Optical magnetic recording medium provided with high coercive force layer and low coercive force layer containing light rare earth

Abstract: An optical magnetic recording medium comprises: a first magnetic layer vertically magnetizable comprising a rare earth-iron group amorphous alloy; and a second magnetic layer having lower coercive force than the first magnetic layer compriging a rare earth-iron group amorphous alloy containing light rare earth, the second magnetic layer being exchange bonded to the first magnetic layer.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to an optical magnetic recording medium which performs recording of an information by forming bits of the reverses magnetic district by use of a laser beam and an external magntic field and performs reading of the information by utilizing the magnetic optical effect by use of a polarized laser beam. Related Background Art In the prior art, in optical magnetic recording medium, it has been difficult to find out a single magnetic material, which is easy in recording due to appropriately low Curie temperature, and yet high in storage stability of information due to high coercive force, and further good in reading characteristic due to large magnetic optical Kerr rotational angle. For this reason, an optical magnetic recording medium having two different magnetic materials laminated with separated necessary functions has been proposed in Japanese Laid-open Publication No. 57-78652 (corresponding U.S. Patent 4,799,114) as reflected in the preamble of claims 1 and 7. This recording medium comprises two layers of a high coercive force layer haivng a low Curie temperature which is vertically magnetizable and a low coercive force having a high Curie temperature, and the high coercive force layer and the low coercive force layer are exchange bonded to each other. In the high coercive force having low Curie temperature, recording and storage of information are performed, the recorded information is transferred to the low coercive force layer, and reading of information is performed in the low coercive force layer having high Curie temperture and has great magnetic optical Kerr rotational angle. As the material of such recording medium, Tb-Fe, Dy-Fe are disclosed for the high coercive force layer, and Gd-Fe, Gd-Co for the low coercive force. Further, Japanese Laid-open Patent Publication No. 61-117747 also discloses an optical magnetic recording medium in which a rare earth-iron group amorphous alloy of Gd-Fe-Co with preferential iron group sub-lattice magnetization is used for the high coercive force layer, and a rare earth-iron group amorphous alloy of Tb-Fe with also preferential iron group sub-lattice magnetization for the low coercive force layer. On the other hand, the recording density of optical magnetic recording medium is restricted by the wavelength of the laser beam used, and the recording density becomes higher as the wavelength becomes shorter. However, a semiconductor laser of short wavelength is presently low in output, and also short in life. Accordingly, although a wavelength of about 800 nm is primarily used at present time, developments of short wavelength laser have been also actively done. Thus, although the effect of higher recording density can be obtained by making the wavelength of laser shorter, by use of a heavy rare earth (e.g. Gd. Tb, Dy, Ho)-iron group (e.g. Fe, Co, Ni) amorphous alloy such as Gd-Fe, Gd-Co, Gd-Fe-Co or Tb-Fe, Dy-Fe, etc., there is the drawback that the magnetic optical effect becomes smaller as the wavelength becomes shorter, whereby the reading characteristic is worsened. Fig. 1 shows the wavelength dependency of the magnetic optical Kerr rotational angle  k of a magnetic film comprising such Gd .33 Fe .67 , Tb .17 Fe .83 and Dy .20 Fe .80 . It can be seen that these show relatively larger Kerr rotational angles at wavlength of about 800 nm, but Kerr angles become smaller as the wavelength becomes shorter. On the other hand, it has been known in an alloy of a light rare earth such as Nd, Pr, Ce, Sm, etc, with an iron group, the magnetic optical effect does not become smaller, or contrariwise become greater even when the wavelength may become shorter. However, such light rare earth-iron group amorphous alloy is a ferromagnetic material, having great saturated magnetization, which can become a vertically magnetized film with low coercive force and therefore is not suitable for optical magnetic recording