Opto-magnetic recording medium

Abstract: An opto-magnetic recording medium having a perpen­ dicularly magnetized film of a dual layer structure com­ prising a first layer composed of a rare earth-transition metal amorphous thin alloy film and a second layer composed of a rare earth-transition metal amorphous thin alloy film of a composition different from that of the first layer and having a lower curie point and a larger coercive force as compared with that of said first layer, and being capable of transferring bits recorded at a curie point in said second layer to the first layer by exchange-coupling force, said first layer being composed of a GdDyFeCo film in which the sublattice magnetization for transition metal is predominant at a room temperature, the second layer being composed of TbFeCo series film in which sublattice magne­ tization for rare earth is predominant at a room tempera­ ture, and the directions of predominant magnetization between the first layer and the second layer being opposed to each other.

Description: The present invention concerns an opto-magnetic recording medium such as an opto-magnetic disk and, more particularly, it relates to an opto-magnetic recording medium having a perpendicularly magnetized film of a dual layer structure comprising a first layer (reading layer) and a second layer (writing layer) each composed of rare earth-transition metal alloy film. An opto-magnetic recording medium using a thin amorphous alloy film comprising a rare earth metal (RE) and a transition metal (TM) has insufficient reading characteristics and various improvements have been studied. As an example, there has been proposed a curie point writing type opto-magnetic recording medium mainly constituted with an exchange-coupled perpendicularly magnetized film of a dual layer structure comprising a writing layer having a low curie point and a large coersive force and a reading layer having a higher curie point and a smaller coersive force as compared with those of the writing layer (Japanese Patent Laid Open Sho 57-78652, "III amorphous multi-layered film", p 294 - 305 in, Collected Technology for Optical Memory-Opto-Magnetic memory, published by Science Forum Co. Ltd, October 31, 1983). In the conventional exchange-coupled dual layer film as described above, TbFe, DyFe, TbFeCo and DyFeCo are used for the writing layer and GdFe, GdFeCo and TbFeCo are used for the reading layer. For improving the C/N ratio which is a ratio between a carrier level and a noise level in an opto-magnetic recording medium thereby improving the reading characteristics, it is necessary to increase the carrier level and reduce the noise level. The carrier level may be increased by making the Kerr rotational angle greater in the reading layer, while it is important to increase the exchange-coupling force between the two layers for reducing the noise level. For improving the carrier level, it is necessary to use a medium having a high Kerr rotation angle for the reading layer, but a medium having a greater Kerr rotation angle generally has increased saturation magnetization. However, as the saturation magnetization is increased, the recording characteristics of the reading layer become predominant during recording, tending to cause writing noise. In this case, although the carrier level is increased, the noise level is also increased by as much, thereby giving no contribution to the improvement of the C/N ratio. The present invention seeks to improve the C/N ratio of an opto-magnetic recording medium by developing a medium for the reading layer having a high Kerr rotation angle while suppressing the increase of the saturation magnetization as much as possible and controlling the sublattice magnetization for each of the rare earth and the transition metal in each of the writing layer and the reading layer, thereby suppressing the occurrence of writing noise. The dual layer medium includes, as shown in Figs. 2(a) and (b), a type in which the magnetization Ms1 and Ms2 in the reading layer RL and the writing layer WL, respectively, are in parallel with each other (hereinafter referred to as P type) and, as shown in Figs. 3(a) and (b), a type in which the magnetization Msl and Ms2 in the reading layer RL and the writing layer WL, respectively, are opposite to each other (hereinafter referred to as A type). For the dual layer film medium of the P type, as shown in Figs 2(a),(b), sublattice magnetization Ms RE for the rare earth RE is predominant in both of the reading layer RL and the writing layer WL (hereinafter the state in which the auxiliary magnetization Ms RE for the rare earth RE is predominant is referred to as RE rich), or