Magneto-optical recording medium of super-resolution type using in-plane magnetic layer, and information reproducing method using the same medium

Abstract: A magneto-optical recording medium comprises an optically transparent substrate, a first magnetic layer having a compensation temperature between room temperature and a Curie temperature, and a second magnetic layer comprised of a perpendicular magnetic layer wherein the first magnetic layer is a magnetic film which is located on the side of the substrate with respect to the second magnetic layer and which is an in-plane magnetic layer at the room temperature and turns into a perpendicular magnetic layer between the room temperature and a Curie temperature of the second magnetic layer and wherein the magnetic film has in-plane anisotropy increasing continuously or stepwise along a direction of a film thickness from the side of the substrate.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a magneto-optical recording medium used in recording and reproducing information with laser light, and particularly to a magneto-optical recording medium and magneto-optical reproducing method of a super-resolution type enabling to achieve high-density recording and reproduction. Related Background Art As a rewritable high-density recording method, attention is being drawn to the magneto-optical recording medium in which information is recorded by writing magnetic domains in a magnetic thin film, using thermal energy of semiconductor laser and from which the information is read out using the magneto-optical effect. Recently, demands are increasing to raise the recording density of the magneto-optical recording medium so as to achieve a larger-capacity recording medium. The linear recording density of an optical disk such as the magneto-optical recording medium greatly depends upon the laser wavelength λ of a reproducing optical system and the numerical aperture NA of an objective lens. Namely, because the size of the beam waist is determined once the wavelength of reproducing light and the numerical aperture of objective lens are determined, the shortest mark length as a reproducible limit becomes approximately about λ/2NA. On the other hand, the track density is limited mainly by crosstalk between adjacent tracks, and it is also dependent upon the spot size of a reproducing beam as the shortest mark length is. Therefore, in order to realize higher-density arrangement for the conventional optical disks, it is necessary to decrease the laser wavelength of the reproducing optical system or to increase the numerical aperture NA of the objective lens. It is, however, not easy to decrease the wavelength of laser because of problems of device efficiency drop and heat generation. Further, an increase of the numerical aperture of objective lens makes machining of lens difficult, and would cause a mechanical problem such as collision of the lens with the disk because of a too short distance between the lens and the disk. Under such circumstances, developed is the technology for improving the recording density as devising the structure of the recording medium or the reading method. As for this technology, the present inventor made an attempt to achieve a magneto-optical recording medium that can realize magnetic super resolution without application of a reproduction magnetic field and a reproducing method of magneto-optical recording medium using the same medium. For example, the present inventor proposed the medium structure as shown in Figs. 1A-1C as a super-resolution technique for realizing a recording density of over the optical resolution of reproducing light in the magneto-optical recording medium and information reproducing method disclosed in the gazette of Japanese Laid-open Patent Application No. 6-124500. Fig. 1A shows a cross section of an optical disk as an example of the super-resolution technique. Arrows in magnetic layers indicate directions of sublattice magnetization of iron group elements in the layers, for example. A memory layer 42 is a film having great perpendicular magnetic anisotropy, for example such as TbFeCo or DyFeCo, and recording information is retained by forming magnetic domains depending upon whether the magnetic domains in this layer are upward or downward relative to the film surface. A reproduction layer 41 is an in-plane magnetic layer at room temperature, but turns into a perpendicular magnetic layer when the temperature is increased to T1-mask. When the light for reproducing information is projected to the disk in this medium structure from the side of substrate 20, the temperature gradient in the center of data track becomes as shown in Fig. 1C. Viewing this from the side of substrate 20, there exists an isothermal line of T1-mask in a spot, as shown in Fig. 1B. Then, the reproduction layer 41 remains as an in-plane magnetic layer in portions of temperatures below T1-mask, as described above, and thus, the portions do not contribute to the polar Kerr effect (as forming a front mask 4), whereby recording domains retained in the memory layer 42 become unseen as masked. On the other hand, the reproduction layer 41 turns into a perpendicular magnetic layer in the portion of temperatures over T1-mask, and directions of magnetization therein become the same as those of recording information in the memory layer because of exchange coupling from the memory layer 42. As a result, the recording domains in the memory layer 42 are transferred only to the portion of smaller aperture 3 than the size of spot 2, thus realizing super resolution. Since in these conventional super-resolution methods the front mask 4 in the low-temperature region also extends toward adjacent tracks, an attempt has also been made to improve the track density as well as the linear recording density. In the magneto-optical recording medium of the super-resolution type in the double layered structure using such an in-plane magnetic layer, greater in-plane anisotropy of the in-plane magnetic layer can adequately mask the magnetic information in the memory layer at room temperature, whereas it, however, becomes difficult to achieve quick transition of from the in-plane magnetic layer into the