JP2002230856A - Magneto-optical recording medium and method of manufacturing for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magneto-optical recording medium of a high recording density which is of a thin type, is small in a tilt angle and has good recording and reproducing characteristics and a method of manufacturing for the same. SOLUTION: The magneto-optical recording medium having a plastic substrate, magnetic layers consisting of at least an alloy of rare earth and transition metals as an essential constitution component on one side of this substrate and a protective layer consisting of a UV curing resin satisfies 0.8 mm<=d<=0.6 mm when the thickness of the substrate is defined as (d) and the diameter as Φ. The magnetic layers are at least >=2 layers and the total thickness of the film thicknesses of the magnetic layers is >=50 nm. The tilt angle in the radial direction in the innermost peripheral recording portions of the magneto- optical recording medium is at least a minus direction as an initial state.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magneto-optical recording medium having a plastic substrate and a method for manufacturing the same.

[0002]

2. Description of the Related Art Conventionally, as a substrate applied to an information recording medium such as an optical disk, a magneto-optical disk, and a magnetic disk, an injection molding method has been used because it is inexpensive, has excellent handleability, and has a required rigidity. A plastic substrate having a thickness of 1.2 mm manufactured by a compression molding method or an injection compression molding method or the like is widely used. In recent years, in a technical field such as an optical disk and a magneto-optical disk, in order to improve a recording density, Therefore, there is a strong demand for a thinner substrate.

That is, in the technical field of optical disks, magneto-optical disks, and the like, improvement of recording density is one of the biggest technical problems, but in order to improve the recording density of optical disks, magneto-optical disks, etc. It is essential to shorten the wavelength of laser light and increase the numerical aperture of the objective lens. But,
Simply shortening the wavelength of the laser beam and increasing the numerical aperture of the objective lens shortens the focal length of the objective lens, necessitating that the objective lens be disposed closer to the substrate surface, causing various technical difficulties. Therefore, it is necessary to reduce the wavelength of the laser beam, increase the numerical aperture of the objective lens, and reduce the thickness of the substrate.

In order to cope with such technical requirements, it has been studied to reduce the thickness of the substrate to 1 mm or less. For example, in the case of a DVD (Digital Parser Tile Disc or Digital Video Disc), A plastic substrate having a thickness of 0.6 mm is used.

[0005]

By the way, a plastic substrate stores an internal stress during molding, and the internal stress is relaxed as time passes. Further, the information recording layer, the protective layer, and the like formed on the surface of the embarrassed substrate also store internal stress during film formation, and the internal stress is relieved as time passes. Therefore, an information recording medium provided with a plastic substrate is deformed with the lapse of time due to the sum of the amount of relaxation of the internal stress of the substrate and the amount of relaxation of the internal stress of the information recording layer, etc., and causes tilt (warpage).

[0006] When a plastic substrate is made thinner, rigidity is reduced, so that a tilt angle generated in an information recording medium becomes larger than when a thick substrate is used. FIG. 8 shows an outer diameter of 2 inches and thicknesses of 0.5 mm and 0.6 m, respectively.
The initial tilt angle of each magneto-optical disk having an information recording layer of the same configuration on a polycarbonate substrate of m, 0.8 mm, and 1.0 mm, and the amount of change in the tilt angle after being left for 100 hours in an environment of 80 ° C. As is clear from this figure, the magneto-optical disk having a thinner substrate has a larger tilt angle change amount after heating, and has a thickness of 0.8.
In the case of a magneto-optical disk having a substrate of not more than 0.1 mm, the total tilt angle obtained by adding the amount of change of the tilt angle after heating to the initial tilt is ± 0.
Exceeds the range of 6 degrees. The values in the table indicate the maximum values of the tilt angles in each magneto-optical disk.

For an information recording medium having a close bonding structure such as a DVD, no special inconvenience occurs even when a plastic substrate having a thickness of 0.6 mm is used.
A single-plate information recording medium such as a (compact disk), a CD-ROM (read-only type CD) and a CD-RW (rewritable type CD) has a thickness of 0.8 m.
When a plastic substrate having a thickness of m or less is provided, a case may occur in which good recording / reproduction characteristics cannot be obtained.

In a magneto-optical recording medium, in order to achieve high-density recording such as magnetic super-resolution, a plurality of magnetic layers are laminated, and when the total thickness of the magnetic layers becomes large and the substrate becomes thin, The problem relating to the tilt angle has increased, and has become a problem in achieving higher density and the like.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has as its object to provide a thin recording medium having a small change in the tilt angle with time and having a high recording density having good recording / reproducing characteristics over a long period of time. An object of the present invention is to provide a magneto-optical recording medium and a method for manufacturing the same.

