JPH0668424A - Thin film magnetic head - Google Patents

Abstract

PURPOSE:To obtain a thin film magnetic head in which the influence of a false gap due to the peeling of a core or deviation in the stacking of the core and resolution can be prevented from being lowered and high performance and high reliability can be provided. CONSTITUTION:This magnetic head is to improve the thin film magnetic head whose lover core 4, upper core 9, and intermediate cores 8a, 8b which connect them are comprised of magnetic materials formed in insulating layers 3, 5, and 11, and in which the surface of the insulating layer including the connecting plane of each core is formed almost in flat shape, and a gap 7 is formed at the connecting part of the lower core 4 and the intermediate core 8a. Only the lower core 4 and the intermediate core 8a holding the gap between out of the cores are faced with a medium confronting plane 6, and the upper core 9 other than the cores holding the gap 7 between is separated from the medium confronting plane 6. Thereby, it is possible to evade a state where the joining plane of the upper core 9 holding the gap 7 between is confronted with the joining plane of the lover core 4 and the intermediate core 8a other than the cores holding the gap between on the medium confronting plane and to prevent the peeling of the core or the false gap from occurring.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film magnetic head, and more particularly to a thin film magnetic head suitable for high density magnetic recording.

[0002]

2. Description of the Related Art First, the structure of a conventional thin film magnetic head 20 will be described with reference to FIG. A magnetic film is formed on the substrate 21, and the lower core 22 is formed by photolithography or etching. The non-magnetic material 24 is formed on the lower core 22 so that the end portion becomes the (magnetic) gap 23. Next, the insulating layer 25 and the conductor layer are formed, and the coil pattern 26 is formed by using photolithography, etching, or the like. An insulating layer 27 and a magnetic layer are formed on the stepped coil forming surface on which the coil pattern 26 is formed to form an upper core 28.

In the conventional thin film magnetic head 20, an insulating layer 27 is formed on a coil pattern 26 having a step with the insulating layer 25, and the upper core 2 is formed on the insulating layer 27.
Since 8 is formed, the step becomes larger as the layers are stacked. For example, the usual thickness of both cores is about 5 μm,
When the coil pattern has a thickness of about 3 μm, the step difference reaches 10 μm immediately before the upper core is formed.

On a surface having such a step, the resolution due to photolithography becomes extremely poor, and the resolution is limited to the size of the step. Therefore, in order to increase the number of turns of the coil, it is not possible to reduce the pitch interval of the coil pattern 26 because the resolution is poor. As a result, it is necessary to increase the lengths of the upper and lower cores 22 and 28 formed above and below the magnetic core, and the magnetic resistance increases due to the increase in the magnetic path length. there were.

As a solution to these problems,
There is a thin film magnetic head described in Japanese Patent Application Laid-Open No. 3-58308 filed earlier by the present applicant. As shown in FIGS. 7A and 7B, this is done by forming a core-shaped groove in the insulating layer by etching, filling the groove with a magnetic material, flattening the surface, and stacking it to form a magnetic layer. It is a thin film magnetic head 30 on which a circuit is formed. 31, 32a, 32b and 33 are cores made of a magnetic material, 35 is a coil pattern, 36, 37 and 3
Reference numeral 8 is an insulating layer, and 39 is a magnetic gap.

[0006]

SUMMARY OF THE INVENTION The thin film magnetic head 3 described above.
0 is all of the upper, lower and middle cores 31, 32a, 32b,
Since 33 is exposed so as to face the medium facing surface 34,
Due to the following problems, it is difficult to provide a thin film magnetic head having excellent performance and high reliability.
When the medium facing surface (slider surface) 34 is processed or the system is operated, peeling easily occurs at the interface of the core. In addition, when peeling occurs, of course, if there is a work-affected layer or the like on the joint surface (interface portion), that portion acts as a pseudo gap, resulting in a reduction in resolution. As is clear from the medium facing surface side of the thin film magnetic head 30 (see FIG. 8), if the core layers do not match each other, for example, as shown in FIG. A facing surface is generated between 31 and the upper core 33, and a pseudo gap NG is formed.

