JPH0676226A - Magnetic head and manufacturing method thereof - Google Patents

Abstract

(57) [Summary] [Purpose] To provide a magnetic head having excellent recording and reproducing characteristics suitable for high-density recording and excellent mass productivity. [Structure] The ferromagnetic metal film 11 is formed on the slope of the core substrate 10.
The magnetic core halves 12 and 12 ′ coated with are bonded and integrated through the gap material 13 so that the ferromagnetic metal films are aligned with each other, and the parts other than the ferromagnetic metal film on the tape sliding surface 15 are integrated. The nonmagnetic filler 16 is formed at least up to the gap depth. The thickness of the ferromagnetic metal film 11 'on the tape sliding surface adjacent to the non-magnetic filler 16 is made smaller than the thickness of the ferromagnetic metal film 11 deposited on the core substrate. [Effect] Since the ferrite is not exposed on the sliding surface, noise is small, and since the cross-sectional area of the rear magnetic circuit is large, the recording / reproducing efficiency is high. Since the track width processing is performed after the joining block is formed, the track width accuracy is high.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic head, and more particularly to a magnetic head which has low noise and is suitable for high density recording, and a method of manufacturing the same.

[0002]

2. Description of the Related Art In recent years, the progress of high-density magnetic recording has been remarkable, and the increase in recording density has led to the miniaturization and higher performance of devices. In order to realize high-density magnetic recording, it is advantageous to make the medium have a high coercive force. Therefore, in recent years, a VTR or the like uses a high coercive force metal tape having a coercive force of 1200 to 1500 Oe. On the other hand, in magnetic heads, in order to sufficiently perform recording on these high coercive force media, ferromagnetic metal materials such as amorphous alloys and FeAlSi alloys having a high saturation magnetic flux density have come to be used as core materials.
As one of magnetic heads using such a ferromagnetic metal material, a so-called composite magnetic head in which the vicinity of the recording / reproducing gap is made of a ferromagnetic metal material and the rear magnetic core is made of a high-permeability ferrite material Is used. As an example of such a head, Japanese Patent Laid-Open No. 62-157306
There is a composite type magnetic head described in Japanese Patent Publication No.

In this head, as shown in FIG. 2, the magnetic core portions 101 and 102 are made of high-permeability ferrite, and the ferromagnetic metal films 103 and 104 are deposited in the vicinity of the gap g to form a recording / reproducing track. Is.

The above conventional head has an advantage of excellent recording / reproducing characteristics for a high coercive force tape, but since ferrite is exposed on the tape sliding surface, when the tape is slid, it slides from the ferrite portion. There is a problem that dynamic noise is generated. In particular, this sliding noise is a serious problem in the case of a digital VTR, a high-definition VTR, etc., which has a high data transfer rate and therefore a high relative speed with the tape.

As one example of means for preventing such sliding noise, there is a head described in Japanese Patent Application Laid-Open No. 60-182507 shown in FIG. In this head, the core near the tape sliding surface is made of non-magnetic material 107, 108 that does not generate noise,
The rear magnetic cores 105 and 106 are made of ferrite, and the ferromagnetic metal films 109 and 110 are deposited in the vicinity of the gaps to form recording / reproducing tracks, thereby preventing noise from being caused by sliding of the tape.

[0006]

However, this magnetic head has the following problems in manufacturing the head. Figure 2
FIG. 4 shows an outline of a method of manufacturing the magnetic head shown in FIG.
In this magnetic head, an inclined V-shaped groove 112 is formed on the surface of a substrate 111 made of high-permeability ferrite in (a), and a ferromagnetic metal film 113 is formed on the inclined surface of the groove in (b). To do. Next, in (c), the groove 112 is filled with a non-magnetic material 114, and the surface of the substrate is ground. Further, in (d), the second groove 115 is formed, the surface of the substrate is mirror-polished to form a gap forming surface, and the magnetic core half block 116 is manufactured. Further, a magnetic core half body 117 having a winding groove formed in a similar shape is manufactured. Next, in (e), the magnetic core half blocks 116 and 117 formed as described above are butted through a gap material so that the end faces of the ferromagnetic metal films are aligned with each other, welded by glass, and formed into a second groove. A block 119 is obtained by filling the non-magnetic material 118. Further, the XX line and the X'-X 'line are cut to obtain the magnetic head shown in FIG.