medium. Accordingly, for example, in "Collected Gists of Academic Lectures in the 11th Meeting of Applied Magnetic Society of Japan (1987) 3aD-6", etc., there is a Nd-Dy-Fe-Co single layer film in which a light rare earth of Nd and a heavity rare earth of Dy are used as the rare earth and formed into an alloy together with Fe, Co of iron group. However, also in such Nd-Dy-Fe-Co singly layer film, in order to obtain sufficient coercive force as the optical magnetic recording medium, Nd can be added only in a slight amount, and therefore the effect of improvement of the magnetic optical effect in short wavelength was small. SUMMARY OF THE INVENTION An object of the present invention is to provide an optical magnetic recording medium which has solved the problems of the prior art as described above, can perform recording easily, is excellent in stability of recorded information due to great coercive force, and yet has good reading characteristic in short wavelength. The above object of the present invention can be accomplished by an optical magnetic recording medium according to claims 1 and 7. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph showing the wavelength dependency of magnetic optical Kerr rotational angle  k of the optical magnetic recording medium by use of a heavy rare earth-iron group amorphous alloy of the prior art. Fig. 2 and Fig. 3 each show a schematic sectional view of an embodiment of the optical magnetic recording medium based on the present invention. Fig. 4 is a graph showing the wavelength dependency of the magnetic optical Kerr rotational angle  k of the Nd-Fe-Co film. Fig. 5 is a graph showing the magnetization curve of a multi-layer film comprising Nd-Co and Tb-Fe. Fig. 6 is a graph showing the magnetic optical Kerr hysteresis curve of a multi-layer film comprising Nd-Co and Tb-Fe. Fig. 7 is an illustration showing the structure of the sample comprising Nd-Tb-Fe-Co and Tb-Fe-Co. Fig. 8 is a graph showing comparison between the magnetic optical Kerr hysteresis curves of the Nd-Tb-Fe-Co single layer film, the Nd-Tb-Fe-Co/Tb-Fe-Co double layer film and the Tb-Fe-Co single layer film. DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 2 is a schematic sectional view showing an embodiment of the optical recording medium based on the present invention. In Fig. 2, 1 is a transparent substrate comprising glass, plastic, etc., 2 is a low coercive force layer comprising a rare earth-iron group amorphous alloy including light rare earth, 3 is a high coercive force layer comprising a rare earth-iron group amorphous alloy having readily magnetizable axis in the direction perpendicular to the film surface, and 4 is a protective layer comprising a dielectric material such as SiO, Si₃N₄, etc. Here, the low coercive force layer 2 and the high coercive force layer 3 are exchange bonded to each other. In carrying out recording on such recording medium, the two magnetic layers are previously magnetized in one direction, and while applying externally a bias magnetic field in the direction opposite thereto, a laser beam modulated corresponding to the information signals is scanned. Then, the portion irradaited with the laser beam is once heated to around the Curie temperature of the high coercive force layer 3, and, when cooled, is magnetized to the opposite direction to the surrounding through the action of the bias magnetic field. Thus, the information is accumulated in the form of bit of the reversed magnetic district. On the other hand, the information thus recorded is irradiated with a laser beam of no modulation, low output linearly polarized from the tranaparent substrate side, and is read from the reflected light thereof which has received modulation to polarized state through the magnetic optical effect of the low coercive force layer 2. In the single layer film comprising a rare earth-iron amorphous alloy including light rare earth and heavy rare earth as shown in the prior art, for making the coercieve force practiclly sufficient value, a considerable amount of heavy rare earth must be added. Whereas, addition of such large amount of heavy rare earth will damage the improvement effect of the magnetic optical effect in short wavelength. In contrast, the present embodiment, by arranging a layer with much light rare earth on the surface where light is incident and having a layer with higher coercive force on the other surface, can maintain magnetization of the low coercive force layer