JPH06150417A - Optical disk and its reproducing system - Google Patents

Abstract

PURPOSE:To increase recording density by forming the thickness of a magnetic layer just above a pit thicker than that of a magnetic layer on a part other than the pit, and detecting the change of a magneto-optical signal between a pit part and a part other than that as an information signal. CONSTITUTION:The pit 105 provided with depth of 200Angstrom is formed on one plane of a resin substrate 101 in advance. The film thickness 107 of a reproduction layer 103 on the pit 105 is stacked thicker than the film thickness 108 of a part other than the pit 105. When an initializing magnetic field is applied to such disk, the magnetization of the layer 103 just above the pit 105 is inverted by the initializing magnetic field. and an interfacial magnetic wall can be formed between an auxiliary layer 104. Then magneto-optical reproduction is performed as adding a reproduction magnetic field. At this time, the magneto-optical signal of the next pit 105 can be read out by applying the continuous irradiation of a laser power in a direction of recording track as inverting the magnetization of the layer 103 of the pit 105. Since a series of such operation are performed in a magnetically ultraresolution state, an information pit of high density can be reproduced as the magneto-optical signal.

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 for reading out information signals by utilizing the magneto-optical effect (Kerr effect). In particular, it relates to high-density recording using magnetic super-resolution.

[0002]

2. Description of the Related Art Currently, optical discs are ROM (Read Only Mem).
ory) has been put to practical use. This is a ROM in which pits (recesses) having information are provided on a transparent resin substrate. Then, a laser beam is incident from the substrate side, and the ROM portion is read out using the change in the reflectance of the diffracted light as a signal. With this method, the recording density can be increased up to the diffraction limit of the laser light used for reproduction. For example, 830n
If a semiconductor laser near m is used, a high recording density of 600 Mbyte can be realized on one side of a 5-inch disk. Further, in this method, the pits provided on the resin substrate are formed by injection molding. Therefore, there is an advantage that the information in the ROM section can be copied in a large amount and at low cost from one stamper. For these reasons, the optical disc is widely accepted in the market as a medium for music. However, recently, as seen in the development of multimedia that handles image data and the like, further increase in storage capacity is desired. However, the recording density of an optical disc is limited by the OTF (Optical Transfer Function), and if you try to reduce the spacing between information pits, waveform interference due to diffracted light from the pits before and after the pit you are trying to read will cause C / There was a problem that N deteriorates.

On the other hand, in the field of magneto-optical recording, there has been proposed a method for realizing high-density recording by eliminating waveform interference from information domains before and after. This technology is Proc.In
It is shown in t.Symp.Optical.Memory page 203 (1991), and will be referred to as a reproduction system by magnetic super-resolution here. In this system, a magnetic medium having two layers of a reproducing layer and a recording layer, which have different Curie temperatures and coercive forces, is used.
External magnetic field (initializing magnetic field) after recording the signal thermomagnetically
To erase the magnetic domain in the reproducing layer. At the time of reproduction, the information stored in the magnetic domains of the recording layer is read while thermally transferring it to the reproduction layer. This super-resolution reproduction method is an important technology for high-density recording because it can avoid C / N deterioration due to waveform interference between adjacent magnetic domains.

[0004]

By the way, in the above-mentioned optical disk, there is no method for realizing the same super-resolution as the magnetic super-resolution in magneto-optical recording. Therefore, when trying to increase the recording pit density, there was a limit set by OTF. The present invention is intended to solve these problems.

[0005]

In an optical disc in which pits having information are formed on a transparent substrate, at least a protective layer, a magnetic layer, and a protective layer are laminated in this order on the side having the pits of the substrate, and directly above the pits. The thickness of the magnetic layer of h1,
When the film thickness of the magnetic layer other than the pit is h2, h1> h2, and the change in the magneto-optical signal between the pit portion and the portion other than the pit portion is detected as an information signal. To do.

[0006]

As shown in FIG. 5, the first substrate is formed on the transparent substrate 101.
The magnetic layer and the second magnetic layer are sequentially laminated to form a reproducing layer 103 and an auxiliary layer 104, respectively. However, in FIG. 5, the protective layers that sandwich the magnetic layer are omitted. Table 1 shows the symbols representing the physical properties of the reproduction layer and the auxiliary layer. However, the magnetization and coercive force are values at room temperature.

[0007]

[Table 1]

At this time, each physical property value is selected so as to satisfy the equations 1 and 2.

[0009]

[Equation 1]

[0010]

[Equation 2]

However, the reproducing layer is selected to be rich in transition metal at room temperature (predominant in sublattice magnetization of transition metal), and the auxiliary layer is rich in rare earth at room temperature (predominant in sublattice magnetization of rare earth). Under this condition, the hysteresis loop of magnetization is shown in Fig. 6 (a).
become that way. FIG. 6 (b) shows a minor loop of the reproduction layer. The switching fields Ha and Hb of the minor loop are

[0012]

[Equation 3]

[0013]

[Equation 4]

It is represented by The state of magnetization at each step of the hysteresis loop is shown in FIGS. However, corresponds to each step described in the hysteresis loop of FIG.

