The present application claims priority to Japanese Applications Nos. P2000-121336 filed Apr. 21, 2000, P2000-260765 filed May 30, 2000, P2000-160766 filed May 30, 2000, P2000-206988 filed Jul. 7, 2000 and P2000-229605 filed Jul. 28, 2000, which applications are incorporated herein by reference to the extent permitted by law.
The present invention relates to a magneto-optical recording medium.
An optical recording medium is a recording medium in which a laser beam is applied to a signal-recording layer by a recording/reproducing apparatus to record and/or reproducing a record signal.
The following are known as optical recording media: a reproducing-dedicated digital audio disk (so called compact disk), an optical video disk (so called laser disk), a magneto-optical disk, and a phase-change-type optical disk.
As an example of the above optical recording media, a magneto-optical recording medium is used, having a multilayer structure in which a first dielectric layer a signal-recording layer, a second dielectric layer, and a metallic reflective layer are formed in order on a transparent substrate made of, for example, polycarbonate.
In the case of the above optical recording medium, a signal is recorded on or reproduced from a signal-recording layer along a recording track, and a groove is formed on a transparent substrate along the recording track, and it is requested to realize a recording medium having a larger capacity by improving the recording density.
For example, an MD (minidisk) which is one of standards of magneto-optical recording media has a configuration which has an easily-magnetized axis in the vertical direction to a substrate surface and in which a magnetic recording layer made of a magnetic film having a large magneto-optical effect is formed, the recording layer, a metallic reflective layer and a dielectric layer are laminated so that a recording section having a laminated structure on a transparent substrate is formed, and a protective layer made of, for example, an ultraviolet-curing resin is laminated on the recording section.
This minidisk is marketed as a re-writable magneto-optical recording medium and a magnetic-field-modulating magneto-optical recording medium in which a user can repeatedly record and erase information.
The following types are realized as the above minidisk (MD): a minidisk in which the pitch between recording tracks is set to 1.6 (xcexcm) to allow music signals to be recorded for 60 or 74 (min) and a minidisk in which the pitch between recording tracks is narrower to set to 1.5 (xcexcm) and thereby, the recording density is improved to allow music signals to be recorded for 80 (min).
Moreover, because a magneto-optical recording medium such as a (MD) minidisk can be used for various cases from home use to use by mounting the recording medium on a car because it is possible to easily record music in the recording medium serving as a household recording unit.
As described above, to accelerate general use of the above minidisk, it is indispensable to downsize an apparatus for recording or reproducing information in or from a magneto-optical recording medium and reduce the power consumption of the apparatus.
Particularly, in the case of a magneto-optical recording medium using a magnetic-field modulating system, it is necessary to modulate an applied magnetic field at a high speed. Therefore, it is necessary to make a magnetic head greatly approach a magneto-optical recording medium by decreasing the distance between the magnetic head and the corresponding face of the recording medium.
As recording systems for the above magnetic-field-modulating magneto-optical recording medium, it is proposed to use various types of fixed heads such as a sliding system and a flying head system.
However, as described above, when narrowing the pitch between recording tracks in order to increase a storage capacity, a problem occurs that a tolerance to a shift of the focus of a laser beam (hereafter referred to as defocus tolerance) to be irradiated for recording or reproducing data is decreased.
Thus, the decrease of the defocus tolerance causes the crosstalk for recording or reproducing data. Moreover, a difference occurs between optical characteristics of optical pickups due to the intrinsic performance of respective various types of recording and reproducing apparatuses. Thus, a problem occurs that a stable C/N characteristic cannot be obtained because a preferable reproduced signal is obtained from a recording/reproducing apparatus but a jitter value (time-axis fluctuation value) increases and an error rate deteriorates in another reproducing apparatus.
To solve the above problems, in the case of a magneto-optical recording medium such as a minidisk for repeatedly recording and reproducing information, a signal characteristic is improved by the so-called enhanced effect capable of enhancing an apparent Kerr rotational angle by adjusting the thickness of a dielectric film having a refractive index higher than that of a transparent substrate and obtaining effects of multiple reflection and interference so that a laser beam accurately traces a recording layer forming a magneto-optical medium.
However, also in the above description, it is difficult to obtain a signal characteristic enough for practical use in accordance with various types of reproducing apparatuses.
