MAGNETO-OPTICAL RECORDING MEDIUM

Abstract: In the magneto-optical recording medium with a transparent substrate (1) a film (2, 5) of dielectric indium oxide and/or tin oxide, or metallic Ti is interposed between the substrate and the recording layer, to prevent deterioration of the properties thereof.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magneto-optical recording medium, in which the information can be written, reproduced, and erased by light, such as a laser or the like. More particularly, the present invention relates to an improvement in the environmental resistance of a magneto-optical recording medium, which comprises a transparent substrate made of synthesized organic resin as well as a recording layer formed thereon and having an easy axis of magnetization perpendicular to the film plane, and which is used for the magneto-optical recording and reproduction of information by the magneto-optical effect. 2. Description of the Related Art Various studies are underway of optical recording media, since it features a high density- and high capacity-recording of information. Also, as the magneto-optical recording material is erasable, it is used for broad field of applications, and various related materials and systems have been disclosed. Actual utilization of the magneto-optical recording has been desired. Various magneto-optical materials have been proposed, such as FeTb in Japanese Unexamined Patent Publication No. 52-31,703, FeTbGd in Japanese Unexamined Patent Publication No. 56-126,907, FeTbCo and FeCoDy in Japanese Unexamined Patent Publication No. 58-73,746, and FeNd in Japanese Unexamined Patent Publication No. 61-165,846. To be able to realize an actual utilization of these materials for erasable magnetooptical recording, it is desirable that the recording and reproducing characteristics thereof be further improved, and further, the durability thereof, including the oxidation-resistance be improved, in view of the probability of corrosion, such as oxidation and the like, of the majority of the above constituent materials of the recording layer. Japanese Unexamined Patent Publication No. 59-110,052 proposes, in line with the above necessity for improvements, to prevent the recording layer from oxidation by sandwiching the recording layer of an optical recording element between two, oxygen-free films, at least one of which is a dielectric layer. Note, it is widely known to enhance the Kerr rotation angle by interposing a dielectric layer between the substrate and the recording layer. The dielectric layer must be free of oxygen and preferably is a nitride and fluoride, such as AlN, MgF₂ , ZnS, CeF₃ , AlF₃·3NaF, Si₃N₄ , and the like. Among these, the nitride layer is considered to have an excellent resistance against environmental effects. The present inventors investigated the nitride films of AlN, Si₃N₄ , and the like, which are considered to exhibit excellent environmental resistance among the dielectric materials, and discovered the existence of a slow film formation speed and a great strain in the film, which gave rise, particularly when depositing a multi-layer film on a plastic substrate, to a problem of peeling along grooves formed for the age resistance test. The peeling occurs because of an insufficient durability, which should be distinguished from the oxidation resistance. SUMMARY OF THE INVENTION In view of the background described above, it is therefore a general object of the present invention to provide a magneto-optical recording medium having an improved durability and which comprises a transparent substrate made of organic resin, a recording layer, and a dielectric layer interposed between the substrate and the recording layer. The first specific object of the present invention is to provide the above mentioned recording medium with an environmental stability, particularly a resistance against deterioration at the interface between the substrate and dielectric layer, thereby preventing cracks and peeling. A second specific object of the present invention is to stabilize the interface between the dielectric layer and magneto-optical alloy layer. A third specific object of the present invention is to alleviate degradation due to an intrusion of degrading materials, particularly H₂O and the like, from or through a plastic substrate to the interface of a magneto-optical recording layer. In accordance with the objects of present invention, there is provided a magneto-optical recording medium comprising a transparent substrate made of synthesized resin, a magneto-optical recording layer, and a dielectric layer interposed between the magneto-optical recording layer and transparent