[0007]

In order to solve the above-mentioned problems, the present invention comprises a lower core, an upper core, and an intermediate core connecting them, which are made of a magnetic material in an insulating layer. In a thin film magnetic head in which the surface of each of the insulating layers including is substantially flat and a gap is formed in the connecting portion of the core, only the core sandwiching the gap is exposed to the medium facing surface. The present invention provides a thin film magnetic head in which cores other than the core sandwiching the core are separated from the medium facing surface.

In the thin-film magnetic head configured as described above, the bonding surface of the core that sandwiches the gap and the bonding surface of the core other than the core that sandwiches the gap do not face each other at the medium facing surface, and peeling or a pseudo gap occurs. Does not happen.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the thin film magnetic head according to the present invention will be described in detail below with reference to the drawings. This thin-film magnetic head is to form a core-shaped groove in an insulating layer by etching, fill the groove with a magnetic material to planarize the surface, and stack it to form a magnetic circuit. Of the (layers), only two layers sandwiching the (magnetic) gap (layer) are exposed to the medium facing surface, and the cores other than the two cores sandwiching the gap layer are recessed from the medium facing surface. is there.

[Embodiment 1] FIG. 1 is a schematic sectional view showing a thin film magnetic head 1 according to the present invention. As shown in the figure, a flat lower insulating layer 3 is formed on the substrate 2,
The groove formed in the lower insulating layer 3 is filled with a magnetic material,
A lower core 4 is formed which is flat and has no step with the lower insulating layer.

An intermediate insulating layer 5 is formed on the lower insulating layer 3, and is made of a magnetic material at an end (the recording medium facing surface 6) of the intermediate insulating layer 5 via a (magnetic) gap 7. An intermediate core 8a is embedded so as to be close to the lower core 4, and an intermediate core 8b made of a magnetic material is embedded so as to be directly bonded to the lower core 12 inside the intermediate core 8a at a distance from the intermediate core 8a. Inside the intermediate insulating layer 5, a planar coil pattern 10 is embedded in a spiral shape so as to surround the intermediate core 8b. One end of the coil pattern 10 is joined to an external lead wire 13 via a conductor 12 buried in a through hole formed in the upper insulating layer 11 and can be electrically connected to an external device. ing.

An upper insulating layer 11 is formed on the intermediate insulating layer 5, and an upper core 9 is formed on the upper insulating layer 11 so that both ends thereof are joined to the intermediate cores 8a and 8b. A magnetic circuit is formed together with the lower core 12. Further, among the plurality of cores 4, 8a, 8b, 12 only the lower core 4 and the intermediate core 8a having the gap 7 sandwiched therebetween are projected (exposed) to the medium facing surface 6 and the upper core 9 other than the core having the gap 7 sandwiched therebetween. Is retracted from the medium facing surface 6 (l is a distance from the recording medium facing surface).

As described above, in the thin-film magnetic head 1 according to the present invention, three flat insulating layers, that is, the lower insulating layer 3, the intermediate insulating layer 5, and the upper insulating layer 11 are stacked, and the insulating layers in these insulating layers are stacked. Since the magnetic layer formed at a predetermined position is connected to form a magnetic circuit, photolithography can be performed on each insulating layer surface without a step. Therefore, since a small coil pattern / magnetic core having excellent dimensional accuracy can be obtained, it is possible to obtain a thin-film magnetic head having low magnetic resistance and good performance.

Further, the joint surface of the upper core 9 other than the core sandwiching the gap 7 does not appear in the medium facing surface 6. Therefore, peeling does not occur unlike the conventional thin film magnetic head. In addition, as shown in FIGS. 2A and 2B, the joint surface of the lower core 4 and the intermediate core 8 a that sandwich the gap 7 and the joint surface of the upper core 9 other than the core that sandwich the gap 7 face the medium. Since the surfaces 6 do not face each other and the influence of the pseudo gap due to the misalignment of the cores is eliminated, the resolution does not decrease.