Next, as shown in FIG. 9, information pits 105 are formed on the substrate 104, a reproducing layer 101 having a different thickness corresponding to the pits is laminated thereon, and then an auxiliary layer 102 is formed. Form. The magnetization state at this time is shown in FIG. FIG. 10A shows a magnetized state which is a starting point of the following process. Here, when the magnetization of the reproducing layer is downward, it is called a "0" state. At this time, there is no interface domain wall between the reproducing layer and the auxiliary layer. And

[0016]

[Equation 5]

If an initialization magnetic field satisfying the above conditions is applied in the direction of the arrow in FIG. 10 (b), only the magnetization of the reproducing layer immediately above the pit can be reversed (FIG. 10 (b)). This inversion unit is called the "1" state. Therefore, with or without pits
It can be detected as a magneto-optical signal. An interface domain wall is formed between the inverted reproducing layer and the auxiliary layer immediately above. When a laser beam having a constant power is applied to the state shown in FIG. 10B, the interface domain wall is eliminated as shown in FIG. 10A and the magnetization of the reproducing layer is reversed. By continuously irradiating such a laser power in the recording track direction, it becomes possible to read the magneto-optical signal of the next pit portion while reversing the magnetization of the pit portion reproducing layer. Since this series of operations is the magnetic super-resolution in magneto-optical recording,
High-density information pits can be reproduced as magneto-optical signals. The above process is illustrated in FIG. At the time of reproduction, if a reproducing magnetic field is applied in the direction shown in FIG. 4, the magnetization reversal of the reproducing layer can be easily performed.

[0018]

EXAMPLES The present invention will be described below based on examples.

As shown in FIG. 1, a transparent resin substrate 101
The protective layer 102, the reproducing layer 103, the auxiliary layer 104, and the protective layer 102 are laminated in this order by magnetron sputtering. The protective layer is made of AlSiN with a thickness of 800Å.

The composition of the reproduction layer consists of transition metal rich Nd7.2Dy20.1F61.7eCo11.0 at room temperature. The composition of the auxiliary layer consists of rare earth-rich Dy25.6Fe43.8Co30.6 at room temperature. Table 2 shows the respective physical properties of the reproduction layer and the auxiliary layer.

[0021]

[Table 2]

The resin substrate 101 is in the shape of a disk having a thickness of 1.2 mm, and a pit 105 having a depth of 200Å is formed in advance on one surface thereof by injection molding. A three-dimensional view of the pit shape is shown in FIG. The pits are repeatedly formed at the same pitch in the track direction. The length of the pit in the track direction and the distance from the adjacent pit are 4000Å.
The width of the pit in the direction perpendicular to the track direction is 4000.
It is Å. As shown in FIG. 1, the film thickness 107 of the reproducing layer on the pit is stacked thicker than the film thickness 108 on the portion other than the pit. This is achieved, for example, as follows. First, after a reproducing layer is laminated by sputtering, a bias is applied and RF sputtering is performed on the substrate side to etch back the reproducing layer. At this time, since the reproducing layer laminated on the pit portion is in the concave portion, it is difficult to be etched. Therefore, as shown in FIG. 1, the film thickness of the reproducing layer can be changed at the pit portion and other places.

The pit portion is detected as a modulation of the magneto-optical signal. The structure of the magneto-optical head is shown in FIG. Laser wavelength is
830nm, NA is 0.55. The magneto-optical signal can be obtained by differential detection as shown in FIG. Tracking servo is performed by sample servo. For this reason, prepits for sample servo are separately formed on the track. A series of processes leading to regeneration is schematically shown in FIG.
However, the protective layer is omitted in FIG. The linear velocity of the disk is 5.7m / sec. While irradiating the medium surface with a 13 mW laser beam by the magneto-optical head 401,
A reproducing magnetic field 402 of is applied at 500 Oe in the direction of the arrow in the figure. Then, the magnetization of the auxiliary layer having a high Curie temperature is first determined according to the first reproducing magnetic field. Next, the magnetization of the reproducing layer having a low Curie temperature freezes in a direction in which an interface domain wall is not formed between the reproducing layer and the auxiliary layer. Since the auxiliary layer is rich in rare earth,
The reproducing magnetic field and the direction of the magnetization of the auxiliary layer are opposite at room temperature. In addition, since the senses of the reproducing layer and the auxiliary layer are opposite to each other, the state where the interface domain wall does not exist is a state where the respective magnetizations are opposite at room temperature. Next, an initializing magnetic field (403) 4.0KOe is applied in the direction shown in FIG. At this time, the magnetization of the reproducing layer immediately above the pit is reversed according to the initializing magnetic field, and an interface domain wall 404 is formed between the reproducing layer and the auxiliary layer. Next, while applying the second reproducing magnetic field (402) 200 Oe, the magneto-optical head
Magneto-optical reproduction is performed by using 401 with laser power of 4.0 mW. The reproduction signal C / N was 45 dB. The C / N of the reproduced signal was 41 dB when the reproducing magnetic field was not applied.

