Magneto-optical recording medium

Abstract: In a magneto-optical disk for recording, reproduction or erasing with a laser beam, which comprises a film of four-layer structure consisting of a first dielectric layer (2), a magneto-optical recording layer (3), a second dielectric layer (4) and a metallic layer (5), laid on a disk substrate (1) provided with guide tracks, temperature distribution of the magneto-optical recording layer (3) is controlled by controlling the thermal diffusivity of the metallic layer (5), and recording/reproduction/erasing repetition characteristics are improved thereby. In the disk of the present structure, the temperature is elevated in a broader area from the center of laser beam, and thus a thoroughly broad erasing domain width taking a track offset into account can be obtained and not recording domains remain after the erasing. The reproduced signal output is increased by 4 dB, as compared with that obtained by a disk of three-layer structure without any metallic layer, because the beam utilization efficiency is increased owing to both Kerr effect and Faraday effect. When the disk of the present structure is subjected to field modulation recording, tails of arrow-feather-shaped recording domains peculiar to the field modulation recording can be shortened and interbit interferences can be overcome, resulting in an increase in C/N (S/N) ratio and the high density recording can be made. By endowing a Kerr enhancement action to the second dielectric layer (4) and a protective action on the magneto-optical layer (3) to the first dielectric layer (2), only the reliability can be improved without lowering the C/N ratio. By improving the corrosion resistance of the magneto-optical recording layer, the first dielectric layer (2) can be omitted to the contrary, and a magneto-optical disk of simple structure can be obtained and used as memory for domestic applica­ tions or communication fields.

Description: BACKGROUND OF THE INVENTION This invention relates to magneto-optical recording capable of recording/reproducing/erasing with laser beam, and more particularly to the structure of a magneto-optical disk effective for the suppression of deterioration in the reproduction output due to repetitions of recording/erasing while maintaining a high S/N and also effective for the control of shape of formed recording domain. With the recent progress of highly informational society, needs for a file memory of larger capacity at a higher density are now increasing and optical disks have been regarded as a response to the needs. Particularly, magneto-optical recording is now going to be used in the form of a practical, rewritable optical disk. The so far known layer structures are classified into two types, i.e. three-layer structure (underlayer/recording layer/protective layer) and four-layer structure (underlayer/recording layer/protective layer/metallic layer). Above all, disks of four-layer structure utilize both of Kerr effect and Faraday effect of magneto-optical recording layer at the same time and thus a large rotational angle can be obtained and also the reproduction output can be increased at the driving. Thus, the disks of four-layer structure are now extensively studied and are under development. In US-A-4 466 035 an information medium having a multi-layer structure is described. The combination of the layers exhibit antireflectivity, that is, incident light is absorbed in the medium and is captured in a light-transmissive layer. In this way, the magneto-optical intensity exciting the magneto-optic medium is enhanced without eliminating normally reflected light, such as e.g. incident read-out light. In EP-A-0 191 226 a rewriteable magneto-optical disc is described which comprises several layers on a substrate. These layers are characterized by their contents of rare earth elements and transition metal elements. According to the document, with the given amount of,rare earth elements and transition metals in the layers, a good anti-oxidation property is achievable. In order to achieve a greater magneto-optical effect without decreasing the quantity of reflected light in JP-A-62-295238 a thin magnetic material film having its axis of easy magnetization perpendicular to the film plane is proposed. The film is deposited on a substrate. The composition of the perpendicularly magnetized film of amorphous ferromagnetic material is given. The film is superposed by resin with belt-like grooves. Either the grooves or the dikes may be used as recording track. They further serve as guide tracks for serval control of the medium. DE-A-3 623 285 concerns the consistency of an amorphous layer and gives the composition coefficients for the rare eath elements. The underlying intention is to provide an amorphous magneto-optical recording medium with sufficiently strong magneto-optical effects for high read-out signals, which can be manufactured with low costs. The given alloys with their specific composition coefficients exhibit great Kerr-rotation angles when present as an amorphous film. In JP-A-62-293539 a magneto-optical recording medium is described and, in particular, the heat conductivity of each of the layers is contemplated. EP-A-0 192 256 discloses another magneto-optical medium of such a four-layer structure of dielectric layer/GdTbFe-layer/dielectric layer/reflecting layer on a substrate. It is intended to provide a magneto-optical storage element, in which recorded signals can be stored without loss in quality, but on the contrary, may even undergo a certain improvement. This can be achieved by a certain composition of elements in the alloy of the recording medium. As regards EP-A-0 139 474 a magneto-optical memory element having an amorphous alloy thin film recording layer consisting of rare earth elements and transition metals is proposed, which exhibits improved stabilization of the amorphous alloy thin film recording layer and enhanced photoelectromagnetic effects. This is achieved by utilization of nitride dielectric layers and an alloy thin film recording layer and a reflection layer, each having a corresponding refraction index. In EP-A-0 233 034 an optical memory element is described which substantially contains, besides other layers, two reflective films. By this two-layer reflective film structure a better signal reproduction and a higher recording sensitivity is intended. This is achieved by various layers of large optical reflectance and small thermal conductivity. In EP-0 228 909 a magnetooptical storage element according to the preamble of claim 1 is described. This magnetooptical storage comprises a multi layered structure including