APPARATUS FOR MAGNETO-OPTICALLY RECORDING, REPRODUCING AND ERASING DATA

Abstract: Herein disclosed is an apparatus for recording, repro­ ducing and erasing data or information on a magneto-optical recording medium, which comprises a means for applying steady magnetic field to the record medium and a means for applying variable magnetic field, these means for applying magnetic field being integrated into a record head and these means and the record head being able to be disposed at the different sides of the record medium, this results in the miniaturization of the means for applying magnetic field to the record medium and the apparatus presents an excellent recording/erasure property.

Description: The present invention relates to an apparatus for magneto-optically recording, reproducing and erasing data on a magneto-optical recording medium with utilizing the laser beam. When treating various kind of data or information, record, display and reproduction are the essential technique for treating data or information and thus, many kinds of techniques therefor have been developed. For instance, there may be mentioned such an image tube, a solid image element, sensor array or the like as the means for reading. As the display means, there may be mentioned such as a display, for instance, a cathode ray tube, optical projector, and plasma displays and a printer, for instance, an electrostatic printer and an ink jet printer. While, as the recording means, there may be mentioned such as a magnetic disc, a tape, a drum, a magnetic bubble element and a semiconductor memory. These means for reading, recording and displaying have substantially been improved in their degree of resolution, the speed of record, reproduction and reading or the like due to the development of the laser technique which provides a strong and fine light beam and thus, many machines and devices utilizing such laser technique have been developed. Among others, an optical memory material capable of rewriting information or data is interested in and particularly many studies to develop magneto-optical recording materials have been done in recent years. When recording and reproducing information on an magneto-optical disc, a static magnetic field is applied to the surface of the disc, on which a crystalline or amorphous layer of magnetic materials is deposited and acts as a record medium, to induce a perpendicular magnetized layer on the disc, the magnetization of which is oriented to the same direction due to the static magnetic field applied. Then, information or data is recorded on the surface by irradiating laser spot while applying a weak magnetic field to a desired position of the record medium having a direction of magnetization opposite to that of the previously magnetized layer. At this stage, the temperature of the layer partially rises over Curie point by the irradiation of laser beam and as a result the magnetization at that point is reversed and the record of information or data is completed. The information recorded on the magneto-optical recording disc may be reproduced by irradiating laser beam to the surface layer of the magnet-optical disc, detecting the degree of rotation of the plane polarized due to Kerr effect or that polarized due to Faraday effect of the recording medium through an analyzer disposed in the light path, as the strength of transmitted or reflected light. The record medium generally comprises a disc of plastics or glass as the substrate and an amorphous alloy layer composed of a rare earth metal such as gadolinium (Gd), terbium (Tb), dysprosium (Dy) and holmium (Ho) and a transition metal such as iron (Fe), cobalt (Co) and nickel (Ni), for instance, Gd-Fe, Gd-Co, Tb-Fe, Tb-Gd-Fe. Such magneto-optical materials must fulfill the following requirements: (i) it must be a material such that the magnetization may easily be reversed at a relatively low temperature to improve the sensitivity of the record medium during recording procedure; (ii) it must have a rather high Kerr rotational angle to increase the S/N ratio of the reproduction signals; (iii) on the surface thereof, a perpendicular magnetized layer may be formed, which makes it possible to record information in more higher record density. In order to carry out the magneto-optical record of information or data, it is necessary to change the direction of the magnetic field to be applied to the disc at the time of recording and erasing information and therefore, as the means for applying magnetic field, an electromagnet, a permanent magnet or an air-core coil is generally used. However, since the strength of magnetic field required for recording information usually differs from that of magnetic field required for erasing information recorded, it is necessary to change not only the direction of the current supplied to the electromagnet and the air-core coil but the strength thereof to switch stage from recording to erasing and vice versa. The performance of the electromagnet and the air-core coil is generally defined by their maximum values of the generated magnetic field requirements and as a result, a large-sized electromagnet and air-core coil are needed as well as the use of a power supply having a complicated construction is needed. Furthermore, if a permanent magnet is used, the apparatus for recording, reproducing and erasing information or data must be provided with a driving mechanism to move the magnet and to reverse the magnetic field and this makes the construction of the apparatus quite complicated. EP-A-0.153.676 discloses an apparatus for recording, reproducing and erasing data on a thin film of magnetic material. The film is irradiated with a laser beam to enable changes to be made to an initial direction of magnetization, thereby to write and/or erase information. The laser beam is focussed by an optical system controlled by an actuator. Included in the actuator is a coil whose leakage field is constant and is applied to the film in the direction of its initial magnetization. A second coil applies a magnetic field in the direction of initial magnetization, during erasing, or in the reverse direction, during recording. Under such circumstances, the inventor of this application has eagerly studied on such apparatus for recording, reproducing and erasing information to solve the problems accompanied by the conventional apparatuses and found that these problems may be solved by dividing the means for applying magnetic field to a record medium into two different elements and integrating them into a record head of the apparatus. A principal purpose of this invention is to provide an apparatus for recording, reproducing and erasing information or data utilizing laser beam, which makes it possible to solve the problems accompanied by the conventional apparatus. Another purpose of this invention is to provide a new apparatus for recording, reproducing and erasing information or data using laser beam, capable of applying a magnetic field required to record and erase information and having a simple construction and a miniaturized means for applying magnetic field. According to the invention there is provided an apparatus for recording, reproducing and erasing data or information in which a thin film of an amorphous magnetic material of a rare earth-transition alloy having a perpendicular magnetic anisotropy and supported on a substrate is used as a Curie- or compensation-temperature recording medium and a laser beam is used as the light source for recording, reproducing and erasing data, the apparatus including an actuator for a lens system driven by a coil, and first means for applying a constant magnetic field to the recording medium in the direction of its initial magnetization and being arranged independently of said actuator, characterised in that the apparatus includes second means also arranged independently from the coil of the actuator, and the second means is used to apply a magnetic field to the recording medium either in the direction of its initial magnetization during erasing or in the direction of the reverse magnetization during recording such that the resultant magnetic field is applied to a limited local area in the neighbourhood of an area where the laser beam impinges on the recording medium so as to allow erasing or recording. The apparatus according to the present invention is now explained in more detail with referring to the accompanying drawings in which: Fig. 1 (a) shows an oblique diagram of part of a conventional apparatus for recording, reproducing and erasing information or data and (b) is the same diagram as (a) showing part of another conventional apparatus; Fig. 2 (a) shows an oblique diagram of a part of a preferred embodiment of the apparatus according to the present invention, Fig. 2 (b) shows a front view of another preferred embodiment of the present invention, Fig. 2 (c) shows the same diagram as (b) of the third preferred embodiment of the present invention and Fig. 2 (d) shows a vertical sectional view of the apparatus shown in Fig. 2 (a); Fig. 4 shows a graph which represents the recording property of the magneto-optical disc used in the present invention when data are recorded utilizing the apparatus according to the present invention; and Fig. 5 is a graph which shows the re-recording property of the magneto-optical disc when applying the apparatus according to the present invention. Referring now to Fig. 1 (a) and (b), two different type of conventional apparatuses, in particular a record disc and a magnetic record head, are disclosed. The apparatus shown in Fig. 1 (a) comprises a means for driving a record disc 1 such as a motor, a magnetic record head 2 which serves to record, reproduce and erase data using laser beam, a means for applying magnetic field, in this case, a permanent magnet 3 being used, which may also be an electromagnet or an air-core coil. The head 2 is provided with an actuator 4 for condenser lens system and focusing the laser beam 5 to a desired region to record, reproduce or erase data. The head 2 is movably supported by a support which provides for radial movement of the head. The radial movement provides for positioning the head 2 over the desired data tracks and permit the random access. According to the apparatus shown in Fig. 1 (a), the record of data on the magnetic record medium is carried out, for instance, by heating the part on which data are recorded, to a temperature higher than the Curie point using laser beam to extinguish magnetization thereon and cooling the portion below the Curie point while applying external magnetic field thereto to change the direction of magnetization at that portion. The record of information or data is also possible by utilizing the fact that ferrimagnetic material has a low coercive force around the compensating temperature thereof i.e., by partially heating the record medium having a compensating temperature near the room temperature and reversing its magnetization by applying an external magnetic field higher than the coercive force of the record medium to record data. On the other hand, the data recorded may be reproduced by irradiating light beam to a desired portion of the record medium and detecting the strength of the transmitted light or reflected light. In this connection, the polarized plane of the transmitted or reflected light is rotated to right or left direction according to the magnetization direction of the recorded portion. Thus, if a polarizer is disposed so that the rotational axis thereof is perpendicular to the rotational direction of the polarized plane, only the recorded pit having a desired direction of magnetization can be detected. In addition, the erasion of data recorded may be carried out by repeating the recording procedure except that the direction of the external magnetic field to be applied is reversed. In Fig. 1 (b), another conventional apparatus for recording, reproducing and erasing data is shown. In this apparatus, an air-core coil 11 is used instead of a permanent magnet 3 [see Fig. 1 (a)]. In this case, the coil 11 is integrated into the head 2 while the magnet 3 and the head 2 are disposed on the opposite sides of the disc 6 [see Fig. 1 (a)]. The other elements of the apparatus are the same as those disclosed in Fig. 1 (a) and therefore, we omit the explanation thereof by assigning the same reference number as in Fig. 1 (a). However, a conventional apparatus typically represented by these two examples shown in Figs. 1 (a) and 1 (b) has a lot of problems to be solved and remains a sufficient room for improvement. As already mentioned above, the direction of the external magnetic field applied during recording data on the record medium differs from that of the magnetic field applied during erasing the data recorded and therefore, it is needed to change the direction of the external magnetic field when switching the stage from recording to erasing. For that purpose, it is necessary to change not only the direction of electric current to be supplied to the electromagnet or the air-core coil used, but the strength thereof. Furthermore, an electromagnet or an air-core coil having a large size must be used since a means for applying magnetic field to the record medium having a high maximum value of the magnetic field requirements is effectively used in the magneto-optical recording apparatus and moreover this results in the complicated construction of the power supply. While, as to the permanent magnet, it must be provided with a driving mechanism to move the magnet and to reverse the direction of the magnetic field. The reversion of the direction of the magnetic field is needed and carried out when data recorded are erased or data are re-recorded after the erasion of the data previously recorded. This also makes the structure of the apparatus more complicated. However, according to an apparatus of the present invention explained below in more concretely, referring to the accompanying drawings which show different preferred embodiments of this invention, these disadvantages of the conventional apparatus can effectively be eliminated. In addition, the features of the invention will become apparent to those skilled in the art as the disclosure is made in the following detailed description of preferred embodiment of this invention. Fig. 2 (a) shows a preferred embodiment of the present invention and the apparatus comprises a magnetic record head 21, a means 22 for driving a disc 23, a means 24 for applying steady magnetic field to the disc surface, which may be a permanent magnet, an air-core coil or an electromagnet and disposed on the head 21 at its extremity, a means 25 for applying variable magnetic field to the disc surface, which comprised of an air-core coil or an electromagnet and disposed below the magnet 24 and an actuator 26 disposed between the head 21 and the magnet 24. In this apparatus, a laser beam 27 is also used to record, reproduce and erase data on the disc 23. The laser beam is adjusted by the actuator 26 for the condenser lens incorporated therein. The apparatus according to the present invention is characterized in that the magnets 24 and 25 are integrated into the head 21. The magnets 24 and 25 integrated into the head 21 may be disposed on the same side of the record disc 23 [see Fig. 2 (a)] or on the different sides of the record disc 23 [see Fig. 2 (b) and (c)]. Referring now to Fig. 2 (d) which presents a vertical sectional view of the mechanism driving the means for applying magnetic field to the record medium shown in Fig. 2 (a), the driving mechanism comprises a support 31 of the magneto-optical head, the magnets 24 and 25 being engaged in or with the support 31 in this order and the support being provided with a condenser lens system 32 in its inner portion and the lens system being supported by a driving mechanism 33 therefor. According to the lens system 32 and the driving mechanism therefor, a laser beam may be shifted and focused on the desired position of the record medium 23 on which data are recorded, reproduced or erased. The record is practically effected on the magneto-optical recording layer 34. In the embodiment, the condenser lens system 32 is supported and driven by an actuator 35 for driving the lens system 32, such as a voice coil. Furthermore, the permanent magnet 24 is mated with the outer surface of the support 31 so that the distance between the magnet 24 and the lower surface of the record medium 23 can be adjusted. In general, the distance between the upper surface of the magnet 25 (or, in some cases, the magnet 24) and the lower surface of the record medium 23 varies depending on the focal length of the lens system 32 used. For example, if the lens having a focal length of 2 to 3 mm, the distance falls within the range of from 0.2 mm to 2 mm, most preferably about 1 mm. If the distance is less than the lower limit of 0.2 mm, the magnet 25 is eventually contact with the surface of the record medium and as a result, the latter causes crush in the worst case, while if the distance is more than the upper limit of 2 mm, it cannot be assured to maintain a sufficient sensitivity in recording, reproducing and erasing data on the record medium. The means 25 for applying a variable magnetic field to the record disc 34 may, for example, be controlled according to a control system shown in Fig. 3 (a), which comprises a pulse generator 41, a device 42 for reversing the polarity of the pulse generated by the generator 41 and a driving device 43 for the coil 25, the device 42 for reversing the polarity of the pulse being provided with a switch 44 which serves to change the recording state to the erasing state and vice versa, while the driving device 43 for the coil being provided with a variable resistance 45 capable of establishing a desired level of the coil driving current. The pulse generator 41 of the control system generates a pulse having an extremely high pulse repetition frequency and synchronized with the disc rotation, based on a pulse 46 synchronized with the disc rotation and inputted to the generator 41 so that the record of a desired pit density may be attained at a given disc rotational velocity. The output of the pulse generator 41 is inputted to the device 42 for reversing the polarity of the pulse inputted. The device 42 is controlled by the switch 44 which serves to switch between the record stage and erasion stage and outputs a pulse having a desired polarity. The driving device 43 receiving the pulse outputted by the device 42 is triggered with a recording/erasing trigger pulse separately inputted at 47 and outputs an amplified pulse. The current level outputted from the driving device 43 is adjusted by the variable resistance 45 attached to the device 43. Thus, a pulse for driving the coil is outputted from the device 43 and the pulse makes it possible to synchronize with the rotation of the disc and to erase data or record data in a desired pit density. In addition, the polarity of the pulse for driving the coil is controlled by the change-over switch 44 and the strength of the pulse for driving the coil is adjusted by the variable resistance 45. While, referring to Fig. 3 (b), a control system for the means 24 for applying steady magnetic field to the record medium is described as a block diagram. If the means 24 is an electromagnet or an air-core coil, a steady current source 51 is provided for the system and the output thereof is inputted to the electromagnet. A variable resistance 52 is provided with the steady current source 51 and controls the current outputted from the source 51. Therefore, it is possible to supply a desired strength of current to the electromagnet or the air-core coil which constitutes the means for applying steady magnetic field to the record medium, due to the control of the variable resistance 52. Moreover, the steady magnetic field can be maintained at a desired strength since the strength of the magnetic field generated by the electromagnet or the air-core coil is proportional to the strength of the current supplied thereto. In the apparatus according to the present invention, the record medium 23 comprises a rather thick substrate and an amorphous thin film of a magnetic material having the perpendicular magnetic anisotropy. Preferred examples of such a magnetic material are an alloy of a rare earth metal and a transition metal, for example, GdFe, GdCo, TbFe, TbGdFe, DyFe. While as the material for the substrate, there may be mentioned such as glass or a plastic disc. The record, reproduction and erasure of data on the disc may be carried out according to the same procedures as those already disclosed herein. As the laser beam used to carry out these procedures, there may be mentioned such as gas lasers, for instance, argon laser, krypton laser, helium-cadmium laser, helium-neon laser, or a semiconductor laser, for example, CSP (channelled-substrate planar) laser. Thus, according to the apparatus of this invention, it is possible to achieve the following effects: (i) the size of the means for applying variable magnetic field can be minimized, since the means for applying magnetic field to the record medium is divided into two different member i.e., a means for applying steady magnetic field and a means for applying variable magnetic field; (ii) the inductance of the means for applying magnetic field is quite low because of the miniaturization of the means for applying variable magnetic field and as a result the velocity of response is also high enough to carry out the magneto-optical recording, reproducing and erasing data or information and it may be possible to record or erase data on every sector unit; (iii) it is possible to simplify the structure of the current supply for the means for applying variable magnetic field; (iv) the apparatus may be applied to a various kinds of magneto-optical discs having a different kind of record and erasion properties by controlling the magnetic field generated from the means for applying steady magnetic field to the record medium and adjusting the peak value of the pulse current supplied to the means for applying variable magnetic field to the medium (such us an air-core coil). Example Using an apparatus according to the present invention having the construction disclosed in Fig. 2 (a), record and erasion test was carried out on a record medium composed of a thin magnetic film of TbFe having the perpendicular magnetic anisotropy. In this example, the air-core coil used as the means for applying variable magnetic field was consisted of a covered copper wire having a diameter of 0.1 mm and the coil has the length of 0.5 mm, the outer diameter of 6.0 mm, the inner diameter of 2.0 mm and the number of turn thereof is 100. This coil provides a magnetic field of 20.67 amperes/metre (260 Oe) at the position of 0.5 mm apart from the extremity of the coil. The magneto-optical disc used was comprised of a plastic substrate having a guide groove (0.8 µm in width) and a thickness of 1.2 mm and a record layer of TbFe film deposited on the substrate by the low temperature sputtering technique. The coercive force of the TbFe film was 159 amperes/metre (2 KOe) at room temperature. Data were recorded on the record disc mentioned above by the use of the apparatus according to the present invention and the resulting record property of the disc is shown in Fig. 4 in which the abscissa represents the applied magnetic field during recording, the left hand side thereof being the direction of initial magnetization and the right hand side thereof being the direction of reversed magnetization and the ordinate is the regenerative C/N during the recording procedure. As seen from the results of Fig. 4, the regenerative C/N is saturated if the magnetic field applied is higher than 1.59 amperes/metre (20 Oe) in the direction of reversed magnetization. While Fig. 5 represents the result of the measurement on re-recording property with respect to the strength of the magnetic field applied during erasion procedure. In Fig. 5, the abscissa is the strength of magnetic field applied during erasing and the ordinate stands for the regenerative C/N during re-recording procedure. According to the result shown in Fig. 5, it is found that the regenerative C/N during re-recording is saturated and that the same level of record property as that observed during the initial recording procedure is obtained when the strength of the magnetic field applied is higher than 21.46 amperes/metre (270 Oe) in the direction of initial magnetization. As is clear from the results shown in Figs. 4 and 5, the strength of the magnetic field required to record data is at least 1.59 amperes/metre (20 Oe) and that required to erase data is at least 21.46 amperes/metre (270 Oe). Then, the position of the head on which the permanent magnet is disposed and the magnetization condition of the permanent magnet which serves as the means for applying steady magnetic field on the disc were adjusted so that 10.33 amperes/metre (130 Oe) of magnetic field can always be applied to the disc surface in the direction of the initial magnetization. Moreover, by supplying + 0.58A of pulse current to the air-core coil. Thus, 1.59 amperes/metre (20 Oe) of magnetic field can be applied to the record layer of the magneto-optical disc in the direction of the reversed magnetization during recording, while 22.26 amperes/metre (280 Oe) of magnetic field can be applied to the record layer in the direction of the initial magnetization during erasing. These magnetic field having the strength mentioned above were found to be applied to the desired position of the record layer, situated at 0.5 mm apart from the extremity of the air-core coil. Thus, we confirmed that the apparatus according to the present invention provides an excellent recording and erasion property when data or information are recorded on a magneto-optical recording medium having the perpendicular magnetic anisotropy. In the light of the above teachings for instance, a permanent magnet is used in the example, as the means for applying steady magnetic field, however, an electromagnet or an air-core coil which are operated under a constant current can also be used effectively and furthermore, the magnets may be disposed within the record head or the means for applying variable magnetic field may coaxially be disposed in the means for applying steady magnetic field.

Claims: An apparatus for recording, reproducing and erasing data or information in which a thin film of an amorphous magnetic material of a rare earth-transition alloy having a perpendicular magnetic anisotropy and supported on a substrate is used as a Curie- or compensation-temperature recording medium (34) and a laser beam (27) is used as the light source for recording, reproducing and erasing data, the apparatus including an actuator (26) for a lens system (32) driven by a coil (35), and first means (24) for applying a constant magnetic field to the recording medium (34) in the direction of its initial magnetization and being arranged independently of said actuator (26), characterised in that the apparatus includes second means (25) also arranged independently from the coil (35) of the actuator (26), and the second means (25) is used to apply a magnetic field to the recording medium (34) either in the direction of its initial magnetization during erasing or in the direction of the reverse magnetization during recording such that the resultant magnetic field is applied to a limited local area in the neighbourhood of an area where the laser beam (27) impinges on the recording medium so as to allow erasing or recording. An apparatus as set forth in claim 1 wherein said first means (24) for applying a constant magnetic field to the medium is a member selected from the group consisting of a permanent magnet and an electromagnet and an air-core coil operated under a constant current condition. An apparatus as set forth in claim 1 or 2, in which said second means (25) for applying variable magnetic field to the record medium is an electromagnet or an air-core coil. An apparatus as set forth in claim 1 in which the said first means for applying constant magnetic field and the said second means (25) for applying a variable magnetic field to the record medium are a permanent magnet and an electromagnet or an air-core coil respectively. An apparatus as set forth in claim 4 in which the permanent magnet and the electromagnet or the air-core coil are disposed at the different sides of the record medium. An apparatus as set forth in claim 4 wherein the permanent magnet and the electromagnet or the air-care coil are disposed on the same side of the record medium. An apparatus as set forth in claim 1 wherein the said second means (25) for applying variable magnetic field is controlled by a control system comprising a pulse generator (41) which generates a pulse having a high pulse repetition frequency and synchronized with the rotation of the disc, a device (42) for reversing the polarity of the pulse generated by the pulse generator (41) and a coil driving device (43), and the device for reversing the polarity of the pulse being provided with a switch (44) capable of switching between the recording stage and the erasing stage, and the coil driving device being provided with a variable resistance (45) capable of establishing a level of coil driving current. An apparatus as set forth in claim 1 wherein the said first means (24) for applying steady magnetic field is an electromagnet or an air-core coil and controlled by a control system comprising a variable resistance.

Inventors: OKADA, MITSUYA
Applicants: NEC CORPORATION
Application year: 1985
Main classification: G11B13
IPC codes: G11B11, G11B5