Magneto-optical recording medium

Abstract: A magneto-optical medium (1) comprises first, second and third layers. The first layer has a thickness of not less than 1 nm and not more than 20 nm and is in the form of a rare earth-transition metal amorphous film including at least one rare earth element selected from a group consisting of Tb, Dy, and Gd and including at least one transition metal element selected from a group consisting of Fe and Co. The lecond layer (2) has a thickness of not less than 1 nm and not more than 20 nm and is in the form of either a rare earth-transition metal amorphous film including at least one rare earth element selected from a group consisting of ND and Pr and including at least one transition metal element selected from a group consisting of Fe and Co, or a crystalline film including at least one element selected from a group consisting of Fe and Co. The third layer (3) has a thickness of not less than 2 nm and is in the form of a rare earth-transition metal amorphous film including at least one rare earth element selected from a group consisting of Tb, Dy, and Gd and including at least one transition metal element selected from a group consisting of Fe and Co. The layers are laminated successively, with the second layer disposed between the first and third layers and with the first layer being the first to be penetrated by light. The medium has an advantageously large magneto-optical effect even at short wavelengths, reorients the magnetization in a direction perpendicular to the film surface, and permits high-density recording.

Description: The present invention relates to a magneto-optical recording medium having a large magneto-optical effect when exposed to short-wavelength light and hence suitable for high-density recording. Magneto-optical recording has been put to practical use as an optical recording method that permits rewriting. Heretofore, single-layered rare earth-transition metal amorphous films having a strong magnetic anisotropy in a direction perpendicular to the film surface have been employed as magneto-optical recording films. In particular, TbFeCo alloy amorphous films are being researched and developed for practical use. An increase in the recording density is a major aim in the field of magneto-optical recording. Shortening of the wavelength of writing/reading light is currently being studied as a method of achieving this. To increase the magneto-optical recording density in future, magneto-optical materials that have a high magneto-optical effect (the Kerr effect and Faraday effect) at short wavelengths are being developed. However, with conventional TbFeCo alloy amorphous films, the magneto-optical effect tends to decrease monotonically as the wavelength of a laser beam becomes shorter, and sufficiently large Kerr rotation or Faraday rotation is not obtained at short wavelengths, which results in a decrease in the output when it is read out by a laser beam. A known amorphous film whichis an alloy of a transition metal consisting mainly of Co and Fe and of rare earth elements consisting mainly of Nd and Pr (see, for instance, T.R. McGuire et al., "Magneto-optical Properties of Nd-Co and Nd-Fe Alloys", J. Appl. Phys. 61(8), 15 April, 1987, pp. 3352-3354), has a large magneto-optical effect at short wavelengths. However, this film has in-plane magnetization but not perpendicular magnetization, and cannot facilitate high-density recording. A double-layered film formed by laminating a Nd alloy rare earth-transition metal amorphous film and a TbFeCo amorphous film to utilize exchange coupling between them is proposed by Ito et al in "Magnetic and Magneto-optical properties of Nd alloy multilayers," Digest of the 13th annual conference on magnetics in Japan (1989), p 325. However, the double-layered film disclosed therein has a Kerr rotation angle of 0.3 degrees at a wavelength of 400 nm, and does not fully facilitate intrinsic Kerr rotation of Nd alloys. EP-A-033 467 describes an optical magnetic recording medium in which a first magnetic layer having a relatively higher coercive force is paired with a second magnetic layer have a relatively lower magnetic coercive force. The first layer is formed from a rare earth-iron group amorphous alloy and the second layer is formed from a rare earth-iron group amorphous alloy including a light rare earth element. Among non-amorphous materials, crystalline Co and Fe are known to have large magneto-optical effects at short wavelengths. However, they are not available in the form of perpendicularly magnetized films and have received almost no attention heretofore as magneto-optical materials. Here again, attempts have been made to obtain a perpendicular-magnetized film by laminating an in-plane magnetized film Co and a perpendicular-magnetized film TbFeCo to utilize exchange coupling between two films (see H Wakabayashi et al, "Magnetic and magneto-optical properties of Co/TbFeCo exchange coupled films," Digest of the 