TREA

Platinum or palladium/cobalt multilayer on a zinc oxide or indium oxide layer for magneto-optical recording

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Information

  • Patent Grant
  • 5082749
  • Patent Number
    5,082,749
  • Date Filed
    Thursday, December 27, 1990
    34 years ago
  • Date Issued
    Tuesday, January 21, 1992
    33 years ago
Information
Abstract
This invention provides a magneto-optical recording medium comprised of a substrate, a base layer of zinc oxide or indium oxide sputtered onto the substrate and a multilayer recording film sputtered onto the base layer of zinc oxide or indium oxide, wherein the base layer or zinc oxide or indium oxide is from about 200 A to about 4500 A (about 20 nm to about 450 nm) thick, preferably from about 200 A to about 2000 A (about 20 nm to about 200 nm) thick, the multilayer recording film is a platinum/cobalt or a palladium/cobalt multilayer film consisting of alternating layers of platinum and cobalt or palladium and cobalt, and the substrate is transparent to the radiation used to record and read.
Description
Claims
  • 1. A magneto-optical recording medium composed of a substrate, a base layer of zinc oxide or indium oxide sputtered onto said substrate and a multilayer recording film sputtered onto said base layer of zinc oxide or indium oxide, wherein said base layer of zinc oxide or indium oxide is from about 200 A to about 4500 A (about 20 nm to about 450 nm) thick, said multilayer recording film is a plantinum/cobalt multilayer film or a palladium/cobalt multilayer film consisting of alternating layers of platinum and cobalt or palladium and cobalt. wherein each of said palladium or plantinum layers has a thickness less than about 24 A and each of said cobalt layers has a thickness less than about 12 A, and said substrate is transparent to the radiation used to record and read, and the sputter gas used in sputtering said multilayer film is argon, krypton, xenon or a mixture thereof.
  • 2. The magneto-optical recording medium of claim 1 wherein said base layer is zinc oxide.
  • 3. The magneto-optical recording medium of claim 2 wherein said multilayer recording film is a platinum/cobalt multilayer film consisting of alternating layers of platinum and cobalt.
  • 4. The magneto-optical recording medium of claim 3 wherein all said cobalt layers of said platinum/cobalt multilayer film have substantially the same thickness d.sub.Co and all said platinum layers of said platinum/cobalt multilayer film have substantially the same thickness d.sub.Pt, nm) and the total thickness of the platinum/cobalt multilayer film is less than about 750 A (75 nm).
  • 5. The magneto-optical recording medium of claim 4 wherein d.sub.Co is from about 2 to about 5 A (0.2 to about 0.5 nm) and d.sub.Pt /d.sub.Co is from about 1 to about 5.
  • 6. The magneto-optical recording medium of claim 2 wherein said base layer of zinc oxide is from about 200 A to about 2000 A (about 20 nm to about 200 nm) thick.
  • 7. The magneto-optical recording medium of claim 6 wherein said multilayer recording film is a platinum/cobalt multilayer film consisting of alternating layers of platinum and cobalt.
  • 8. The magneto-optical recording medium of claim 7 wherein all said cobalt layers of said platinum/cobalt multilayer film have substantially the same thickness d.sub.Co and all said platinum layers of said platinum/cobalt multilayer film have substantially the same thickness d.sub.Pt, and the total thickness of the platinum/cobalt multilayer film is less than about 750 A (75 nm).
  • 9. The magneto-optical recording medium of claim 8 wherein d.sub.Co is from about 2 to about 5 A (0.2 to about 0.5 nm) and d.sub.Pt /d.sub.Co is from about 1 to about 5.
  • 10. The magneto-optical recording medium of claim 2 wherein said sputter gas used in sputtering said multilayer film is krypton, xenon or a mixture thereof.
  • 11. The magneto-optical recording medium of claim 10 wherein the gas pressure of said sputtering gas is about 2 to about 12 mTorr (about 0.27 to about 1.6 Pa).
  • 12. The magneto-optical recording medium of claim 11 wherein said multilayer recording film is a platinum/cobalt multilayer film consisting of alternating layers of platinum and cobalt.
  • 13. The magneto-optical recording medium of claim 12 wherein all said cobalt layers of said platinum/cobalt multilayer film have substantially the same d.sub.Co and all said platinum layers of said platinum/cobalt multilayer film have substantially the same thickness d.sub.Pt, and the total thickness of the platinum/cobalt multilayer film is less than about 750 A (75 nm).
  • 14. The magneto-optical recording medium of claim 13 wherein d.sub.Co is from about 2 to about 5 A (0.2 to about 0.5 nm) and dPt/dCo is from about 1 to about 5.
  • 15. The magneto-optical recording medium of claim 6 wherein said sputter gas used in sputtering said multilayer film is krypton, xenon or a mixture thereof.
  • 16. The magneto-optical recording medium of claim 15 wherein the gas pressure of said sputtering gas is about 2 to about 12 mTorr (about 0.27 to about 1.6 Pa).
  • 17. The magneto-optical recording medium of claim 16 wherein said multilayer recording film is a platinum/cobalt multilayer film consisting of alternating layers of platinum and cobalt.
  • 18. The magneto-optical recording medium of claim 17 wherein all said cobalt layers of said platinum/cobalt multilayer film have substantially the same thickness d.sub.Co and all said platinum layers of said platinum/cobalt multilayer film have substantially the same thickness d.sub.Pt, and the total thickness of the platinum/cobalt multilayer film is less than about 750 A (75 nm).
  • 19. The magneto-optical recording medium of claim 18 wherein d.sub.Co is from about 2 to about 5 A (0.2 to about 0.5 nm) and d.sub.Pt /d.sub.Co is from about 1 to about 5.
  • 20. The magneto-optical recording medium of claim 1 wherein said base layer is indium oxide.
  • 21. The magneto-optical recording medium of claim 20 wherein said multilayer recording film is a platinum/cobalt multilayer film consisting of alternating layers of platinum and cobalt.
  • 22. The magneto-optical recording medium of claim 21 wherein all said cobalt layers of said platinum/cobalt multilayer film have substantially the same thickness d.sub.Co and all said platinum layers of said platinum/cobalt multilayer film have substantially the same thickness d.sub.Pt, and the total thickness of the platinum/cobalt multilayer film is less than about 750 A (75 nm).
  • 23. The magneto-optical recording medium of claim 22 wherein d.sub.Co is from about 2 to about 5 A (0.2 to about 0.5 nm) and d.sub.Pt /d.sub.Co is from about 1 to about 5.
  • 24. The magneto-optical recording medium of claim 20 wherein said base layer of indium oxide is from about 200 A to about 2000 A (about 20 nm to about 200 nm) thick.
  • 25. The magneto-optical recording medium of claim 24 wherein said multilayer recording film is a platinum/cobalt multilayer film consisting of alternating layers of platinum and cobalt.
  • 26. The magneto-optical recording medium of claim 25 wherein all said cobalt layers of said platinum/cobalt multilayer film have substantially the same thickness d.sub.Co and all said platinum layers of said platinum/cobalt multilayer film have substantially the same thickness d.sub.Pt, and the total thickness of the platinum/cobalt multilayer film is less than about 750 A (75 nm).
  • 27. The magneto-optical recording medium of claim 26 wherein d.sub.Co is from about 2 to about 5 A (0.2 to about 0.5 nm) and d.sub.Pt /d.sub.Co is from about 1 to about 5.
  • 28. The magneto-optical recording medium of claim 20 wherein said sputter gas used in sputtering said multilayer film is krypton, xenon or a mixture thereof.
  • 29. The magneto-optical recording medium of claim 28 wherein the gas pressure of said sputtering gas is about 2 to about 12 mTorr (about 0.27 to about 1.6 Pa).
  • 30. The magneto-optical recording medium of claim 29 wherein said multilayer recording film is a platinum/cobalt multilayer film consisting of alternating layers of platinum and cobalt.
  • 31. The magneto-optical recording medium of claim 30 wherein all said cobalt layers of said platinum/cobalt multilayer film have substantially the same thickness d.sub.Co and all said platinum layers of said platinum/cobalt multilayer film have substantially the same thickness d.sub.Pt, and the total thickness of the platinum/cobalt multilayer film is less than about 750 A (75 nm).
  • 32. The magneto-optical recording medium of claim 31 wherein d.sub.Co is from about 2 to about 5 A (0.2 to about 0.5 nm) and d.sub.Pt /d.sub.Co is from about 1 to about 5.
