Magneto-optical recording medium and method of producing the same

Abstract: An amorphous magnetic thin film (3) which is formed on a substrate (1) and has an axis of easy magnetization perpendicular to a film surface, wherein, when the composition of the magnetic thin film is represented by Tb x Dy y (Fe 1-z Co z ) 1-(x+y) and when x, y and z are within ranges of 0 < x ≦ 0.12, 0 < y ≦ 0.12, and 0 < z ≦ 0.30, a condition to be satisfied by x and y is 0.25 ≦ x/(x+y). Consequently, an inexpensive magneto-optical recording medium with an improved C/N ratio is manufactured.

Description: FIELD OF THE INVENTION The present invention relates to a magneto-optical recording medium on/from which information is recorded, reproduced and erased by laser light, and to a method of producing the magneto-optical recording medium. BACKGROUND OF THE INVENTION In recent years, research and development of optical memory devices have been actively carried out to meet various demands, for example, high-density recording, large memory capacity, and high-speed access. In particular, a magneto-optical magnetic thin film using a magnetic film with an axis of easy magnetization perpendicular to a film surface is utilized for a magneto-optical recording medium for use in the optical memory devices. The following are conventionally well known magneto-optical magnetic thin films. Polycrystal thin films such as MnBi and MnCuBi. Amorphous thin films such as GdTbFeCo, TbFeCo, TbDyFeCo, DyFeCo, GdTbFe, GdDyFe, GdFeCo, TbFe, DyFe, GdFe and GdCo. Compound monocrystal thin films, for example, GIG. Among these magneto-optical magnetic thin films, the amorphous thin films are suitable for magneto-optical recording media. The reason for this is that the amorphous thin films are practical because the thin films with a large area are manufactured at temperatures in the vicinity of room temperature and that the amorphous thin films are writable and readable, for example, by a practical semiconductor (GaAlAs) laser. Table 1, below, shows the relation between the amorphous thin film and the Kerr rotation angle (degree) as an index of performance. Material Kerr Rotation Angle (degree) TbFeCo 0.38 TbDyFeCo 0.37 DyFeCo 0.35 GdTbFeCo 0.40 GdFeCo 0.43 As shown in Table 1, TbFeCo and GdFeCo exhibited large Kerr rotation angles. Namely, it is found that these amorphous thin films are suitable for use in magneto-optical recording media. In order to achieve high-density recording, various amorphous thin films with compositions which decrease the writing magnetic field, i.e., increase the perpendicular magnetic anisotropic energy have been proposed. There is a thin film of amorphous TbFeCo with a composition which gives the highest C/N (carrier-to-noise) ratio even when the writing bit length (recording bit length) is around 0.76 µm. This is achieved because the perpendicular magnetic anisotropic energy of TbFeCo is large. Table 2, below, shows the relation between the perpendicular magnetic anisotropic energy (Ku) and the C/N ratio of magneto-optical magnetic thin film. Material Ku (erg/cm 3 ) C/N Ratio (dB) TbFeCo 6.5 × 10 5 49.2 DyFeCo 4.5 × 10 5 46.5 GdTbFeCo 3.2 × 10 5 46.0 GdFeCo 1.9 × 10 5 45.0 As shown in Table 2, TbFeCo which exhibits a Kerr rotation angle smaller than that of GdFeCo gives a greater perpendicular magnetic anisotropic energy and an improved C/N ratio. Thus, in order to improve the C/N ratio, it is necessary to increase the perpendicular magnetic anisotropic energy rather than the Kerr rotation angle shown in Fig. 1. Namely, the value of the perpendicular magnetic anisotropic energy is predominant over the degree of the Kerr rotation angle for improving the C/N ratio. However, if TbFeCo is used as a magneto-optical magnetic thin film, the price of the magneto-optical recording medium is increased because Tb (terbium) is most expensive among rare-earth elements. Furthermore, if DyFeCo which is inexpensive and has relatively large perpendicular magnetic anisotropic energy is used as the magneto-optical magnetic thin film, the magneto-optical recording medium suffers from another problem that the C/N ratio is impaired. Then, in order to maintain the C/N ratio at a practical level and to prevent an increase in the price, for example, Japanese Publication for Unexamined Patent Application No. 3-209647 discloses a magneto-optical recording medium using a magneto-optical