13th annual conference on magnetics in Japan (1989), p 326). However, to obtain a double-layered film that has good symmetry according to the proposed approach, the thickness of Co must be 25 A or less. Therefore, this film can have only a low Kerr rotation angle derived from the TbFeCo film alone at short wavelengths. In accordance with the present invention, there is now provided a magneto-optical recording medium comprising: a glass substrate; a first magnetic layer with a thickness of not less than 1 nm and not more than 20 nm in the form of a rare earth-transition metal amorphous film including at least one rare earth element and at least one transition metal element, the or each rare earth metal element being selected from Tb, Dy, and Gd, and the or each transition metal element being selected from Fe and Co; a second magnetic layer with a thickness of not less than 1 nm and not more than 20 nm in the form of either a rare earth-transition metal amorphous film including at least one rare earth element selected from Nd and Pr and including at least one transition metal element selected from Fe and Co, or a crystalline film including at least one element selected from Fe and Co; and, a third magnetic layer with thickness of not less than 2 nm in the form of a rare earth-transition metal amorphous film including at least one rare earth element and at least one transition metal element, the or each rare earth metal element being selected from Tb, Dy, and Gd, and the or each transition metal element being selected from Fe and Co; the layers being laminated successively on the glass substrate, with the first layer being adjacent the glass substrate and the second layer disposed between the first and third layers, and with the first layer being the first to be penetrated by light incident on the glass substrate. This advantageously provides a magneto-optical recording medium that has sufficiently large magneto-optical effect even at short wavelengths, reorients the magnetization in a direction perpendicular to the film surface, and permits high-density recording. The thickness of the first layer is required to be not more than 20 nm, to allow a sufficient amount of light to be transmitted and to make the best use of the excellent magneto-optical effect of the second layer, and not less than 1 nm, to exert magnetic exchange coupling on the second layer and thus to reorient a sufficient strength of magnetization perpendicularly. The thickness of the second layer is required to be not more than 20 nm, to allow magnetic exchange coupling to be received from the first layer and the third layer, and not less than 1 nm, to ensure an excellent magneto-optical effect at short wavelengths. The thickness of the third layer is required to be not less than 2 nm, or preferably not less than 5 nm, to allow a sufficient strength of magnetic exchange coupling to be exerted. From the viewpoint of the cost, the supper limit is preferably 200 nm. The preferred curie temperatures of the first and third layers are not more than 300°C, making them suitable for writing under heat by a laser beam. The more preferable temperature range is not less than 100°C and not more than 200°C. The preferred composition of the second magnetic layer is represented by the formula (A 1-a B a ) x (Fe 1-b Co b ) y T z ) where A is at least one element selected from a group consisting of Nd and Pr, B is at least one element selected from a group consisting of Tb, Dy, and Gd, T is at least one element selected from a group of transition metals for improving anticorrosion, 0 ≦ a ≦ 0.9, 0 ≦ b ≦ 1, 0 ≦ x ≦ 40,0 ≦ z ≦ 10, and x + y + z = 100. Examples of the first and third magnetic layers are TbFeCo, GdTbFeCo, DyTbFeCo, DyFeCo, and DyGdFeCo films. The group T of transition metal elements consists of Ti, Zr, V, Nb, Ta, Cr, Mo, W, Pd, Pt, Au, and Al, which are known to improve anticorrosion. These elements may be included in the first or third layer. Since excessive addition of these elements would destroy the expected properties of the respective magnetic films, the additional amount, in either film, must be within 10 atomic percentage points. Preferred embodiments of the present invention will now be described with reference to the accompanying drawings, in which Figure 1 is a cross-sectional view of the general construction of a magneto-optical recording medium; Figure 2 is a graph showing the relationship between the thickness of the first layer and the Kerr rotation angle in a TbFeCo/NdCo/TbFeCo trilayer film; Figure 3 is a graph showing the Kerr hysteresis curve of an example of such a trilayer film; Figure 4 is a graph showing the relationship between the thickness of the second layer and the Kerr rotation angle in a TbFeCoCr/NdFeCo/TbFeCo trilayer film; Figure 5 is a graph showing the relationship between the thickness of the third layer and the Kerr rotation angle in a TbFeCoTa/PrFeCo/TbFeCoTa trilayer film; Figure 6 is a graph showing the relationship