Composite magneto-optical information recording media

Abstract: A composite magneto-optical information recording medium comprising a substrate (1) and a composite magneto-optical recording layer deposited on said substrate. The composite magneto-optical recording layer comprises a magneto-optical layer (2), a metallic layer (3) and an electron transition layer (4) which exhibits transition of electrons in a wave length range of a laser beam used for recording and/or regenerating information, the three layers being layered successively in this order so that said laser beam (L) impinges on a side of said magneto-optical layer (2).

Description: Background of the Invention Field of the Industrial The present invention relates to magneto-optical information recording media having an improved composite recording layer. The magneto-optical information recording media according to the present invention are advantageously applicable for magneto-optical recording disks, cards, tapes or the like which are used in document filing memories, video filing memories, external memories for computers. Description of the related art Magmeto-optical recording disks (MO disks) are used in actual applications as an erasable recording memory for computers. In the magmeto-optical recording disks, information is recorded by the thermo-optical effect in which the orientation of a magnetic domain is changed by heat given by a laser bean, while regeneration of the information is effected by the magneto-optical effect in which a small change of a polarization angle of a laser beam is detected optically so that the direction of magnetization in the domain is determined. The intensity of regeneration signals of the magneto-optical recording media is strongly influenced by the reflectance from a recording layer and by the change of polarization angle (polar Kerr rotational angle : ϑ k ) of a laser beam. In other words, in order to improve the performance of regeneration of the magneto-optical recording media, it is indispensable to increase the polar Karr rotational angle : ϑ k and/or to increase the reflectance. Heretofore, several ideas for increasing an apparent value of ϑ k and/or the reflectance were proposed. For example, more than one thin layers of dielectric material is deposited on the magneto-optical recording layer so that a light-pass of a laser beam is extended when the laser beam travels at plural times in the magneto-optical layer due to multi-reflection of the laser beam, resulting in increment of the value of ϑ k (this structure is called "an interference film type"). While, in order to increase the reflectance, a relatively thick reflective layer of metal such as aluminum is arranged under the magneto-optical recording layer (this structure is called "a reflective layer type"). The interference film type structure, however, is difficult to be produced in a scale of mass production because the thickness of the interference film layer of dielectric material must be controlled precisely. The film thickness of the interference film layer must be optimized if the wave length of a semiconductor laser is changed or improved because the interference effect depend on the wave length. In the case of the reflective layer type structure, the recording sensitivity is spoiled because thermal energy of a laser beam is absorbed by the metallic layer whose thermal conductivity is very high. Still more, increment of the value of ϑ k is determined by the reflectance of the reflective layer used. The present applicant proposed, in Japanese patent application No. 223,743/1988, a new concept of a composite magneto-optical recording medium. In this medium, a layer made of a substance which exhibits electron transition in a wave length range of a light source used (hereinafter called "electron transition layer") is combined with a magneto-optical recording layer. Although this magneto-optical recording medium itself functions satisfactorily, there is such a problem that the electron transition layer diffuses or migrates into an adjacent magneto-optical recording layer, resulting in that the magneto-optical recording layer is oxidized or deteriorated, because the electron transition layer is deposited directly on the magneto-optical recording layer. The present inventors tried to solve the problem and found an improved composite magneto-optical recording medium which is improved in the value of ϑ k and which is applicable in practical uses without oxidation or deterioration of the magneto-optical recording layer, and completed the present invention. Therefore, an object of the present invention is to provide an composite magneto-optical recording medium which is based on the above-mentioned new