Magneto-optical recording medium

Abstract: A magneto-optical recording medium is provided which has high magneto-optic effect even under a short-wavelength light, high anisotropy in the direction perpendicular to the film surface, and permits high-density recording, the medium having a first magnetic layer (1) laminated onto a second magnetic layer (2), the first magnetic layer being a rare-earth-transition-metal amorphous film with mainly a light rare-earth element as rare-earth element, the light rare-earth element being at least one element selected from an element group consisting of Nd and Pr, and the second magnetic layer being a rare-earth-transition-metal amorphous film with at least one element selected from an element group consisting of Tb and Dy as rare-earth element.

Description: This invention relates to a magneto-optical recording medium for recording information by use of a laser beam, and particularly to a magneto-optical recording medium which has a high magneto-optic effect under short-wavelength light and is suitable for high-density recording. Magneto-optical recording has been put to practical use through a rewritable optical recording approach. Until now, a single amorphous rare-earth-transition-metal film with a strong magnetic anisotropy in the direction perpendicular to the film surface has been used as a magneto-optical recording film. In particular, TbFeCo alloy system amorphous films have been researched and developed for practical use. In magneto-optical recording, there is a trend toward higher recording density, and use of a short-wavelength read/write light is now being researched as a possible approach. For this purpose, the development of magneto-optical materials with higher magneto-optic effects such as Kerr rotation and Faraday rotation, is essential in order to achieve higher density magneto-optical recording in the future. However, in the conventional TbFeCo alloy series amorphous film, as the wavelength of the laser beam becomes shorter, the magneto-optic effect tends to decrease monotonically. Thus, a sufficiently large Kerr rotation angle or Faraday rotation angle cannot be obtained on the short wavelength side, and thus there is a problem that the output in reading with the laser beam decreases greatly. It was recently proposed that a two-layer film be used as a new magneto-optical film with magnetic exchange coupling between the layers. See for example Japanese Published Unexamined Patent Application No. 60-171652). However, since these are also substantially TbFe alloy system laminated films, the Kerr rotation angle decreases for shorter wavelengths. They are therefore unsuitable for read/write films at short wavelengths. A rare-earth-transition-metal amorphous film consisting mainly of Nd or Pr is known. See for example T.R.McGuire et al, "Magneto-optical Properties of Nd-Co and Nd-Fe alloys", J. Appl. Phys. 61 (1987) pp. 3352-3354. Although this film has a characteristic higher magneto-optical effect for short wavelengths, it is not a perpendicular magnetized film with stong magnetic anisotropy in the direction perpendicular to the film surface, but merely an in-plane magnetized film with strong magnetic anisotropy in the direction parallel to the film face. Thus, high-density recording cannot be attained. The object of the present invention is to provide a magneto-optical recording medium which has a sufficiently high magneto-optic effect even for short wavelengths, strong magnetic anisotropy in the direction perpendicular to the film surface, and permits high-density recording. Therefore in accordance with the present invention there is provided a magneto-optical recording medium with strong magnetic anisotropy in the direction perpendicular to the surface having a first magnetic layer comprising a first rare-earth-transition metal amorphous alloy, the rare-earth component of the first alloy predominently comprising a light rare-earth element or elements, the light rare-earth element or elements being Nd and/or Pr; a second magnetic layer comprising a second rare earth-transition metal amorphous alloy, the rare-earth component of the second alloy being Tb and/or Dy. EP-A-0333467 having priority date 18 March 1988 discloses such a medium and forms part of the state of the art by virtue of article 54(3)EPC for contracting states DE FR GB. The proposed magneto-optical recording