a reflecting film which is made of a metal or metalloid having a low thermal conductivity coefficient, preferably tantalum or titanium. On the other hand, studies on higher density and higher speed are also underway. For example, it has been proposed to utilize a field modulation recording system, a system for using a double layer based on an exchange couple, etc. in order to attain the overwritting. In order to attain a higher density, it has been studied to form micromagnetic domains or use pit edge recording. Anyway, disks having a high S/N and a good controllability of magnetic domains are required. If magneto-optical disks can be used not only in computer file memory, but also for domestic uses for example, as rewritable compact disks, a larger impact will be posed on the development. The first condition for this purpose is a lower cost. To satisfy the condition, it is necessary to use low cost plastics as substrate materials and make the disk structure as simple as possible. This can be attained only on the basis of improvements in disk technology such as (1) technology of preparating substrates, (2) technology of making a magneto-optical recording layer of higher corrosion resistance and (3) optimization of disk structure, etc. Particularly, the technology of making a magneto-optical recording layer of higher corrosion resistance, as mentioned in (2), is important. In the above-mentioned disk technology, research and development have been so far concentrated particularly on an increase in the rotational angle, and no particular consideration has been paid to the decrease in disk characteristics, especially reproduction output, when recording/erasing are repeated, and no higher reliability has been obtained. This is because the amorphous state of magneto-optical recording layer is relaxed with a laser beam used for the recording/erasing. In the case of higher density recording by field modulation recording, recording domains themselves interfere with one another or no better shapes of recording domains are obtained, and thus no satisfactory reproduction output is obtained. This is because parts in a larger ratio of the domain wall to the domain area exist and consequently the energy of the domain wall is so high that the shapes of recording domains become unstable. In the case of domestic applications, etc. on the other hand, no satisfactory simplification of layer structure has been made yet, resulting in poor mass production and less realization of lower cost. That is, the magneto-optical disks have been found to be unsuitable for the domestic applications. Still furthermore, the underlayer made mainly of inorganic compounds must have a thickness of at least 75 nm in view of the optical effect, which increases an overall stress of the magneto-optical recording medium and causes the underlayer to peel off, especially when formed on a plastic substrate. In order to prevent such a peeling, it has been attempted to reduce the stress on the underlayer of inorganic compounds or give some treatment to the substrate surface. The thickness of the underlayer of inorganic compound has been restricted to 70-90 nm in view of the optical characteristics, and in that case the recording layer has been sometimes corroded through pinholes existing in the underlayer. SUMMARY OF THE INVENTION The object of the present invention is to increase density and reliability of the magneto-optical recording. This object is accomplished by a magneto-optical medium according to claim 1. The subsequent dependent claims are directed to particular embodiments of the magneto-optical medium according to the invention and its use. In an optical disk that conducts recording/reproduction/erasing with a laser beam, the reproduction output is sometimes lowered when recording/reproduction/erasing (W/R/E) are repeated. Thus, there has been a problem of low reliability, causing a disk error, etc. Thus, it is an urgent need to suppress the decrease in the reproduction output when recording/reproduction/erasing are repeated. To this end, a film of 4-layer structure comprising a first dielectric layer, a magneto-optical recording layer, a second dielectric layer and a metallic layer, laid successively in this order on a disk substrate has been proposed to cause a heat flow from the magneto-optical recording layer while controlling the heat flow in the film. The thermal conductivity of the metallic protective layer can be controlled by making an alloy from Al and Ti. In order to cause a heat flow from the magneto-optical recording layer, it is also possible to adjust the thickness of the magneto-optical recording layer, etc. With the film of four-layer structure, the magneto-optical layer is apparently very liable to be heated or cooled. In the three-layer film without the metallic layer, on the other hand, heat is accumulated in the dielectric layer as the third layer and thus the maximum attainable temperature of the magneto-optical recording layer becomes higher, and the structural relaxation is liable to take place on the magneto-optical recording layer, further because the high temperature region is broader. Thus, the heat flow will have an important meaning. This effect is pronounced at the erasing. At the erasing, an erasing magnetic domain having a large width must be formed in view of the track offset. In order to obtain a magnetic domain width of 1.3 µm, the beam spot center region, 0.5 µm in diameter, has a temperature of 300°C or higher in the case of the film of three-layer structure, whereas there is no such a region having a temperature of 300°C or higher in the case of the film of four-layer structure. This difference is shown as a difference in the change of reproduction output when recording/erasing are repeated. In addition, at least one compound selected from aluminum nitride, silicon nitride, germanium nitride, silicon oxide, zinc sulfide, zirconium oxide and stabilized zirconia or double oxides, or oxide-nitrides of these compounds are used as materials for the first and second dielectric layers in the film. Amorphous alloys of rare earth elements and iron group elements having a perpendicular magnetization anisotropy, more specifically alloys comprising one or two elements selected from Tb, Dy, Ho and Gd, and at least one element selected from Fe, Co, Nb, Ti, Ta, Cr, Rh, Pd, Au and Pt, the alloys containing one or both of Fe and Co are used as materials for the magneto-optical recording layer. In addition, alloys in which a portion of the above-mentioned rare earth element is replaced with at least one element selected from Nd, Pr, Ce and Sm may be used. Furthermore, the