Abstract:
A magneto-optic memory device includes a transparent dielectric film of a non-oxygen containing nitride material deposited by sputtering on a substrate such as glass, a rare-earth/transition metal alloy recording medium deposited by sputtering on the dielectric film and a reflecting film deposited over the recording medium with or without an intervening transparent dielectric film of non-oxygen containing nitride material. This arrangement prevents the recording medium from being oxidized, and stablizes coercive force characteristics and information recording characteristics of the recording medium.

Description:
This application is a divisional of application Ser. No. 08/055,911, filed on Apr. 30, 1993, which was a continuation of Ser. No. 07/730,528 filed on Jul. 15, 1991, which was a continuation of Ser. No. 07/218,259 filed Jul. 13, 1988, which was a divisional of Ser. No. 07/068,775 filed Jun. 29, 1987, which was a continuation of Ser. No. 06/697,027 filed on Jan. 31, 1985, which was a continuation of Ser. No. 06/489,889 filed Apr. 29, 1983; the entire contents of which are hereby incorporated by reference, all now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a magneto-optic memory device capable of recording, reproducing and erasing information thereon with the use of light waves such as laser beams. 
     Recent years have seen various efforts to develop optical memory devices since they are expected to be a good memory candidate because of their higher storage density and capacity. Particularly, much attention has been given to those optical memory devices in which additional information can be recorded by the user (the recorded information is not 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 which have heretofore been made to solve this problem. 
     Difficulties experienced with optical memory mediums when the latter are corroded or oxdized will be described. The Inventors have studied optical memory devices (so-called &#34;magneto-optic memory devices&#34;) 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 of from 100 to 200 Å deposited on the glass substrate 1 by sputtering, a film 3 of SiO 2  (transparent dielectric film) having a thickness ranging from 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 Å deposited on the film 3 by sputtering. 
     The magneto-optic memory device of the above construction has been measured by an optical system shown in FIG. 2 for the degree of dependency of a magneto-optic rotational angle on a 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 arranged perpendicularly to its surface. The light beam enters the recording medium 2 of the magneto-optic memory device 8 through the glass substrate 1. 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 an angle at which the plane of polarization of light is rotated. The magnetic field is rendered variable by an electromagnet 10. 
     FIG. 3 illustrates a measurement of the degree of dependency of the magneto-optic rotational angle on the magnetic field as measured by 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 force (magnetic force at which magnetization is inverted). The value of the coercive force is quite important in the magneto-optic memory device. If the coercive force is 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 applied to record information would have to be excessively great. Conversely, if the coercive force is too small the, 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. 
     When the recording medium of the above magneto-optic memory device is made of an alloy of rare earth elements and transition metals, 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 of the thin film of an 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, and Tb are equal in their amounts. FIG. 4 shows that the percentage of the rare earth elements in the thin film of an amorphous alloy of GdTbFe is about 26.3% at a compensation point of 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%. When 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 of the respective curve in FIG. 4. When 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 of the respective curve in FIG. 4. 
     Applicants have prepared a magneto-optic memory device having a recording medium of a composition as shown at point A in FIG. 4. The prepared magneto-optic memory device has been left at 70° C. for 42 hours to study its reliability. The recording medium changes to a recording medium having a composition as shown at point B where the coercive force Hc is 3K Oe, in a pattern as indicated by the arrowheads. This is considered to be brought about by the fact that the rare earth elements (Gd, Tb) in the film of the GdTbFe amorphous alloy are oxidized by oxygen separated from the SiO 2  film adjacent to the film of GdTbFe amorphous alloy, and the oxidized amount of rare earth elements no longer accounts for the magnetic characteristics. 
     As described above, the conventional magneto-optic memory devices fail to prevent a time-dependent change in the recording medium due to oxidization, and hence are incapable of ensuring a constant coercive force. 
     Accordingly, the conventional magneto-optic memory devices cannot record information in a stable manner. 