perpendicular magnetic layer at reproduction temperature. Conversely, smaller in-plane anisotropy can permit quick transition of from the in-plane magnetic layer into the perpendicular magnetic layer at the reproduction temperature, but it becomes difficult to adequately mask the magnetic information in the memory layer at room temperature. This is because an interfacial magnetic domain wall appearing between the reproduction layer and the memory layer mainly appears on the reproduction layer side, as shown in Fig. 4, that is, because vertical magnetic components according to the magnetic information in the memory layer are formed in portions of the reproduction layer near the memory layer. Accordingly, with the magneto-optical recording medium of the super-resolution type in the double layered structure using the conventional in-plane magnetic layer, it was not easy to obtain good reproduction signals when recording marks and the track width were made shorter. Under the above circumstances, the present inventor proposed a magneto-optical recording medium of the super-resolution type in a three layered structure in which an intermediate layer, having greater in-plane anisotropy at room temperature and a lower Curie temperature than the reproduction layer, was provided between the reproduction layer and the memory layer, and a reproducing method using the same medium. Using this medium, the transition of the reproduction layer from the in-plane magnetic layer into the perpendicular magnetic layer became quicker, which thus enabled to improve reproduction characteristics. (Japanese Patent Application No. 06-45594) However, the magneto-optical recording medium of the super-resolution type in the three layered structure using the above in-plane magnetic layer was improved in the transition of the reproduction layer from the in-plane magnetic layer into the perpendicular magnetic layer, but the improvement was not enough yet. Accordingly, it was not easy with the above medium to obtain good reproduction signals when the recording marks and the track width were made shorter. SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to further improve the super-resolution medium of the above in-plane magnetic layer type, whereby the magnetic information in the memory layer is perfectly masked at temperatures ranging from the room temperature to a high-temperature region in the reproducing beam spot and the transition of the reproduction layer from the in-plane magnetic layer into the vertical magnetic layer is quicker so as to enable to reproduce the recording information well at the high-temperature region of spot, thereby achieving further characteristic improvements, such as improvements in the linear recording density and the track density. According to a first aspect of the present invention, there is provided a magneto-optical recording medium comprising: an optically transparent substrate; and a first magnetic layer and a second magnetic layer laminated on the substrate in this order; said first magnetic layer having a compensation temperature between room temperature and the Curie temperature thereof and being an in-plane magnetic layer at room temperature turning into a perpendicular magnetic layer between room temperature and the Curie temperature of the second magnetic layer; said second magnetic layer comprising a perpendicular magnetic layer; said first magnetic layer comprising three or more magnetic films laminated in the direction of thickness of the layer and having compositions different from each other; the medium being characterised in that said three magnetic films or more have an in-plane anisotropy increasing stepwise in the direction from said substrate to said second magnetic layer. According to a second aspect of the present invention, there is provided an information reproducing method for reproducing information stored in the second magnetic layer of the magneto-optical recording medium according to the first aspect of the present invention, the method comprising: directing a light beam onto the medium so as to form a light spot on the medium, thereby changing the first magnetic layer into a perpendicular magnetic layer in a high-temperature portion of the medium inside said light spot so as to magnetically couple the first magnetic layer with said second magnetic layer and thereby transfer the information stored in said second magnetic layer to said first magnetic layer, whilst maintaining said first magnetic layer as an in-plane magnetic layer in a low-temperature portion of the medium inside said light spot; and performing reproduction of said information, using reflected light from said light spot. EP-A-0619577 which is reflected in the preamble of claim 1 describes instead a first magnetic layer for reproducing in which three magnetic layers have magnetization direction transition temperatures from in-plane to perpendicular magnetization increasing stepwise in the direction to another magnetic layer with perpendicular magnetization; this provides for a larger read-out domain when heated for reproduction. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A-1C are drawings to show an information reproducing method in one form of the magneto-optical recording medium, wherein Fig. 1A is a drawing to show a state of magnetization directions in the layers, Fig. 1B is a drawing to show a mask region and an aperture region in the light spot on the top surface of medium, and Fig. 1C is a drawing to show a temperature profile along the track direction; Fig. 2 is a drawing to show a basic layer structure of magnetic layers in the magneto-optical recording medium of the present invention; Fig. 3 is a cross section of a magnetic structure of the magneto-optical recording medium of the super-resolution type according to the present invention; Fig. 4 is a cross section of a magnetic structure of the magneto-optical recording medium of the super-resolution type using the conventional in-plane magnetic layer as the reproduction layer; Fig. 5 is a drawing to show an example of temperature characteristics of demagnetizing energy 2πMs 2 and perpendicular magnetic anisotropy constant Ku of the reproduction layer in the