JPS63285738A - Magneto-optical recording medium - Google Patents

Abstract

PURPOSE:To improve the recording sensitivity and preservable stability of a medium by forming a protective film into multi-layered structure consisting of the same material and alternately varying the refractive indices or densities of the layers. CONSTITUTION:The protective film 2, a magneto-optical recording layer 3 consisting of an amorphous rare earth-transition metal material, etc., the protective film 2' and a reflecting layer 4 consisting of Al, etc., are successively laminated on a light transmittable substrate 1. The layers 2, 2' are formed of the plural layers, 2(1), 2(2),...2(n) and 2(1)', 2(2)'...2(n)' which consist of the same material and vary alternately in the refractive indices. An oxide such as SiO, sulfide such as ZnS or nitride such as Si3N4 is usable for the material of the layers 2, 2'. The respective layers of the layers 2, 2' are formed by using a sputtering method and changing the film forming conditions.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to the recording of information using light such as laser light (the light here refers to energy rays of various wavelengths including the above-mentioned laser light). -Relates to a method of manufacturing an optical magnetic recording medium that performs playback, erasure, etc.

[Prior Art] In recent years, research and development of optical memory elements using laser light as high-density, large-capacity memories have been carried out at a rapid pace. Among these, magneto-optical recording has attracted attention as an old-fashioned and rewritable recording method, and the optical-magnetic recording medium used for this recording is highly anticipated as a rewritable optical memory element.

Conventionally, MnB has been used as a material constituting the magneto-optical recording layer of an optical-magnetic recording medium used for such magneto-optical recording.
Typical examples are i-based, garnet-based, rare earth-transition metal amorphous-based, and the like. MnB i
The disadvantage of the garnet type is that it requires a high-power laser for recording due to its high Curie temperature, and that reproduction with a high S/N ratio cannot be achieved due to the large amount of grain boundary noise. Because of its high transmittance, it had the disadvantage of requiring a high-power laser for recording. Among them, the rare earth-transition metal amorphous system has a low Curie temperature and relatively low light transmittance, so
It is expected that it will compensate for the shortcomings of both.

This type of technology will be described in more detail below with reference to the drawings.

FIG. 3 is a schematic cross-sectional view of a typical conventionally used optical magnetic recording medium.

In Figure 3, 1 is polymethyl methacrylate (PM
It is a light-transmitting substrate made of plastic such as MA), polycarbonate (PC), or glass, and plate-shaped substrates of various shapes such as a donut shape are generally used. 2
a is Sin, 5i02. This is a protective layer made of AIN%ZnS or the like. 3 is a magneto-optical recording layer, and for the reasons mentioned above, it is currently made of, for example, TbFe, GdTbF.
A rare earth-transition metal 7-[-rufus system such as e%TbFe(:o) is widely used.

Recording, reproducing, and erasing on such an optical magnetic recording medium is generally performed as follows.

First, the recording medium is magnetized in a certain direction perpendicular to the substrate 1, and then a spot of laser light is irradiated from the substrate 1 side.

The magnetization direction may be any desired direction as long as it is constant. When the laser beam irradiated onto the substrate 1 passes through the substrate 1 and the protective layer 2a and reaches the magneto-optical recording layer 3, the magneto-optical recording layer 3
In the area irradiated with laser light, light absorption occurs and the temperature locally increases. As a result, only this portion reaches one or more Curie points of the layer-constituting material, and the magnetization disappears. At this time, when a magnetic field is applied in the direction opposite to the magnetization direction of the record to the part of the magneto-optical recording layer 3 where the magnetization has disappeared, the magnetization in that part is reversed and the magnetization direction is different from the part not irradiated with the laser beam. Inverted magnetic domains are formed, and information is recorded. Erasing the recording is done by re-irradiating the recorded portion of the magneto-optical recording layer 3 with a laser beam to raise the temperature of this portion to over one Curie point, and applying magnetization in the opposite direction to that during recording. This is done by returning the magnetization direction to the state before the start of recording. In such recording and erasing, a protective layer 2a is provided as shown in the figure, and the thickness of the layer is set to a thickness that exhibits an antireflection function for the wavelength of the laser beam used. The temperature increase of the magneto-optical recording layer 3 can be made extremely effective for recording and erasing.

In addition, for reproduction of recording, a laser beam whose power is lowered to such an extent that the temperature of the magneto-optical recording layer 3 does not rise above one Curie point is irradiated from the substrate l side, and the magnetization direction of the recorded portion is read out using the magnetic Kerr effect. To do this.