     SUMMARY OF THE INVENTION 
     With the foregoing difficulties in mind, it is an object of the present invention to provide an improved magneto-optic memory device having a recording medium protected from oxidizataion for a stabilized coercive force and stable information recording characteristics. 
     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 examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     To achieve the above objects, a magneto-optic memory device according to an embodiment of the present invention comprises a substrate, a thin film recording medium of a rare earth transition metal alloy, a film of aluminum nitride serving as a transparent dielectric film with no oxygen content, and a reflecting film, all layered in turn. The reflecting film is made of stainless steel. 
     The above objects can also be achieved by a magneto-optic memory device according to another embodiment of the invention, comprising a substrate, a thin film of a rare earth transition metal alloy, a film of transparent dielectric film composed of a film of nitride such as of aluminum nitride or silicon nitride having no oxygen content, and a reflecting film of titanium or titanium nitride, all layered in turn. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be better 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 fragmentary cross-sectional view of a conventional magneto-optic memory device; 
     FIG. 2 is a schematic view of a measurement optical system; 
     FIG. 3 is a graph showing a degree of dependency of the magneto-optic rotational angle on a magnetic field; 
     FIG. 4 is a graph showing the relationship between a composition ratio of rare earth elements in a thin film of GdTbFe amorphous alloy and the coercive force of the film; 
     FIG. 5 is a fragmentary cross-sectional view of a magneto-optic memory device according to an embodiment 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 according to another embodiment of the invention; 
     FIG. 8 is a fragmentary cross-sectional view of a magneto-optic memory device according to still another 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. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 5 shows in fragmentary cross section a magneto-optic memory device construction according to an embodiment of the present invention. The 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 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 AlN (Aluminum nitride) having a thickness ranging from about 400 to about 500 Å by reactive sputtering of aluminum 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 Å is deposited on the AlN film 13 by sputtering. 
     Applicants 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 ∘, x, •, Δ, respectively. The mark Δ is superimposed on the mark • in many instances. As shown in FIG. 6, the coercive force becomes slightly larger by about 0.4 kOe than an initial coercive force (in the range of from about 1.8 to about 2.2 kOe) after about 100 days of the storage test on 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 aluminum nitride has no oxygen content in itself while conventional transparent dielectric films of SiO 2  have an oxygen content. Stated otherwise, when the transparent dielectic film is formed by way of reactive sputtering in a nitrogen gas using an aluminum target, there is no tendency for oxygen to enter the recording medium at the time the film is deposited. From this standpoint, the transparent dielectric film may be made of other materials with no oxygen content such as MgF 2 , ZnS, CeF 3 , AlF 3  ·3NaF. 
     However, most of the targets comprising these materials are porous, so that in case these materials are employed to fabricate the transparent dielectric film by conventional sputtering, oxygen and water for penetrating into the pores in the materials are liable to oxidize the recording medium during sputtering. To the contrary, where the transparent dielectric film is fabricated of aluminum nitride, the target used is made of aluminum only and is advantageous from the standpoint of material savings. The aluminum target is not porous and hence does not trap therein oxygen and water. Due to the structural features, aluminum nitride can form highly tight films which render themselves impermeable to oxygen and water, a property which also prevents the recording medium from being oxidized. 
     The film 14 of stainless steel is highly advantageous for use as a reflecting film in the magneto-optic memory device. The advantages of the reflecting film of stainless steel will be described below. 
     (1) Corrosion resistance: 
     As is well known, stainless steel is excellent in its corrosion resistance capability. For example, no pinhole will formed in a reflecting film of stainless steel when a fingerprint is put on the film, while a minute pinpole will be form in a reflecting film of Cu at a 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 resistivity 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, and 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. Aluminum nitride of which the transparent dielectric film is made has relatively good thermal conductivity and hence allows much heat to escape. Where the transparent dielectric film is formed of aluminum nitride, therefore, the reflecting film of stainless steel is particularly suitable for use in combination therewith. 
