JPH07220221A - Magnetic head - Google Patents

Abstract

PURPOSE:To improve the apparatus wear resistant characteristics of the magnetic metallic layers of magnetic cores and to lessen the unequal wear by providing a magnetic head with fine powder supply parts disposed with a high polymer material. CONSTITUTION:Sliding surfaces 16 with a magnetic recording medium M of a least either of magnetic core half bodies 3A, 3b constituting the magnetic cores are provided with the high polymer supply parts 9 formed of the high polymer material. As a result, the fine powder of the high polymer material is generated on the sliding surfaces where the magnetic cores 3 and the magnetic recording medium M, such as magnetic recording tape, face each other when the magnetic recording medium M travels. This fine powder is included into relative sliding surfaces of the magnetic recording medium M and the magnetic core half bodies 3A, 3B and forms extremely thin films on the surfaces of the magnetic metallic layers 6, 7 of the magnetic core half bodies 3A, 3B facing these sliding surface. These films function as protective films against wear of the magnetic metallic layers 6, 7 facing the sliding surfaces, thereby improving the apparent wear resistant characteristics of the magnetic metallic layers and lessening the unequal wear.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention, for example, in a video tape recorder (VTR) or a digital audio tape recorder (R-DAT), magnetically records audio signals and video signals on recording tracks of a magnetic recording medium such as a magnetic tape. The present invention relates to a magnetic head used for high density recording as a signal.

[0002]

2. Description of the Related Art Conventionally, magnetic recording and / or recording of VTR or R-DAT using a magnetic recording tape as a magnetic recording medium.
Alternatively, the reproducing device reads the information signal recorded on the magnetic recording tape or records the information signal on the magnetic recording tape, so that the magnetic flux generated in the magnetic gap by the coil of the magnetic head causes the magnetic recording tape to be recorded on the signal recording surface. And a magnetic head for reading and reproducing the information signal and writing and recording the information signal on the signal recording surface of the magnetic recording tape.

The magnetic head is generally composed of a magnetic core and a member such as a coil wound around the magnetic core. The magnetic core has a magnetic gap, which is a minute gap. . The coil serves to transfer an information signal current for recording or reproduction as a magnetic flux to the magnetic core. The magnetic core serves as a passage for transmitting a magnetic flux from the coil to the magnetic recording medium during recording and from the magnetic recording medium to the coil during reproducing, to the magnetic recording tape. The magnetic gap serves to narrow the range of the spread of the magnetic field in order to record an information signal on the magnetic recording medium and to serve as an inlet for the magnetic flux from the magnetic recording medium during reproduction.

By the way, in recent years, a method of recording or reproducing a short wavelength of a video information signal or the like has been proposed in order to record or reproduce a larger number of information signals for the purpose of improving the image quality of recording or reproduction of a video signal or the like. Has been. Therefore, a high coercive force such as a metal tape in which a ferromagnetic metal powder is applied as a magnetic powder for a magnetic layer of a magnetic recording tape or a vapor deposition tape in which a ferromagnetic metal material is directly evaporated on the base film is used. Magnetic recording media are used.

In order to enable recording or reproduction on these high coercive force magnetic recording media, a metal magnetic layer having a high magnetic permeability and a high saturation magnetic flux density, for example, a Fe magnetic layer, is used as a magnetic core material of a magnetic head. A laminated magnetic head or an MIG magnetic head using an alloy, an Fe-Ni alloy, or the like is used. A laminated magnetic head is a magnetic head configured by sandwiching a metallic magnetic layer between substrates made of a non-magnetic material to form a magnetic core half body, and abutting the magnetic core half bodies together and joining and integrating them by glass fusion. is there. The MIG type magnetic head is a magnetic head using a ferrite material as a magnetic core material, and is a magnetic head having a metal-based magnetic layer disposed near the magnetic gap.

[0006]

By the way, in the above-mentioned laminated magnetic head or MIG magnetic head using a metal magnetic material as a magnetic core material for high density magnetic recording, the wear resistance of the metal magnetic layer is reduced. Since the characteristics are inferior to those of the other members constituting the magnetic core, that is, the guard material and the ferrite material, the portion of the metal-based magnetic layer of the magnetic core is worn earlier than the other portions, resulting in uneven wear. Therefore, in these laminated magnetic heads or MIG type magnetic heads, a gap is generated between the magnetic gap and the magnetic recording medium due to this uneven wear, so that the magnetic field strength of the magnetic gap with respect to the magnetic recording medium becomes weak, especially in the high frequency range. However, there was a problem that the electromagnetic conversion characteristics at the point of time deteriorated.

