Magneto-optical recording method

Abstract: A magneto-optical recording method is disclosed. A first recording domain is formed by the application of light of predetermined energy to a magneto-optical recording medium. A portion of the first recording domain is erased to thereby form a crescent second recording domain by the application of light of energy smaller than the predetermined light energy after the application of light of predetermined energy.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a magneto-optical recording method of recording information on a magneto-optical recording medium. Related Background Art The information overwriting method using a magneto-optical recording medium is broadly divided into the light modulation system and the magnetic field modulation system. The light modulation system includes the light modulation overwrite system utilizing the difference in thermal magnetic characteristic between two layers of magnetic film, i.e., a recording layer and an auxiliary layer, as described, for example, in Japanese Patent Application Laid-Open No. 62-175948. On the other hand, the magnetic field modulation system in a system as described, for example, in Japanese Patent Application Laid-Open No. 57-186248 wherein a light beam is applied to magneto-optical recording film and at the same time, an extraneous magnetic field modulated in conformity with a recording signal is applied to the region of the film to which the light beam is applied, whereby the direction of magnetization is changed in conformity with recording information to thereby accomplish the recording of information. However, in the overwriting system using two layers of magnetic film and the light modulation overwriting system utilizing an anti-magnetic field, when the recording domain length becomes short as indicated by a solid curve in Figure 1 of the accompanying drawings, the carrier level corresponding to the amplitude of a reproduction signal is greatly reduced. Figure 2 of the accompanying drawings show the relation between the temperature distribution of a recording layer by the application of light and the recorded domain. When the laser recording wavelength is 780 nm and the N.A. of an objective lens is 0.53, the laser beam diameter is approximately 1.4 µm. Also, this is an example in which the recording land width is 1.0 µm and the domain was formed at a duty 50%. In the light modulation recording system, when laser light is applied, the temperature distribution of the magneto-optical recording layer becomes substantially Gaussian distribution and domains are formed in areas wherein temperature is about T C or higher. Accordingly, the recording domain length and the recording domain width were made substantially the same. Figure 2(a) shows an example in which the recording domain length is 0.8 µm, Figure 2(b) shows an example in which the recording domain length is 0.6 µm, and Figure 2(c) shows an example in which the recording domain length is 0.5 µm. There has been the problem that when the recording domain length decreases thus, the carrier level is greatly reduced as already noted. On the other hand, the magnetic modulation system is a system as shown in Figure 3(b) of the accompanying drawings wherein upward and downward modulation extraneous magnetic fields are applied to a magneto-optical recording layer to thereby change the direction of magnetization to upward and downward directions. Accordingly, domains formed are of a substantially constant domain width, irrespective of the magnitude of the recording domain length, as shown in Figure 3(a) of the accompanying drawings. Consequently, as indicated by a dotted curve in Figure 1, the reduction in the carrier level caused by a decrease in the recording domain length becomes smaller than in the light modulation recording. However, in the magnetic field modulation system, the recording speed is limited by the magnetic field modulation speed and therefore, high-speed recording has been difficult. Not only there has been the possibility of head crash by a floating magnetic head or the like, but also additional devices such as a magnetic head and a driver therefor have been increased, and this has led to the problem that the apparatus becomes complicated and bulky. EP-A-0328667 discloses a magnetooptical storage medium in which the recording medium is irradiated with a first light spot of a small diameter and with a second light spot of a larger diameter concentric with the first spot. This causes the temperature around the recording position to increase to higher than the compensation temperature of the recording film due to irradiation with the second light spot. The intensity of the first light spot is modified dependent upon the recording data in order to increase the temperature at the recording position to the Curie point of the recording medium. EP-A-0383386 discloses a method and apparatus for writing and reading information on a magnetooptical recording medium. The recorded data is recorded in elliptical domains which are produced by switching the applied magnetic field in time intervals which are shorter than the time interval required to move the irradiating beam over the recording medium. SUMMARY OF THE INVENTION The present invention has been made to eliminate such problems and the object thereof is to provide a magneto-optical recording method which is free of a reduction in carrier level and can obtain a good reproduction signal amplitude. Such object of the present invention is achieved by a magneto-optical recording method and a magneto-optical recording apparatus according to the subject-matter of independent claims 1 and 10 respectively. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graph showing the relation between recording domain length and carrier level by a comparison between light modulation recording and magnetic field modulation recording. Figure 2 illustrates the relations between the temperature distribution of a recording layer by the application of light and the recorded domain in light modulation recording. Figure 3 illustrates the relations between the direction of an applied magnetic field and the recorded domain in the magnetic field modulation system. Figure 4 is a graph showing the characteristic of a magneto-optical recording medium used in the present invention. Figure 5 is a graph showing the relation between recording pulse width and residual domain diameter when there is a history forming domain. Figure 6 is a graph showing the relation between recording pulse width and recording domain diameter when there is no history forming domain. Figure 7 shows a magneto-optical recording apparatus used to carry out the magneto-optical recording method of the present invention. Figure 8 shows the magneto-optical recording method of the present invention, and particularly the relation between the pulse of light irradiation and the formed domain. Figure 9 and 10 show further embodiments of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Some embodiment of the present invention will hereinafter be described in detail with reference to the drawings. Description will first be made of a magneto-optical recording medium used in the present embodiment. The magneto-optical recording film used is GdTbFeCo film which is a rare earth-transition metal thin film. Figure 4 shows the temperature characteristic of this film, and particularly shows the relations among magnetization Ms (A/m cc and emu/cc) and coercive force Hc (A/m and (KOe) and temperature. As is apparent from Figures 4, the recording film in the present embodiment is ferrimagnetic material thin film having a compensation point temperature of about 80°C and a curie point temperature of about 200°C. A recording domain of about 1.0 µm was prerecorded on such magneto-optical recording film, whereafter overwriting was effected with the pulse width of a laser varied at the same recording laser power. Figure 5 shows the relation between the recording pulse width obtained at that time and the residual domain diameter. The recording bias magnetic field at that time was about 1591 A/m (20 Oe) in the recording direction. The result is divided into three areas (A), (B) and (C), as shown in Figure 5. The area (A) in which the recording pulse width is 18 ns or less is an area in which light irradiation energy for erasing is little and the history domain cannot be erased. The area (B) in which the recording pulse width is 18-30 ns is an area of recording condition in which the history domain can be completely erased. The area (C) in which the recording pulse width is 30 ns or more is an area of recording condition in which a domain can be formed independently of the history domain, Figure 6 shows the relation between recording pulse width and recording domain diameter when there is no history recording domain. As is apparent from Figure 6, no domain is formed in an area (D) wherein the recording pulse width is 0-30 ns, and a domain is formed in an area (E) wherein the recording pulse width is 30 ns or more. From the results of Figures 5 and 6, it is seen that if the recording pulse width is set to 18 - 30 ns, there can be brought about a state in which no domain is formed, i.e., an erased state, after overwriting, irrespective of the history state. Also, if the recording pulse width is set to 30 ns or more, a domain will be formed after overwriting, irrespective of the history state, and recording can be effected. The overwriting utilizing the movement of the magnetic wall by the effect of the anti-magnetic field in Figures 5 and 6 is an overwriting technique using two phenomena, i.e., the erasing by light irradiation by low energy, and the formation of a domain by light irradiation of high energy. In Figures 5 and 6, the control of the low and high light irradiation energies is accomplished by changing the recording pulse width, but the control of light energy is possible even if the recording pulse width is made constant and the light energy, in other words, the crest value, is changed. An overwriting system utilizing a movement of the magnetic wall of the formed domain by an anti-magnetic field in the temperature rise and cooling process by the laser light irradiation as described above is described in Japanese Patent Application Laid-open No. 2-33749. This system utilizing the movement of the magnetic wall forms a recording domain by recording laser power determined by recording conditions and the pulse width thereof, with an extraneous magnetic field applied or a relatively weak extraneous magnetic field applied. The present invention is a magneto-optical recording method utilizing the above-described overwriting system utilizing the movement of the magnetic wall of the formed domain by the anti-magnetic field in the temperature rise and cooling process by laser light irradiation. Description will now be made of a magneto-optical recording apparatus used in the magneto-optical recording method of the present invention. In Figure 7, laser light 4 emitted from a laser source 3 is modulated by a recording signal made by a recording signal modulator 2. The laser light 4 which has left the laser source 3 passes through a collimator lens 5 and an objective lens 6 and irradiates a magneto-optical recording medium 7. At the same time, a magnetic head 8 provided in opposed relationship with an optical head with the recording medium interposed therebetween is driven by a driving circuit 