  • 33. The magneto-optical recording medium of claim 24 wherein said sputter gas used in sputtering said multilayer film is krypton, xenon or a mixture thereof.
  • 34. The magneto-optical recording medium of claim 33 wherein the gas pressure of said sputtering gas is about 2 to about 12 mTorr (about 0.27 to about 1.6 Pa).
  • 35. The magneto-optical recording medium of claim 34 wherein said multilayer recording film is a platinum/cobalt multilayer film consisting of alternating layers of platinum and cobalt.
  • 36. The magneto-optical recording medium of claim 35 wherein all said cobalt layers of said platinum/cobalt multilayer film have substantially the same thickness d.sub.Co and all said platinum layers of said platinum/cobalt multilayer film have substantially the same thickness d.sub.Pt, and the total thickness of the platinum/cobalt multilayer film is less than about 750 A (75 nm).
  • 37. The magneto-optical recording medium of claim 36 wherein d.sub.Co is from about 2 to about 5 A (0.2 to about 0.5 nm) and d.sub.Pt /d.sub.Co is from about 1 to about 5.
SUMMARY OF THE INVENTION

This application is a continuation of Application Ser. No. 07/494,207 filed 3/15/90 is now abandoned. 1. Field of the Invention This invention relates to a magneto-optical recording medium comprised of a platinum or palladium/cobalt multilayer film with a zinc oxide or indium oxide base layer between the multilayer film and the substrate. 2. References Thin films with perpendicular magnetic anisotropy are potential candidates in high density magnetic and magneto-optical recording. Examples of materials useful in preparing such films include the oxide garnets and ferrites, amorphous rare-earth transition metal alloys, metal alloys such as CoCr, and metal multilayers of platinum/cobalt (Pt/Co) and palladium/cobalt (Pd/Co), as disclosed in P. F. Carcia, U.S. Pat. No. 4,587,176. In order to be useful for magneto-optical recording, a material must have additional attributes besides perpendicular magnetic anisotropy. These requirements include a square hysteresis loop, a sufficient Kerr effect, a large room temperature coercivity H.sub.c, and switching characteristics compatible with available laser power and magnetic field strength. Pt/Co multilayers are a promising new candidate for a magneto-optical (MO) recording medium. They have perpendicular magnetic anisotropy, square hysteresis loops, a modest Kerr rotation, and excellent environmental stability. These attributes are discussed in detail in several recent publications, W. B. Zeper et al., J. Appl. Phys. 65, 4971 (1989), F. J. A. M. Greidanus, Appl. Phys. Lett. 54, 2481 (1989) and P. F. Carcia et al., Mat. Res. Soc. Symp. Proc., 159,115 (1989). When compared with the rare-earth transition-metal (RE-TM) alloys, the Pt/Co multilayer has superior corrosion resistance and a larger Kerr effect, i.e., a larger magneto-optical signal, at shorter wavelengths which allows higher density recording. A magnetic coercivity H.sub.c of about 1000 Oe is adequate for some recording applications, but higher coercivities would be more attractive and facilitate practical applications. Pt/Co multilayers prepared by vapor-deposition, (electron-beam evaporation from separate Pt and Co sources) have H.sub.c of about 1000 Oe. Sputtering is the preferred manufacturing process for preparing these multilayer films because it is simpler than other processes and the results obtained are more reproducible. However, sputtered Pt/Co multilayers have coercivities too small for magneto-optical recording. For example, Ochiai et al., Jap. J. Appl. Phys. 28, L659 (1989) and Ochiai et al., Digest of the Int'l. Mag. Conf.