On the other hand, in order to manufacture the magnetic head shown in FIG. 3, the composite substrate 125 in which the wear-resistant non-magnetic material 121 is bonded to the high magnetic permeability ferrite 120 is prepared in (a), and V is prepared in (b). The V-shaped groove 122 is formed, and the ferromagnetic metal film 123 is formed thereon in (c). Next, in (d), the upper surface of the composite substrate is ground and polished to produce the magnetic core half block 126. Further, in (e), the winding groove 124 is formed, and the magnetic core half block 127 is manufactured. Next, these magnetic core half blocks 126 and 127 are abutted with each other through a gap material, welded with glass, and then cut along lines c and c'to obtain the magnetic head shown in FIG.

In the method of manufacturing the magnetic head shown in FIG. 4, the substrate 111 has a large shape and is cut in the direction perpendicular to the direction of the second groove 115 in FIG. A plurality of substrates 116 and 117 can be manufactured from the substrate 111. This is effective in improving the mass productivity of the magnetic head. However, since the composite substrate 125 is used in the method of manufacturing the magnetic head shown in FIG. 5, a plurality of magnetic core half blocks 126 and 127 are cut by cutting in the direction perpendicular to the direction of the groove 122 in FIG. It cannot be made. Therefore, the magnetic head shown in FIG. 3 whose sliding surface is made of a non-magnetic material, which is manufactured using the composite substrate 125, is inferior in mass productivity to the magnetic head shown in FIG. There is.

Further, another problem of the magnetic head shown in FIGS. 2 and 3 is as follows. That is, in these heads, the magnetic head core halves are mechanically aligned and joined to each other, so that a track deviation is likely to occur as in the tape sliding surface of FIG. This poses a serious problem for a narrow track head used for high density recording.

[0010]

In order to solve the above problems, the present invention provides a magnetic head which is excellent in recording / reproducing characteristics and mass productivity, does not generate sliding noise, and is excellent in track width accuracy. The following means are used. That is, prepare a magnetic core half block in which a ferromagnetic metal film is formed on a slope of a groove formed on a substrate made of high-permeability ferrite so as to have a thickness larger than a required track width. A joining block is produced by joining the magnetic core half blocks together so that the end faces coincide with each other.After that, the ferromagnetic metal film is left on the tape sliding surface of the joining block in parallel with the track width. A groove is formed at least to the depth of the gap, the groove is filled with a non-magnetic filling material, and the joining block is cut to obtain a magnetic head. As a result, it is possible to obtain a magnetic head in which the rear magnetic core has a high magnetic permeability ferrite, the vicinity of the gap is made of a ferromagnetic metal film, and the tape sliding surface other than the ferromagnetic metal film is made of a non-magnetic material.

The ferromagnetic metal film used in the magnetic head of the present invention has Fe, Co and Ni as magnetic elements and Z.
Metal elements such as r, Ta, Nb, Ti, Hf, or S
A ferromagnetic amorphous alloy containing a metalloid element such as i or C as an amorphizing element, FeAlSi-based (sendust),
Ni-Fe system (permalloy), Fe-C system crystalline alloy,
Further, Fe, Co contain C, N, B, and further Zr, T
Examples thereof include microcrystalline alloys to which i, Nb, Ta, Hf and the like are added. These ferromagnetic metal films are formed by a thin film forming method such as sputtering or vacuum evaporation.

As a non-magnetic filler for filling the groove formed on the tape sliding surface, forsterite (2MgO-S) is used.
iO _2), steatite (MgO-SiO _2), Al
Materials such as ₂ O ₃ and SiO ₂ can be used. These are formed by a vacuum vapor deposition method or a sputtering method. Further, by melting and filling various kinds of glass, it can be used as the non-magnetic filler.

[0013]

In the above-described magnetic head structure and manufacturing method, the thickness of the ferromagnetic metal film is made larger than the track width, and the track width is set when the groove is formed on the tape sliding surface after the joining block is manufactured. By restricting it, it is possible to prevent the deterioration of the track width accuracy due to the track deviation shown in FIG. 6 in the conventional head. Further, by filling the groove with a non-magnetic filler, the high-permeability ferrite is not exposed at the portion where the tape is in sliding contact near the gap, and sliding noise can be prevented. Further, it is not necessary to fabricate the composite substrate of the high magnetic permeability ferrite and the non-magnetic material shown in FIG. 5, and therefore, as described above, a plurality of magnetic core half blocks can be fabricated from a large substrate, Excellent mass productivity. Further, as compared with the method of manufacturing the magnetic head shown in FIG. 5, it is not necessary to form the second groove for regulating the track width, and therefore the productivity is excellent.