Therefore, as a result of earnestly repeating studies, the present inventor et al. provide an over-writable magneto-optical recording medium from which a superior signal characteristic and a high durability are obtained by sufficiently securing the defocus tolerance of a laser beam to be irradiated, improving the jitter characteristic, magnifying a power margin and thereby improving the C/N characteristic, and holding the surface strength of a disk even if a track pitch is narrowed.
An optical recording medium of the present invention has a configuration in which a recording section constituted of at least a first dielectric layer, a first recording layer, a second recording layer, and a second dielectric layer is sequentially formed, and a reflective layer and a protective layer are formed on the recording section.
In the case of an optical recording medium in which a groove is formed on the transparent substrate along a recording track and both sidewalls of the groove are respectively formed into an independent wobbling shape, the relation of 0.470xe2x89xa6W/Øxe2x89xa60.610 is effectuated when assuming the width of the groove as W and the relation of 0.120xe2x89xa6D/(xcex/n)xe2x89xa60.142 is effectuated when assuming the depth of the groove as D, and the angle xcex8 between the wall faces of the groove is kept at 150xc2x0 xc2x110xc2x0.
In this case, Ø denotes the spot diameter of a laser beam, xcex denotes the wavelength of a laser beam in a vacuum state, and n denotes the refractive index of a transparent substrate.
Then, a refractive index in the in-plane direction is made to range between 140 and 25 (nm) and a double-refractive index in the vertical direction is made to range between 200 and 300 (nm).
Then, the first recording layer is made of Gd, Fe, and Co and contains 28 to 30 (at %) of Gd and the thickness of the layer is selected in a range of 50 to 100 xc3x85, and the second recording layer is made of at least Tb, Fe, and Co, contains 22 to 24 (at %) of Tb and has a thickness of 180 to 250 xc3x85.
Moreover, the relation of 0.55xe2x89xa6(d2/d1)xe2x89xa60.9 is effectuated when assuming the thickness of a first dielectric layer as d1 and that of a reflective layer as d2, and the reflective layer is made of Al, Au, and Ti and contains 20 to 40 (wt %) of Au and 0.5 to 2.0 (wt %) of Ti.
According to the present invention, also when narrowing the track pitch of a magnetic-field-modulating over-writable magneto-optical recording medium, a superior signal characteristic and a high durability can be obtained by sufficiently securing the defocus tolerance of a laser beam for recording or reproducing a signal, improving the jitter characteristic, magnifying a power margin and thereby improving the C/N characteristic, and holding the surface strength of a disk.
A magneto-optical recording medium of a first aspect of the present invention has a configuration in which a groove is formed on a transparent substrate made of, for example, polycarbonate along a recording track and both sidewalls of the groove are respectively formed into an independent wobbling shape. The relation of 0.470xe2x89xa6W/Øxe2x89xa60.610 is effectuated when assuming the width of the groove as W, and the relation of 0.120xe2x89xa6D/(xcex/n)xe2x89xa60.142 is effectuated when assuming the depth of the groove as D and the angle xcex8 between the wall faces of the groove is kept at 150xc2x0 xc2x110xc2x0.
(In this case, Ø denotes the spot diameter of a laser beam to be irradiated, xcex denotes the wavelength of a laser beam in a vacuum state, and n denotes the refractive index of a transparent substrate.)
A magneto-optical recording medium is described below as an optical recording medium of the present invention by referring to the accompanying drawings. However, an optical recording medium of the present invention is not restricted to the example below.
That is, an optical recording medium of the present invention can be used as an optical recording medium for recording and/or reproducing data by using a laser beam as long as the recording medium has a configuration in which a groove is formed along a recording track and both sidewalls of the groove are respectively formed into a wobbling (zigzag) shape. For example, it can be used as an optical recording medium dedicated to reproducing in which a predetermined fine uneven pattern corresponding to a record signal is formed on a signal-recording layer, which can be used as one of various optical recording media such as a phase-change optical disk having a signal-recording layer made of a phase-change material.
FIG. 1 shows a schematic perspective view of an optical recording medium 10 of the present invention when a part of the medium 10 is cut out and FIG. 2 shows a schematic perspective view showing the layer structure of the optical recording medium 10.
The optical recording medium 10 of the present invention allows a record signal to be recorded or reproduced a plurality of times, which is accommodated in a disk cartridge (not illustrated) and removably used for a recording/reproducing apparatus (not illustrated).
As shown in FIG. 2, the magneto-optical recording medium 10 of the present invention has a configuration in which a first dielectric layer 3, a first recording layer 4, a second recording layer 5, a second dielectric layer 6, a reflective layer 7, and a protective layer 8 are sequentially laminated on a transparent substrate 2 made of a thermoplastic resin such as polycarbonate.