     (3) Easy film formation: 
     Since stainless steel is easily capable of sputtering, and since a film can be fabricated thereof with ease, this is advantageous for fabrication purposes. 
     While in the foregoing embodiment 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 recording medium may be made of TeOx (0≦x&lt;1), TeC, TeSe or the like with Te being a main component, or of MnBi, MnBiCu, MnB or the like. The dielectric film may be formed of silicon nitride instead of aluminum nitride. The reflecting film 14 may be formed of Ni, Ti, or TiN instead of stainless steel. In addition, a transparent dielectric film having no oxygen content may be disposed between the substrate and the recording medium film. 
     FIG. 7 is illustrative, in fragmentary cross section, of a magneto-optic memory device according to 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 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 (aluminum nitride) having a thickness ranging from about 400 to about 500 Å by reactive sputtering of aluminum 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 aluminum nitride, it is quite 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 considered to be due to the fact that the effect of nitrogen in the aluminum nitride on Cu or Al is not negligible. To the contrary, the reflecting film of titanium or titanim nitride can easily be deposited on the transparent dielectric film of aluminum 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 in case the reflecting film is deposited of titanium or titanium nitride by way of sputtering, the target used is made of Ti, which is relatively less costly and easily available. 
     The magneto-optic memory device of the present invention is not limited to the construction of the illustrated embodiment. More specifically, a transparent dielectric film of aluminum nitride may be interposed between the glass substrate 11 and the thin film 12 of a GdTbFe amorphous alloy, so that the thin film can be sandwiched between the transparent dielectric films of aluminum nitride. 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 aluminum nitride (serving as a first transparent dielectric film) is deposited on a transparent substrate 16 made of glass, polycarbonate, acrylic resin or the like. On the film 17, there is formed a thin film 18 of a rare earth transition metal alloy such as GdTbFe, TbDyFe, GdTbDyFe, TbFe, GdFeCo, GdCo, or such alloys with a content of Sn, Zn, Si, Bi, B or the like. In addition, a transparent film 19 of aluminum 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 was subjected to an Auger electron spectroscopy. FIG. 9 shows the result of such an Auger electron spectroscopy, indicating Auger electron intensities in Al, Fe, and O of the three films, that is, the first film of aluminum nitride, the film of GdTbFe, and the second film of aluminum 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 aluminum nitride and the interface between the film of aluminum nitride and the transparent substrate. This indicates that there is oxygen entering into the film of aluminum nitride from the exterior and into the film of aluminum nitride from the glass substrate after the films have been fabricated. This experimental result shows that the sandwich structure of a thin film 18 of a rare earth transition metal alloy between the films of aluminum nitride is effective to prevent the thin film 18 from being oxidized during the process of fabricating the films. 
     Instead of aluminum nitride, the transparent dielectric films may be formed of Si 3  N 4  by way of reactive sputtering in a nitrogen gas using an Si target. The transparent dielectric films may be formed of MgF 2 , ZnS, CeF 3  ·3NaF by vacuum deposition. 
     The first transparent dielectric film in the above magneto-optical memory device should have a thickness of at least 100 Å for the reason that when the first transparent dielectric film is deposited on the glass substrate, oxygen in the glass substrate enters the first transparent dielectric film to a depth of about 50 Å. If the thickness of the first transparent dielectric film were smaller than 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 to improve the quality of reproduced signals. In the event of reproduced signals already available of sufficent quality, the second transparent dielectric film may be dispensed with, and a reflecting film of Cu, Ag, Al, Au or the like may be deposited directly on the thin film 18 of rare earth transition metal alloy. 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), an alloy of a rare earth element and transition metal (GdTbFe, TbDyFe, GdCo, GdTbDyFe, etc) to provide a complete device of a construction which prevents entry of oxygen from the exterior. 
     The film of aluminum 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. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope if the invention, and all such modifications are intended to included within the scope of the following claims.