[Embodiment 2] FIG. 3 shows a thin film magnetic head 14 in which a (magnetic) gap 7 is set between an upper core 9 and an intermediate core 8a. The thin film magnetic head 14 has a structure in which the upper core 4 and the intermediate core 8a are projected (exposed) on the medium facing surface 6, and the lower cores 4 other than the core sandwiching the gap 7 are retracted from the medium facing surface 6. There is.

[Embodiment 3] FIG. 4 shows two layers of coil patterns 10-1, 10-2 and two layers of lower intermediate cores 8a-1, 8b-.
1. The thin film magnetic head 15 is composed of the upper intermediate cores 8a-2 and 8b-2, and the (magnetic) gap 7 is set between the lower core 4 and the lower intermediate core 8a-1. This thin film magnetic head 15
Is a structure in which the lower core 4 and the lower intermediate core 8a-1 are projected (exposed) from the medium facing surface 6, and the upper core 9 and the upper intermediate core 8a-2 other than the cores sandwiching the gap 7 are the medium facing surface 6 It has a structure retracted from.

[Embodiment 4] FIG. 5 shows two layers of coil patterns 10-1, 10-2 and two layers of lower intermediate cores 8a-1, 8b-.
1. The thin film magnetic head 16 is composed of the upper intermediate cores 8a-2 and 8b-2, and the (magnetic) gap 7 is set between the lower intermediate core 8a-1 and the upper intermediate core 8a-2. The thin film magnetic head 16 has a structure in which the lower intermediate core 8a-1 and the upper intermediate core 8a-2 are projected (exposed) from the medium facing surface 6, and the upper core 9 and the lower core 4 other than the core sandwiching the gap 7 are provided. Has a structure retracted from the medium facing surface 6.

[Embodiment 5] FIG. 6 shows two-layer coil patterns 10-1, 10-2 and two-layer lower intermediate cores 8a-1, 8b-.
1. The thin film magnetic head 17 is composed of the upper intermediate cores 8a-2 and 8b-2, and the (magnetic) gap 7 is set between the upper core 9 and the upper intermediate core 8a-2. The thin film magnetic head 17 has a structure in which the upper core 9 and the upper intermediate core 8a-2 are projected (exposed) on the medium facing surface 6, and the lower core 4 and the lower intermediate core 8a-1 other than the core sandwiching the gap 7 are provided. Has a structure retracted from the medium facing surface 6.

The thin film magnetic head 14 shown in Examples 2-5.
Also in # 17, since flat insulating layers are stacked and magnetic layers formed at predetermined locations in these insulating layers are connected to form a magnetic circuit, photolithography is possible on each insulating layer surface without steps. Becomes Therefore, since a small coil pattern / magnetic core having excellent dimensional accuracy can be obtained, it is possible to obtain a thin-film magnetic head having low magnetic resistance and good performance.

Further, the joint surface of the core other than the core with the gap sandwiched does not appear in the medium facing surface. Therefore, unlike the conventional thin film magnetic head, the influence of the pseudo gap due to the peeling or the misalignment of the core is eliminated, so that the resolution is not lowered.

[0021]

In the thin-film magnetic head according to the present invention, the lower core, the upper core, and the intermediate core connecting them are made of a magnetic material in the insulating layer, and the insulating layer including the connecting surface of each core is formed. In a thin film magnetic head having a substantially flat surface and forming a gap at the connecting portion of the core, only the core sandwiching the gap among the cores faces the medium facing surface, and the cores other than the core sandwiching the gap are formed. Since it is separated from the medium facing surface, the bonding surface of the core that sandwiches the gap and the bonding surface of the core other than the core that sandwiches the gap do not face each other at the medium facing surface, and peeling or a pseudo gap may occur. Absent. Therefore, unlike the conventional thin film magnetic head, the influence of the pseudo gap due to the peeling of the core and the misalignment of the core is eliminated, the resolution is not deteriorated, and the thin film magnetic head having high performance and high reliability is provided.

[Brief description of drawings]

FIG. 1 is a schematic sectional view showing an embodiment of a thin film magnetic head according to the present invention.