A signal is recorded in the optical recording medium 10 shown in FIGS. 1 and 2 by condensing a laser beam having a predetermined wavelength, e.g. 780 (nm) by an objective lens and applying the laser beam from the transparent-substrate-2-forming-face-2f side and applying a predetermined magnetic field by the recording magnetic head of a recording/reproducing apparatus.
In this case, the first recording layer 4 and second recording layer 5 of the optical recording medium 10 are raised in temperature by the laser beam and thereby decreased in coercive force. Therefore, a magnetic domain corresponding to a record signal is recorded in the second recording layer 5 by a magnetic field applied by a recording magnetic head.
Moreover, to reproduce the record signal recorded in the second recording layer 5, a laser beam is applied which has an output smaller than that of a laser beam for recording a signal. In this case, in the optical recording medium 10, return light of the laser beam reflected from the second recording layer 5 is polarized due to the magneto-optical effect such as the Kerr effect. The recording/reproducing apparatus detects the direction of a magnetic field of the magnetic domain recorded in the second recording layer 5 by detecting the polarized direction of the return light and thereby, reproduces a record signal.
The transparent substrate 2 is formed almost like a disk with a hard material capable of passing a laser beam. As the material for forming the transparent substrate 2, it is possible to use one of polycarbonate resin, acrylic resin, polyolefin resin, epoxy resin, and quartz glass.
Moreover, a groove 2b is spirally formed on the transparent substrate 2 in the circumferential direction. The optical recording medium 10 is constituted so that a signal is recorded or reproduced in or from the signal recording layer 4 along the groove 2b. That is, in the optical recording medium 10, a recording track is formed along the groove 2b. 
Moreover, in the optical recording medium 10 of the present invention, a groove 2b is formed zigzag in the radius direction at a constant cycle and a small width as shown in FIG. 3. That is, both sidewalls 21 and 22 of a groove 2a are respectively formed into an independent wobbling (zigzag) shape.
The optical recording medium 10 is constituted so that the zigzag cycle of the groove 2b is read by a recording/reproducing apparatus when data is recorded or reproduced by a laser beam. Thereby, the recording/reproducing apparatus can always stably keep the rotational speed of the optical recording medium 10 in accordance with the groove 2b zigzagging at a constant cycle.
Moreover, the groove 2b is used as the criterion for positioning when recording or reproducing a record signal in or from the second recording layer 5 by a recording/reproducing apparatus and has a function for showing the recording position of a record signal, that is, the address of the signal in the optical recording medium 10. Thereby, the recording/reproducing apparatus can quickly perform accurate positioning when recording or reproducing data in or from the optical recording medium 10.
FIG. 4 shows a sectional view of a simplified layer structure of the optical recording medium 10 of the present invention. In this case, the first dielectric layer 3, second dielectric layer 6, and reflective layer 7 are omitted.
In the case of the optical recording medium 10 of the present invention, the relation of 0.470xe2x89xa6W/Øxe2x89xa60.610 is effectuated when assuming the width of the groove 2a as W, the relation of 0.120xe2x89xa6D/(xcex/n)xe2x89xa60.142 is effectuated when assuming the depth of the groove 2a as D, and the angle xcex8 between wall faces of the groove 2a is kept at 150xc2x0xc2x110xc2x0.
In this case, Ø denotes the spot diameter of a laser beam to be irradiated, xcex denotes the wavelength of a laser beam L in a vacuum state, and n denotes the refractive index of a transparent substrate.
Moreover, the track pitch TP between recording tracks of the optical recording medium 10 of the present invention is set to approx. 1.5 xcexcm.
Furthermore, the groove 2a is formed so that the relation of 0.200xe2x89xa6[{D/(xcex/n)}/(W/Ø)]xe2x89xa60.330 is effectuated when assuming the width of the groove 2a as W and the depth of the groove 2a as D.
In this case, Ø denotes the spot diameter of a laser beam to be irradiated, xcex denotes the wavelength of a laser beam in a vacuum state, and n denotes the refractive index of a transparent substrate.
The first dielectric layer 3 and second dielectric layer 6 improve the C/N characteristic and prevent the first recording layer 4 and second recording layer 5 from corroding.