sublattice magnetization Ms TM for the transition metal TM is predominant in both of the layers (hereinafter the state in which the sublattice magnetization Ms TM for the transition metal TM is predominant is referred to as TM rich). On the other hand, for the dual layer film medium of the A type, as shown in Figs. 3(a) and (b), one of the layers is RE rich and the other of the layers is TM rich in the two layers of the reading layer RL and the writing layer WL. For the sake of the simplicity, explanation will be made hereinafter assuming that both of the reading layer RL and the writing layer WL are TM rich for the P type and that the reading layer RL is TM rich, while the writing layer WL is RE rich for the A type. In the case of conducting recording on a dual layer film medium of the P type, the directions for the respective magnetization Ms₁ and Ms₂ in the reading layer RL and the writing layer WL of the medium are identical as shown in Fig. 4(a) and then an external auxiliary magnetic field Hex is applied downwardly while heating a recording area shown by hatched lines by means of a laser beam, by which the magnetization Ms₂ and Ms₁ in the writing layer WL and the reading layer RL are reversed. Then, following temperature reduction, downward magnetization Ms₂ and Ms₁ appears the writing layer W1 and the reading layer RL. On the other hand, in the case of conducting recording on a dual film medium of the A type, magnetization Ms₁ in the reading layer RL and the magnetization Ms₂ in the writing layer WL of the medium are in opposite directions. Then, when an external auxiliary magnetic field Hex is applied while heating the hatched area by means of the laser beam as shown in Fig. 5(b), the magnetization Ms₂ in the writing layer WL is reversed downwardly, whereas the magnetization Ms₁ in the reading layer RL remains in the downward direction. Then, following temperature reduction after completion of the laser beam irradiation, magnetization Ms₁ in the reading layer RL is turned upwardly due to the exchange coupling between the two layers and information is recorded on the hatched portion in the reading layer RL. In order to obtain excellent recording characteristics in the opto-magnetic recording medium, it is necessary that the bits written into the reading layer RL have a regular and well-arranged shape. For this purpose, it is necessary in the case of the dual layer film medium of the P type that bits, when recorded in the reading layer RL, do not diffuse into other regions than the area of the writing layer WL that reaches the temperature near the curie point (laser beam irradiated area) by the exchanging force between the two layers. In the case of the dual layer film medium of the A type, it is necessary to form regular and well-arranged bits in the writing layer WL and transfer them to the reading layer RL. Then, necessary conditions required for each of the P and A types in each of the recording and temperature lowering processes are as shown in Table 1 according to theoretical calculations. From Table 1, it is necessary as a measure for the development of the dual layer film medium, to make the magnetization Ms₁ in the reading layer RL smaller, the magnetization Ms₂ in the writing layer WL larger and, further, to establish a relationship between Ms₁ and Ms₂ including H₁, H₂, h₁, h₂ as: Ms₂ x H₂ x h₂ > Ms₁ x H₁ x h₁ H₁ ... coersive force of the reading layer RL at that temperature. H₂ ... coersive force of the writing layer WL at that temperature. h₁ ... film thickness of the reading layer RL h₂ ... film thickness of the writing layer WL δw ... boundary magnetic wall energy at a temperature in this case H ex ... external auxiliary magnetic field (applied in the direction opposite to that for the magnetization in the writing layer WL) The film thickness h₂ of the writing layer WL has to be within such a range as capable of undergoing the exchanging force from the boundary between both of the layers and, if the magnetization Ms₂ in the writing layer WL is excessively large, a stray magnetic field or demagnetizing field is increased to cause distortion in the shape of the recorded bits even if in the curie point recording. On the other hand, the reading layer RL is required to have a high Kerr rotation angle ϑk, and small saturation magnetization Ms and coersive force Hc. GdFeCo among the reading layer media used at present has a rather high Kerr rotational angle ϑk of about 0.46° (at a measuring wavelength of 780 nm), in which the saturation magnetization Ms and the coersive force Hc are about 50 emu/cm³ and 0.5 KOe, respectively. Accordingly, for improving the C/N ratio further, it is necessary to develop