JPH06333280A - Magneto-optical recording medium which enables overwriting - Google Patents

Abstract

PURPOSE:To improve durability for repetition of writing by forming layers in such a manner that at least one of the memory layer or writing layer has a quasi-crystalline structure and the other layer having no quasi-crystalline structure has a multilayer structure. CONSTITUTION:The M layer and/or W layer of this magneto-optical recording medium has a quasi-crystalline structure. When the W layer and M layer are formed by sputtering from heavy rare earty-transition metal alloy targets, N2, O2, Al, Cu, Cr, etc., are added by sputtering from gas or targets. The obtd. medium shows little change in the window margin (the range of beam intensity to obtain desired C/N) while a medium produced by a conventional method shows large changes in the window margin. Namely, the obtd. medium has high durability for repetition of writing.

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 which can be overwritten only by intensity modulation of a light beam without modulating the direction of a recording magnetic field, and in particular, recording having high durability against repeated recording and reproduction. Regarding the medium. Overwrite (ove
r write) is the act of recording new information without erasing the previous information. In this case, when played back, the previous information should not be played back. The term "overwrite" as used in the present specification refers to overwrite by simply irradiating the laser beam while modulating the pulse according to the information to be recorded, without modulating the direction and intensity of the recording magnetic field Hb. Tell what to do.

[0002]

2. Description of the Related Art Recently, an optical recording / reproducing method satisfying various requirements including high density, large capacity, high access speed, and high recording / reproducing speed, and a recording apparatus, reproducing apparatus and recording medium used therefor. Efforts are being made to develop. Among a wide range of optical recording / reproducing methods, the magneto-optical recording / reproducing method has a unique advantage that information can be recorded and then erased, and new information can be recorded again and again. For being full of the greatest attraction.

A recording medium used in this magneto-optical recording / reproducing method has a perpendicular magnetic layer (perpendicular magnetic layer or layers) consisting of one layer or multiple layers as a layer for recording. This magnetic film is made of, for example, amorphous GdFe or Gd.
It is composed of Co, GdFeCo, TbFe, TbCo, TbFeCo and the like. The perpendicular magnetic film generally has concentric or spiral tracks, and information is recorded on the tracks. There are two types of tracks, explicit and implicit. [Explicit Track] The magneto-optical recording medium has a disk shape. In a disc having explicit tracks, tracks for recording information are formed in a spiral shape or a concentric circle shape when viewed in a direction perpendicular to the disk plane. There is a groove for tracking and separation between two adjacent tracks. Conversely, the space between the grooves is called a land. In reality, the land and groove are reversed on the front and back of the disc. Therefore, looking at the disk in the same way as the beam enters, the front side is called a groove and the back side is called a land. Since the perpendicularly magnetized film is formed over both the groove and the land, the groove portion may be the track or the land portion may be the track. There is no particular relationship between the width of the groove and the width of the land.

In order to form such a land and groove,
Generally, a substrate has a land formed in a spiral or concentric shape on the surface and a groove sandwiched between two adjacent lands. A thin perpendicular magnetic film is formed on such a substrate. As a result, the perpendicular magnetization film has lands and grooves. [Mark] In the present specification, “upward (upwar
d) "or" downward "
Orientation "and the other is defined as" reverse A orientation ".

The information to be recorded is binarized in advance, and this information has a mark (B ₁ ) having a magnetization in "A direction".
Then, two signals of the mark (B ₀ ) having the magnetization in the “reverse A direction” are recorded. These marks B ₁ and B ₀ correspond to either one or the other of the digital signals 1 and 0, respectively. However, generally, the magnetization of the track to be recorded is aligned in the "reverse A direction" by applying a strong external magnetic field before recording. Act to align the magnetization direction is referred to as the "initialization ^* (initialize ^*)" in the old sense. Then, the mark (B
₁ ) to form. Information is represented by the presence or absence of the mark (B ₁ ), the position, the front end position, the rear end position of the mark, the mark length, and the like. In particular, a method in which the edge position of a mark represents information is called mark length recording. A mark has been called a pit or a bit in the past, but is recently called a mark.