through the exchange bonding without damaging the improvement effect of the magnetic optical effect in short wavelength. In other words, in the present embodiment, the low coercive force layer is increased in apparent coercive force through the effect of exchange bonding from the high coercive force layer, whereby the same fine magnetic district as the high coercive force layer can be retained and also stability of the recorded information to the magnetic field from outside is increased. The greatness of the effect of the exchange bonding is proportional to the greatness of the exchange force between the two layers, and inversely proportional to saturated magnetization and film thickenss thereof. The greatness of the exchange force between the two layers will vary with the combination of those materials, and there exist combinations of materials with greater exchange force and combinations of materials with smaller exchange force. One specific feature of the present invention resides in discovery of the fact that even in a combination of rare earth-iron group amorphous alloy including light rare earth and a rare earth-iron amorphous alloy, a great effect of exchange bonding to the same extent as the combination mutually between the heavy rare earth-iron groups amorphous alloys can be obtained. Concerning film thickness, since it may considered that both the rare earth-iron group amorphous alloy and the heavy rare earth-iron group alloy have the same extent of absorption, no sufficient magnetic optical effect can be obtained with a film thickness less than about some 100 Å. Concerning saturated magnetization, since the heavy rare earth-iron group amorphous alloy is a ferrimagnetic material, the saturated magnetization can be made smaller while maintaining the magnetic optical effect large, by bringing its composition to near the compensation composition. Whereas, since the light rare earth-iron group amorphous alloy is a ferromagnetic material, it is impossible to make the saturated magnetization small, while maintaining the magnetic optical effect large. Accordingly, it may be also considered that no stable vertically magnetized state can be obtained, if the light rare earth-iron group amorphous alloy is made a low coercive force layer of two layer exchange bonded film. An embodiment further improved in this point is shown below. Fig. 3 is a schematic sectional view showing another embodiment of the optical magnetic recording medium based on the present invention. In Fig. 3, the same members as in Fig. 2 are affixed with the same symbols, and detailed description thereof is omitted. This embodiment comprises N sets (N is an integer of 2 or more) of low coercive force layers 2₁, 2 ₂, ..., 2 N and high coercive force layers 3₁, ..., 3 N which are the same as the low coercive force layer 2 and the high coercive force layer 3 in Fig. 2, alternately laminated. These magnetic layers are also exchange bonded to each other. The effect of exchange bonding is inversely proportional to saturated magnetization and film thickness. This embodiment compensates for the small effect of exchange bonding with great saturated magnetization by making the film thickness thinner. Also, in this embodiment, the low coercive force layer sandwiched between high coercive force layers receives exchange force from the both high coercive force layers existing above and below thereof, whereby there is the advantage that the exchange effect is doubled. In Fig. 3, it is now supposed to read by irradiation of a laser beam from the substrate 1 side. The film thickness of the low coercive force layer 2₁ is not so thick as to obtain great magnetic optical effect by reflecting sufficiently the light, and considerable light reaches the high coercive force layer 3₁. Since the high coercive force layer has inherently little magnetic optical effect, the light reflected against this layer is reflected without so great magnetic optical interaction. However, if this high coercive force layer 3₁ is considerably thin, the light further transmits through this layer to reach the low coercive force layer 2₂ having great magnetic opticl effect. Thus, by laminating alternately thin low coercive force layer and thin high coercive force layer, light can interact with a plurality of low coercive force layers having great magnetic optical effect, and consequently a great magnetic optical effect can be obtained. The film thickness of the layer to be laminated alternately