However, among the various materials constituting the protective layer 2a and the magneto-optical recording layer 3, the rare earth-transition metal amorphous material described above, which is widely used as the material constituting the magneto-optical recording layer 3, does not have the above-mentioned excellent characteristics. However, since its thermal conductivity is relatively high, thermal energy absorbed by the magneto-optical recording layer 3 is transferred to the protective layer 2 by laser beam irradiation.
The disadvantage is that the temperature of the recording layer 3 decreases, resulting in a decrease in recording sensitivity.

In order to improve the recording layer, a laser beam with higher power is required, but there is a limit to the power, both economically and due to the durability of the recording medium. It is possible to suppress heat dissipation by using a protective layer constituent material with low thermal conductivity, but the conventionally used 5iO1Si
It was difficult to suppress heat dissipation using materials such as 02, AIN, and ZnS.

In addition, as one method for improving the recording sensitivity, for example, as shown in FIG. Another possible method is to use the magnetic Faraday effect to increase the apparent Kerr rotation angle. However, the magneto-optical recording layer 3 generally has the disadvantage that it is easily oxidized when left in a constant temperature and humidity atmosphere in the presence of oxygen, and this is particularly noticeable when the layer is made thin.

This results in an increase in errors during recording and reproduction, signal deterioration, etc., and there is a natural limit to the improvement of recording sensitivity by thinning the film.

The present invention was made in view of the above points, and its main purpose is to improve recording sensitivity and also have excellent storage stability. The purpose of the present invention is to provide an optical recording medium.

(Means for Solving the Problems) The present inventors have completed the present invention as a result of extensive research in order to achieve the above object.That is, an optical system having a magneto-optical recording layer on a transparent substrate. In a conventional recording medium, a protective film is laminated in contact with the magneto-optical recording layer or via another layer, and this protective film has a multilayer structure made of the same material, and the refraction of these layers It is an optical recording medium characterized in that the magnitude of the refractive index or density is alternately different. Note that "alternately different" means that the refractive index or density is ABABA... or ABCD... ( A,
B, C, and D are refractive index or density).

(Embodiment of the invention) Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 is a schematic cross-sectional view showing a basic aspect of the optical magnetic recording medium of the present invention.

In Figure 1, 1 is the aforementioned glass, PMMA, PC
It is a translucent base material made of various materials such as. In this example, the shape is disk-shaped, but it is not particularly limited and can be any desired shape.

2.2' is a protective film that is a feature of the present invention.

Each of the two image protectors 2.2' consists of a plurality of layers 2(1), 2(2), .
・2 (b) or 2 (1) ', 2 (2)',...
...2(n)'. Materials for the protective film 2.2 include: Sin, 5i02, ZrO2, MgO
oxide, ZnS, Bi253, Is2.
Sulfides such as SeS, Aj2S, ZrS2, htN
Examples include nitrides such as %513x4, ZrN, (:rN, ON), carbides such as SiC, Tie, 11G, and 84C, and fluorides such as MgF2 and BiF3.

3 is a magneto-optical recording layer, the material of which is TbFe,
GdTbFe, TbFeCo, GdTbFeCo
A rare earth-transition metal amorphous system such as the following is preferably used. Of course, the above-mentioned MnBi type, garnet type, etc. can also be used.

4 is a reflective layer made of a metal such as Au, Ag, Cu, or A).

The method of laminating the magneto-optical recording layer 3 and the reflective layer 4 on the base material is not particularly limited, but specific examples include thin film formation such as vapor deposition, CVD sputtering, and ion plating. A typical example is the law.

The following method can be used to form the protective film 2.2' consisting of multiple layers of the same material and having alternately different refractive indexes. That is, this is a method of changing the refractive index by using a sputtering method and changing the manufacturing conditions during the sputtering process.

The following two types of targets are used in this method.

■ When using a compound target that will be the material that makes up the protective film ■ A metal target that will be part of the material that makes up the protective film ■ The above creation conditions are the sputtering pressure during sputtering, These include sputtering rate, substrate temperature, target-substrate distance, etc.

When using the target (2), it is necessary to use reactive sputtering, but the above manufacturing conditions include the ratio of reactive gas to Ar, etc., sputtering pressure, sputtering rate, substrate temperature, target The distance between boards, etc.

[Function] As mentioned above, during recording, laser light is irradiated from the substrate 1 side, and most of the optical energy of the irradiated light is absorbed in the magneto-optical recording layer 3, converted into thermal energy, and at the same time 1 and the reflective layer 4. Therefore, in order to improve recording sensitivity, it is considered effective to efficiently convert optical energy into thermal energy and to suppress thermal diffusion from the magneto-optical recording layer 3. As shown in the Examples below, the optical magnetic recording medium of the present invention has significantly superior recording sensitivity compared to conventional media. The cause of this recording sensitivity E is not clear, but the reason is that the multilayer structure of the protective layer suppresses heat diffusion from the magneto-optical recording layer 3 at the interface of the protective film. It is inferred that Further, in the optical recording medium of the present invention using an organic material as a substrate, there was no occurrence of cracking, film peeling, etc. This is also assumed to be due to stress relaxation caused by the multilayer structure of the protective film, which is a contributing factor to the elimination of cracks and film peeling.

[Example] The present invention will now be described in detail based on examples.

Example 1 An optical magnetic recording medium was produced in the same manner as illustrated in FIG. 1 in the following manner. Protective films 2 [2(1), 2(2), 2(3)] each have a refractive index of 2.2 (film thickness: 450 A) on a disk-shaped polycarbonate substrate 1.

S of 1.9 (film thickness 300 people), 2.3 (@thickness 150 people)
A thin N film was formed by sputtering.

On top of that, as a magneto-optical recording layer, a film thickness of 150 mm is made of TbFe.
A Co thin film was formed by sputtering.

Furthermore, a protective film 2'[2(1)', 2(2)
', 2(3)'], the refractive index is 2.3 (thickness: 150), 1.9 (@thickness: 350), 2.2 (thickness: 400).
A Si3N4 thin film with a thickness of 800 nm and finally an Ajl film with a thickness of 800 nm were respectively formed by sputtering to obtain the optical magnetic recording medium of this example.

Comparative Example Protective Film 2 Instead of [2(1), 2(2), 2(3)], a Si3N thin film with a refractive index of 2.2 and a film thickness of 900, protective film 2'[2(1)', The optical magnetic recording of the conventional example was carried out in the same manner as in Example 1, except that a Si3N4 film with a refractive index of 2.2 and a film thickness of 900 mm was formed instead of 2(2)', 2(3)']. A medium was prepared.

(Evaluation) ■ The relationship between the recording power and C/N at a radius of 60 mm when the above image recording medium is rotated at 1800 rpm, the semiconductor laser is oscillated at the recording frequency IM)Iz, and the duty ratio is 50%. It is shown in FIG. 2 (solid line: Example 1, broken line: Comparative example). Please note that playback requires laser output 2.
The test was conducted under the conditions of mW and a bandwidth of 30 KHz.

As is clear from the results in FIG. 2, compared to the comparative example using a single-layer Si3N4 thin film as the protective film, in Example 1,
It can be seen that recording can be performed with low recording power and recording sensitivity is improved.・ ■ As a result of a storage test in which the above image recording medium was left in an atmosphere with a temperature of 60°C and a relative humidity of 90% for 500 hours, cracks occurred in the recording medium of the comparative example, whereas cracks occurred in the recording medium of Example 1. There were no changes in appearance such as pinholes or cracks, and a decrease in coercive force of only about 10% compared to the initial coercive force was observed.

Examples 2 to 6 Various optical recording media having the same configuration as in Example 1 were produced in the same manner as in Example 1, except that the protective layer was constructed from the various materials shown in Examples 2 to 6 in Table 1. .

(Evaluation) Recording and reproduction were performed on these recording media in the same manner as in Example 1, and the recording sensitivity (recording laser power that minimizes the second harmonic) and coercive force change H due to storage tests were evaluated.
c/Hco (ratio of coercive force Hc after standing to 1co coercive force before standing) was determined. The results are shown in Table 1. It is clear that the sample exhibits excellent sensitivity and storage stability similar to Example 1.

[Effects of the Invention] As explained in detail below, according to the present invention, it is possible to provide an optical recording medium that improves recording sensitivity, prevents the occurrence of cracks, etc., and also has excellent storage stability. is now possible.

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional view showing the basic aspect of the optical recording medium of the present invention, FIG. 2 is a diagram showing the relationship between recording power and C/N in Example 1 and Comparative Example, and FIG. FIG. 4 is a diagram showing an example of a conventional optical recording medium. DESCRIPTION OF SYMBOLS 1...Substrate, 2.2'-...Protective film of multilayer structure with alternately different refractive indexes 2a...Protective film 3...Magneto-optical recording layer, 4...Reflection layer.

Claims (1)

[Claims] (1) In an optical recording medium having a magneto-optical recording layer on a transparent substrate, a protective film is laminated in contact with the magneto-optical recording layer or via another layer, and this protective film is made of the same material. 1. An optical recording medium having a multilayer structure consisting of layers, wherein the refractive index or density of these layers is alternately different.