concept but which is improved in the value of ϑ k and hence is improved in the sensitivity at regeneration and which has a resistance to oxidation and degradation. Summary of the Invention A composite magneto-optical information recording medium according to the present invention comprises a substrate and a composite magneto-optical recording layer deposited on the substrate and is characterized in that the composite magneto-optical recording layer comprises a magneto-optical layer, a metallic layer and an electron transition layer which exhibits transition of electrons in a wave length range of a laser beam used for recording and/or regenerating information, the three layers being layered successively in this order so that the laser beam impinges on a side of the magneto-optical layer. The magneto-optical recording medium used in the present invention can have any shape including disk, drum, tape, card or the like and its configuration depends on a shape of the substrate. The composite magneto.optical recording layer can be formed on a surface or on both surfaces of the substrate. The media may have any known mode and structure including so-called "a reflection mode" or "a transmission mode" and "an air-sandwich type" or "tightly bonded double substrates type" etc. Generally, the reflection mode with the tightly bonded double substrates type is used mostly. According to the present invention, a laser beam which is used for recording and/or reading information enters firstly the magneto-optical layer. In other words, the three layers of magneto-optical layer, metallic layer and electron transition layer are layered successively in this order along a direction of incidence of a laser beam. Therefore, in the case of the "reflection mode", the magneto-optical layer is deposited on the transparent substrate directly or through a suitable protective layer. This protective layer.may be made of organic glass which resist to penetration of moisture through the transparent plastic substrate. The substrate can be made of any material including metal, glass, plastic and ceramics. In the case of the "reflective mode" which is the most popular mode, both of recording and read-out are effected through a transparent substrate, so that the substrate must be made of transparent material, preferably transparent plastics such as polycarbonate, polymethylmethacrylate, amorphous polyolefins. It is possible to use light-transmitting ceramics such as GGG or tempered glass. The substrate has preferably guide grooves for guiding a laser beam and/or control-signal pits for addressing or the like on a surface thereof. Usually, the substrate has pre-grooves or pre-pits for tracking a laser beam and pits corresponding pre-format for addressing, error correction and synchronization. The magneto-optical recording layer used in the present invention is made of any known material which exhibit satisfactory Kerr effect and/or Faraday effect such as amorphous alloys of rare earth / transition (R/T) metal, spinel ferrite, hexagonal ferrite or the like. The magneto-optical layer has preferably a thickness from 10 nm to 100 nm (100 Å to 1,000 Å). As defined above, the electron transition layer means a layer made of a substance which exhibits electron transition. This substance can be any material in which transition of electrons occur in the ranges of wave length of ultra-violet zone, visible zone and infra-red zone. In usual practice, substances which exhibits electron transition in the ranges of wave length of visible zone and infra-red zone are preferably used, because the laser sources which are available today for recording and/or reading-out information are semiconductor lasers, argon laser and helium laser which have their maximum absorption in the wave length ranges of visible zone and near infra-red zone. Semiconductor lasers are the most preferable because they are small in size. It is apparent that future semiconductor lasers which will have a shorter wave length of about 400 nm than about 800 nm of today's model can be applicable to the present invention. The substances for the electron transition layer which satisfy the above-mentioned conditions are easily found in dyes and/or pigments which exhibit absorption in the visible wave length zone. But, the present invention is not limited to so-called dyes and/or pigments but include any material which exhibits electron transition and absorption in the ranges of wave length from ultra-violet zone to infra-red zone. The layer of dyes and/or pigments can have a thickness from 0.1 µm to 5 µm. Now, the present invention will be described with reference to attached drawings. Brief Description of Drawings Fig. 1 is an illustrative cross section of