substrate, and optionally, a protecting layer, characterized in that the dielectric layer consists of a dielectric transparent oxide film containing at least one element selected from the group consisting of In and Sn. This recording medium is hereinafter referred to as the In/Sn-oxide recording medium. In the In/Sn-oxide-recording medium, the dielectric layer consisting of dielectric transparent oxide is formed by physical vapor deposition. Preferably, in the In/Sn-oxide recording medium, a protecting layer consisting of a metallic titanium film is provided between the oxide film and the magneto-optical film (this medium is hereinafter referred to as the In/Sn-oxide/Ti recording medium), since the resistances against oxidation and humidity are enhanced by the titanium film. The titanium film has a thickness of preferably 50 Å or less, more preferably 20 Å or less, in view of the recording and reproducing properties. The present invention is described in more detail with reference to the drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figures 1(A) and 1(B) show the layer structure and a top plan view of the recording media according to Examples 1, 3, and Comparative Examples 1,3; Fig. 2 illustrates the layer structure of the recording medium of Example 2; and Fig. 3 illustrates the layer structure of the recording medium of Comparative Example 2. DESCRIPTION OF THE PREFERRED EMBODIMENTS The dielectric layer consisting of dielectric transparent oxide film may be any material provided that it contains In and/or Sn and is dielectric. However, in view of the recording characteristics, the dielectric layer is preferably highly oxidized to form a stoichiometric oxide, and thus to provide an electric resistance as high as a volume resistivity of 1 x 10⁻¹ Ω·cm or more. Such oxide is indium oxide (In₂O₃) and/or tin oxide (SnOx) having a high oxygen concentration and a low concentration of carriers (electrons) due to defects and the like. The In₂O₃ and/or SnOx may contain impurities such as Ta, Sb, F, and the like therein. However, In₂O₃ and/or SnO₂ films free of impurities are most preferable because of their greater reflectivity, which is an important property for the optical interference-layer of magneto-optics. The oxide film of consisting of In₂O₃ or consisting mainly of In₂O₃ and containing a small amount of Sn oxide is preferred in the light of durability and low laser power for reading the information. The content of Sn oxide can be 30% by weight or less, and 7% by weight or less. The In₂O₃ and/or SnOx layers can be formed by the vacuum evaporation method and the sputtering method, which are known per se. The sputtering method is preferred, because it provides a condition such that the adhesion with a plastic substrate is sufficiently high that peeling will not occur in an environmental test under a high temperature- and high humidity-condition. The dielectric transparent film containing In as the constituent member of the dielectric layer according to the present invention, does not include the indium oxide film known as the transparent conductive film, because this film has a volume resistivity in the range of from 10⁻² to 10⁻⁴ Ω·cm, and thus is electrically conductive. This film contains the metallic In and has an excellent heat conductivity, with the result that, when used in contact with the recording layer and the recording layer is irradiated with a laser light spot, a heat dissipation from the recording layer to the transparent electrode occurs. This, in turn, makes it necessary to strengthen the laser power, and leads to a decrease in the S/N ratio due to distortion of the bit shape. Furthermore, an electrochemical current flows between an Fe-base alloy and a conductive oxide film at a high humidity environment. It accelerates the environmental degradation of a magneto-optical disc. Accordingly, the conductive oxide film containing In is inappropriate for application in the magneto-optical recording medium. In addition, the oxide film containing metallic In and having the resistivity of 10⁻⁴ Ω·cm has a high carrier concentration and exhibits a reflection loss when irradiated with near infrared radiation, due to plasma oscillation. Furthermore, to form a transparent conductive film by using an oxide film containing In on the transparent plastic substrate made of polycarbonate, acrylic resin and the like, the substrate usually must be heated to a temperature of approximately 100°C and, therefore, the grooves on the surface of the plastic substrate are deformed due to the thermal factor to an extent that it is difficult to attain an excellent medium. Therefore, the transparent conductive film