By the way, in order to reuse the recorded medium, (1) initialize the medium again ^* initialize it with the device ^* , or
Or (2) it is necessary to add an erasing head similar to the recording head to the recording device, or (3) it is necessary to erase the recorded information in advance by using the recording device or the erasing device as a pre-stage process. Therefore, in the magneto-optical recording system, it has hitherto been impossible to perform overwriting capable of recording new information on the spot regardless of the presence or absence of recorded information.

However, if the direction of the recording magnetic field Hb can be freely modulated between "A direction" and "reverse A direction" as required, overwriting becomes possible. However, it is impossible to modulate the direction of the recording magnetic field Hb at high speed. For example, when the recording magnetic field Hb is a permanent magnet, it is necessary to mechanically reverse the direction of the magnet. However, it is impossible to reverse the direction of the magnet at high speed. Even when the recording magnetic field Hb is an electromagnet, it is impossible to modulate the direction of a large-capacity current at such a high speed.

However, the technical progress is remarkable, and the intensity of the irradiation light beam should be recorded without modulating the intensity of the recording magnetic field Hb (including ON and OFF) or without modulating the direction of the recording magnetic field Hb. A magneto-optical recording method capable of overwriting, a magneto-optical recording medium capable of being overwritten, and an overwritable recording apparatus also employed therefor have been invented only by modulating in accordance with binary information. A patent application has been filed (JP-A-62-175948 = DE3,619,61)
8A1 = US patent pending Ser. No. 453,255). Hereinafter, this invention is referred to as a "basic invention". [Description of Basic Invention] In the basic invention, "a recording / reproducing layer basically composed of a magnetic thin film capable of perpendicular magnetization
(In this specification, this recording / reproducing layer is referred to as a memory layer Memory
layer or M layer) and a recording auxiliary layer consisting of a perpendicularly magnetizable magnetic thin film reference layer (in this specification, this recording auxiliary layer is a writing layer Writing layer or W).
Layer) and both layers are exchange-coupled (exchange-coupl
ed), and at the room temperature, the overwritable multilayer magneto-optical recording medium capable of keeping only the magnetization of the W layer in a predetermined direction without changing the magnetization direction of the M layer is used.

Information is represented and recorded by a mark having an "A direction" magnetization and a mark having an "inverse A direction" magnetization in the M layer (and also in the W layer in some cases).
In this medium, the W layer has an external means (for example, an initial auxiliary magnetic field Hin.
By i.), the direction of the magnetization can be aligned in the “A direction”. Moreover, at that time, the magnetization direction of the M layer is not reversed, and further, the magnetization direction of the W layer once aligned in the “A direction” is not reversed even when the exchange coupling force from the M layer is received. On the contrary, the magnetization direction of the M layer is not reversed even when receiving the exchange coupling force from the W layer aligned in the “A direction”.

The W layer has a lower coercive force H _C and a higher Curie point T _C than the M layer. According to the recording method of the basic invention, only the magnetization direction of the W layer of the recording medium is aligned in the “A direction” by external means before recording. This act is specifically referred to herein as "initialize". This "initialization" is unique to overwritable media.

Then, the medium is irradiated with a laser beam pulse-modulated according to the binarized information. The intensity of the laser beam has a high level P _H and a low level P _L , which correspond to the high level and low level of the pulse. This low level is higher than the reproduction level P _R that illuminates the medium during reproduction. As is already known, the laser beam may be turned on at a <very low level> even when recording is not performed, for example, to access a predetermined recording position on the medium. This <very low level> is also the same as or close to the reproduction level P _R.

For example, "initialize" for "A direction"
When the medium subjected to e) ”is irradiated with a laser beam of a low level P _L , the temperature of the medium is increased and the coercive force H _{c1 of the} M layer is increased.
Becomes very small or extremely zero. It becomes zero when the temperature of the medium is equal to or higher than the Curie point of the M layer. At this time, the coercive force H _c2 of the W layer is sufficiently large,
It is not reversed by the recording magnetic field Hb in the "reverse A direction".
Then, the force of the W layer reaches the M layer via the exchange coupling force. M
Layers and W layers are generally composed of heavy rare earth metals.
tal: hereinafter abbreviated as RE) -transition metal (transition meta
l: abbreviated as TM hereinafter). The exchange coupling force is composed of a force that aligns the RE magnetic moments of both layers and a force that aligns the TM magnetic moments of both layers. In the alloy, the sublattice magnetization of RE and the sublattice magnetization of TM have opposite directions, and the direction of the larger sublattice magnetization determines the direction of magnetization of the alloy. When both sublattice magnetizations are equal, the composition is called a compensation composition and the temperature is called a compensation temperature. Above the compensation temperature, the TM sublattice magnetization is stronger, and below the compensation temperature, the RE sublattice magnetization is stronger.