magneto-optical recording medium of the present invention; Fig. 6 is a drawing to show a composition dependence of saturation magnetization Ms at room temperature for Gd x (Fe 60 Co 40 ) 100-x ; Fig. 7 is a drawing to show composition dependencies of compensation temperature Tcomp and Curie temperature Tc for Gd x (Fe 60 Co 40 ) 100-x ; Fig. 8 is a drawing to show an example of the layer structure of the magneto-optical recording medium of the present invention; and Fig. 9 is a drawing to show a temperature dependence of residual Kerr rotation angle in Example 1 and Comparative Examples 1, 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The magneto-optical recording medium and information reproducing method using the same medium according to the present invention will be explained in detail with reference to the drawings. The magneto-optical recording medium of the present invention has such a structure that at least two magnetic layers, a first magnetic layer and a second magnetic layer being a perpendicular magnetic film, are laminated on an optically transparent substrate in order from the substrate side (Fig. 2). Hereinafter, the first magnetic layer will be called as a reproduction layer and the second magnetic layer as a memory layer. The reproduction layer has such a magnetization mode that the reproduction layer is an in-plane magnetic layer at room temperature and turns into a perpendicular magnetic layer at temperatures between the room temperature and the Curie temperature of the memory layer. Then the reproduction layer is arranged so that the in-plane anisotropy at room temperature increases continuously (in a smooth change) or stepwise (in a stepped change) in the thickness direction from the entrance side of light (or from the transparent substrate side). A preferred material for the reproduction layer transferring from the in-plane magnetic film into the perpendicular magnetic film is a magnetic film having its compensation temperature between the room temperature and the Curie temperature thereof, as shown in Fig. 5. It is because the magnetic film of this type has such a property that saturation magnetization decreases to zero at a temperature higher than the room temperature but lower than the Curie temperature and an intersecting point occurs between demagnetizing energy and perpendicular magnetic anisotropy constant, whereby the transition from the in-plane magnetic layer into the perpendicular magnetic layer becomes easier to take place. It is thus to be desired that the reproduction layer have a compensation temperature, for example, between the room temperature and the Curie temperature and the magnetic anisotropy so small as to become smaller than the demagnetizing energy 2πMs 2 at room temperature. The reproduction layer in this medium has in-plane anisotropy increasing continuously or stepwise in the thickness direction from the entrance side of light, and the change of in-plane anisotropy in the thickness direction may be realized by changing saturation magnetization Ms at room temperature along the thickness direction, for example. A specific method for the cases where the reproduction layer is made of rare earth-iron group metal with dominant sublattice magnetization of rare earth element is to change the content of the rare earth element in the thickness direction. In this case, the in-plane anisotropy becomes greater with an increase of the content of the rare earth element. In another example, an amount of addition of an element to improve in-plane anisotropy, such as Co, may be changed in the thickness direction so as to give a slope to the perpendicular magnetic anisotropy energy Ku. In this case the in-plane anisotropy becomes greater as Ku becomes smaller or negative. In still another example, types of elements may be changed in the thickness direction. For example, supposing rare earth-iron group element alloys are used for the reproduction layer, because perpendicular magnetic anisotropies of the rare earth-iron group element alloys increase in the order of Gd, Dy, and Tb of rare earth elements contained therein, from the light entrance side of the reproduction layer, for example, layers of GdFeCo, GdDyFeCo, DyFeCo, and DyTbFeCo are laminated in this order as changing the types of rare earth elements. In a further example, the in-plane anisotropy of the reproduction layer in the thickness direction may be changed by changing film-forming conditions with same elements and a same composition. The reproduction layer is a layer in charge of reproducing the magnetic information retained in the memory layer, which is located closer to the entrance side of light than the memory layer is and which has a higher Curie temperature than that of the memory layer in order to prevent Kerr rotation angles from being degraded upon reproduction. It is also necessary to set a coercive force of the reproduction layer smaller than that of the memory layer. This is for enabling the recording information in the memory layer to be transferred on a stable basis upon reproduction. Specific materials for the reproduction layer may be, for example, rare earth-iron group amorphous alloys, for example GdFeCo, GdTbFeCo, GdDyFeCo, NdGdFeCo, and DyFeCo. More desired among them are materials mainly containing GdFeCo, which have high Curie temperatures and low coercive forces. The memory layer is a layer for storing the recording information, which is required to retain magnetic domains on a stable basis. Materials for the memory layer may be those having great perpendicular magnetic anisotropy and capable of stably retaining magnetic states, for example rare earth-iron group amorphous alloys such as TbFeCo, DyFeCo, and TbDyFeCo, garnet, or platinum group-iron group periodic structure films, for example Pt/Co, Pd/Co, or platinum group-iron group alloys, for example PtCo, PdCo. An element for improving corrosion resistance, such as Al, Ti, Pt, Nb, or Cr, may be added to the reproduction layer or the memory layer. In addition to the above reproduction layer and memory layer, a layer made of a dielectric such as SiNx, AlOx, TaOx, or SiOx may be provided adjacent thereto in order to enhance interference effect or protection