In such a high-density magnetic recording type magnetic head, in order to improve the wear resistance of the metal magnetic layer portion constituting the magnetic core, a material having wear resistance is used for the metal magnetic layer. Contamination is considered. However, in the magnetic core configured as described above, the content rate of the metal-based magnetic layer is reduced, and the magnetic characteristics are degraded. When a material having wear resistance is not mixed in order to maintain the magnetic characteristics of the metal magnetic layer, uneven wear occurs in the metal magnetic layer portion because the wear resistance is small as described above. There is such a problem.
That is, in the metal-based magnetic layer forming the magnetic core, an optimum metal-based magnetic layer satisfying both the wear resistance property and the magnetic property has not been obtained.

Therefore, according to the present invention, in a magnetic head using a metal-based magnetic layer for high density magnetic recording, the apparent wear resistance of the metal-based magnetic layer of the magnetic core is improved to reduce uneven wear. And, it is proposed for the purpose of providing a magnetic head capable of maintaining the magnetic characteristics.

[0009]

In order to achieve the above-mentioned object, a magnetic head according to the present invention comprises a pair of magnetic layers made of a metallic magnetic material facing a sliding surface with a magnetic recording medium. A magnetic core formed by combining magnetic core halves,
A coil wound around this magnetic core is provided, and at least one of the magnetic core halves is made of a polymer material and is provided with a polymer fine powder on the sliding surface between the magnetic core and the magnetic recording medium. It is characterized in that a fine powder supply unit for supplying is formed.

Further, the magnetic head according to the present invention is characterized in that the fine powder supply section is arranged in the magnetic core half on the entry side of the magnetic recording medium, or the fine powder supply section is a magnetic core. Is arranged along both widthwise edges of the sliding surface with respect to the magnetic recording medium.

Further, the polymer material forming the fine powder supply part is polyvinyl chloride, polyvinyl acetate, polyvinyl alcohol, polyvinylidene chloride, polyacrylonitrile, polybutadiene, polyvinyl butyral, nitrocellulose, acetyl cellulose, epoxy resin, It is characterized in that it is any one kind selected from a phenoxy resin and a urethane resin, or a copolymer of two or more kinds thereof.

[0012]

In the magnetic head according to the present invention constructed as described above, at least one of the magnetic core halves constituting the magnetic core has a high surface formed of a polymer material on the sliding surface with respect to the magnetic recording medium. By providing the molecule supply unit, when a magnetic recording medium such as a magnetic recording tape is run, fine powder of the above-mentioned polymer material is generated on the sliding surfaces of the magnetic core and the magnetic recording medium facing each other.

The fine powder of the polymer material is caught in the relative sliding surface between the magnetic core half body and the magnetic recording medium, and is extremely adhered to the surface of the metallic magnetic layer of the magnetic core half body facing the sliding surface. Form a thin film. This film functions as a protective film against wear of the metallic magnetic layer facing the sliding surface, improves the apparent wear resistance of the metallic magnetic layer, and reduces uneven wear.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific embodiments of the present invention will be described below with reference to the drawings. The laminated magnetic head 1 shown in FIG. 1 as the first embodiment according to the present invention is a magnetic device formed by a pair of magnetic core halves 3A and 3B forming a closed circuit being butted against each other and joined together. It has a core 3. In addition, the magnetic core half 3 that constitutes the magnetic core 3
A magnetic gap G, which is a minute gap, is formed between the opposite side surfaces of A and 3B. One magnetic core half 3A
In the side surface portion facing the magnetic core half body 3B, substantially U-shaped winding windows 8 are formed which are opened on both side surfaces in the width direction and through which a coil wire (not shown) is wound.