medium with strong magnetic anisotropy in the direction perpendicular to the surface has a first magnetic layer laminated on a second magnetic layer, the first magnetic layer, of (Nd, Pr)- transition metal amorphous alloy, having high magneto-optical effect at short wavelengths but low perpendicular anisotropy, and the second magnetic layer, of (Tb, Dy)- transition metal amorphous alloy, having low magneto-optical effect at short wavelengths but high perpendicular anisotropy. By exerting magnetic exchange coupling between these two layers on the said first layer to induce strong perpendicular anisotropy in it, a perpendicular magnetized magneto-optical recording medium is obtained that has overall high magneto-optical effect, even at short wavelengths. The first magnetic layer of the proposed magneto-optical recording medium is preferably a rare-earth-transition-metal amorphous film whose composition is expressed by a general formula (A 1-a B a ) l (Fe b Co 1-b ) m T n , wherein A is at least one element selected from a light rare-earth element group consisting of Nd and Pr, B is at least one element selected from a group of heavy rare-earth elements, and T is at least one element selected from a group of transition-metal elements for increasing corrosion resistance, 0 ≦ a ≦ 0.5, 0 ≦ b ≦ 1, and in atomic percentage, 10 ≦ l ≦ 50, 50 ≦ m ≦ 90, ≦ n ≦ 15, and l+m+n=100. Nd and Pr are specifically selected as light rare-earth elements for A, since they have the effect of making the Kerr rotation high for short wavelengths. Note that the first magnetic layer is mainly composed of these light rare-earth elements. Addition of a heavy rare-earth element to the light rare-earth elements Nd and Pr in the first magnetic layer facilitates magnetic exchange coupling between the first layer and the second layer, makes it easier to obtain a perpendicular magnetized film, and makes the shape of the written magnetic domain uniform to reduce medium noise due to magnetic domain shape. Heavy rare-earth elements for such purposes can be selected from typical heavy rare-earth elements such as Tb, Dy, Gd, Ho, and Er. Alternatively, heavy rare-earth elements may be quite absent. A transition-metal element such as Ti, Zr, V, Nb, Ta, Cr, Mo, w, Pd, Pt, or Al known to have the property of increasing corrosion resistance can be selected specifically for T. The second magnetic layer is preferably a rare-earth-transition-metal amorphous alloy film whose composition is expressed by a general formula R x M y U z , wherein R is at least one element selected from an element group consisting of Tb and Dy, M is at least one element selected from an element group consisting of Fe and Co, and U is at least one element selected from a group of transition-metal elements for increasing corrosion resistance, and in atomic percentage, 15 ≦ x ≦ 35, 65 ≦ y ≦ 85, 0 ≦ z ≦ 15, and x+y+z=100. In order to obtain higher magnetic anisotropy,higher corrosion resistance, and higher performance in recording and writing, the preferable composition is 20 ≦ x ≦ 30, 70 ≦ y ≦ 80, and 0 ≦ z ≦ 10, either on the transition-metal rich side where the sublattice magnetization of transition metals is predominant, with the Curie temperature Tc being in the range 150°C to 250°C, or on the rare-earth rich side where the sublattice magnetization of the rare earth element is predominant, with the Curie temperature Tc as above, and the compensation temperature 50°C to 150°C. A transition metal similar to the above T can be selected for U. In order to obtain a film suitable for magneto-optical recording which behaves like a perpendicular magnetized film with a coercive force of 1 kOe or more as a whole by laminating the first magnetic layer and the second magnetic layer alternately to utilize magnetic exchange coupling, it is preferable to set the ratio (t₁/t₂) of the total film thickness (t₁) of the first magnetic layer to the total thickness (t₂) of the second magnetic layer to 0.5 or less, more preferably in the range 0.001 to 0.2, so that the magnetic exchange coupling may be exerted on the entire first magnetic layer. Arrangement of a thermal diffusion film made of a material such as metal, alloy, or ceramic with a high heat conductivity on the proposed multilayer magnetic film, either directly or through a transparent dielectric protective film, gives an optical recording medium with higher