[0010]

In order to solve the above-mentioned problems, the present invention provides a plastic substrate, a magnetic layer mainly composed of an alloy of at least a rare earth and a transition metal on one side of the substrate, In a magneto-optical recording medium provided with a protective layer made of a resin, when the thickness of the substrate is d and the diameter is Φ, 0.8 mm ≧ d ≧ 0.6 mm;
Φ ≦ 65 mm, the number of the magnetic layers is at least two or more, and the total thickness of the magnetic layers is 50 nm.
As described above, the tilt (inclination) angle in the radial direction at the innermost recording portion of the magneto-optical recording medium is at least a minus direction as an initial state.

In a magneto-optical recording medium using a plastic substrate, no recording layer is formed at the center. In this case, between the deposited area of the recording layer and the non-deposited area,
A large stress acts on the substrate. Therefore, the tilt in the radial direction is the largest at the innermost peripheral portion of the recording portion, and is particularly remarkable for a disk using a thin substrate having a thickness of 0.6 mm to 0.8 mm.

Since the tilt angle in the innermost recording area is the largest, if the tilt angle in the area is regulated, the amount of tilt in other areas becomes smaller than this.
Good recording / reproducing characteristics can be exhibited as a whole.

The magnetic film is usually formed by a magnetron spar method or the like. However, the magnetic film made of a rare earth / transition metal alloy formed by such a physical method has a large internal stress, and causes a large tilt in a magneto-optical recording medium with time. It is a factor that is generated in the future. Therefore, the tilt change amount is restricted to the above range for a magneto-optical recording medium in which a plurality of magnetic films are laminated on one surface of a plastic substrate and an information recording layer in which the total thickness of the magnetic layers is 50 nm or more is formed. Thus, the life of this type of magneto-optical recording medium can be extended.

The magneto-optical recording medium is usually coated with an ultraviolet curable resin film as a protective film for protecting the information recording layer from mechanical shock and chemical damage. The ultraviolet curable resin film also causes a stress on the plastic substrate, which causes a large tilt with time in the magneto-optical recording medium. Therefore, if the tilt angle variation of the magneto-optical recording medium in which the front or back surface of the plastic substrate is coated with the ultraviolet curable resin film is regulated within the above range, the life of this type of magneto-optical recording medium can be extended. it can.

[0015] A simple evaluation method is as follows.
By conducting an acceleration test for 100 hours, performance sufficient for practical use can be generally confirmed.

Further, in the magneto-optical recording medium in which the tilt angle in the radial direction in the innermost recording portion of the magneto-optical information recording medium is set to at least one direction as an initial state, x is a reference to the disk surface. When a displacement amount in the vertical direction from the surface and r is a radial position, a second derivative d2x / dr2 of x by r, that is, d (tilt)
/ Dr is configured to take a positive value substantially over the entire surface of the disk.

Regarding the method of manufacturing a magneto-optical recording medium, first, a plastic substrate having a thickness of 0.6 mm or more and 0.8 mm or less is molded, and then the plastic substrate is subjected to a required film formation. After the film formation was completed, the plastic substrate was left in an environment of 50 ° C. or more and 120 ° C. or less to perform heat aging treatment.

When the molded plastic substrate is subjected to the heat aging treatment, the internal stress of the plastic substrate is alleviated, so that the subsequent change in the tilt angle with time can be reduced. In this case, if the heating temperature is 50 ° C. or lower, it takes a long time for the heat aging treatment, and thus it is not practical. On the other hand, when the temperature is higher than 120 ° C., the substrate is softened to cause thermal deformation. Therefore, if the heating is performed at a temperature of 50 ° C. or more and 120 ° C. or less, the heat aging treatment of the plastic substrate can be performed quickly and safely.

If the heat aging treatment is performed after the required film formation is completed, the heat aging treatment can be performed not only on the plastic substrate but also on the information recording layer and the protective film at the same time. Can be done.

In this case, more preferably, the heat aging treatment is started at an initial temperature of 80 ° C. or lower, and the temperature is raised to a target temperature of 100 ° C. to 120 ° C. over 30 minutes or more.
After the target temperature is maintained for a predetermined time, the temperature may be gradually cooled to an end temperature of 80 ° C. or less over 30 minutes or more. The time is suitably 1 hour to 10 hours, but preferably 4 to 8 hours.

