MAGNETO-OPTIC MEMORY DEVICE

Abstract: A magneto-optic memory device comprises a recording medium layer (18) sandwiched between two dielectric layers (17,19), with one of the dielectric layers (17) between the recording medium layer (18) and a substrate (16), and a reflective layer (20) provided on the other dielectric layer (19). The two dielectric layers (17,19) are made of a nitride material thus inhibiting oxidation of the recording medium (18) and thereby stabilizing coercive force characteristics and information recording characteristics of the recording medium.

Description: The present invention relates to a magneto-optic memory device on and from which information can be recorded, reproduced and erased using light waves such as laser beams. In recent years much effort has been put into studying and developing optical memory devices as their potential capability for high storage density and capacity has been appreciated. In particular, much attention has been given to those optical memory devices in which additional information can be recorded by the user (the recorded information being non-erasable) and those optical memory devices in which information can be recorded and erased by the user. Various recording mediums and optical memory systems have been proposed for such memory arrangements. The known recording mediums for the former optical memory devices include TeOx, TeSe, TeC, etc. Among the known recording mediums for the latter optical memory devices are GdTbFe, GdTbDyFe, TbFe, etc. Most of such recording mediums are poor in their resistance to corrosion and efforts which have heretoforce been made to solve this problem have been mostly unsuccessful. The difficulties which arise when the optical memory mediums suffer corrosion or oxidation will be described. The inventor has studied optical memory devices (so-called "magneto-optic memory devices") having a memory medium made of a rare earth/transition metal alloy and in which information can be recorded and erased by the user. FIG. 1 shows in detail the arrangement of one such conventional magneto-optic memory device. The illustrated magneto-optic memory device includes a glass substrate 1, a thin film 2 (recording medium) of amorphous alloy of GdTbFe having a thickness in the range from 100 to 200 x 10⁻¹⁰m (100 to 200 Å) deposited on the glass substrate 1 by sputtering, a film 3 of SiO₂ (transparent dielectric film) having a thickness ranging from 300 to 400 x 10⁻¹⁰m (300 to 400 Å) deposited on the thin film 2 by sputtering, and a film 4 of Cu (reflecting film) having a thickness ranging from 300 to 500 x 10⁻¹⁰m (300 to 500 Å) deposited on the film 3 by sputtering. The characteristics of the magneto-optic memory device of the above construction have been measured by an optical system shown in FIG. 2 to determine the degree of dependence of a magneto-optic rotational angle on the magnetic field. As shown in FIG. 2, a monochromatic light beam such as a laser beam emitted from a light source 5 is changed by a polarizer 6 into a linearly polarized light beam, which passes through a half-mirror 7 and falls on a magneto-optic memory device 8 perpendicularly to its surface. The light beam enters the recording medium 2 of the magneto-optic memory device 8 through the glass substrate. A light beam reflected from the magneto-optic memory device 8 is deflected by the half-mirror 7 into a light detector 9 which detects the angle through which the plane of polarization of light is rotated. The magnetic field is rendered variable by an electromagnet 10. FIG. 3 illustrates the results of such measurement of the degree of dependence of the magneto-optic rotational angle on the magnetic field, obtained using the illustrated optical system. The graph of FIG. 3 has a horizontal axis indicating the intensity of a magnetic field H and a vertical axis indicating a Kerr rotational angle ϑk. Designated at Hc is the value of a coercive (magnetic force with which magnetization is inverted). The value of the coercive force is quite important in the magneto-optic memory device. If the coercive force were too large, an excessive amount of heat would be required for recording information, and no information could be recorded with a small-size laser such as a semiconductor laser. The magnetic field which would have to be applied for recording information would be excessively large. Conversely, if the coercive force were too small, recorded information would tend to be erased by a relatively small increase in the external temperature or the external magnetic field. As the coercive force of the recording medium varies with time, the values of the temperature and magnetic field necessary for information recording also vary in general. Therefore, it is desirable to keep the coercive force constant. Where the recording medium of the above magneto-optic memory device is made of an alloy of rare earth elements and transition metal, the coercive force of the recording medium varies widely with the composition ratio of rare earth elements. FIG. 4 is a graph showing the relationship between the composition ratio of rare earth elements (Gd, Tb) and the coercive force in a thin film of amorphous alloy of GdTbFe. The graph has a horizontal axis indicative of an area ratio of the rare earth elements placed on an iron target upon sputtering. The elements Gd, Tb are equal in their amounts. FIG. 4 shows that the percentage of the rare elements in the thin film amorphous alloy of GdTbFe is about 26. 