performance suitable for higher speed writing. Embodiments of this invention are described below with reference to the drawings, wherein: Figure 1 is a cross-sectional view of an embodiment of the magneto-optical recording medium; Figure 2 is a Kerr loop diagram of the Nd₂₀Co₈₀ single film; Figure 3 is a Kerr loop diagram of the Tb₂₅Fe₇₅ single film; Figure 4 is a Kerr loop diagram of the Nd₂₀Co₈₀/Tb₂₅Fe₇₅ laminated film; Figure 5 is a graph showing the relationship between the wavelength of the laser beam and the Kerr rotation angle for the Nd₂₀Co₈₀/Tb₂₅ Fe₇₅ laminated film; Figure 6 is a graph showing the relationship between the temperature and the coercive force for the Nd₂₀Co₈₀/Tb₂₅Fe₇₅ laminated film; Figure 7 is a graph showing the relationship between the temperature and Kerr rotation angle for the Nd₂₀Co₈₀/Tb₂₅Fe₇₅ laminated film; Figure 8 is a graph showing the relationship between the ratio of coercive force HC₀/Hc₂ and the film thickness ratio (t₁/t₂) and the relationship between the Kerr rotation angle and the film thickness ratio (t₁/t₂) for an embodiment of the invention, where the thickness of the first magnetic layer is t₁, the thickness of the second magnetic layer is t₂, the coercive force of the single second magnetic layer alone is Hc₂, and the coercive force of the whole laminated film is HC₀. Figure 1 shows the sectional structure of the laminated film of an embodiment of the invention. On substrate 3, made of glass or plastic, a 100-angstrom-thick in-plane magnetized film 1 of Nd₂₀Co₈₀ was deposited as a first magnetized layer by a known sputtering process or vacuum deposition method, and on top of that a 1000-angstrom-thick perpendicular magnetized film 2 of Tb₂₅Fe₇₅ was deposited as a second magnetic layer. Figures 2 to 4 show the measured Kerr hysteresis loops of an Nd₂₀Co₈₀ single-layer film, a Tb₂₅Fe₇₅ single-layer film, and the laminated film of this embodiment, respectively. For the measurement of these Kerr hysteresis loops, an He-Ne laser beam of 630 nm in wavelength was used. As shown in Figure 2, with respect to the Nd₂₀Co₈₀ single-layer film, the obtained Kerr rotation angle is large but the perpendicular anisotropy is low. The latter property is shown by the fact that the coercive force is small and the loop lies almost obliquely. As shown in Figure 3, the obtained Kerr rotation angle is small with respect to the Tb₂₅Fe₇₅ single-layer film, but the perpendicular anisotropy is high. The latter property is shown by the large coercive force and the loop intersecting the H axis nearly at right angles, that is, by good squareness. Figure 4, in contrast, shows that a perpendicular magnetized film suitable for magneto-optical recording is obtained with good squareness of 2 kOe in coercive force and a large Kerr rotation angle in this embodiment. Such properties are considered to result from the magnetic exchange coupling between the two layers. In this connection, magnetic exchange coupling has been reported in some papers. However, no case has been reported where the lowness of the perpendicular anisotropy of a first film is covered by magnetic exchange coupling with a second film without a change in the magnitude of the magneto-optic effect of the first film. In Japanese Published Unexamined Patent Application No. 60-171652, for example, no more than an attempt to stabilizing pits by composing the recording film of two layers is disclosed. Figure 5 compares the dependence on the wavelength of the Kerr rotation angle k of the multilayer film of the present invention and that of the amorphous film of the conventional material, Tb₂₂Fe₆₈Co₁₀. As is apparent from the figure, this multilayer film has a much larger Kerr rotation angle than the conventional material, even at a wavelength of 500 nm, and therefore high-density recording with a short wavelength laser beam gives a high read output. Figures 6 and 7 show the dependence of the coercive force Hc and the Kerr rotation k of the double-layer film on the temperature. As shown in Figure 7, since a large Kerr rotation angle can be obtained even at high temperatures, the drop of output is low even if the temperature of the film is elevated by the laser beam at the time of reading, resulting in a high C/N ratio. The value shown as the Kerr rotation angle in the graph at a wavelength Xnm is the value of the Kerr rotation angle obtained by reading with