Source: http://www.google.com/patents/US5764600?dq=6289460
Timestamp: 2017-07-21 18:58:26
Document Index: 421946520

Matched Legal Cases: ['art 1', 'art 12', 'Application No. 240418', 'art 12', 'Application No. 240418', 'art 1', 'art 1']

Patent US5764600 - Overwritable, high-density magneto-optical recording medium and recording ... - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inPatentsA recording/reproduction method for recording or reproducing information using the magneto-optical recording medium, the magneto-optical recording medium comprising a first magnetic layer comprising a rare earth-transition metal alloy or a ferromagnetic material containing a magnetic transition metal,...http://www.google.com/patents/US5764600?utm_source=gb-gplus-sharePatent US5764600 - Overwritable, high-density magneto-optical recording medium and recording/reproduction method thereforAdvanced Patent SearchTry the new Google Patents, with machine-classified Google Scholar results, and Japanese and South Korean patents.Publication numberUS5764600 APublication typeGrantApplication numberUS 08/464,371Publication dateJun 9, 1998Filing dateJun 5, 1995Priority dateSep 9, 1992Fee statusLapsedAlso published asUS5449566Publication number08464371, 464371, US 5764600 A, US 5764600A, US-A-5764600, US5764600 A, US5764600AInventorsYoshio Fujii, Tatsuya Fukami, Takashi Tokunaga, Yoshiyuki Nakaki, Kazuhiko TsutsumiOriginal AssigneeMitsubishi Denki Kabushiki KaishaExport CitationBiBTeX, EndNote, RefManPatent Citations (9), Non-Patent Citations (12), Referenced by (3), Classifications (25), Legal Events (5) External Links: USPTO, USPTO Assignment, EspacenetOverwritable, high-density magneto-optical recording medium and recording/reproduction method therefor
US 5764600 AAbstract
A recording/reproduction method for recording or reproducing information using the magneto-optical recording medium, the magneto-optical recording medium comprising a first magnetic layer comprising a rare earth-transition metal alloy or a ferromagnetic material containing a magnetic transition metal, a second magnetic layer comprising a rare earth-transition metal alloy, a third magnetic layer comprising a rare earth-transition metal alloy, a fourth magnetic layer comprising a rare earth-transition metal alloy, a fifth magnetic layer comprising a rare earth-transition metal alloy, and a sixth magnetic layer comprising a rare earth-transition metal alloy, the second to sixth magnetic layers being stacked in this order on the first magnetic layer, adjacent layers of the second to sixth magnetic layers being coupled by an exchange force thereof.
1. A recording/reproduction method for recording or reproducing information using the magneto-optical recording medium, said magneto-optical recording medium comprising a first magnetic layer comprising a rare earth-transition metal alloy or a ferromagnetic material containing a magnetic transition metal, a second magnetic layer comprising a rare earth-transition metal alloy, a third magnetic layer comprising a rare earth-transition metal alloy, a fourth magnetic layer comprising a rare earth-transition metal alloy, a fifth magnetic layer comprising a rare earth-transition metal alloy, and a sixth magnetic layer comprising a rare earth-transition metal alloy, said second to sixth magnetic layers being stacked in this order on said first magnetic layer, adjacent layers of said second to sixth magnetic layers being coupled by an exchange force thereof, wherein said first magnetic layer as a Curie temperature higher than that of said second magnetic layer; said third magnetic layer has a Curie temperature higher than that of said second magnetic layer; said fourth magnetic layer has a Curie temperature higher than that of said third magnetic layer; said sixth magnetic layer has a Curie temperature higher than that of said third magnetic layer; said fourth magnetic layer has a Curie temperature higher than that of said fifth magnetic layer; said fifth magnetic layer has a Curie temperature higher than or equal to that of said second magnetic layer, said sixth magnetic layer has a Curie temperature higher than that of said fifth magnetic layer, and wherein magnetization of said third magnetic layer is not inverted by an inversion of magnetization of said fourth magnetic layer at room temperature; a transition metal sub-lattice magnetization direction of said first magnetic layer and a transition metal sub-lattice magnetization direction of said second magnetic layer are aligned in an upward direction within a region where a transition metal sub-lattice magnetization direction of said third magnetic layer is aligned in the upward direction, while aligned in a downward direction within a region where the transition metal sub-lattice magnetization direction of said third magnetic layer is aligned in the downward direction, and respective transition sub-lattice magnetization directions of said fourth, fifth and sixth magnetic layers are aligned in either the upward or downward direction at room temperature, said method comprising applying external magnetic fields of the same direction for recording information and reproducing information, respectively.