As described above, when the temperature of the magneto-optical recording medium is gradually increased and cooled, the thermal shock applied to the magneto-optical recording medium can be reduced, so that the internal stress generated on the plastic substrate or the like can be reduced uniformly. Can be eased.

When a magneto-optical recording medium having the structure of the present invention is heated, a plastic substrate is curved so that the surface on which the information recording layer is formed becomes concave due to the balance between the internal stress of the plastic substrate and the internal stress of the information recording layer. Is selected to form the information recording layer, the tilt angle of the magneto-optical recording medium after the heat aging treatment, that is, the tilt angle in the initial stage used as a product can be suppressed, and the time of the subsequent tilt can be reduced. Since the change can be suppressed to a small value, a magneto-optical recording medium having good recording and reproducing characteristics can be obtained.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In order to perform high-density recording / reproduction, the diameter of a focused spot of laser light is generally reduced. For that purpose, it is necessary to use an objective lens having a large aperture ratio (NA).

However, as a disadvantage when a lens having a large NA is used, the influence of coma aberration on the tilt (tilt) of the medium with respect to the incident optical axis increases. In order to compensate for this, it is necessary to use a thin substrate. Then, this time, the rigidity of the medium decreases, so that a problem of so-called decrease in mechanical strength occurs. In particular, this problem is remarkable in a medium using a plastic substrate such as polycarbonate and polyolefin. The problem of the decrease in rigidity appears as an increase in the tilt of the medium when the temperature of the medium increases or decreases. This increase in tilt causes a decrease in the effective laser power and defocus when performing recording and reproduction, and eventually leads to errors in recording and reproduction. This increase in tilt is more remarkable as the diameter of the disk increases and as the thickness of the substrate decreases.

In particular, in a magneto-optical recording medium, in order to add a special function such as magnetic super-resolution or direct overwrite, in a medium in which a plurality of magnetic layers are laminated, an inorganic dielectric or metal formed by a sputtering method is used. Since the internal stress of the thin film is added, the deformation with time tends to occur, especially in combination with a thin substrate. Therefore, disks of various diameters and thicknesses were manufactured and the medium temperature was set to 80.
° C, and the amount of change in tilt after 100 hours was measured.

An embodiment of a magneto-optical recording medium according to the present invention will be described below with reference to FIGS. FIG. 1 is a cross-sectional view of a magneto-optical recording medium according to an embodiment, FIG. 2 is an enlarged cross-sectional view of a main part showing a film structure of the magneto-optical recording medium according to the embodiment, and FIG. FIG. 3 is a sectional view of a recording medium.

As shown in FIG. 1, the magneto-optical recording medium of this embodiment has a polycarbonate substrate 1 having a thickness of 0.6 mm and a diameter of 50 mm, an information recording layer 2 formed on one surface of the substrate 1, An ultraviolet curable resin protective layer 3 having a thickness of about 5 μm and covering the information recording layer 2.

The polycarbonate substrate 1 is molded by an injection compression zone method, and a required preformat pattern is formed on the information recording layer forming surface of the substrate 1 in the form of fine irregularities. The preformat pattern is formed in a recording area set in an intermediate area excluding the innermost peripheral part and the outermost peripheral part of the polycarbonate substrate 1, and the information recording layer 2 covers the entire recording area. It is formed. The protective layer 3 is formed so as to cover the entire information recording layer 2. Information recording layer 2
Has a thickness of 50 nm from the substrate 1 side, as shown in FIG.
Film 11 made of SiN and Gd having a thickness of 40 nm
A reproducing layer 12 of FeCo and a 10 nm thick Si
A nonmagnetic layer 13 made of N and TbFe having a thickness of 40 nm.
A recording layer 14 made of Co, a SiN layer 15 having a thickness of 20 nm, and a reflection layer 16 made of AlTi having a thickness of 40 nm
Are laminated in this order. Each of these films is formed by the magnetron sutter method.