There are two types of marks before irradiation with the laser beam: a state in which an interface domain wall exists between the M layer and the W layer and a state in which no interface magnetic wall exists. The mark which does not exist corresponds to the mark to be formed.
The existing mark does not match the mark to be formed. In the latter case, as a result of the force of the W layer reaching the M layer via the exchange coupling force, the magnetization of the M layer having a very small coercive force H _c1 is determined in a predetermined direction (eg, "A direction"). As a result, a mark having no interface domain wall (target mark) is formed between the M layer and the W layer.

Even if the magnetization of the M layer is zero (T _c1 or more), the irradiation of the laser beam is stopped, and the temperature of the medium naturally lowers to slightly lower than the Curie point T _c1.
Magnetization appears in the layer. At this time, similarly, the force of the W layer reaches the M layer via the exchange coupling force. Therefore, the magnetization appearing in the M layer has a predetermined direction (for example, “A
Direction "). From this state, the temperature returns to room temperature, but the predetermined orientation is maintained. However, if there is a compensation temperature in the M layer and the W layer during returning to room temperature, the magnetization direction of the layer is reversed when the temperature exceeds the compensation temperature. This process is called a cold cycle or cold process.

On the other hand, for example, "Initialization (ini
tialize) "is media receives the irradiation of the laser beam of high level P _H, the coercive force H _c1 of the M layer is improved temperature of the medium is zero, the coercive force H _c2 of W layer is very Therefore, the magnetization of the W layer, which has a very small coercive force H _c2 , loses the recording magnetic field Hb and faces a predetermined direction (for example, the “reverse A direction”). Even if the magnetization of the W layer is zero, the irradiation of the laser beam is stopped and the temperature of the medium is naturally lowered to the Curie point T _c2.
When it goes down a little, magnetization appears in the W layer, but at this time,
Similarly, when the recording magnetic field Hb is lost, the magnetization of the W layer is oriented in a predetermined direction (for example, "reverse A direction"). Further, when the temperature of the medium cools and falls slightly below the Curie point T _c1 , magnetization appears in the M layer. At this time, the force of the W layer reaches the M layer via the exchange coupling force. Therefore, the magnetization appearing in the M layer is oriented in a predetermined direction (for example, “reverse A direction”) dominated by the W layer.
From this state, the temperature returns to room temperature, but the predetermined orientation is maintained. However, if there is a compensation temperature in the M and W layers on the way back to room temperature,
When it exceeds that, the magnetization directions of the M layer and the W layer are reversed. This process is called a high temperature cycle or high temperature process.

The above low temperature cycle and high temperature cycle are M
It occurs regardless of the direction of magnetization of the layer and the W layer. anyway,
Before the laser beam irradiation, the W layer is “initialized (initializ
e) ”, so that overwriting is possible. In the basic invention, the laser beam is pulse-modulated according to the information to be recorded. However, the means for pulse-modulating the beam intensity according to the binary information to be recorded is a known means, for example, THE BELL SYSTEM T.
It is described in detail in ECHNICAL JOURNAL, Vol.62 (1983), 1923-1936. Therefore, given the required high and low levels of beam intensity, they are readily available with some modifications to conventional modulation means. For those skilled in the art, such modification would be easy given the high and low levels of beam intensity.

One of the features of the basic invention is
There are high and low levels of beam intensity. That is, when the beam intensity is at a high level, the "A direction" magnetization of the W layer is reversed (reverse A direction) by the recording magnetic field Hb or other external means (re
verse), and the "reverse A direction" magnetization of the W layer forms a mark having "reverse A direction" magnetization (or "A direction" magnetization) in the M layer. When the beam intensity is low, the magnetization direction of the W layer is the same as in the "initialized" state, and
By the action of the layer (this action is transmitted to the M layer through the exchange coupling force), the M layer is magnetized "A direction" [or "reverse A direction"].
Magnetization] is formed.