a medium having a higher Kerr rotational angle ϑk than that of GdFeCo while suppressing Ms and Hc. The present inventor has sought for media adaptable to the conditions described above among quaternary rare earth-transition metal opto-magnetic recording media and has found that GdDyFeCo is particularly excellent as the reading layer medium. More specifically, in the following formula: (Gd 100-X Dy X ) z (Fe 100-Y CO Y ) 100-Z , the Kerr rotation angle ϑk is increased to greater than 0.48° within a range: X = 5 - 25, Y = 18 - 30 and Z = 16 - 24. In particular, the Kerr rotational angle ϑk can be increased to 0.54° while suppressing the saturation magnetization Ms to less than 250 emu/cm³ in a TM rich composition ratio in which X is about 20 and the coersive force Hc is less than 2 KOe. The Kerr rotational angle ϑk is increased by 17.4% as compared with that of conventional CdFeCo. Although the value for the saturation magnetization Ms can be suppressed to as relatively low as 250 emu/cm³, it is increased as compared with that of the conventional reading layer medium. Therefore, in order to form regular and well-arranged bits while satisfying the conditions in Table 1, it is necessary that the saturation magnetization Ms in the writing layer WL is also made greater, relatively. Further, a large coersive force Hc is required for the writing layer medium and, at the same time, a lower curie temperature Tc is required for the improvement of the recording sensitivity. Accordingly, a medium having a curie temperature Tc between 100 to 150°C has predominantly been used. However, according to the study of the present inventor, it has been found that there is no practical problems in view of the recording sensitivity so long as the curie temperature Tc is up to about 190°C. Accordingly, the range for selecting the writing layer can be widened as compared with the usual case. Then, as a result of our search under the foregoing conditions, the inventors have found that TbFeCo is excellent as the writing layer. In particular, in the following formula representing the composition ratio: Tb X (Fe 100-Y Co Y ) 100-X both of the saturation magnetization Ms and the coersive force Hc could be increased while restricting the curie temperature Tc to 155 - 195°C by defining X as 15 to 30 and Y as 6 to 12. In view of the relationship with the recording process described below, if the saturation magnetization Ms for the reading layer medium is large, it is necessary that TbFeCo has RE rich composition ratio near the compensation composition with X being greater than 25, and such a composition ratio that predominant magnetization does not turn from RE into TM in the temperature elevation process due to the irradiation of laser beams upon recording. If a portion of Tb in TbFeCo is substituted with other rare earth element such as Ge, Dy, Ho or Nd, for controlling the curie temperature, TbFeCo can be used sufficiently as the writing layer medium, so long as the RE rich composition ratio is maintained. Then, for suppressing writing noise, the inventors have examined as to which of the P or A type film medium is suitable. In the dual layer film medium of the P type in which the directions of magnetization Ms₁, Ms₂ in the reading layer RL and the writing layer WL are in parallel, bits formed by the external auxiliary magnetic field and the stray magnetic field to the laser beam irradiated area of the reading layer RL upon recording undergo the effect of exchanging force between both of the layers and, accordingly, are not diffused to the outside of the area of the writing layer WL that reaches a temperature near the curie point (laser beam irradiation area), but can take any optional shape within a range near the curie temperature. Accordingly, if the saturation magnetization Ms of the reading layer RL is large, the bits formed in the reading layer RL are disturbed at the outer periphery or tend to form multiple polymagnetic domains so as to reduce the magnetostatic energy within the region near the curie temperature of the writing layer WL. The bits thus formed irregularly also form irregular shape of bits in the writing layer WL in the temperature lowering process. Accordingly, in the case of using a medium of large saturated magnetization Ms as in the present invention, the P type dual layer film medium is not suitable. Then, in the case of using the A type dual layer film medium in which the directions of the magnetization Ms₁, Ms₂ of the reading layer RL and the writing layer WL are opposite to each other, since the magnetization Ms₁ of the reading layer RL has the same direction with that of the external auxiliary magnetic field Hex