Magneto-optical recording medium

Abstract: A magneto-optical recording medium which comprises, on a transparent substrate (1) a magneto-optical recording layer (10) having an exchange-interacted, multi-layered structure which consists of a first magnetic film (11) serving as a reproduction layer, a second magnetic film (12) serving as an intermediate layer, and a third magnetic film (13) serving as a record holding layer, these films being magnetically coupled to one another. The Curie points and coercive forces of the respective films are so set that the magnetic pattern formed in the first magnetic film is partially expanded, shrunk or inverted depending on the temperature distribution in a reproduction laser beam spot. Further, the first magnetic film is made of a rare earth element-transition metal amorphous alloy film mainly composed of GdFeCo with a saturation magnetization, Ms, of not greater than 200 emu/cc. The second magnetic film is made of a rare earth element-transition metal amorphous alloy film mainly composed of TbFeCo. In a transition metal sublattice magnetization-rich case, the saturation magnetization, Ms, is not greater than 300 emu/cc and in a rare earth element sublattice magnetization-rich case, the saturation magnetization, Ms, is not greater than 300 emu/cc. Reproduction methods are also described.

Description: BACKGROUND OF THE INVENTION Field Of The Invention This invention relates to a magneto-optical recording medium wherein information bits (magnetic domains) are read out by the magneto-optical effect and more particularly, to a magneto-optical recording medium wherein information bits are partially extinguished or expanded depending on the temperature distribution in a reproduction beam spot at the time of reproduction. The invention also relates to reproduction methods using the medium mentioned above. Description Of The Prior Art The magneto-optical recording system makes use of the fundamental principle that a magnetic thin film is partially heated to a temperature which exceeds the Curie point or compensation temperature so that the coercive force of the heated portion is lost and the direction of magnetization is inverted to conform to the direction of a recording magnetic field which is applied from outside. A known magneto-optical recording medium has such an arrangement which includes a transparent substrate such as of, for example, polycarbonate and a recording portion formed on one side of the substrate. The recording portion includes laminated layer structure which has a magnetic recording layer, which has an easy axis of magnetization in a direction vertical to the film plane and good magneto-optical characteristics and is made, for example of a rare earth element-transition metal amorphous alloy, a reflection layer, and a dielectric film. Signals are read out by irradiation of a laser beam from the side of the transparent substrate. Not only in the magneto-optical recording medium, but also in digital audio disks (so-called compact disks) or optical disks such as video disks, the linear recording density is determined by the S/N value at the time of reproduction. The quantity of reproduction signals depends greatly on the pitch or frequency of recorded signal bit rows and the laser beam wavelength and the numerical aperture of an objective lens of a reproducing optical system. At present, when the laser beam wavelength, λ, and the numerical aperture, N.A. are determined, the bit frequency, f, which becomes a detection limit is determined as follows: f = λ/2 N.A. Accordingly, in order to make high density recording of optical disks, it is fundamentally essential to shorten the laser beam wavelength, λ, and increase the numerical aperture, N.A. of the objective lens. However, existing techniques place limitation on the improvements in the laser beam wavelength, λ, and the numerical aperture, N.A. of the objective lens. Another approach of improving the recording density has been made wherein efforts have been made on the construction of the magneto-optical recording medium and the manner of reading out from the medium. For instance, the present applicant proposed in Japanese Laid-open Patent Application Nos. 1-143041 and 1-143042 systems wherein an information bit (magnetic domain) is partly expanded, shrunk or extinguished, thereby improving reproducing resolution. In the system, the magnetic recording layer is made of an exchanging-coupled multi-layer film which includes a reproduction layer, an intermediate layer and a record-holding layer. At the time of reproduction, a magnetic domain of the reproduction layer which has been heated by means of a reproducing light beam is expanded or shrunk or distinguished at the high temperature portion, making it possible to reproduce a signal with a pitch which exceeds a diffraction limit. This is reflected in the preambles of claims 1 and 7. In the procedure wherein information bits are partially expanded, shrunk or distinguished at the time of reproduction, record information with a density higher than a record density which is determined