In this embodiment, SiN was formed as a base layer by the Spack method. The underlayer blocks water that penetrates and penetrates the substrate, and prevents the magnetic layer from being deteriorated by the water. Si as target
Was used to introduce an Ar gas mixed with 30% of N ₂ gas to laminate a dielectric layer of SiN. The reflectance of the medium can be adjusted by controlling the refractive index and the film thickness. For example, the refractive index of the dielectric of the underlayer is suitably 1.9 to 2.3, and the film thickness A at this time is preferably λ / 15 ≦ A ≦ λ / 5, where λ is the recording / reproducing laser wavelength. Further, when λ / 12 ≦ A ≦ λ / 9, the effect of increasing the Kerr rotation angle can be obtained particularly well, and the reflectance can be controlled. In the present embodiment, the SiN dielectric layer has a refractive index of 2.1 and a thickness of 5
It was set to 0 nm.

As shown in FIG. 2, the information recording layer 2 of this embodiment has a TbFeCo recording layer and a GdFe
A Co reproducing layer was laminated via a non-magnetic layer. By utilizing the fact that a temperature distribution is generated in the spot of the laser beam irradiated at the time of reproduction, the resolution is improved by setting only the central portion of the reproduction layer where the temperature is high as an opening. In order to make this work, the GdFeCo reproducing layer is adjusted to have special magnetic properties. First, at around room temperature, this GdFeCo reproducing layer is in an in-plane magnetization state. And, it has magnetism that changes to a perpendicular magnetization state at a certain critical temperature (T _cr ) or higher. With such a configuration, the temperature near the center of the laser beam spot _rises to _Tcr or more during reproduction, so that the area is vertically magnetized. The direction of the perpendicular magnetization at this time transfers the magnetization direction of the recording layer. As a result, the mask portion outside by in-plane magnetization of the isotherm T _cr, becomes inside it were magnetic aperture, thereby improving the reproduction resolution. Since the method introduced here is a type in which an opening is formed in the center as described above, C
AD type (Center Aperture Decec)
Tion (center opening detection type) is called magnetic super-resolution. By adopting this technology, in the iD photo disc, 0.47 μm, which is less than half the size of the light spot,
The repetition of the recording mark of m (2T signal) can be reproduced with a sufficient signal output (CNR; 47 dB).

However, in order to add this function, a plurality of magnetic layers are required, and as a result, the stress applied to the substrate increases. In a conventional magneto-optical recording medium, a magnetic layer made of a rare-earth transition metal alloy is usually formed as a single layer as a recording layer, and its thickness is at most about 50 nm. For this reason,
There was no need to apply the structure and heat treatment as in the present invention.

In this embodiment, the reflection layer is provided directly on the recording layer or via another layer. Gold for the reflective layer,
Metals such as silver, aluminum, copper, and platinum and alloys containing these metals are used, and aluminum or an alloy containing these as a main component is preferable in terms of reflectance and durability.
The thickness of the reflective layer is 10 to 100 nm, preferably 2 to 100 nm.
0 to 60 nm. Examples of the film forming method include a Spack method, a vacuum evaporation method, and an ion plating method. In order to improve the characteristics, a light interference layer may be provided between the recording layer and the reflection layer. As a material for forming the light interference layer, an inorganic dielectric is mainly used. In the present embodiment, the same material as the base SiN was used, and the film thickness was 10 nm.

In this embodiment, the protective layer is laminated with the protective layer 4 in order to protect the recording layer 2 and the reflective layer 3 or to improve the recording characteristics. The protective layer 4
Usually, the film is formed by spin-coating an ultraviolet curable resin to form a film, and then irradiating the film with ultraviolet light using an ultraviolet irradiator to cure the coating film. The thickness of such a protective coat is usually about 0.1 to 100 μm, preferably 4 to 15 μm. In this embodiment, 5 μm
It was. In the magneto-optical disk of this embodiment, a hub can be provided in the center hole. This hub is a magnetic clamping hub made of a material that can be attracted by a magnet, and has a central portion protruding in a convex shape to form an edge, an inclined portion, and a central flat portion from the outer peripheral side. The inclination angle φ that the inclined part forms with the flat part is about 145 degrees. In this embodiment, the outer diameter (diameter) of the hub is 31.72 mm (the disk diameter is 26 mm).
%). The hub is attached to the board in a state where the hub is separated (movable) from the board. SUS430 was used as a material for the hub. Further, the effect of reducing the influence is obtained because the holding force of the substrate is increased even with respect to surface run-out caused by uneven rotation and turbulence generated in the cartridge due to rotation.

When the optical recording disk is loaded into the drive, the magnetic hub arranged in the center of the optical recording disk is magnetically attracted to the magnetic magnet of the drive and chucked.

This medium was not laminated and used as a single plate.

