US3656229A

US3656229A - Process for producing magnetic head

Info

Publication number: US3656229A
Application number: US874292A
Authority: US
Inventors: Yo Sakurai; Teizo Tamura; Norikazu Hashimoto
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1968-11-08
Filing date: 1969-11-05
Publication date: 1972-04-18
Anticipated expiration: 1989-04-18
Also published as: NL148171B; DE1956217C3; DE1956217B2; DE1956217A1; NL6916827A

Abstract

An improved magnetic head is produced by utilizing as a spacer for regulating the width of the head gap thereof a non-magnetic oxide film formed by applying a vapor of at least one volatile metal alkoxide capable of forming a non-magnetic metal oxide by thermal decomposition reaction, e.g. an alkoxide of such a metal as A1, Ti, Zr, Si or Hf, onto the surface of a ferrite core heated to above the thermal decomposition temperature of said alkoxide, thereby bringing about on the core surface the thermal decomposition reaction of said alkoxide.

Description

United States Patent Sakurai et al.

[151 3,656,229 [451 Apr. 18, 1972 PROCESS FOR PRODUCING MAGNETIC HEAD Inventors: Yo Sakural, Kunitachi-shi; Telzo Tamura, Katsuta-shi; Norlkazu Hashlmoto, l-lachioji-shi, all of Japan Assignee: Hitachi, Ltd., Tokyo, Japan Filed: Nov. 5, 1969 Appl. No.: 874,292

Foreign Application Priority Data Nov. 8, 1968 Japan ..43/8l256 Feb. 28, 1969 Japan ..44/14621 U.S. Cl. ..29/603, 117/106, 1 17/106 D, 117/234, 179/100.2 C

Int. Cl. ..G11b 5/42, H011 7/06 Field of Search ..29/603; 117/234, 106, 107.2, 117/106 D; 340/l74.1 F; 179/1002 C; 346/74 MC References Cited UNITED STATES PATENTS 12/1958 Kornei ..179 /1 ,2 c

Black et al. .J ..l17/106 D 3,098,126 7/1963 Kaspaul.... 1 79/1002 C 3,126,615 3/1964 Duinker ..29/603 3,458,926 8/1969 Maissel et al. ..29/603 Primary Ltaminer-John F. Campbell Assistant Examiner-Carl E. Hall Attorney-Craig, Antonelli and Hill [57] ABSTRACT 8 Claims, 8 Drawing Figures i I L153? PATENTEDAPR I8 i972 SHEET 1 OF 3 INVENTORS rerzo TAMI-4H8 J Y0 sAkuRAr,

NORIKAZH HASHIMaTo ATTOR NEYS PROCESS FOR PRODUCING MAGNETIC HEAD This invention relates to an improvement in a process for producing a magnetic head employed in magnetic recording devices, including memory devices for electronic computers and the like, and particularly to an improvement in the struc ture of the magnetic gap portion of such magnetic head.

With the development of magnetic recording devices such as video tape recorders for color televisions, memory devices for electronic computers, etc., the recording devices are desired to be increased in storage capacity or to be made higher in storage density in order to make the devices compact. In response to such desire, there have been made various improvements concerning magnetic tapes and magnetic disks which are used as recording media. What is of great importance, however, is an improvement of magnetic heads which constitute the most important terminals in subjecting the recording devices to recording-reproduction on memoryreading, and particularly an improvement relating to the head gap portions of magnetic heads.

That is, the thickness of the head core of magnetic head and the width of the head gap thereof are critical factors for making the storage density higher. A magnetic head employed in a video tape recorder is different from that employed in a disk file for computer, or the like. However, the two are substan tially the same in geometrical structure at the head core and head gap portions, and it has recently been required that the head gap portion should be precisely restricted to a width of about 0.8-1 .Zu.

Further, in connection with the questions to make the storage density higher and to make the speed of recordingreproduction higher, it is required that the head should be uniformly contacted with such a recording medium as magnetic tape or magnetic disk; that the head should be made greater in bonding strength to a spacer, which is inserted in the head gap portion of the head core to regulate the width of the head gap, so that the head core is increased in wear resistance and is prevented from breakage; and that the difference in hardness or thermal expansion coefficient between the two should be made smaller as far as possible.

The head gap portion of the above-mentioned head core is formed by inserting between two thin plates of magnetic substances a non-magnetic spacer having a thickness corresponding to the width of the gap and bonding them together by means of a suitable adhesive.