depending on the laser beam wavelength, λ, and the numerical aperture, N.A., can be reproduced. We made further investigations and, as a result, found that mere selection of the Curie point and the coercive force does not always lead to reproduction with a high C/N value. SUMMARY OF THE INVENTION It is an object of the invention to provide a magneto-optical recording medium wherein high density recording bits, which exceed a recording density determined by the laser beam wavelength, λ, and the numerical aperture, N.A., of an objective lens, can be reproduced with a high C/N value. It is another object of the invention to provide a method for reproducing a signal of a recorded magnetic domain from the medium of the type mentioned above whereby the size of the magnetic domain can be expanded, shrunk or distinguished. The present inventors made intensive studies over a long term in order to attain the above objects. As a result, it was found that in the above-stated recording system, the types of magnetic materials used as the first and third magnetic films and the saturation magnetizations of these films greatly influence the C/N value. According to one embodiment of the invention, there is provided a magneto-optical recording medium which comprises the features of appended claim 1. According to other embodiments of the invention, there are provided methods for reproducing a signal of a recorded magnetic domain having the features of appended claims 3, and 7, respectively. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an enlarged, sectional view of an essential part of a magneto-optical recording medium according to the invention; Fig. 2 is a schematic view showing an initially magnetized state of the magneto-optical recording medium; Fig. 3 is a schematic view showing a magnetic pattern in a first magnetic film of the medium in the initially magnetized state; Fig. 4 is a characteristic view showing a temperature profile of a beam spot diameter of a laser beam at the time of reproduction; Fig. 5 is a schematic view showing a magnetized state of a magneto-optical recording layer at the time of reproduction: Fig. 6 is a characteristic view illustrating spontaneous magnetization; Fig. 7 is a characteristic view showing a temperature change of the respective sublattice magnetizations and spontaneous magnetization in case where transition metal magnetization is rich; Fig. 8 is a characteristic view showing a temperature change of the respective sublattice magnetizations and spontaneous magnetization in case where rare earth element magnetization is rich at low temperatures and transition metal magnetization is rich at high temperatures; Fig. 9 is a characteristic view showing a temperature change of the respective sublattice magnetizations and spontaneous magnetization in case where rare earth element magnetization is rich; Fig. 10 is a characteristic view showing the relation between the spontaneous magnetization of the first magnetic film and C/N; and Fig. 11 is a characteristic view showing the relation between the spontaneous magnetization of the third magnetic film and C/N. DETAILED DESCRIPTION AND EMBODIMENTS OF THE INVENTION In the magneto-optical recording medium of the invention, a record or information magnetic domain is formed by a photo or magnetic modulation system, like ordinary magneto-optical recording mediums. On the other hand, for reproduction, a laser beam is irradiated on the first magnetic film which serves as a reproducing layer, by which magnetization signals of the first magnetic film are read out after conversion into optical signals by the magneto-optical effect (magnetic Kerr effect or Faraday effect). At this time, the magnetic coupling between the first magnetic film serving as the reproducing layer and the second magnetic film used as an intermediate layer is partially interrupted depending on the temperature distribution within a spot diameter of the laser. When an external magnetic field is applied, the magnetic domain at the portion where the magnetic coupling has been interrupted is deformed, i.e. expanded, shrunk or inverted, by application of an external magnetic field. The type of deformation depends on the direction of the external magnetic field. As a result, the magnetic domain pattern in the spot of the laser beam is read out partially in a masked form, so that even a signal formed at a pitch which is smaller than the spot diameter of the laser beam can be reproduced. The first magnetic film is mainly composed of GdFeCo wherein the saturation magnetization, M s1 , is not greater than 200 emu/cc when the sublattice of the transition metals is more predominant than that of the rare earth element or when the sublattice of the rare earth element is more predominant than that of the transition metals. The third magnetic film is mainly composed of TbFeCo, in which the saturation magnetization, M s3 , is not greater than 300 emu/cc when the sublattice of the transition metals is more predominant than that of the rare earth element, or not greater than 160 emu/cc when the sublattice of