magnetic thin film of a quaternary alloy including DyFeCo plus Tb. However, in this magneto-optical recording medium, the composition of TbDyFeCo was arranged so that the coercive force becomes not larger than 5x 10 6 / 4π A/m (5 kOe) so as to increase the sensitivity of the recording magnetic field. Therefore, a multiplicity of error occurred after a durability test in which around 10 6 cycles of recording, reproduction and erasing operations were performed on the magneto-optical recording medium. Patent Abstracts of Japan vol. 015 no. 435 (P-1272), 6 November 1991 & JP-A-03 181 041 (Ricoh Co Ltd) 7 August 1991, discloses a magneto-optical recording medium having a magnetic thin film layer made of a four element alloy such as (Tb 0.5 Dy 0.5 ) 0.22 (Fe 0.8 Co 0.2 ) 0.78 . SUMMARY OF THE INVENTION It is desirable to provide an inexpensive magneto-optical recording medium using a magneto-optical magnetic thin film of a quaternary alloy including TbDyFeCo, which is capable of maintaining the C/N ratio at a practical level and reducing the occurrence of error after a durability test, and a method of manufacturing the magneto-optical recording medium. The present invention provides a magneto-optical recording medium as set out in claim 1. With this structure, when the composition of the quaternary alloy thin film is represented by Tb x Dy y (Fe 1-z Co z ) 1-(x+y) and when x, y and z are within ranges of 0 < x ≤ 0.12, 0 < y ≤ 0.12, and 0 < z ≤ 0.30, if a condition to be satisfied by x and y is 0.25 ≤ x/(x+y), the magnetization is stably oriented in a direction perpendicular to the film surface and the C/N ratio is held at a practical level. As a result, the minimum Tb content of the TbDyFeCo alloy thin film for holding the C/N ratio at a practical level is found. It is therefore possible to manufacture an inexpensive magneto-optical recording medium with an improved C/N ratio. Moreover, with the magneto-optical recording medium having a TbDyFeCo alloy thin film of the above-mentioned composition, since the coercive force is not lower than 6 kOe at room temperature, it is possible to prevent a multiplicity of error from occurring after a durability test in which 10 6 cycles of recording, reproduction and erasing operations are performed. The invention also provides a method of manufacturing a magneto-optical recording medium as set out in claim 7. For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic drawing showing the structure of a magneto-optical recording medium according to one embodiment of the present invention. Fig. 2 is a schematic drawing showing the structure of an amorphous magnetic thin film in the magneto-optical recording medium shown in Fig. 1. Fig. 3 is a schematic drawing showing the structure of a revolving sputtering device for forming the amorphous magnetic thin film shown in Fig. 2. Fig. 4 is a graph showing the relation between the Tb content of the amorphous magnetic thin film and the C/N ratio. Fig. 5(A) shows the correlation between the perpendicular magnetic anisotropic energy (Ku) and the shape of recording bit, and explains the shape of recording bit when Ku is not larger than 4.5 x 10 4 J/m 3 (4.5 x 10 5 erg/cm 3 ). Fig. 5(B) explains the shape of recording bit when Ku is not smaller than 6.3 x 10 4 J/m 3 (6.3 x 10 5 erg/cm 3 ). DESCRIPTION OF THE PREFERRED EMBODIMENTS The following description discusses one embodiment of the present invention with reference to Figs. 1 to 5. As illustrated in Fig. 1, a magneto-optical disk (magneto-optical recording medium) of this embodiment has a magneto-optical recording film 6 and a protective coating film 5 on one side of a substrate 1. The substrate 1 is made of an optically transparent material such as glass and polycarbonates. The magneto-optical recording film 6 is a recording layer for recording information. The magneto-optical recording film 6 is formed by laminating four layers, i.e., a dielectric film 2, an amorphous magnetic film 3, a dielectric film 2 and a reflective film 4 in this order on the substrate 1. More specifically, the dielectric film 2 is 100 nm in film thickness and made of AlN. The amorphous magnetic thin film 3 is 30 nm in film thickness and made of TbDyFeCo. The dielectric film 2 is 30 nm in film thickness and made of AlN. The reflective film 4 is 50 nm in film thickness. The amorphous magnetic thin film 3 is formed by a thin film of an amorphous