performance. A layer with high thermal conductivity such as Al, AlTa, AlTi, AlCr, or Cu may be provided in order to improve thermal conductivity. Another additional layer may be an initialization layer in which magnetization is aligned in one direction for optical modulation overwrite, or an auxiliary layer for recording assist or reproducing assist to adjust exchange coupling force or magnetostatic coupling force. Further, a protection coating of the above dielectric layer or a polymeric resin may be provided as a protection layer. Recording and reproducing processes of the present invention are next explained. First, recording magnetic domains are formed according to data signals in the memory layer of the magneto-optical recording medium of the present invention. The recording method is, for example, a method for once erasing written data and thereafter modulating the laser power as applying a magnetic field in the recording direction. Another recording method is arranged to modulate the laser power as applying an external magnetic field and to overwrite new data over old data. In the case of the light intensity modulation recording of this type, if the intensity of laser light is determined taking the linear velocity of the recording medium into consideration so that only the predetermined region in the light spot may reach temperatures near the Curie temperature of the memory layer, recording magnetic domains can be formed in a diameter smaller than the diameter of the light spot, and as a result, signals can be recorded in a period lower than the diffraction limit of light. Alternatively, overwrite recording may be carried out by modulating the external magnetic field as radiating laser light of such power as to raise the temperature of the memory layer to over the Curie temperature thereof. In this case, recording magnetic domains can be formed in a diameter smaller than the diameter of the light spot by increasing the modulation speed in accordance with the linear velocity, and as a result, signals can be recorded in a period lower than the diffraction limit of light. Next described are the medium and reproducing method according to the present invention. The present invention realizes magnetic super resolution by apparently optically masking a partial region in the light spot by the in-plane magnetic film without applying the external magnetic field. An aperture area is produced when the reproduction layer turns into the perpendicular magnetic layer with an increase of the temperature and the magnetic information in the memory layer is transferred thereto by magnetic coupling force. Namely, in the mask area, the magnetic information in the memory layer cannot be perfectly masked unless the reproduction layer is sufficiently in a state of the in-plane magnetic layer; whereas, unless the reproduction layer quickly turns into the perpendicular magnetic layer at the transition temperature, the border becomes unclear between the mask and the aperture, thus becoming a cause of noise. Originally, the transition from the in-plane magnetic layer into the perpendicular magnetic layer occurs quickly when the magnetic layer exists as a single layer, and the reproduction layer can exist as an almost perfect in-plane magnetic layer at low temperatures. However, when it is laminated on the memory layer, occurrence of the transition becomes not so quick, and it thus becomes difficult to change it into a perfect in-plane magnetic layer at room temperature. The reason is that portions of the reproduction layer closer to the memory layer are subject to stronger exchange coupling force from the memory layer in the case of the double layered structure where the reproduction layer is laminated on the memory layer being the perpendicular magnetic layer, so that effective perpendicular magnetic anisotropy becomes greater in those portions. Namely, Ku in (Eq. 1) as detailed below becomes apparently greater due to the exchange force. Further, the magnetic domain wall between the reproduction layer and the memory layer permeates into smaller anisotropy region in the reproduction layer, and this domain wall gradually spreads toward the light entrance side of the reproduction layer with a rise of the temperature. The magneto-optical recording medium of the present invention is featured in that, improving the conventional medium of the double layered structure, the reproduction layer is arranged so that the in-plane anisotropy may become stronger continuously or stepwise in the thickness direction from the entrance side of light. This arrangement makes the in-plane anisotropy stronger in portions of the reproduction layer close to the memory layer, and those portions become less affected by the perpendicular exchange force from the memory layer. As a result, even if the reproduction layer is laminated with the memory layer, the reproduction layer may have uniform magnetization directions independent of the thickness direction. Accordingly, the reproduction layer acts as if to be in a state of a single layer which is not laminated with the memory layer, so that the transition from the in-plane magnetic layer into the perpendicular magnetic layer occurs quickly and it can be changed into a nearly perfect in-plane magnetic layer at low temperatures. Thus, the reproduction layer is a perfect in-plane magnetic layer at room temperature, but turns quickly from the in-plane magnetic layer into the perpendicular magnetic layer with a rise of the temperature. This improves the reproduction characteristics. For the above medium, the most preferred arrangement is to continuously enhance the in-plane anisotropy in the thickness direction, but it is also acceptable to enhance the in-plane anisotropy stepwise from the viewpoint of control on production. However, the stepwise increase of the in-plane anisotropy is preferably to be realized by a three or more layer structure of the reproduction layer in which magnetic anisotropies thereof are increased stepwise. This is because a two layer structure cannot fully exhibit the effect as described above. In the magneto-optical recording medium of the present invention, the reproduction layer is the magnetic layer which is