the rare earth element is more predominant than those of the transition metals. By this, a high C/N value is ensured. Reference is now made to accompanying drawings and, particularly, to Fig. 1 to illustrate embodiments of the invention. Initially, the fundamental arrangement of the magneto-optical recording medium of the invention and the principle of reproduction of the medium are described. Fig. 1 shows a magneto-optical recording medium in accordance with an embodiment of the invention. The medium includes a transparent substrate 1, on which a magnetic film 11 serving as a reproducing layer, a second magnetic film 12 used as an intermediate layer, and a third magnetic film 13 used as a record holding layer are built up, thereby forming a magneto-optical recording layer 10. The magnetic films 11, 12, 13 are all made of rare earth element-transition metal amorphous alloys. In this embodiment, the first magnetic film 11 is formed of a rare earth element-transition metal amorphous alloy mainly made of GdFeCo. The third magnetic film 13 is formed of a rare earth element-transition metal amorphous alloy mainly made of TbFeCo. The Curie points and coercive forces of these magnetic films 11, 12. 13 are set as follows. When the Curie point is taken as T c1 for the first magnetic film 11, as T c2 for the second magnetic film 12 and as T c3 for the third magnetic film 13, 60°C ≦ T c2 ≦ 200°C, T c2 < T c1 and T c2 < T c3 . Moreover, the coercive force, H c1 , of the first magnetic film 11 is so selected as to be small In the vicinity of the Curie point, T c2 , of the second magnetic film 12 and the coercive force, H c3 , of the third magnetic film 13 is so selected as to be larger than a required minimum magnetic field intensity within a temperature range between room temperature T RT , and a predetermined temperature, T PB , which is higher than the Curie point, T c2 , of the second magnetic film 12. In this embodiment, the surface of the magneto-optical recording layer 10 is covered with a protective film 20. The protective film 20 may be omitted. In addition, a transparent dielectric layer may be provided between the transparent substrate 1 and the magneto-optical recording layer 10 in order to enhance the Kerr angle of rotation. For the reproduction of the magneto-optical recording medium having such an arrangement as stated above, a recording magnetic domain pattern of the first magnetic film 11 is deformed so as to lower an apparent space frequency of record bits, thereby realizing high density recording. The principle of the reproduction is as described in Japanese Laid-open Patent Application Nos. 1-143041 and 1-14304, and the fundamental principle is briefly described below. As shown in Figs. 2 and 3, given magnetic signals are recorded in the first magnetic film 11, second magnetic film 12 and third magnetic film 13, thereby forming a magnetic domain pattern P 2 . In Fig. 3, the regions where the direction of magnetization is upward are indicated as solid. In case of reading out of the signals by means of a laser beam, LB, when the spot diameter of the laser beam, LB, is larger than a pitch of record bits (magnetic domain pattern), a plurality of record bits exist in the spot. This makes it impossible to read out individual bits by an ordinary reproduction procedure. In the magneto-optical recording medium of this embodiment, however, the magneto-optical recording layer 10 has a multi-layer structure including the first magnetic film 11, the second magnetic film 12 and the third magnetic film 13. The Curie points and coercive forces of the magnetic films 11, 12 and 13 are set at predetermined values, so that the magnetic domain pattern of the first magnetic film 11 can be deformed, thus making it possible to read out individual record bits even when the spot diameter is larger than pitches of the record bits. More particularly, when the laser beam, LB, is irradiated on the magneto-optical recording layer 10, which has the multilayer structure as stated above, a temperature distribution takes place in the spot. As shown in Fig. 4, the front portion of the laser beam (shaded portion in the figure) as viewed along the direction of running of the medium (i.e. the direction of arrow X in the figure) becomes higher in temperature. When the temperature, T H , of the shaded portion is higher than the Curie point, T c2 , of the second magnetic film 12, the magnetization of the second magnetic film 12 is lost, so that the magnetic coupling (exchange interaction) between the first and third magnetic films 11 and 13 are interrupted. In this state, when an external magnetic field, H PB , which is larger than the coercive force, H c1 , of the first magnetic film 11 is applied, the direction of magnetization of the first magnetic film 11 in the shaded portion is inverted and aligned with the direction of the external magnetic field, H PB , (downward direction in the figure). In portions other than the shaded portion, i.e. in portions whose temperature is lower, the magnetic coupling between the first magnetic film 11 and the third