing table that in all the working examples of this invention, the relative values A of initial C/N exceed 1.25 and the relative values B of C/N after aging exceed 0.95- Thus, the magneto-optical disks produced in these working examples possess very high magneto-optical recording and reproducing properties and excel in long stability and that in the example of prior art and the comparative experiments, the magneto-optical disks are not excellent in both magneto-optical recording and reproducing properties and long durability. This invention is not limited to the stratal compositions indicated in the working examples cited above. For example, stratal compositions having a layer of a resin capable of being set with ultraviolet light superposed additionally on the dielectric film or the metallic film serving as the third .or fourth layer and stratal composition obtained by opposing two identical magneto-optical disks of the foregoing compositions across a space and filling this intervening space with an inert gas can be obtained as embodiments of the present invention. Industrial Applicability As described above, the magneto-optical disks according to the present invention enjoy a great advantage that they possess notably high magneto-optical recording and reproducing properties and excel in long durability.

Claims: CLAIMS: " 1. A magneto-optical disk having superposed on a transparent substrate three films, i.e. a film of magneto- optical recording medium, an interference film formed on one side of said film, and a protective film formed on the other side of said film, characterized by the fact that said interference film is formed of a dielectric material of the composition of AO , LiAO , or BiD 0 (wherein A stands for at least one member selected from the group consisting of Ta and Nb, D for at least one member selected from the group consisting of Ge, Si, Ti, Pb, Zn, Al, and B, and x, y, z, and u stand for numerical values respectively satisfying the formulas 1.5 _>x <2.5, 1.5 -y < 3, 11 *≥ z f 13, and 16 _fu _?23). - 2. The magneto-optical disk according to Claim 1, wherein the first layer contiguous to the transparent substrate is in interference film, the second layer is formed of a magneto- o optical recording medium in a thickness exceeding 300 A, and the third layer is a protective film. 3- The magneto-optical disk according to Claim 2, wherein said protective film is made of a dielectric substance or a metallic substance. 4. The magneto-optical disk according to Claim 1, wherein the first layer contiguous to the transparent substrate is a protective film, the second layer is formed of a magneto- o optical medium in a thickness of not more than 300 A, and the third ' layer is an interference film. 5. The magneto-optical disk according to Claim 4, wherein said protective film is made of a dielectric substance. 6. The magneto-optical disk according to Claim 1, wherein said interference film has a thickness in the range of 200 to 1,200 A. 7. The magneto-optical disk according to Claim 1, wherein said protective film has a thickness of not less than 200 A. 8. The magneto-optical disk according to Claim 1, wherein said protective film is formed of a composition of Si0 2 , SiO, or Ce0 2 9- The magneto-optical disk according to Claim 1, wherein said p r rotective film has a composition of (wherein E stands for Zr or Cr, F for Hf or Zn, and a and v stand for numerical values respectively satisfying the formulas 0 _f a _> 1 and v> 0.5). 10. The magneto-optical disk according to Claim 1, wherein said protective film has a composition of G- Zri O (wherein G stands for Y or Al and b and w stand for numerical values respectively satisfying the formulas 0 < b < 1 and 0.6 <w < 1.9). 11. The magneto-optical disk according to Claim 1, wherein said protective film has a composition of JOs (wherein J stands for Gd, Tb, Dy, Ho, Yb, or Y and s for a numerical value satisfying the formula 0.6 _f s "__- 1.3)- 12. " The magneto-optical disk according to Claim 1, wherein said protective film has a composition of L ^_ N, (wherein L stands for Zr, B, or Cr, M for Hf, Al, or Ti, and c and t stand for numerical values respectively satisfying the formulas 0 _ c * _. 1 and 0.4 _r t T 0.9). 13. The magneto-optical disk according to Claim 1, wherein said protective film has a composition of A'O , , LiA'Oy.,' or Biz,D'0u. (wherein A' stands for at least one member selected from the group consisting of Ta and Nb, D' for at least one member selected from the group con¬ sisting of Ge, Si, Ti, Pb, Zn, Al, and B, x' , y', z', and u' stand or numerical values respectively satisfying the formulas 1.5 x' < 2.5, 1.5 ^y' < 3, 11 ' z 1 ≤ 13, and 16 u' ^23). 14. The magneto-optical disk according to Claim 3 ? wherein said protective film is a film of a metallic element selected from the group consisting of Al, Bi, and Pb. 15. The magneto-optical disk according to Claim 4, wherein a metallic film is formed additionally as the fourth layer on said interference film.

Inventors: ITO MIKIHIKO, KAWAMOTO ATSUSHI, NATE TATSUO, YONEYAMA YUKINOBU, YOROZU TAKEHIKO, ITO, MIKIHIKO, KAWAMOTO, ATSUSHI, NATE, TATSUO, YONEYAMA, YUKINOBU, YOROZU, TAKEHIKO
Applicants: SUMITOMO METAL MINING CO, SUMITOMO METAL MINING CO., LTD.
Application year: 1987
Main classification: G11B5
IPC codes: G11B5, G11B11