The magneto-optical disks of various diameters and thicknesses manufactured in this example were heated to 80 ° C., and the amount of change in tilt after 100 hours was measured.
Shown in In Table 1, the amount of tilt change is ± 3 mr.
For magneto-optical disks of ad or less, a mark ◎ is displayed as a magneto-optical disk suitable for practical use, and the tilt change amount is ± 5 m.
For a magneto-optical disk of rad or less, a circle is displayed as a usable magneto-optical disk, and the tilt change amount is ± 5 m.
For the magneto-optical disk of rad or more, an x mark was displayed as an unsuitable magneto-optical disk.

[0037]

[Table 1] The change in tilt greatly differs depending on the structure of the laminated thin film layers. The above embodiment is an example of a magneto-optical recording medium using magnetic super-resolution. The feature of this embodiment is that a magnetic layer has a plurality of stacked layers, and the thin film has a strong stress.
As a result of the experiment, in most cases, the magnetic layer of the rare earth transition metal alloy has a strong compressive stress. Therefore, when the total film thickness is 50 nm or more, the disk diameter is 65 mm or less and the substrate thickness is 0.6 mm or more. As a result, it was found that the amount of change in the tilt with respect to the change over time was small, and that an appropriate disk that could be used stably could be constructed.

The thicker the plate, the better. However, in the case of a medium having a diameter of 65 mm or less, the change of the tilt with the passage of time is almost 0 when the thickness is about 2 mm, so that the upper limit of 2 mm is sufficient. .

However, in order to increase the recording density, it is necessary to use a lens having a high aperture ratio (NA), and accordingly, it is necessary to reduce coma aberration with respect to tilt. At present, a substrate having a thickness of 1.2 mm is used in many disks. However, in order to achieve a higher density, the substrate needs to be thinner than 1.2 mm.
mm or less.

An example of the above-mentioned medium is shown. A magneto-optical recording medium having a disk diameter of 50 mm and a substrate thickness of 0.6 mm
It shows a change with time of the tilt when left in an environment of 0 ° C. for 100 hours. FIG. 4 shows no heat treatment and FIG.
Is a sample which has been subjected to a heat treatment at 100 ° C. for 5 hours.

In a magneto-optical disk having a recording layer including a rare earth transition metal alloy magnetic film on one side of a substrate and a protective coat of an ultraviolet curable resin, the total thickness of the magnetic film is 50 nm or more. And the disc diameter is 65
mm and the thickness of the substrate is 0.6 mm or more, the amount of tilt change with time is small, and an appropriate disk that can be used stably can be constructed. However, the slight change of tilt with time is always in the plus (+) direction. (The side on which the recording layer and the protective coat layer are laminated). This is because the strong compressive stress of the magnetic layer of the rare earth transition metal alloy is relaxed with time, and the polymerization reaction of the ultraviolet curable resin of the maintenance coat layer further progresses with time, so that the resin layer contracts. The heat treatment has a function of accelerating these changes, and when used in combination with the disc structure of the present invention, it is possible to obtain further stability over time of the tilt.

Further, it is an important method that the disk is tilted in one direction in consideration of the fact that the temporal change of the tilt is +. In order to include the time lapse especially in the range of ± 4 mrad of the target value,
It is desirable that the tilt at the time of product shipment be controlled in one direction, and a more preferable range is -4 to 0 m
rad is controlled.

FIG. 6 shows the tilt change rate of the magneto-optical disk without heat treatment, and FIG. 7 shows the tilt change rate of the magneto-optical disk subjected to heat treatment. The change rate of the tilt also changes in the + direction in the same manner as the change with time in the tilt.

An example in which recording / reproduction is performed by a magneto-optical disk drive using the magneto-optical disk manufactured as described above will be described. The medium of the present invention can record on one or both of the land and the group. In the present embodiment, recording was performed only in the group portion and its performance was examined.

The medium is rotated so that the linear velocity becomes 5.0 m / s, a semiconductor laser beam having a wavelength of 650 nm is condensed by an objective lens having an NA of 0.6, and irradiated on a recording film through a substrate. Recording and playback were performed on the group. The recording laser power was 10 mW. At this time, a multi-pulse recording waveform for dividing the recording pulse into a plurality was used. A NRZ-modulated random signal having a shortest mark length of 0.35 μm was recorded and PRML reproduction was performed. As a result, a bit error rate of 2 × 10 ⁻⁵ was obtained.