quaternary alloy including amorphous terbium, dysprosium, iron, and cobalt (hereinafter referred to as the TbDyFeCo alloy thin film) having an axis of easy magnetization perpendicular to a film surface. As illustrated in Fig. 2, the amorphous magnetic thin film 3 is a multi-layer film formed by repeatedly laminating a TbFeCo layer (2.5 x 10 -10 m (2.5Å) in thickness) and a DyFeCo layer 8 (2.5 x 10 -10 m (2.5Å) in thickness) by turns. Therefore, in the amorphous magnetic thin film 3, by repeatedly laminating the TbFeCo layer 7 and the DyFeCo layer 8 by turns as mentioned above, a Tb-rich layer and a Dy-rich layer of the TbDyFeCo alloy thin film alternately appear in the direction of film thickness. When the composition of the TbDyFeCo alloy thin film is represented by Tb x Dy y (Fe 1-z Co z ) 1-(x+y) , if x, y and z are within ranges 0 < x ≤ 0.12, 0 < y ≤ 0.12, and 0 < z ≤ 0.30, the magnetization is stably oriented to a direction perpendicular to the film surface. Additionally, if z is 0.3 < z, since the dependance of the Curie temperature or compensation temperature on composition becomes larger, the film becomes impracticable. On the other hand, if x and y are not in the above-mentioned ranges, it becomes difficult to stably orient the magnetization to a direction perpendicular to the film surface. Consequently, practical recording and reproduction characteristics can not be obtained. Table 3 and Fig. 4 show the C/N ratio when x, y and z of Tb x Dy y (Fe 1-z Co z ) 1-(x+y) were within the above-mentioned ranges and the composition of the TbDyFeCo alloy thin film was (Tb x Dy y ) 0.215 (Fe 1-z Co z ) 0.785 . The C/N ratio was measured by a dc sputtering device by performing sputtering using a TbDyFeCo alloy target while varying x, y, and z, where x represents the Tb amount, y is the Dy amount, and z is the a Co amount. x/(x+y) indicates a ratio of Tb amount in the TbDy alloy. z was adjusted so that the Curie temperature became 200 °C in the respective compositions of the TbDyFeCo alloy thin films. FILM COMPOSITION C/N Ratio (dB) x/(x+y) z 0 0.20 46.5 0.10 0.19 47.5 0.25 0.17 48.1 0.47 0.14 48.7 0.50 0.135 48.7 0.75 0.10 49.0 1.00 0.07 49.2 As shown in Table 3 and Fig. 4, the relation between the Tb amount and the C/N ratio is not necessarily linear, and can be non-linear so that the C/N ratio becomes nearly saturated when x/(x+y) is not smaller than about 0.25. Accordingly, if x/(x+y) is at least 0.25, the C/N ratio is maintained at a practical level. Namely, if TbDy contains Tb by at least 25 %, the C/N ratio becomes a practical level. Therefore, the minimum Tb content of the TbDyFeCo alloy thin film for holding the C/N ratio at a practical level is found. It is thus possible to manufacture an inexpensive magneto-optical recording medium with a large C/N ratio. Moreover, in a magneto-optical recording medium using a thin film of TbDyFeCo alloy with any of the above-mentioned compositions, the coercive force does not become lower than 6 x 10 -6 / 4π A/m (6 kOe) at room temperature. It is therefore possible to prevent a multiplicity of error from occurring after the durability test in which 10 6 cycles of recording, reproduction and erasing operations are performed. The dielectric film 2 and the reflective film 4 are formed by a sputtering method using a vacuum device (not shown) or a deposition method, while the amorphous magnetic thin film 3 is formed, for example, by a revolving sputtering device 9 shown in Fig. 3. As illustrated in Fig. 3, the revolving sputtering device 9 includes a cage-like sputtering room 10 having therein a palette 12, a TbFeCo target 13 and a DyFeCo target 14. A plurality of bases 11 are set on the palette 12. Each of the bases 11 includes the substrate 1 and the dielectric layer 2 formed thereon. The palette 12 is rotated on a shaft 12a in the direction of arrow X by driving means, not shown. Meanwhile, the bases 11 set on the palette 12 are rotated in the direction of arrow Y. Namely, the palette 12 revolves on the shaft 12a, while the bases 11 revolve around the shaft 12a. The bases 11 are disposed on the palette 12 so that they are positioned on the apexes of an equilateral polygon whose center is located on the shaft 12a, respectively. The TbFeCo target 13 and the DyFeCo target 14 are disposed on any two opposite apexes of the equilateral polygon. Namely, when the TbFeCo target 13 is positioned to