a composited magneto-optical recording medium according to the present invention. Fig. 2 is an illustrative drawing showing the wave length dependency of dielectric constants of a pigment or dye layer, which is used for explaining the principle of the present invention Fig. 3 is a drawing showing the wave length dependency of the polar Kerr rotational angle (ϑ k ) of a magneto-optical recording medium of Example 1 according to the present invention. Fig. 4 is a drawing showing the wave length dependency of percent transmission of "Rhodamin B" which is shown for a comparison. Fig. 1 illustrates a typical construction of a composited magneto-optical recording medium according to the present invention. This composite magneto-optical information recording medium comprises a transparent substrate (1) and a series of layers deposited on the substrate and comprising a magneto-optical layer (2), a metallic layer (3) and an electron transition layer (4). A laser beam (L) for recording and reading is directed from a side of the substrate (1). A surface protective layer (5) may be formed on the electron transition layer (4) with any suitable material used in this field of technology. The present invention is an improvement in the Japanese patent application No. 223,743/1988. A basic mechanism of the magneto-optical recording medium described in this patent application is described in Fig. 2. which illustrates the change of complex dielectric constant of the substances which exhibit electron transition in a function of resonance wave lengths (ε* = ε′ - jε″). Fig. 1 shows such a fact that a reflectance of the substance increase in such a wave length range that a real component (ε′) of the complex dielectric constant becomes nearly zero while an imaginary component (jε′) of the complex dielectric constant becomes satisfactorily lower. In other words, a film made of this substance functions as a reflective layer having a high reflectance which depends on the wave length used. The wave length dependency of dielectric constant of the substances which exhibit electron transition as well as their specific wave length depending reflective effect are known in the field of optical physics. For example, I. Porkrand et al. reports, in J. Chem. Phys, 69 (9), 1978, p 4001-4011, "Optical properties of organic dyes by surface plasmon spectroscopy", specific reflection effect of a composite thin film consisting of silver and organic dye. This wave length depending reflection effect can be utilized as a reflective layer for magneto-optical recording media to enhance apparent Kerr effect by multiple reflection of a laser beam in the magneto-optical recording layer. In fact, the composite recording layer according to the present invention exhibits remarkably higher ϑ k value. Although we can't explain the reason theoretically, the inherent Karr rotational angle ϑ k might increase apparently by a combination of the above-mentioned specific wave-length depending reflection effect which is described in the Japanese patent application No. 223,743/1988 with the metallic layer. In the composite magneto-optic recording layer according to the present invention, the enhancement effect of the ϑ k value is not influenced by variation of thickness which is inevitable in actual manufacturing condition and by variation of incident angle of the laser beam. This is an important advantage of the present invention. Still more, oxidation or deterioration of the magneto-optical recording material caused by diffusion or migration of the electron transition layer into the magneto-optical layer is prevented effectively by the presence of the metallic layer interposed between the electron transition layer and the magneto-optical layer. Still more, the magneto-optical recording layer according to the present invention possesses a higher recording sensitivity than the conventional "reflection mode", because the thickness of the metallic layer can be reduced relatively and hence the thermal diffusion is minimized. Description of the Preferred Embodiments Materials for the magneto-optical layer and the metallic layer and substances for the electron transition layer used in the present invention can be selected from those that are known themselves. Followings are their examples but the scope of the present invention should not be limited to the following examples. Materials for the magneto-optical layer The magneto-optical materials used in the present invention are preferably those that have such Curie temperatures or compensation temperatures that magnetic domains can be held stably at about room temperatures and that are magnetizable perpendicularly in order to reduce the size of magnetic domains for increasing recording