MAGNETIC RECORDING MEDIUM

Abstract: This improved magnetic recording medium comprises a non-magnetisable substrate on which a magnetisable layer (1) is formed. (1) is composed of (a) one or more than one rare earth element chosen from Y, La, Ce, Pr, Nd, Sm, Gd, Tb and Dy(upto 13 wt.%), (b) oxygen (3-13 wt.%), and (c) Co and unavoidable impurities (the remaining wt.%). It may also contain, if necessary Ni (up to 22 wt.%). The unavoidable impurities are at least one of B, C, Al, Si, Sn, S, Cu, Zn, Fe, Ti, Cr. They must be present to less than 1 wt.%. The thickness of (1) is 0.1-1.0 microns. The non-magnetizable substrate is polyester, acetate, polycarbonate, glass or Al. Its thickness is 5-25 microns. (1) is formed by a spattering, ion-plating or vaccum-evapn. method after the surface of the substrate is treated with primer or by a carbon discharge.

Description: This invention concerns a magnetic recording medium. Magnetic recording media comprising a product obtained by kneading magnetic powder such as of gamma-Fe₂0₃ of Fe with a synthetic resin binder coated on a non-magnetic substrate has been used so far. Magnetic recording medium capable of high density recording in which a thin metal magnetic film is formed on a substrate has been developed in the coated-type magnetic recording medium. Magnetic recording medium of the thin metal film type, has been formed by chemical plating (a wet process) and by sputtering method, ion plating or vacuum evaporation (dry processes). It has generally been considered that the dry processes can produce better magnetic recording medium than the wet process in view of the primary treatment applied to the substrate and the homogenity of the magnetic layer in the longitudinal direction and the lateral direction. However, the thin metal film type magnetic recording medium formed by the dry process has much lower thermodynamic stability, and more pores as compared with oxide magnetic powder, and tends to be readily corroded in air to cause reduction in the performance. With an aim of improving the corrosion resistance without reducing the magnetic properties to the magnetic recording medium, it has been proposed in JP-A-56-15014, JP-A-57-196507 and JP-A-58-134414 to improve the corrosion resistance of the magnetic recording medium by adding metals, supplying a gaseous mixture of ozone and oxygen to the magnetic layer portion, or quantitatively setting the number of oxygen atoms. JP 57-198615 discloses a magnetic deposition film having a composition containing 0-30 wt % Ni, 0.02-1.5 wt % Y, 3.0-11.0 wt % O and the residue of Co formed on a tape base material. The yttrium is included to improve the corrosion resistance of the film of Ni- Co series metal. More than 1.5% yttrium leads to deterioration of the magnetic characteristics of the film. However, only slight improvements have been achieved in the corrosion resistance and the saturation magnetization or the residual magnetization is decreased resulting in a reduction in the reproduced signal level of the magnetic recording medium. One object of this invention is to avoid the drawbacks of conventional magnetic recording media and provide media of the Co-Ni series metal film type and corrosion resistance properties. The invention provides a magnetic recording medium comprising a non-magnetisable or non-magnetic substrate and a magnetic layer formed on the substrate characterised in that the composition of the magnetic layer comprises apart from impurities, not more than 13% by wt of at least one rare earth element, selected from Y, La, Ce, Pr, Nd, Sm, Gd, Tb or Dy, 3-13% by wt of oxygen, 0-22% by weight of Ni the balance being Co but not including such compositions having an oxygen content of 3-11% by wt and containing Y as the only rare earth element in an amount 0.02-1.5% by wt. This invention will further be explained specifically. At first, the reason for defining the composition of the magnetic layer formed on the non-magnetic substrate will be explained. When Ni is used it has the tendency of reducing the residual magnetization of the magnetic recording medium and has an effect of improving the corrosion