EQUIPMENT FOR MAGNETO-OPTIC READING AND WRITING

Abstract: Equipment for use with apparatus for magneto-optic reading and writing of information on an information-­ carrying medium with three ferromagnetic layers (3, 4, 10) comprises a housing (11), in which an initializing magnet (8) is disposed for magnetizing the second layer (4) of the information-carrying medium.

Description: BACKGROUND OF THE INVENTION This invention relates in general to a magneto-optic medium for reading and writing of information which comprise a substrate and three distinct layers magnetized uniformly in a direction parallel to the thickness of the layer. Examples of such media are described in EP-A-0217067, EP-A-0258978 and EP-A-0285241. More particularly the invention relates to equipment for magneto-optic reading and writing of information that enables direct overwriting of new information on old information stored on an information-carrying medium. Figures 5A and 5B illustrate the principle of the prior art apparatus for magneto-optic information reading and writing as shown in the Extended Abstracts (The 34th Spring Meeting, 1987): The Japan Society of Applied Physics and Related Societes, 28p-ZL-3. Figure 5A shows an oblique view of the apparatus and Figure 5B is a cross-sectional view showing the main parts. In these drawings a magneto-optic information-carrying medium 1, comprises a glass or plastic substrate 2 and two ferromagnetic 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. 7 designates regions in the first layer 3 in which the magnetic alignment points upward in Figure 5B, which represents binary data '1'. 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 as direction a). The essential components of the information-carrying medium 1 are those noted previously: a glass or plastic substrate 2 and two ferromagnetic layers, a first layer 3 and a second layer 4. The first layer 3, which is illuminated by 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 respectively, and let Hc1 and Hc2 be the coercive forces of first layer 3 and second layer 4. These parameters of the two layers satisfy the following relationships: Tc1 < Tc2 Hc1 > Hc2 Let us first consider the reading of information recorded in the recording layer, namely 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 focused on the recording layer and followed on an information-carrying track. As shown in Fig. 5B, 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 the "0" and "1" of a binary code. Magnetization in the up direction in the drawing represents a binary "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 read operation is equivalent to intensity a in Fig. 6. At this intensity the regions of the first layer 3 and second layer 4 illuminated by the focused spot 6 do not reach their Curie temperature Tc1 or Tc2, 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 Figs. 5A and 5B applies an external field Hini (indicated by arrow b in the drawing) to the information-carrying medium 1. The external field is related to the coercive forces of the first layer 3 and second layer 4 as follows: Hini < Hc1 Hini > Hc2 As the information-carrying medium 1 rotates in direction a in Fig. 5B, 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. 6. 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 portion of the first layer 3 illuminated by the focused spot 6 cools, it therefore acquires the upward magnetic alignment 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. 6. In this case the temperature in the region illuminated by the focused spot 6 rises until it exceeds the Curie temperature in the first layer 3 (Tc1) and the second layer 4 (Tc2), causing both layers to lose their magnetic alignment. Due to a weak external magnetic field Hb 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 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 direction. The intensity of the external bias field Hb is weak but within the range consistent with the above operation. The operations described above enable new information to be overwritten in real time on old information by modulation of the laser beam between intensities b and c in Fig. 6, which write the binary codes "1" and "0". When the magneto-optic information-carrying medium is structured as in the prior art described above, however, the initializing magnet must be located external to the housing of the information-carrying medium, in a space in the optical reading and writing apparatus. Consequently, the optical reading and writing apparatus must be complex in structure and large in size. Our European patent application 88 301 071.2 filed 9 February 1988 describes a modified form of information-carrying medium which has an additional pre-magnetized layer. SUMMARY OF THE INVENTION An object of this invention is to provide equipment for magneto-optic reading and writing information that is simple and compact and is particularly intended for use with an information-carrying medium comprising a substrate, a first layer, a second layer, and a third layer magnetized in a uniform direction parallel to the thickness of the layer. These layers are stacked on a substrate in the above order as seen from the side illuminated by a laser beam. The Curie temperature of the second layer is higher than the Curie temperature of the first layer but lower than the Curie temperature of the third layer, and the coercive force of the second layer is less than the coercive forces of the first layer and the third layer. In accordance with the invention the equipment comprises a housing for receiving the information-carrying medium; and magnetizing means disposed in the housing facing the information-carrying medium, for initializing the direction of magnetization of the second layer of the information-carrying medium in a designated direction. In this invention the bias magnetic field is generated by the third layer of the information-carrying medium, and the second layer is initialized by the magnetizing means e.g. an initializing magnet, built into the housing of the equipment. The invention thus yields a simple structure and a simple, compact optical reading and writing apparatus. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1A is an oblique view of a first embodiment of the present invention showing how the invenetion is used. Fig. 1B is a cross-sectional drawing of the first embodiment 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 drawing of the second embodiment indicating how information is written. Fig. 3 is a partial cross-sectional drawing showing a third embodiment of this invention. Fig. 4 is a partial cross-sectional drawing showing a fourth embodiment of this invention. Fig. 5A is an oblique view of prior art showing how the prior art is used. Fig. 5B is a cross-sectional drawing of prior art indicating how information is written. Fig. 6 indicates the intensity modulation of the laser beam. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of this invention will be described with reference to Figure 1 of the drawings. Elements 1 through 8 in Fig. 1A are the same as in the prior art. Element 10 is a third layer of ferromagnetic material in accordance with the aforementioned European patent application 88 301 071.2. The information-carrying medium 1 basically comprises a substrate 2 and three ferromagnetic layers 3, 4 and 10. Element 11 is a housing that encloses the information-carrying medium 1; in the drawing the housing is partly cut away. Magnetizing means in the form of an initializing magnet 8 is built into the housing 11 for receiving the information-carrying medium. Element 12 is a shutter which is mounted on the housing 11 so that it is free to slide, and is opened when the information-carrying medium 1 is used. This embodiment operates as follows. The information-carrying medium 1 is rotated in the direction of the arrow a in the drawing. The three ferromagnetic 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. Here, for example, each ferromagnetic layer possesses an axis of easy magnetization extending in the thickness direction of the layer. 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 temperature of the first layer 3, the second layer 4, and the third layer 10 respectively, and let Hc1, Hc2, and Hc3 be the coercive force of the first layer 3, the second layer 4, and the third layer 10. 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 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 focused on the recording surface and followed 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 when information is read is equivalent to intensity a in Fig. 6. 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 temperature Tc1, Tc2, or 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, which is built into the housing 11 as already described, applies 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 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 the 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. 6. 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 and the third layer 10, however, remain below their Curie temperatures Tc2 and Tc3, so the second layer 4 retains the upward magnetic alignment given it 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. When the portion of the first layer 3 which was illuminated by the focused spot 6 and raised above its Curie temperature Tc1 cools, it therefore acquires the magnetic alignment of the second layer 4. 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. 6. Due to the higher beam intensity, in this case the temperature in the region illuminated by the focused spot 6 rises until it exceeds the Curie temperature in the first layer 3 (Tc1) and the second layer (Tc2), causing both the first layer 3 and the second layer 4 to lose their magnetic alignment in the region illuminated by the focused spot 6. The temperature of the third layer 10 in the region illuminated by the focused spot 6, however, does not reach the Curie temperature Tc3 of that layer, so it retains its magnetic field Hb. The second layer 4 is therefore remagnetized 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] After 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 alignment of the first layer 3 and the third layer 10. [3] After 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. 6. 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 bias magnetic field Hb on the second layer 4 is constant; it also eliminates the need for an external bias magnet 9 as in the prior art. 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. Furthermore, since the initializing magnet 8 is built into the housing 11 of the information-carrying medium in this invention, there is no need to mount an initializing magnet inside the optical reading and writing apparatus. This gives a simple, uncomplicated structure and a compact optical reading and writing apparatus. A second embodiment of this invention is shown in Figs. 2A and 2B. In this embodiment the initializing magnet 8 is built into the housing 11 on the same side of the information-carrying medium 1 as the objective lens 5. This enables the apparatus to be made similarly compact. A third embodiment of this invention is shown in Fig. 3, which gives a cross-sectional view of the main parts of the housing 11. In this drawing the numeral 13 denotes a U-shaped yoke of a magnetic material. The yoke 13 is attached to the inside of the housing 11 as shown in the drawing, and the initializing magnet 8 is attached to the yoke 13. The initializing magnet 8 and yoke 13 form a closed magnetic loop. The information-carrying medium 1 is located in the gap of this closed magnetic loop. This configuration not only yields a simple, compact structure, but also increases and stabilizes the effect of the magnetic field Hini produced by the initializing magnet 8. A fourth embodiment of this invention is shown in Fig. 4, in which the information-carrying medium 1 is located between two initializing magnets 8a and 8b. This configuration similarly yields a simple, compact structure and increases and stabilizes the effect of the magnetic field Hini. In the above embodiments the initial magnetic field Hini pointed up and the bias magnetic field Hb pointed down, but the opposite arrangement, in which Hini points down and Hb points up, is also possible. The effect of the invention remains the same.