2. A recording/reproduction method of claim 1, wherein said first magnetic layer comprises a rare earth-transition metal alloy in which the transition metal sub-lattice magnetization thereof is predominant, and said fourth magnetic layer comprises a rare earth-transition metal alloy having a compensation temperature between room temperature and the Curie temperature thereof.
3. A recording/reproduction method of claim 1, wherein said first magnetic layer comprises a ferromagnetic material containing a magnetic transition metal, and said fourth magnetic layer comprises a rare earth-transition metal alloy having a compensation temperature between room temperature and the Curie temperature thereof.
4. The magneto-optical recording medium of claim 1, wherein said second magnetic layer is composed of a rare earth-transition metal alloy containing Ho, represented by the general formula (I): (RE11-x Hox)y (Fe1-z Coz)1-y (I) wherein RE1 is a rare earth metal other than Ho, 1≧x≧0.02, 0.05≦y≦0.2, and 0≦z≦1. 5. A recording/reproduction method of claim 1, wherein said first magnetic layer comprises a rare earth-transition metal alloy containing Nd, represented by the general formula (II): RE21-u Ndu)v (Fe1-w Cow)1-v  (II) where RE2 is a rare earth metal other than Nd, 1≧u≧0.1, 0.05≦v≦0.2, and 0≦w≦1. 6. A recording/reproduction method of claim 1, wherein said first magnetic layer comprises a multilayered film of said ferromagnetic material in which a platinum layer and a cobalt layer, or a palladium layer and a cobalt layer are alternately stacked, and wherein the thickness of the platinum layer or palladium layer is within the range of 0.7 to 1.8 nm, while the thickness of the cobalt layer is within the range of 0.3 to 1.4 nm.
7. A recording/reproduction method of claim 1, wherein said first magnetic layer is formed by a substrate bias sputtering process.
This application is a divisional of application Ser. No. 08/107,157, filed Aug. 17, 1993, now U.S. Pat. No. 5,449,566.
The method of superresolutive reproduction is described in, for example, Japanese Journal of Applied Physics, Vol. 31, Part 1, No. 2B, February 1992, pp. 568-575. FIG. 7 is an explanatory view showing the layered structure of an magneto-optical recording medium allowing superresolutive reproduction, with indication of the magnetization direction of each layer by an arrow. The operation in the method of superresolutive reproduction will be described with reference to FIG. 7 wherein numeral 11 denotes a reproduction layer, numeral 12 denotes a switching layer, numeral 13 denotes a memory layer, numeral 14 denotes a mask region, numeral 15 denotes a light spot, numeral 16 denotes a record bit domain, and numeral 17 denotes an unrecorded region. The magnetization of the reproduction layer 11 is aligned in the same direction as that of the memory layer 13 at room temperature by exchange-coupling through the switching layer 12. When the magnetic layer is given the energy of reproduction light, a temperature distribution is produced of which peak appears on the medium-advancing side of the reproduction light spot. Within such a temperature distribution of the magnetic layer the portion of the switching layer 12 which is heated above the Curie temperature thereof cuts off the exchange-coupling between the reproduction layer 11 and the memory layer 13. Hence, the magnetization direction of the reproduction layer 11 at the region coincident with that portion of the switching layer 12 is no longer restrained by the memory layer 13 and is, therefore, aligned with the direction of an external magnetic field so as to be identical with each other. At this time that region of the reproduction layer 11 within the reproduction light spot, of which magnetization direction is aligned with the direction of the external magnetic field, becomes a mask region 14, which will not contribute to a reproductive signal component. Therefore, the reproductive signal is detected from the region other than the "mask" region. This means that the diameter of the light spot is virtually reduced. In other words, it is possible to achieve reproduction from a minute magnetic bit domain which is beyond the limit of an optical resolving power ruled by the diameter of a light spot. That is, superresolutive reproduction is feasible. To form the "mask" required for the superresolutive reproduction a reproduction light beam needs to have a certain degree of intensity. The intensity of a reproduction light beam for the superresolutive reproduction is represented by PR hereinbelow.
On the other hand, the exchange-coupled four layer film allowing direct overwriting is described in, for example, Japanese Journal of Applied Magnetics, Vol. 14, No. 2, 1990, pp. 165 to 170. FIG. 8 is an explanatory view for illustrating a direct overwriting operation of the aforesaid four layer magneto-optical memory medium allowing direct overwriting based only on modulation of light intensity. In FIG. 8, a large arrow indicates the magnetization direction of each layer, while a small arrow in the large arrow the magnetization direction of the transition metal sub-lattice of each layer. The four layer magnetic film includes, from the top, a memory layer 21, recording layer 22, switching layer 23 and an initializing layer 24. The Curie temperatures of the layers 21 to 24 are represented by Tc1, Tc2, Tc3 and Tc4) respectively. Troom represents room temperature, Tcomp2 the compensation temperature of the recording layer 22, and Tcomp4 the compensation temperature of the initializing layer 24. The direct overwriting operation will be described in the order of (A) initializing operation, (B) high-temperature operation and (C) low-temperature operation.