3% at a compensation point of a room temperature. The magneto-optic effect (the Kerr rotational angle as shown in FIG. 3) is changed at the point where the percentage of the rare earth elements is about 26.3%. Where the percentage of the rare earth elements is greater than 26.3%, that is, the rare earth elements are relatively rich, the magneto-optic effect is progressively greater at the left in FIG. 4. Where the percentage of the rare earth elements is smaller than 26.3%, that is, the iron is relatively rich, the magneto-optic effect is progressively greater at the right in FIG. 4. The inventor prepared a magneto-optic memory device having a recording medium of a composition as shown at the point A in FIG. 4. The prepared magneto-optic memory device was left at 70°C for 42 hours to study its reliability. The composition of the recording medium changed to that shown at the point B where the coercive force Hc is about +2.4 x 10⁵ amp/m (3K Oe), in a manner as indicated by the arrowheads. This is assumed to be brought about by the fact that the rare earth elements (Gd, Tb) in the film of GdTbFe amorphous alloy are oxidized by oxygen which separates from the adjacent dielectric SiO₂ film, and the oxidized rare earth elements no longer contribute to the magnetic characteristics. As described above, the construction of the conventional magneto-optic memory devices fail to prevent a time-dependent change in the composition of the recording medium due to oxidation, and hence is incapable of ensuring a constant coercive force. Accordingly, the conventional magneto-optic memory devices cannot record information stably. In our published French patent application No. 2485241, we describe a magneto-optic memory medium in which a layer of an optical memory recording medium comprising an amorphous rare earth/transition metal alloy is formed on a transparent substrate, a dielectric layer being overlaid on this recording layer, and a reflector layer being overlaid on the dielectric layer. In use, the dielectric layer acts to prevent heat escaping from the recording medium layer into the reflector layer, the latter serving to increase the Kerr rotation angle. Further increase of the Kerr rotation angle can be obtained by providing a further dielectric layer between the substrate and the recording medium layer. This prior art does not recognise the aforementioned problem of deterioration of the opto-magnetic characteristics of the recording medium due to oxidation and accordingly does not teach the selection of materials for the dielectric layer or layers specifically to avoid this problem. IBM Technical Disclosure Bulletin, Vol. 16, No. 5, October 1973, page 1442 discloses the use of antireflection coatings for the enhancement of magneto-optic effects in amorphous rare earth-transition metal alloys. Although this reference mentions the requirement that such an antireflection coating should not degrade the magnetic properties of the magnetic film, it does not disclose that oxidation of the rare earth-transition metal alloy is a likely cause of degradation. The range of preferred materials given in this prior art document for the antireflection coating, several of them including oxygen, clearly indicates the absence of any recognition of the cause of the problem which is addressed by the present invention. Published European patent application 0062075 discusses the problem of oxidation by atmospheric oxygen of an optical recording medium of the type on which information can be recorded using a laser beam to form pits in the recording layer surface. The particular material disclosed for the recording layer is tellurium and the problem is alleviated by including carbon in the recording layer. The recording layer may be divided into two sub-layers, the carbon content being in only one of them. In one arrangement the Te layer is sandwiched between a Te-C layer and a Te-O layer. Since this prior art is concerned only with recording by melt-deformation in the recording layer the problem of oxidation of a magneto-optical memory recording medium by oxygen from an adjacent dielectric layer is not recognized. Accordingly, the present invention aims to provide an improved magneto-optic memory device having a recording medium protected from oxidization for providing a stabilized coercive force and stable information recording characteristics. According to the present invention there is provided a magneto-optical memory device having a layer of an optical memory recording medium comprising an amorphous rare earth/transition metal alloy sandwiched between a first dielectric layer and a second dielectric layer, characterised