between the incident light wavelengths and the figures of merit in a TbFeCo/NdCo/TbFeCo trilayer film and a TbFeCo single layer film; Figure 7 is a graph showing the relationship between the thickness of the first layer and the Kerr rotation angle of a TbFeCo/Fe/TbFeCo trilayer film; and Figure 8 is a graph showing the relationship between the thickness of the second layer and the Kerr rotation angle of a TbFeCo/Co/TbFeCo trilayer film. Figure 1 is a cross-sectional view of the general construction of a triple-layered structure consisting of a first layer 1, a second layer 2, and a third layer 3 made on a glass substrate 4. A laser beam L first penetrates the first layer of the trilayer film. All films in experimental examples referred to hereinafter were made by a DC magnetron sputtering method. Several samples were made in which Tb21Fe75Co4 (coercivity Hc = 5 kOe, Curie temperature Tc = 200° C) amorphous films were used for the first and third layers and a Nd20Co80 amorphous layer for the second layer, with a constant thickness of 10 nm for the second layer, a constant thickness of 100 nm for the third layer, and a variable thickness tl for the first layer. Figure 2 shows the values of the remanent Kerr rotation angle measured by a laser beam of wavelength 400 nm incident from the glass substrate. This figure shows that the first layer is required to have a thickness of not less than 1 nm and not more than 20 nm in order to obtain a Kerr rotation angle of 0.3 or more degrees, which is suitable for practical use. Figure 3 shows a representative Kerr hysteresis loop measured by a laser beam of wavelength 400 nm incident from the glass substrate of a sample whose first layer is 5 nm thick. The figure shows that a perpendicular-magnetized film suitable for magneto-optical recording is obtained, showing an excellent squareness with a coercivity of 2 kOe. The other characteristics of Tb21Fe75Co4(t1 = 5 nm)/Co(t2 = 5 nm)/Tb21Fe75Co4(t3 = 100 nm) were as follows: coercivity = 2 kOe, and Kerr rotation angle ϑk = 0.4 degrees (measured at 400 nm wavelength). The characteristics of Tb21Fe73Co6(t1 = 7 nm)/Fe(t2 = 7nm)/Tb21Fe73Co6(t3 = 100 nm) were as follows: coercivity =3 kOe, and Kerr rotation angle ϑk = 0.35 degrees (measured at 400 nm wavelength). The Kerr hysteresis loop in both triple-layered films had excellent symmetry. Also, several kinds of samples were also made in which a Tb20Fe45Co29Cr6 (coercivity Hc = 7 kOe, Curie temperature Tc = 350°C) amorphous film was used as the first layer, a Nd20(fe0.65Co0.35)80 amorphous layer as the second layer, and a Tb21Fe75Co4 amorphous film as the third layer, with a constant thickness of 5 nm for the first layer, a constant thickness of 100 nm for the third layer, and a variable thickness t2 for the second layer. Figure 4 shows the values of the remanent Kerr rotation angle measured by a laser beam of wavelength 400 nm incident from the glass substrate. The figure shows that a Kerr rotation angle of 0.3 or more degrees, suitable for practical use, is obtained when the range of thickness of the second layer is not less than 1 nm and not more than 20 nm. Also, several kinds of samples were made in which Tb24Fe65Co6Ta5 (coercivity Hc = 8 kOe, Curie temperature Tc = 150°C) amorphous films were used for the first and third layers and a Pr20(Fe0.5Co0.5)80 amorphous layer for the second layer, with a constant thickness of 5 nm for the first layer, a constant thickness of 15 nm for the second layer, and a variable thickness t3 for the third layer. Figure 5 shows the values of the remanent Kerr rotation angle measured by a laser beam of wavelength 400 nm incident from the glass substrate. The figure shows that a Kerr rotation angle of 0.3 or more degrees, suitable for practical use, is obtained when the range of thickness of the third layer is not less than 2 nm. For a triple-layered film with a first layer in the form of a 5-nm-thick Tb19Fe67Co10Nb4 film, a second layer in the form of a 10-nm-thick Pr20Fe24Co56 film, and a third layer in the form of 50-nm-thick Tb19Fe67Co10Nb4 film, the Kerr rotation angle measured under the same conditions was 0.4 degrees, and the coercivity was 2 kOe. The Kerr hysteresis loop had excellent symmetry. Figure 6 shows the figure of merit (the product of the square root of the reflection ratio and the Kerr rotation angle) of a triple-layered film of Tb18Fe49Co33 (5 nm thick)/Nd20Co80 (10 nm thick)/ Tb18Fe49Co33 (100 nm thick) as a function of the wavelength of incident light (1). The figure also shows the figure of merit of a conventional Tb25Fe65Co10 film for comparison (2). It is evident from the figure that the triple-layered film according to the invention has a higher figure of merit than the conventional material throughout the entire wavelength range, and is an excellent magneto-optical recording material. The following explanation concerns experimental examples using crystalline Co, Fe, or an alloy of them as the second layer. Several kinds