Since the magnetization direction of the memory layer 21 is aligned in the upward direction by the high-temperature operation (B) or in the downword direction by the low-temperature operation, direct overwriting can be achieved if the intensity of recording light beam is modulated in a binary fashion, i.e., high or low in accordance with binary-coded information "0" or "1". Hereinafter the high-intensity of the recording light beam for the high-temperature operation (B) will be represented by PH, while the low-intensity thereof for the low-temperature operation will be represented by PL.
As described above, there have been proposed, on one side, a magneto-optical recording medium capable of superresolutive reproduction and, on the other side, one allowing direct overwriting. However, either the former or the latter does not allow both superresolutive reproduction and direct overwriting. To make these merits compatible with each other in one magneto-optical recording medium, the light beam needs to have three degrees of intensity, i.e., a light beam intensity PR for the superresolutive reproduction in addition to the two light beam intensities for overwriting, PH for the high-temperature operation and PL for the low-temperature operation. Further, it is desired that an expected operation be assuredly achieved in accordance with each light beam intensity so as to make satisfactory superresolution behavior and satisfactory overwriting behavior compatible with each other, and that the light beam intensities PH, PL and PR each have a sufficient margin (or allowance). A phenomenon must not occur such that during superresolutive reproduction at the light beam intensity PR the low-temperature operation happens thereby changing the recorded information. In addition, where a magneto-optical material, such as a NdFeCo film, Pt/Co multilayered film or the like, which produces a large reproductive signal output in response to light of a short wavelength is used in the medium allowing both superresolutive reproduction and overwriting, it is not clarified yet how and what to do in order to improve both superresolution behavior and overwriting behavior as well as the behavior in response to light of a short wavelength.
According to the present invention, there is provided a magneto-optical recording medium comprising a first magnetic layer made of a rare earth-transition metal alloy or a ferromagnetic material containing a magnetic transition metal, a second magnetic layer made of a rare earth-transition metal alloy, a third magnetic layer made of a rare earth-transition metal alloy, a fourth magnetic layer made of a rare earth-transition metal alloy, a fifth magnetic layer made of a rare earth-transition metal alloy, and a sixth magnetic layer made of a rare earth-transition metal alloy, said second to sixth magnetic layers being stacked in this order on said first magnetic layer, adjacent layers of said second to sixth magnetic layers being coupled by an exchange force thereof, wherein said first magnetic layer has a Curie temperature higher than that of said second magnetic layer; said third magnetic layer has a Curie temperature higher than that of said second magnetic layer; said fourth magnetic layer has a Curie temperature higher than that of said third magnetic layer; said sixth magnetic layer has a Curie temperature higher than that of said third magnetic layer; said fourth magnetic layer has a Curie temperature higher than that of said fifth magnetic layer; said sixth magnetic layer has a Curie temperature higher than that of said fifth magnetic layer, and wherein magnetization of said third magnetic layer is not inversed by an inversion of magnetization of said fourth magnetic layer at room temperature; a magnetization direction of transition metal sub-lattice of said first magnetic layer made of said rare earth-transition metal alloy or a magnetization direction of said first magnetic layer made of said ferromagnetic material and a magnetization direction of transition metal sub-lattice of said second magnetic layer are aligned in an upward direction within a region where a magnetization direction of transition metal sub-lattice of said third magnetic layer is aligned in the upward direction, while aligned in a downward direction within a region where the magnetization direction of transition metal sub-lattice of said third magnetic layer is aligned in the downward direction; and respective magnetization directions of transition metal sub-lattice of said fourth, fifth and sixth magnetic layers are aligned in either the upward or downward direction.
Further, it is preferable that the first magnetic layer is made of a rare earth-transition metal alloy in which magnetization of transition metal sub-lattice thereof is predominant, or of a ferromagnetic material containing a magnetic transition metal, while at the same time the fourth magnetic layer is made of a rare earth-transition metal alloy having a compensation temperature between room temperature and the Curie temperature thereof.
Further, the second magnetic layer of the magneto-optical recording medium according to the present invention is preferably made of a rare earth-transition metal alloy containing Ho, represented by the general formula (I)
where RE2 is a rare earth metal other than Nd, u≧0.1, 0.05≦v≦0.2, and 0≦w≦1.