FIG. 2 is a diagram of the thin-film magnetic head as viewed from the medium facing surface, and is a diagram illustrating a case where misalignment of the pattern of the core occurs.

FIG. 3 is an example of a thin film magnetic head in which a magnetic gap is set between an upper core and an intermediate core.

FIG. 4 is an example of a thin film magnetic head in which a coil (and an intermediate core) has two layers and a magnetic gap is set between the lower core and the lower intermediate core.

FIG. 5 is an example of a thin film magnetic head in which a coil (and an intermediate core) has two layers and a magnetic gap is set between an upper intermediate core and a lower intermediate core.

FIG. 6 is an example of a thin film magnetic head in which a coil (and an intermediate core) has two layers and a magnetic gap is set between the upper core and the upper intermediate core.

FIG. 7 is a schematic sectional view and a plan view showing a conventional thin film magnetic head.

FIG. 8 is a diagram of a conventional thin-film magnetic head as viewed from the medium facing surface, and is a diagram for explaining a case where a core pattern misalignment occurs.

FIG. 9 is a schematic sectional view showing a conventional thin film magnetic head.

[Explanation of symbols]

 1 thin film magnetic head 2 substrate 3 lower insulating layer 4 lower core 5 intermediate insulating layer 6 recording medium facing surface 7 (magnetic) gap 8 intermediate core 8a-1, 8b-1 lower intermediate core 8a-2, 8b-2 upper intermediate core 9 upper core 10 coil pattern 10-1 and 10-2 coil pattern 12 lower core 14 to 17 thin film magnetic head l separation distance from recording medium facing surface

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[Procedure amendment]

[Submission date] August 3, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be amended] Claims

[Correction method] Change

[Correction content]

[Claims]

[Procedure Amendment 2]

[Document name to be amended] Statement

[Name of item to be amended] Detailed explanation of the invention

[Correction method] Change

[Correction content]

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film magnetic head, and more particularly to a thin film magnetic head suitable for high density magnetic recording.

[0002]

2. Description of the Related Art First, the structure of a conventional thin film magnetic head 20 will be described with reference to FIG. A magnetic film is formed on the substrate 21, and the lower core 22 is formed by photolithography or etching. The non-magnetic material 24 is formed on the lower core 22 so that the end portion becomes the (magnetic) gap 23. Next, the insulating layer 25 and the conductor layer are formed, and the coil pattern 26 is formed by using photolithography, etching, or the like. An insulating layer 27 and a magnetic layer are formed on the stepped coil forming surface on which the coil pattern 26 is formed to form an upper core 28.

In the conventional thin film magnetic head 20, an insulating layer 27 is formed on a coil pattern 26 having a step with the insulating layer 25, and the upper core 2 is formed on the insulating layer 27.
Since 8 is formed, the step becomes larger as the layers are stacked. For example, the usual thickness of both cores is about 5 μm,
When the coil pattern has a thickness of about 3 μm, the level difference reaches 10 μm immediately before forming the upper core.

On a surface having such a step, the resolution due to photolithography becomes extremely poor, and the resolution is limited to the size of the step. Therefore, in order to increase the number of turns of the coil, it is not possible to reduce the pitch interval of the coil pattern 26 because the resolution is poor. As a result, it is necessary to increase the lengths of the upper and lower cores 22 and 28 formed above and below the magnetic core, and the magnetic resistance increases due to the increase in the magnetic path length. there were.

As a solution to these problems,
There is a thin film magnetic head described in Japanese Patent Application Laid-Open No. 3-58308 filed earlier by the present applicant. As shown in FIGS. 7A and 7B, this is done by forming a core-shaped groove in the insulating layer by etching, filling the groove with a magnetic material, flattening the surface, and stacking it to form a magnetic layer. It is a thin film magnetic head 30 on which a circuit is formed. 31, 32a, 32b and 33 are cores made of a magnetic material, 35 is a coil pattern, 36, 37 and 3
Reference numeral 8 is an insulating layer, and 39 is a magnetic gap.