Typical examples of procedures employed in the prior art in constituting head gap portions are as follows:

1. A foil of a non-magnetic substance, such as titanium, Be-

Cu alloy or mica, which has a thickness corresponding to a desired gap width, is inserted between two thin plates of magnetic substances, and the resulting composite is fused together by use of glass.

2. A glass foil having a thickness corresponding to a desired gap width is inserted between two thin plates of magnetic substances and the resulting composite is fused by heating to the softening temperature of the glass under a mechanically pressurized state.

. A metal halide or the like of a non-magnetic substance is coated onto the surface of a thin plate of a magnetic sub stance; the coating is oxidized by heating to convert said coating to a non-magnetic oxide; and then another thin plate of a magnetic substance is adhered to the thus treated coating.

As the aforesaid adhesive, there is sometimes used a synthetic resin adhesive in place of the glass.

All of the above-mentioned conventional processes, however, are extremely disadvantageous for the formation of the aforesaid narrow head gap of 0.8-1 .2p..

For example, in the above-mentioned conventional process l the use of a foil having a thickness of less than 1p. is necessary in order to form a head gap of less than 1 t. However, it is extremely difficult to obtain such a thinly processed foil. Further, the operation of inserting such a thin foil between two magnetic substances is markedly low in efficiency. Thus,

the conventional process (1) is not suitable for mass productron.

According to the conventional process (2), the thin magnetic plates tend to be distorted when mechanical pressure is applied to the composite. Further, the softening of the glass foil results in uneveness in thickness of the spacer. Moreover, the process has such drawbacks that in most cases, air bubbles are formed in the glass spacer which lower the bonding strength between the glass and the magnetic substances and induce the breakage of the gap to make the life of the head shorter.

The conventional process (3) necessarily brings about such drawbacks that the control in thickness of the spacer is difficult; and that due to the heat treatment and oxidation reaction in converting the halide to an oxide, even the thin plates of magnetic substances are subjected to chemical change or thermal stress to form cracks.

In order to overcome such drawback of the prior art processes as mentioned above, the present inventors made various studies to find a process for producing a new magnetic head which is based particularly on an improvement in procedures for forming the head gap portion of the magnetic head.

It is therefore an object of the present invention to provide a process for producing a magnetic head having a head gap which is narrow and is uniform and well-controlled in width.

Another object is to provide a process for producing a mag netic head long in life.

A fundamental improvement in the: present process resides in the point that the spacer, which regulates the width of the head gap of magnetic head, is composed of a non-magnetic oxide film formed by applying a vapor of a metal alkoxide onto the surface of a magnetic substance and thermally decomposing said metal alkoxide.

Examples of the metal alkoxides employed in the present invention which give non-magnetic metal oxides by thermal decomposition are alkoxides of such metals as aluminum, titanium, silicon, zirconium and hafnium. These may be used either singly or in admixture of two or more.

The process of the present invention is explained in detail below with reference to the accompanying drawings and preferred embodiments.

In the accompanying drawings:

FIG. 1 is a rough plane view of a magnetic head for video tape recorder.

FIG. 2 is a rough sketch of the head tip portion of the magnetic head.

FIG. 3 is a rough plane view of a magnetic head employed in a disk file for electronic computer.

FIG. 4 is a flow sheet showing the steps for producing the magnetic head according to the present invention.

FIG. 5 is a block diagram showing an example of a thermal decomposition apparatus for depositing a non-magnetic oxide onto a thin magnetic plate.

FIG. 6 is a curve showing the relationship between the heating temperature of aluminum isopropoxide and the growth rate of aluminum oxide on a thin magnetic plate.

FIG. 7 is a curve showing the relationship between the growth rate of a mixed aluminum-silicon oxide film obtained by thermally decomposing on a thin magnetic plate a mixed vapor of silicon tetraethoxide and aluminum triethoxide and the heating temperature of said aluminum triethoxide.

FIG. 8 is a curve showing the relationship between the thermal expansion coefficient and the composition of a film com posed of a mixture of aluminum oxide and silicon oxide.