in that each of said dielectric layers is made of a nitride material and has substantially no oxygen content, thereby substantially to prevent selective oxidation of the rare earth component of said rare earth/transition metal alloy. The first dielectric layer may be disposed between the optical memory recording medium layer and a transparent substrate of the device. This transparent substrate is preferably made of a material selected from the group comprising glass, polycarbonate, acrylic resin and epoxy resin. The first dielectric layer is preferably at least 100 x 10⁻¹⁰m (100Å) thick, thereby to prevent oxygen from the transparent substrate from reaching the optical memory recording medium layer. Preferably, each of the dielectric layers is made from a material selected from the group consisting of the nitrides of aluminium and silicon. An example of a rare earth/transition metal alloy used for the optical memory recording medium layer is GdTbFe. Some embodiments of the invention will now be described by way of example with reference to Figures 5 to 9 of the accompanying drawings. In the drawings:- FIG. 1 is a fragmentary cross-sectional view of a known magneto-optic memory device; FIG. 2 is a schematic view of a measurement optical system; FIG. 3 is a graph showing the degree of dependence of a magneto-optic rotational angle on applied magnetic fields; FIG. 4 is a graph showing the relationship between a composition ratio of rare earth elements in a thin film GdTbFe amorphous alloy and a coercive force; FIG. 5 is a fragmentary cross-sectional view of a magneto-optic memory device which is not in itself an embodiment of the invention but is described hereinafter to facilitate a full understanding of the present invention; FIG. 6 is a graph showing the result of a storage test on the magneto-optic memory device illustrated in FIG. 5; FIG. 7 is a fragmentary cross-sectional view of a magneto-optic memory device which again is not in itself an embodiment of the invention but is described hereinafter to facilitate a full understanding of the invention; FIG. 8 is a fragmentary cross-sectional view of a magneto-optic memory device according to an embodiment of the invention; and FIG. 9 is a graph showing the result of an Auger electron spectroscopy of the magneto-optic memory device shown in FIG. 8. The magneto-optic memory device of Figure 5 which, as mentioned above, is not in itself an embodiment of the invention includes a thin film 12 (serving as a recording medium) made of GdTbFe amorphous alloy having a thickness in the range of from about 150 to about 200 x 10⁻¹⁰m (150 to 200Å) deposited on a glass substrate 11 by sputtering. On the GdTbFe film 12, there is deposited a film 13 (serving as a transparent dielectric film) made of AlN (aluminium nitride) having a thickness ranging from about 400 to about 500 x 10⁻¹⁰m (400 to 500Å) by reactive sputtering of aluminium in nitrogen gas. A film 14 (serving as a reflecting film) of stainless steel (such as SUS 304) having a thickness ranging from about 500 to about 600 x 10⁻¹⁰m (500 to 600 Å) is deposited on the AlN film 13 by sputtering. The inventor prepared four magneto-optic memory devices of the foregoing construction, and conducted a storage test on the memory devices at a temperature of 70°C. FIG. 6 illustrates the result of such a storage test, in which test results for the four memory devices are indicated by the marks o, x, ·, ▵, respectively. The mark ▵ is superimposed on the mark · in some instances. As shown in FIG. 6, the coercive force becomes slightly larger by about 3.2 x 10⁴ amp/m (0.4 kOe) than an initial coercive force (in the range of from about 1.4 to about 1.7 x 10⁵ amp/m (1.8 to about 2.2 kOe) after about 100 days of the storage test of the magneto-optic memory device. The increase in the coercive force is extremely small as compared with that in magneto-optic memory devices of a conventional structure. This is because the transparent dielectric film of aluminium nitride has no oxygen content in itself while conventional transparent dielectric films of SiO₂ have an oxygen content. Stated otherwise, when the transparent dielectric film is formed by way of reactive sputtering in a nitrogen gas using an aluminium target, there is no tendency for oxygen to enter the recording medium at the time the film is deposited. Although it may be thought that the transparent dielectric film may be made of other materials with no oxygen content such as MgF₂, ZnS, CeF₃, AIF₃ 3NaF, the use of such materials may present a problem. More particularly, most targets comprising these materials are porous, so that if these materials are employed to fabricate the transparent dielectric film by conventional sputtering, oxygen and water penetrating into the pores in the materials are liable to oxidize the recording medium during sputtering. However, where the transparent dielectric film is made of aluminium nitride, the target used is of aluminium only and is advantageous from the standpoint of material savings. The aluminium target is not porous and hence does