In the present specification, ○○○ [or △△△]
With the expression, when XX outside [] is read first, XX outside [] will be read also in the following XX [or ΔΔΔ ]. On the contrary, ○○○
If you select and read the △△△ one in [] without reading, you can also read △ ○ △ in [] without reading ○○○ even in the following ○○○ [or △△△ ] Should be read.

The medium used in the basic invention is roughly classified into a first embodiment and a second embodiment. In any of the embodiments, the recording medium has a multilayer structure including an M layer and a W layer. The M layer is a magnetic layer having a high coercive force and a low magnetization reversal temperature at room temperature. The W layer is a magnetic layer having a lower coercive force and a higher magnetization reversal temperature at room temperature than the M layer. Both the M layer and the W layer may themselves be composed of a multilayer film. In some cases, an intermediate layer (for example, between the M layer and the W layer)
Exchange coupling force σ _W adjusting layer ... (Hereinafter, this layer is abbreviated as Int. Layer). For the Int. Layer, see JP-A-64-50257 and JP-A-1-273248.

Further, regarding overwritable magneto-optical recording, in addition to the above, JP-A-4-123339 and JP-A-4-13
Since many materials such as No. 4741 have been issued, here,
Omit further explanation. The disc having a four-layer structure disclosed in Japanese Patent Laid-Open No. 4-123339 has, in addition to the M layer and the W layer,
"Initializing layer" (hereinafter abbreviated as I layer), and exchange coupling between I layer and W layer is turned on.
Switching layer to turn off (below: S
Layer).

In order to increase the C / N ratio, a read-out layer (Readout layer: below) having a higher Curie point and a higher Kerr effect than the M layer on the M layer (that is, on the laser beam incident side):
A laminate of R layers) is also proposed. For example, see JP-A-63-64651 and JP-A-63-48637. The proposed R layer is also composed of a RE-TM based alloy.

Conventionally, the M layer and the W layer have been formed by sputtering a perpendicularly magnetized film having an amorphous structure composed of a heavy rare earth metal and a transition metal.

[0023]

Conventionally, an overwritable magneto-optical recording medium has a C content when repeatedly written.
There is a problem that / N decreases and reliability of reproduced information deteriorates. SUMMARY OF THE INVENTION It is therefore an object of the present invention to solve these problems and to obtain an overwritable magneto-optical recording medium in which the C / N does not decrease even after repeated writing.

[0024]

DISCLOSURE OF THE INVENTION The present inventor has investigated the cause of a decrease in C / N of a reproduction signal of an overwritable magneto-optical recording medium. Therefore, the relationship between the crystal structure of the M layer and the W layer and the recording conditions was investigated. As a result, it was found that the crystal structure of the M layer or W layer, which is an amorphous structure, changes due to the influence of the heat of the laser beam applied during recording and reproduction. As a result, the magnetization reversal operation becomes unstable, and it becomes impossible to form the recording mark according to the original information.
When rewriting old information to new information, the crystal structure in the layer is unstable, so the old information remains as it is,
It was found that this becomes noise and C / N is lowered.

As a result of earnest research, the present inventor has found that the M layer and W
It was proposed that the crystal structure of the layer be formed of a material having a stable structure with a lower free energy than the conventional amorphous structure. Therefore, the present invention is firstly “(Claim 1)”.
I will provide a. Secondly, “(Claim 2)” is provided.

[0026]

According to the present invention, the M layer and / or the W layer has a quasi-crystal structure. The quasi-crystal structure is an intermediate structure between an amorphous structure (amorphous structure) and a crystal structure, and means a structure in which amorphous and crystal are mixed. When a small angle X-ray scattering analysis is performed on the quasicrystal structure, a diffraction pattern appears,
No diffraction pattern appears when wide-angle X-ray scattering analysis is performed. As described above, the W layer or the M layer having a quasi-crystal structure has a lower free energy of the layer as compared with the case of having a completely amorphous structure. Since the free energy becomes low, the structural change in the layer due to heat upon irradiation with laser light during recording / reproduction is reduced before and after repeated writing, and the thermal durability of the medium is improved.