of samples were made, in which Tb18Fe49Co33 amorphous films were used for the first and third layers and an Fe crystalline film for the second layer, with a constant thickness of 7 nm for the second layer, a constant thickness of 100 nm for the third layer, and a variable thickness t1 for the first layer. Figure 7 shows the values of the remanent Kerr rotation angle measured by a laser beam of wavelength 400 nm incident from the glass substrate. This shows that the first layer is required to have a thickness of not less than 1 nm and not more than 20 nm in order to obtain a Kerr rotation angle of 0.3 or more degrees, suitable for practical use, as when the second layer is a rare earth-transition metal amorphous film. Also, several kinds of samples were made in which Tb18Fe49Co33 amorphous films were used for the first and third layers and a Co crystalline film for the second layer, with a constant thickness of 10 nm for the first layer, a constant thickness of 100 nm for the third layer, and a variable thickness t2 for the second layer. Figure 8 shows the values of the remanent Kerr rotation angle measured by a laser beam of wavelength 400 nm incident from the glass substrate. The figure shows that the second layer is required to have a thickness of not less than 1 nm and not more than 20 nm in order to obtain a Kerr rotation angle of 0.3 or more degrees, suitable for practical use, as when the second layer is a rare earth-transition metal amorphous film. The characteristics of Tb18Fe49Co33(t1 = 10 nm)/Fe70Co30(t2 = 5 nm)/Tb18Fe49Co33(t3 = 100 nm) samples were as follows: coercivity = 1.4 kOe, and Kerr rotation angle ϑk = 0.45 degrees (measured at 400 nm wavelength). The characteristics of Tb18Fe49Co33(t1 = 20 nm)/Fe70Co30(t2 = 5 nm)/Tb18Fe49Co33(t3 = 100 nm) were as follows: coercivity = 1.9 kOe, and Kerr rotation angle ϑk = 0.30 degrees (measured at 400 nm wavelength). The characteristics of Tb18Fe49Co33(t1 = 10 nm)/Fe50Co50(t2 = 7 nm)/Tb18Fe49Co33(t3 = 100 nm) were as follows: coercivity = 1.7 kOe, and Kerr rotation angle ϑk = 0.40 degrees (measured at 400 nm wavelength). All of the samples have high Kerr rotation at short wavelengths, and high coercivity, making them suitable for practical use. The Kerr hysteresis loop in both triple-layered films had excellent symmetry. Specific experimental examples have been given for various cases in which the first and third layers are TbFeCo alloy films and the second layer includes a rare earth element of a group consisting of Nd or Pr (hereafter called the Nd group). An objective of the present invention is to obtain a magneto-optical recording film with a high perpendicular magnetic anisotropy and a high Kerr rotation angle at short wavelengths, by sandwiching a magneto-opticsl recording film that has an in-plane magnetization but a high Kerr rotation angle at short wavelengths between magneto-optical recording films with a high perpendicular magnetic anisotropy (even with a low Kerr rotation angle at short wavelengths) and by optimizing their respective thickness. Therefore, any magneto-optical recording medium that has substantially the same characteristics as those of TbFeCo alloys may be used for the first and third layers. For instance, a GdTbFeCo film may be used for the first or third layer, since GdTbFeCo is not inferior to TbFeCo in Kerr rotation angle and perpendicular magnetic anisotropy, as shown by D.K. Hairston et al. in Figure 2 of "The TM dependence of the magneto-optic signal in GbTb-TM thin films," J. Appl. Phys. 63(8), 15 April 1988, pp. 3621-3623. Further, evaluation of a DyTbFeCo disk and a TbFeCo disk in Figure 4 of "Compositional dependence of recording noise in amorphous rare-earth-transition metal magneto-optical disks," J. Appl. Phys. 63(8), 15 April 1988, pp. 3856-3858, shows that both are substantially equal in performance. In view of this, the TbFeCo film may also be replaced by a DyTbFeCo film. Moreover, as shown by Endo et al. in "Magnetic and magneto-optic properties of amorphous Dy-Fe-Co and Tb-Fe-Co films," Journal of the Magnetics Society of Japan, Vol. 8, No. 2, 1984, pp. 101-104, a DyFeCo film has magnetic and magneto-optical properties similar to those of a TbFeCo film. Therefore, a DyFeCo film may be used for the first or third layer instead of the TbFeCo film. As shown by Sumi et al. in "Read/write characteristics of GdDyFeCo magneto-optical disk," Digest of the 11th annual conference on magnetics in Japan (1987), P. 273, a GdDyFeCo film is a magneto-optical recording material that has substantially the same properties as those of a TbFeCo film. Therefore, the TbFeCo film may be replaced by a GdDyFeCo film. At least one rare earth element selected from a group consisting of Tb, Dy, and Gd (hereafter called the Tb group) may be added to the second layer. This is because addition of an element of the Tb group to the second layer enhances the perpendicular magnetic anisotropy of the second layer itself and hence increases