METHOD FOR FORMING FERRITE FILM

Abstract: A method for forming a ferrite film by causing a ferrite crystallization reaction on a substrate, comprises simultaneously bringing into contact with the substrate, in a deoxidized atmosphere, an aqueous solution of an oxidizing agent and a deoxidized aqueous solution containing ferrous ions.

Description: Field of the Invention The present invention relates to a method for forming a spinel structure ferrite film containing Fe³⁺ which is widely applied to magnetic recording media, photomagnetic recording media, magnetic heads and the like. Background of the Invention Conventional methods for forming a ferrite film on a solid surface are generally classified into a coating method wherein a binder is employed and a method wherein no binder is employed. The coating method is usually applied to a production of the ferrite film for a magnetic tape, a magnetic disc and the like, but it has some defects. For example, magnetic recording density is low, because a nonmagnetic binder is present between ferrite particles and therefore it is difficult to apply to an electronic element which is required to be polycrystalline. It is also limited to gamma-Fe₂O₃ and Fe₃O₄ type ferrite which are generally obtained in needle shape, since shape anisotropy of the ferrite particles is utilized for obtaining magnetic anisotropy of the ferrite film. The method wherein no binder is employed includes a solution coating method; an electrophoretic electrodeposition method; a dry plating method such as spattering, vacuum deposition and arc discharge; a melt spray method, and a gas-phase growth method. However, these methods require high temperatures (300 °C or more) and cannot use as a substrate a material having low melting point, low decomposition temperature or poor heat stability. Therefore, the inventors have proposed a method (Japanese Laid-open Patent Application No. 111, 929/1984) quite different in technical concept from the conventional ferrite film forming methods described above. In this method, the surface of a solid is brought in contact with an aqueous solution containing at least ferrous ions, to absorb FeOH⁺ or a combination of FeOH⁺ and other metal hydroxyoxide ions and the absorbed FeOH⁺ is oxidated to conduct ferrite crystallization reaction. This method is hereinafter referred to "wet plating method" in contrast to the conventional methods. The wet plating method, however, has not attained satisfactory rate of formation in the industrial scale production. Thus, various improvements have been proposed to obtain a higher rate of formation and better quality of ferrite film. The formation of the ferrite film proceeds by alternately occurring absorption of ferrous ions or other metal ions and oxidation of the adsorbed ions, as described above. Accordingly, a method has been proposed in which an immersion of a substrate into an aqueous metal ion solution and an oxidation which may be conducted by contacting with a mixed N₂ and O₂ gas are alternately performed. This method, however, is not practicable because the process is complicated. It can be considered to conduct the immersion and oxidation simultaneously on the surface of the substrate, but this leads to poor quality of the ferrite film since small particles of ferrite are produced at the same time as the formation of the ferrite film. Large effort must be exerted to remove such ferrite particles. For dissolving the above problems, Japanese Laid-open Patent Application No. 179877/1986 proposes that an aqueous solution containing Fe²⁺ be sprayed on a rotating substrate. However, since this method is conducted in an oxidizing condition, oxidization is not proceeded uniformly on the substrate and therefore it is difficult to obtain a uniform ferrite film on the substrate. Summary of the Invention The inventors have found that the origin of these difficulties is in that the reaction is always effected in an oxidizing atmosphere, and the control of oxidation is relatively difficult. Thus, the inventors have studied ways to facilitate the control of the reaction by changing the oxidizing conditions. The present invention provides a method of forming ferrite film comprising flowing downward