resistance of the magnetic layer of metal mainly composed of Co. If it is contained in excess of 22 % by weight, the residual magnetization of the magnetic layer is remarkably reduced to lower the reproduced signal level. The rare earth element R consisting one or more of Y, La, Ce, Pr, Nd, Sm, Gd, Td, and Dy is added in order to improve the magnetic properties and the corrosion resistance of the Co - Ni series metal magnetic layer. If it is added in excess of 13 % by weight, the coercive force and the saturation magnetization are increased to reduce the corrosion resistance as well. For obtaining the magnetic layer of the above-mentioned composition, one of B, C, Al, Si, Sn, S, Cu, Zn, Fe, Ti, Cr, Zr may be contained inevitably in purifying starting material or alloying. However, if these inevitable impurities are contained by more than 1 % by weight, the magnetic properties are reduced. Oxygen is contained in the magnetic recording medium with an aim of improving the coercive force of the magnetic recording medium by partially reacting it chemically with the metal to form a metal oxide membrane and causing the other portion to be adsorbed in the magnetic layer of the magnetic recording medium. If it is less than 3 % by weight, no effect for adding oxygen is recognized, the coercive force of the magnetic recording medium is not improved and the corrosion resistance is reduced. While on the other hand, if it is contained in excess of 13 % by weight, a significant effect of oxidization appears and the ratio of metal oxide is increased thereby failing to obtain sufficient corrosion resistance and the saturation magnetization Ms is reduced. Magnetic layer of the composition according to this invention as described above is desirably about from 0.1 µm to 1.0 µm in thickness. If it is less than 0.1 µm, no sufficient reproduced signal level is obtained, whereas if the thickness exceeds 1.0 µm the flexibility of the film is decreased and it is difficult to increase the recording density. As the non-magnetic substrate on which such a magnetic layer is formed, there may be used those plastic films having appropriate flexibility and stretch resistance and heat resistance upon vapor deposition, for example, made of polyester, acetate, polycarbonate and polyimide with a thickness of 5 - 25 µm, as well as glass substrate and metal substrate such as of Al as a disc-like substrate. Then, explanation will be made to the method of preparing the magnetic recoding medium according to this invention. In this case, since it is required that the magnetic layer to be formed has a high coercive force and a high saturation magnetic flux density due to the requirement for high density recording, it is desirable to employ the dry process such as sputtering, ion plating or vacuum evaporation. In the dry process, it is possible to improve the coercive force by entering the composite atoms obliquely to the substrate thereby forming a thin metal film. When the composite atoms are entered obliquely to the substrate, an acicular structure of thin metal film grows with the orientation direction of crystals being oblique to the substrate. In such a state, the magnetacrystalline anisotropy and the shape magnetic anisotropy of crystals cause the magnetic anisotropic properties and the coercive force of the magnetic recording medium is improved by the increase in the magnetic anisotropy. Further, the dry process is excellent for the reproducibility of the magnetic properties of the magnetic layer and also has an advantage in view of the manufacture in that a layer uniform in the longitudinal direction and the lateral direction can be manufactured with ease. It is desirable to apply treatment such as corona discharging treatment or primer treatment to the non-magnetizable substrate in order to improve the adhesiveness with the magnetic layer. Figure 1 is a cross sectional view showing the constitution for a main portion of a device for manufacturing magnetic recording media; Figure 2 is a graph showing the corrosion resistance of magnetic recording media in accordance with this invention and conventional example; Figures 3(a) and (b) are