-1989. Wash., D. C., report H.sub.c of only 100-350 Oe for sputtered Pt/Co multilayers which they prepared by using argon as the sputter gas. The commonly assigned patent application, "Improved Process for Sputtering Multilayers for Magneto-optical Recording", Ser. No. 441,499, filed Nov. 27, 1989, in the name of P. F. Carcia, discloses that sputtering Pt/Co multilayers in krypton or xenon gas instead of argon results in coercivities of the order of 1000 Oe, comparable to vapor deposited films. A practical magneto-optical disk structure incorporates a transparent dielectric layer, i.e., a base layer, between the substrate and the magnetic layer. In addition, a similar dielectric layer may be deposited on the magnetic layer. The primary role of the dielectric layer is to enhance by optical interference the Kerr rotation of the magnetic layer and the reading efficiency. For example, Bernstein et al., IEEE Transactions on Magnetics, 21, 1613 (1985), discuss different dielectric configurations to increase the Kerr signal of rare-earth transition metal alloys. When the MO layer is a RE-TM alloy, e.g. TbFeCo, a dielectric layer deposited on the magnetic layer also serves to protect the very sensitive rare earth elements from oxidation, which can compromise their MO properties. Usually nitrides (e.g., Si.sub.3 N.sub.4 or AlN) are used because most oxides are easily chemically reduced by the rare earth elements and are consequently no longer protective. Because Pt/Co multilayers are much less sensitive to oxidation, the choice of a dielectric enhancement layer is not restricted to nitrides and a dielectric layer is not needed to protect the magnetic layer from the atmosphere. As a result other materials can be used as the dielectric and a dielectric with a larger optical index of refraction can further increase the Kerr signal. In addition, the dielectric may be used as a base layer. It is highly preferred to read and write with laser light incident from the substrate side of the MO structure because the relatively thick substrate defocuses the effects of any surface foreign particles so that they do not obstruct or distort the read/write process. Recording on the air incident side would require adding an additional thick defocusing layer on the magnetic layer. Some attempts have been made to increase the coercivity of multilayers by using base layers. Y. Ochiai et al., EP 0304873 disclose studies of sputtered Pt/Co multilayers including the use of underlayers to increase H.sub.c. The underlayer (base layer) was formed from a metal, at least one of Cu, Rh, Pd, Ag, Ir, Pt and Au having face-centered cubic structure and W having a body-centered cubic structure, or from dielectric materials, including oxides such as A1.sub.2 O.sub.3, Ta.sub.2 O.sub.5, MgO, SiO.sub.2, TiO.sub.2, Fe.sub.2 O.sub.3, ZrO.sub.2, or Bi.sub.2 O.sub.3, nitrides such as ZrN, TiN, Si.sub.3 N.sub.4, AlN, AlSiN, BN, TaN or NbN, or oxide-nitride composite materials, oxynitride compounds. However, generally only marginal improvements in H.sub.c were achieved and the singular best result of 725 Oe requires a 1000 A (100 nm) thick Pt underlayer. This is impractical for most magneto-optical recording applications because it prohibits reading and writing information from the substrate side, and the large heat capacity and thermal diffusivity of such a thick Pt layer will likely prevent writing with the limited power available with current solid-state lasers. An Hc of 700 Oe was obtained with a 350 A (35 nm) underlayer of Ta.sub.2 O.sub.5 in conjunction with an about 600 A (60 nm) Ta.sub.2 O.sub.5 upper layer, i.e., on top of the multilayer, plus a Pt reflective layer on the Ta.sub.2 O.sub.5 upper layer. Large increases in the H.sub.c of Pd/Co multilayers were achieved with Pt or Pd underlayers and H.sub.c increased from 825 Oe with no Pd underlayer to 3750 Oe with a 400 A (40 nm) Pd underlayer. However, the Kerr effect for the Pd/Co multilayers is lower and therefore Pd/Co multilayers are less attractive than Pt/Co multilayers. S. Hashimoto et al., Proceedings of the 13th Conference of the Applied Magnetics Association of Japan, p. 56 (1989), disclose the increase in H.sub.c of sputtered Pt/Co and Pd/Co multilayers by a metal base layer when the metal is a face-centered cubic metal or the body-centered cubic W. Pt and Pd were most effective. The maximum H.sub.c of about 1000 Oe for Pt/Co multilayers was obtained when the base layer thickness reached 400-500 A (40-50 nm). Thicker base layers resulted in no further increase in H.sub.c. If the multilayers are sputtered in krypton or xenon gas instead of argon as disclosed in the commonly assigned application discussed above, much larger coercivities can be achieved with the same Pt base layer thickness. However, as discussed above, such thick Pt layers are undersirable in most MO structures. Y. Ochiai