upon forming bits in the writing layer WL during recording, as shown in Fig. 5(b), bits are not formed in the reading layer RL during irradiation of the laser beam. Then, the bits can be formed in the reading layer RL only by means of the transfer of regular and well-arranged bits having been formed in the writing layer WL by the curie point recording, in the course of the temperature lowering process to the reading layer RL by the exchanging force. Thus, the conditions shown in Table 1 should be satisfied so as to realize the transfer for the A type. As has been described above, improvement for the exchange-coupling force between the two layers is necessary for reducing writing noise in the dual layer film structure as has been pointed out, but it is still insufficient. The A type dual layer film structure is considered to be preferable for a medium having a large saturation magnetisation with an aim of improving the Kerr rotation angle Θk for thereading layer RL. The present invention now provides an optomagnetic recording medium having a perpendicularly magnetized film of a dual layer structure comprising a first layer composed of a rare earth-transition metal amorphous alloy thin film and a second layer composed of a rare earth-transition metal amorphous alloy thin film of a different composition and having a lower curie point and a larger coercive force as compared with those of the first layer and being capable of transferring bits recorded at a curie point in the second layer to the first layer by exchange-coupling, wherein the first layer is composed of a GdDyFeCo film of an alloy represented by the formula: (Gd 100-X Dy X ) Z (Fe 100-Y Co Y ) 100-Z in which X = 5 - 25, Y = 18 - 30 and Z = 16 - 24, in which the sub-lattice magnetisation for transition metal is predominant at room temperature, the second layer is composed of a TbFeCo series film in which sub-lattice magnetisation for rare earth is predominant at room temperature, and the directions of predominant magnetisation between the first layer and the second layer are opposed. The invention is described below in greater detail by way of example only with reference to the accompanying drawings, in which: Fig. 1 is a schematic perpendicularly cross sectional view for an opto-magnetic disk according to the present invention; Figs 2(a) and (b) are explanatory views illustrating the arrangement of sublattice magnetization in the P type dual layer film; Figs. 3(a) and (b) are explanatory views illustrating the arrangement of sublattice magnetization in the A type dual layer film; Figs. 4(a)-(c) are explanatory views illustrating the recording step of information to the P type dual layer film; Figs. 5(a) -(c) are explanatory views illustrating the recording step of information to the A type dual layer film; Fig. 6 is a graph showing a relationship between the temperature and the saturation magnetization in GdDyFeCo and GdTbFeCo alloys used in the present invention. Figs. 7 and 8 are, respectively, graphs showing a relationship, between the Tb content and the saturation magnetization, the Kerr rotational angle and the coersive force in TbFeCo used in the present invention. Fig. 9 is a graph showing a relationship between the rare earth content and the saturation magnetization, the Kerr rotational angle and the coercive force in GdTbFe used in the present invention; Fig. 10 is a graph showing X-dependency of Ms, Hc and ϑk in the alloy of the composition: (Gd₈₀Gy₂₀) X (Fe 81.5 Co 18.5 ) 100-X used in the present invention. Figs. 11 and 12 are, respectively, graphs illustrating the dependency of the recording/reproducing characteristics of the opto-magnetic disk according to the present invention on the auxiliary magnetic field. The GdDyFeCo film for the first layer in the opto-magnetic recording medium according to the present invention comprises a rare earth-transition metal alloy of the composition represented by the following formula: (Gd 100-X Dy X ) Z (Fe 100-Y Co Y ) 100-Z in which X, Y and Z are, respectively, within the range: X = 5 - 25 atm%, Y = 18 - 30 atm% and Z = 16 - 24 atm%. The second layer preferably comprises a rare earth-transition metal alloy of TbFeCo or TbFeCo in which Tb is partially (ca. 5 - 20 %) substituted with rare earth element other than Tb such as Gd, Dy, Nd and Ho, and in which the curie point is set to higher than 155°C and sublattice magnetization for rare earth remains predominant upon temperature elevation for recording. Each of the layers can be formed by means of a known method such as vapor deposition, sputtering or CVD. Upon recording, the recording medium comprising the second layer (writing layer) previously magnetized