With the magneto-optical recording medium wherein the first magnetic layer is composed of a rare earth-transition metal alloy containing neodymium, represented by the general formula (II), or wherein the first magnetic layer is made of a multilayered film of a ferromagnetic material in which a platinum layer and a cobalt layer, or a palladium layer and a cobalt layer are alternately stacked; and the thickness of the platinum or palladium layer as a unit is within the range of 0.7 to 1.8 nm, while the thickness of the cobalt layer is within the range of 0.3 to 1.4 nm, superresolutive reproduction and direct overwriting are possible with light of a short wavelength since a magneto-optical recording material is used which is advantageous in reproduction with light of such a short wavelength.
TABLE 1______________________________________Layer: Material          Thickness (nm)                     Curie temperature______________________________________Dielectric layer: SiNx          65First magnetic layer:          25         above 300° C.Gd0.19 Fe0.69 Co0.12Second magnetic layer:          8          120° C.Tb0.15 Fe0.84 Co0.01Third magnetic layer:          40         210° C.Tb0.21 Fe0.70 Co0.09Fourth magnetic layer:          40         250° C.Dy0.25 Fe0.50 Co0.25Fifth magnetic layer:          20         170° C.Tb0.15 Fe0.78 Co0.07Sixth magnetic layer:          40         above 300° C.Tb0.25 Fe0.15 Co0.60Protective layer: SiNx          80______________________________________
The magnetic layers are designed as follows: the first magnetic layer has a Curie temperature higher than that of the second magnetic layer; the third magnetic layer has a Curie temperature higher than that of the second magnetic layer; the fourth magnetic layer has a Curie temperature higher than that of the third magnetic layer; the sixth magnetic layer has a Curie temperature higher than that of the third magnetic layer; the fourth magnetic layer has a Curie temperature higher than that of the fifth magnetic layer; and the sixth magnetic layer has a Curie temperature higher than that of the fifth magnetic layer. Preferably, the fifth magnetic layer is made to have a Curie temperature higher than that of the second magnetic layer.
&#963;4/(2·Ms·t)-&#963;2/(2·Ms·t)&lt;Hc(1)
(Gd1-p-q Tbp Dyq (r (Fe1-s Cos)1-r(III)
The second magnetic layer satisfying the above condition is composed of a rare earth-transition metal alloy. Examples of such an alloy include, as well as the above-noted Tb0.15 Fe0.84 Co0.01 alloy, a rare earth-transition metal alloy represented by the general formula (IV), a rare earth-transition metal alloy containing Ho, represented by the general formula (I), and an alloy containing each of those alloys as a main ingredient and a nonmagnetic element such as Al, Ti, Cr, Si, B or the like. The thickness of the second magnetic layer is preferably in the range of 3 to 20 nm,
where 0 ≦a 1, 0.05≦b≦0.4, and 0≦c≦0.5,
After the construction of the magneto-optical recording medium, the respective sub-lattice magnetization directions of the fourth to sixth magnetic layers are made to align in one direction, for example, in the downward direction. This is achieved by, for example, applying first a sufficiently large magnetic field or when an extremely large inversed magnetic field is present, heating or cooling the whole magnetic film while applying a magnetic field thereto. In this case the sub-lattice magnetization direction of the third magnetic field may be aligned either upward (state in FIG. 2(a)), or downward (state in FIG. 2(b)). The sub-lattice magnetization direction of the second magnetic layer is aligned with that of the third magnetic layer by the exchange force from the third magnetic layer and, similarly, the sub-lattice magnetization direction of the first magnetic layer is aligned with that of the second magnetic layer by the exchange force from the second magnetic layer. As a result, the respective sub-lattice magnetization directions of the first to third magnetic layers are aligned in the same direction. If the transition metal sub-lattice magnetization or the rare-earth-metal sub-lattice magnetization is predominant in both the fourth and sixth magnetic layers, the magnetization directions of these layers can be conveniently aligned by uniform magnetic fields of the same direction. To prevent an inversion of the magnetization of the sixth magnetic layer within the temperature range for operation, it is desired that the magnetization of the rare-earth-metal sub-lattice having a relatively large coercive force be predominant up to a relatively high temperature. It is, therefore, desired that the rare-earth-metal sub-lattice magnetization be predominant in both the fourth and sixth magnetic layers. The predominance of the rare-earth-metal sub-lattice magnetization herein means that the magnetization of rare-earth-metal sub-lattice is larger than that of the transition metal sub-lattice at room temperature, and that the direction of the resulting magnetization appearing outside is aligned with the rare-earth-metal magnetization direction.
At this time, regardless of the initialized state, in FIG. 2(a) or (b), the magnetizations of the second, third and fifth magnetic layers are lost and, hence, the fourth magnetic layer is not influenced by the exchange force from other magnetic layers. As a result, the sub-lattice magnetization direction of the fourth magnetic layer is aligned with the direction of the external magnetic field, i. e., the upward direction (refer to the state in FIG. 3(a)).
