Optical memory element

Abstract: An optical memory element contains a first reflector film made of a material of sufficiently large light reflectance in the relative sense and a second reflector film made of a material of sufficiently small thermal conductivity in the relative sense.

Description: The present invention relates to an optical memory element which optically records, reproduces and/or erases information. It has recently been a trend in the field of information processing technology that greater magnetic memory capacity is demanded as information is diversified and increases in volume. To meet this demand, an optical recording system using an optical recording medium has been developed and put into practise, replacing the conventional recording system. However, an erasable and reloadable optical disc for practical use has not yet been realised but is expected to be developed imminently. There are various kinds of erasable and reloadable optical memories: one with a magneto-optical medium, based on the so-called magnetic Kerr effect, one based on the photo-darkening phenomenon of amorphous chalcogenide and one based on the light reflectance variation effected by a phase change in the recording medium. The most promising amongst these is the system using the magneto-optical medium. Some optical magnetic media use rare earth metal-transition metal amorphous alloys. They are, however, defective in that the Kerr rotation angle, which is vital for the signal reading performance, is still small. A number of methods have been tried for increasing the magnetic force-rotation angle of a magneto-optical medium. One of these is to use multi-layer film having the reflecting film structure thereby increasing the apparent Kerr rotation angle and improving the signal quality. In this method, part of the incident light is reflected by the magnetic film due to the Kerr effect. The rest goes through the magnetic film and is reflected back into the magnetic film by the reflector film on the other side of the magnetic film due to the Faraday effect in the magnetic film. The combination of the Faraday effect and the Kerr effect causes the apparent Kerr rotation angle of the total incident light to increase, thus raising the C/N ratio. Fig. 2 is a structural drawing of a known magneto-optical disc, in which 1 is a glass or plastic plate, 2 is an AℓN film, 3 is a GdTbFe film, 4 is an Aℓ N film and 5 is an Aℓ reflector film. In the magneto-optical disc of the above structure, the reflector film desirably has as large a reflectance as possible. Therefore, Aℓ, Cu, Au or Ag which has high reflectance in the semiconductor laser wavelength range, is used as reflector film material. These elements, however, provide large thermal conductivity in general and therefore impair the recording sensitivity of the magneto-optical disc; since heat absorption by the reflector film in recording by laser is large due to the high thermal conductivity of Aℓ, larger laser power is required for recording, which inversely affects the recording sensitivity of the magneto-optical disc. In view of the above, it is an aim of the present invention to provide an optical recording medium which alleviates some of the aforementioned problems. Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific samples, while indicating preferred embodiments of the invention, are given by way of illustration only. In accordance with one aspect of the present invention there is provided a magneto-optical memory element which includes a film of optical recording medium and a plural-layer reflector film comprising a first layer of relatively high optical reflectivity and a second layer of relatively low thermal conductivity, characterised in that the reflector film is disposed with its first layer closest to the optical recording medium film. The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention and wherein: Fig. 1 is a sectional view of a magneto-optical recording medium of the present invention; Fig. 2 is a sectional view of the conventional magneto-optical recording medium; and Fig. 3 is a graph showing Kerr rotation angle, reflectance and ellipticity for various Aℓ film thicknesses. Detailed Description of the Invention As an embodiment of an optical memory element of the present invention,a magneto-optical disc is described below in detail with reference to the drawings. Fig. 1 shows the structure of the magneto-optical disc of the present invention. Referring to Fig. 1, the magneto-optical disc comprises about 800Å thick AℓN film 2 as a first dielectric layer, about 200Å thick GdTbFe film 3 as a magneto-optical recording medium, about 250Å thick AℓN film 4 as a second dielectric layer, about 100Å thick Aℓ film 6 as a first reflector film and about 500Å thick Ni film 7 as a second reflector film all of which are formed in this order on a transparent substrate 1 made of a glass or plastic plate, whereby the apparent Kerr rotation angle is increased without deteriorating the recording sensitivity. Using Aℓ with sufficiently large light reflectance in relative sense as the first layer of the reflector film, and Ni with sufficiently small thermal conductivity in relative sense as the second layer of the reflector film, the recording medium of the present invention can provide high reflectance and recording sensitivity that could not be attained by the conventional reflector film of single metal layer construction. Fig. 3 shows changes in the Kerr rotation angle, reflectance and ellipticity of the above-mentioned two-layer reflector film with the Aℓ film thickness varying in the range between 0 and 200Å and with the Ni film thickness fixed at about 500Å. The graph shows that as the Aℓ film thickness increases, optical effect of the Ni layer reduces, approaching that of an Aℓ-single layer film. At about 100Å thickness of Aℓ film, the two-layer reflector film presents Kerr rotation angle of 1.1 degree, reflectance of 17% and ellipticity of 0.02, which are almost the same level as those of the Aℓ single layer film. Since the Aℓ layer which causes recording sensitivity deterioration is only 100Å thick, the Ni layer whose thermal conductivity is not higher than a half that of Aℓ can constitute substantial portion of the reflector film. The reflector film first layer made of Cu, Au, Ag or the like metal of high reflectance in combination with the second layer made of Th, Ta, Pb, Zr, Bi, Te, Se or like metal of low thermal conductivity can provide the same effect as the above embodiment. Since low thermal conductivity can be achieved by adding Ni to Aℓ, the second layer of the reflector film may be made of two-element alloy such as AℓNi instead of single element of Ni. The first layer of the reflector film may be any material of high reflectance. The second layer of the reflector film may be any material of low thermal conductivity in place of the above metal. As understood from the above, the primary object of the present invention is to provide an optical recording medium with good signal reproduction characteristic and high recording sensitivity by employing the two-layer reflector film in the optical memory. As appreciated from the above description, the reflector film of the present invention is superior to the conventional single-layer reflector film in the point that it can improve the recording sensitivity of the optical disc without sacrificing the light reflectance. The transparent substrate 1 may be made of polycarbonate, acrylic or epoxy resin. The optical magnetic recording medium 3 may be made of any material selected from rare earth metal-transition metal alloys; GdTbFeCo, TbFe and TbFeCo are the examples. The first and second dielectric films 2 and 4 may be made of SiN, ZnN, SiO or SiO 2 . The thickness of each layer is not limited to those presented in the above embodiment. Due to the low thermal conductivity of the material used for the second layer of the reflector film, the present invention minimizes heat loss in recording by laser, thus saving laser power.