Ordinarily, the structure of a magnetic head for video tape recorder is as shown in FIG. 1. That is, the magnetic head of this kind is composed of a head tip 11, a coil 12 wound around said tip, a base plate 14 provided with lead terminals 13, 13' for said coil, and a head base for attaching the tip 11 and the base plate 14 to the main body of tape recorder (not shown). The mark 16 shows a magnetic tape. The detailed structure of the above-mentioned head tip 11 is as shown in FIG. 2. That is, the head tip comprises two plate-like

semi-core bodies

21 and 22, and a spacer 25 is inserted between said two semi-core bodies to form a magnetic gap having a gap width 26. The semi-core body 21 is provided on the interior and exterior sides with two V-

shaped notches

23 and 24, and a coil 24 is wound around said notches. Further, the

semi-core bodies

21 and 22 are cut to the form of an are at the end of the interfacial portion thereof so as to minimize the track width.

FIG. 3 shows a head of disk file. This head is composed of a core 31, two

coils

32 and 33 wound around said core, and a slider 34 for supporting said core. In the case of this head also, the structure of the head gap portion of the core is entirely identical with that of the aforesaid head for video tape recorder.

In accordance with the present invention, such a magnetic head as mentioned above is produced according to, for example, such steps as shown in FIG. 4.

As a material 41 of the head core, there is chiefly used a single crystal or a hot-pressed sintered article of a ferrite of the Mn-Zn system or Ni-Zn system. In case the material 41 is a single crystal ferrite, a definite crystal orientation is selected, and the material is cut to a wafer 42 in the form of a parallel flat plate. Further, the water is processed in a given axial direction into a strip 43 in the form of a rectangular parallelopiped. Thereafter, the strip 43 is lapped on the surface 44, which is to be provided with a spacer to be inserted into a head gap portion, and the surface is polished to a plane degree of about 0.1;,t/ mm and a face coarseness of less than about 0. I ;L.

Subsequently, the thus obtained strip 43 is engraved on the interior and exterior sides thereof with V-shaped

grooves

45 and 46 to form notches (refer to FIG. 2), through which a coil is wound around a core. Thereafter, the strip 43 is heated in such a heating furnace as mentioned later in a neutral atmosphere, e.g. in a nitrogen or argon gas, to a temperature higher than the thermal decomposition temperature of the metal alkoxide employed, and the polished surface thereof is contacted with a vapor of the metal alkoxide to form on said surface a metal oxide film 47. On said metal oxide film 49 formed on said strip 43 is further placed another strip 43 which has been polished on the surface, and the two strips are bonded together by means of a suitable adhesive, while interposing said film between the two, to obtain a composite. The thus obtained composite is thinly cut in the direction perpendicular to the lengthwise direction to prepare a head tip 48.

Alternatively, the above-mentioned strip 43 is further provided with a metal oxide film 47' and is then adhered to the strip 43 to obtain a composite, which is then thinly cut in the same manner as above to prepare a head tip 49.

The thus

prepared head tip

48 or 49 is ground or cut in order to make the track width smaller, is wound with a coil and is then assembled to such a head as shown in FIG. 1 or 3.

FIG. 5 is a rough sketch of an apparatus employed for forming on the aforesaid strip a non-magnetic oxide film by the thermal decomposition of metal alkoxide. This apparatus comprises a vessel 51 for a carrier gas of alkoxide vapor, a deoxidation trip 52 for the carrier gas, a dehydration trap 53, a

gas filter 54, a gas flow-controlling valve 55, a flowmeter 56, a

thermal decomposition furnace 50, a main pipe 57 connecting in series various means arranged from said vessel 51 to said thermal decomposition furnace 50, at least one branch flow path 58 which is branched from the main pipe 57 at a portion between the gas filter 54 and the thermal decomposition furnace 50 and is again connected to the main pipe, and a flowcontrolling valve 59, a flowmeter 60, a metal alkoxide trap 61 and a heater therefor 62 which are provided on said branch flow path 58.

The thermal decomposition furnace 50 is composed of a furnace vessel in the form of a temple bell, a support stand therefor 64, a sample-placing stand 65 inserted through a hole provided in said support stand 64 into the interior of the furnace vessel 63, a cylinder for supporting said sample-placing stand 65, a heater 67 attached to the lower part of said sample-placing stand 65, and a gas-discharging opening 68 provided in said support stand 64.

As the above-mentioned disoxidation trap is used DIOXO, which is a disoxidant produced by Baker & Co., Inc. U.S.A., and as the dehydration trap is used such a cooling trap as dry ice or the like.

Using such apparatus as mentioned above, an aluminum oxide film is formed on a single crystal ferrite strip according to such procedures as explained below.