In the magnetic head of the present invention, a wear resistant non-magnetic material may be used as the substrate instead of using the high magnetic permeability ferrite as the substrate. In this case, the magnetic circuit is composed only of the ferromagnetic metal film. Also in the present magnetic head, the track width can be regulated by forming the groove on the sliding surface of the joining block, which is effective in improving the track width accuracy. As the wear-resistant non-magnetic material used in the magnetic head, MnO-NiO system, NiO-T
Ceramics such as i ₂ O ₃ series and αFe ₂ O ₃ are used.

As an example of a head in which a groove is formed on the sliding surface of a joining block and a nonmagnetic filler is filled to regulate the track width, there is a head described in Japanese Patent Laid-Open No. 4-30308. In this head, the track width regulating groove is provided parallel to the side surface of the magnetic head. For this reason, when high-permeability ferrite is used for the substrate, the ferrite may remain in a portion of the tape sliding surface adjacent to the ferromagnetic metal film, which may cause noise. When a wear-resistant non-magnetic substrate is used as the substrate, the length of the ferromagnetic metal film on the tape sliding surface becomes short, which is disadvantageous in terms of recording / reproducing efficiency.

[0016]

EXAMPLES Next, the present invention will be described in detail with reference to examples.

<Embodiment 1> A perspective view of an embodiment of the magnetic head of the present invention is shown in FIG. 1 (a), and tape sliding surfaces are shown in (b), (c). In this head, magnetic head core halves 12 and 1 in which a ferromagnetic metal film 11 having a high saturation magnetic flux density is coated on a slope formed on a core substrate 10 made of high-permeability ferrite
2'is integrated with the bonding material 14 via the gap material 13. The nonmagnetic filler 16 constitutes at least the gap depth other than the ferromagnetic metal film portion on the tape sliding surface 15. The thickness of the ferromagnetic metal film 11 ′ on the tape sliding surface adjacent to the non-magnetic filler 16 is
It is smaller than the thickness of the ferromagnetic metal film 11 deposited on the core substrate portion. A coil winding window 17 is provided on the core substrate, and the coil is wound to input / output a recording / reproducing signal.

Due to the structure of this head, the rear magnetic circuit is composed of a high-permeability ferrite and a ferromagnetic metal magnetic film. Therefore, a head having high recording / reproducing efficiency can be obtained because the rear magnetic circuit has a large cross-sectional area. The front magnetic circuit constituting the recording / reproducing track is composed of a ferromagnetic metal film having a high saturation magnetic flux density. Therefore, a sufficient recording magnetic field can be generated for the high coercive force tape. Since the tape sliding surface is composed of a ferromagnetic metal film and a non-magnetic filler, sliding noise due to ferrite does not occur. Further, as shown in FIG. 6, no track deviation occurs in the left and right magnetic core halves, and
As shown in (b), a magnetic head with high track width accuracy can be obtained.

In the magnetic head of the present invention, as shown in FIG. 1B, a multilayer film laminated with a non-magnetic insulating material can be used as the ferromagnetic metal film 11 '. As a result, an eddy current can be prevented and a magnetic head having excellent recording and reproducing characteristics up to high frequencies can be obtained. Further, in the magnetic head of the present invention, a core substrate made of high-permeability ferrite may remain at the end of the head depending on the manufacturing method, as shown in FIG. Even in such a configuration, the tape mainly slides in the vicinity of the gap, and the head end does not slide or the contact force is extremely weak even if the tape slides. If the distance L to is greater than the width W of the sliding surface, it is considered that sliding noise is not affected.