9, and an extraneous magnetic field is applied to the recording medium. The reference numeral 1 designates a control unit for controlling the recording signal modulator 2 and the driving circuit 9. An embodiment of the magneto-optical recording method of the present invention will now be described with reference to Figure 8. In Figure 8, recording is effected so that the duty of the reproduction signal of the recording domain after overwriting and the reproduction signal in the erased state may be 50%. Figure 8(a) shows laser light applied to the recording medium, and the laser light is applied with the recording pulse width thereof changed over alternately to 50 ns and 25 ns. Figure 8(b) shows recording domains formed by the application of this laser light. In the present embodiment, recording was effected so that as shown in Figure 8, after overwriting, the recording domain length and the unrecorded domain length might be approximately 0.5 µm and approximately 0.5 µm, respectively, on a track having a recording land width of 1.0 µm. In this case, the recording pulse width of the laser irradiation when the recording domain is formed is 50 ns as shown in Figure 8(a), and as the result, a circular recording domain 1 having a diameter of approximately 1.0 µm is formed as shown in Figure 8(b). At this time, recording domains corresponding to the recording conditions of the area (C) of Figure 5 are formed. Next, as shown in Figure 8(a), laser light irradiation having a recording pulse width of 25 ns is effected immediately after the laser light having a recording pulse width of 50 ns. This corresponds to the area (B) of Figure 5, and effects erasing. By this laser light irradiation, as shown in Figure 8(b), a circular erasing portion 2 is formed in an area including a part of the recording domain 1. As the result, a crescent domain indicated by hatching in Figure 8(b) remains, and this remains as a recording domain. The domain width of the domain formed by such recording can be widely recorded up to the full recording land width, and the recording domain length does not decrease with a decrease in the recording domain width. Accordingly, the characteristic of the carrier level to the recording domain length becomes similar to that in the magnetic field modulation system indicated by the dotted curve in Figure 1, and as compared with the prior art, the reduction in the carrier level can be greatly suppressed. When recording domains are to be formed not in the continuous form as shown in Figure 8, but in spaced apart relationship with one another, light irradiation of 25 ns can be effected a necessary number of times after the light irradiation of 50 ns. Figure 9 shows another embodiment of the present invention. In this embodiment, the control of light energy was effected by keeping the pulse width constant and changing its crest value, instead of changing the pulse width of recording and erasing. That is, as shown in Figure 9(a), laser light of light power P w was applied and immediately after that, laser light of light power P e weaker than the light power P w was applied. By this application of laser light, circular recording domains 1 are formed as shown in Figure 9(b), and circular erasing portions 2 are formed. As the result, crescent domains are formed and an effect entirely similar to that of the previous embodiment is obtained. Figure 10 shows still another embodiment of the present invention. This embodiment is a modification of the embodiment of Figure 9, and is an embodiment in which as shown in Figure 10(a), a constant amount of light power P e for erasing is normally applied and light power P w for forming domains is superposed thereon. When the light power is controlled in this manner, the formation and erasing of recording domains are effected as shown in Figure 10(b), and as in the embodiments of Figures 8 and 9, crescent domains are formed. Accordingly, again in this embodiment, there can be obtained an effect entirely similar to that of the aforedescribed two embodiments. As described above, the magneto-optical recording method of the present invention is a method in which a first recording domain is formed by the application of light of predetermined energy, and after the application of said light, a portion of said first recording domain is erased by the application of light of energy smaller than said light energy to thereby form a crescent second recording domain. Also, in the above-described method, said first recording domain is formed over the substantially full recording land width. Also, in the above-described method, the control of the light energies of the light irradiation for forming said first recording domain and the light irradiation for effecting said erasing is accomplished by changing the pulse width of the light irradiation. Also, in the above-described method, the control of the light energies of the light irradiation for forming said first recording domain and the light irradiation for effecting said erasing is accomplished by changing the crest value of the pulse of the light irradiation. As described above, according to the present invention, there can be formed recording domains which are formed by light modulation recording and yet are free of a reduction in carrier level. Accordingly, there can be obtained a good reproduction signal and thus, there can be provided an apparatus of low error rate. There is no possibility of the head crash by a floating magnetic head or the like as is experienced in the magnetic field modulation system, and further any additional device such as a magnetic head is unnecessary and therefore, there can be realized a highly reliable, compact and simple light modulation overwriting apparatus.