density. Among following magneto-optical materials, rare earth / transition metal alloys are most preferable: (1) Garnets : R ₃ Fe ₅ O ₁₂ , R ₃ (FeAl) ₅ O ₁₂ or R ₃ (FeGa) ₅ O ₁₂ in which "R" is a rare earth element such as Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Tb or Lu. (2) Spinel ferrites : MFe ₂ O ₄ in which "M" is Mn, Fe, Co, Ni, Cu, Mg or Li 0.5 Fe 0.5 (3) Hexagonal ferrites : MFe ₁₂ O ₁₉ in which "M" is Ba, Pb, Sr, Ca, Ni 0.5 Fe 0.5 or Al 0.5 La 0.5 (4) Rare earth / transition metal alloys : GdCo, GdFe, DyFe, TbFe, GdTbFe, GdDyFe, TbFeCo, GdTbFeCo, (Gd, Fe)Bi, (Gd, Fe)Sn or BdDyFeCo (5) Polycrystals : MnBi, MnCuBi, MnAlGe, MnGaGe, PtCo, EuO or PtMnSb Materials for the metallic layer The metallic layer used in the present invention can be any metal provided that they are stable so that very unstable metals such as alkali metals are excluded. Preferable metals are Al, Au, Cr, Cu, Ti, Zr, Nb, Ag, Pt, Fe, Co, Mn, Ni, Zn and V. The metallic layer must be as thin as possible in order to reduce heat loss caused by high thermal conductivity and has preferably a thickness of less than 20 nm (200 Å), more preferably less than 10 nm (100 Å). In order to compromise the enhancement effect of the value of ϑ k with the recording sensitivity, a thickness of about 50 Å is preferable. Electron transition substances Electron transmission substance can be easily selected from so-called pigments and or dyes. Followings are non-limiting examples of the substance which exhibits electron transition in the wave length range from ultra-violet zone to infra-red zone: (1) For infra-red zone Phthalocyaninine type pigments or dyes such as Co-phthalocyanine, Vo-phthalocyanine or Pb-phthalocyanine, Naphthoquinone type dues, Naphthalocyanine type dyes, Squalium type dues such as 4,6-diphenylpyrane 2 ylidene-squalium, 1,1-diethyl,4,4′-quinocarbo cyanine iodide, Cyanine type dies, Azurenium type dyes, Chroconium type dyes or Dithiol nickel complexes. (1) For visible zone Crystal violet, Acridine dyes, Rhodamin B, Pigment Yellow 37, CI 77199, Pigment Yellow 35, CI 77117, Pigment Orange 20, CI 77196, Pigment Red 108, CI 77202, Pigment Orange 21, CI 77601, Pigment Yellow 34, CI 77603, Pigment Green 17, CI 77288, Pigment Green 18, CI 77289, Pigment BLue 28, CI 77346, Pigment Blue 27, CI 77510, Pigment Violet 16, CI 77442, Pigment Orange 23, CI 77201, Pigment Brown 6, CI 77499, Pigment Red 113, CI 77201, Pigment Red 104, CI 77605, Pigment Yellow 53, CI 77788 Pigment Red 101, CI 77491, Pigment Red 105, CI 77578, Pigment Blown 7, CI 77499, Pigment Yellow 32, CI 77893, Pigment blue 29, CI 77007, Pigment Blown 7, CI 77499, Pigment Red 10d, CI 77766, Pigment Yellow 36, CI 77955, Pigment Blown 11, CI 77495, Pigment Yellow 36, CI 77955, Prucyane blue or Antimony Red. Each of the magneto-optical layer, the metallic layer and the electron transition layer can be formed or deposited on the substrate by physical vapour deposition (PVD) technique such as vacuum deposition, sputtering or the like. The electron transition layer can be formed also by coating technique such as spin-coating, dipping, spray coating or the like. In this case, the pigment or dyes can be dispersed or dissolved in suitable solvents, binders or the like. It is apparent from above that, in the composite magneto-optical recording media according to the present invention, following advantages are expected: (1) Production management in a plant for producing the magneto-optical recording media can be facilitated owing to substantially no independence on wall thickness of the respective layer. (2) The Kerr rotational angle (ϑ k ) can be increased greatly. (3) Loss of sensitivity at recording phase caused by increased thermal conductivity due to the existence of the metallic layer can be reduced greatly because the metallic layer can be thinner than the conventional reflection model media. (4) Oxidation of magneto-optical layer caused by diffusion or migration of pigments or dyes can be prevented because the metallic layer is interposed between the magneto-optical layer and the pigments or dyes layer. Now, examples of the present invention will be described but the scope of the invention should not be limited to the example. Example 1 Preparation of Composite Magneto-Optical Disk A reflection mode magneto-optical recording disk shown in Fig. 1 was produced. At first, a magneto-optical recording layer of Tb₂₃Fe₆₇Co₁₀ and having a thickness of 15 nm (150 Å) was deposited on a disk substrate made of glass having a diameter of 12 cm and a thickness of 1.2 mm by well-known radio-frequency (RF) sputtering method. Then, a metallic layer of aluminum (A1) having a thickness of 5 nm (50 Å) was deposited on the magneto-optical recording