a laser beam of wavelength Xnm after recording with a laser beam of the same wavelength Xnm. In the experiment, the data were collected while the wavelength of the laser beam was continuously varied. Embodiment 2. On a polycarbonate substrate is formed a laminated film consisting of a 400-angstrom-thick silicon nitride film, a 200-angstrom-thick Nd₂₀(Fe 0.8 Co 0.2 )₈₀ amorphous film, a 2000-angstrom-thick Tb₂₂Fe₆₈Co₁₀ amorphous film, a 1000-angstrom-thick silicon nitride film, and a 1000-angstrom-thick Al film to provide a perpendicular magnetized film with good squareness, as in Embodiment 1. Even at a wavelength of 500 nm, the Kerr rotation does not deteriorate, and the Kerr rotation angle of 0.75 degree is measured from the PC substrate side. The coercive force is as large as 10 kOe, and the material is suitable for practical magneto-optical recording. Embodiment 3. On a glass substrate is formed a laminated film comprising a 450 angstrom-thick silicon nitride film, a 20-angstrom-thick Nd₄₀(Co 0.5 Fe 0.5 )₅₅Cr₅ amorphous film, a 2000-angstrom-thick Tb₂₂Fe₆₀Co₁₀Pt₈ amorphous film, and a 1000-angstrom-thick aluminum nitride film to provide a perpendicular magnetized film with good squareness of 4 kOe in coercive force Hc, as in Embodiment 1. The Kerr rotation does not deteriorate even at a wavelength of 400 nm, and the Kerr rotation angle of 0.60 degree is measured from the PC substrate side. Embodiment 4. On a polycarbonate film is formed a laminated film comprising a 200-angstrom-thick Nd₁₀Pr₁₅Co₇₂Cr₃ amorphous film, a 2000-angstrom-thick Tb18Fe65Co12Cr5 amorphous film, and a 1000-angstrom-thick ZrO₂ film to provide a perpendicular magnetized film with good squareness of 4 kOe in coercive force Hc, as in Embodiment 1. The Kerr rotation does not deteriorate even at a wavelength of 400 nm, and a value as large as 0.5 degree is obtained without interference film. Embodiment 5. On a glass substrate are laminated alternately a 20-angstrom-thick Pr₂₃Co₇₄Ta₃ amorphous film and a 100-angstrom-thick Tb₁₈Fe₆₅Co₁₂Cr₅ amorphous film to a total thickness of about 1000 angstrom. The results of the experiment ensure that a perpendicular magnetized film with good squareness of 5 kOe (0,4 A/m) in coercive force Hc is obtained, as in Embodiment 1. The Kerr rotation angle is 0.45 degree at 500 nm wavelength. Embodiment 6. On a glass substrate are successively laminated a 200-angstrom-thick Pr₃₀Tb₁₀(Fe 0.7 Co 0.3 )₆₀ amorphous film, a 1000-angstrom-thick Dy₂₀( Fe 0.8 Co 0.2 )₇₆Nb₄ amorphous film, and a 1000-angstrom-thick SiN film. This gives a perpendicular magnetized film with good squareness of 5 kOe (0,4 A/m) in coercive force Hc, as in Embodiment 1. The Kerr rotation angle at 500 nm wavelength is as large as 0.55 degree without interference film. Embodiment 7. On a glass substrate is formed a first layer of 1000-angstrom-thick Tb₂₂Fe₆₈Co₁₀ amorphous film, then an Nd₁₈Tb₂(Fe 0.7 Co 0.3 )₈₀ amorphous film with a different thickness is laminated onto it to form nine types of double-layer film. Figure 8 shows, based on the data obtained on the double-layer film, the residual Kerr rotation angle k and the ratio of the coercive force of the double layer film to that of the single second layer alone HC₀/Hc₂ as a function of the film thickness ratio of the first layer to the second layer (t₁/t₂). Here, the residual Kerr rotation angle is measured by using a laser beam with a wavelength of 822 nm. The ratio of the coercive force indicates good squareness. That is, it shows the extent to which the double-layer film as a whole inherits the favorable quality of the coercive force (and thus the squareness) of the second layer. As can be seen from the figure, the residual Kerr rotation angle k has the highest values in the range of film thickness ratios from 0.001 to 0.2, while it decreases greatly when the ratio exceeds 0.5. The coercive force ratio HC₀/Hc₂ significantly decreases when the film thickness exceeds 0.5. Thus,in order to obtain a perpendicular magnetized film suitable for high-density magneto-optical recording, the ratio of film thickness (t₁/t₂) should be set to 0.5 or less, preferably in the range of 0.001 to 0.2. A magneto-optical recording medium is provided which has high magneto-optic effect even under a short-wavelength light, high anisotropy in the direction perpendicular to the film surface, and permits high-density recording.