perpendicularly in one direction and the first layer (reading layer) magnetized in the direction opposite to that of the second layer (writing layer) by means of the exchange-coupling force is irradiated with a laser beam emitted from a semiconductor laser, etc. and, at the same time, applied with an external auxiliary magnetic field in the direction opposite to that of the magnetization in the second layer (writing layer). In this case, the area in the second layer (writing layer) irradiated with the laser beam is heated to a temperature near the curie point and the coercive force is reduced. As a result, in the laser beam irradiated area of the second layer (writing layer), bits magnetized in the direction opposite to the peripheral area are formed by the external auxiliary magnetic field and the stray magnetic field. Subsequently, in the temperature lowering process after the completion of the laser beam irradiation, the bits recorded in the second layer (writing layer) are transferred by the exchange-coupling force to the first layer (reading layer). Reproduction is conducted by reading the bits recorded in the first layer (reading layer) by the laser beam. In this instance, since a GdDyFeCo film of predominant sublattice magnetization for transition metal at a room temperature (10 - 50°C, typically 25°C) is used as the first layer (reading layer) in the present invention, the Kerr rotational angle in the first layer (reading layer) can be increased and, at the same time, increase of the saturation magnetization and the coercive force along with the increase of the Kerr rotational angle can be suppressed. In addition, since a TbFeCo film mainly comprising TbFeCo of predominant sublattice magnetization for rare earth at a room temperature is used as the second layer (writing layer), it is possible to obtain a large coercive force and a relatively low curie temperature necessary for the second layer. Further, the saturation magnetization for the second layer (writing layer) can also be increased corresponding to the increase of the saturation magnetization in the first layer (reading layer). Since the directions of predominant magnetization are set opposite to each other between the first layer and the second layer, regular and well-arranged bits after being recorded in the second layer (writing layer) can be transferred by means of the exchange force with a regular and well-arranged shape to the first layer (reading layer), thereby reducing writing noises. The following Examples illustrate the invention. EXAMPLES As shown in Fig. 1, an opto-magnetic disk as an opto-magnetic recording medium comprises a disk substrate 1 made of glass, and a first SiO layer 2 for enhancing the Kerr rotational angle ϑk, a reading layer 3 as the first layer, a writing layer 4 as the second layer 2 and a second SiO layer 5 as a protection layer laminated on the substrate 1. The film thickness for each of the layers is set, for example, to 82 nm for the first SiO layer 2, 40 to 50 nm for the reading layer 3, 100 nm for the writing layer 4 and 55 nm for the second SiO layer 5 respectively. The reading layer 3 and the writing layer 4 were formed by using two electron beam heating vacuum deposition devices and vapor depositing transition metal (TM) from one direction and rare earth (RE) from the other direction simultaneously. Each of the layers 2 - 5 was laminated on the disc substrate 1 within one identical vacuum vessel without changing the vacuum state. Further, for preparing the opto-magnetic disk described above, a glass disk substrate of 130 mm diameter manufactured by Corning Co. in USA was used as the disk substrate 1. As the disk substrate 1, a resin substrate made of polycarbonate, PMMA (acrylic resin) or epoxy resin may be used in place of the glass substrate. Instead of the SiO layer, dielectric material such as AlN, SiN or SiO₂ may also be used. Table 2 described below shows the results of measurement for coercive force Hc, Kerr rotational angle ϑK and the saturation magnetization Ms, while varying the composition ratio in the GdDyFeCo layer as the reading layer 3. For the measurement of the reading layer 3, a micro-sheet (0211) manufactured by the Corning Co. was used as the substrate and only the reading layer 3 and the protection layer 5 were formed at 100 nm and 55 nm thickness respectively on the substrate. The Kerr rotational angle was measured at a wavelength of 780 nm from the side of the substrate. In the table, specimens Nos. 1, 2, 4 through 8, 11 and 12 are included within the range of the composition of the present invention and remaining specimens Nos. 3, 9, 10 and 13 through 15 are out of the range of