For example, in the case of an information recording medium that curves with time so that the surface on which the information recording layer is formed is concave, a magneto-optical disk is manufactured using a plastic substrate that is curved so that the surface on which the information recording layer is formed is concave. I do.

Further, in the above embodiment, a polycarbonate substrate was used, but the gist of the present invention is not limited to this, and other plastic materials such as polymethyl methacrylate resin, polyolefin resin or ethoxy resin are used. The present invention can also be applied to a magneto-optical recording medium having a substrate made of.

[0048]

According to the present invention, a plurality of magnetic layers are stacked on a thin plastic substrate to achieve high-density recording such as magnetic super-resolution, and the total thickness of the magnetic layers is 50 nm.
As described above, even if a maintenance coat made of an ultraviolet curable resin is further laminated on the magnetically permeable layer, the change with time of the tilt angle is small, and a high recording density magneto-optical recording medium having good recording / reproducing characteristics for a long time. The manufacturing method can be provided.

[Brief description of the drawings]

FIG. 1 is a sectional view of a magneto-optical recording medium according to an embodiment.

FIG. 2 is an essential part enlarged cross-sectional view showing a film structure of a magneto-optical recording medium according to an embodiment.

FIG. 3 is a sectional view of a magneto-optical recording medium showing a tilt angle measuring method.

FIG. 4 is a diagram showing a change with time of tilt when a magneto-optical recording medium without heat treatment according to an embodiment is left in an environment of 80 ° C. for 100 hours.

FIG. 5 is a diagram showing a change with time of the tilt when the magneto-optical recording medium subjected to the heat treatment according to the embodiment is left in an environment of 80 ° C. for 100 hours.

FIG. 6 is a diagram showing a change over time in a tilt change rate when a magneto-optical recording medium without a heat treatment according to an embodiment is left in an environment of 80 ° C. for 100 hours.

FIG. 7 is a diagram showing a temporal change of a tilt change rate when the magneto-optical recording medium subjected to the heat treatment according to the embodiment is left in an environment of 80 ° C. for 100 hours.

FIG. 8 is a table showing the initial tilt angle of a magneto-optical disk according to an experimental example and the amount of change in the tilt angle after being left for 100 hours in an environment of 80 ° C.

[Explanation of symbols]

 Reference Signs List 1 polycarbonate substrate 2 information recording layer 3 ultraviolet curable resin protective layer 4 SiN counterbalance film 11 SiN film 12 GdFeCo film 13 SiN film 14 TbFeCo film 15 SiN film 16 AlTi film

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Koichi Otsuka 1-88 Ushitora, Ibaraki-shi, Osaka F-term in Hitachi Maxell, Ltd. 5D075 FF11 FG17 GG01 GG16

Claims (5)

[Claims] 1. A magneto-optical recording medium comprising: a plastic substrate; a magnetic layer mainly composed of an alloy of at least a rare earth and a transition metal on one side of the substrate; and a protective layer made of an ultraviolet curable resin. When the thickness of the substrate is d and the diameter is Φ, 0.8 mm ≧ d ≧ 0.6 mm,
Φ ≦ 65 mm, the number of the magnetic layers is at least two or more, and the total thickness of the magnetic layers is 50 nm.
The magneto-optical recording medium is characterized in that the tilt angle in the radial direction at the innermost recording portion of the magneto-optical recording medium is at least a minus direction as an initial state. 2. The magneto-optical recording medium according to claim 1, wherein a radial tilt angle in an innermost recording portion of the magneto-optical recording medium changes in a positive direction by heating at 80 ° C. for 100 hours. Characteristic magneto-optical recording medium. 3. The magneto-optical recording medium according to claim 1, wherein x is a displacement amount in a vertical direction from a reference surface of the disk surface, and r is a radial position, where x is a second derivative of x with r. d2x / dr2, that is, d (tilt) / dr
A magneto-optical recording medium characterized in that it takes a positive value over substantially the entire surface of the disk. 4. A plastic substrate having a thickness of 0.6 mm or more and 0.8 mm or less and a diameter of 65 mm or less is formed, and then a required film is formed on the plastic substrate. A method for manufacturing a magneto-optical recording medium, wherein a heat aging treatment is performed by leaving a substrate in an environment of 50 ° C. or more and 120 ° C. or less. 5. The method for manufacturing a magneto-optical recording medium according to claim 4, wherein the information recording layer is formed by selecting, as the plastic substrate, one that is curved so that the surface on which the information recording layer is formed is concave. A method for manufacturing a magneto-optical recording medium, comprising: