MAGNETO-OPTIC INFORMATION-CARRYING MEDIUM

Abstract: A magneto-optic information-carrying medium comprises three magnetic layers (3,4,10). Two of these layers (3,4) are accessed by a laser beam to read out or write in information while the third layer (10) is premagnetized to create a magnetic bias field without the need for external magnets.

Description: This invention relates to a magneto-optic information-carrying medium that enables direct overwriting of new information on old information. Figs. 3 A and 3B illustrate the principle of the prior art of magneto-optic information reading and writing as shown in " Single Beam Over Write Method Using Multilayered Magneto Optical Media," 28p-ZL-3, Extended Abstracts (The 34th Spring Meeting, 1987); The Japan Society of Applied Physics and Related Societies. Fig. 3A shows an oblique view; Fig. 3B is a cross-sectional view showing the main parts. The apparatus in these drawings comprises a magneto-optic information-carrying medium 1, which in turn comprises a glass or plastic substrate 2 and two ferromagnetic or ferrimagnetic layers: a first layer 3 and a second layer 4. The apparatus also comprises an objective lens 5 for focusing a laser beam onto the information-carrying medium 1, where it forms a focused spot 6. Reference numeral 7 designates a region on the first layer 3 where binary data " 1 " is recorded by magnetization in the upward direction in Fig. 3b. The apparatus also comprises two magnets: an initializing magnet 8 for creating an initial magnetic alignment in the second layer 4; and a bias magnet 9 located opposite the objective lens 5 on the other side of the information-carrying medium 1. This apparatus operates as follows. A support and drive mechanism not shown in the drawings turns the information-carrying medium 1 in a fixed direction (indicated by arrow a in the drawing). The first layer 3, which is positioned on the incident side of the laser beam, has properties similar to those of the recording layer of the information-carrying media used in ordinary magneto-optic discs, and also operates as a recording layer in the apparatus under discussion. The second layer 4, called the supplementary layer, is provided to enable overwriting; that is, to enable new data to be written over old data directly. Let Tc1 and Tc2 be the Curie temperatures of the first layer 3 and second layer 4, and let Hc1 and Hc2 to the coercive forces of first layer 3 and second layer 4, respectively. These parameters of the two layers satisfy the following relationships: Tc1 < Tc2 Hc1 > Hc2 Let us first consider the reading of information recorded on the recording layer, that is, the first layer 3. The objective lens 5 is driven by a driving mechanism not shown in the drawings in the direction of its optic axis and in a direction perpendicular to its optic axis (the radial direction of the information-carrying medium) so as to keep the spot 6 in focus and on the track on the recording layer. As show in Fig. 3B, the recording layer is magnetized parallel to its thickness direction. The magnetic alignment is either up or down in the drawing, these two directions corresponding to " 0 " and " 1 " of binary codes. Magnetization in the up direction in the drawing represents " 1. " When the information is read, the spot 6 is focused onto the first layer 3. The magnetic alignment of the first layer 3 is converted to optical information through a well-known magneto-optic effect (such as the Kerr effect); thus the information on the information-carrying medium 1 is detected. The intensity of the laser beam on the information-carrying medium 1 in this reading operation is equivalent to intensity in a Fig. 4. At this intensity the temperatures of regions of the first layer 3 and second layer 4 illuminated by the focused spot 6 do not reach their Curie temperatures Tc1 and Tc2, so the focused spot 6 do not erase the recorded information, that is, the magnetic alignment. Information is overwritten as follows. The information-carrying medium 1 is subject to an external magnetic field Hini (indicated by arrow b in the drawing) generated by initializing magnet 8 in Fig. 3B. The external magnetic field is related to the coercive forces of the first layer 3 and the second layer 4 as follows: Hini < Hc1 Hini > Hc2 As the information-carrying medium 1 rotates in direction a in Fig. 3B, when the second layer 4 passes over the initializing magnet 8 it is uniformly magnetized in the up direction, regardless of the magnetic alignment of the first layer 3. At room temperature the first layer 3 retains its existing magnetic alignment, being unaffected by the magnetic field generated by the initializing magnet 8 or the magnetic field generated by the magnetization of the second layer 4. To write the information " 1, " that is, to magnetize the first layer 3 in the up direction, the laser beam is modulated to an intensity equivalent to b in Fig. 4. The temperature of the region of the first layer 3 where the spot 6 is focused by the objective lens 5 then rises until it exceeds the Curie temperature Tc1 of the first layer 3, destroying the magnetic alignment of the first layer 3. The second layer 4, however, remains below its Curie temperature Tc2, so it retains the upward magnetic alignment given it by the initializing magnet 8. When the region of the first layer 3 illustrated by the focused spot 6 cools, it therefore acquires the upward magnetic alignment in accordance with that of the second layer 4. To write the information " 0, " that is, to magnetize the first layer 3 in the down direction, the laser beam is modulated to an intensity equivalent to c in Fig. 4. In this case the temperatures in the regions illuminated by the focused spot 6 on the first layer 3 and the second layer 4 rise until they exceed the Curie temperatures Tc1 and Tc2, respectively, causing both layers to lose their magnetic alignment. Due to a weak external magnetic field Hb, with magnetic alignment indicated by arrow c in the drawing, generated by the bias magnet 9 located opposite the objective lens 5 on the other side of the information-carrying medium 1, however, the second layer 4 is remagnetized in the direction of the magnetic field Hb, namely the down direction. When the first layer 3 cools, it acquires the downward magnetic alignment of the second layer 4. In this way the first layer 3 is magnetized in the down directions. The intensity of the external bias field Hb is weak but within the range consistent with the above operation. As described above, new information can be overwritten directly on old information by modulating the laser beam between intensities b and c in Fig. 4 in accordance with the binary codes " 1 " and " 0 ". When the magneto-optic information-carrying medium is structured as in the prior art described above, however, it is difficult to achieve a simple, compact apparatus configuration because the bias magnet must be placed in alignment with the objective lens on the opposite side of the information-carrying medium. US-A-4649519 describes an alternative magneto-optic storage medium comprising a substrate and three magnetic layers. No bias magnet is utilised but the medium relies on the reversal of the magnetic orientation of the bias layer at its compensation temperature. One drawback of this prior art is that to have each area of the medium magnetised in a particular direction, it is necessary to know the magnetic orientation of such an area before recording is to be performed. An object of the invention is to solve the problems of the prior art as stated above. According to the invention, and as is known from US-A-4649519 there is provided a magneto-optic storage medium comprising: a substrate, a first recording magneto-optic layer having a first Curie temperature (Tc1) and a first coercive force (Hc1); a second auxiliary magnetic layer having a second Curie temperature (Tc2) and a second coercive force (Hc2), a third bias magnetic layer having a third Curie temperature (Tc3) and a third coercive force (Hc3) The first, second and third layers are stacked on top of each other on the substrate which permits the transmission of a laser beam for heating the layers and thereby reading and writing information. In contrast to US-A-4649519 the present invention is characterised in that: i) the third magnetic layer is premagnetized to generate a magnetic field weaker than is necessary to reverse the direction of magnetization of the first layer and the second layer when at a temperature below their respective Curie temperatures (Tc1, Tc2), but sufficiently strong to magnetize said first layer and said second layer when said first and second layers have reached their respective Curie temperatures and to destroy the magnetic alignments therein; ii) the second layer has a weaker coercive force (Hc2) than said first layer and said third layer so that when an external initializing magnetic field Hini satisfying the relationships: Hc2 < Hini < Hc1 and Hini < Hc3 is applied to said storage medium in a predetermined direction, the second layer is magnetized in said predetermined direction of the external initializing magnetic field while the direction of the magnetic field of the first and third layers remain unchanged; iii) the first layer has a lower Curie temperature (Tc1) than said second and third layers so that when the layers are heated to a first temperature (T1) satisfying: Tc1 < T1 < Tc2, T1 < Tc3 then the first layer reaches its respective Curie temperature (Tc1), destroying the magnetic alignment therein, while said second and third layers do not reach their respective Curie temperatures (Tc2, Tc3) and hence maintain their magnetic alignments; and iv) the second layer has a lower Curie temperature (Tc2) than said third layer (Tc3) so that when the layers are heated to a second temperature (TO) satisfying the relationship: Tc2 < TO < Tc3 then said second layer reaches its Curie temperature (Tc2), destroying the magnetic alignment therein, while said third layer does not reach its respective Curie (Tc3) temperature and hence maintains its magnetic alignment therein. In a further embodiment, the magnetic field the second layer can be stronger than the magnetic field in the third layer so that when the layers are heated to said first temperature (T1), the first layer can be magnetized by the magnetic field in said second layer. When the second layer is allowed to cool and when the layers are heated to said second temperature (TO), said first layer and said second layer can be magnetized in the direction of the field of the third magnetic layer. It is preferable that the layers have axes susceptible to magnetization in directions perpendicular to the plane of the layers. It is also preferable that at least one of the first, second and third layers comprises a layer of ferromagnetic material. The third layer can be premagnetized in a direction opposite to the direction of the external initializing magnetic field. The third layer may be premagnetized either in the direction of the substrate or in an opposite direction. The provision of a third layer completely eliminates the need for an external bias magnet. The optical reading and writing apparatus can therefore be small in size and simple in structure, without the complexity of the configuration in which a bias magnet is mounted opposite the objective lens. In addition, the bias magnetic field has a uniform effect on the second layer, because the third layer that generates the bias field in the direction contact with the second layer. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1A is an oblique view of a first embodiment of the present invention showing how the invention is used. Fig. 1B is a cross-sectional view of the first embodiment of the present invention indicating how information is written. Fig. 2A is an oblique view of a second embodiment of the present invention showing how the invention is used. Fig. 2B is a cross-sectional view of the second embodiment of the present invention indicating how information is written. Fig. 3A is an oblique view of the prior art showing how the prior art is used. Fig. 3B is a cross-sectional view of the prior art indicating how information is written. Fig. 4 indicates the intensity modulation of the laser beam. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of this invention will now be described with reference to the drawings. Elements 1 through 8 in Figs. 1A and 1B are the same as in the prior art. The additional element is a third layer 10 of magnetic material. The information-carrying medium 1 basically comprises a substrate 2 and three ferromagnetic layers 3, 4 and 10. These layers have axes of easy magnetization in the direction parallel to the thickness thereof. This embodiment operates as follows. The information-carrying medium 1 is rotated in the direction of the arrow a in the drawing. The three magnetic layers of the information-carrying medium 1 are stacked so that, as seen from the side on which the laser beam is incident, the first layer 3 is on top, the second layer 4 is in the middle, and the third layer 10 is at the bottom. The first layer 3 possesses the same properties as the recording layer in the information-carrying media used in ordinary magneto-optic discs. Information is recorded in this layer. The second layer 4 and third layer 10, which are called supplementary layers, are provided to enable overwriting. Let Tc1, Tc2 and Tc3 be the Curie temperatures of the first layer 3, the second layer 4 and the third layer 10, and let Hc1, Hc2 and Hc3 be the coercive forces of the first layer 3, the second layer 4 and the third layer 10 respectively. These parameters of the three layers satisfy the following relationships: Tc1 < Tc2 < Tc3 Hc1 > Hc3 > Hc2, or Hc1 > Hc2 and Hc3 > Hc2 The third layer 10 is uniformly premagnetized preferably during fabrication of the medium, so that the resulting magnetic field lines of the magnetic field Hb generated in the third layer 10, point in the down direction (the direction of arrow c in Fig. 1B). The magnetic field Hb resulting from the magnetization of the third layer 10 is too weak to affect the magnetic alignment of the first layer 3 and second layer 4 at room temperature. Information recorded in the recording layer (the first layer 3) is read as follows. The objective lens 5 is driven as in the prior art to keep a spot 6 in focus on the recording surface and keep it on an information track. Information is then read from the information-carrying medium 1 by detecting the direction of magnetic alignment (parallel to the thickness of the layer; up or down in the drawing) of the first layer 3 by means of a magneto-optic effect. The intensity of the laser beam at the time of reading is equivalent to the intensity a in Fig. 4. At this intensity the regions of the first layer 3, the second layer 4 and the third layer 10 illuminated by the focused spot 6 do not reach their Curie temperatures Tc1, Tc2 and Tc3, so the focused spot 6 does not erase the recorded information by destroying the magnetic alignment. Information is overwritten as follows. The initializing magnet 8 in Fig. 1B generates an external field Hini (indicated by arrow b in the drawing). The external field Hini is related to the coercive forces of the first layer 3, the second layer 4 and the third layer 10 as follows: Hini < Hc1 Hini > Hc2 Hini < Hc3 Accordingly, as the information-carrying medium 1 rotates in direction a in Fig. 1B, when the second layer 4 passes over the initializing magnet 8 it is uniformly magnetized in the up direction in the drawing, regardless of the magnetic alignment of the first layer 3 and third layer 10. The magnetic field generated by the second layer 4 is weaker than the coercive force Hc1 of the first layer 3 and the coercive force Hc3 of the third layer 10, so at room temperature it does not affect the magnetic alignment of the first layer 3 and the third layer 10. To write the information " 1, " that is, to magnetize the first layer 3 in the up direction, the laser beam is modulated to an intensity equivalent to b in Fig. 4. The temperature of the region of the first layer 3 where the spot 6 is focused by the objective lens 5 then rises until it exceeds the Curie temperature Tc1 of the first layer 3, destroying the magnetic alignment of the first layer 3. The temperatures of second layer 4 and third layer 10, however, remain below their Curie temperatures Tc2 and Tc3, so the second layer 4 retains the upward magnetic alignment by the initializing magnet 8 and the third layer 10 retains its original down alignment. The first layer 3 is subject to the magnetic fields generated by both the second layer 4 and the third layer 10, but as stated previously, the magnetic field Hb of the third layer 10, which is aligned in the downward direction of the arrow c, is weak; the magnetic field of the second layer 4, which is aligned in the upward direction of the arrow b, has a stronger effect on the first layer 3. Therefore, the region of the first layer 3, which was illuminated by the focused spot 6 and the temperature of which raised above its Curie temperature Tc1, acquires the magnetic alignment of the second layer 4 at the time of cooling. The result is that the first layer 3 is magnetized in the up direction. To write the information " 0, " that is, to magnetize the first layer 3 in the down direction, the laser beam is modulated to an intensity equivalent to c in Fig. 4. Due to the higher beam intensity, in this case the temperatures in the regions illuminated by the focused spot 6 on the first layer 3 and the second layer 4 rise until they exceed the Curie temperatures Tc1 and Tc2, causing both the first layer 3 and the second layer 4 to lose their magnetic alignments in the region illuminated by the focused spot 6. The temperature in the region illuminated by the focused spot 6 on the third layer 10, however, does not reach the Curie temperature Tc3 of that layer, causing the third layer 10 to retain the magnetic field Hb. The second layer 4 is therefore remagnetized in reverse in the down direction by the magnetic field Hb. When the first layer 3 cools, it acquires the downward magnetic alignment of the second layer 4. As a result the first layer 3 is magnetized in the down direction. To summarize the above explanation: [1] When the Curie temperature Tc1 is exceeded, the magnetic alignment of the second layer 4 is transferred to the first layer 3. [2] At room temperature, the second layer 4 is uniformly magnetized in the up direction by the magnetic field Hini of the initializing magnet 8, regardless of the magnetic alignments of the first layer 3 and the third layer 10. [3] When the Curie temperature Tc2 is exceeded, the magnetic field Hb of the third layer 10 magnetizes the second layer 4 in the down direction. Information can thus be overwritten in the manner explained above by modulating the intensity of the laser beam between intensities b and c in Fig. 4. The provision of a third layer 10 in the information-carrying medium 1 of this invention and its uniform premagnetization in the down direction as described above not only ensures that the effect of the magnetic bias field Hb on the second layer 4 is constant; but also eliminates the need for an external bias magnet 9. The optical reading and writing apparatus can therefore be small in size and simple in structure, without the complexity of the configuration in which a bias magnet 9 is mounted in a position opposite the objective lens 5. Another embodiment of this invention is shown in Figs. 2A and 2B. In this embodiment the initializing magnet 8 is placed on the same side of the information-carrying medium 1 as the objective lens 5. This enables the apparatus to be made still more compact. In the above embodiments the initializing magnetic field Hini pointed up and the magnetic bias field Hb pointed down(in the direction of the substrate 2), but the opposite arrangement, in which Hini points down and Hb points up(in the direction opposite the substrate 2), is also possible. The effect of the invention remains the same.

Claims: A magneto-optic storage medium comprising: a substrate (2), a first recording magneto-optic layer (3) having a first Curie temperature (Tc1) and a first coercive force (Hc1); a second auxiliary magnetic layer (4) having a second Curie temperature (Tc2) and a second coercive force (Hc2), a third bias magnetic layer (10) having a third Curie temperature (Tc3) and a third coercive force (Hc3); the first, second and third layers (3, 4, 10) being stacked on each other on the substrate (2) which permits transmission of a laser beam for heating the layers and thereby reading and writing information; characterized in that: said third magnetic layer (10) is premagnetized to generate a magnetic field weaker than is necessary to reverse the direction of magnetization of said first layer (3) and said second layer (4) at a temperature below their respective Curie temperatures (Tc1, Tc2) but sufficiently strong enough to magnetize said first layer (3) and said second layer (4) when said first and second layers (3, 4) have reached their respective Curie temperatures to destroy the magnetic alignments therein; said second layer (4) has a weaker coercive force (Hc2) than said first layer (3) and said third layer (10) so that when an external initializing magnetic field Hini satisfying the relationships: Hc2 < Hini < Hc1 and Hini < Hc3 is applied to said medium in a predetermined direction, said second layer (4) is magnetized in said predetermined direction of said external initializing magnetic field while said first and third layers (3, 10) remain unchanged; said first layer (3) has a lower Curie temperature (Tc1) than said second and third layers (4, 10) so that when the layers are heated to a first temperature (T1) satisfying: Tc1 < T1 < Tc2, T1 < Tc3 said first layer (3) reaches its respective Curie temperature (Tc1) to destroy the magnetic alignment therein, while said second and third layers (4, 10) do not reach their respective Curie temperatures (Tc2, Tc3) to maintain the magnetic alignment therein; and said second layer (4) has a lower Curie temperature (Tc2) than said third layer (Tc3) so that when the layers are heated to a second temperature (TO) satisfying the relationship: Tc2 < TO < Tc3 then said second layer (4) reaches its Curie temperature (Tc2) to destroy the magnetic alignment therein, while said third layer (10) does not reach its respective Curie (Tc3) temperature to maintain the magnetic alignment therein. A medium according to claim 1 wherein the magnetic field in said second layer (4) is stronger than the magnetic field in said third layer (10) whereby when the layers (3, 4, 10) are heated to said first temperature (T1), said first layer (3) is magnetized by the magnetic field in said second layer (4) upon said second layer (4) being allowed to cool and when the layers (3, 4, 10) are heated to said second temperature (TO) said first layer (3) and said second layer (4) are magnetized in the direction of the field in said third magnetic layer (10). A medium according to claim 1, wherein the magnetic field in said second layer (4) is stronger than the magnetic field in said third layer (10) whereby when the layers are illuminated with a laser beam of a first intensity to heat the layers to the first temperature (T1) said first layer (3) is magnetized by the magnetic field in said second layer (4) upon said second layer being allowed to cool and when the layers are illuminated with a laser beam of a second intensity to heat the layers to the second temperature (TO) first layer (3) and said second layer (4) are magnetized in the direction of the magnetic field in said third layer (10). A medium according to claim 1, 2 or 3 wherein said layers (3, 4, 10) have axes of easy magnetization in the direction of the thickness thereof. A medium according to any one of claims 1 to 4 wherein at least one of said first, second and third layers (3, 4, 10) comprises a layer of ferromagnetic material. A medium according to any one of claims 1 to 5 wherein said third layer (10) is premagnetized in a direction opposite to said predetermined direction of said external initializing magnetic field. A medium according to claim 1, wherein the third layer (10) is premagnetized in the direction of the substrate (2). A storage medium according to claim 1, wherein the third layer (10) is premagnetized in the direction opposite the substrate.

Inventors: IMAI, HITOSHI MITSUBISHI DENKI KABUSHIKI, OKADA, KAZUO MITSUBISHI DENKI KABUSHIKI, TADOKORO, MICHIHIRO MITSUBISHI DENKI KABUSHIKI
Applicants: MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1988
Main classification: G11B13
IPC codes: G11B11