cannot be used in the present invention, for the reasons described above. In the In/Sn oxide recording medium, the transparent oxide film may consist of either indium or tin oxide. It may also consists both of indium and tin oxides. In the In/Sn-oxide recording medium, the tin oxide layer may be any tin-containing oxide, provided that it is dielectric. Preferably, the tin oxide is SnOx, with 1 ≦ x ≦ 2, since it exhibits a small absorption of the laser light used for recording and regenerating. The refractive index of the dielectric layer containing In and/or Sn must be more than 1.7 for reading-out laser wavelength, and is preferably more than 1.9 so as to obtain adequate enhancement of the Kerr rotation angle. The effects of the In and/or Sn-containing oxide are now described in comparison with the prior art. In the prior art, in which a dielectric film is interposed between the transparent plastic substrate of a recording disc and the magneto-optical recording layer, to enhance the Kerr rotational angle due to reflection from the film surface, the insulating film represented by films of SiO, ZnS, AlN, Si₃N₄ and the like is used. When such a disc is subjected to a durability test under a high temperature- and high humidity-condition, or by heat cycles, the disc will crack, thereby drastically impairing the magneto-optical properties. This is attributable, according to researches by the present inventors, mainly to the peeling of the dielectric layer at the interface thereof with the plastic substrate. Surprisingly, however, peeling of the In and/or Sn containing oxide layer does not occur due to deterioration at the interface thereof with the substrate, thereby considerably improving the durability, particularly against heat cycles and heat shock. This improvement seems to be a result of a high affinity of the In and/or Sn-containing oxide with the plastic substrate, particularly the polycarbonate substrate. The titanium contained in the protecting layer of the In/Sn-oxide/Ti recording medium, is now described. Any adsorbed oxygen present on the surface of the dielectric layer will react with titanium, thereby preventing the degradation of the recording layer due to oxygen intrusion. Since the coefficient of thermal expansion of titanium is an intermediate value between that of dielectrics and the Fe-based alloy of recording layer, the stress which is induced due to thermal shock at the interfaces can be reduced, thereby stabilizing the recording medium. The titanium of the protecting layer may be oxidized, but even in this case, the refractive index, which is originally 1.8 or more, exhibits little change. The titanium may be alloyed, provided that the above described small dependence of refractive index upon oxidation is not impaired. The titanium protecting layer preferably has a thickness of 50 Å (1 Å = 10⁻¹⁰ m) or less, since the light absorption in the layer is 20% or less and small. A thickness of the titanium protecting layer of 20 Å or less is more preferable, since the light absorption thereof is 5% or less. The recording media according to the present invention, i.e., the In/Sn-oxide recording medium and the In/Sn-oxide/Ti recording medium are now described with regard to the magneto-optical layer, the synthetic resin- or plastic-substrate, the layer protecting a magneto-optical recording layer on the side thereof opposite to the substrate. The magneto-optical recording layer according to the present invention may be made of any thin film of magnetic metal, provided that information can be written therein or reproduced therefrom by the magneto-optical effect. For example, the thin film of magnetic metal may have an axis of easy magnetization perpendicular to the film surface and form any inverse magnetic domains, such as FeTb series alloys, e.g., FeTbCo alloy, FeTbGd alloy, and FeNd alloy, FeNd series alloys, e.g., FeCoDyNd, FeCoDyNdTi, FePr series alloys, FeSm series alloy, and FeCe series alloys. The synthetic resin substrate according to the present invention may be made of polycarbonate resin, acrylic resin, epoxy resin, and 4-methylpentene resin, as well as copolymers thereof. The polycarbonate resin is preferred in view of the mechanical properties, weathering qualities, heat resistance and vapor-permeable amount thereof. In the present invention, a protecting layer consisting of dielectrics or metal may be provided on the magneto-optical recording layer on the side thereof opposite to the substrate. Since the adhesion property of the protective layer with the substrate is of no importance, nitrides, e.g., AlN, AlSiN, and Si₃N₄ , fluorides, e.g., MgF₂ , ZnS, CeF₃ , and AlF₃·3NaF, and oxides: e.g., SiO, SiO₂ , AlSiO, and TiO₂ , can be used for the protecting layer. As the metal, Ti or Zr is preferable because of the low heat conductivity thereof. The present invention is now described in more detail by way of examples, with reference to the drawings. Example 1 (Dielectric In₂O₃) A substrate in the form of a disc having a diameter of 200 mm, a thickness of 1.2 mm and having grooves at a pitch of 2.5 µm, was made of acrylic resin (PMMA). The substrate is denoted by 1 in Fig. 1. The substrate 1 was fixed in the vacuum chamber of a high frequency magnetron sputtering device with three targets (SPF-430 type produced by ANELVA Co., Ltd.). The vacuum chamber was evacuated to a pressure of 4 x 10⁻⁷ Torr (1 Torr = 133,3 Pa) or less, and the substrate 1 was rotated at 15 rpm and water-cooled while the sputtering was carried out. Subsequently, a gaseous mixture of Ar and O₂ (20 vol%) was admitted into the vacuum chamber and the flow rate thereof was adjusted to provide a pressure of 1 x 10⁻² Torr. The target of the magnetron sputtering device for the indium oxide deposition was an In (indium) disc having a diameter of 100 mm and a thickness of 5 mm. A radio frequency reactive sputtering was carried out at a discharge power of 100 W and a discharge frequency of 13.56 MHz. As a result, an In₂O₃ film was deposited as the dielectric layer 2 having a thickness of approximately 800 Å. Subsequently, the target to be sputtered was replaced with the target for depositing the recording layer, i.e., the Fe₆₉Tb₂₃Co₈ alloy target (number indicates the composition by atomic %), and argon gas (5N) was then admitted into the vacuum chamber. The FeTbCo alloy film was deposited under the same discharge conditions as above to a thickness of approximately 1000 Å. Finally, as the protecting layer 4, the In₂O₃ film was deposited in the same way as the dielectric layer 2, to a thickness of approximately 800 Å. By the procedure as described above a magneto-optical recording medium having a layer structure of PMMA/In₂O₃/TbFeCo/In₂O₃ was obtained. The circumferential part at a point A (Fig. 1(B)) was observed by an optical microscope, and neither defects nor peeling were revealed. The layer structure as described above was allowed to stand in a constant-temperature and humidity condition of 60°C and an RH of 90% for 1000 hours. Subsequently, the circumferential part at the point A was observed by an optical microscope, and neither defects nor peeling were revealed, thus showing no change from the condition before standing. The discrete film of In₂O₃ was prepared under the same conditions as above, and the electric resistance thereof was measured. The measurement revealed the film to be an insulating film having surface resistance of 10 MΩ/sq or more and a volume resistivity of 8 x 10 Ω·cm or more. Further, a refractive index was measured by an ellipsometer (Mizoziri Co., Ltd). The measurement revealed the film to be a high refractive film having an index of refractive of 2.1 at 830 mm wave length. Comparative Example 1 (SiN) A substrate in the form of a disc having a diameter of 200 mm and a thickness of 1.2 mm and having grooves at a pitch of 2.5 µm, was made of acrylic resin (PMMA). The substrate 1 was fixed in the vacuum chamber of a high frequency magnetron sputtering device with three targets (SPF-430 type produced by ANELVA Co., Ltd.). The vacuum chamber was evacuated to a pressure of 4 x 10⁻⁷ Torr or less, and the substrate 1 was rotated at 15 rpm and water-cooled while the sputtering was carried out. Subsequently, a gaseous mixture of Ar and N₂ (50 vol%) was admitted into the vacuum chamber and the flow rate thereof was adjusted to provide a pressure of 1 x 10⁻² Torr. The target of the magnetron sputtering device for the dielectrics-deposition was an Si disc having a diameter of 100 mm and a thickness of 5 mm. A radio frequency reactive sputtering was carried out at a discharge power of 100 W and a discharge frequency of 13.56 MHz. As a result, an SiN film was deposited as the dielectric layer 2 having a thickness of approximately 800 Å. Subsequently, the target to be sputtered was replaced with the target for depositing the recording layer, i.e., the Fe₆₉Tb₂₃Co₈ alloy target. The FeTbCo alloy film was deposited under the same discharge conditions as above to a thickness of approximately 1000 Å. Finally, as the protecting layer 4, the SiN film was deposited, in the same way as the dielectric layer 2, to a thickness of approximately 800 Å. By the procedure as described above a magneto-optical recording medium having a layer structure of PMMA/SiN/TbFeCo/SiN was obtained. The circumferential part at a point A (Fig. 1(B)) was observed