In order to solve the above problems, in the present invention, when the W layer and the M layer are formed, N ₂ , O ₂ and N ₂ are added together with sputtering from a heavy rare earth-transition metal alloy target.
Al, Cu, Cr, etc. are added by sputtering from a gas or a target.

[0028]

Embodiment 1 (1) First, an ultraviolet curable resin (PP: Photo polymer) is formed on a glass substrate (G) having a diameter of 130 mm and a thickness of about 1.2 mm.
A 2P substrate on which is formed is prepared. This resin layer (PP)
Above, from the inner circumference side (radius r = 30 mm position) to the outer circumference side (r =
A large number of grooves are formed in a spiral shape up to the 60 mm position). Groove dimensions are 0.35 μm wide and 1.4 μ pitch
m, depth is about 700 Å. (2) Next, sputtering is performed. FIG. 1 is a schematic view schematically showing a sputtering apparatus for manufacturing a magneto-optical recording medium according to this embodiment.

For the sputtering, once the inside of the chamber
It is performed after evacuation to a vacuum of × 10 ^-5 Pa or less. The sputtering apparatus has three vacuum chambers, two of which are sputtering chambers (6) each having a ternary cathode,
(7) is installed. First, using a Si target in the sputtering chamber (7), N ₂ gas was introduced into the chamber in addition to Ar gas with a gas pressure of 0.3 Pa to carry out reactive sputtering, and 700 Å film thickness of silicon nitride was deposited on the resin. To form a protective layer. Sputtering deposition rate is 100Å /
Minutes This medium was moved to the sputtering chamber (6), and two-source simultaneous sputtering was performed from the two targets of FeCo alloy of the target (1) and Tb of the target (2) to form the M layer. The sputtering conditions are an Ar gas pressure of 0.5 Pa and a deposition rate of 200 Å / min. Since the M layer is thus formed by the two-source simultaneous sputtering, the substrate holder is rotated so that the substrate passes over each target. Therefore, the M layer has a multilayer structure in which extremely thin films are laminated in layers.

Next, as an intermediate layer (Int. Layer), the target (3) has better overwrite characteristics.
A GdFeCo film was formed. These sputtering conditions are as described above. Protective layer, M layer, Int. The medium on which the layer is formed is moved to the sputtering chamber (7). The target (4) is a DyFeCo alloy target for forming the W layer,
The target (5) is a Cr target. Co-sputtering is performed using each of these targets to form a W layer. The power (applied voltage) of the Cr target (5) at this time was 0.5% of the target (4). Also,
Ar gas pressure during sputtering is 0.2 Pa, deposition rate is 150
Å / min.

Finally, reactive sputtering is performed under the same forming method and sputtering conditions as for the protective layer to form a silicon nitride protective layer having a film thickness of 700Å on the W layer.

[0032]

[Evaluation Test 1] DO prepared by the manufacturing method of Example 1
Durability against repetitive writing between a W medium (direct overwrite) and a magneto-optical recording medium in which an M layer and a W layer are formed from an alloy target as in the conventional case and which has an amorphous structure and does not have a quasi-crystal structure Comparative measurement was performed.

The durability test was conducted as follows. First,
Duty ratio = 50%, frequency 7.0MHz reference signal linear velocity
A recording mark was formed at 11.3 m / s, a reproducing beam having an intensity of each condition was irradiated, and the C / N write power fluctuation (profile) of the reproducing signal from the recording mark was measured. Next, write continuously for 15 million times at a duty ratio of 50% and a frequency of 2.6 MHz, and then write again at a duty ratio of 50% and a C / frequency of 7 MHz.
The write power fluctuation of N was measured. Contour maps of the results are shown in FIGS. 3 and 4. FIG. 3 shows the results of measurement of the medium prepared according to this example.
Is the result of measurement on a conventional medium. 3 and 4
The horizontal axis represents the low level beam intensity of the laser beam during writing, and the vertical axis represents the high level beam intensity.
In each figure, a curve representing the beam intensity at the time of writing, which gives C / N of 46 dB, 50 dB, and 54 dB, is shown. In the figure, the solid line is before repeated writing and the broken line is after repeated writing.