On the sample-placing stand 65 of the thermal decomposition furnace 50, a ferrite strip, which has been polished as mentioned above on the surface to be provided with an oxide film, is horizontally placed with the polished surface up. Subsequently, the air in the apparatus is completely replaced with nitrogen introduced into the apparatus from the vessel 51. Thereafter, the valve 59 on the branch flow path 58 is closed, and nitrogen gas is flowed only through the main pipe. The strip in the furnace 50 is heated by means of the heater 67 and is maintained at a definite temperature of 500 C.

On the other hand, the metal alkoxide trap 61 provided on the branch flow path 58, which trap has been previously charged with aluminum isopropoxide, is heated to l30 C by means of the heater 62. In such a state, the valve 59 of the branch flow path 58 is opened, and the gas flow rate in the branch flow path is maintained at l liter/min while that in the main pipe is maintained at 4 l/min. When the apparatus is operated in the above manner, a vapor of the aluminum isopropoxide in the alkoxide trap 61 is sent to and contacted with the surface of the ferrite strip 69 in the thermal decomposition furnace 50 to form a thin aluminum oxide film on said surface. In this case, the thickness growth rate of the aluminum oxide films is about 50 A./min. Accordingly, the thickness of an aluminum oxide film to be formed on the surface of the ferrite strip 69 can be precisely adjusted to a desired thickness by controlling the time of the thermal decomposition reaction.

FIG. 6 is a curve showing the relationship between the above-mentioned heating temperature of the aluminum isopropoxide and the growth rate of the aluminum oxide film formed on the strip. As is clear from FIG. 6, the growth rate of the aluminum oxide film can be accelerated by increasing the vapor pressure of the aluminum isopropoxide.

In the present process, the growth rate of the oxide film can be increased, in general, when the heating temperature of the alkoxide is elevated. However, if the growth rate is excessively accelerated, the resulting oxide film is lowered in density to make it impossible to obtain an oxide film sufficiently high in hardness. In practicing the present process, therefore, the heating temperature of alkoxide is desirably within such a range that the resulting oxide film is not lowered in hardness. When, the above-mentioned embodiment, the heating temperature of aluminum isopropoxide is C, the resulting aluminum oxide film is substantially the same in hardness as a stainless steel film.

The ferrite strip, on which the aluminum oxide film has been formed in the above manner, is processed into a head tip according to such steps as shown in FIG. 4.

The thermal expansion coefficient of the thus formed aluminum oxide film is 7-8 X I0' C and is close to that of the ferrite 9 X l0/ C. Accordingly, there are scarcely brought about the peeling of said aluminum oxide film and the like damages due to abrasion heat or the like applied to the head.

As mentioned previously, the present invention can be practiced by use of any metal alkoxides so far as they are volatile and can give non-magnetic oxides as thermal decomposition products thereof. For example, a titanium oxide film can be formed on a ferrite strip in the same manner as in the abovementioned embodiment by heating titanium isopropoxide to 60-29 C, using nitrogen gas as a carrier gas. The thermal expansion coefficient of the thus obtained titanium oxide film is 8-9 X l0"/ C and is quite close to that of the ferrite.

Table 1 shows the boiling points and preferable heating temperatures of various metal alkoxides and the heating temperatures of ferrite cores.

TABLE 1 Metal alkoxide preferable for use in the present invention and preferable heating temperature Heating terppplii'atuo Tempppiture a on e o errite Alkoxides 13.1. 0. (mm. Hg) trap, C. strip, C. A1(o ci)a-- 145 0. (50 mm. Hg) 120-140 40%00 i- Al(O-C3H1)a 176 C. (80 mm. Hg)" 160-180 400-500 S1 (OC2H5)4.. 30 C. (50 mm. Hg) -30 700-800 fIi( 0CzHs)4- 104 C. (1 mm. Hg)... 00-100 400-500 i-Ti(OC3H1)4 189 C. (18 mm. Hg) 60-00 400-500 -Ta(OO4Hn)4. 94 C. (5 mm. Hg) 80-00 500-000 l-ZI(OO4HD)I... 80 C. (5 mm. Hg) 70-85 500-000 70-85 500-600 In case, in the present invention, .a vapor of a mixture of metal alkoxides is contacted with a ferrite strip, there is formed an oxide film composed of the mixture of said metals. In such a case, the aforesaid apparatus is further provided with another branch flow path 58, alkoxide trap 61', heater therefor 62, flow-meter 60 and control valve 59' as shown by the broken line in FIG. 5. Using this apparatus, the strip in the thermal decomposition furnace is maintained at about 500 C, and, for example, aluminum triethoxide is charged into one alkoxide trap 61 and is heated to about 160-180 C while silicon tetraethoxide is charged into the other alkoxide trap 61' and is heated to -30 C. Subsequently, nitrogen gas is flowed through each of the main pipe, the branch flow path 58 and the branch flow path 58 at rates of 5 l/min, 3 l/min and 10 l/min, respectively, and the individual alkoxides are thermally decomposed to contact a mixed vapor of the two alkoxides with the ferrite strip, whereby a non-magnetic oxide film composed of aluminum oxide and silicon oxide can be formed on the ferrite strip. In this case, the growth rate in thickness of the film is about 250 A./min.