by an optical microscope, and neither defects nor peeling were revealed. The layer structure as described above was allowed to stand in a constant-temperature and humidity condition of 60°C and an RH of 90% for 90 hours. Subsequently, the circumferential part at point A was observed by an optical microscope, which revealed peeling along the grooves. The peeled parts were subjected to a surface analysis of the elements by means of an X-ray microprobe analysis (XMA), which revealed peeling of the dielectric layer 2. The superiority of the present invention is demonstrated by Example 1 and Comparative Example 1. Example 2 (Dielectric In₂O₃ and Protecting Ti) A substrate in the form of a disc having a diameter of 200 mm and a thickness of 1.2 mm and having grooves at the pitch of 1.6 µm, was made of polycarbonate resin (PC). The substrate 1 (Fig. 2) was fixed in the vacuum chamber of a radio frequency magnetron sputtering device with three targets (SPF-430 type produced by Anerva Co., Ltd.). The vacuum chamber was evacuated to a pressure of 4 x 10⁻⁷ Torr or less. The substrate 1 was rotated at 15 rpm and water-cooled while the sputtering was carried out. Subsequently, the gaseous mixture of Ar and O₂ (10 vol%) was admitted into the vacuum chamber and the flow rate thereof was adjusted to provide a pressure of 1 x 10⁻² Torr. The target of the magnetron sputtering device for the indium oxide deposition was an In₂O₃ sintered body having a diameter of 100 mm and a thickness of 5 mm. The high frequency reactive sputtering was carried out at a discharge power of 100 W and a discharge frequency of 13.56 MHz. As a result, an In₂O₃ film was deposited as the dielectric layer 2 to a thickness of approximately 800 Å. Subsequently, the target to be sputtered was replaced with the target for depositing the first protecting layer 5, i.e., the Ti layer, and then the Ar gas (5N) was admitted into the vacuum chamber. The Ti film 5 was deposited under the same discharge conditions as above, to a thickness of approximately 10 Å. The target to be sputtered was then replaced with the target for depositing the recording layer, i.e., Fe₆₉Tb₂₃Co₈ alloy target, and then the argon gas (5N) was admitted into the vacuum chamber. The FeTbCo alloy film was deposited under the same discharge conditions as above, to a thickness of approximately 1000 Å. Note, the substrate having a diameter of 200 mm was masked so that the above respective, In₂O₃ film, Ti film and FeTbCo alloy film were deposited on the substrate from the center to a radius of 90 mm. The mask was finally removed to enable a film to be deposited on the entire surface of substrate. The target to be sputtered was then replaced with the Ti target for forming the second protecting layer 6. The Ti film was deposited under the same conditions as described above to a thickness of approximately 200 Å. By the procedure as described above, the magneto-optical (In/Sn-oxide/Ti) recording medium having a layer structure of PC/In₂O₃/Ti/FeTbCo/Ti was obtained. In this medium, the second protecting layer 6 covers all of the layers, including the recording layer 3. The C/N ratio (carrier to noise ratio) (= S/N + 10 log (frequency band width/resolution band width) of the layer structure was measured. In this measurement, a magneto-optical recording and reproducing unit (OMS-1000 Type (III) by Nakamichi) was used. The disc was rotated at 900 rpm, and signal of 1.024 MHz was recorded in the disc by means of a laser diode light of 5.0 mW. The signal was then read out by means of a laser diode light of 0.8 mW. The applied magnetic field was 500 Öe. The result is given in Table 1 as "Example 2". The layer structure as described above was allowed to stand in a constant-temperature and humidity condition of 60°C and an RH of 90% for 1000 hours. Subsequently, the C/N ratio was measured. The result is given in Table 1 as "Example 2". The discrete film of In₂O₃ was prepared under the same conditions above and the electric resistance thereof was measured. The measurement revealed the film to be an insulating film having a surface resistance of 10 MΩ/sq or more and a volume resistivity of 8 x 10 Ω·cm or more. Comparative Example 2 A substrate in the form of a disc having a diameter of 200 mm, a thickness of 1.2 mm and having grooves at the pitch of 1.6 µm was made of polycarbonate resin (PC). The substrate 1 (Fig. 2) was fixed in the vacuum chamber of a radio frequency magnetron sputtering apparatus with three targets (SPF-430 type produced by ANELVA Co., Ltd.). The vacuum chamber was evacuated to a pressure of 4 x 10⁻⁷ Torr or less. The substrate 1 was rotated at 15 rpm and water-cooled while the sputtering