<Embodiment 2> FIG. 7 shows another magnetic head of the present invention. In the present head, magnetic head core halves 12 and 12 ′ having a ferromagnetic metal film 11 having a high saturation magnetic flux density deposited on a slope formed on a core base portion 10 made of a wear-resistant non-magnetic material are used as a gap material. It has a structure integrated by using a bonding material 14 via 13. The nonmagnetic filler 16 constitutes at least the gap depth other than the ferromagnetic metal film portion on the tape sliding surface 15. The thickness of the ferromagnetic metal film 11 'on the tape sliding surface adjacent to the non-magnetic filler 16 is smaller than the thickness of the ferromagnetic metal film 11 deposited on the core substrate. A coil winding window 17 is provided on the core substrate, and the coil is wound to input / output a recording / reproducing signal.

In this head, both the front and rear magnetic circuits are made of a ferromagnetic metal film having a high saturation magnetic flux density, and a sufficient recording magnetic field can be generated for the high coercive force tape. The thickness of the ferromagnetic metal film 11 'on the tape sliding surface that determines the track width is smaller than the thickness of the ferromagnetic metal film 11' on the rear magnetic core.
Since the thickness of the rear magnetic core can be increased, the magnetic resistance of the rear magnetic core can be reduced, and the recording / reproducing efficiency can be improved. Further, no track shift occurs in the left and right magnetic core halves, and a magnetic head with high track width accuracy can be obtained.

<Embodiment 3> Taking the magnetic head shown in Embodiment 1 as an example, a method of manufacturing the magnetic head of the present invention will be described with reference to FIGS.
It is shown in FIG.

A) As shown in FIG. 8A, a plurality of substantially V-shaped grooves 21 each having a slope 20 are formed in parallel with each other on a substrate 19 made of high-permeability ferrite which is to be a core substrate of a magnetic head. Form a book.

Here, the high magnetic permeability ferrite is Mn-Z.
A single crystal or polycrystalline material of n-ferrite is used. In addition, Ni-Zn ferrite can also be used. To form the groove 21 in the substrate 19, a diamond blade is used and mechanical processing is performed by a high speed dicer. Also,
There are a chemical etching method using a corrosive liquid, an electrolytic polishing method, and a method of processing by a thin film processing method such as ion milling.

B) Next, as shown in FIG. 8B, a ferromagnetic metal magnetic film is deposited and formed on the entire surface of the substrate 19.

As described above, the ferromagnetic metal magnetic film is made of Co.
Ferromagnetic amorphous alloys such as NbZr and CoTaZr, Fe
Crystalline magnetic alloys such as AlSi-based and FeGaSi-based, FeTaC, FeZrC, FeHfC, CoTaC, C
A microcrystalline magnetic alloy such as oZrC, FeTaN, FeZrN is used. To form these films, a sputtering method such as a bipolar sputtering method, a high speed sputtering method or an ion beam sputtering method is mainly used.

C) Next, as shown in FIG.
Is filled with a non-magnetic material 23 such as low melting point glass, and the surface of the substrate 19 is ground and polished to form a gap forming surface 24. In this way, the magnetic core half block 25 is manufactured. Also,
As shown in FIG. 9D, the winding groove 26 is formed in the magnetic core half block 25 ′ similar to that of FIG. Furthermore, Si is formed on either or both of the gap forming surfaces 24, 24 '.
A gap material such as O ₂ is deposited by sputtering or the like.

In this embodiment, the magnetic core half blocks 25 and 25 'are manufactured one by one from the substrate 19, but as described above, the large magnetic core half blocks are formed from the large substrate. A plurality of magnetic core half blocks 25, 2 are manufactured and cut in a direction perpendicular to and parallel to the groove 21.
5'can be produced. As a result, mass productivity can be significantly improved.

D) Next, as shown in FIG. 9 (e), the magnetic film end faces 27, 27 'of the magnetic core half blocks 25, 25'.
The gap forming surfaces 24 and 24 'are abutted to each other via the gap material so that they coincide with each other, the low melting point glass 23 is heated and melted, and the magnetic core half blocks are bonded to each other to form a bonded block 28.

In order to bond the magnetic core half blocks, in addition to the method of using the low melting point glass filled in the groove 21 as the bonding material as described above, a low melting point glass rod is inserted in the winding groove 26 to bond the bonding material. , A method of forming a coupling groove other than the winding groove and filling with low melting point glass for coupling, a method of forming a thin film of low melting point glass on the gap surface and using as a binder, a method of using these in combination is there.