the in-plane magnetic layer at room temperature and which turns into the perpendicular magnetic layer at high temperatures. An example of the magnetic layer of this type is explained in the following. It is generally known as to the single-layer magnetic film that the main direction of magnetization is determined by the effective perpendicular magnetic anisotropy constant K defined by the following equation when the saturation magnetization is Ms and the perpendicular magnetic anisotropy energy is Ku. K = Ku - 2πMs 2 Here, 2πMs 2 is the demagnetizing energy, and the layer is a perpendicular magnetic film for positive K while an in-plane magnetic film for negative K. Thus, a magnetic layer changing the relation of magnitude between Ku and 2πMs 2 depending upon the temperature is effective to the transition from the in-plane magnetic film into the perpendicular magnetic film, as shown in Figs. 1A-1C. In the reproduction layer of this type, (Eq. 2) holds in the low-temperature region below the temperature T1-mask before reaching the area for reproducing the magnetic information in the memory layer (the aperture area 3 in Figs. 1A-1C), and, therefore, that region remains as an in-plane magnetic layer to mask the magnetic information in the memory layer (as a front mask 4, by reference to Figs. 1A-1C). Ku < 2πMs 2 , K < 0 (T < T1-mask) Since Ms becomes smaller with an increase of the medium temperature T, 2πMs 2 becomes rapidly smaller, thus reversing the relation of magnitude with Ku into the condition of (Eq. 3), whereby the reproduction layer turns into the perpendicular magnetic layer to form the aperture area 3 (see Figs. 1A-1C). Ku > 2πMs 2 , K > 0 (T1-mask < T) Here, it is necessary for the reproduction layer that the in-plane anisotropy thereof be increased, that is, Ki be decreased in the thickness direction from the entrance side of light. A method for realizing it is to continuously increase Ms along the thickness direction from the entrance side of light as in (Eq. 4), or to decrease Ku along the thickness direction from the entrance side of light as in (Eq. 5). An effective method for decreasing Ku is, for example, to increase the Co content. Among them, the method to increase Ms is more preferred because the perpendicular magnetic anisotropy can readily be enhanced with a decrease of Ms upon a rise of the temperature, for example, by controlling the compensation temperature. In the case where Ku is decreased, it is relatively difficult to change the reproduction layer into the perpendicular magnetic layer upon a rise of the temperature. If Ms is increased in the thickness direction, for example in the case of use of an RE rich rare earth-iron group alloy (for example, Gd xi (Fe 100-y Co y ) 100-xi etc.), x may be increased in the thickness direction from the entrance side of light as in (Eq. 6). For the magnetic layer of Gd x (Fe 60 Co 40 ) 100-x as a specific example, Fig. 6 shows a composition dependence of Ms at room temperature and Fig. 7 shows composition dependencies of the compensation temperature Tcomp and the Curie temperature Tc. Ms1 < Ms2 < Ms3 Ku1 > Ku2 > Ku3 x1 < x2 < x3 Further, specific values of K i in the thickness direction are determined as follows: on the entrance side of light of the reproduction layer it is preferably between -6 × 10 5 erg/cc and -5 × 10 4 erg/cc both inclusive, more preferably between -4 × 10 5 erg/cc and -8 × 10 4 erg/cc both inclusive; and on the memory layer side it is preferably between -6 × 10 6 erg/cc and -8 × 10 5 erg/cc both inclusive, more preferably between -4 x 10 6 erg/cc and -1 × 10 6 erg/cc both inclusive. Further, K i of the memory layer is preferably not less than 8 × 10 5 erg/cc, more preferably not less than 1 × 10 6 erg/cc. In order to satisfy the above conditions, saturation magnetizations Ms and compositions are desired to be set to the following values where the reproduction layer and memory layer are made of rare earth metal-iron group metal alloys, for example GdFeCo and TbFeCo. (1) Ms of reproduction layer On the entrance side of light it is between 100 emu/cc and 300 emu/cc both inclusive, more preferably between 150 emu/cc and 280 emu/cc both inclusive; on the memory side it is between 340 emu/cc and 1000 emu/cc both inclusive, more preferably between 450 emu/cc and 800 emu/cc both inclusive. (2) Composition of reproduction layer (in units of at %) In Gd x (Fe 100-y Co y ) 100-x , on the light entrance side: 24 ≤ x ≤ 35, more preferably 26 ≤ x ≤ 32; on the memory layer side: 38 ≤ x ≤ 60, more preferably 40 ≤ x ≤ 50; as the whole, 20 ≤ y ≤ 60. (3) Ms of memory layer It is not less than 0 emu, but not more than 300 emu. (4) Composition of memory layer (in units of at %) In Tb x (Fe 100-y Co y ) 100-x , 15 ≤ x ≤ 35, more preferably 18 ≤ x ≤ 22; 10 ≤ y ≤ 30. (5) Curie temperature of reproduction layer On the light entrance side: not less than 280 °C, more preferably not less than 300 °C; on the memory layer side: between 100 °C and 250 °C both inclusive, more preferably between 120 °C and 200 °C both inclusive. For reproducing data, the laser light is continuously radiated toward the medium, and reflected light from the medium is detected. On this occasion, the temperature of the portion irradiated by the laser increases to form a temperature distribution on the medium in a shape extending in the moving direction of medium, where a part in the light spot is raised to high temperatures. Namely, if the intensity of the laser light upon reproduction is set so that the part of the light spot as shown may reach temperatures above the temperature T1 at which the reproduction layer turns from the in-plane magnetic layer into the perpendicular magnetic layer, such a state is realized that the reproduction layer is the perpendicular magnetic layer (the aperture area) in the part of the light spot and the in-plane magnetic layer (the mask region) in the other portions, as shown in Figs. 1A-1C. Since the perpendicular magnetic layer in the reproduction layer is magnetically coupled with the memory layer by exchange coupling, signals in the memory layer are transferred to the reproduction layer. Magnetic signals transferred are converted into optical signals by the