magnetic film 13 are kept, in which the magnetic domain pattern recorded in the third magnetic film 13 is retained as it is in the first magnetic film 11. Accordingly, the shaded portion within the spot is kept as if it is masked, so that the magnetic domain pattern recorded in this portion apparently disappears, leaving a single magnetic domain pattern to exist in the beam spot. With regard to the space frequency of the record bit as viewed from the reproducing light at the time of reproduction, the absolute value of the optical transmission function becomes greater, thereby improving the reproducing resolution. In this condition, if the coercive force, H c3 , of the third magnetic film 13 is made greater than the intensity of the external magnetic field, H PB , the magnetization (magnetic domain pattern) of the third magnetic film 13 does not change by application of the external magnetic field, H PB . When the temperature, T, of the medium is equal to T = T c2 after cooling, the exchange force again works. When the following conditions are satisfied, the magnetization pattern of the third magnetic film 13 is transferred to the second magnetic film 12 and the first magnetic film 11, thus returning to the state prior to the reproduction. T ≦ T c2 - ΔT 2M s1 H c1 h 1 + 2M s1 H PB h 1 < σ w 2M s3 H c3 h 3 + 2M s3 H PB h 3 < σ w In the above inequalities, the thickness of the second magnetic film 12 is taken as negligible as compared with an interfacial magnetic wall width established between the first magnetic film 11 and the third magnetic film 13, M s1 represents a saturation magnetization of the first magnetic film 11, h 1 is a thickness of the first magnetic film 11, M s3 is a saturation magnetization of the third magnetic film 13, h 3 is a thickness of the third magnetic film 13, and σ w is an interfacial magnetic wall energy (unit: erg/cm 2 ). The above inequalities indicate the case where the same sublattice magnetization for the first and third magnetic films 11, 13 are rich or predominant. The reproduction procedure has been described above using the case where the first magnetic film 11 and the third magnetic film 13 are both rich or predominant in the same sublattice magnetization and where the signals are read out while inverting the record magnetic domain of the first magnetic film by application of the external magnetic field. If the first magnetic film 11 and the third magnetic film 13 are different from each other with respect to the predominant sublattice magnetization, the inversion, shrinkage or expansion of the magnetic domain pattern is possible by proper selection of the direction of the external magnetic field, H PB , at the time of reproduction depending on whether the respective layers are a transition metal sublattice magnetization-rich film or a rare earth element sublattice magnetization-rich film. By this, an apparent space frequency can be suppressed with the reproducing resolution being improved. In the magneto-optical recording medium which is subjected to the reproduction procedure as stated above, the Curie points and coercive forces of the first, second and third magnetic films should satisfy the conditions set forth before. For reproduction at a high C/N value, the following conditions should also be satisfied. The first magnetic film 11 is a rare earth element-transition metal amorphous alloy film which is mainly composed of GdFeCo. The saturation magnetization, M s1 , of the first magnetic film 11 at room temperature should be not larger than 200 emu/cc when the transition metal sublattice magnetization is rich (hereinafter referred to TM rich) or not larger than 200 emu/cc when the rare earth element sublattice magnetization is rich (hereinafter referred to as RE rich). The reason for this will be described hereinafter. The first magnetic film 11 may further comprise Cr or the like in order to ensure good reliability and Nd or the like for improvement at short wavelengths. The Curie point, T c2 . of the second magnetic film should be in the range of from 60°C to 200°C. The third magnetic film 13 is a rare earth element-transition metal amorphous alloy film which is mainly made of TbFeCo. The saturation magnetization, M s3 , at room temperature should be not larger than 300 emu/cc when TM is rich and not larger than 160 emu/cc when RE is rich. The third magnetic film 13 may further comprise additive elements, if necessary. The reason why the optimum ranges of the saturation magnetization of the first and third magnetic films 11 and 13 are different for the TM rich and RE rich cases is described. GdFeCo and TbFeCo are called ferrimagnetic materials and the sublattice magnetization of the rare earth elements such as Tb, Gd and the like and sublattice magnetization of the transition metals such as Fe, Co and the like are invariably opposite in direction to each other. The difference between the magnetizations of the sublattices is spontaneous magnetization (saturation magnetization, Ms). For the same value of the saturation magnetization Ms, there are two cases including one wherein the