or spraying an aqueous oxidizing agent solution on a substrate in a deoxidized atmosphere and simultaneously supplying an aqueous deoxidized solution containing at least ferrous ions, to effect ferrite crystallization reaction on the substrate. Brief Explanation of the Drawings Figure 1 is a schematical view showing one embodiment of the apparatus for the present invention. Figure 2 is a schematical view showing another embodiment of the apparatus for the present invention. Detailed Description of the Invention According to the invention, the aqueous oxidizing agent solution (1) is flowed downward or sprayed on the surface of the substrate (2) for forming ferrite film thereon. Figure 1 shows one embodiment of spraying from the spray nozzle (3), but the solution can be let to flow dropwise. The oxidizing agent solution (1) is an aqueous solution containing an oxidizing agent such as a nitrite, a nitrate, a perchlorate, hydrogen peroxide, an organic peroxide and oxygen present in a dissolved condition. Preferred are an aqueous solution containing a nitrite or disolved oxygen in view of oxidizing ability. Most preferred are an aqueous nitrite solution, because, in case of oxygen, oxygen must be introduced in a large amount. The aqueous oxidizing agent solution may contain a buffer solution for stabilizing pH. When the aqueous oxidizing agent solution is flowed downward or sprayed as in the invention, the oxidation reaction can proceed only on the substrate surface, and the progress of oxidation reaction and the decrease of the amount of the ferrite particles produced in the reaction solution can be controlled by controlling the quantity (concentration) and kind of the oxidizing agent solution. Further, the oxidation reaction on the substrate becomes independent of the thickness of the aqueous solution. In the oxidizing atmosphere so far, the oxidation reaction varies depending upon the thickness of the aqueous solution, because it is dependent upon the diffusion of the oxidizing gas. In the present invention, it is preferable to give fluidity to both the aqueous solutions so that the reaction by the aqueous oxidizing agent solution (1) or by the aqueous deoxidized solution (4) containing ferrous ions occurs uniformly and homogeneously on the substrate (2) surface. the fluidity may be given, for example, by the action of centrifugal force with the substrate placed on a rotary disk (5) as shown in Figs. 1 and 2, or by the action of gravity with the substrate placed inclined or vertically. Further, it is particularly preferable to make the aqueous solutions continuously flow out (overflow) of the substrate surface by the fluidity, since the aqueous solution on the substrate is always renewed with new supply solution and the ferrite particles produced in the solution are discharged by the flow out of the system. It is preferable in the invention to place the substrate on the rotary disk and give fluidity by centrifugal force to the aqueous solution for uniforming the thickness of the supplied solutions and making it thin. By continuously flowing the aqueous solution on the substrate out of the system by centrifugal force, ferrite production can at all times be effected by using newly replenished aqueous solution. It is desirable in this case to rotate the substrate surface at from 100 to 2000 rpm, preferably from 150 to 500 rpm. The quantities of the aqueous oxidizing solution (1) and the aqueous deoxidized solution (4) supplied on the substrate surface, are generally selected depending upon the temperature of the substrate surface and the degree of the fluidity given to the aqueous solutions. They are generally from 0.01 to 10 ml/cm² min, preferably from 0.1 to 1 ml/cm² min. Amounts more than the above range are unfavorable for the uniformity and homogeneity of the ferrite film produced, and amounts less than the above range make the consumption of the aqueous solution larger and maintenance of the temperature difficult. The aqueous deoxidized solution (4) for supplying iron atoms (Fe) and oxygen atoms (O) used in forming the ferrite film, and for supplying other metal atoms (M) as required is obtained by dissolving the metal salts in water or dissolving the metals