magneto-optical effect of the reproduction layer and the optical signals are then detected. Considering that the area of the high-temperature portion of the light spot shown in the drawing can be determined by the setting intensity of the laser light in this manner, signals recorded in the period below the diffraction limit of light in the memory layer can be transferred in units of recording marks, and as a result, the signals of the period below the diffraction limit of light can be reproduced without inter-symbol interference. Further, if the temperature distribution is determined so that the temperature Tr at borders between a reproducing track and adjacent tracks upon reproduction may satisfy the condition of Tr < T 1 , signals recorded in memory layers of adjacent tracks are not transferred to reproduction layers thereof, which can stop crosstalk and which can permit an increase in the track density. This state is shown in Figs. 1A-1C. If a medium has a relatively large thermal conductivity, the center of the temperature profile approaches the center of the light spot, and the portion near the center of the light spot becomes the aperture area as shown in Figs. 1A-1C. Crosstalk with adjacent tracks can also be suppressed similarly in this case. The present invention will be explained in further detail with examples, but it should be noted that the present invention is not intended to be limited to the following examples, but involves all arrangements and modifications within the scope not departing from the essence of the invention. First, magneto-optical recording media were produced in the form of the reproduction layer being the in-plane magnetic layer at room temperature and becoming the perpendicular magnetic layer between the room temperature and the Curie temperature and were evaluated, which are shown in the following Examples 1, 2. (Example 1) In a dc magnetron sputtering apparatus, targets of Si, Gd, Tb, Fe, and Co were mounted, a glass substrate and a pre-grooved polycarbonate substrate in the diameter of 130 mm were fixed on a substrate holder set at a distance of 150 mm to the targets, and the inside of the chamber was evacuated by a cryo-pump to a high vacuum of below 1 × 10 -5 Pa. As evacuating the chamber, an Ar gas was introduced into the chamber up to 0.4 Pa, and thereafter a SiN interference layer was formed in the thickness of 900 Å, and the reproduction layer of GdFeCo was formed as composition-modified films consisting of a magnetic layer 1 1 of 100 Å, a magnetic layer 1 2 of 100 Å, a magnetic layer 1 3 of 100 Å, and a magnetic layer 1 4 of 100 Å in this order. Next, the memory layer of TbFeCo and a SiN protection layer were formed in order in the thickness of 350 Å and in the thickness of 700 Å, respectively, thus obtaining the magneto-optical recording medium of the present invention in the structure of Fig. 8. Upon forming each SiN dielectric layer, N 2 gas was also introduced in addition to the Ar gas, and the film was formed by reactive sputtering while adjusting a mixture ratio of the gases so as to achieve a refractive index of 2.2. The composition of the reproduction layer of GdFeCo was as follows. The magnetic layer 1 1 : Gd 26.5 (Fe 65 Co 35 ) 73.5 , which was RE rich with Ms of 130 emu/cc at room temperature and which had the compensation temperature of 160 °C and the Curie temperature of not less than 300 °C. The magnetic layer 1 2 : Gd 27 (Fe 63 Co 37 ) 73 , which was RE rich with Ms of 180 emu/cc at room temperature and which had the compensation temperature of 188 °C and the Curie temperature of not less than 300 °C. The magnetic layer 1 3 : Gd 28 (Fe 60 Co 40 ) 72 , which was RE rich with Ms of 240 emu/cc at room temperature and which had the compensation temperature of 225 °C and the Curie temperature of not less than 300 °C. The magnetic layer 1 4 : Gd 32 (Fe 58 Co 42 ) 68 , which was RE rich with Ms of 340 emu/cc at room temperature and which had the compensation temperature of 270 °C and the Curie temperature of not less than 300 °C. The composition of the memory layer of TbFeCo was Tb 20 (Fe 80 Co 20 ) 80 , which was TM rich with Ms of 180 emu/cc at room temperature and which had the Curie temperature of 250 °C. The sample thus formed by film formation on the glass substrate was irradiated with a semiconductor laser of 830 nm from above the glass substrate, and an applied magnetic field dependence of Kerr rotation angle was measured. Measurements were carried out as heating the sample from the room temperature to about 200 °C. A temperature dependence of Kerr rotation angle at the external magnetic field of zero (the residual Kerr rotation angle;  K R ) is shown by a curve of (A) in the graph of Fig. 9. It is seen from this figure that  K R is zero from the room temperature to approximately 140 °C and that  K R becomes suddenly increasing. Next, magnetic domains were recorded in the mark length of 0.78 µm in this magneto-optical recording medium, and thereafter the magnetic domains were observed with a polarizing microscope under irradiation with the semiconductor laser of 830 nm. It was confirmed that as increasing the laser power, the recording magnetic domains contracted in the central portion (high-temperature region) of the light spot at a certain laser power, where magnetization was oriented in the erasing direction. Next, using this magneto-optical recording medium, recording and reproduction characteristics were measured. The measurements were carried out so as to achieve the highest C/N ratio by setting the numerical aperture NA of the objective lens to 0.55, the laser wavelength to 780 nm, the recording power in the range of 7 to 13 mW, and the reproducing power in the range of 2.5 to 3.5 mW. The linear velocity was 9 m/s. After the entire surface of the medium was erased at the beginning, carrier signals of 5.8, 10, and 15 MHz (corresponding to mark lengths of 0.78 µm, 0.40 µm, and 0.30 µm, respectively) were recorded in the memory layer, and a mark length dependence of the C/N ratio was checked. Next conducted was a measurement of crosstalk with adjacent tracks (hereinafter referred to as crosstalk). This was expressed as follows. Signals of the mark length 0.78 µm were recorded in a land portion by the above method and carrier signals (called C1) were measured. Then tracking was aligned with a groove portion after erased adjacent to the land portion and carrier signals (called C2) were measured in the same manner. A difference (C2 - C1) was taken between them to express crosstalk. Namely, experiments were carried out as assuming that data was recorded both in lands and grooves, and thus, the effective track pitch was 0.8 µm. The C/N ratio and crosstalk both were measured without applying an initialization magnetic field or a reproduction magnetic field. Results are shown in Table 1. (Example 2) Using the same apparatus and the same method as in Example 1, the SiN interference layer was formed in the thickness of 900 Å, the reproduction layer of GdFeCo was formed as composition-modified layers consisting of a magnetic layer 1 1 of 100 Å, a magnetic layer 1 2 of 100 Å, and a magnetic layer 1 3 of 100 Å, and then the memory layer of TbFeCo and the SiN protection layer were formed in order in the thickness of 350 Å and in the thickness of 700 Å, respectively, thus obtaining the magneto-optical recording medium of the present invention in the structure of Fig. 8. Upon forming each SiN dielectric layer, the N 2 gas was introduced in addition to the Ar gas, and the film was formed by reactive sputtering so as to achieve the refractive index of 2.2 as controlling the mixture ratio of the gases. The composition of the reproduction layer of GdFeCo was as follows. The magnetic layer 1 1 : Gd 27 (Fe 65 Co 35 ) 73 . The magnetic layer 1 2 : (Gd 50 Dy 50 ) 30 (Fe 65 Co 35 ) 70 . The magnetic layer 1 3 : Dy 30 (Fe 60 Co 40 ) 70 . The composition of the memory layer of TbFeCo was Tb 20 (Fe 80 Co 20 ) 80 , which was TM rich with Ms of 180 emu/cc at room temperature and which had the Curie temperature of 250 °C. Dynamic characteristics were next evaluated in the same manner as in Example 1. Results are shown in Table 1. (Example 3) Using the same apparatus and the same method as in Example 1, the SiN interference layer was formed in the thickness of 900 Å, the reproduction layer of GdFeCo was formed as a continuously compositionmodified layer in the thickness of 400 Å as gradually increasing the power supplied to the Gd target so as to gradually increase the Gd content, and then the memory layer of TbFeCo and the SiN protection layer were formed in order in the thickness of 350 Å and in the thickness of 700 Å, respectively, thus obtaining the magneto-optical recording medium of the present invention in the structure of Fig. 8. Upon forming each SiN dielectric layer, the N 2 gas was introduced in addition to the Ar gas, and the film was formed by reactive sputtering so as to achieve the refractive index of 2.2 by controlling the mixture ratio of the gases. In the composition of the reproduction layer of GdFeCo, the magnetic layer 1 was Gd x (Fe 60 Co 40 ) 100-x and x was continuously changed from 27 at% to 32 at% upon film formation. The composition of the memory layer of TbFeCo was Tb 18 (Fe 88 Co 12 ) 82 , which was TM rich with Ms of 120 emu/cc at room temperature and which had the Curie temperature of 220 °C. Dynamic characteristics were next evaluated in the same manner as in Example 1. Results are shown in Table 1. Comparative examples of magneto-optical recording media of the magnetic super-resolution type were next produced, and they were measured and evaluated by the same apparatus as in the above examples. (Comparative Example 1) Next, a medium, similar to Examples 2 to 10 as described in the gazette of Japanese Laid-open Patent Application No. 6-124500, was produced and then evaluated. Using the same film-forming apparatus and film-forming method as in Example 1, similarly on a glass substrate, the SiN interference layer was formed in the thickness of 900 Å, the GdFeCo reproduction layer in the thickness of 400 Å, the TbFeCo memory layer in the thickness of 400 Å, and the SiN protection layer in the thickness of 700 Å in this order, thus obtaining the magneto-optical recording medium of comparative example. The composition of the GdFeCo reproduction layer was determined so as to be RE rich with Ms of 130 emu/cc at room temperature and to have the compensation temperature of 188 °C and the Curie temperature of not less than 300 °C. The composition of the TbFeCo memory layer was determined so as to be TM rich with Ms of 200 emu/cc at room temperature and to have the Curie temperature of 220 °C. For this magneto-optical recording medium, a temperature dependence of residual Kerr rotation angle  K R was measured in the same manner as in Example 1. Results are shown by a graphic curve (R1) in Fig. 9. Comparing the results with Example 1 (curve (A)), the magneto-optical recording medium of Example 1 is better improved in the in-plane magnetic anisotropy in the low temperature region near the room temperature while maintaining the state of perpendicular magnetic layer at high temperatures. Next, recording and reproduction characteristics were measured for this magneto-optical recording medium in the same manner as in Example 1, and results are shown in Table 1. As seen from the above measurement results of Examples 1 to 3, particularly from the measurement results in the short mark lengths, high C/N ratios were obtained in the short mark lengths without applying the reproduction magnetic field with either one of the media. With the media in which the reproduction layer was the in-plane magnetic layer at room temperature and turned into the perpendicular magnetic layer between the room temperature and the Curie temperature, an improvement in crosstalk was observed as well as the improvement in C/N. Comparative Example 1 failed to obtain sufficient C/N. With the magneto-optical recording media of the present invention, it thus became possible to achieve an improvement in C/N or improvements both in C/N and crosstalk without applying the reproduction magnetic field or with applying neither the initialization magnetic field nor the reproduction magnetic field, and it also became possible to achieve an increase in the linear recording density or increases both in the linear recording density and the track