E) Next, as shown in FIG. 9F, a ferromagnetic metal magnetic film 22 is left on the tape sliding surface of the coupling block 28 at a depth of at least the gap depth, and a sliding parallel to the ferromagnetic metal magnetic film 22 is left. The moving surface groove 30 is formed.

For forming the groove, the machining method using the diamond blade and the high speed dicer described in the step a) is used. Further, various etching methods and ion milling methods are also possible.

In this step, as shown in FIG. 10G, the ferromagnetic metal magnetic film 22 is formed into the magnetic core half block 2.
Even if there is a deviation between 5, 25 ', since the processing is performed through both blocks as indicated by the alternate long and short dash line, the final track width is highly accurate with no track deviation. Therefore,
The magnetic head of the present invention is suitable for a narrow track head for high density recording.

F) Next, as shown in FIG. 10G, the sliding surface groove 30 is filled with a non-magnetic filling material 31, and unnecessary non-magnetic material is removed by grinding or the like.

As the non-magnetic filler in this step, an oxide such as forsterite or steatite can be used as described above. These materials can be formed at a high speed by the vacuum deposition method and have a coefficient of thermal expansion close to that of ferrite, so that they are useful for maintaining head characteristics and preventing cracks due to internal stress. . In addition, Al ₂ O ₃ formed by the sputtering method,
Oxides such as SiO ₂ can also be used. Since these have extremely high hardness and excellent wear resistance, a head having a long wear life can be obtained. Further, instead of using the film formed by the thin film forming method as described above, low melting point glass or the like can be used by heating and melting and filling the groove in the sliding surface. If a low melting point glass is used as the non-magnetic filler, this softening point must be lower than the softening point of the binder used to make the bonded block in step d) above.

G) Next, the broken line portion of FIG. 10C is cut and the sliding surface is processed into a curved surface to obtain the magnetic head shown in FIG.

In a magnetic head used for a VTR, it is common to provide an azimuth angle that inclines the gap by 5 to 20 degrees with respect to the traveling direction of the head. As shown in the tape sliding surface of FIG. 10 (i), when the head is obtained by cutting the coupling block 28 by tilting the azimuth angle θ, tilt it in the same direction as the tilt of the ferromagnetic metal film 22 as indicated by the broken line a. There are a method of cutting and a method of cutting by tilting in the direction opposite to the tilt of the ferromagnetic metal film 22 as indicated by a broken line b. Of these, tilting in the same direction as the ferromagnetic metal film as indicated by the broken line a makes the length of the ferromagnetic metal film 22 in the magnetic head larger, which is excellent in recording / reproducing characteristics. Since the inclination of the film in the direction of the film is small, it is also advantageous in running property.

<Embodiment 4> FIG. 11 shows another example of the magnetic head of the present invention. In this embodiment, a narrow track cutout 32 is provided near the gap of the ferromagnetic metal film 11 'on the tape sliding surface 15. As a result, the film thickness of the ferromagnetic metal film in the front magnetic circuit is larger than the track width in the front magnetic circuit, so that the magnetic resistance of the front magnetic circuit is reduced and the recording / reproducing efficiency can be increased. . The narrow track notch portion can be formed by various etching methods or ion milling methods after or at the same time as processing the sliding surface groove 30 in FIG. 9F of the manufacturing method shown in the third embodiment. Further, with the recent development of the focused ion beam generator, it has become possible to locally form a narrow track notch near the gap.

FIGS. 12A and 12B show another example of the magnetic head of the present invention. (B) is an enlarged view of the vicinity of the gap of (A). In this embodiment, Cr, Mo, W, Ta, N are provided between the non-magnetic filler 16 and the ferromagnetic metal film 11 '.
A non-magnetic metal film 33 of b, Ti, Zr or the like is provided. This film improves the adhesion between the non-magnetic filler and the ferromagnetic metal film,
Or prevent the reaction between the two.

When a multilayer film for preventing eddy current is used as the ferromagnetic metal film 11 ', the non-magnetic metal film 33 is used.
By directly depositing on the ferromagnetic metal film, the ferromagnetic metal films of the left and right magnetic cores may be electrically conducted and a problem may occur. As shown in FIG. 12C, the above problem can be solved by depositing the nonmagnetic insulating film 34 between the nonmagnetic metal film 33 and the ferromagnetic metal film 11 ′.