the composition of the present invention. As can be seen from the table, within the range of the composition of the present invention of the following formula: (Gd 100-X Dy X ) Z (Fe 100-Y Co Y ) 100-Z in which X = 5 - 25 atm%, Y = 18 - 30 atm% and Z = 16 - 24 atm% respectively, a Kerr rotational angle ϑk of greater than 0.48° was obtained as measured from the side of the substrate with a laser beam used for the measurement at a wavelength λ=780 nm. In particular, the specimen No. 8 is suitable as the reading layer 3 since a great Kerr rotational angle ϑk and a relatively small coercive force Hc can be obtained while suppressing the saturation magnetization Ms to less than 250 emu/cm³. In this experiment, it has been found that CdTbFeCo also has a Kerr rotational angle ϑk of greater than 0.50° with the same range composition as that of GdDyFeCo. However, since the saturation magnetization Ms of GdTbFeCo under the identical coercive force Hc with that of GdDyFeCo is much larger than that of GdDyFeCo, it is difficult to satisfy the conditions for the recording process shown in Table 1. Fig. 6 shows a relationship between the temperature and the saturation magnetization for (Gd₉₀Dy₁₀) 21.4 (Fe 81.5 Co 18.5 ) 78.6 by a curve I and relationship between the temperature and the magnetization for (Gd₉₀Tb₁₀) 20.3 (Fe 81.5 Co 18.5 ) 79.7 by a curve II. The coercive force Hc at a room temperature of the two kinds of media are equal to each other. Further, when taking notice only on the Kerr rotational angle ϑk, the Kerr rotational angle ϑk can be increased to greater than 0.55° in any of the medium of GdDyFeCo and GdTbFeCo in which the composition ratio Fe: Co between the transition metals is 60 - 80 : 40 - 20 and the composition ratio between the rare earth elements is about 50:50, the status is TM rich and the coercive force Hc is less than about 2 KOe. However, since the saturation magnetization Ms is extremely increased as larger than 300 emu/cm³ in such a composition region, the conditions for the recording process shown in Table 1 can not be satisfied. Then, relationship for composition ratio, and coercive force Hc, Kerr rotational angle ϑk and saturation magnetization Ms of TbFeCo as the writing layer 4 was measured in this experiment. Also in this case, the microsheet (0211) manufactured by Coning Co. was used as the substrate and only the writing layer 4 and the protection layer 5 were formed on the substrate for measurement. Fig. 7 shows the change of characteristics in Tb X (Fe 93.8 Co 6.2 ) 100-X when the Tb content X was varied and Fig. 8 shows the change of the characteristics of Tb X (Fe 89.5 Co 10.5 ) 100-X when the Tb content X was varied. Further, as other example of the composition, Fig. 9 shows the changes for coercive force Hc, Kerr rotational angle ϑk, and saturation magnetization Ms of: (Gd₅₀Tb₅₀) X Fe 100-X when X was varied. In Figs. 7 - 9, when the saturation magnetization Ms at an identical coercive force Hc is compared, the saturation magnetization Ms for TbFeCo is larger than that for GdTbFe. Further, in TbFeCo, saturation magnetization Ms is larger as the amount of Co is increased. Further, curie temperature To for each of the media, the characteristics of which are shown in Figs. 7 - 9, are 150 to 175°C for (Gd₅₀Tb₅₀) X Fe 100-X in Fig. 9, 155 - 160 °C for Tb X (Fe 93.8 Co 6.2 ) 100-X in Fig. 7 and 175 - 185°C for Tb X (Fe 89.5 Co 10.5 ) 100-X in Fig. 8. As has been described above, since it has been found that the curie temperature Tc may be about 190°C in view of the practical recording sensitivity, Tb X (Fe 89.5 Co 10.5 ) 100-X is excellent for the writing layer 5. If higher recording sensitivity is required, an element of Tb in TbFeCo may be partially substituted with Dy. In the opposite case, Tb may be partially replaced with Gd. Table 3 shows the result of measurement for saturation magnetization Ms, coercive force Hs and writing noises in each of the layers when the composition and the film thickness for the reading layer 3 and the writing layer 4 were varied in the dual layer film medium. Each of the dual layer film media has a structure as shown in Fig. 1. The reading layer (RL) 3 was formed to a film thickness of 40 to 50 nm on the first SiO layer 2. Since the thickness of the reading layer (RL) 3 was reduced in the disk formation as compared with the case of the measurement in Table 2 (formed to a thickness of 100 nm on the micro-sheet), the conditions as the reading layer shown in Table 1 can sufficiently be satisfied. In this case, it is desirable that the entire thickness of the dual layer film is from 15 to 300 nm and the film thickness of the writing layer 4 is made greater