In a subsequent cooling step, when the temperature of the magnetic film is first made to drop down to below the Curie temperature of the third magnetic layer, the magnetization of the third magnetic layer appears. In this case the sub-lattice magnetization direction of the third magnetic layer is aligned with that of the fourth magnetic layer, i.e. the upward direction, by the exchange force from the fourth magnetic layer (refer to the state (b) in FIG. 3).
Subsequently, when the temperature of the magnetic film is made to drop down to below the Curie temperature of the fifth magnetic layer, the magnetization of the fifth magnetic layer appears. In this case the sub-lattice magnetization direction of the fifth magnetic layer is aligned with that of the sixth magnetic layer, i.e., the downward direction, by the exchange force from the sixth magnetic layer. Successively the sub-lattice magnetization direction of the fourth magnetic layer is aligned with that of the fifth magnetic layer, i.e., the downward direction (refer to the state in FIG. 3(c)).
In the cooling step, when the temperature of the magnetic film is made to drop down to below the Curie temperature of the second magnetic layer, the magnetization of the second magnetic layer appears. In this case the sub-lattice magnetization direction of the second magnetic layer is aligned with that of the third magnetic layer, i.e. the upward direction, by the exchange force from the third magnetic layer. Successively the sub-lattice magnetization direction of the first magnetic layer is aligned with that of the second magnetic layer, i.e., the upward direction, by the exchange force from the second magnetic layer (refer to the state in FIG. 3(d)).
At this time the magnetization of the second magnetic layer is lost regardless of the state in FIGS. 2(a) or (b) which is initialized by the initializing operation (A). Thus the sub-lattice magnetization direction of the third magnetic layer is not influenced by the exchange force from the second magnetic layer and is, hence, aligned with the sub-lattice magnetization direction of the fourth magnetic layer, i.e. the downward direction, by the exchange force from the fourth magnetic layer (refer to the state in FIG. 4(a)).
Subsequently, when the temperature of the magnetic film is made to drop down to below the Curie temperature of the second magnetic layer, the magnetization of the second magnetic layer appears. At this time the sub-lattice magnetization direction of the second magnetic layer is aligned with that of the third magnetic layer, i. e., the downward direction, by the exchange force from the third magnetic layer. Likewise, the sub-lattice magnetization direction of the first magnetic layer is aligned with that of the second magnetic layer, i.e. the downward direction, by the exchange force from the second magnetic layer (refer to the state in FIG. 4(b).
As described above, regardless of the initial state in FIGS. 2(a) or (b), the low-temperature operation realizes the state in FIG. 4(b), which is identical with the state in FIG. 2(b). This state can be made to correspond to, for example, information "0".
To be described next is the superresolutive reproduction operation. FIG. 5 illustrates the operation of reproducing with superresolution the information recorded as magnetization patterns in FIGS. 2(a) and (b) by the direct overwriting operation including the high-temperature operation (B) and low-temperature operation (C). In FIG. 5, same reference characters denote like or corresponding parts shown in FIG. 1, indication of a magnetization direction is omitted, and the recording medium advances to the right in the drawing. The temperature of the magnetic film is raised when it is given reproduction light energy, and there is produced a temperature distribution having a peak on the medium-advancing side of the reproduction light spot. In this temperature distribution of the magnetic film, the exchange coupling between the first and third magnetic layers is cut off at a region of which temperature is raised up to above the Curie temperature of the second magnetic layer. As a result in such a region the magnetization direction of the first magnetic layer is not restrained any more by the third magnetic layer and is, hence, aligned with the direction of an external magnetic field. Then such a region in the light spot where the magnetization direction of the reproductive layer is aligned with the direction of the external magnetic field becomes a "mask" region, which will not contribute to a reproductive signal component. Therefore, a reproductive signal is detected from the region other than the "mask" region. This means that the diameter of the light spot is virtually reduced. Consequently it becomes possible to reproduce a microscopic bit domain which is beyond the limit of the optical resolution power dependent on the diameter of a light spot. In other words, the superresolutive reproduction becomes feasible.
The recording medium of the present example wherein the combination of Curie temperatures of the second and fifth magnetic layers was variously varied was examined for its reproduction property, or a carrier to noise ratio (CN ratio), and the results were as shown in Table 2. The respective Curie temperatures of the second and fifth magnetic layers were varied by varying the contents of each layer. Specifically, in the Tbn (Fe1-m Com)1-n film was varied the content of Tb (n) or the ratio of Co (m) to FeCo including the case where Co was not contained. The contents (m) and (n) corresponding to different Curie temperatures of each of the second and fifth magnetic layers were as shown in Table 3. The thicknesses of the second and fifth magnetic layers were 8 nm and 20 nm, respectively. In Tables 2 and 3, Tc2 denotes the Curie temperature of the second magnetic layer, and Tc5 denotes that of the fifth magnetic layer. Further, in FIG. 1, for the medium giving no reproductive signal a dash (horizontal line) is given instead of showing the CN ratio.