temperature distribution of the magneto-optical recording layer can be controlled by adjusting the Kerr enhancement effect by light multiple interference effect of the first and second dielectric layers. In the case of adjusting the Kerr enhancement effect, a (Kerr + Faraday) rotational angle sometimes changes largely, and thus the adjustment must be carefully made. In the formation of the metallic layer as a fourth layer, a corrosion resistance can be obtained (higher reliability) by combining a material of high reflectivity such as Al with a material of high corrosion resistance such as Ti, thereby producing a concentrating gradient in the composition or by providing a layer of the material of high reflectivity and another layer of the material of high corrosion resistance to make two layers, while maintaining the high reflectivity (an increase in C/N), and furthermore the change in the reproduction output due to repetitions of recording/reproduction/erasing (W/R/E) can be controlled. Still furthermore, when a magneto-optical recording disk of the structure of the present invention is subjected to recording by field modulation recording, the "tail" of recording domain peculiar to the field modulation recording can be eliminated, and recording at a higher density and with better disk characteristics (higher C/N ratio) can be obtained. When a magneto-optical disk of four-layer structure having a metallic layer, as mentioned above, is subjected to recording by field modulation recording, the controllability of shapes of recording domains can be improved and high density recording can be obtained by making the length of tails of the recording domain in the moving direction of a high beam smaller than 0.7 times the width of the recording domains in the direction perpendicular to the moving direction of the light beam. In order to control the thermal diffusivity of a magneto-optical recording medium, that is, to cause a heat flow from the magneto-optical recording medium, or to make the length of the tail of recording domain smaller, the magneto-optical disk must comprise a magneto-optical recording layer and a metallic layer. Furthermore, in order to control the thermal diffusivity of the magneto-optical recording medium or makes the length of the tail of recording domain smaller, the magneto-optical disk must comprise at least a magneto-optical recording layer, a metallic layer and a dielectric layer, where the dielectric layer is provided between the magneto-optical recording layer and the metallic layer. The magneto-optical disk or magneto-optical recording medium, as referred to in the specification, comprises a magneto-optical recording layer, a dielectric layer and a metallic layer, laid successively on a disk substrate. In order to control the thermal diffusivity of the magneto-optical recording medium or make the length of the tail of recording domain smaller, the magneto-optical recording medium of the present invention must satisfy the following condition: K D < K M wherein K D : thermal diffusivity of dielectric layer K M : thermal diffusivity of metallic layer Still furthermore, in order to control the thermal diffusivity of the magneto-optical recording medium of the present invention and make the length of the tail of recording domain smaller, the magneto-optical recording medium of the present invention must satisfy the following condition: K R · ℓ R < K M · ℓ M wherein K R : thermal diffusivity of magneto-optical layer ℓ R : thickness of magneto-optical recording layer K M : thermal diffusivity of metallic layer ℓM : thickness of metallic layer Still furthermore, in order to control the thermal diffusivity of the magneto-optical recording medium of the present invention which makes the length of the tail of the recording domain smaller, the magneto-optical recording medium of the present invention must satisfy the following condition: K M > K R Still furthermore, in order to control the thermal diffusivity of the magneto-optical recording medium of the present invention or make the length of the tail of recording domain smaller, the metallic layer is made of Al and Ti. Still furthermore, in order to control the thermal diffusivity of the magneto-optical recording medium of the present invention or make the length of the tail of magnetic domain smaller, the dielectric layer is composed of one or a mixture or compound of two or more of AlN x , SiN x , ZrO x , SiO x , TaO x , ZnS, ZnO x , TiO x , Y 2 O 3 and Sm 2 O 3 . On the other hand, in a magneto-optical disk comprising a first dielectric layer, a recording layer, second dielectric layer and a metallic layer, laid successively on a disk substrate having guide tracks, the decrease in the reliability can be prevented by endowing a Kerr enhancement action by multiple interference to the second dielectric layer and endowing a protective action mainly to the first dielectric layer, whereby the reliability can be made higher and the duration longer. The depth of guide tracks depends on the wavelength of light beam used. Since a semiconductor laser having a wavelength of 822 nm is now used, the depth of the guide tracks is then about 80 nm. Thus, the necessary thickness of the first dielectric layer for complete coverage of the guide tracks is 100 nm or more and 130 nm or more for more effective coverage. The thickness of the second dielectric layer having a Kerr enhancement action depends on the wavelength of the light beam and the refractive index of the layer. In that case, the thickness of the magneto-optical recording layer is important and if there is much absorption of light beam, the disk characteristics are deteriorated. Thus, care must be paid to this fact. The magneto-optical recording layer is not more than 40 nm. At the same time, a change in the magnetic characteristics is also important, because the Kerr rotational angle and the coercive force change only by a change in the layer thickness, and thus care should be paid also to this fact in the preparation of the disks. It is not the gist of the present invention what materials are to be used for the magneto-optical recording layer and the metallic layer, but the gist of the present invention is to endow a Kerr enhancement action to the second dielectric layer and a protective action to the first dielectric layer to increase the reliability and duration of the disk without deteriorating the disk characteristics. By endowing the Kerr enhancement action to the second dielectric layer, a difference in the refractive index in the upper and lower layers provided at both sides of the second dielectric layer can be made larger as a second effect