than that of the reading layer 3. Evaluation conditions for recording/reproduction were set as: number of rotation of the disk to 1800 rpm, recording diameter to 110 mm⊘, recording frequency to 1 MHz, recording duty to 50% and reproducing laser power to 1.5 mW, and the recording laser power Pw and the external auxiliary magnetic field Hex were set so as to minimize the writing noises in each of the specimens. In Table 3, specimens Nos. 1 - 4 are disks of P type in which both the reading layer (RL) 3 and the writing layer (WL) 4 are TM rich (refer to Figs. 2(a), (b) and Figs, 4(a) - (c)) and the writing noises are reduced so reduced as to satisfy the conditions for the recording process in the P type shown in Table 1. In particular, occurrence of the writing noises is suppressed substantially to 0 dB in the specimen No. 4. It can be understood from the comparison between the specimen No. 1 and the specimen No. 2 that TbFeCo is more excellent over GdTbFe as the writing layer (WL) 4. In the specimen No. 5 of Table 3, GdDyFeCo of the same composition as that for the specimen No. 4 was used for the reading layer (RL) 3, while TbFeCo for the writing layer (WL) 4 was changed from the TM rich composition ratio to the RE rich composition ratio. The specimen No. 5 can satisfy the conditions for the recording process of the A type shown in Table 1 (refer to Figs. 3(a), (b) and Figs 5(a) - (c)), in which the writing noises are substantially reduced to 0 dB like that in the specimen No. 4. The boundary magnetic wall energy σw as a measure for the exchange-coupling force between the reading layer (RL) 3 and the writing layer (WL) 4 in the specimens Nos. 1 - 5 is 2.0 erg/cm² for the specimen No. 1, 2.5 erg/cm² for the specimen No. 2, 1.8 erg/cm² for the specimen No. 3, 2.5 erg/cm² for the specimen No. 4, and 1.4 erg/cm² for the specimen No. 5. Accordingly, it can be understood that the magnitude of the writing noises can not be defined only based on that of the boundary magnetic wall energy σw. It can be seen from the foregoings that the writing noises can be suppressed sufficiently with any one of the A type or type P constitution if the conditions for the recording process in Table 1 are satisfied sufficiently in the reading layer (RL) 3 = (Gd₉₀Dy₁₀)(Fe 81.5 Co 19.5 )/writing layer (WL)4 = Tb(Fe 93.8 Co 6.2 ) system (specimens Nos. 2 - 5). However, in the reading layer (RL) 3 = (Gd₉₀Dy₁₀)(Fe 81.5 Co 18.5 )/writing layer (WL) 4 = Tb(Fe 93.8 Co 6.2 ) system, saturation magnetization Ms of the reading layer (RL) 3 is relatively decreased and, there is left a problem that the Kerr rotational angle ϑk of the reading layer (RL) 3 can not be increased sufficiently. Next, in the specimens Nos. 6 - 8, a composition of (Gd₈₀Dy₂₀)(Fe 81.5 Co 18.5 ) is used as the reading layer (RL) 3 for increasing the saturation magnetization Ms of the reading layer (RL) 3 to obtain a sufficiently great Kerr rotational angle ϑk. On the other hand, Tb(Fe 89.5 Co 10.5 ) is used for the writing layer (WL) 4. The specimen No. 6 uses a composition ratio which is always TM rich, the specimen No. 7 uses a composition ratio which is TM rich at a normal temperature and RE rich upon recording (at high temperature) and the specimen No. 8 uses a composition ratio which is always RE rich. Although any of the specimen Nos. 6 - 8 can sufficiently satisfy the conditions for the recording process in Table 1, relatively large writing noises are generated in the composition of the specimens Nos.6, 7. On the other hand, in the specimen No. 8, since the writing noises are substantially reduced to 0 dB and the saturation magnetization Ms for the reading layer (RL) 3 is relatively large, a great Kerr rotational angle can be obtained. Accordingly, among each of the specimens shown in Table 3, the specimen No. 8 gives most excellent composition. However, the composition for the reading layer (RL) 3 and the writing layer (WL) 4 is not necessarily restricted to that for the specimen No. 8, and any of the compositions may be used so long as it is within the composition range according to the present invention as described above. As has been described above, in the case of keeping the a Kerr rotational angle ϑk greater by increasing the saturation magnetization Ms to some extent for the reading layer (RL) 3, it has been found that if the composition of the reading layer (RL) 3 is TM rich at a room temperature, it is necessary that the composition of the wiring layer (WL) 4 is RE rich at a room temperature and that the RE rich is not reversed to a TM rich composition in the temperature elevating process upon recording. For the reference, Fig. 10 shows the X-dependency of Ms, Hc and