TABLE 2______________________________________Tc2 /Tc5  110° C.          120° C.                   135° C.                         145° C.                                155° C.                                      170° C.______________________________________120° C.  28.5 dB 43.3 dB  48.7 dB                         50.3 bB                                51.2 dB                                      50.8 dB130° C.  19.7 dB 23.2 dB  46.3 dB                         50.2 bB                                51.8 dB                                      49.3 dB140° C.  --      17.6 dB  21.1 dB                         47.3 bB                                49.6 dB                                      50.2 dB150° C.  --      --       --    18.5 bB                                15.3 dB                                      45.4 dB165° C.  --      --       --    --     10.6 dB                                      42.1 dB180° C.  --      --       --    --     --    --______________________________________
TABLE 3______________________________________Second magnetic layer            Fifth magnetic layer   Content             ContentTc2 (°C.)    n        m      Tc5 (°C.)                             n    m______________________________________120      0.15     0.012  110      0.12 0.012130      0.15     0.024  120      0.15 0.012140      0.15     0.038  135      0.15 0.030150      0.15     0.052  145      0.15 0.045160      0.15     0.066  155      0.15 0.060180      0.13     0.082  170      0.15 0.082______________________________________ Note: m and n are each a proportion of the number of atoms.
In Example 1 the first magnetic layer was composed of a rare earth-transition metal alloy in which the transition metal sub-lattice magnetization was predominant, and the fourth magnetic layer was composed of a material having a compensation temperature between room temperature and the Curie temperature thereof. For comparison, a magneto-optical recording medium as a comparative example was constructed wherein the fourth magnetic layer was composed of a material not having a compensation temperature between room temperature and the Curie temperature thereof and in which the rare-earth sub-lattice magnetization was predominant at room temperature. The constitution of Comparative Example 1 was the same as that of Example 1 except for the fourth magnetic layer. The fourth magnetic layer was 40 nm thick and composed of Dy0.28 Fe0.47 Co0.25 having a Curie temperature of 255° C.
FIGS. 6(a) and (b) illustrate the media of Example 1 and Comparative Example 1, respectively, in the "mask" state where the magnetization direction of the first magnetic layer is aligned with the direction of an external magnetic field in the superresolutive reproduction operation. In these figures the magnetization condition of each layer in such media is shown, and same reference characters are used to denote like or corresponding parts shown in FIG. 1. In the case of Comparative Example 1, the sub-lattice magnetization direction of the first magnetic layer is aligned with a downwardly-orienting external magnetic field. In cooling the magnetic film to room temperature, upon appearance of the magnetization of the second magnetic layer the second magnetic layer is influenced by the exchange forces from the first and third magnetic layers simultaneously. In the case of Example 1 (FIG. 6 (a)), the respective sub-lattice magnetization directions of the first and third magnetic layers are both in the upward direction and, hence, the sub-lattice magnetization direction of the second magnetic layer is aligned upward by the exchange forces from the first and third magnetic layers. On the other hand, in Comparative Example 1 (FIG. 6 (b)) the sub-lattice magnetization direction of the first magnetic layer is aligned downward while that of the third magnetic layer is aligned upward. Accordingly, upon appearance of the magnetization of the second magnetic layer in Comparative Example 1, the sub-lattice magnetization direction of the second magnetic layer depends upon the balance of the three factors: the exchange forces from the first and third magnetic layers and the magnetization force of external magnetic field. An interfacial domain wall is produced between the first and second magnetic layers if the sub-lattice magnetization direction of the second magnetic layer is aligned upward, or between the second and third magnetic layers if it is aligned downward. In addition, an interfacial domain wall is also present between the third and fourth magnetic layers. Therefore, two interfacial domain walls in total appear in the cooling step. Since exchange energy is accumulated in each interfacial domain wall, Comparative Example 1 in the state where two interfacial domain walls are present has a large energy as compared with Example 1, which renders the medium instable. In particular, the case is not preferable where interfacial walls are produced between the second and third magnetic layers and between the third and fourth magnetic layers, respectively and the third magnetic layer is interposed between two intertacial walls because the magnetization direction of the third magnetic layer storing information becomes instable and it is possible, in the worst case, that recorded information is changed by the superresolutive reproduction operation. With Example 1, in contrast, the number of interfacial domain walls which can be produced is at most one and, thus, the superresolutive reproduction operation can be achieved stably without apprehension of change of recorded information. Thus, if the first magnetic layer is composed of a rare earth-transition metal alloy in which the transition metal sub-lattice magnetization thereof is predominant or of a ferromagnetic material, it is desired that the fourth magnetic layer have a compensation temperature between room temperature and the Curie temperature thereof.