Claims: A magneto-optical recording medium with strong magnetic anisotropy in the direction perpendicular to the surface having a first magnetic layer (1) comprising a first rare-earth-transition metal amorphous alloy, the rare-earth component of the first alloy predominently comprising a light rare-earth element or elements, the light rare-earth element or elements being Nd and/or Pr; a second magnetic layer (2) comprising a second rare earth-transition metal amorphous alloy, the rare-earth component of the second alloy being Tb and/or Dy wherein the transition metal component of the first alloy and/or the second alloy includes a transition metal selected from a group of transition metals for increasing the corrosion resistance wherein the group of transition metal elements for increasing the corrosion resistance comprises Ti, Zr, V, Nb, Ta, Cr, Mo, W, Pd, Pt, Au, and Al. A magneto-optical recording medium as claimed in Claim 1, wherein the ratio of the thickness of the first magnetic layer (1) to the second magnetic layer (2) is 0.5 or less. A magneto-optical recording medium as claimed in Claim 2, wherein the ratio of the thickness of the first magnetic layer (1) to the second layer (2) is in the range 0.001 to 0.2. A magneto-optical recording medium as claimed in any preceding claim, wherein the transition metal components of the first and/or second alloys comprise Fe and/or Co. A magneto-optical recording medium as claimed in any preceding claim, wherein the composition of the first alloy has the general formula (A 1-a B a ) l (Fe b Co 1-b ) m T n , wherein A is at least one of Nd and Fr, B is at least one element selected from a group of heavy rare earth elements, T is at least one element selected from a group of transition metal elements for increasing the corrosion resistance, 0 ≦ a ≦ 0.5, 0 ≦ b ≦ 1 and 0.1 ≦ l ≦ 0.5, 0.5 ≦ m ≦ 0.9 and l+m+n=1 and wherein the composition of the second alloy has the general Formula R x M y U z , wherein R is at least one of Tb and Dy, M is at least one of Fe and Co, U is at least one element selected from the group of transition metal elements for increasing the corrosion resistance, 0.15 ≦ x ≦ 0.35, 0.65 ≦ y ≦ 0.85, 0 ≦ z ≦ 0.15 and x+y+z=1. A magneto-optical recording medium as claimed in Claim 5 wherein the group of heavy rare-earth elements comprises Tb, Dy, Gd, Ho, and Er. A magneto-optical recording medium as claimed in any of Claims 1 to 4 wherein the first alloy is Nd 0.2 Fe 0.75 and the second alloy is Tb 0.25 Fe 0.75 . A magneto-optical recording medium as claimed in any of Claims 1 to 6 wherein the first alloy is Nd 0.4 (Co 0.5 Fe 0.5 )₀ .55 Cr 0.05 and wherein the second alloy is Tb 0.22 Fe 0.6 Co 0.1 Pt 0.08.

Inventors: KATO YOSHIMINE, SHIMIZU TERUHISA, TAKAYAMA SHINJI, TANAKA HIROSHI, KATO, YOSHIMINE, SHIMIZU, TERUHISA, TAKAYAMA, SHINJI, TANAKA, HIROSHI
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1990
Main classification: G11B11
IPC codes: G11B11