not trap therein oxygen and water. Due to its structural features, aluminium nitride can form very tight films which are impermeable to oxygen and water, a property which further inhibits oxidation of the recording medium. It is highly advantageous to make the reflecting film 14 in the magneto-optic memory device from stainless steel. The advantages of making the reflecting film of stainless steel will be described below. (1) Corrosion resistance: As is well known, stainless steel has excellent properties of resistance to corrosion. For example, no pinholes will be formed in a reflecting film of stainless steel when a fingerprint is put on the film, while a minute pinhole will be formed in a reflecting film of Cu at some later time after a fingerprint is impressed on the film. (In an experiment, no pinhole was formed in a reflecting film of Ni). The excellent corrosion resistance of stainless steel can contribute to the corrosion resistance capability of the recording medium. (2) Thermal conductivity: Stainless steel has a poor thermal conductivity as compared with Cu, Au, Ag, Al. For this reason, when the recording medium is heated by a laser beam irradiated thereon, the heat conductivity is reduced and the laser beam energy required for information recording is reduced. Aluminium nitride of which the transparent dielectric film is made has relatively good thermal conductivity and hence allows much heat escape. Thus, while the transparent dielectric film is formed of aluminium nitride, the use of stainless steel for the reflecting film in combination therewith is particularly suitable. (3) Easy Film formation: Since the sputtering of stainless steel is relatively easy, a stainless steel film can be easily made, and this is advantageous for fabrication purposes. While in the above-described device the substrate 11 is formed of glass, it may be in the form of a transparent substrate of polycarbonate, acrylic resin, or epoxy resin, for example. The dielectric film may be formed of silicon nitride instead of aluminium nitride. The reflecting film 14 may be formed of Ni, Ti, or TiN instead of stainless steel. Adaptation of the above-described device by way of insertion of another transparent dielectric film with no oxygen content between the substrate and the recording medium film produces a structure according to the present invention. FIG. 7 is illustrative, in fragmentary cross section, of a magneto-optic memory device which again is not in itself an embodiment of the invention but which may readily be adapted as described later to produce another embodiment of the present invention. The illustrated magneto-optic memory device includes a thin film 12 (serving as a recording medium) made of GdTbFe amorphous alloy having a thickness in the range of from about 150 to about 200 x 10⁻¹⁰m (150 to 200 Å) deposited on a glass substrate 11 by sputtering. On the GdTbFe film 12, there is deposited a film 13 (serving as a transparent dielectric film) made of AlN (aliminium nitride) having a thickness ranging from about 400 to about 500 x 10⁻¹⁰m (400 to 500 Å) by reactive sputtering of aluminium in a nitrogen gas. A film 15 (serving as a reflecting film) of Ti (titanium) or TiN (titanium nitride) is deposited on the AlN film 13 by sputtering. The reflecting film of titanium or titanium nitride exhibits the following advantages: Where the dielectric film is made of aluminium nitride, it is difficult to cover the dielectric film with a conventional reflecting film of Cu, Al, or the like with a high reflectivity in a laser wavelength range (about 800 nm). This difficulty is thought to be due to the fact that the effect of nitrogen in the aluminium nitride on Cu or Al is significant. However, the reflecting film of titanium or titanium nitride can easily be deposited on the transparent dielectric film of aluminium nitride with a high reflectivity in the laser wavelength range. This allows a large amount of light to be reflected from the reflecting film, resulting in an improved reproduced signal. Another advantage is that if a reflecting film of titanium or titanium nitride is deposited by way of sputtering, the target used is made of Ti, which is relatively cheap and easily available. Adaptation of the magneto-optic memory device described with reference to Figure 7 by way of inclusion of a transparent dielectric film of aluminium nitride between the glass substrate 11 and the thin film 12 of GdTbFe amorphous alloy, so that the latter film can be sandwiched between two transparent dielectric films of aluminium nitride, produces another embodiment of the present invention. The glass substrate may be replaced with a substrate of polycarbonate, acrylic resin, epoxy resin or the like. FIG. 8 shows a magneto-optical memory device according to still another embodiment of the present invention. A film 17 of aluminium nitride (serving as a first transparent dielectric film) is deposited on a transparent substrate 16 made as of glass, polycarbonate, acrylic resin or the like. On the film 17, there is formed a thin film 18 of