than by endowing the Kerr enhancement action to the first dielectric layer, and consequently the enhancement ratio of Kerr rotational angle can be increased, resulting in an increase in the reproduced signal output and improvements in disk characteristics. With this structure, both Kerr rotational angle  k and reflectivity R become larger than by endowing the Kerr enhancement action to the first dielectric layer, and also the figure of merit R ·  k can be increased. Generally, the larger  k , the lower R. With this structure, both  k and R can be increased at the same time, and this is one of the features of the present invention. The reflectivity is obtained between the first dielectric layer and the recording layer and the Kerr rotational angle is enhanced in the second dielectric layer. In order to obtain a low end magneto-optical recording medium for domestic applications, a low cost disk substrate made of, for example, plastics must be used and also the disk structure must be simplified. The foregoing objects of the present invention can be attained by forming a magneto-optical recording layer of high corrosion resistance on a disk substrate directly or through a thin nitride layer and then by successively forming a dielectric layer as an optical effect-enhancing layer and a metallic layer as a reflective layer thereon to form a magneto-optical disk. The thus prepared disk has equivalent reliability and characteristics to those of the conventional magneto-optical disk. That is, the Kerr rotational angle  k peculiar to the magneto-optical recording layer increases and the reflectivity is corresponding lowered. According to the present invention, formation of an underlayer requiring processwise most time can be omitted and the production efficiency can be much increased, because the magneto-optical recording layer having a higher corrosion resistance is provided in the present invention. The higher corrosion resistance can be given to the magneto-optical recording material by adding 0.5 to 15 at.% of either or both of at least one element selected from Cr, Ti, Ta, Nb and Al and at least Pt, Pd, Rh and Au to the alloy of rare earth element and the iron group element. The afore-mentioned materials can be used as such, irrespective of disk structures. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 and 6 are schematic views showing cross-sectional structures of magneto-optical disks. Figs. 2 and 4 show changes in the represented signal output when recording/reproduction/ erasing (W/R/E) are repeated. Figs. 5 and 8 show the dependence of reproduced signal output on recording laser power. Fig. 3 shows the shape of recording domain formed when recording is carried out by field modulation recording. Fig. 7 shows the relationship between the thermal diffusivity of a metallic layer as a fourth layer and the thermal diffusivity of dielectric layer when the tail length of arrow feather-type magnetic domain is 0.7 times the domain width. Fig. 9 shows changes with time in the bit error rate (BER) when the magneto-optical disk is held at 80°C -85% RH. Fig. 10 in a schematic view showing a field modulation recording apparatus with a flying magnetic head. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS When a magneto-optical disk has a four-layer structure comprising a first dielectric layer, a magneto-optical recording layer, a second dielectric layer and a metallic layer, laid successively on a disk substrate, as already described above, heat can readily be dissipated, and the dissipating heat is diffused in the direction parallel to the substrate, so that broad magnetic domains can be formed without excessive elevation of the temperature of the magneto-optical recording layer. Furthermore, the state of heat flow can be freely changed by controlling the thermal diffusivity and the thickness of the metallic layer and furthermore the thickness of the magneto-optical recording layer, and thus any desired recording/erasing sensitivities to the magneto-optical disk can be selected. As a result, the maximum attainable temperature of the magneto-optical recording layer can be controlled as described and the structural relaxation of the amorphous magneto-optical recording layer can be suppressed. Thus, a decrease in the reproduction output due to repetitions of recording/erasing can be eliminated. When the track offset is taken into consideration, an erasing domain having a domain width larger than the recording domain width is required. To this end, a higher erasing laser power than the recording laser power is required, and in that case only problem is the maximum attainable temperature of the magneto-optical recording layer. However, in the case of the afore-mentioned four-layer structure there is no such problem, because the temperature distribution of the magneto-optical recording layer can be selected as desired by the presence of the metallic layer as a fourth layer. In the magneto-optical recording of a magneto-optical recording medium of the four-layer structure by field modulation recording, the magneto-optical recording layer is irradiated with a continuous (DC) beam to elevate the temperature of the magneto-optical recording layer. In the magneto-optical recording layer for use in the magneto-optical recording, the coercive force H c decreases with increasing temperature in the writing temperature range. Thus, there is a temperature T W at which H c is less than the total sum of the external magnetic field intensity Hext and the leakage magnetic field intensity H D generated by the magnetization of the magneto-optical recording layer itself, and the direction of magnetization is in accordance to the direction of modulation field, when the temperature T is above the temperature T W . That is, there is a temperature T W , where H c = |Hext + H D |. In the region where T > T W , the magnetization of the magneto-optical recording layer occurs in the direction of modulation field, irrespective of information written beforehand. When T is lowered from the region where T > T W and reaches the T W region where H c = |Hext + H D |, the direction of magnetization is immediately set to the direction of the applied modulation field with Hext. Thus the recording domain obtained by field modulation recording reflects an isotherm of T = T W to give an arrow feature shape. The length of the tail parts at the arrow feather shape depends on the shape of spot part at the right half in the isothem of T = T W . The shape of the isothem at that part shows a change in the temperature of the magneto-optical recording layer after the passage of the light beam spot. Thus, when the