was carried out. Subsequently, Ar gas (5N) was admitted into the vacuum chamber and flow rate thereof was adjusted to provide a pressure of 1 x 10⁻² Torr. The target of the magnetron sputtering device for the dielectrics-deposition was a ZnS disc having a diameter of 100 mm and a thickness of 5 mm. The radio frequency reactive sputtering was carried out at a discharge power of 100 W and a discharge frequency of 13.56 MHz. As a result, a ZnS film was deposited as the dielectric layer 2, to a thickness of approximately 800 Å. Subsequently, the target to be sputtered was replaced with the target for depositing the recording layer, i.e., the Fe₆₉Tb₂₃Co₈ alloy target, and then the FeTbCo alloy film was deposited under the same discharge conditions as above, to a thickness of approximately 1000 Å. The target to be sputtered was then replaced with the ZnS target for forming the protecting layer 4. The ZnS film was deposited under the same conditions as described above, to a thickness of approximately 800 Å. By the procedure as described above the magnetooptical recording medium having a layer structure of PC/ZnS/FeTbCo/ZnS was obtained. The C/N ratio of this layer structure was measured as in Example 2. The result is shown in Table 1 as "Comparative Example 2". The layer structure as described above was allowed to stand in a constant-temperature and humidity condition of 60°C and an RH of 90% for 500 hours. The C/N ratio was then measured, and an observation of the disc revealed a number of cracks on the surface of recording medium. The results are shown in Table 1 as "Comparative Example 2". C/N ratio Before Standing After Standing Example 2 51 dB 51 dB Comparative Example 2 51 dB 44 dB As apparent from Table 1, the C/N ratio does not change even after standing for 500 hours, but the C/N ratio decreases from 51 dB to 44 dB in Comparative Example 2. No change in the appearance occurred in Example 2, but wrinkles, presumably due to corrosion deterioration, were numerous in Comparative Example 2. Comparative Example 3 The procedure of Example 2 for producing a recording disc was repeated except that: the substrate temperature was raised to 100°C; the target was made of a sintered body of In₂O₃ (5 wt% SnO₂); and the sputtering gas was pure Ar gas (5N). The obtained In₂O₃ (SnO₂) film was transparent and conductive, and had a surface resistance of 500 Ω/sq and a volume resistivity of 4 x 10⁻³ Ω·cm. The obtained disc was rotated at 900 rpm and a signal of 1.024 MHz was recorded by means of semiconductor laser light of 5 mW. The reproducing was then carried out by means of a laser diode light of 0.8 mW, and the resulting C/N ratio was 44 dB. The power of the laser light for writing was increased to 8.5 mW, and then an C/N ratio of 49 dB was obtained. As is apparent from Comparative Example 3, the conductive transparent In₂O₃ (SnO₂) film, known as the ITO electrode, is disadvantageous in that it produces an energy loss of the laser when writing. Example 3 A substrate in the form of a disc having a diameter of 200 mm, a thickness of 1.2 mm and having grooves at the pitch of 1.6 µm, was made of polycarbonate resin (PC). The substrate 1 (Fig. 2) was fixed in the vacuum chamber of a radio frequency magnetron sputtering device with three targets (SPF-430 type produced by ANELVA Co., Ltd.). The vacuum chamber was evacuated to a pressure of 4 x 10⁻⁷ Torr or less. The substrate 1 was rotated at 15 rpm and water-cooled while the sputtering was carried out. Subsequently, a gaseous mixture of Ar and O₂ (20 vol%) was admitted into the vacuum chamber and the flow rate thereof was adjusted to provide a pressure of 2 x 10⁻² Torr. The target of the magnetron sputtering device for the tin oxide deposition was a SnO₂ sintered disc having a diameter of 100 mm and a thickness of 5 mm. A high frequency reactive sputtering was carried out at a discharge power of 50 W and a discharge frequency of 13.56 MHz. As a result, an SnO₂ film was deposited as the dielectric layer 2, to a thickness of approximately 800 Å. Subsequently, the target to be sputtered was replaced with the target for depositing the first protecting layer 5, i.e., a Ti target, and then the argon gas (5N) was admitted into the vacuum chamber. The Ti film was deposited under the same discharge conditions as above, to a thickness of approximately 10 Å. The target to be sputtered was then replaced with the TbFeCo composite target, and then the Ar gas (5N) was admitted into the vacuum chamber. The FeTbCo alloy film was deposited under the same discharge conditions as above, to a thickness of approximately 1000 Å. The target to be sputtered was then replaced with a Ti target for depositing