FIG. 7 shows the variation in growth rate of the oxide film when, in the above-mentioned embodiment, the heating temperature of the silicon tetraethoxide is made 25 C and the heating temperature of the aluminum ethoxide is varied within the rage from 160 to 180 C.

The thus obtained film comprising the mixture of aluminum oxide and silicon oxide is higher in hardness and more chemically stable than a film of pure aluminum oxide or pure silicon oxide obtained in the same manner as above. Accordingly, when a spacer for head gap is constructed by such mixed oxide film as mentioned above, the head gap portion can be enhanced in wear resistance.

FIG. 8 is a curve showing the relationship between the thermal expansion coefficient and the composition of a mixed oxide film comprising aluminum oxide and silicon oxide. According to FIG. 8, the thermal expansion coefficient of pure silicon oxide is 6.7 X l0' C and considerably differs from that of the ferrite 90 X 10 C. However, the thermal expansion coefficient of a mixed oxide film comprising said silicon oxide and an equimolar amount of aluminum oxide is about 40 X 10'/ C, and thus is considerably close to that of the ferrite.

In addition thereto, various combinations of metal alkoxides may be used so far as they can form oxide films, which are non-magnetic, are high in hardness and mechanical strength and have a thermal expansion coefficient close to that of the ferrite. For example, a film of a mixture comprising substantially equal amounts of zirconium oxide and silicon oxide, which has been formed according to the same process as in the above-mentioned embodiment by use of zirconium tetraethoxide and silicon tetraethoxide, has a thermal expansion coefficient of about 8-9 X l0"/ c, and is substantially the same in other mechanical and chemical properties as the aforesaid film of a mixture comprising aluminum oxide and silicon oxide.

In the present invention, particularly preferable combinations and heating temperatures of alkoxides, and preferable heating temperatures of ferrite strip are as set forth in Table 2.

A ferrite tip bearing thereon a film consisting of at least one kind of magnetic oxides formed in the above-mentioned manner is processed into a head tip according to such steps as shown in FIG. 4.

TAB LE 2 Combinations of alkoxides preferable for use in the present invention and preferable heating temperatures Hunting temperature of ferrite Combination and heating temperature of alkoxides strip, C,

AI(OC2II5)J (160 (1,); Si(OCzII5)4 (room temperature)" 400-500 Zr(0C li )4 0,); Sl(OCzlis)4 (room termpeaturo)... 500-600 i-Ti(0C ll1)4 (80 0,); 81(002115) (room temperature) 400-500 i-Al(0() lI1)3 0,); t-Ti(()C;l-I (80 0,) 400-500 l-Tl1(0C4Ilp)3 (80 SI(OC2H5)4 (room temperature) 500-600 For the adhesion of strips, adhesives such as glass, synthetic resins, etc. are used. However, synthetic resins swell undesirably due to organic substances, and therefore the use of glass is preferable.

In order to bond two ferrite strips by use of glass, the two ferrite strips 43 and 43' are contacted each other while interposing therebetween the non-magnetic oxide film 47, as shown in FIG. 4, and then fused glass is flowed into the gap between the two ferrite strips.

The magnetic head obtained by practicing the present invention in the above-mentioned manner bears the non-magnetic oxide film directly on the thoroughly polished surface of the ferrite. Further, as said film, there may be selected an oxide film which is close in thermal expansion coefficient to the ferrite. Accordingly, it is possible to form a spacer for head gap which is firmly bonded to the core body and is difficulty peeled off therefrom.