graphs showing the relationship between the rare earth element content Rc and the coercive force Hc in various magnetic recording media of different compositions; Figures 4 to 11 are graphs showing the effects of each of the rare earth elements on the corrosion resistance of the magnetic recording media; and Figures 12 to 19 are graphs similar to Figures 3(b) showing the relationship between the rare earth element content and the coercive force. Explanation will be made specifically to the magnetic recording medium according to this invention referring to examples thereof in conjunction with the drawings and in accordance with the production method. Figure 1 shows the cross sectional structure of the embodiment of a manufacturing device for the magnetic recording medium according to this invention, in which vacuum evaporation chamber 11 substantially of a bell-like shape is formed. The vacuum evaporation chamber 11 is connected with vacuum pump 12 and adapted such that the inside of the vacuum evaporation chamber 11 can be set to a predetermined vacuum degree by driving the vacuum pump 12. Electron beam gun 14 is disposed on the bottom plate 13 of the vacuum evaporation chamber 11, and ferromagnetic metal 15 is disposed within the electron beam gun 14. The ferromagnetic metal 15 is mainly composed of Co,Ni and, further, one or more of rate earth element R from Y, La,Ce, Pr, Nd, Sm, Gd, Tb and Dy and the composition is set so as to provide a predetermined composition of the film. While on the other hand, introduction pipe 18 is let out from oxygen cylinder 16 by way of variable leak valve 17 and the end of the introduction pipe 18 is inserted passing through the bottom 13 to the inside of the vacuum evaporation chamber 11. Unwinder 21 and winder 22 are rotatably disposed in the vacuum evaporation chamber 11 and can 23 is rotatably disposed between the unwinder 21 and the winder 22. Shutter 25 is disposed between the electron beam gun 14 and the can 23. The shutter 25 is rotatable and opened only upon film forming. The maximum incident angle ϑ₂ and the minimum incident angle ϑ₁ of the evaporated matters to the substrate can be adjusted by shutter member 26. Non-magnetic substrate 30 is wound around the unwinder 21, the end of the non-magnetic substrate 30 is let out and taken-up by way of the can 23 around the winder 22. A polyester film of 12 µm thickness was used as the non-magnetic substrate 30. The unwinder 21 and the winder 22 are driven and the non-magnetic substrate 30 is sent in contact with the circumferential surface of the can 23 at a speed, for example, of 15 m/min. The vacuum pump 12 is driven to evacuate the inside of the vacuum evaporation chamber 11 to less than 5 x 10⁻⁶ torr (6.6 x 10⁻⁴ Pa) thereby evaporating the ferromagnetic metal 15 in the electron beam gun 14 at a predetermined speed. At the same time, ON-OFF cock 17 is opened to release oxygen gas from the introduction pipe 18 to the inside of the vacuum evaporation chamber 11. The supply of the oxygen gas to the inside of the vacuum evaporation chamber 11 is also set such that a predetermined oxygen composition in the film is obtained. By the opening of the shutter 25, in the ferromagnetic metal 15 released from the electron beam gun 14, only the portion entering obliquely within a predetermined range of angle relative to the non-magnetic substrate 30 passes through the shutter member 26 and reaches the non-magnetic substrate 30. That is, among the ferromagnetic metal 15 released from the electron beam gun 14, only the portion within the range of angle from the minimum angle ϑ₁ to the maximum angle ϑ₂ as shown in Figure 1 relative to the normal line to the plate surface of the non-magnetic substrate 30 passes through the shutter 25 and enters obliquely to the non-magnetic substrate 30. In this way, a magnetic layer of 0.2 µm thickness was formed. The thus obtained magnetic recording media were cut and served for the analysis of the composition and the evaluation for the magnetic properties and the corrosion resistance respectively. For the composition analysis, EPMA (Electron Probe Microanalyzer) analysis and chemical