may be about 30 to 300 Å, preferably about 50 to 150 Å, with 1 Å = 10⁻¹⁰ m In other words, no great magnetic optical effect can be obtained with only one layer of thin low coercive force layer, by alternate lamination together with high coercive force layers, a great magnetic optical effect can be obtained, and further due to thin film thickness of the respective low coercive force layers, great effect of exchange bonding can be also obtained. As the low coervice force layer as described above, for example, an amorphous alloy of at least one element selected from Nd, Pr, Ce and Sm with at least one element selected from iron group elements such as Fe, Co and Ni can be used. The light rare earth element may be desirably Nd, Pr, preferably Nd from the point that the magnetic optical effect is great in short wavelength. As the iron group, Fe, Co may be desirable, more preferably Fe-Co alloy from the point that greater magnetic optical effect with great magnetic moment can be obtained. When light rare earth element is generally represented by LR, this alloy is represented by the following formula. LR X (Fe 1-Y Co Y ) 1-X . Here, X and Y represent atomic ratios, with X being 0.1 ≤ X ≤ 0.4, desirably 0.2 ≤ X ≤ 0.3, Y being 0 ≤ Y ≤ 1, desirably 0.2 ≤ Y ≤ 1, more desirably 0.2 ≤ Y ≤ 0.8. If X is small, saturated magnetization becomes too great, while if X is too large, magnetic optical effect will be reduced. Fig. 4 shows the dependency of the magnetic optical Kerr rotational angle  k of the magnetic layer comprising Nd .22 Fe 0.78' Nd .22 (F .65 Co .35 ) .78' Nd .22 (Fe .35 Co .65 ) .78 and Nd .22 Co .78 on the wavelength of reading light. Nd-Fe and Nd-Co have Kerr rotational angles  k with the same extent of largeness in long wavelength (700 to 800 nm), but Nd-Co is larger in Kerr rotational angle in short wavelength (to 600 nm). Nd-Fe-Co is larger in Kerr rotational angle than Nd-Co in the whole region of measured wavelength (300 to 800 nm). Accordingly, as the material for low coercive force layer, Nd-Fe, desirably Nd-Co, more desirably Nd-Fe-Co may be employed. When compared with Fig. 1, it can be seen that even by use of Nd-Fe, the Kerr rotational angle is larger in short wavelength than heavy rare earth-iron group amorphous alloy. Also, for lowering saturated magnetization, a heavy rare earth may be also added in the low coercive force layer in an amount which does not lower the magnetic optical effect in short wavelength so much. In this case, an amorphous alloy of at least one element selected from light rare earth elements such as Nd, Pr, Ce, Sm, etc., at least one element selected from heavy rare earth elements such as Gd, Tb, Dy, Ho, etc. and at least one element selected from iron group elements such as Fe, Co, Ni, etc. may be employed. As the light rare earth element, Nd, Pr may be desirable, more desirably Nd. As the heavy earth element, Tb, Dy may be also available, but desirably Gd from the point that the magnetic optical effect is not lowered in short wavelength. As the iron group element, Fe, Co are desirable, but more desirably Fe-Co alloy. When light rare earh element is represented by LR, and heavy rare earth element generally by HR, such alloy is represented by the following formula. LR x HR y (Fe 1-z Co z ) 1-x-y Here, x, y and z are atomic ratios. x may be 0.1 ≤ x ≤ 0.4, desirably 0.1 ≤ x ≤ 0.3, y may be 0.05 ≤ y ≤ 0.3, desirably 0.05 ≤ y ≤ 0.2, and z may be 0 ≤ z ≤ 1, desirably 0.2 ≤ z ≤ 0.8. When a rare earth-iron group amorphous alloy including light rare earth and heavy rare earth is used as single layer for the low coercive force layer, the thickness of the low coercive force layer may be about 100 to 500 Å, desirably about 150 to 300 Å. On the other hand, as the high coercive force layer, heavy rare earth-iron group amorphous alloy such as Tb-Fe-Co, Dy-Fe-Co, Tb-Dy-Fe-Co, etc. may be suitably employed, and a light earth element may be slightly contained therein. The film thickness of the high coercive force layer may be about 100 to 2000 Å, desirably about 500 to 1000 Å. In the present invention, the coercive force of the low coercive force layer should be desirably about 1 KOe or lower with 1KOe = (10⁶/4π) A/m ≈ 79,6·10³ A/m. The coercive force of the high coercive force layer is set at several KOe or higher, desirably 5 KOe or higher, more desirably 10 KOe or higher. The Curie temperature may be higher for either of the magnetic fibers, but it is desirable that the lower coercive