Claims: Equipment for magneto-optic reading and writing of information with a laser beam on an information-carrying medium (1) comprising a substrate (2) having a first layer (3), a second layer (4), and a third layer (10) magnetized in a uniform direction parallel to the thickness of the layer with the layers stacked on the substrate in the above order as seen from the side illuminated by the laser beam and the Curie temperature of the second layer being higher than the Curie temperature of the first layer but lower than the Curie temperature of the third layer, and the coercive force of the second layer being less than the coercive forces of the first layer and the third layer; said equipment comprising a housing (11) for receiving the information-carrying medium; and the magnetizing means (8) disposed in the housing facing the information-carrying medium, for initializing the direction of magnetization of the second layer of the information-carrying medium in a designated direction. Equipment according to claim 1, wherein the magnetizing means comprises an initializing magnet (8) for confronting the third layer. Equipment according to claim 1, wherein the magnetizing means comprises an initializing magnet (8) for confronting the substrate. Equipment according to claim 1, wherein the magnetizing means comprises a U-shaped yoke (13) disposed on both sides of the information-carrying medium and an initializing magnet (8) attached to the yoke facing the information-carrying medium. Equipment according to claim 1, wherein the magnetizing means comprises a U-shaped yoke (13) disposed on both sides of the information-carrying medium and two initializing magnets (8a, 8b) attached to the yoke facing the information-carrying medium.

Inventors: OKADA, KAZUO MITSUBISHI DENKI K.K., TAKOKORO, MICHIHIRO MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1988
Main classification: G11B11
IPC codes: G11B11