Claims: A magneto-optical memory element which includes a film (3) of optical recording medium and a plural-layer reflector film (6, 7) comprising a first layer (6) of relatively high optical reflectivity and a second layer (7) of relatively low thermal conductivity, characterised in that the reflector film (6, 7) is disposed with its first layer (6) closest to the optical recording medium film (3). An optical memory element according to claim 1 characterised in that said first layer (6) of the reflector film is of relatively high thermal conductivity and in that said second layer (7) of the reflector film is of relatively low optical reflectivity. An optical memory element according to claim 1 or 2 characterised by the position of a dielectric film (2) between a light-transmissive substrate (1) of the element and the optical recording medium film (3) and/or a dielectric film (4) between the optical recording medium film (3) and the reflector film (6, 7). An optical memory element according to any of claims 1 to 3 characterised in that said first layer comprises a metal of relatively high reflectance. An optical memory element according to claim 4 characterised in that said metal of the first layer is selected from the group comprising Cu, Al, Au and Ag. An optical memory element according to any of the preceding claims characterised in that the second layer comprises Ni, Th, Ta, Pb, Zr, Bi, Te, Se or an alloy of low thermal conductivity, such as AlNi.

Inventors: AKIRA TAKAHASHI, HIROYUKI KATAYAMA, KENJI OHTA, TOMOYUKI MIYAKE, YOSHITERU MURAKAMI, AKIRA, TAKAHASHI, HIROYUKI, KATAYAMA, KENJI, OHTA, TOMOYUKI, MIYAKE, YOSHITERU, MURAKAMI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1987
Main classification: G11B7
IPC codes: G11B11, G11B13, G11B7