force layer should be higher so that the magnetic optical effect may not be reduced even when the temperature may be elevated by irradiation of reading light. Example 1 As described below, an optical magnetic recording medium having a constitution as shown in Fig. 3 was prepared. First, according to the vacuum vapor deposition method, on a glass substrate were vapor deposited a Nd-Co film of a light rare earth-iron group amorphous alloy as the low coercive force layer and a Tb-Fe film as the high coercive force layer alternately with a thickness of about 50 Å, followed finally by attachment of Tb-Fe with a thickness, of 150 Å to a total film thickness of about 400 Å. Then, SiO was attached as the protective film. These were all formed continuously without breaking vacuum so that effective exchanged bonding could work between the magnetic layers. The vacuum degree during vapor deposition was 10⁻⁵ Torr. The compositions of the respective layers were made Nd .27 Co .73 and Tb .24 Fe .76 . Next, the magnetic characteristics of this medium were examined. The results are shown in Fig. 5 and Fig. 6. The Kerr effect was measured from the substrate side. In Fig. 5, the axis of abscissa shows magnetic field and the axis of ordinate magnetization. The solid line indicates the characteristic in the direction in parallel to the film surface and the broken line the characteristic in the direction perpendicular thereto. As can be seen from the magnetization curve, the magnetic layer of the present invention has become a vertically magnetized film with an angular ratio approximate to 1. Winc the saturated magnetization substantially coincides with the difference in magnetization between the Nd layer and the Tb layer, they may be considered to be ferrimagnetially bonded. Fig. 6 is a Kerr loop when the wavelength of the reading light is 400 nm (shown by the solid line) and 800 nm (shown by the broken line). Here, the axis of abscissa shows magnetic field and the axis of ordinate Kerr rotational angle. In this Example, saturated Kerr rotational angle is larger in the case of 400 nm than in the case of 800 nm. This is because there is contribution to the Kerr effect by Nd-Co on the shorter wavelength side. Example 2 A sample was prepared on a glass substrate according to the vacuum vapor deposition method, by vapor depositing Nd-Tb-Fe-Co film of a rare earth-iron group amorphous alloy including light rare earth and heavy rare earth as the low coercive force layer to a thickness of 150 Å and a Tb-Fe-Co film as the high coercive force layer to a thickness of 850 Å. Then, as the protective film, SiO was attached. These were all formed continuously without breaking vacuum so that effective exchange could work between the magnetic layers. The vacuum degree during vapor deposition was 10⁻⁶ Torr or less. The sample was prepared by use of a mask so that a singly layer portion could be also formed at the same time, as shown by the cross-section in Fig. 7, for comparison with the single layer portion. The compositions of the respective layers were Nd .15 Tb .10 (Fe .78 Co .22 ) .75 for one and Tb .24 (Fe .75 Co .25 ) .76 for the other. Next, Kerr loop of this sample was measured. Measurement was conducted from the glass substrate side. The results are shown in Fig. 8. In Fig. 8, the respective curves indicate Kerr loops of the Nd-Tb-Fe-Co single layer portion A, the Nd-Tb-Fe-Co/Tb-Fe-Co double layer portion B and the Tb-Fe-Co single layer portion C in the case when the reading light has a wavelength of 400 nm. The Nd-Tb-Fe-Co single layer portion A contains much Nd to have great magnetization, whereby coercive force is very small and also angular ratio is bad. On the other hand, Nd-Tb-Fe-Co of the Nd-Tb-Fe-Co/Tb-Fe-Co double layer portion B has greater coercive force through the exchange bonding with Tb-Fe-Co, and angular ratio is improved to 1. Further, the Kerr rotational angle  k of the Nd-Tb-Fe-Co/Tb-Fe-Co double layer portion B is considerably larger than that of the Tb-Fe-Co single layer portion C. This is because three is contribution to the Kerr effect by Nd on the shorter wavelength side. The present invention is not limited to the above Examples, but various applications are possible. For example, between the substrate and the low coercive force layer, reflection preventive layer, insulating layer, etc. may be also provided. The present invention includes all of such application examples without departing from the scope of claims.