As a comparative example, a reproduction experiment on a conventional optical disk is conducted. On a transparent resin substrate with a thickness of 1.2 mm,
Pits with a depth of 2000Å, a width of 4000Å and a length of 4000Å are repeatedly formed at intervals of 4000Å using injection molding. An aluminum reflective film is laminated on this substrate to a thickness of 800Å. This optical disc is reproduced by using the optical head shown in FIG. At this time, the sum signal of the differential optical system is taken as the reproduction signal. The laser power is 1.0mW. At this time, the C / N of the reproduced signal is 20 dB. The superiority of the present invention is clear from this example and the comparative example.

In this embodiment, the pit depth, pit width, and pit length are not limited to the above values. The effects similar to those of the present embodiment are achieved without departing from the gist of the present invention. Further, in the present embodiment, the interface between the reproducing layer and the auxiliary layer does not necessarily have to have the unevenness corresponding to the pit as shown in FIG. 1, and even if the interface is on the same plane other than the pit portion, This is the same as the result of this example.

[0026]

As described above, according to the present invention, it is possible to provide an optical disc capable of high density.

[Brief description of drawings]

FIG. 1 is a side sectional view of an embodiment.

FIG. 2 is a schematic diagram of pits provided on a resin substrate.

FIG. 3 is a configuration diagram of a magneto-optical head.

FIG. 4 is a diagram showing a series of processes in reproduction.

FIG. 5 is a schematic diagram of pits provided on a resin substrate.

FIG. 6 is a diagram showing a relationship between magnetization and coercive force.

FIG. 7 is a diagram showing the magnetization direction of each magnetic layer.

FIG. 8 is a view illustrating a magnetization direction of each magnetic layer following FIG. 7.

FIG. 9 is a view showing a shape in a thickness direction of a magnetic layer laminated on a substrate having a pit portion.

FIG. 10 is a diagram showing a magnetized state of a magnetic layer laminated on a substrate having a pit portion.

[Explanation of symbols]

 101 Substrate 102 Protective layer 103 Reproducing layer 104 Auxiliary layer 105 Pit 106 Pit depth 107 Thickness of the reproducing layer directly above the pit 108 Thickness of reproducing layer other than the pit 401 Magneto-optical head 402 Reproducing magnetic field 403 Initialization magnetic field

Claims (5)

[Claims] 1. In an optical disc having pits having information formed on a transparent substrate, at least a protective layer, a magnetic layer, and a protective layer are laminated in this order on the side having the pits of the substrate, and a magnetic layer immediately above the pits is formed. When the film thickness is h1 and the film thickness of the magnetic layer other than the pit is h2, h1> h2, and the change in the magneto-optical signal between the pit portion and the portion other than the pit portion is detected as an information signal. An optical disc characterized by: 2. The optical disc according to claim 1, wherein the magnetic layer is composed of two magnetic layers having different magnetic characteristics, and the first magnetic layer is a reproducing layer and the second magnetic layer is an auxiliary layer when viewed from the substrate side. When called, an optical disc which simultaneously satisfies the following (1), (2) and (3). (1) When the film thickness of the reproducing layer directly above the pit is h3 and the film thickness of the reproducing layer other than the pit portion is h4, h3> h4. (2) The reproduction layer is composed of an alloy of rare earth and a transition metal, the auxiliary layer is composed of an alloy of rare earth and a transition metal, the reproduction layer has a sublattice magnetization of the transition metal predominant at room temperature, and the auxiliary layer is composed of a rare earth metal at room temperature. Sub-lattice magnetization predominates. (3) The coercive force of the reproducing layer and that of the auxiliary layer are
HC1 <HC2 when 1 and HC2, and TC1 <TC2 when the Curie temperatures of the regeneration layer and the auxiliary layer are TC1 and TC2, respectively. 3. An optical disc reproducing system according to claim 2, wherein the following (1), (2), (3) and (4) are sequentially performed. (1) A first state in which no interface domain wall is generated is created between the reproducing layer and the auxiliary layer. (2) Before the reproducing operation, an initialization magnetic field Hinit in the direction opposite to the magnetization of the auxiliary layer, where HC1 <Hinit <HC2 is added, and an interface domain wall is formed between the reproducing layer and the auxiliary layer directly above the pit portion. Create a second state that creates. (3) Using a magneto-optical head, a change in the magneto-optical signal between the pit portion and a place other than the pit portion is detected and reproduced as an information signal. (4) When reproducing, eliminate the interface magnetic wall in the pit portion,
The magnetization of the reproduction layer is reversed to achieve the first state. 4. The optical disk reproducing method according to claim 3, wherein a magnetic field in a direction opposite to the initialization magnetic field is applied during reproduction. 5. The optical disk reproducing method according to claim 3, wherein a magnetic field in a direction opposite to the initialization magnetic field is applied, and at the same time, the temperature of the magnetic layer is sufficient to reach the Curie temperature of the auxiliary layer. A method of reproducing an optical disk, characterized in that the first state is achieved by irradiating a laser of various powers.