density. (Comparative Example 2) Next produced and evaluated was a medium including an intermediate layer to improve the reproduction characteristics. Using the same film-forming machine and film-forming method as in Example 1, the SiN interference layer was formed in the thickness of 900 Å, the GdFeCo reproduction layer in the thickness of 400 Å, the GdFe intermediate layer in the thickness of 100 Å, the TbFeCo memory layer in the thickness of 300 Å, and the SiN protection layer in the thickness of 700 Å in order in the same manner on the glass substrate, thus obtaining the magneto-optical recording medium of Comparative Example 2. The composition of the GdFeCo reproduction layer was set so as to be RE rich with Ms of 160 emu/cc at room temperature and to have the compensation temperature of 205 °C and the Curie temperature of not less than 300 °C. The composition of the GdFe intermediate layer was set so as to be RE rich with Ms of 450 emu/cc at room temperature and to have the Curie temperature of 205 °C. Here, this intermediate layer has no compensation temperature between the room temperature and the Curie temperature. The composition of the TbFeCo memory layer was set so as to be TM rich with Ms of 200 emu/cc at room temperature and to have the Curie temperature of 220 °C. For this magneto-optical recording medium, a temperature dependence of residual Kerr rotation angle  K R was measured in the same manner as in Example 1. Results are shown by a graphic curve (R2) in Fig. 9. Comparing the results with Example 1 (curve (A)) of the magneto-optical recording medium, it is seen that the medium of Comparative Example 2 is improved as compared with Comparative Example 1 (R1), but is not improved to the level of Example 1, and that the magneto-optical recording medium of the present invention shows the best in-plane anisotropy in the low-temperature region near the room temperature and the transition from the in-plane magnetic layer into the perpendicular magnetic layer occurs steeply with the medium of the present invention. Next, recording and reproduction characteristics were observed with this magneto-optical recording medium in the same manner as in Example 1, and results are shown in Table 1. As seen from the above measurement results of Examples 1 to 3, particularly from the measurement results in the short mark lengths, high C/N ratios were obtained in the short mark lengths without applying the reproduction magnetic field with either one of the media. Further, an improvement in crosstalk was observed together with an improvement in C/N for the media in which the reproduction layer was the in-plane magnetic layer at room temperature and turned into the perpendicular magnetic layer between the room temperature and the Curie temperature. C/N (dB) Crosstalk (dB) 0.78 µm 0.40 µm 0.30 µm 1.6 µm Example 1 48 46 40 -40 Example 2 48 45 40 -42 Example 3 49 45 38 -45 Comp. Ex. 1 48 30 24 -29 Comp. Ex. 2 48 42 34 -36 Employing the magneto-optical recording medium and reproducing method according to the present invention, magnetic domains smaller than the diameter of the beam spot can be reproduced using a simple apparatus (conventional apparatus) not necessitating either the reproduction magnetic field or the initialization magnetic field or necessitating neither of them, and the linear recording density or both the linear density and the track density can be greatly improved so as to achieve high-density recording.

Claims: A magneto-optical recording medium comprising: an optically transparent substrate; and a first magnetic layer and a second magnetic layer laminated on the substrate in this order; said first magnetic layer having a compensation temperature between room temperature and the Curie temperature thereof and being an in-plane magnetic layer at room temperature turning into a perpendicular magnetic layer between room temperature and the Curie temperature of the second magnetic layer; said second magnetic layer comprising a perpendicular magnetic layer; said first magnetic layer comprising three or more magnetic films laminated in the direction of thickness of the layer and having compositions different from each other; the medium being characterised in that said three magnetic films or more have an in-plane anisotropy increasing stepwise in the direction from said substrate to said second magnetic layer. The magneto-optical recording medium according to Claim 1, wherein said first and second magnetic layers each comprise rare earth-transition metal element amorphous alloys. The magneto-optical recording medium according to Claim 2, wherein said first magnetic layer comprises GdFeCo. The magneto-optical recording medium according to Claim 2, wherein the second magnetic layer comprises TbFeCo. The magneto-optical recording medium according to Claim 3, wherein, where the first magnetic layer comprises Gd x (Fe 100-y Co y ) 100-x , 24 ≤ x ≤ 35 adjacent to the substrate and 38 ≤ x ≤ 60 adjacent to the second magnetic layer. The magneto-optical recording medium according to Claim 3, wherein the saturation magnetization Ms of the first magnetic layer satisfies 100 emu/cc ≤ Ms ≤ 300 emu/cc adjacent to the substrate and 340 emu/cc ≤ Ms ≤ 1000 emu/cc adjacent to the second magnetic layer. An information reproducing method for reproducing information stored in the second magnetic layer of the magneto-optical recording medium of claim 1, comprising: directing a light beam onto the medium so as to form a light spot on the medium, thereby changing the first magnetic layer into a perpendicular magnetic layer in a high-temperature portion of the medium inside said light spot so as to magnetically couple the first magnetic layer with said second magnetic layer and thereby transfer the information stored in said second magnetic layer to said first magnetic layer, whilst maintaining said first magnetic layer as an in-plane magnetic layer in a low-temperature portion of the medium inside said light spot; and performing reproduction of said information, using reflected light from said light spot.

Inventors: NISHIMURA NAOKI, NISHIMURA, NAOKI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1995
Main classification: G11B11
IPC codes: G11B11