As is apparent from the figure, the conventional magneto-optical recording medium has a window margin (desired C / N) before and after repeated writing of the medium in which the W layer is formed by the conventional manufacturing method.
The change in the window margin is not significant in the medium prepared in this example. That is, it was confirmed that the medium has high durability against repeated writing.

[0035]

Example 2 (1) The same substrate as in Example 1 was prepared. (2) Next, sputtering is performed. The sputtering apparatus for manufacturing the magneto-optical recording medium in this example is the same as that in Example 1.
It is the same as the device used in. Next, a protective layer, an M layer, Int.
The medium on which the layer is formed is moved to the sputtering chamber (7).
The target (4) is a DyFeCo alloy target. Thereby, the W layer is formed. DyFeCo is introducing O ₂ into the chamber during which sputtering, addition of the reactive sputtering a third element O _2. The gas pressure of O ₂ introduced at this time was 1/160 of the Ar gas pressure. The gas pressure of O ₂ is preferably in the range of 1/200 to 1/100 of the Ar gas pressure. The flow rate of O ₂ was 1.0 SCCM.

Finally, as in Example 1, a protective layer of silicon nitride having a film thickness of 700 Å is formed.

[0037]

[Evaluation Test 2] The repetitive writing durability of the medium prepared according to this example and the medium in which the W layer was formed by only the sputtering from the alloy target, which is the conventional method, were measured for comparison. The durability test was performed in the same manner as in Example 1, and the C / N was measured at 4 MHz and 50%.

The results are shown in Table 1. DISK in Table 1
A is a conventional recording medium in which the W layer does not contain the third element, and DISKB is a recording medium in which the W layer according to the present embodiment contains oxygen as the third element.

[0039]

[Table 1]

Looking at this, DISKA is C after the durability test.
/ N is lower than that after the test, whereas C / N of DISKB of this example is almost unchanged. Furthermore, MsHc was evaluated by measuring the Kerr rotation angle of the recording medium. It was found that M _S H _C of DISK A and B were getting larger. Further, its size depends on the flow rate of O ₂ which is the third element, and the size of M _S H _C is the third.
It can be controlled by the flow rate of elemental O ₂ .

In this embodiment, the M layer is a layer having a multi-layer structure and the W layer has a quasi-crystal structure. However, both the M layer and the W layer have a quasi-crystal structure. Is preferred. Further, in the present embodiment, the W layer and / or the M layer has a quasi-crystal structure formed by containing a predetermined substance, but the present invention is not limited to this, and the W layer and / or the M layer may have a quasi-crystal structure. Good.

[0042]

As described above, according to the present invention, since the recording medium is less deteriorated by repeated writing, the durability is improved, and the C / N of the reproduced signal is not lowered, the repeated writing is particularly frequent. It is effective for the overwritable magneto-optical recording medium.

[Brief description of drawings]

FIG. 1 is a schematic view of a sputtering apparatus according to this embodiment.

FIG. 2 is a vertical sectional view of an overwritable magneto-optical recording medium according to the present embodiment.

FIG. 3 is a graph showing changes before and after repeated writing durability of the contour map of the recording medium according to the first embodiment.

FIG. 4 is a graph showing changes in a contour map of a conventional recording medium before and after a repeated writing durability test.

[Explanation of symbols]

 1 ... FeCo alloy target 2 ... Tb target 3 ... GdFeCo alloy target 4 ... DyFeCo alloy target 5 ... Target for third element 6,7 ... Sputtering chamber and above

Claims (2)

[Claims] 1. A memory layer formed of a perpendicular magnetization film and a writing layer formed of a perpendicular magnetization film have at least two-layer structure, and the magnetization direction of the writing layer is changed without reversing the magnetization direction of the memory layer. In an overwritable magneto-optical recording medium that can be aligned on one side, at least one of the writing layer and the memory layer has a quasi-crystal structure, and one of the two layers that is not a quasi-crystal structure has a multilayer structure. An overwritable magneto-optical recording medium, which is a layer formed of a material. 2. The memory layer and / or the writing layer having the quasi-crystal structure according to claim 1, wherein Al, Cu, Cr, Si,
A magneto-optical recording medium containing a substance which is at least one of Ti, Ge, Pt, oxygen and nitrogen, or a combination thereof.