Claims: A magnetooptical recording medium including a vertically magnetizable first magnetic layer (3) comprising a rare earth-iron group amorphous alloy and a second magnetic layer (2) comprising a rare earth-iron group amorphous alloy, the second magnetic layer having a lower coercive force than that of the first magnetic layer and being exchange-coupled to the first magnetic layer, the medium being characterised in comprising at least two sets of layers with said first (3₁, 3₂ ... 3 N ) and second (2₁, 2₂ ... 2 N ) layers alternating such that each said second layer is sandwiched between two of said first layers, and said second magnetic layers (2₁, 2₂ ... 2 N ) have the following composition: LR X (Fe 1-Y Co Y ) 1-X where LR is Nd, Pr, Ce or Sm and 0.1 ≤ X ≤ 0.4 and 0 ≤ Y ≤ 1. A magnetooptical recording medium according to claim 1, in which the magnetooptical signal is primarily derived from a plurality of second layers. A magnetooptical recording medium according to claim 1 or 2, wherein each of said first (3₁, 3₂ ... 3 N ) and second (2₁, 2₂ ... 2 N ) magnetic layers has a thickness in the range of 3 to 30 nm. A magnetooptical recording medium according to claim 3, wherein each of said first and second magnetic layers has a thickness in the range 5 to 15 nm. A magnetooptical recording medium according to any one of claims 1 to 4, wherein the following condition is satisfied: 0.2 ≤Y ≤ 0.8. A magnetooptical recording medium according to claim 5, wherein each second magnetic layer comprises Nd-Fe-Co, Nd-Co or Nd-Fe. A magnetooptical recording medium including a vertically magnetizable first magnetic layer (3) comprising a rare earth-iron group amorphous alloy and a second magnetic layer (2) comprising a rare earth-iron group amorphous alloy, a second magnetic layer having a lower coercive force than that of first magnetic layer and being exchange-coupled to the first magnetic layer, the medium being characterised in at least two sets of layers with said first (3₁, 3₂, ... 3 M ) and second (2₁, 2₂, ... 3 N ) layers alternating such that each said second layer is sandwiched between two of said first layers, and said second magnetic layers (2₁, 2₂, ... 2 N ) having the following composition: LR x HR Y (Fe 1-z Co z ) 1-x-y where LR is Nd, Pr, Ce or Sm, HR is Gd, Tb, Dy or Ho and 0.1 ≤ X ≤ 0.4; 0.05 ≤ Y ≤ 0.4 and 0 ≤ Z ≤ 1. A magnetooptical recording medium according to claim 7, wherein said first magnetic layer has a thickness in the range of 10 to 200 nm, and said second magnetic layer has a thickness in the range of 10 to 50 nm. A magnetooptical recording medium according to claim 8, wherein said second magnetic layer has a thickness in the range of 15 to 30 nm. A magnetooptical recording medium according to any one of claims 7 to 9, wherein the following condition is satisfied: 0.1 ≤ X ≤ 0.3, 0.05 ≤ Y ≤ 0.2 and 0.2 ≤ Z ≤ 1. A magnetooptical recording medium according to claim 10, wherein the following condition is satisfied: 0.2 ≤ Z ≤ 0.8. A magnetooptical recording medium according to any one of claims 7 to 11, in which HR is Gd. A magnetooptical recording medium according to any one of the preceding claims, wherein each second magnetic layer (2) has a coercive force of 1 KOe or less. A magnetooptical recording medium according to any one of the preceding claims, wherein each first magnetic layer (3) has a coercive force of several KOe or more. A magnetooptical recording medium according to claim 14, wherein each first magnetic layer (3) has a coercive force of 5 KOe or more. A magnetooptical recording medium according to claim 15, wherein each first magnetic layer (3) has a coercive force of 10 KOe or more. A magnetooptical recording medium according to any preceding claim, wherein the medium further comprises a transparent substrate layer (1). A magnetooptical recording medium according to claim 17, wherein said transparent substrate layer (1) comprises glass or plastic. A method of recording information on a medium as defined in any of claims 1 to 18, which comprises the steps of: (a) premagnetising said magnetic layers in one direction, (b) applying externally a biased magnetic field in an opposite direction thereto, (c) directing a laser beam onto said medium, said laser beam being modulated in accordance with the information to be recorded incident on said medium. A method of reading information, recorded on a medium as defined in any of claims 1 to 18, which comprises the steps of: irradiating the medium with a beam of radiation, and detecting changes in the polarisation direction of the reflected beam caused by magneto-optical modulation of the second magnetic layer (3).

Inventors: TSUNASHIMA SHIGERU, TSUNASHIMA, SHIGERU
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1989
Main classification: G11B11
IPC codes: G11B11