temperature-decreasing rate of the magneto-optical recording layer is very low, the isotherm takes a shape extended in the diameter direction of the disk. Thus, the temperature-decreasing rate of the magneto-optical recording layer can be changed, and also the shape of magnetic domains can be controlled by controlling the thermal diffusivity of the magneto-optical recording medium on the whole. Particularly by increasing the thermal diffusivity of the magneto-optical recording medium to facilitate heat release, the temperature-decreasing rate of the magneto-optical recording layer can be accelerated and the shape of the isotherm of T = T W approaches a true circle. That is, the tail length of the arrow feather shape of recording domain can be made smaller. The procedure and principle of increasing the thermal diffusivity of the magneto-optical recording medium will be described in detail below. According to one of the procedures for increasing the thermal diffusivity of the magneto-optical recording medium, a metallic layer of high thermal diffusivity is used in the disk structure. By controlling the thermal diffusivity of the metallic layer itself, the tails of the arrow feather-shaped recording domains can be shortened. By providing a dielectric layer between the metallic layer and the magneto-optical recording layer, and changing the thickness or the thermal diffusivity of the dielectric layer, the heat flow from the magneto-optical recording layer to the metallic layer can be controlled, and thus the shape of the recording domains can be controlled. By making the thickness of the dielectric layer smaller, the heat flow from the magneto-optical recording layer to the metallic layer can be increased, that is, the thermal diffusion through the metallic layer can be increased and thus the tails of the recording domains can be shortened. In order to make the thermal diffusion through the metallic layer dominant or not to lower the recording sensitivity, it is necessary to make the thickness of the magneto-optical recording layer smaller to lower the thermal capacity thereof. Thus, the thickness of the magneto-optical recording layer must be made smaller. The condition for making the thermal diffusion by the metallic layer dominant is that the product (multiplied value) of the thermal diffusivity and the thickness of the metallic layer is larger than the product of the thermal diffusivity and the thickness of the magneto-optical recording layer. In order to increase the thermal diffusivity of the magneto-optical recording medium on the whole by the metallic layer, it is preferred that the thermal diffusivity of the metallic layer itself is lower than that of the magneto-optical recording layer. Furthermore, in order to control the heat flow from the magneto-optical recording layer to the metallic layer by the dielectric layer, it is preferred that the thermal diffusivity of the dielectric layer is lower than that of the magneto-optical recording layer. In order to effectively utilize the laser beam in the temperature increase of magneto-optical recording layer, a metallic layer having a higher reflectivity is preferable. The light beam passed through the magneto-optical recording layer is reflected on the metallic layer and is effectively absorbed in the magneto-optical recording layer. This is important also for an increase in the Xerr rotational angle and the quantity of reflected light beam at the reproduction, and the reflectivity of the metallic layer is preferably 80% or higher particularly for obtaining better recording/reproduction characteristics. The thermal diffusivity of the metallic layer can be controlled by adding a metal having a relatively low thermal diffusivity to a metal having a relatively high reflectivity. As materials for the metallic layer, such materials having a high relfectivity such as Al is used and the thermal diffusivity of the metallic layer is controlled preferably by adding a metal having a low thermal diffusivity such as Ti and this additional element has an action to increase the corrosion resistance of the metallic layer. The dielectric layer is preferably made of one of AlN, SiN x , ZrO x , SiO x TaO x and ZnS or two or more thereof in mixture in view of the thermal diffusivity, corrosion resistance and optical characteristics. In the case using two or more thereof, not only double compounds such as oxide-nitrides having direct bonds between the metal and the oxygen and nitrogen, but also mere mixture can be used. In a magneto-optical disk of four-layer structure comprising a first dielectric layer, a magneto-optical recording layer, a second dielectric layer and a metallic layer, laid successively on a disk substrate provided with guide tracks, on the other hand, the disk characteristics, particularly S/N (signal/noise) ratio can be increased, and a higher reliability and a longer duration can be obtained by endowing the Kerr enhancement action by multiple interference effect to the second dielectric layer and also endowing mainly a protective action to the first dielectric layer without endowing it a substantial optical effect, because a larger difference in the refractive index and a higher Kerr enhancement action can be obtained by sandwiching the second dielectric layer between the magneto-optical recording layer and the metallic layer. Thus, in the magneto-optical disk of this structure, a higher effect can be obtained when the refractive index of the second dielectric layer is as approximate to unity (1) as possible. The boundary surface of the magneto-optical recording layer can be nitrided by deposition of the layer by sputtering, etc. in a N 2 -containing atmosphere. By controlling the atmosphere, the magneto-optical recording layer having a corrosion resistance can be simply obtained. It is satisfactory that the nitrided layer has a thickness of 5 to 10 nm. When the magneto-optical recording layer having the nitrided layer is formed directly on a plastic substrate such as a polycarbonate substrate, the adhesion between the substrate and the magneto-optical recording layer is effectively increased. For example, a magneto-optical disk of such a simple structure consisting only of a magneto-optical recording layer having the nitrided layer on both boundary surfaces, formed on a plastic substrate could be obtained, where C/N ratio was 50 dB when recording was carried out with the most dense pattern at a position, 30 mm in radius from the center of the 13,3cm (5.25") disk (number of revolutions: 1,800 rpm). In that case, care must be paid to the occurrence of structural relaxation of the magneto-optical recording layer