[0041]

As described above, since the core base portion other than the ferromagnetic metal film is removed on the tape sliding surface and the structure is filled with the non-magnetic filler, the recording and reproducing characteristics are excellent. We were able to provide a magnetic head with no sliding noise, high mass productivity, and high track width accuracy.

[Brief description of drawings]

FIG. 1 is a perspective view and a plan view showing a magnetic head of the present invention.

FIG. 2 is a perspective view showing a conventional magnetic head.

FIG. 3 is a perspective view showing a conventional magnetic head.

FIG. 4 is a sectional view showing a conventional method of manufacturing a magnetic head.

FIG. 5 is a perspective view showing a conventional method of manufacturing a magnetic head.

FIG. 6 is a plan view showing a problem of a conventional magnetic head.

FIG. 7 is a perspective view showing an example of a magnetic head of the present invention.

FIG. 8 is a perspective view showing a method of manufacturing a magnetic head of the present invention.

FIG. 9 is a perspective view showing a method of manufacturing a magnetic head of the present invention.

FIG. 10 is a perspective view showing a method of manufacturing a magnetic head of the present invention.

FIG. 11 is a plan view showing an example of a magnetic head of the present invention.

FIG. 12 is a plan view showing an example of a magnetic head of the present invention.

[Explanation of symbols]

 10 ... Core substrate part 11, 11 ', 22 ... Ferromagnetic metal film 12, 12' ... Magnetic core half body 13 ... Gap material 14 ... Bonding material 15, 29 ... Tape sliding surface 16, 31 ... Nonmagnetic filler 17 ... Winding window 18 ... Non-magnetic insulating material 19 ... Substrate 20 ... Slope 21 ... Groove 23 ... Non-magnetic material 24, 24 '... Gap forming surface 25, 25' ... Magnetic core half block 26 ... Winding groove 27, 27 '... Magnetic film end surface 28 ... Coupling block 30 ... Sliding surface groove 32 ... Narrow track notch 33 ... Non-magnetic metal film 34 ... Non-magnetic insulating film

Claims (5)

[Claims] 1. A magnetic structure in which two magnetic core halves each having a ferromagnetic metal film deposited on a slope of a core substrate are joined and integrated via a gap material so that the ferromagnetic metal films are aligned with each other. In the head, a groove that is parallel to the ferromagnetic metal film and extends to at least the gap depth up to at least the gap depth is formed on the tape sliding surface so that the ferromagnetic metal film only constitutes a recording / reproducing track, thereby reducing the thickness of the ferromagnetic metal film and reducing the core. A magnetic head, characterized in that a base portion is removed and a groove on the sliding surface is filled with a non-magnetic filling material. 2. The magnetic head according to claim 1, wherein the core substrate is a high magnetic permeability ferrite. 3. A magnetic head according to claim 1, wherein the core substrate is made of a wear-resistant non-magnetic material. 4. A magnetic head having an azimuth angle,
4. The magnetic head according to claim 1, wherein the direction of inclination of the ferromagnetic metal film with respect to the gap surface and the direction of inclination of the side surface of the magnetic head are the same. 5. A magnetic structure in which two magnetic core halves each having a ferromagnetic metal film deposited on the slope of a core substrate are joined and integrated via a gap material so that the ferromagnetic metal films are aligned with each other. In the method of manufacturing a head, a step of forming a groove having an inclined surface on a substrate to be the core base portion, a step of depositing a ferromagnetic metal film on the inclined surface, a step of filling the groove with a non-magnetic material, Excessive non-magnetic material and ferromagnetic metal film on the substrate are removed by grinding and polishing to form a gap forming surface, and a magnetic core half block is manufactured. At least one magnetic core half block has a groove for coil winding. The step of forming
The step of depositing a gap material on the gap forming surface of the magnetic core half block, the two magnetic core half blocks are joined and integrated via the gap material so that the ferromagnetic metal films are aligned with each other, and the joined block is manufactured. Forming step, forming a groove on the tape sliding surface of the joining block in parallel with the ferromagnetic metal film up to at least the gap depth, filling the groove on the sliding surface with a non-magnetic filling material, extra non-magnetic A method of manufacturing a magnetic head, comprising a step of removing a filler and a step of cutting a joining block into a predetermined core width to form a magnetic head.