The magnetic core half body 3A is formed by joining the non-magnetic guard materials 11 and 12 with the metallic magnetic layer 6 sandwiched therebetween. Similarly, the magnetic core half body 3B is formed of the non-magnetic guard material 1.
3 and 14 are formed by joining with the metal-based magnetic layer 7 sandwiched therebetween. These non-magnetic guard materials 11, 12, 13, 1
No. 4 may be a material that is non-magnetic, has an appropriate expansion coefficient, and has abrasion resistance to the magnetic recording medium. As the non-magnetic guard material, generally, non-magnetic ferrite, zirconium oxide-based ceramics, crystallized glass, non-magnetic iron oxide-based ceramics, titanate-based ceramics, or the like, or a bonding material thereof is used.

The material forming the metallic magnetic layers 6 and 7 is F
e-based alloy, Co-based alloy, Fe-Ni-based alloy, Fe-C-based alloy, Fe-Al-Si-based alloy, Fe-Ga-Si-based alloy, Fe-Al-Ge-based alloy, Fe-Ga-Ge-based alloy Alloy, Fe-Si-Ge based alloy, Fe-Co-Si based alloy, Fe-Ru-Ga-Si based alloy, Fe-Co-Si
-Al-based alloy or other crystalline alloy material or Co-Zr
There are amorphous alloys such as -Nb and Co-Zr-Nb-Ta.

Further, a commonly used amorphous alloy or the like can also be used. For example, 1 out of Fe, Ni, Co
Alloy consisting of at least one element and at least one element of P, C, B and Si, or Al, B containing this as a main component
e, Sn, In, Mo, W, Ti, Mu, Cr, Zr,
Amorphous metal-metalloid alloys such as alloys containing Hf, Nb, etc., or alloys containing transition elements such as Co-Zr, Co-Hf as the main component, or alloys containing rare earth elements added to these alloys. Alloys can also be used.

The metal-based magnetic layers 6 and 7 are not limited to a single layer in order to increase output and avoid eddy current loss in a high frequency band. It is also possible to use a multilayer structure in which the layers are laminated. As a method for forming the metal-based magnetic layer and the insulating layer, in addition to the above-described sputtering method, a vacuum thin film forming technique typified by a vacuum vapor deposition method, an ion plating method, a cluster ion beam method or the like is used. .

The magnetic recording medium M is arranged in one of the magnetic core halves 3A constituting the magnetic core 3 arranged on the traveling direction of the magnetic recording medium M, that is, on the traveling side from the arrow A to the arrow B in FIG. A plurality of fine powder supplying portions 9 are formed in the width direction of the magnetic core 3 so as to face the sliding surface 16 on which M slides. The fine powder supply unit 9 is formed by filling a polymer material by the manufacturing method described later.

The polymeric material forming the fine powder supply section 9 is
For example, selected from polymer resin materials such as polyvinyl chloride, polyvinyl acetate, polyvinyl alcohol, polyvinylidene chloride, polyacrylonitrile, polybutadiene, polyvinyl butyral, nitrocellulose, acetyl cellulose, epoxy resin, phenoxy resin, or urethane resin. Any one kind or a copolymer of two or more kinds of these polymer resin materials is used.

In the magnetic head 1 constructed as described above, when the magnetic recording medium M slides on the sliding surface 16 of the magnetic core 3, the sliding surface 16 gradually wears and the fine powder supplying portion 9 A fine powder 30 is generated. The fine powder 30 of the polymer material is wound around the sliding surface 16 facing the magnetic core 3 and the magnetic recording medium M, and the metal magnetic layers 6, 7 are formed.
A very thin film is formed on the surface of. This film is the sliding surface 1
It functions as a protective film against wear of the metal-based magnetic layers 6 and 7 facing 6, and improves the apparent wear resistance of these metal-based magnetic layers 6 and 7 and reduces uneven wear.

Next, the laminated magnetic head 1 of the above-described embodiment
The manufacturing method will be described with reference to the process drawings shown in FIGS. There are two types of magnetic heads, a double window type in which a coil winding window is provided on both of the magnetic core halves, and a single window type in which a coil winding window is provided only in one side. The magnetic head 1 employs a single window type which is relatively easy to manufacture.

EXAMPLE The manufacture of the laminated magnetic head 1 is as follows.
The process starts with the film forming process shown in FIG. This film-forming step is performed in the strip-shaped non-magnetic guard material substrate 2
This is a step of forming a metal-based magnetic layer 6 having a predetermined thickness on one side of No. 0. That is, on one surface of the non-magnetic guard substrate 20, a sputtering method, a vacuum deposition method, an ion plating method,
A film-forming substrate 21 is manufactured by forming a film of the above-mentioned metal-based magnetic material or ferromagnetic metal material into a predetermined thickness by a vacuum thin film forming technique typified by a cluster ion beam method or the like.