Claims: A magneto-optical recording method wherein recording of information on a magneto-optical recording medium (7) is performed by applying modulated light to the recording medium (7) comprising the step of: applying a pulse of light of a predetermined energy to the recording medium (7) to form a first recording domain (1); the method being characterised in including the further step of: applying a pulse of light of less energy than said predetermined energy after said application of the pulse of light of predetermined energy so as to erase a portion of the first recording domain (1) to form a crescent shaped recording domain within the first recording domain (1). A method according to Claim 1, wherein said first recording domain (1) is formed by applying a first light pulse having a first pulse width to the magneto-optical recording medium (7), and said crescent shaped recording domain is formed by applying a second light pulse having a second pulse width different from the first pulse width to erase a portion of the first recording domain (1). A method according to Claim 2, wherein the second pulse width is smaller than the first pulse width. A method according to Claim 3, wherein the power of the first light pulse having the first pulse width is the same as the power of the second light pulse having the second pulse width. A method according to Claim 1, wherein the first recording domain (1) is formed by applying a first light pulse having a first pulse width to the magneto-optical recording medium (7), and the crescent shaped recording domain is formed by applying a second light pulse of a second pulse width, the crest of the second light pulse being different from the crest of the first light pulse, to erase a portion of the first recording domain (1). A method according to Claim 5, wherein the first pulse width is the same as the second pulse width. A method according to Claim 5, wherein the power of the first light pulse applied to the recording medium is greater than the power of the second light pulse applied to the recording medium (7). A method according to Claim 1, wherein the first recording domain (1) is formed by applying a first light pulse having a predetermined pulse width to the magneto-optical recording medium (7), and the recording domain having the crescent shape is formed by applying a second light pulse having a power smaller than that of the first light pulse to erase a portion of the first recording domain. A method according to any one of the preceding claims, wherein the first recording domain is formed over substantially the full width of a recording land on the magneto-optical recording medium (7). A magneto-optical recording apparatus for recording information on a recording medium (7) by application of modulated light pulses on the recording medium, the apparatus comprising means (2,3) for applying a light pulse of a predetermined energy to the recording medium (7) to form a first recording domain (1); the apparatus being characterised in further including means for applying a light pulse of less energy than said predetermined energy after the application of the light pulse of predetermined energy to form a crescent shaped recording domain within the first recording domain (1). A magneto-optical recording apparatus according to Claim 10, wherein said means for applying a light pulse of a first predetermined energy is arranged to provide a first light pulse having a first pulse width and the means for providing a light pulse of less energy is arranged to provide a second light pulse having a second pulse width different from the first pulse width. A magneto-optical recording apparatus according to Claim 11, wherein the second pulse width is smaller than the first pulse width. A magneto-optical recording apparatus according to Claim 12, wherein the power of the first light pulse having the first pulse width is the same as the power of the second light pulse having the second pulse width. A magneto-optical recording apparatus according to Claim 10, wherein the means for applying a first light pulse is arranged to apply a light pulse having a first pulse width to the magneto-optical recording medium (7) and the means for applying a second light pulse is arranged to apply a light pulse having a second pulse width, and a crest which is different from that of the light pulse of the first pulse width. An apparatus according to Claim 14, wherein the first pulse width is the same as the second pulse width. An apparatus according to Claim 12, wherein the means for applying the first light pulse is arranged to apply a light pulse having a power which is greater than the power of the second light pulse of the second pulse width. An apparatus according to Claim 10, wherein the means for applying the second light pulse is arranged to apply a light pulse of less power. An apparatus according to any one of Claims 10 to 17, wherein the first recording domain (1) is formed over substantially the full width of the recording land on the magneto-optical recording medium (7). A recording carrier comprising a magneto-optical recording medium including a plurality of crescent shaped magnetic recording domains each comprising the difference between a first circular domain (1) and a portion formed by a second partially overlapping circular region of smaller diameter than the first circular domain and displaced along the direction of recording.

Inventors: HASEGAWA KOYO, HASEGAWA, KOYO, HASEGAWA, KOYO, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: G11B11
IPC codes: G11B11