Method of producing a core for magnetic head

Abstract: A method of producing a ferrite/Sendust composite core for magnetic head (18), in which a metal film (16) made of any one of metals of Zn, Cr, Fe, Cd, Co, Ni, Sn and Pb is formed on a surface, where a track of the core is formed, of a gapped bar (4) composed of ferrite and Sendust, before the gapped bar (4) is subjected to a laser-induced etching in an alkali metal hydroxide aqueous solution. The metal film (16) serves to prevent to form a dull at an edge portion of the track, so that the magnetic core being free from the dull and high in reliability can be obtained.

Description: The present invention relates to a method of machining, by laser induced etching, of a substrate formed of a ferrite material and Sendust, and more particularly relates to a method of producing a core for magnetic head having a track formed by a laser-induced etching wherein a thermochemical reaction is induced by a laser beam. Recently, magnetic recorders, such as floppy disc drive (FDD), rigid magnetic disc drive (RDD), VTR or the like, have been steadily high-densified and, accordingly, the track width of the magnetic head tends to become narrower with a higher accuracy. Nowadays, the track width requires a dimensional accuracy lower than ±2 µm for VTR and FDD, or lower than ±1 µm for RDD. In forming such cores for magnetic head, there have heretofore been known a laser machining in air of a track portion, which is disclosed in Japanese Patent Application Laid-open Nos. 29,118/76, 212,617/82, etc. Further, Japanese Patent Application Nos. 117,726/80, 260,408/86, etc. disclose the formation of the track, by laser machining in air, of a core for magnetic head that is made of a high magnetic permeable alloy, such as Sendust or the like, or of a composite core for magnetic head that is composed of ferrite and a high magnetic permeable alloy. However, according to these methods, since the temperature of the material to be machined reaches above the melting point temperature of the material, a processing strain due to heat is formed on the machined surface, accompanied with the formation of cracks on the surface, and properties of the resulting magnetic head are deteriorated. Further, these methods have presented problems such that melted and solidified substances or melted and scattered substances adhere to the machined surface and the vicinity thereof, as well as problems, such as a thermal strain, cracks or the like, to deteriorate the surface roughness or dimensional accuracy, which do not allow a track processing of high accuracy to be conducted with a dimensional accuracy lower than ±2 µm, which has recently been strongly required. Meanwhile, Japanese Patent Application Laid-open No. 60,995/85, the Precision Machine Society, 1985, Spring Symposium, Scientific Lecture Articles (Presentation No. 404) and Engineering Materials, 33 , No. 14 (P.57∼p.62) disclose, in a field other than the magnetic head processing, three methods of machining ceramics, such as ferrite, Si 3 N 4 , SiC or the like, by a laser-induced etching wherein a laser beam is irradiated in a potassium hydroxide aqueous solution. However, any of these methods disclosed in the above references lack requirements for obtaining a high accuracy which is necessary for the magnetic head processing, so that the machining with such an accuracy as required in the magnetic head processing has not been performable. Japanese laid-open patent applications no. 60-229283 and 61-76689 describe laser-etching of magnetic head slider material made of alumina-TiC ceramic in KOH, using a stainless steel mask not corroded by the etching solution. IBM Technical Disclosure Bulletin No. 10A, Vol. 26 describes laser-etching of a recording head of ferrite elements in a liquid, such as KOH solution, using a mask of unspecified metal. Japanese laid-open patent application no. 57-100620 describes laser-etching of ferrite, to form grooves adjacent a track or slider of a magnetic head, using a metal mask. The object of the present invention is to obviate the drawbacks of the above-described conventional methods, for example to obviate the drawback that burrs are formed at edge portions of a track formed on the magnet core, and to provide a method wherein a track with a high accuracy is machined by a laser-induced etching method to produce cores for magnetic head with a high reliability. According to the invention there is provided a method of machining as set out in claim 1. The invention further provides a method of producing a core for magnetic head having a track, as set out in claim 2. In this method of producing a core, typically, at least on the portion, where the track is formed, of the gapped-bar having a coil turn hole and a magnetic gap is firstly formed a metal film made of any one of Zn, Cr, Fe, Cd, Co, Ni, Sn and Pb; and thereafter, the gapped bar is subjected to the laser beam to form grooves defining the track width of the track of the magnetic core on the surface of the gapped bar in the alkali metal hydroxide aqueous solution. Therefore, a highly accurate track that is free from adhesion of solidified molten substances and free from dulls formed at the edge portions thereof can be obtained. The dimensional accuracy of the grooves to define the track width is required to be lower than ±2 µm, preferably lower than ±1 µm. Accordingly, it is required to restrain the dimension of the burr formed at the edge portions of the track to be lower than 0.5 µm in radius, preferably lower than 0.25 µm in radius. In the laser-induced etching method applied to the present invention, the alkali metal hydroxide aqueous solution serves as an etching solution for etching the surface of the gapped bar. Iron, which is a main composition of ferrite and Sendust, is etched in the alkali metal hydroxide aqueous solution with a high temperature in a good manner. However, aluminum, which is one of the compositions of Sendust, is etched in the alkali metal hydroxide aqueous solution in an excessive manner. Additionally, since Sendust has a high heat conductivity, burrs are often formed at the edge portion of the track made of Sendust. In such a case, the dimensional accuracy for the track is so decreased that the above-mentioned requirement for forming the track cannot be carried out. In the present invention, in order to restrain that the burrs are formed at the edge portions of the track, the metal film is firstly formed at least on the portion, where the track is formed, of the gapped bar; and thereafter the gapped bar is machined by a laser beam in the alkali metal hydroxide aqueous solution. The metal film serves to protect the track portions of the gapped bar from over etching of aluminum included in Sendust, to restrain that big burrs are formed at the edge portion of the track; and to make the dimension of the burrs formed at the edge portion of tracks to be lower than 0.5 µm in radius. The reason for selecting Zn, Cr, Fe, Cd, Co, Ni, Sn and Pb as the material of the metal film is that: when the gapped bar is immersed in the alkali metal hydroxide aqueous solution a potential difference occurs between ferrite and Sendust (a battery is formed); since a natural potential of Sendust is lower than that of ferrite in alkali metal hydroxide aqueous solution and an area fraction of ferrite is larger than that of the Sendust in the gapped bar, the natural potential of the gapped bar as a whole becomes close to the nature potential of ferrite; therefore, the potential difference between ferrite and Sendust is not so large with respect to the natural potential of ferrite but large to that of Sendust; thus when the Sendust constituting of the gapped bar is exposed in the aqueous solution for etching, an electric current flows to Sendust side, so that only Sendust constituting of the gapped bar is etched. Therefore, it is required for the metal film formed on the gapped bar to make the potential of the gapped bar as a whole close to the natural potential of Sendust and it is also required that the natural potential of the metal film is lower than that of ferrite. On the other hand, the metal having a natural potential lower than that of Sendust is not appropriate for the metal film, because such metal is etched by the battery formed due to the potential difference between ferrite and Sendust, and the metal does not serve as a protection film. Therefore, Zn, Cr, Fe, Cd, Co, Ni, Sn and Pb, whose natural potentials are higher than Sendust but lower than ferrite, are selected as the material of the metal film. The metal film is formed on the surface of the gapped bar by spattering, deposition with the aid of electron beam, metal plating, etc. The desired thickness of the metal film is 200Å to 5000Å, particularly 300Å to 1500Å. If the thickness is smaller than the lower limit of this range, the metal film may not serve to prevent to form the big burrs due to the small film strength thereof; on the other hand, if the thickness is larger than the upper limit of the range, the desired etching depth of the grooves may not be obtained or the dimensional accuracy may be decreased, because the machining efficiency is deteriorated due to the thickness. After forming the grooves in the gapped bar, the metal film is removed by means of dry etching or wet etching, etc. For a better understanding of the invention, reference is taken to the accompanying drawings, in which: Fig. 1 is a diagrammatic view of one embodiment of apparatuses to be used for carrying out the present invention; and Figs. 2a to 2g are perspective views showing the procedure in the case where the present invention is applied to the production of a ferrite Sendust composite core for VTR magnetic head. (1) Structure of apparatus: Fig. 1 illustrates an embodiment of an apparatus to be used for carrying out the present invention. In this embodiment, on an X-Y stage 1, placed a container 2. In the container 2, there are arranged a sample holder 3 and a gapped bar 4 to be processed. The container 2 is filled with an alkali metal hydroxide aqueous solution 5, such as potassium hydroxide, sodium hydroxide or the like. The liquid level of the aqueous solution 5 above the gapped bar 4 is controlled by a quartz window 7 whose position is adjustable by means of a micrometer 6. If the liquid level of the aqueous solution 5 is established to be too low, the flow rate of the aqueous solution is decreased whereby the aqueous solution is heated up excessively and bubbles are apt to be generated. Therefore, the etched depth is decreased and the dimensional accuracy becomes to be lowered. When the liquid level is set at 200 µm or more, there may scarcely be such an influence. However, when the liquid level is too high, the light amount of the laser beam is decreased during the laser beam passes through the aqueous solution, the etched depth is decreased, or a convection is formed in the aqueous solution due to upward and downward flows of the solution. In case the connection is formed in the aqueous solution, wastage ejected from the etching point or bubbles, are raised by the convection and the laser beam is scattered by the wastage and bubbles, resulting in decreases of the etched depth and dimensional accuracy. Therefore, it is preferred to set the liquid level at least 200 µm, more preferably 300∼10,000 µm. Further, the quartz window 7 serves to control the liquid level and eliminate the fluctuation of the liquid surface, resulting in the improvement of the dimensional accuracy. A laser beam 8 is emitted from a laser source 9 to irradiate onto te gapped bar 4 through a lens system 10 and the quartz window 7. Then, an etching of a predetermined pattern can be carried out by moving the X-Y stage 1. (2) Production of a core for magnetic head Figs. 2a to 2g show a flow diagram illustrating the procedure in the case where the present invention is applied to the production of a ferrite/Sendust composite core for VTR magnetic head. On the outset, as shown in Fig. 2a, a ferrite bar 11a and a ferrite bar 11b having a coil turn hole 12 are prepared; and on surfaces 13, 13 opposite to each other of the respective bars 11a, 11b are formed films made of Sendust each having a thickness of 5 µm, as shown in Fig. 2b. Further, on the film of the ferrite bar 11a, is formed non-magnetic material layer 15, such as SiO 2 layer, with the same thickness as a predetermined gap length, as shown in Fig. 2c; and thereafter these bars 11a and 11b are bonded with each other by means of a glass-cementing to form a gapped bar 4 having magnetic gap 15 as shown in Fig. 2d. After the gapped bar 4 is ground to a predetermined dimension, a metal film 16 made of Cr is formed on the one surface, where the track is formed, of the gapped bar 4 with a thickness of 500Å. Then, the prepared gapped bar 4 is set in the apparatus shown in Fig. 1; and a plural number of grooves 17 defining a track width 13 are machined on the gapped bar 4 as shown in Fig. 2e, under conditions such that the laser power is 100 mW, the scanning speed is 10 µm/sec, the focused laser beam diameter is 4 µm, the concentration of potassium hydroxide aqueous solution is 25% by weight, and the liquid level is 500 µm. Thereafter, the film of Cr 16 is removed and cores 18 having a predetermined width are cut out from the processed gapped bar. Thus, cores for VTR magnetic head 18 as shown in Fig. 2g are obtained. On the thus obtained core for magnetic head 18, a highly accurate track that is free from microcracks or adhesion of solidified molten substances and free from burrs at the edge portion of the track is formed, which is high in reliability. Additionally, the present invention is not intended to be limited to the manufacturing method of cores for VTR magnetic head and can be suitably applied to the cores for diversified magnetic heads, such as RDD, FDD, or the like. Further, the present invention is not limited to the track machining and also can be suitably utilized in various processings of further composite materials of ferrite and Sendust, such as machining of coil turn holes, machining of air bearing surface, or the like in magnetic cores. In the above mentioned embodiment, the Cr film is used as the metal film, however, the other metals cited in this specification be also able to use as the metal film. As described above, according to the producing method of the present invention, tracks having a narrow width and being free from burrs can be formed on a gapped bar in a high accuracy by irradiating a predetermined laser beam at a predetermined scanning speed in an alkali metal hydroxide aqueous solution of a predetermined concentration after forming the metal film on the surface of the ferrite/Sendust composite magnetic gapped bar, so that cores for magnetic head high in reliability can be produced.

Claims: A method of machining, by laser-induced etching, of a substrate (4) formed of a ferrite material and Sendust, when the substrate is in an aqueous solution of alkali metal hydroxide, characterized by, prior to the machining, forming a metal film (16) on the surface to be machined, said metal film being of a metal having a natural potential higher than that of Sendust and lower than that of ferrite and selected from Zn, Cr, Fe, Cd, Co, Ni, Sn and Pb, and said metal film (16) being partially removed by said laser-induced etching. A method according to claim 1, for producing a core (18) for a magnetic head, in which a track is formed by the laser machining, comprising the steps of: forming a composite gapped bar (4) composed of ferrite and Sendust; forming said metal film (16) on at least a surface, where the track is formed, of the composite gapped bar; subjecting the gapped bar (4) to the laser-induced etching in the alkali metal hydroxide aqueous solution to form grooves (17) which define the width of the track of the core for magnetic head, said metal film being partially removed by the laser-induced etching; removing unetched parts of said metal film (16) from the surface of the composite gapped bar; and cutting the core (18) having a predetermined width from the gapped bar (4).

Inventors: IKEDA KOJI, ONISHI YUJI, SANDAIJI HIDETO, IKEDA, KOJI, ONISHI, YUJI, SANDAIJI, HIDETO
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1990
Main classification: G11B5
IPC codes: C04B41, G11B5, C23F1