ϑk in the composition of: (Gd₈₀Dy₂₀) X (Fe 81.5 Co 18.5 ) 100-X . In this case, the compositions with X = 20 - 21 and with greater ϑk correspond to the specimen Nos. 6, 7 and 8 (reading layer) in Table 3. Figs. 11 and 12 show, respectively, the dependency of the recording/reproducing characteristics on the auxiliary magnetic field for the specimens Nos. 6 and 8. In this case, evaluation conditions for the recording/reproducing characteristics were set as: number of rotation for the optical disk to 1800 rpm, recording diameter to 110 mm⊘, recording frequency to 3 MHz, recording duty to 50% and reproducing laser power to 1.5 mW. In the figures, the recording laser power Pw means a value at which the reproducing secondary harmonic wave level takes a minimum value, and C/N (noise level (N) and the carrier level (C)) means the values at the minimum value described above. As can be seen from the results, the specimen No. 8 has a high C/N, shows no dependency of the recording laser power Pw on the auxiliary magnetic field and is excellent as the optical magnetic disk. As has been described above, according to the present invention, since a GdDyFeCo film in which sublattice magnetization for the transition metal is predominant at a room temperature is used for the first layer (reading layer), it is possible to increase the Kerr rotational angle for the first layer (reading layer) while suppressing the increase of the saturation magnetization and the coercive force along with the increase of the Kerr rotational angle. Further, since a TbFeCo series film in which sublattice magnetization for rare earth at a room temperature is predominant is used for the second layer (writing layer), a large coercive force and a relatively low curie temperature necessary for the second layer can be obtained. Further, the saturation magnetization for the second layer (writing layer) can be increased along with the increase in the saturation magnetization of the first layer (reading layer). Then, since directions of the predominant magnetization between the first layer and the second layer are set opposite to each other, after the regular and well-arranged bits are formed in the second layer (writing layer), the bits in the second layer can be transferred with regular and well-arranged shape into the first layer (reading layer) by means of the exchanging force in the temperature lowering step thereby enabling to reduce the writing noises.

Claims: An opto-magnetic recording medium having a perpendicularly magnetized film of a dual layer structure comprising a first layer (3) composed of a rare earth-transition metal amorphous alloy thin film and a second layer (4) composed of a rare earth-transition metal amorphous alloy thin film of a different composition and having a lower curie point and a larger coercive force as compared with those of the first layer and being capable of transferring bits recorded at a curie point in the second layer to the first layer by exchange-coupling, wherein the first layer is composed of a GdDyFeCo film of an alloy represented by the formula: (Gd 100-X Dy X ) Z (Fe 100-Y CO Y ) 100-Z in which X = 5 - 25, Y = 18 - 30 and Z = 16 - 24, in which the sub-lattice magnetisation for transition metal is predominant at room temperature, the second layer is composed of a TbFeCo series film in which sub-lattice magnetisation for rare earth is predominant at room temperature, and the directions of predominant magnetisation between the first layer and the second layer are opposed. A recording medium according to claim 1, in which the TbFeCo series film is of TbFeCo or a TbFeCo- based alloy in which Tb is partially substituted with another rare earth element such as Gd, Dy, Nd or Ho. A recording medium according to claim 1 or claim 2, in which the total thickness of the magnetized film is 15 to 300 nm. A recording medium according to any one of claims 1 to 3, in which the thickness of the second layer is greater than that of the first layer. A recording medium according to any one of claims 1 to 4, in which the magnetized film is formed over a transparent substrate such as a glass or resin substrate. A recording medium according to claim 5, in which the magnetised film is interposed between a pair of dielectric material layers. A recording medium according to any one of claims 1 to 6, having a Kerr rotation angle ϑK of more than 0.48°. A recording medium according to any one of claims 1 to 7, having a saturation magnetization Ms of less than 250 emu/cm³.

Inventors: ISE TOMOKAZU, TAKASE TAKEO, ISE, TOMOKAZU, TAKASE, TAKEO
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1990
Main classification: G11B11
IPC codes: G11B11