magnetization of the rare earth elements (RE) such as Tb, Gd and the like is rich and the other wherein the magnetization of the transition metals (TM) such as Fe, Co and the like is rich. The former case is taken herein as RE rich and the latter case is taken as TM rich. This is particularly shown in Fig. 6 wherein solid arrow indicates Ms of TM and white-on-black arrow indicates Ms of RE. If rare earth elements are added to transition metals portion by portion, Ms of TM is decreased with an increasing amount of the rare earth element, resulting in an increase in Ms of RE. The Ms values of TM and Re are equal to each other when a certain amount of RE is added, whereupon the spontaneous magnetization (saturation magnetization Ms) becomes zero. This point is a boundary for the TM rich region where Ms of TM is larger than Ms of RE (l.e. the left-side region in the figure) and the RE rich region where Ms of Re is larger than Ms of TM (i.e. the right-side region in the figure). In the magneto-optical recording medium of the invention, the saturation magnetization, M s3 , of the third magnetic film 13 is important. The value of M s3 should be not larger than 300 emu/cc for the TM-rich case and not larger than 160 emu/cc for the RE-rich case. This is because when the saturation magnetization, M s3 , of the TM-rich region exceeds 300 emu/cc, the magnetization of the third magnetic film 13 used as the record holding layer is inverted by the application of the external magnetic field, H PB , for the reason that the saturation magnetisation, M s3 , is great and the coercive force, H c1 , is small at the reproduction temperature. On the other hand, when M s3 exceeds 160 emu/cc, RE-rich and TM-rich regions coexist at a recording temperature, so that the directions of magnetization at the time of reproduction become random. It will be noted that the saturation magnetization, M s3 , defined above is a value at room temperature and whether a given region is TM-rich or RE-rich is defined at room temperature. The temperature characteristics (temperature change) of the magnetization of RE and TM, respectively, vary. With certain compositions, the rich state of RE or TM may vary depending on the temperature. For instance, Fig. 7 shows the respective sublattice magnetizations (indicated by solid line) and the spontaneous magnetization (indicated by broken line) in case where the magnetization of TM is invariably rich. Fig. 8 shows the respective sublattice magnetizations and the spontaneous magnetization in case where RE is rich at low temperatures and TM is rich at high temperatures. Fig. 9 shows the respective sublattice magnetizations and the spontaneous magnetization in case where the sublattice magnetization of RE is invariably rich. Magneto-optical recording mediums wherein the spontaneous magnetizations of the first magnetic film 11 and the third magnetic film 13 were changed were fabricated to measure C/N values. The measuring conditions (reproducing conditions) are as follows. Numerical aperture, N.A. of objective 0.53 Wavelength of reproduction laser beam 780 nm Reproduction power 3.0 mW Land width 1.0 µm Group width 0.6 µm Linear velocity 8 m/second Fig. 10 shows a change of C/N relative to the spontaneous magnetization of the first magnetic film 11 serving as a reproduction layer. In Fig. 10, the spontaneous magnetization is represented by magnetization of Gd (Ms - Gd), so that the plus side is RE rich and the minus side is TM rich. As will be apparent from Fig. 10, the first magnetic film 11 has an optimum range of the magnetization, i.e., a high C/N value is obtained in the range of 200 emu/cc in the TM-rich region to 200 emu/cc in the RE-rich region. Fig. 11 shows a change of C/N relative to the spontaneous magnetization of the third magnetic film 13 serving as a record-holding layer. In Fig. 10, the spontaneous magnetization is represented by magnetization of Tb (Ms - Tb), so that the plus side is RE rich and the minus side is TM rich. With regard to the third magnetic film 13, the C/N value is abruptly lowered in a TM-rich region exceeding 300 emu/cc (in a region less than -300 emu/cc in the figure) or in a RE-rich region exceeding 160 emu/cc (in a region exceeding +160 emu/cc). As will become apparent from the foregoing description, the magneto-optical recording medium has an exchange-interacted, multi-layered film as a recording magnetic layer consisting of a reproduction layer, an intermediate layer and a record-holding layer in such a way that Curie points and coercive forces of the respective layers are set to satisfy certain conditions. Accordingly, an information bit (magnetic domain) can be partially enlarged, reduced or distinguished at the time of reproduction, so that signals with a pitch or interval which is greater than a diffraction limit of light can be reproduced. Since the spontaneous magnetizations of the first magnetic layer used as a reproduction layer and the third magnetic film serving as a record holding layer are defined within certain ranges, respectively, reproduction with a high C/N value is possible.