the second protecting layer 6, and the Ti film was deposited under the same discharge conditions as above, to a thickness of approximately 200 Å. By the procedure as described above, the magnetooptical (In/Sn-oxide/Ti) recording medium having a layer structure of PC/SnOx/Ti/Fe TbCo/Ti was obtained. In this medium, the second protecting layer 6 covers all of the layers, including the recording layer 3. The C/N ratio (= S/N + 10 log (frequency band width/resolution band width) of the layer structure was measured. In this measurement, a magneto-optical recording and reproducing unit (OMS-1000 Type (III) by Nakamichi) was used. The disc was rotated at 900 rpm, and a signal of 1.024 MHz was recorded in the disc by means of a laser diode light of 7.5 mW. The signal was then read out by means of a laser diode light of 0.8 mW. The applied magnetic field was 500 Oe. The result is given in Table 2 as "Example 3". The layer structure as described above was allowed to stand in a constant-temperature and humidity condition of 60°C and an RH of 90% for 500 hours. Subsequently, the C/N ratio was measured. The result is given in Table 2 as "Example 3". The discrete film of SnO₂ was prepared under the same conditions as above, and the electric resistance thereof was measured. The measurement revealed the film to be an insulating film having a surface resistance of 10 MΩ/sq or more and a volume resistivity of 8 x 10 Ω·cm or more. Further, a refractive index was measured by an ellipsometer (Mizoziri Co., Ltd.). The measurement revealed the film to be a high refractive film having an index of refraction of 2.0 at 830 nm wavelength. C/N ratio Before Standing After Standing Example 3 51 dB 50 dB Comparative Example 3 51 dB 44 dB As apparent from Table 2, the C/N ratio does not change even after standing for 500 hours in Example 2, but the C/N ratio decreases from 51 dB to 44 dB in Comparative Example 3. A change in the appearance did not occur in Example 3, but wrinkles, presumably due to the corrosion deterioration, were numerous in Comparative Example 2. Higher power of laser diode than in Example 1 appears to result from a lower refractive index of the SnOx layer than In₂O₃ layer. Example 4 A 1000 Å thick ITO (In₂O₃-SnO₂) film was formed on a water-cooled, glass substrate by means of a high frequency sputtering method, in which the pressure of an Ar-O₂ (20 vol%) gaseous mixture in the vacuum chamber was 1 x 10⁻² Torr and the target was made of an In-Sn (Sn 5 wt%) alloy. The ITO film had a surface resistance of 100 MΩ/sq or more and was dielectric. A magneto-optical layer was formed on a polycarbonate substrate 10 x 20 mm square and 1.2 mm thick by the high frequency sputtering in the argon gas atmosphere, using an Fe₆₉Co₈Tb₂₃ alloy. The above two substrates treated by the high frequency sputtering were dipped in an aqueous solution of NaCl having a concentration of 0.1 mol/l, as electrodes in a opposed position. The current conducting between the electrodes was measured for 120 minutes. The current conduction was slight and the total quantity of electric charge was 1.4 x 10⁻² Coulomb. Comparative Example 6 A 1000 Å thick ITO (In₂O₃-SnO₂) film was formed on a water-cooled, glass substrate by means of a high frequency sputtering method, in which the pressure of Ar-gas in the vacuum chamber was 1 x 10⁻² Torr, the target was made of a sintered In₂O₃-SnO₂ (5% SnO₂), and, the substrate temperature was 130°C. The ITO film had a surface resistance of 500 Ω/sq, and was conductive and transparent. The above substrate treated by the high frequency sputtering and the polycarbonate substrate treated in Example 6 were dipped in an aqueous solution of NaCl having a concentration of 0.1 mol/l, as electrodes in a opposed position. The current conducting between the electrodes was measured for 120 minutes. The current conduction was noticeable and the total quantity of electric charge was 1.3 Coulomb. From Example 4 and the Comparative Examples, it will be apparent that a local electrochemical cell will be formed at the interface between the FeCoTb film and the conductive ITO film under an extremely humid condition. Example 5 (Dielectric In₂O₃ (SnO₂ 7 wt%) and Protecting Ti) The process of Example 2 was repeated except that instead of the In₂O₃ film the In₂O₃ containing SnO₂ was formed by using a target consisting of a In₂O₃ - 7% by weight of SnO₂. The obtained layer structure was PC/In₂O₃ (SnO₂)/Ti/FeTbCo/Ti. The same tests as in Example 2 were carried out and the same results as in Example 2 were obtained. Example 6 (Dielectric In₂O₃ (SnO₂ 30 wt%) and Protecting Ti) The process of Example 2 was repeated except that instead of the In₂O₃ film