face one of the apexes of the equilateral polygon, the DyFeCo target 14 faces other apex thereof. With this arrangement, when the base 11 is moved to the position facing the TbFeCo target 13 by the revolution of the palette 12, the TbFeCo layer is sputtered. Then, when the base 11 is moved to the position facing the DyFeCo target 14, the DyFeCo layer is sputtered. More specifically, the amorphous magnetic thin film 3 is formed by repeatedly laminating the TbFeCo layer 7 and the DyFeCo layer 8 by turns in the direction of film thickness. The TbFeCo layer 7 and the DyFeCo layer 8 are formed by simultaneously sputtering two elements for about 120 seconds by discharging the TbFeCo target 13 and the DyFeCo target 14 with respect the bases 11 including the substrate 1 and the dielectric film 2 formed thereon while revolving the bases 11 in the direction of arrow X around the shaft 12a at a revolving speed of about 30 rpm and simultaneously revolving each of the bases 11 on its own axis. Consequently, thirty TbFeCo layers 7 and thirty DyFeCo layers 8 are totally formed. As a result, the amorphous magnetic thin film 3 becomes a multi-layer film with 60 layers. In this case, Tb 21 (Fe 90 Co 10 ) 79 is used as the TbFeCo target 13, and Dy 22 (Fe 82 Co 18 ) 78 is used as the DyFeCo target 14. The perpendicular magnetic anisotropic energy (Ku) and the C/N ratio of a magneto-optical recording medium using the above multi-layer TbDyFeCo alloy thin film and of a magneto-optical recording medium using a TbDyFeCo alloy thin film having evenly mixed Tb, Dy, Fe, and Co were measured. The results are shown in Table 4. In this case, the thin films of magnetic alloys were arranged to have the same composition, (Tb 0.46 Dy 0.54 ) 0.215 (Fe 0.86 Co 0.14 ) 0.785 . MAGNETIC FILM STRUCTURE Ku(erg/cm 3 ) C/N Ratio(dB) TbDyFeCo Alloy Thin Film 6.0 × 10 5 48.7 TbFeCo/DyFeCo Multi-layer Film 6.3 × 10 5 49.0 As shown in Table 4, the multi-layer TbDyFeCo alloy thin film (TbFeCo/DyFeCo multi-layer film) had an increased Ku and an improved C/N ratio compared with the evenly mixed TbDyFeCo alloy thin film. Namely, when the amorphous magnetic thin film 3 made of the TbDyFeCo alloy thin film is arranged to be a multi-layer film having the Tb-rich layer and the Dy-rich layer alternately in the direction of film thickness, Ku is increased. Therefore, when the TbDyFeCo alloy thin film has a composition represented by Tb x Dy y (Fe 1-2 Co z ) 1-(x+y) where x, y and z are within ranges 0 < x ≤ 0.12, 0 < y ≤ 0.12, and 0 < z ≤ 0.30, if a condition to be satisfied by x and y is 0.25 ≤ x/(x+y) and if the TbDyFeCo alloy thin film is a multi-layer film formed by the TbFeCo layers and the DyFeCo layers, the C/N ratio is held at a practical level with the minimum Tb amount. It is thus possible to reduce the manufacturing cost and provide a magneto-optical recording medium showing satisfactory results in the durability test. Moreover, it is known that, when number of laminated layers forming the TbDyFeCo alloy thin film is about twenty, i.e, when the number of the TbFeCo layers 7 and of the DyFeCo layers 8 are respectively ten, the perpendicular magnetic anisotropic energy is increased compared with the evenly mixed TbDyFeCo alloy thin film. If the number of laminated layers is less than twenty, there is substantially no difference in the perpendicular magnetic anisotropic energy between the multi-layer TbDyFeCo alloy thin film and the evenly mixed TbDyFeCo alloy thin film. In this embodiment, therefore, the TbDyFeCo alloy thin film is formed by 60 layers of alternately laminated TbFeCo layers 7 and the DyFeCo layers 8. However, it is not necessary to limit the number of laminated layers to sixty. Considering the above-mentioned results, the number of laminated layers is desired to be set not smaller than twenty. Table 5 and Fig. 5 show the relation between the perpendicular magnetic anisotropic energy (Ku) and the distribution of recording bit length. In this case, the average recording bit length was 650 nm. Ku of 4.5 x 10 4 J/m 3 (4.5 x 10 5 (erg/cm 3 )) was obtained by the DyFeCo alloy thin film. Ku of 6.0 x 10 4 J/m 3 (6.0 x 10 5 (erg/cm 3 )) was obtained by the (evenly mixed) TbDyFeCo alloy thin film. Ku of 6.3 x 10 5 J/m 3 (6.3 x 10 5 (erg/cm 3 )) was obtained by the (multi-layer) TbDyFeCo alloy thin film. Ku (erg/cm 3 ) STANDARD DEVIATION OF RECORDING BIT LENGTH (nm) 4.5 x 10 5 