unless the recording or erasing laser power is controlled, as already described before. The disk characteristics can be further improved by making the thickness of the magneto-optical recording layer a thickness through which the light beam can pass, forming a metallic layer acting as both of a light reflecting layer and a thermal diffusion layer on the opposite side of the magneto-optical recording layer and endowing the Kerr effect and Faraday effect and also the Kerr enhancement action to the magneto-optical recording layer, whereby the structural relaxation of the magneto-optical recording layer can be suppressed and the disk reliability can be improved. This is because the metallic layer has a high corrosion resistance and thus acts as a protective layer. In the foregoing description, amorphous alloys of a rare earth element and an iron group element have been explained as materials for the magneto-optical recording layer. The effects of the present invention do not depend on the species of materials, and a magneto-optical recording layer made of alternately laminated layers of noble metal elements such as Pt, Pd or Rh and iron group elements can be also used. As the structure of the magneto-optical recording layer, a double layer structure such as (Pt/Co) n /TbFeCo or Nd(Tb)FeCo/TbFeCo or multiple layer structures of three or more layers can be also used. In that case, the double layer structure or multiple layer structure has an effect on the suppression not only of structural relaxation of the amorphous magneto-optical recording layer, but also of diffusion between the layers. The so called exchange couple layer, where magneto-optical recording layers of different species are magnetically coupled to one another, has an effect on the suppression of changes in the exchange coupling force. This is because the effect depends not on the structure or materials of magneto-optical recording layer, but on the structure of the magneto-optical recording medium on the whole. The present invention will be described in detail below, referring to Examples and drawings. Example 1 The schematic view showing the cross-sectional structure of a magneto-optical disk prepared according to the present invention is given in Fig. 1. The disk was prepared in the following manner. On a disk substrate 1 having guide tracks, a first dielectric layer 2 of silicon nitride was at first formed by sputtering. Sintered Si 3 N 4 target was used and an Ar/N 2 mixed gas (mixing ratio in partial pressure: 80 : 10) was used as a sputtering gas. The sputtering conditions were a sputtering gas pressure of 1.3Pa (1 x 10 -2 Torr)and a discharge RF power density of 42 kW/m 2 . The first dielectric layer 2 had a thickness of 85 nm. Then, a Tb 23 Fe 62 Co 12 Nb 3 magneto-optical recording layer 3 was formed thereon by sputtering. The target was an alloy of the above-mentioned composition and the sputtering gas was a pure Ar gas. The sputtering conditions were a sputtering gas pressure of 0.67Pa (5 x 10 -3 Torr) and a discharge RF powder density of 42 kW/m 2 . The magneto-optical recording layer thus formed had a thickness of 30 nm. Then, a second silicon nitride dielectric layer 4 was formed thereon by sputtering. The sputtering gas was Ar and a sintered silicon nitride target was used. The sputtering conditions were the same as those used in the formation of the first dielectric layer except that the sputtering gas pressure was changed to 2.7Pa (2 x 10 -2 Torr). Finally, a metallic Al 90 Ti 10 layer 5 was formed thereon by sputtering. A target of Al-Ti alloy having the above-mentioned composition and an Ar sputtering gas were used. The sputtering conditions were a sputtering gas pressure of 1.3Pa (1 x 10 -2 Torr) and a discharge RF powder density of 42 kW/m 2 , and the metallic layer thus formed had a thickness of 50 nm. Example 2 A magneto-optical disk having the same structure as in Example 1 was used in this example, except that the surface was overcoated with a UV resin having a low water absorbability. The disk was subjected to repetitions of recording/reproduction/erasing (W/R/E) by field modulation recording. The results are shown in Fig. 2. No change was observed in the reproduction output even after 1 x 10 7 repetitions. Comparative Example was the same comparative disk as used in Example 1 except that the surface was overcoated with the same UV resin as above. The conditions for the determination are as follows: Laser power 6 mW (continuous beam) Number of disk revolution 2,400 rpm Recording frequency f 5.0 MHz Switching magnetic field ±24 kA/m (±300 Oe) Band width 30 kHz Thus, the decrease in the reproduction output by repetitions of W/R/E could be suppressed, and the effects were due to the disk structure, irrespective of the recording system. The recording domains obtained by the field modulation recording were investigated by a polarizing microscope, and the results of observation are schematically shown in Fig. 3. The magneto-optical disk of the structure of the present invention had shortened tails of the domain as shown by circles in Fig. 3, which were useful for the high density recording. In the present magneto-optical disk, the C/N ratio was 62 dB, whereas in the comparative disk it was 57 dB, which was by about 5 dB smaller. This is because the tails in the domains were eliminated in the present disk and the resolving power was increased thereby. That is, the recording could be made at smaller bit pitches. In other words, a high density recording could be carried out. Furthermore, the recording and reproduction could be effectively made with a laser beam of shorter wavelength, Example 3 A magneto-optical disk having a schematic cross-sectional structure shown in Fig. 1, prepared in the same procedure as in Example 1 was used. Materials and sputtering conditions are summarized in Table 1. The refractive index n 1 of the first dielectric layer was 2.05 and the refractive index n 2 of the second dielectric layer was 2.15, where it was necessary that n 2 ≥ n 1 . The disk was subjected to recording/reproduction/ erasing (W/R/E) repetitions to investigate changes in the reproduced signal outputs. The results are shown in Fig. 4. Comparative Example was the same comparative magneto-optical disk as used in Example 1 and was subjected to the investigation under the same conditions and the procedure as in Example 1. It can be seen from Fig. 4 that no decrease was observed in the reproduction output of the present magneto-optical disk even after 1 x 10 7 W/R/E or more repetitions, whereas a decrease by 70% was observed from the initial reproduction output of the comparative magneto-optical disk after 10 7 W/R/E repetitions. As already described before, the decrease in the reproduction output by W/R/E repetitions could be much suppressed by controlling the temperature distribution of the magneto-optical recording medium of four-layer structure as in the present magneto-optical disk. Furthermore,  k was 0,76° in the case of the present magneto-optical disk, whereas that of the comparative disk'was 0.55°. Thus, the four-layer structure of the present magneto-optical disk largely contributed to an increase in the Kerr rotational angle, and thus to a higher C/N ratio. The results of the measurements supported this fact. That is, the C/N ratio of the present disk was 51 dB, whereas that of the comparative disk was 45 dB, and thus the C/N ratio of the present disk was by about 7 dB higher. Fig. 5 shows the dependences of the reproduced output on the laser power of the present disk and the comparative disk, respectively. Furthermore, the recording sensitivity could be controlled by controlling the Ti concentration in the Al-Ti system of metallic layer 5 . That is, the higher the Ti concentration, the higher the recording sensitivity of the magneto-optical disk. When the thickness of the metallic layer was made smaller, the similar effect was also obtained, but when the reproducing laser power approaches the minimum recording power, there is a possibility to cause data breakage by the reproducing beam. Thus, care must be paid to this fact. The Ti concentration is a result of fine adjustment in view of the matching to the disk drive as well as the care to the aforementioned fact. Example 4 As shown in Fig. 6, SiN x was deposited as a dielectric layer 2 on a transparent glass substrate 1 provided with guide tracks for tracking in a vacuum chamber after evacuation of the chamber by sputtering with a gas mixture of Ar and N 2 under a gas pressure of 1.3Pa (1 x 10 -2 Torr). The dielectric layer 2 had a thickness of 85 nm a thermal conductivity of 13 W/m·deg and a refractive index of 2.0. Then, after the similar evacuation of the vacuum chamber, an Ar gas was introduced into the chamber and a magneto-optical recording layer 3 was deposited thereon to a thickness of 20 nm by sputtering a TbFeCoNb alloy target under a gas pressure of 0.67Pa (5 x 10 -3 Torr). Then, after the similar evacuation of the chamber, an Ar gas was introduced into the chamber, and a dielectric layer 4 was deposited thereon to a thickness of 50 nm by sputtering one of AlN, SiN, SiO, TiO 2 and ZrO 2 as targets. Then, after the similar evacuation of the chamber, an Ar gas was introduced into the chamber, and a metallic layer 5 was deposited thereon to a thickness of 50 nm by sputtering one of stainless steel, Al-Ti alloy, Al, Au and Cu as targets under a gas pressure of 0.4Pa (3 x 10 -3 Torr). Then, an ultraviolet ray-curing resin (UV resin) was spin coated thereon to a thickness of 10 µm as an overcoat layer 6 , and cured by exposure to ultraviolet rays in vacuum for one minute. The thus prepared magneto-optical recording media were subjected to field modulation recording. The results are shown in Fig. 7, where the axis of abscissa shows the thermal conductivity of the metallic layer 5 and the axis of ordinate shows the thermal conductivity of the dielectric layer 4 and the marks "○" and "Δ" show that the tail length is less than 0.7 times the domain width, and the marks "○" and "□ " shows that the optimized recording power is less than 10 mW. The tails of the recording domains could be shortened by controlling the thermal diffusivity of the metallic layer 5 and dielectric layer 4 , and thus magneto-optical recording media suitable for high density recording could be obtained. The layer structure of the present magneto-optical recording medium is not restricted to those disclosed in the foregoing Examples and can include the following modifications. (1) One or a mixture or compound of two or more of AlN, SiN x , ZrO x , SiO x , TaO x , ZnS, ZnO x and TiO x can be used as materials for the dielectric layer 4 . (2) Al is used as a host component for the metallic layer 5 . (3) Ti is added to the metallic layer 5 . (4) An X-Y amorphous alloy can be used as materials for the magneto-optical recording layer 3 , where X is one or an alloy of two or more of Tb, Dy, Gd, Nd and Pr, and Y is Fe or Co or an alloy thereof. (5) An element for increasing the corrosion resistande such as Ti, Ta, Cr, V, Pt, etc. can be added to the magneto-optical recording layer 3 besides Nb. (6) Glass, polycarbonate, polyolefin, acrylic resin, ultraviolet-curing resin, etc. can be used as materials for the disk substrate 1 . (7) Media of such a structure as shown in Figs. 1 and 6 are bonded to each other so as to face the UV resin layer. (8) Film of alternately laminated layers of X and layers of Y (thickness of one layer is 0.5 to 5 nm and thickness of the film is such that the light beam can pass therethrough) can be used as a magneto-optical recording layer 3, where X is Pt, Pd or Rh and Y is Co or Fe or an alloy of Co or Fe with X. Example 5 A magneto-optical disk having a schematic cross-sectional structure shown in Fig. 1 was prepared in the following manner. A Si 3 N 4 layer was formed on a disk substrate 1 provided with guide tracks as a first dielectric layer 2 by sputtering a sintered Si 3 N 4 target in a vacuum chamber with a sputtering gas of Ar and N 2 (a mixing ratio of Ar/N 2 in partial pressure: 80/20). The sputtering was carried out by evacuating the vacuum chamber to the order of 10·10 -6 Pa (10 -7 Torr), introducing the sputtering Ar/N 2 gas into the chamber and sputtering under a gas pressure of 1.3Pa (10 -2 Torr) with a discharge RF power density of 42 kW/m 2 for 10 minutes to form the first dielectric layer to a thickness of 130 nm. As materials for the first dielectric layer 2 , SiO 2 , SiO, AlN, Al 2 O 3 , TiO 2 , ZrO 2 , ZnS, Nb 2 O 5 , Cr 2 O 3 , WO 3 , MoO 3 , Ta 2 O 2 or any other materials having a high stability could be used. Then, a Tb 23 Fe 62 Co 12 Nb 3 layer was formed by sputtering as a magneto-optical recording layer 3 . The target was an alloy of the above-mentioned composition and the sputtering gas was an Ar gas. The sputtering was carried out by evacuating the chamber to the order of 10 -5 (10 -7 ), introducing the Ar gas into the chamber up to 0.67Pa (5 x 10 -3 Torr) and sputtering with a discharge RF power density of 42 kW/m 2 for 23 minutes, whereby the magneto-optical recording layer 3 having a thickness of 30 nm was formed. In the foregoing embodiment, TbFeCoNb was used as the material for the magneto-optical recording layer 3 , but materials having a perpendicular magnetization anisotropy, such as (Gd 0.7 Tb 0.3 ) 24 Fe 61 Co 12 Cr 3 , GdDyFeCo, GdHoFeCo, TbNdFeCo, DyNdFeCo, etc. could be used. Then, a second dielectric layer 4 was deposited thereon by sputtering a sintered Si 3 N 4 target with a sputtering Ar gas. The sputtering was carried out by evacuating the chamber to the order of 10 -5 (10 -7 ), introducing the sputtering Ar gas therein up to 2.7 Pa (2 x 10 -2 Torr) and sputtering with a discharge RF power density of 42 kW/m 2 for 23 minutes, whereby the second dielectric layer 4 having a thickness of 50 nm was formed. In this connection, the thickness and the refractive index were found important and the effect on the Kerr enhancement was abruptly lowered when any of the foregoing parameters was deviated from the optimized conditions. Thus, care must be paid to this fact. Finally, a metallic layer 5 of Al-Ti (Al: 85 at.