That is, because components including chlorine ions or the like for corroding metals are frequently contained in the transparent substrate 2 and the protective layer 7, it is possible to avoid the first recording layer 4 and second recording layer 5 from being directly influenced by components for corroding metals by forming the first dielectric layer 3 and the second dielectric layer 6 so as to hold the first recording layer 4 and second recording layer 5 between the layers 3 and 6.
It is necessary to form the first dielectric layer 3 and second dielectric layer 6 with a material having a small absorbing power for an applied wavelength of the laser beam L for recording or reproducing data.
For example, the first dielectric layer 3 and second dielectric layer 6 are respectively formed on the transparent substrate 2 as a thin film by a thin-film forming art such as one of various sputtering methods by using one of such materials as Si3N4, SiN, AlN, Al2O3, AlSiNO, HfO2, ZnS, ZrO2, Y2O3, MgO, SiO2, MgF2, and LiF. However, it is preferable to use a material having a performance for passing the laser beam L incoming to record or reproduce a record signal but not passing oxygen or moisture, and not containing oxygen as the material for forming the first dielectric layer 3 and use SiN, Si3N4, or AlN as the material.
In the above case, the first recording layer 4 and second recording layer 5 constituting the optical recording medium 10 of the present invention respectively include a magneto-optical recording layer and the magneto-optical layer is formed as a thin film on the first dielectric layer 3.
The magneto-optical layer is made of a material in which the coercive force is decreased due to a temperature rise exceeding Curie temperature and which causes magnetic reversal in the direction of an external magnetic field and has a magneto-optical characteristic such as Kerr effect or Faraday effect. For example, the layer is made of a rare-earth and transition-metal alloy such as TbFeCo, TbFeCoCr, or GdFeCo.
It is permitted that a recording section 9 is formed with a single layer such as only a magneto-optical layer or with a multilayer structure obtained by further laminating a dielectric layer as proposed for a CAD (Center Aperture Detector) disk or MSR (Magnetically induced Super Resolution) disk.
Moreover, the uneven shape on the groove 2a of the transparent substrate 2 is transferred to the first recording layer 4 and second recording layer 5. In the case of the optical recording medium 10, recording tracks in or from which a record signal is recorded or reproduced are formed on the first recording layer 4 and second recording layer 5 along the groove 2a. Moreover, the optical recording medium 10 is constituted so that the pitch between recording tracks becomes 1.5 xcexcm or 1.5xc2x10.01 xcexcm by considering an error range. In the case of the optical recording medium 10, it is possible to improve the recording density compared to the case of a conventional optical recording medium having a track pitch of 1.6 xcexcm. Therefore, the optical recording medium 10 allows the capacity to be increased while keeping the outside diameter equal to that of a conventional optical recording medium.
Moreover, in the case of the optical recording medium of the present invention, because of numerically specifying the width and depth of the groove 2a and the spot diameter Ø and wavelength xcex of a laser beam for irradiating the angle between wall faces of the groove, it is possible to sufficiently secure the defocus tolerance of a laser beam for recording or reproducing a signal even if the track pitch is further narrowed than ever and keep a high reliability.
The reflective layer 7 constituting the optical recording medium 10 is formed on the second dielectric layer 5 as a thin film. The reflective layer 7 has a function for reflecting the laser beam L passing through the first recording layer 4, second recording layer 5 and second dielectric layer 6 as well as a function as a heat sink for preventing heat from being accumulated in the first recording layer 4 and second recording layer 5 due to a laser beam irradiated toward the first recording layer 4 and second recording layer 5.
The optical recording medium 10 makes it possible to improve the utilization efficiency of the laser beam L when recording or reproducing data because the reflective layer 7 has a function for reflecting a laser beam. As a material for forming the reflective layer 7, it is preferable to use a non-magnetic metal element serving as a thermally good conductor or its compound independently or compounding metal elements. The reflective layer 7 is made of, for example, Au, Al, and Ti.
The protective layer 8 is formed on the reflective layer 7 as a thin film by curing an ultraviolet-curing resin applied by a spin coater or the like.
The optical recording medium 10 makes it possible to avoid the first recording layer 4, second recording layer 5, and reflective layer 7 from being deteriorating due to oxidation or the like because of including the protective layer 8.
Moreover, the medium 10 makes it possible to prevent a crack from being produced on each layer formed on the transparent substrate 2.
The protective layer 8 may contain various lubricants or various lubricants may be applied to the surface of the layer 8.
Thereby, the optical recording medium 10 can avoid the recording head of a recording/reproducing apparatus or the protective layer 8 from abrading or producing heat when sliding the head on the protective-layer-8 forming surface.