layer by the same method as above. As the substrate which exhibit electron transition, "Rhodamin B" was selected and was deposited on the metallic layer of aluminum by well-known vacuum evaporation method. The thickness of the "Rhodamin B" was about 1 µm. Finally, a protective layer of silicon of 40 nm (400 Å) thick was deposited on the "Rhodamin B" layer. Evaluation of the Disk The resulting optical disk was magnetized perpendicularly with respect to a surface of the substrate in an external magnetic field of 10 K Oe. After the external magnetic field was removed, a light beam from a monochromator was directed onto an opposite surface of the substrate at an incident angle of about 90 degree with respect to the surface of the disk and the wave length was shifted continuously to determine change of the value of ϑ k (polar Kerr rotational angle). Fig. 3 shows a wave length dependency of the ϑ k value (minute). Fig. 3 reveals such a fact that the composite magneto-optical recording layer according to the present invention shows a very high apparent polar Kerr rotational angle (ϑ k ) of 21 minute at a wave length of about 500 nm. For comparison, only a layer of "Rhodamin B" was deposited on a glass disk substrate by the same method as above and the percent transmission of the resulting "Rhodamin B" layer was measured by the monochromator. Fig. 4 shows the result of wave length dependency of the percent transmission of the "Rhodamin B" layer. From Fig. 4, it was understood that the maximum absorption of "Rhodamin B" layer occur at about 580 nm. Comparing Fig. 3 with Fig. 4, it is apparent that the enhancement effect of the polar Karr rotational angle (ϑ k ) of the composite magneto-optical recording layer at the wave length of about 500 nm is contributed to the existence of "Rhodamin B" layer. This experimental result reveals such a fact that the Karr rotational angle (ϑ k ) can be increased greatly at predetermined wave lengths by adopting the composite recording medium having the magneto-optical layer, the metallic layer and the dye or pigment layer which are laminated in this order from the incident direction of a laser beam. The wave length depends on nature of pigments or dyes used. In other words, magneto-optical recording media which are the most suitable for a selected wave length can be produced by selecting nature of pigments or dyes.

Claims: A composite magneto-optical information recording medium comprising a substrate (1) and a composite magneto-optical recording layer deposited on said substrate, a magneto-optical layer (2), a metallic layer (3) and an electron transition layer (4) which exhibits transition of electrons in a wave length range of a laser beam used for recording and/or regenerating information, the three layers (2, 3, 4) being layered successively in this order so that said laser beam (L) impinges on a side of said magneto-optical layer (2). The composite magneto-optical information recording medium set forth in Claim 1 wherein said metallic layer (3) has a thickness of less than 20 nm (200 Å). The composite magneto-optical information recording medium set forth in Claim 2 wherein said metallic layer (3) has a thickness of less than 10 nm (100 Å). The composite magneto-optical information recording medium set forth in any one of Claim 1 to 3 wherein said metallic layer (3) is made of a metal selected from the group comprising Al, Au, Cr, Ti, Zr, Nb, Ag, Pt, Fe, Co, Mn, Ni, Zn and V. The composite magneto-optical information recording medium set forth in any one of Claim 1 to 4 wherein said electron transition layer (4) is made of dyes and/or pigments. The composite magneto-optical information recording medium set forth in Claim 5 wherein the layer of dyes and/or pigments has a thickness from 0.1 µm to 5 µm. The composite magneto-optical information recording medium set forth in any one of Claim 1 to 6 wherein said magneto-optical layer (2) is made of alloy of rear earth / transition metal. The composite magneto-optical information recording medium set forth in Claim 7 wherein said magneto-optical layer has a thickness from 10 nm to 100 nm (100 Å to 1,000 Å). The composite magneto-optical information recording medium set forth in any one of Claim 1 to 8 wherein said magneto-optical layer (2) is deposited on a surface of a substrate (1) made of transparent material. The composite magneto-optical information recording medium set forth in Claim 9 wherein a surface protective layer (5) is further deposited on said electron transition layer (4).

Inventors: KITAGUCHI TOORU, YONEDA MIKIO, KITAGUCHI, TOORU, YONEDA, MIKIO
Applicants: DAICEL CHEM, DAICEL CHEMICAL INDUSTRIES, LTD.
Application year: 1990
Main classification: G11B11
IPC codes: G11B11