analysis were used in combination, and the magnetic properties were measured by a vibrating sample magneto meter. Then, the evaluation of the corrosion resistance was conducted by maintaining the magnetic recording medium to be measured in an atmosphere prepared by a thermostable and humidity stable tank (temperature 50°C , humidity 90%) over a predetermined of corrosion time T, then taking out the medium from the temperature stable and humidity stable tank, irradiating laser beam to the magnetic recording medium, measuring the amount of light permeating through the magnetic recording medium and determining the transmittance thereof. The results thus obtained are shown in Figure 2 and Figure 3 and Table 1 (transmittance after 100 hours of corrosion time). B, C, Al, Si, Sn, S, Cu, Zn, Fe, Ti, Cr and Zr were analyzed as inevitable impurity elements other than Ni, R, and oxygen in the medium and the total amount therefor was less than 1 wt %. Figure 2 shows the relationship between the corrosion time I (hour) and the transmittance D (%) of the magnetic recording media each of an identical film thickness having 5 types of compositions, in which the magnetic layer shown by the symbol O has a composition of 21.3 wt % Ni, 9.5 wt% O and the balance of Co, the magnetic layer shown by the symbol ▵ has a composition of 14.2 wt% Ni, 4.4 wt% Gd as the rare earth element R, 3.7 wt% O and the balance of Co, the magnetic layer shown by the symbol ▪ has a composition comprising 18.4 wt % Ni, 5.4 wt % Tb, 3.5 wt% O and the balance of Co and, further, the magnetic layer shown by the symbol □ has a composition comprising 10.5 wt% Ni, 1.7 wt% Pr as the rare earth element, 1.2 wt% Sm, 10.4 wt% O and the balance of Co. In the magnetic recording medium not incoporated with the rare earth element R shown by O for the measuring points the transmittance D is increased along with the increase in the corrosion time T and the transmittance D is increased to about 5 % if the corrosion time exceeds 100 hours. In the magnetic recording medium shown by ▵, □, ▪ for the measuring points in Figure 2, the transmittance D does not increase in excess of about 3.5 % even if the corrosion time T increases by more than about 100 hours showing that the corrosion resistance is excellent. These magnetic recording media correspond to examples shown by specimen numbers 47, 24 and 33 respectively in Table 1. It is considered that the corrosion resistance of the magnetic recording medium is increased under the presence of the rare earth element R, due to the gettering effect in which the rare earth element R combines the oxygen introduced in the vacuum evaporation chamber 11 to form an oxide of the rare earth element R and thus the oxide film covers the magnetic layer to protect the magnetic layer, Figure 3(a) shows the relationship between the content Rc of the rare earth element R, the coercive force Hc of the magnetic recording medium and the saturation magnetization Ms of magnetic recording media having two type of compositions, in which the magnetic layer represented by the symbols O (Ms) and ▵ (Hc) has a composition comprising 12.0 wt% Ni, 10.6 wt% O and the balance of Co + Nd + Dy (Nd and Dy weight ratio is 5 : 2), and the magnetic layer represented by the symbols  (Ms) and ▴ (Hc) has a composition comprising 7.6 wt% Ni, 11.6 wt% O and the balance of Co + Y + Pr (Y and Pr weight ratio is 5 : 3). Figure 3(b) shows the coercive force Hc of the magnetic recording medium in the case where the content Rc of the rare earth element R is increased as compared with that in Figure 3(a). The magnetic layer represented by the symbol has a composition comprising 15 wt% Ni, 5 wt% O and the balance of Co + Sm + Tb (Sm and Tb weight ratio is 3 : 2). As apparent from the result of the measurement in Figures 3(a)(b), although the magnetic properties are excellent with the content of the rare earth element R being not more than 13 wt%, the coercive force Hc of the magnetic recording medium is remarkably reduced if it exceeds 13 wt%. Then, the relationships between the corrosion time T and the transmittance D measured for the magnetic recording media having