et al., Proceedings of the 13th Conference of the Applied Magnetics Association of Japan, p. 57 (1989), disclose the use of dielectric films as base layers for sputtered Pt/Co multilayers and present data that appears to be essentially the same as that discussed above in connection with Y. Ochiai et al., EP 0304873. Y. Iwasaki et al., Proceedings of the 13th Conference of the Applied Magnetics Association of Japan, p. 129 (1989), state that if it were possible to increase the coercivity of Pt/Co multilayers using a nonmetallic transparent base layer to the degree shown by S. Hashimoto et al. (discussed above) using a Pt base layer, such films would be useful as MO disk materials. Y. Iwasaki et al. disclose the use of optically isotropic cubic metal oxides such as Co, Fe and Ni oxides as base layer materials. When a 400-600 A (40-60 nm) base layer of cubic wustite-phase CoO is used, the coercivity of Pt/Co multilayer has increased to about 1000 Oe. If prepared properly a 200 A (20 nm) base layer of CoO results in an Hc of 800 Oe. Fe or Ni oxide base layers resulted in an H.sub.c of 600 Oe. In the commonly co-assigned application, "Method of Thermomagnetic Recording of Information and Optical Readout of the Stored Information and Also A Recording Element Suitable for Use in this Method", Ser. No. 384,587, filed July 24, 1989, dielectric layers of an inorganic oxide, nitride, selenide, etc. between the multilayer and the substrate are discussed. Examples of suitable materials are Si.sub.3 N.sub.4, AlN, SiO, SiO.sub.2, ZnO, Zn.sub.3 N.sub.2, ZnSi.sub.3 N.sub.2, ZnSe, ZrO.sub.2, TiO.sub.2, and AlZrN.sub.2. Only AlN is exemplied. The instant invention provides a base layer that is transparent, has a relatively large index of refraction (n>1.8) and enhances the coercivity of Pt/Co multilayers to thereby provide a structure especially suitable for magneto-optical recording. This invention provides a magneto-optical recording medium comprised of a substrate, a base layer of zinc oxide or indium oxide sputtered onto the substrate and a multilayer recording film sputtered onto the base layer, wherein the zinc oxide or indium oxide film is from about 200 A to about 4500 A (about 20 nm to about 450 nm) thick, preferably from about 200 A to about 2000 A (about 20 nm to about 200 nm) thick, the multilayer recording film is a platinum/cobalt (Pt/Co) or a palladium/cobalt (Pd/Co) multilayer film consisting of alternating layers of platinum and cobalt or palladium and cobalt, and the substrate is transparent to the radiation used to record and read. Argon, krypton, xenon or a mixture thereof can be used as the sputter gas when sputtering the multilayer, but it is preferred to use krypton, xenon or a mixture thereof as the sputter gas and to use a sputtering gas pressure of about 2 to about 12 mTorr (about 0.27 to about 1.6 Pa). Preferred is a platinum/cobalt multilayer film and more preferred is a platinum/cobalt multilayer film wherein all the cobalt layers have substantially the same thickness d.sub.Co and all the platinum layers have substantially the same thickness d.sub.Pt, d.sub.Co is less than about 12 A (1.2 nm), d.sub.Pt is less than about 24 A (2.4 nm) and the total thickness of the platinum/cobalt multilayer film is less than about 750 A (75 nm). Especially preferred is the platinum/cobalt multilayer film wherein d.sub.Co is from about 2 to about 5 A (0.2 to about 0.5 nm) and d.sub.Pt /d.sub.Co is from about 1 to about 5. Surprisingly, the zinc oxide or indium oxide base layer not only enhances the Kerr rotation of the multilayer recording film but also enhances the magnetic coercivity. High coercivities and square magnetic hysteresis loops have been achieved with the magneto-optical recording medium of the invention. Coercivities as high as 3000 Oe have been observed with a Pt/Co multilayer film.

US Referenced Citations (5)
Number Name Date Kind
4587176 Carcia May 1986
4670323 Nakamura et al. Jun 1987
4678721 de Broeder et al. Jul 1987
4690861 Nakamura et al. Sep 1987
4788095 Machida et al. Nov 1988
Foreign Referenced Citations (5)
Number Date Country
304927 Mar 1989 EPX
199236 Sep 1986 JPX
242321 Oct 1986 JPX
128019 Jun 1987 JPX
169757 Jul 1989 JPX
Continuations (1)
Number Date Country
Parent 494207 Mar 1990