where RE1 is a rare earth metal other than Ho; and the thickness thereof was 10 nm. The second magentic layer of this magneto-optical recording medium is characterized by containing Ho in the rare earth metal. Such a characteristic allows the second magnetic layer to have a decreased Curie temperature and the superresolutive reproduction operation to be achieved with reproduction light beam of a decreased intensity PR. This is favorable for avoiding interference between the low-temperature operation of the direct overwriting operation and the superresolutive reproduction operation because there is provided a large difference between the recording light beam intensity PL. for the former and the reproduction light beam intensity PR for the latter. Also, the second magnetic layer containing Ho is suited for the superresolutive reproduction operation because a large exchange force is exerted at the interface with the first magnetic layer and with the third magnetic layer.
Table 5 shows examples of the combination of the content u (proportion of Nd in the rare earth metal), content v (proportion of the whole rare earth metal) and content w (proportion of Co in FeCo) in the above general formula for the first magnetic layer, and a CN ratio for each of the examples. In Example 3 the rare earth metal other than Nd, represented by RE2 in the above formula was Gd, but Dy and Tb as well as Gd can be used either alone or as a mixture of two or more of these. The rare earth-transition metal alloy containing Nd exhibits a large magneto-optical effect in response to light of a relatively short wavelength as compared with a rare earth-transition metal not containing Nd and is, hence, known to be a high-density recording material. The magneto-optical medium having the first magnetic layer of any content combination shown in Table 5 exhibited a larger magneto-optical effect than one having the first magnetic layer composed of a material not containing Nd in response to light of a wavelength shorter than infrared light which is frequently used at present. Among the combinations shown in Table 4 the combination satisfying u≧0.1 and 0.05≦v≦0.2 exhibited a particularly favorable CN ratio. Hence, this range for the combination of such contents is suitable for the first magnetic layer of the magneto-optical recording medium according to the present invention.
TABLE 7______________________________________First magnetic layer   Hca     Hcb______________________________________Gd0.19 Fe0.69 Co0.12                  280 Oe  120 Oe(Gd0.90 Nd0.10)0.20 (Fe0.85 Co0.15)0.80                  160 Oe   90 OePt/Co (Pt unit layer thickness: 1.2 nm,                  720 Oe  220 OeCo unit layer thickness: 0.5 nm)______________________________________
TABLE 8__________________________________________________________________________Reproduction light beam        0.8           1.0              1.2                 1.4                    1.8                       2.0                          2.2                             2.4intensity (mW)CN ratio (dB)     (a)        48.2           49.6              50.2                 50.8                    51.0                       51.2                          51.2                             18.1     (b)        46.3           47.2              49.5                 50.3                    22.7                       22.1                          21.2                             19.8__________________________________________________________________________
In the condition (a), the CN ratio dropped steeply when the reproduction light beam intensity exceeded 2.2 mW. This is conceivably because with a large reproduction light beam intensity the low-temperature operation of the direct overwriting operation occurred even in the reproduction operation thereby erasing recorded information. In the condition (b), on the other hand, the CN ratio dropped steeply when the reproduction light beam intensity exceeded 1.4 mW. This indicates that reproduction light beam having an intensity lower than the intensity at which the low-temperature operation occurs confuses recorded information, and which is based on the fact that part of the recorded information was erased since the direction of a magnetic field for reproduction was opposite to that of a magnetic field for recording. That is, if there is applied an external magnetic field for reproduction of the same direction with that for recording, good reproduction characteristics can be obtained in response to reproduction light beam of a wide intensity range. In addition thereto, as means for producing an external magnetic field a stationary magnet can be used instead of an electromagnet, thereby offering secondary effects such as a simplified device configuration and reduced costs for production.