the compositions as shown in Table 2 described below are shown in Figures 4 through 11 in the same manner as above. It can be seen that in all of the Figures that the magnetic recording media according to this invention containing specific amounts of rare earth elements R shows extremely small increase in the transmittance D even after the elapse of 200 hours of corrosion time T thus having excellent corrosion resistance as compared with the magnetic recording medium not containing the rare earth element R. Further, the result obtained by varying the content Rc of the rare earth element R and examining the effect on the coercive force Hc for the magnetic recording media in Table 3 described below are shown in Figures 12 through 19. As apparent from Figures 12 - 19, although excellent coercive force Hc is provided if the rare earth element R is added by not more than 13 wt%, the coercive force is remarkably reduced if it exceeds 13 wt%. Symbol Co(wt%) Ni(wt%) R(wt%) O(wt%) Fig. 12 ○ 79-Rc 16 Y=Rc 5 "  80-Rc 15 (La:Ce=3:7)=Rc 5 Fig. 13 ○ 79-Rc 16 Pr=Rc 5 Fig. 14 ○ " 16 Nd=Rc 5 Fig. 15 ○ " 16 Sm=Rc 5 Fig. 16 ○ " 16 Gd=Rc 5 Fig. 17 ○ " 16 Tb=Rc 5 Fig. 18 ○ " 16 Dy=Rc 5 Fig. 19 ○ " 16 (Pr:Gd=10:1)=Rc 5 "  80-Rc 15 (Gd:Dy=2:1)=Rc 5 From Table 1, the followings can be seen, that is conventional example Nos. 56 and 57 lack in the rare earth element R in the thin film composition, and go out of the composition range of this invention for oxygen in conventional Examples Nos. 58, 59, 60, 62, 63, 64, 65, 66, 68, 70, 72, 73, 75, 76, 78, 80, 81, 83 and 84, for Ni in No. 79 and for rare earth element in Nos. 61, 64, 67, 69, 71, 74, 77, 82 and 85 and they are poor in either or both of the magnetic properties and the corrosion resistance, whereas it is shown that examples Nos. 1 - 31 are excellent in the magnetic properties and the corrosion resistance. As apparent from the foregoings, this invention can provide those which are excellent in the magnetic properties and the corrosion resistance as compared with conventional Co - Ni series magnetic recording media.

Claims: A magnetic recording medium comprising a non-magnetisable or non-magnetic substrate and a magnetic layer formed on the substrate characterised in that the composition of the magnetic layer comprises apart from impurities, not more than 13% by wt of at least one rare earth element, selected from Y, La, Ce, Pr, Nd, Sm, Gd, Tb or Dy, 3-13% by wt of oxygen, 0-22% by weight of Ni the balance being Co but not including such compositions having an oxygen content of 3-11% by wt and containing Y as the only rare earth element in an amount 0.02-1.5% by wt. A medium as defined in claim 1, wherein the inevitable impurities are less than 1 % by weight of one or more of B, C, A1, Si, Sn, S, Cu, Zn, Fe, Ti, Cr or Zr. A medium according to any preceding claim, wherein the magnetic layer has a thickness of 0.1 µm - 1.0 µm. A medium according to any preceding claim, wherein the non-magnetic substrate has flexibility, stretch resistance and thickness of 5 - 25 µm. A medium according to claim 4, wherein the non-magnetic substrate is composed of polyester, acetate, polycarbonate or polyamide. A medium according to claim 1, wherein the non-magnetic substrate is glass. A medium according to claim 1 or claim 2, wherein the non-magnetic substrate is Aℓ. A medium according to claim 1 or claim 2, wherein the magnetic layer is formed by sputtering method, ion plating method or vacuum deposition method. A medium according to claim 1 or claim 2, wherein the non-magnetic substrate is subjected to a primary treatment of corona discharging treatment or primer treatment.

Inventors: AKIHIRO MAKOTO - NAKAKOKUBUN -, OKA KOUICHI - NAKAKOKUBUN -CHO, SATO KAKEHIKO - KYONAN-CHO -CH, TADA JUNYA - NAKAKOKUBUN -CHOM, WATANABE TOMOHISA - KAMISHIZU, AKIHIRO, MAKOTO 18-35, NAKAKOKUBUN 3-CHOME, OKA, KOUICHI 18-35, NAKAKOKUBUN 3-CHOME, SATO, KAKEHIKO 17-7, KYONAN-CHO 3-CHOME, TADA, JUNYA 18-35, NAKAKOKUBUN 3-CHOME, WATANABE, TOMOHISA 1080-213, KAMISHIZU
Applicants: SUMITOMO METAL MINING CO, SUMITOMO METAL MINING COMPANY LIMITED
Application year: 1986
Main classification: G11B5
IPC codes: G11B5