Next, the joining step shown in FIG. 3, which is the second step, is performed. This bonding step is a step of bonding a plurality of film formation substrates 21 manufactured in the above-described film formation step.
That is, the film formation substrates 21 are bonded to each other by, for example, low temperature thermal diffusion bonding in which noble metals provided on respective bonding surfaces are bonded by thermal diffusion, or a bonding method such as glass fusion or thermocompression bonding. 22 is produced.

Further, the cutting step shown in FIG. 4, which is the third step, is performed. This cutting step is a step of individually cutting the bonded substrates 22 manufactured in the above-described bonding step. That is, the joint substrate 22 is individually cut in a direction forming a predetermined angle X with respect to the joint surface of the joint substrate 22,
The cut individual magnetic core half blocks 23 are produced. The cutting step of the embodiment shown in FIG. 4 shows a case where the angle X with respect to the joint surface of the joint substrate 22 is 90 degrees.

Then, the cutting step shown in FIG. 5 which is the fourth step is performed. This cutting step is a step of forming the winding groove 8 around which the coil (not shown) is wound in the magnetic core half block 24 manufactured in the above cutting step. That is, the side surfaces of the magnetic core half block 24 facing each other with respect to the one magnetic core half 23 shown in FIG. The magnetic core half block 24 in which the winding groove 8 around which the unwound coil is wound is provided is produced.

Next, the butting step shown in FIG. 6 which is the fifth step is performed. This butting process is performed by the magnetic core half block 24 manufactured in the above-described cutting process and the magnetic core half block 23 manufactured in the third process.
This is the process of matching and. That is, the joining surfaces of the magnetic core half block 23 and the magnetic core half block 24 that are butted against each other are mirror-finished. After that,
These magnetic core half blocks 23 and 24 are butted against each other by using the respective metal-based magnetic layers 6 and 7 as a position reference and joined by a joining method such as the above-mentioned low temperature thermal diffusion joining, glass fusion or thermocompression bonding. The magnetic core block 25 is manufactured by being integrated. Then, a magnetic gap G is formed as a minute gap between the metal-based magnetic layers 6 and 7 abutted by the magnetic core half blocks 23 and 24.

Further, a shaving step shown in FIG. 7 which is a sixth step is performed. This grinding step is a step of forming the fine powder supply portion 9 of the polymer material on the magnetic core block 25 manufactured in the above-mentioned butting step. That is,
The magnetic core block 25 is arranged on the sliding surface of the magnetic core half block 24, which is the approaching side of the magnetic recording medium, on which the magnetic recording medium rubs, in a direction parallel to the abutting surfaces of the magnetic core half blocks 23, 24. A plurality of fine powder supply parts 9 are engraved respectively. Then, the polymer material is disposed by being filled in these fine powder supply parts 9 and curing.

Next, the grinding step shown in FIG. 8 which is the seventh step is performed. This grinding step is a step of forming a cylindrical sliding surface on the magnetic core block 25 manufactured in the sixth step described above. That is, the magnetic core block 25
In order to secure a sliding surface on which the magnetic recording medium of FIG.

Then, the cutting step shown in FIG. 9, which is the eighth step, is performed. This cutting step is performed by the magnetic core block 25.
Is a step of manufacturing each of the laminated magnetic heads 1 of the respective examples by cutting. That is, the magnetic core block 25
On the sliding surface on which the magnetic recording medium rubs, between the layers of the non-magnetic guard material substrate 20, the magnetic core blocks 23, 2
Width-regulating grooves 10 are engraved in the direction orthogonal to the abutting surface of No. 4, respectively. Then, the laminated magnetic heads 1 of the embodiment shown in FIG. 1 are manufactured by being cut along the center of the width regulating groove 10.

The laminated magnetic head 2 shown in FIG. 10 as the second embodiment of the present invention is similar to the example of FIG.
The magnetic core 4 is formed by a pair of magnetic core halves 4A and 4B forming a closed circuit being abutted against each other and joined together. A magnetic gap G, which is a minute gap, is formed between the facing side surfaces of the magnetic core halves 4A and 4B that form the magnetic core 4. One side of the magnetic core half 4A facing the magnetic core half 4B has a substantially U-shaped winding window that is opened on both side surfaces in the width direction and has a coil wire (not shown) wound therein. 8 is recessed.

The magnetic core half body 4A is formed by joining the non-magnetic guard materials 11 and 12 with the metal magnetic layer 6 sandwiched therebetween. Similarly, the magnetic core half body 4B is formed of the non-magnetic guard material 1.
3 and 14 are formed by joining with the metal-based magnetic layer 7 sandwiched therebetween. These non-magnetic guard materials 11, 12, 13, 1
No. 4 may be a non-magnetic material having an appropriate coefficient of thermal expansion and having abrasion resistance to the magnetic recording medium. As the non-magnetic guard material, non-magnetic ferrite, zirconium oxide-based ceramics, crystallized glass, non-magnetic iron oxide-based ceramics, titanate-based ceramics, or the like, or a bonding material thereof is generally used.

Further, in this magnetic head 2, the fine powder supply section 9 is arranged in the fine powder supply section 9 provided along both edges in the width direction of the sliding surface of the magnetic core with respect to the magnetic recording medium. ing. These fine powder supply parts 9 are constituted by the respective fine powder supply parts 9 which are provided so as to be continuous with both widthwise end edges of the sliding surfaces of the magnetic core halves 4A and 4B by the manufacturing method described later. Then, the polymer material is filled in the fine powder supply unit 9 and arranged. Further, this polymer material is selected from the above-mentioned polymer resin materials, but a thermosetting polymer resin material is preferable in the production of the magnetic head.

The polymer material forming the fine powder supply section 9 is polyvinyl chloride, polyvinyl acetate, polyvinyl alcohol, polyvinylidene chloride, polyacrylonitrile, polybutadiene, polyvinyl butyral, nitrocellulose, acetyl cellulose, epoxy resin. Any one selected from polymer resin materials such as phenoxy resin and urethane resin, or a copolymer of two or more of these polymer resin materials is used.

In the magnetic head 2 constructed as described above, when the magnetic recording medium M runs on the sliding surface 16 of the magnetic core 4, wear of the sliding surface 16 causes wear of the magnetic recording medium M and fine powder of the above-mentioned polymer material. 30 occurs. Fine powder of this polymeric material 3
0 is wound on the sliding surface 16 to form an extremely thin film on the surfaces of the metallic magnetic layers 6 and 7. This film functions as a protective film against wear of the metallic magnetic layers 6 and 7 facing the sliding surface 16, improves apparent wear resistance of these metallic magnetic layers 6 and 7, and reduces uneven wear. Let

Next, the laminated magnetic head 2 of the above-described embodiment.
The manufacturing method will be described with reference to the process diagrams shown in FIGS. In the above-mentioned embodiment, since the film forming process shown in FIG. 2 which is the first process to the matching process which is the fifth process shown in FIG. 6 are the same, the description thereof will be omitted.

Therefore, after the magnetic core block 25 is manufactured in the fifth step, the cutting step shown in FIG. 11 which is the sixth step is performed. This grinding step is a step of forming the fine powder supply portion 9 of the polymer material on the magnetic core block 25. That is, the magnetic core block 25 has the magnetic core half block 2 on the sliding surface on which the magnetic recording medium rubs.
The fine powder supply portion 9 is engraved between the respective metal magnetic layers in a direction orthogonal to the abutting surfaces of 3, 24. Then, the polymer material is disposed by being filled in these fine powder supply parts 9 and curing.

Next, a grinding step shown in FIG. 12, which is a seventh step, is performed. This grinding step is a step of forming a cylindrical sliding surface on the magnetic core block 25 manufactured in the sixth step described above. That is, the magnetic core block 25
In order to secure a sliding surface on which the magnetic recording medium of FIG.