%, Ti: 15 at.%) was formed thereon by sputtering a Al-Ti alloy target of the above-mentioned composition with a sputtering Ar gas. The sputtering conditions were the same as in the case of forming the first dielectric layer except that the sputtering gas was an Ar gas and the sputtering time was 23 minutes, whereby the metallic layer having a thickness of 50 nm was formed. The composition of elements in the metallic layer was set to optimum conditions for obtaining a desired recording or erasing sensitivity in view of the matching to the disk drive. In this Example, the sensitivity could be adjusted by controlling the Ti concentration. For example, the minimum recording power could be made 2.5 mW by adjusting the Ti concentration to 30 at.%. The thus prepared disk was subjected to determination of disk characteristics. As Comparative Example, a disk having the following structure was used: Disk substrate/SiN (8,5 nm)/TbFeCoNb (30 nm)/SiN (20 nm)/Al-Ti (50 nm) was subjected to the determination under the same conditions. In the comparative disk the Kerr enhancement action was endowed to the first dielectric layer. The results are shown in Fig. 8 showing the dependence of carrier level (C) and noise level (N) on the recording laser power. It can be seen from Fig. 8 that there is no large difference in the recording sensitivity between the present disk in which the Kerr enhancement action was endowed to the second dielectric layer and the comparative disk in which the Kerr enhancement action was endowed to the first dielectric layer, and that the recording could be made with about 3 mW. The C/N ratio of these disks was 53 dB in the case of the present disk, and 47 dB in the case of the comparative disk, and thus the present disk could have a higher C/N ratio by 5 dB, because the Kerr rotational angle  k was 0.86° in the case of the present disk and 0.75° in the case of the comparative disk, and the Kerr rotational angle was larger by 0.11° in the case of the present disk. Thus, the Kerr rotational angle enhanced by the second dielectric layer was in a higher ratio than by the first dielectric layer, though the reflectivity R was 21% in both cases. Then, the reliability test of the present disk and the comparative disk was carried out in the following manner. The disks were held at 80° - 85% RH and changes with time in the bit error rate (BER) were measured. The results are shown in Fig. 9. It can be seen from Fig. 9 that no increase in BER was observed in the present disk in which the Kerr enhancement action was endowed to the second dielectric layer even after holding in the aforementioned atmosphere for 2,000 hours, whereas in the comparative disk in which the Kerr enhancement action was endowed to the first dielectric layer an increase in BER slowly appeared 1,000 hours after holding and reached 15 - fold 2,000 hours thereafter. Thus, the magneto-optical disk with a high reliability and a long duration could be obtained by endowing the Kerr enhancement action to the second dielectric layer and endowing a protective action to the first dielectric layer without lowering the disk characteristics. As described above, a magneto-optical disk of high reliability capable of controlling the structural relaxation of amorphous magneto-optical recording layer can be provided according to the present invention by forming a metallic layer of heat diffusion as a fourth layer made of Al - Ti.

Claims: A magneto-optical recording medium comprising a substrate (1), a first dielectric layer (2) on the substrate, a magneto-optical recording layer (3) on the first dielectric layer, a second dielectric layer (4) on the magneto-optical recording layer and a metallic layer (5) on the second dielectric layer, characterized in that the metallic layer is of Al-Ti. A magneto-optical medium according to claim 1 characterized in that the thermal diffusivity K D of the dielectric layer (2) is smaller than the thermal diffusivity K M of the metallic layer (5). A magneto-optical medium according to claim 1 or 2, characterized in that the product of the thermal diffusivity and the thickness l R of the magneto-optical recording layer (3) is smaller than the product of the thermal diffusivity K M and the thickness l M of the metallic layer (5). A magneto-optical medium according to claim 1 or 2, characterized in that the thermal diffusivity of the metallic layer (5) is larger than the thermal diffusivity of the magneto-optical recording layer (3). A magneto-optical medium according to claim 1 or 2, characterized in that the dielectric layer (2) is composed of one or a mixture or compound of two or more of AlN x , SiN x , ZrO x , SiO x , TaO x , ZnS, ZnO x , TiO x , Y 2 O 3 and Sm 2 O 3 . Use of a magneto-optical recording medium according to one of the claims 1 to 5 for information recording by magnetic field modulation. Use of a magneto-optical recording medium according to claim 6 characterized in that the thermal diffusivity of the metallic layer (5) and the thermal diffusivity of the second dielectric layer (4) are selected so as to render a ratio of less than 0.7 between the length of the trailing projections of the recording domains in the moving direction of the light beam and the width of the recording domains in the direction perpendicular to the moving direction of the light beam.

Inventors: KIRINO FUMIYOSHI, MIYAMOTO HARUKAZU, NIIHARA TOSHIO, OGIHARA NORIYUKI, OHTA NORIO, OKAMINE SHIGENORI, TAKAHASHI MASAHIKO, KIRINO, FUMIYOSHI, MIYAMOTO, HARUKAZU, NIIHARA, TOSHIO, OGIHARA, NORIYUKI, OHTA, NORIO, OKAMINE, SHIGENORI, TAKAHASHI, MASAHIKO, KIRINO, FUMIYOSHI SANIKOPO TAKAIDOHIGASHI 401
Applicants: HITACHI LTD, HITACHI MAXELL, HITACHI MAXELL LTD., HITACHI, LTD.
Application year: 1989
Main classification: G11B11
IPC codes: G11B11