the In₂O₃ containing SnO₂ was formed by using a target consisting of a In₂O₃ - 30% by weight of SnO₂. The obtained layer structure was PC/In₂O₃ (SnO₂)/Ti/FeTbCo/Ti. The same tests as in Example 2 were carried out and the same results as in Example 2 were obtained. Example 7 (NdDyFeCo) A substrate in the form of a disc having a diameter of 200 mm, a thickness of 1.2 mm and having grooves at a pitch of 1.6 µm, was made of acrylic resin (PMMA). The substrate is denoted by 1 in Fig. 1. The substrate 1 was fixed in the vacuum chamber of a high frequency magnetron sputtering device with three targets (SPF-430 type produced by ANELVA Co., Ltd.). The vacuum chamber was evacuated to a pressure of 4 x 10⁻⁷ Torr or less, and the substrate 1 was rotated at 15 rpm and water-cooled while the sputtering was carried out. Subsequently, a gaseous mixture of Ar and O₂ (20 vol%) was admitted into the vacuum chamber and the flow rate thereof was adjusted to provide a pressure of 1 x 10⁻² Torr. The target of the magnetron sputtering device for the indium oxide deposition was an In (indium) disc having a diameter of 100 mm and a thickness of 5 mm. A radio frequency reactive sputtering was carried out at a discharge power of 100 W and a discharge frequency of 13.56 MHz. As a result, an In₂O₃ film was deposited as the dielectric layer 2 having a thickness of approximately 800 Å. Subsequently, the target to be sputtered was replaced with the target for depositing the recording layer, i.e., the Nd 7.5 Dy 17.5 Fe 52.5 Co 22.5 alloy target (number indicates the composition by atomic %), and argon gas (5N) was then admitted into the vacuum chamber. The NdDyFeCo alloy film was deposited under the same discharge conditions as above to a thickness of approximately 1000 Å. Finally, as the protecting layer 4, the In₂O₃ film was deposited in the same way as the dielectric layer 2, to a thickness of approximately 800 Å. By the procedure as described above a magneto-optical recording medium having a layer structure of PMMA/In₂O₃/NdDyFeCo/In₂O₃ was obtained. The circumferential part at a point A (Fig. 1(B)) was observed by an optical microscope, and neither defects nor peeling were revealed. The layer structure as described above was allowed to stand in a constant-temperature and humidity condition of 60°C and an RH of 90% for 200 hours. Subsequently, the circumferential part at the point A was observed by an optical microscope, and neither defects nor peeling were revealed, thus showing no change from the condition before standing. The discrete film of In₂O₃ was prepared under the same conditions as above, and the electric resistance thereof was measured. The measurement revealed the film to be an insulating film having surface resistance of 10 MΩ/sq or more and a volume resistivity of 8 x 10 Ω·cm or more.

Claims: A magneto-optical recording medium comprising a transparent substrate (1) made of synthesized resin, a magneto-optical recording layer (3), and a dielectric layer (2) interposed between the magneto-optical recording layer (3) and transparent substrate (1), characterized in that the dielectric layer (2) consists of a dielectric transparent oxide film containing at least one element of In and Sn. A magneto-optical recording medium according to claim 1, further comprising a protecting layer (5) consisting of a metallic titanium film between the dielectric layer (2) and the magneto-optical recording layer (3). A magneto-optical recording medium according to claim 2, wherein the protecting layer (5) has a thickness of 5 nm (50 Å) or less. A magneto-optical recording medium according to any one of claims 1 through 3, further comprising a protecting layer (4, 6) covering the magneto-optical recording layer (3) on its side opposite to the transparent substrate (1). A magneto-optical recording medium according to any one of claims 1 through 4, wherein said dielectric layer (2) consists of stoichiometric In₂O₃. A magneto-optical recording medium according to any one of claims 1 through 4, wherein said dielectric layer (2) consists mainly of indium oxide and additionally of tin oxide. A magneto-optical recording medium according to claim 6, wherein said indium oxide is In₂O₃ and said tin oxide is SnO₂. A magneto-optical recording medium according to claim 6 or 7, wherein the content of said tin oxide is 30% by weight or less. A magneto-optical recording medium according to any one of claims 1 through 4, wherein said dielectric layer (2) consists of SnO x , x ranging 1 ≦ x ≦ 2.

Inventors: CHIBA, KIYOSHI, SATO, TETSUO, SEKIYA, MASAHIKO, SUZUKI, KAZUTOMI
Applicants: TEIJIN LIMITED
Application year: 1987
Main classification: G11B11
IPC codes: G11B11