Claims: A magneto-optical recording medium which has, on a transparent substrate (1), a magneto-optical recording layer (10) consisting of a first magnetic film (11), a second magnetic film (12) and a third magnetic film (13) which are magnetically coupled to one another at room temperature, T RT , wherein the Curie points, T c1 , T c2 , and T c3 of the first, second and third magnetic films, respectively, have the relationships of T c2 > T RT , T c2 < T c1 and T c2 < T c3 , and the coercive force, H c1 , of the first magnetic film is so selected as to be small in the vicinity of the Curie point, T c2 , of the second magnetic film and the coercive force, H c3 , of the third magnetic film is so selected as to be larger than a required minimum magnetic field intensity within a temperature range between said room temperature, T RT , and a predetermined temperature, T PB , which is higher than the Curie point, T c2 , of the second magnetic film, wherein the first magnetic film is mainly composed of GdFeCo, and the third magnetic film is mainly composed of TbFeCo, characterized in that the Curie point, T c2 , of the second magnetic film has the relationship of 60°C ≤ T c2 ≤ 200°C, and the saturation magnetization, M s1 , of the first magnetic film is not greater than 0,25T (200 emu/cc), and the saturation magnetization, M s3 , of the third magnetic film is not greater than 0.38T (300 emu/cc) when the sublattice of the transition metals is more predominant than the sublattice of the rare earth element or not greater than 0.2 T (160 emu/cc) when the sublattice of the rare earth element is more predominant than the sublattice of the transition metals. A magneto-optical recording medium according to Claim 1, further comprising a transparent dielectric film provided between the substrate and the magneto-optical recording layer, and a protective layer formed on the magneto-optical layer. A method for reproducing a signal of a recorded magnetic domain from a magneto-optical recording medium as defined in claim 1, which method comprising, for the reproduction of the signal from the magneto-optical recording medium, heating the medium to the predetermined temperature, T PB , to interrupt the magnetic coupling between the first and third magnetic films with application of a magnetic field for demagnetizing magnetic fields or stray magnetic fields from the recording medium and an external magnetic field sufficient to provide the required magnetic intensity to change a domain size in the first magnetic film. The method of claim 3, wherein an external magnetic field is applied sufficient to provide the required magnetic intensity to the first magnetic film to cause shrinking of the recorded magnetic domain. The method of claim 3, wherein an external magnetic field is applied sufficient to provide the required magnetic intensity to the first magnetic film to expand the recorded magnetic domain. A method according to any of Claims 3, 4 and 5, wherein the signal is obtained by detecting signal change upon change of the domain size. A method for reproducing information signals from a magneto-optical recording medium having a first magnetic film (11), a second magnetic film (12) and a third magnetic film (13) magnetically coupled to each other at room temperature, T RT , wherein the Curie points, T c1 , T c2 , and T c3 of the first, second and third magnetic films, respectively, have the relationships of T c2 > T RT , T c2 < T c1 and T c2 < T c3 , wherein the first magnetic film is mainly composed of GdFeCo, and the third magnetic film is mainly composed of TbFeCo, said magneto-optical recording medium being characterized in that the Curie point, T c2 , of the second magnetic film has the relationship of 60°C ≤ T c2 ≤ 200°C, and the saturation magnetization, M s1 , of the first magnetic film is not greater than 0.25T (200 emu/cc), and the saturation magnetization, M s3 , of the third magnetic film is not greater than 0.38T (300 emu/cc) when the sublattice of the transition metals is more predominant than the sublattice of the rare earth element or not greater than 0.2 T (160 emu/cc) when the sublattice of the rare earth element is more predominant than the sublattice of the transition metals, which method comprising irradiating the medium with a laser beam to cause a first portion where magnetic coupling of the first, second and third magnetic films are extinguished and a second portion where magnetic coupling of the first, second and third magnetic films is maintained as it is to be formed within a laser beam spot, and reading out the signals from the second portion of the first magnetic film.

Inventors: ARATANI KATSUHISA, OHTA MASUMI, ARATANI, KATSUHISA, OHTA, MASUMI
Applicants: SONY CORP, SONY CORPORATION
Application year: 1991
Main classification: G11B11
IPC codes: G11B11