Then, a cutting step shown in FIG. 13, which is an eighth step, is performed. This cutting step is performed by the magnetic core block 25.
Is a step of manufacturing each of the example laminated magnetic heads 2 by cutting. That is, the magnetic core block 25
Is engraved inside the groove width of the fine powder supply portion 9 with a width narrower than the width of the fine powder supply portion 9 in the sliding surface. Then, by cutting the center of the regulation groove 10 of the width of the sliding surface along this groove, the polymer material disposed in the fine powder supply portion 9 along both edges of the sliding surface in the width direction is removed. As a result, the laminated magnetic head 2 of the embodiment shown in FIG. 10 is manufactured.

[0040]

As described above, according to the magnetic head of the present invention, by providing the magnetic head with the fine powder supplying section in which the polymer material is arranged, the high level of the information signal can be achieved on the magnetic recording medium such as the magnetic recording tape. In a magnetic head using a metal-based magnetic layer for density magnetic recording, it is possible to reduce uneven wear and maintain magnetic characteristics by improving the apparent wear resistance of the metal-based magnetic layer of the magnetic core. The magnetic head can be provided. As a result, the life of the magnetic head is extended, and the recording or reproduction of the information signal of the high quality image or the high quality sound in the magnetic recording and / or reproducing apparatus is stabilized.

[Brief description of drawings]

FIG. 1 is a perspective view showing an embodiment of a magnetic head according to the present invention.

FIG. 2 is a conceptual diagram showing a step of forming a metal-based magnetic layer on a non-magnetic guard material substrate.

FIG. 3 is a conceptual diagram showing a step of joining film-forming substrates.

FIG. 4 is a schematic view showing a step of cutting a bonded substrate.

FIG. 5 is a conceptual diagram showing a process of forming a winding groove in a magnetic core half block.

FIG. 6 is a conceptual diagram showing a step of butting magnetic core half blocks to each other.

FIG. 7: Engraving the fine powder supply part on the magnetic core block,
It is a conceptual diagram which shows the process of arrange | positioning a polymeric material.

FIG. 8 is a conceptual diagram showing a step of cylindrically grinding a magnetic core block.

FIG. 9 is a conceptual diagram showing a step of engraving a width regulating groove of a sliding surface on a magnetic core block and cutting the magnetic head into individual magnetic heads.

FIG. 10 is a perspective view showing an embodiment of a magnetic head according to the present invention.

FIG. 11 is a conceptual diagram showing a process of engraving a fine powder supply unit on a magnetic core block and disposing a polymer material.

FIG. 12 is a conceptual diagram showing a step of cylindrically grinding a magnetic core block.

FIG. 13 is a conceptual diagram showing a step of engraving a width regulation groove of a sliding surface on a magnetic core block and cutting it for each magnetic head.

[Explanation of symbols]

 1 Laminated Magnetic Head 2 Laminated Magnetic Head 3, 4 Magnetic Cores 3A, 3B, 4A, 4B Magnetic Core Half 6,7 Metallic Magnetic Layer 8 Winding Window 9 Fine Powder Supply Section 10 Regulation of Width of Sliding Surface Grooves 11, 12, 13, 14 Non-magnetic guard material 16 Sliding surface 20 Non-magnetic guard material substrate 21 Film-forming substrate 22 Bonding substrate 23, 24 Magnetic core half block 25 Magnetic core block 30 Fine powder G Magnetic gap M Magnetic recording Medium

Claims (4)

[Claims] 1. A magnetic core formed by combining a pair of magnetic core halves each having a magnetic layer made of a metallic magnetic material formed facing a sliding surface with a magnetic recording medium, and a magnetic core wound around the magnetic core. At least one of the magnetic core halves is provided with a fine powder supply unit that supplies the polymer fine powder to the opposing sliding surfaces of the magnetic core and the magnetic recording medium. A magnetic head characterized by being formed. 2. The magnetic head according to claim 1, wherein the fine powder supply section is arranged in the magnetic core half on the entry side of the magnetic recording medium. 3. The magnetic head according to claim 1, wherein the fine powder supply portion is arranged along both widthwise edges of the sliding surface of the magnetic core with respect to the magnetic recording medium. 4. The polymer material forming the fine powder supply part is polyvinyl chloride, polyvinyl acetate, polyvinyl alcohol, polyvinylidene chloride, polyacrylonitrile,
2. Any one selected from polybutadiene, polyvinyl butyral, nitrocellulose, acetyl cellulose, epoxy resin, phenoxy resin, urethane resin, or a copolymer of two or more thereof. Magnetic head.