themselves in an acid. For ferrous ions and other metal ions (M n+ ) to be absorbed to the surface of a specified substrate to effect a series of ferrite film forming reaction, it is necessary to make the metal hydroxide ions present in the above aqueous solution. The anions in the aqueous solution are, for example, inorganic acid ions such as chloric ion and sulfuric ion, and organic acid ions such as acetic ion and tartaric ion; preferably chloric ion and acetic ion. The pH value in the aqueous solution is adequately selected and controlled depending upon the type of metal ion, and is preferably adjusted to a range of 6 to 11, more preferably 7 to 11. For stabilizing the pH value, a buffer solution or a salt having a buffer effect such as sodium acetate may be added. If M n+ ions to be coexistent with the ferrous ions develop a precipitation, the aqueous deoxidized solution (4) may be divided into two or more as shown, for example, by 4 and 4′ in Fig. 2. When the aqueous deoxidized solution (4) contains only the Fe²⁺ ion as the metal ion, the spinel ferrite which contains only iron as the metal element, that is, magnetite Fe₃O₄ or maghemite gamma-Fe₂O₃ film is obtained. When the aqueous solution (4) contains, in addition to the Fe²⁺ ion, other metal ions M n+ such as Zn²⁺, Co 2,3+ , Ni²⁻ , Mn 2,3+ , Fe³⁺, Cu²⁺, V 3,4,5+ , Sb⁵⁺, Li⁺, Mo 4,5+ , Ti⁴⁺, Pd²⁺, Mg²⁺, Al³⁺, Si⁴⁺, Cr³⁻, Sn 2,4+ , Ca²⁺, Cd²⁺, and In³⁺, ferrite films containing metal elements such as cobalt ferrite (Co x Fe 3-x O₄) and nickel ferrites (Ni x Fe 3-x O₄) (when M is of one type) are obtained, or mixed crystalline ferrites such as Mn-Zx ferrites (Mn x Zn y FE 3-x-y O₄) are obtained (when M is of several types). The invention can be applied to the preparation of any of these films. It is essential that the aqueous deoxidized solution containing ferrous ions and used in the invention is prepared and stored in non-oxidizing conditions. For this reason, the water used for the preparation of the aqueous solution is usually deaerated and distilled water. The substrate (5) used in the present invention may be a metallic or nonmetallic material, provided that it has surface activity to FeOH⁺ absorption and resistance to said aqueous solutions. Such materials usable as the substrate include an alloy containing iron such as stainless steel; an iron oxide such as Fe₃O₄, gamma-Fe₂O₃ alpha-Fe₂O₃ or ferrite; a noble metal such as gold, platinum or paradium, one composed of saccharide such as sucrose and cellulose; and a plastic in the form of block, cylinder, and film, or attached to the surface of other substrate, and metal ions such as steel bonded to the surface of a solid. It is desirable that the surface of the substrate has a smoothness as well as the surface activity. The surface activity of the substrate in the invention, if not originally possessed by the substrate, may be provided as a surface active layer on the substrate which does not originally have such a property. To form such a surface active layer, a coat of a material originally having such properties may be provided on the surface. To form the coat layer, if the substrate withstands high temperatures, it may be coated with a material such as iron oxides by a method such as spattering, vacuum decomposition, arc discharge, and welding. Otherwise, the surface layer may be formed by means of ordinary chemical plating. In addition to these methods, the pretreating method used to give surface activity by means of a plasma treatment on the substrate is also effective. The plasma treatment mentioned here means production of surface activity caused by plasma produced by glow discharge, corona discharge, and microwave discharge. It can be caused under either atmospheric pressure or reduced pressure. The gases available are, for example, nitrogen, argon, helium, hydrogen, ammonia, and oxygen containing gases such as oxygen, carbon dioxide, carbon monoxide, nitrogen dioxide, sulfur dioxide, and air, such reactive and nonreactive gases, or one or two or more types of these gases; preferably oxygen containing gases are used. Materials showing effective surface activity by plasma treatment are either metallic and nonmetallic ones, and for the application