Patent CitationsCited PatentFiling datePublication dateApplicantTitleUS4882231 *Nov 30, 1988Nov 21, 1989Sony CorporationMagneto-optical recording mediumUS4908809 *May 20, 1988Mar 13, 1990Mitsubishi Denki Kabushiki KaishaInformation-carrying medium equipment for magneto-optic reading and writingUS5016232 *Mar 8, 1990May 14, 1991Mitsubishi Denki Kabushiki KaishaMagneto-optic information-carrying medium including three magnetic layersUS5164926 *Sep 6, 1991Nov 17, 1992Nikon CorporationOver-write capable magnetooptical recording medium with four magnetic layered structure dispensing with external initializing fieldUS5449566 *Aug 17, 1993Sep 12, 1995Mitsubishi Denki Kabushiki KaishaOverwritable, high-density magneto-optical recording medium and recording/reproduction method thereforEP0258978A2 *Jul 8, 1987Mar 9, 1988Canon Kabushiki KaishaMagnetooptical recording medium allowing overwriting with two or more magnetic layers and recording method utilizing the sameEP0304873A1 *Aug 23, 1988Mar 1, 1989Sony CorporationMagneto-optical recording mediumJPS609855A * Title not availableJPS61165847A * Title not available* Cited by examinerNon-Patent CitationsReference1Fujii et al., "DOW and SR Readout by Exchange Coupled Multilayer Film", J. Mag. Soc. Jpn. vol. 17 (1993) pp. 167-170.2 *Fujii et al., DOW and SR Readout by Exchange Coupled Multilayer Film , J. Mag. Soc. Jpn. vol. 17 (1993) pp. 167 170.3Fujii et al., Extract of "Direct Overwriting and Super Resolution Readout by Exchange-Coupled Multilayer Film".4 *Fujii et al., Extract of Direct Overwriting and Super Resolution Readout by Exchange Coupled Multilayer Film .5J. Saito et al., "Direct Overwrite by Light Power Modulation on Magneto-Optical Multi-Layered Media", Japanese Journal of Applied Physics, vol. 26, 1987, Supplement 26-4, pp. 155-159.6 *J. Saito et al., Direct Overwrite by Light Power Modulation on Magneto Optical Multi Layered Media , Japanese Journal of Applied Physics , vol. 26, 1987, Supplement 26 4, pp. 155 159.7 *Japanese Journal of Applied Magnetics , vol. 14, 1990, Part 12, pp. 165 170 Verified English translation of Japanese Patent Application No. 240418/1992.8Japanese Journal of Applied Magnetics, vol. 14, 1990, Part 12, pp. 165-170 Verified English translation of Japanese Patent Application No. 240418/1992.9M. Kaneko et al., "Multilayered Magneto-Optical Disks for Magnetically Induced Superresolution", Japanese Journal of Applied Physics, vol. 31, 1992, Part 1, No. 2B, Feb. 1992, pp. 568-575.10 *M. Kaneko et al., Multilayered Magneto Optical Disks for Magnetically Induced Superresolution , Japanese Journal of Applied Physics , vol. 31, 1992, Part 1, No. 2B, Feb. 1992, pp. 568 575.11Nakada et al., "MO Properties of (Pr, Nd)-(Tb,Dy)-FeCo Amorphous Films" IEEE Trans. Mag. vol. 25, No. 5 (Sep. 1989) pp. 3767-3769.12 *Nakada et al., MO Properties of (Pr, Nd) (Tb,Dy) FeCo Amorphous Films IEEE Trans. Mag. vol. 25, No. 5 (Sep. 1989) pp. 3767 3769.* Cited by examinerReferenced byCiting PatentFiling datePublication dateApplicantTitleUS6096444 *Feb 2, 1998Aug 1, 2000Fujitsu LimitedMagneto-optical recording medium capable of double mask readoutEP1480210A1 *Feb 25, 2002Nov 24, 2004Fujitsu LimitedMagneto-optical recording medium and recording/reproduction apparatus thereofEP1480210A4 *Feb 25, 2002Dec 5, 2007Fujitsu LtdMagneto-optical recording medium and recording/reproduction apparatus thereof* Cited by examinerClassifications U.S. Classification369/13.4, G9B/11.048, 428/819.3, 369/286, G9B/11.049, 369/13.42, 428/900, 369/13.51, G9B/11.016, G9B/11.019, 369/283, G9B/11.012International ClassificationG11B11/10, G11B11/105Cooperative ClassificationG11B11/10584, G11B11/10506, G11B11/10515, G11B11/10521, G11B11/10586, Y10S428/90European ClassificationG11B11/105B1L, G11B11/105M2, G11B11/105B2, G11B11/105B3B1, G11B11/105M1Legal EventsDateCodeEventDescriptionNov 15, 2001FPAYFee paymentYear of fee payment: 4Nov 14, 2005FPAYFee paymentYear of fee payment: 8Jan 11, 2010REMIMaintenance fee reminder mailedJun 9, 2010LAPSLapse for failure to pay maintenance feesJul 27, 2010FPExpired due to failure to pay maintenance feeEffective date: 20100609RotateOriginal ImageGoogle Home - Sitemap - USPTO Bulk Downloads - Privacy Policy - Terms of Service - About Google Patents - Send FeedbackData provided by IFI CLAIMS Patent Services