rare earth/transition metal alloy such as GdTbFe, TbDyFe, GdTbDyFe, TbFe, GdFeCo, GdCo, or of such alloys with a content of Sn, Zn, Si, Bi, B or the like. In addition, a transparent film 19 of aluminium nitride (serving as a second transparent dielectric film) is deposited on the thin film 18. A reflecting film 20 of Cu, Ag, Al, Au or the like is formed on the transparent film 19. The magneto-optic memory device of the above structure is subjected to an Auger electron spectroscopy. FIG. 9 shows the result of such an Auger electron spectroscopy, indicating Auger electron intensities in Al, Fe, O of the three films, that is, the film of aluminium nitride, the film of GdTbFe, and the film of aluminium nitride. As illustrated, no oxygen is contained in the GdTbFe film, and a slight amount of oxygen is present in the interface between the surface and the film of aluminium nitride and the interface between the film of aluminium nitride and the transparent substrate. This indicates that there is oxygen entered into the film of aluminium nitride from the exterior and into the film of aluminium nitride from the glass substrate after the films have been fabricated. This experimental result shows that the sandwich structure of the thin film 18 of rare earth/transition metal alloy between the films of aluminium nitride is effective to prevent the thin film 16 from being oxidized during the process of fabricating the films. Instead of aluminium nitride, the transparent dielectric films may be formed of Si₃N₄ (silicon nitride) by way of reactive sputtering in a nitrogen gas using an Si target. The first transparent dielectric film in the above magneto-optical memory device should be of a thickness of at least 100 x 10⁻¹⁰m (100 Å) for the reason which follows: When the first transparent dielectric film is deposited on the glass substrate, oxygen in the glass substrate enters the first transparent dielectric film to the depth of about 50 x 10⁻¹⁰m (50 Å). If the thickness of the first transparent dielectric film were smaller than 100 x 10⁻¹⁰m (100 Å), it would be possible for oxygen to find its way into the thin film of rare earth/transition metal alloy when the latter is being sputtered on the first transparent dielectric film. The second transparent dielectric film is deposited mainly for the purposes of increasing the magneto-optic rotational angle and improving the quality of reproduced signals. The reflecting film 20 in the magneto-optic memory device of FIG. 8 may be covered with a film of an easily oxidizable metal such as Ti, Mg, a rare earth metal (Gd, Tb, Dy, Ho, Y, etc), or an alloy of a rare earth element and transition metal (GdTbFe, TbDyFe, GdCo, GdTbDyFe, etc) to provide a complete device construction which prevents entry of oxygen from the exterior. The film of aluminium nitride serving as a transparent dielectric film may be formed by deposition or other fabrication processes. The thicknesses of the films in the magneto-optic memory device are not limited to those described above in the illustrated embodiments. Reference is hereby directed to European Patent No. 0111988 from which this patent is divided, and to European Patents Nos. 0319636 and 0316508 which are also divided from European Patent No. 0111988. Reference is also directed to copending European Patent Application No. 92110340.4 which is divided from this patent.

Claims: A magneto-optical memory device having a layer of an optical memory recording medium (18) comprising an amorphous rare earth/transition metal alloy sandwiched between a first dielectric layer (17) and a second dielectric layer (19), characterised in that each of said dielectric layers is made of a nitride material and has substantially no oxygen content, thereby substantially to prevent selective oxidation of the rare earth component of said rare earth/transition metal alloy. A magneto-optical memory device according to claim 1, wherein said first dielectric layer (17) is disposed between said optical memory recording medium layer (18) and a transparent substrate (16) of the device. A magneto-optical memory device according to claim 2, wherein said transparent substrate (16) is made of a material selected from the group comprising glass, polycarbonate, acrylic resin and epoxy resin. A magneto-optical memory device according to claim 2 or claim 3, wherein said first dielectric layer (17) is at least 100 x 10⁻¹⁰m (100Å) thick, thereby to prevent oxygen from the transparent substrate (16) from reaching the optical memory recording medium layer. A magneto-optical memory device according to any preceding claim, wherein each of said dielectric layers is made from a material selected from the group consisting of the nitrides of aluminium and silicon. A magneto-optical memory device according to any preceding claim wherein said rare earth/transition metal alloy is GdTbFe.

Inventors: HIROKANE, JUNJI, KATAYAMA, HIROYUKI, OHTA, KENJI, TAKAHASHI, AKIRA, YAMAOKA, HIDEYOSHI
Applicants: SHARP KABUSHIKI KAISHA
Application year: 1983
Main classification: G11B11
IPC codes: G11B7, C23C14, G11B11