58.5 6.0 x 10 5 26.0 6.3 x 10 5 22.5 Average bit length: 650 nm As shown in Table 5, when Ku becomes larger, the standard deviation of the recording bit length becomes smaller, i.e., when Ku becomes smaller, the standard deviation of the recording bit length becomes larger. Namely, as illustrated in Fig. 5(a), when Ku is not larger than 4.5 x 10 5 J/m 3 (4.5 x 10 5 (erg/cm 3 )), the distribution of the recording bit length spreads over the area shown by the broken line. Whereas, as illustrated in Fig. 5(B), when Ku is not smaller than 6.3 x 10 4 J/m 3 (6.3 x 10 5 (erg/cm 3 )), the distribution of the recording bit length becomes smaller. It is found that the distribution of the recording bit length becomes even with an increase of Ku. Moreover, as shown in Tables 2 and 4, since it is generally considered that the C/N ratio is improved with an increase of Ku, if the distribution of the recording bit length is made even, the C/N ratio is also improved. In addition, the recording density on the magneto-optical recording medium is improved by the even distribution of the recording bit length. Thus, by using the multi-layer TbDyFeCo alloy thin film with large Ku as the amorphous magnetic thin film 3, the recording density of the magneto-optical recording is improved and the C/N ratio is increased.

Claims: A magneto-optical recording medium comprising: a substrate (1); and a quaternary alloy thin film structure (3) including amorphous terbium, dysprosium, iron, and cobalt formed on said substrate (1), said quaternary alloy thin film structure (3) having an axis of easy magnetization perpendicular to the film surface, wherein, when the composition of said quaternary alloy thin film structure (3) is represented by Tb x Dy y (Fe 1-z Co z ) 1-(x+y) and when x, y and z are within ranges of 0 < x ≤ 0.12, 0 < y ≤ 0.12, and 0 < z ≤ 0.30, a condition to be satisfied by x and y is 0.25 ≤ x/(x+y), characterized in that said quaternary alloy thin film structure (3) is made of a multi-layer film formed of alternate layers of TbFeCo and DyFeCo. The magneto-optical recording medium according to claim 1, wherein said composition is (Tb x Dy y )0.215 (Fe 1-z Co z ) 0.785 . The magneto-optical recording medium according to claim 1, wherein said quaternary alloy thin film structure (3) is such that the Curie temperature is about 200 °C. The magneto-optical recording medium according to claim 1, wherein said quaternary alloy thin film structure (3) is such that the coercive force is not lower than 6 x 10 -6 / 4π A/m (6 kOe). The magneto-optical recording medium according to claim 1, wherein said multi-layer film (3) includes at least 20 layers. The magneto-optical recording medium according to claim 1 wherein said quaternary multi-layer film structure (3) has a perpendicular magnetic anisotropic energy of not less than 6.3 x 10 4 J/m 3 (6.3 x 10 5 erg/cm 3 ). A method of manufacturing a magneto-optical recording medium comprising the step of forming a quaternary alloy thin film structure (3) including amorphous terbium, dysprosium, iron, and cobalt on a substrate (1) by repeatedly performing deposition using a TbFeCo target and a DyFeCo target alternately to laminate a TbFeCo layer and a DyFeCo layer, said quaternary alloy thin film structure (3) having an axis of easy magnetization perpendicular to a film surface. A method according to claim 7 wherein the step of forming a quaternary alloy thin film structure (3) by deposition comprises repeatedly performing sputtering using a TbFeCo target and a DyFeCo target alternately. The method according to claim 8, further comprising the step of setting said substrate (1) on at least one of apexes of an equilateral polygon whose center is positioned on a rotation axis of a rotating palette, wherein said TbFeCo target and said DyFeCo target are disposed so that, when said TbFeCo target faces one of the apexes of said equilateral polygon, said DyFeCo target faces another apex thereof. The method according to claim 7, wherein said quaternary alloy thin film structure (3) has a perpendicular magnetic anisotropic energy of not less than 6.3 x 10 4 J/m 3 (6.3 x 10 5 erg/cm 3 ). The method according to claim 7, wherein the number of laminated layers formed is not less than 20.

Inventors: TAKAMORI NOBUYUKI, TAKAMORI, NOBUYUKI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1995
Main classification: G11B11
IPC codes: G11B11