Furthermore, it is also possible according to the present invention that the thickness of the oxide film constituting the above-mentioned spacer can be controlled uniformly and precisely. For example, in the case of an aluminum oxide film obtained by contacting the aforesaid aluminum isopropoxide vapor with a ferrite strip, the variation in thickness of said film is about :t 0.02;.t. Further, in the case where glass had been used as an adhesive, the variation in gap width of the whole gap is i 0.1;tper 5 mm. According to the present invention, therefore, it is quite easy to obtain ahead core having a head gap of less than l;1..

What is claimed is:

1. An improvement in a process for producing a magnetic head by using a head tip having a magnetic gap regulated in width by means of a spacer composed of a non-magnetic oxide film, which improvement comprises:

a step in which a ferrite strip shaped into a desired form is polished sufficiently smooth on the surface to be provided with a magnetic gap;

a step in which the above-mentioned ferrite strip is heated in an inert or neutral atmosphere to a thermal decomposition temperature of a metal alkoxide employed for forming a spacer for the gap;

a step in which at least one volatile metal alkoxide capable of forming a non-magnetic metal oxide by thermal decomposition is vaporized;

a step in which the vapor of said vaporized alkoxide is contacted with the above-mentioned heated ferrite strip, using an inert or neutral gas as a carrier, and said alkoxide is thermally decomposed on the polished surface of said ferrite strip to form on said surface a non-magnetic oxide film having a desired thickness; and

a step in which'the ferrite strip bearing the above-mentioned oxide film is adhered by use of an adhesive to another ferrite strip, interposing therebetween the said non-magnetic oxide film.

2. An improvement according to claim 1, wherein the metal alkoxide is at least one member selected from the group consisting of alkoxides of metals of aluminum, silicon, titanium, zirconium and hafnium.

3. An improvement according to claim 1, wherein the adheide.

6. An improvement in a process for producing a magnetic head by using a head tip having a magnetic gap regulated in width by means of a spacer composed of a non-magnetic metal oxide film, which improvement comprises:

a step in which a ferrite strip shaped into a desired shape is polished sufficiently smooth on the surface to be provided with a magnetic gap;

a step in which the above-mentioned ferrite strip is heated in an inert or neutral atmosphere at temperature of from 400 to 600 C.;

a step in which at least one aluminum alkoxide selected from the group consisting of Al(-C l-l and i-Al(0-C,,l-l is vaporized;

a step in which the vapor of said vaporized alkoxide is conselected from the group consisting of vaporized alkoxides of metals of silicon, titanium, zirconium and hafnium.

8. An improvement according to claim 1, wherein the process is carried out in a non-vacuum, flow atmosphere.

Claims

2. An improvement according to claim 1, wherein the metal alkoxide is at least one member selected from the group consisting of alkoxides of metals of aluminum, silicon, titanium, zirconium and hafnium.
3. An improvement according to claim 1, wherein the adhesive for adhering the two ferrite strips is glass.
4. An improvement according to claim 1, wherein the metal alkoxide is aluminum alkoxide.
5. An improvement according to claim 1, wherein the metal alkoxide is a mixture of aluminum alkoxide and silicon alkoxide.
6. An improvement in a process for producing a magnetic head by using a head tip having a magnetic gap regulated in width by means of a spacer composed of a non-magnetic metal oxide film, which improvement comprises: a step in which a ferrite strip shaped into a desired shape is polished sufficiently smooth on the surface to be provided with a magnetic gap; a step in which the above-mentioned ferrite strip is heated in an inert or neutral atmosphere at temperature of from 400* to 600* C.; a step in which at least one aluminum alkoxide selected from the group consisting of Al(0.C2H5)3 and i-Al(0.C3H7)3, is vaporized; a step in which the vapor of said vaporized alkoxide is contacted with the above-mentioned heated ferrite strip, using an inert or neutral gas as a carrier, and said alkoxide is thermally decomposed on the polished surface of said ferrite strip to form on said surface a Al2O3 film having a desired thickness; and a step in which the ferrite strip bearing said Al2O3 film is adhered by use of adhesive to another ferrite strip interposing therebetween said Al2O3 film.
7. An improvement according to claim 1, wherein to said aluminum alkoxide vapor is added at least one member selected from the group consisting of vaporized alkoxides of metals of silicon, titanium, zirconium and hafnium.
8. An improvement according to claim 1, wherein the process is carried out in a non-vacuum, flow atmosphere.