to magnetic recording media metals such as aluminum and stainless steel, and nonmetals, for example, plastics such as polyethylene terephthalate, Teflon, acryl, and polycarbonate, silica glass, and mica are mentioned. The plasma treatment remarkably improves the wettability of the substrate by the aqueous solution and allows the formation of a thin film of aqueous solution on the surface of the substrate. The heating condition for allowing the reaction to proceed is at a range below the boiling point, preferably at a range from 60 to 90 °C. The reaction of the invention is preferably attained in a substantially deoxidized atmosphere. In the presence of a large quantity of oxygen, unnecessary oxidation proceeds, which is contrary to the spirit of the present invention, i.e. to control oxidation. Specifically, the reaction proceeds in a nitrogen atmosphere. According to the invention, ferrite film can be formed at a very high efficiency. The formed ferrite film has uniform properties and uniform thickness of all area by controlling quantity and concentration of the supplying solution. Examples The present invention is illustrated by the following examples which, however, are not to be construed as limiting the invention to their details. Preparation of Aqueous FeCl₂ Solution Aqueous FeCl₂ solution was prepared by dissolving 6g of FeCl₂ in 1 liter of ion-exchanged water which had been deoxidized with N₂ gas and by adjusting the solution to pH 6.9 with ammonia. Preparation of Mixed FeCl₂ and CoCl₂ Solution A mixed FeCl₂ and CoCl₂ solution was prepared by dissolving 6g of FeCl₂ and 3g of CoCl₂ in 1 liter of ion-exchanged water which had been deoxidized with N₂ gas and adjusting the solution to pH 6.9 with ammonia. Preparation of Aqueous MnCl₂ Solution Aqueous MnCl₂ solution was prepared by dissolving 3g of MnCl₂ in 1 liter of ion-exchanged water which had been deoxidized with N₂ gas and adjusting the solution to pH 10 with ammonia. Preparation of Aqueous ZnCl₂ Solution Aqueous ZnCl₂2 solution was prepared by dissolving 3g of ZnCl₂ in 1 liter of ion-exchanged water which had been deoxidized with N₂ gas and adjusting the solution to pH 6.5 with ammonia. Preparation of a Mixed FeCl₂ and MnCl₂ Solution Six grams of FeCl₂ and 3g of MnCl₂ were dissolved in 1 liter of ion-exchanged water which had been deoxidized with N₂ gas and adjusted to pH 7.2 with ammonia. If the pH was elevated higher than this, the solution became turbid. Preparation of Aqueous Oxidizing Agent Solution 1 Aqueous NaNO₂ solution was obtained by dissolving 1g of NaNO₂ in 1 liter of ion-exchanged water which had been deoxidized with N₂ gas and adjusting the solution to pH 6.9 with ammonia. Preparation of Aqueous Oxidizing Agent Solution 2 A dissolved oxygen containing water was obtained by applying air bubbling to 1 liter of ion-exchanged water adjusting the solution to pH 6.9 with ammonia. The dissolved oxygen content was 15 ppm. Example 1 A polycarbonate disk (120 mm diameter, 1.2 mm thick) subjected to hydrophilic treatment by corona discharge was mounted on the apparatus shown in Fig. 1. While being rotated at 400 rpm and being sprayed with deoxidized ion-exchanged water, the disk was heated to 70 °C. Then, N₂ gas was introduced into the apparatus to form a deoxidized atmosphere. The aqueous FeCl₂ solution and aqueous oxidizing agent solution 1 were supplied on the polycarbonate disk each from a spray nozzle at a rate of 50 cc/min. When the polycarbonate disk was taken out after approximately 15 min, a black mirror film of approximately 0.15 micrometer had been formed on the disk surface. The results of X-ray diffraction showed that it was magnetite. Example 2 A silica glass disk (130 mm diameter, 2 mm thick) with polished surface was mounted on the apparatus shown in Fig. 1. While being rotated at 400 rpm and being sprayed with deoxidized ion-exchanged water, the disk was heated to 70 °C. Then, N₂ gas was introduced into the apparatus to establish a deoxidized atmosphere. The mixed FeCl₂ and CoCl₂ solution and oxidizing agent solution were supplied on the silica glass disk each from a spray nozzle at a rate of 50 ml/min. When the silica glass disk was taken out after 15 min, a black mirror film of