the perpendicular magnetic anisotropy of the entire trilayer film. However, if the additional amount of the Tb group element is excessive, the amount of the Nd group element becomes insufficient, and the second layer and hence the entire trilayer film cannot have a high Kerr rotation angle at short wavelengths. Therefore, the additional amount of the Tb group element should be not more than 90 atomic percent of all the rare earth elements. One example of a rare earth-transition metal amorphous film with an additional element of the Tb group that can be used for the second layer is an NdGdFeCo film. As shown by Ito et al. in Figure 3 of "Magnetic and Magneto-optical properties of Nd alloy multilayers," Digest of the 13th annual conference on magnetics in Japan (1989), p. 325, a single-layered film of Nd5Gd22(Fe68Co32)73 actually has a Kerr rotation angle of 0.336 degrees at 400 nm. This is larger than the value of TbFeCo at 400 nm (approximately 0.2 degrees), and the shape of its spectrum demonstrates that the decrease in the Kerr rotation angle at short wavelengths is small. Therefore, it is expected that the use of this film as the second layer of the trilayer film of the present invention will also result in a sufficiently large Kerr rotation angle at a short wavelength range. Another example of a rare earth-transition metal amorphous film with an additional element of the Tb group that can be used for the second layer is a NdTbFeCo film. As shown by Ota in Figure 7 of "High density recording of optical memory," Journal of the Magnetics Society of Japan, Vol. 14, No. 4, 1990, pp. 617-623, an Nd12Tb16Fe36Co36 film has substantially the same properties as the NdGdFeCo film, and its use as the second layer is expected to result in a sufficiently large Kerr rotation angle at short wavelengths. A NdDyFeCo film may also be used for the second layer, because its magneto-optical properties do not vary much, even when Tb is replaced by Dy in th rare earth-transition metal amorphous layer, as described in the foregoing article by Endo et al. It will now be appreciated that the present invention provides a magneto-optical recording medium that has a sufficiently large magneto-optical effect even at short wavelengths and is a perpendicular-magnetized film.

Claims: A magneto-optical recording medium comprising: a glass substrate (4); a first magnetic layer (1) with a thickness of not less than 1 nm and not more than 20 nm in the form of a rare earth-transition metal amorphous film including at least one rare earth element and at least one transition metal element, the or each rare earth metal element being selected from Tb, Dy, and Gd, and the or each transition metal element being selected from Fe and Co; a second magnetic layer (2) with a thickness of not less than 1 nm and not more than 20 nm in the form of either a rare earth-transition metal amorphous film including at least one rare earth element selected from Nd and Pr and including at least one transition metal element selected from Fe and Co, or a crystalline film including at least one element selected from Fe and Co; and, a third magnetic layer (3) with thickness of not less than 2 nm in the form of a rare earth-transition metal amorphous film including at least one rare earth element and at least one transition metal element, the or each rare earth metal element being selected from Tb, Dy, and Gd, and the or each transition metal element being selected from Fe and Co; the layers being laminated successively on the glass substrate, with the first layer being adjacent the glass substrate and the second layer disposed between the first and third layers, and with the first layer being the first to be penetrated by light incident on the glass substrate. A magneto-optical medium as claimed in claim 1 wherein said second layer is represented by the formula (A 1-a B a ) x (Fe 1-b Co b ) y T z , where A is at least one element selected from a group consisting of Nd and Pr, B is at least one element selected from a group consisting of Tb, Dy, and Gd, T is at least one element selected from a group of transition metals for improving anticorrosion, 0 ≦ a ≦ 0.9, 0 ≦ b ≦ 1, 0 ≦ x ≦ 40, 0 ≦ z ≦ 10, and x + y + z = 100. A magneto-optical medium as claimed in claims 1 and 2 wherein said third layer is a TbFeCo film, a GdTbFeCo film, a DyTbFeCo film, a DyFeCo film, or a DyGdFeCo film. A magneto-optical medium as claimed in claims 1 to 3 wherein said first and third magnetic layers include at least one element selected from a group of transition metals for improving anticorrosion. A magneto-optical medium as claimed in claims 1 to 4 wherein said group of transition metals for improving anticorrosion consists of Ti, Zr, V, Nb, Ta, Cr, Mo, W, Pd, Pt, Au, and Al.

Inventors: IIYORI HIDEO C O MR AKAO, TAKAYAMA SHINJI, IIYORI, HIDEO, C/O MR AKAO, TAKAYAMA, SHINJI
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1991
Main classification: G11B11
IPC codes: H01F10, G11B11, C22C45