approximately 0.15 micrometer in thickness was found formed on the disk. The results of the composition analysis showed it was C 0.3 Fe 0.7 O₄ film. Example 3 A silica glass disk (130 mm diameter, 2 mm thick) with polished surface was mounted on the apparatus shown in Fig. 2. While being rotated at 400 rpm and being sprayed with deoxidized ion-exchange water, the disk was heated to 70 °C. Then, N₂ gas was introduced into the apparatus to establish a deoxidized atmosphere. The aqueous FeCl₂ solution, aqueous MnCl₂ solution, and aqueous oxidizing agent solution 1 were supplied on the polycarbonate disk each from a spray nozzle at a rate of 30 ml/min. When the polycarbonate disk was taken out after approximately 15 min, a black mirror film of approximately 0.15 micrometer in thickness was found formed on the disk. The composition analusis showed it was Mn 0.5 Fe 2.5 O₄ film. Control 1 A silica glass disk (130 mm diameter, 2 mm thick) with polished surface was mounted on the apparatus shown in Fig. 1. While being rotated at 400 rpm and being sprayed with deoxidized ion-exchanged water, the disk was heated to 70 °C. Then, N₂ gas was introduced into the apparatus to form a deoxidized atmosphere. The mixed FeCl₂ and MnCl₂ solution and oxidizing agent solution 1 were supplied on the polycarbonate disk each from a spray nozzle at a rate of 60 ml/min. When the polycarbonate disk was taken out after approximately 15 min, a black mirror film was found formed at a thickness of approximately 0.2 micrometer. The composition analysis showed little Mn contained in the film. This shows that the introduction of the metal species into the ferrite film is different depending upon the adsorption to the ferrite film and pH dependence of the metal ion, and in certain circumstances the method of supplying to the substrate should be changed. Example 4 A silica glass disk (130 mm diameter, 2mm thick) with polished surface was mounted on the apparatus shown in Fig. 2 having an additional spray nozzle provided. While being rotated at 400 rpm and being sprayed with deoxidized ion-exchanged water, the disk was heated to 70 °C. Then, N₂ gas introduced into the apparatus to establish a deoxidized atmosphere. The aqueous FeCl₂ solution, aqueous MnCl₂ solution, aqueous ZnCl₂ solution, and aqueous oxidizing agent solution 2 were supplied on the silica glass disk from four spray nozzles each at a rate of 20 ml/min. When the polycarbonate disk was taken out after approximately 15 min, a black mirror film was found formed on the disk at a thickness of approximately 0.15 micrometer. The compositions analysis showed that it was Mn 0.2 Zn 0.5 Fe 2.3 O.

Claims: A method for forming a ferrite film by causing a ferrite crystallization reaction on a substrate, which comprises simultaneously bringing into contact with the substrate, in a deoxidized atmosphere, an aqueous solution of an oxidizing agent and a deoxidized aqueous solution containing ferrous ions. A method according to claim 1, wherein the oxidizing agent is a nitrite. A method according to claim 1, wherein the oxidizing agent is oxygen. A method according to any prceding claim, wherein the deoxidized solution contains ferrous ions as the only metal ions. A method according to any of claims 1 to 3, wherein the deoxidized solution contains, in addition to the ferrous ions, one or more of Zn²⁺, Co²⁺, Co³⁺, Ni²⁺, Mn²⁺, Mn³⁺, Fe³⁺, Cu²⁺, V³⁺, V⁴⁺, V⁵⁺, Sb⁵⁺, Li⁺, Mo⁴⁺, Mo⁵⁺, Ti⁴⁺, Pd²⁺, Mg²⁺, Al³⁺, Si⁴⁺, Cr³⁺, Sn²⁺, Sn²⁺, Sn⁴⁺ Ca²⁺ and Cd²⁺ A method according to any preceding claim, wherein the two solutions are applied on to the substrate from different nozzles. A method according to any preceding claim, wherein the two solutions are caused to flow in contact with the substrate under gravity or centrifugal force. A method according to any preceding claim, wherein the two solutions are caused to flow off the substrate continuously. A method according to any preceding claim, wherein the surface of the substrate is pretreated with plasma.

Inventors: ABE, MASANORI, ISHIKAWA, KATSUKIYO, OISHI, MASAO, SAITO, TAKAO, TAMAURA, YUTAKA
Applicants: NIPPON PAINT CO., LTD.
Application year: 1987
Main classification: G11B5
IPC codes: C23C22, C01G49, C23C18, G11B5, H01F41