[0006]

SUMMARY OF THE INVENTION The thin film magnetic head 3 described above.
0 is all of the upper, lower and middle cores 31, 32a, 32b,
Since 33 is exposed so as to face the medium facing surface 34,
Due to the following problems, it is difficult to provide a thin film magnetic head having excellent performance and high reliability. When the medium facing surface (slider surface) 34 is processed or the system is operated, peeling easily occurs at the interface of the core. In addition, when peeling occurs, of course, if there is a work-affected layer or the like on the joint surface (interface portion), that portion acts as a pseudo gap, resulting in a reduction in resolution. As is clear from the medium facing surface side of the thin film magnetic head 30 (see FIG. 8), if the core layers do not match each other, for example, as shown in FIG. A facing surface is generated between 31 and the upper core 33, and a pseudo gap NG is formed.

[0007]

In order to solve the above-mentioned problems, the present invention comprises a lower core, an upper core, and an intermediate core connecting them, which are made of a magnetic material in an insulating layer. In a thin-film magnetic head in which the surface of each of the insulating layers including is substantially flat and a gap is formed in the connecting portion of the core, only the core sandwiching the gap is exposed to the medium facing surface, The present invention provides a thin film magnetic head in which cores other than the core sandwiching the core are separated from the medium facing surface.

In the thin-film magnetic head configured as described above, the bonding surface of the core that sandwiches the gap and the bonding surface of the core other than the core that sandwiches the gap do not face each other at the medium facing surface, and peeling or a pseudo gap occurs. Does not happen.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the thin film magnetic head according to the present invention will be described in detail below with reference to the drawings. This thin-film magnetic head is to form a core-shaped groove in an insulating layer by etching, fill the groove with a magnetic material to planarize the surface, and stack it to form a magnetic circuit. Of the (layers), only two layers sandwiching the (magnetic) gap (layer) are exposed to the medium facing surface, and the cores other than the two cores sandwiching the gap layer are recessed from the medium facing surface. is there.

[Embodiment 1] FIG. 1 is a schematic sectional view showing a thin film magnetic head 1 according to the present invention. As shown in the figure, a flat lower insulating layer 3 is formed on the substrate 2,
The groove formed in the lower insulating layer 3 is filled with a magnetic material,
A lower core 4 is formed which is flat and has no step with the lower insulating layer.

An intermediate insulating layer 5 is formed on the lower insulating layer 3, and is made of a magnetic material at an end (the recording medium facing surface 6) of the intermediate insulating layer 5 via a (magnetic) gap 7. An intermediate core 8a is embedded so as to be close to the lower core 4, and an intermediate core 8b made of a magnetic material is embedded so as to be directly bonded to the lower core 12 inside the intermediate core 8a at a distance from the intermediate core 8a. Inside the intermediate insulating layer 5, a planar coil pattern 10 is embedded in a spiral shape so as to surround the intermediate core 8b. One end of the coil pattern 10 is joined to an external lead wire 13 via a conductor 12 buried in a through hole formed in the upper insulating layer 11 and can be electrically connected to an external device. ing.

An upper insulating layer 11 is formed on the intermediate insulating layer 5, and an upper core 9 is formed on the upper insulating layer 11 so that both ends thereof are joined to the intermediate cores 8a and 8b. A magnetic circuit is formed together with the lower core 12. Further, among the plurality of cores 4, 8a, 8b, 12 only the lower core 4 and the intermediate core 8a having the gap 7 sandwiched therebetween are projected (exposed) to the medium facing surface 6 and the upper core 9 other than the core having the gap 7 sandwiched therebetween. Is retracted from the medium facing surface 6 (1 is a distance from the recording medium facing surface).

As described above, in the thin-film magnetic head 1 according to the present invention, three flat insulating layers, that is, the lower insulating layer 3, the intermediate insulating layer 5, and the upper insulating layer 11 are stacked, and the insulating layers in these insulating layers are stacked. Since the magnetic layer formed at a predetermined position is connected to form a magnetic circuit, photolithography can be performed on each insulating layer surface without a step. Therefore, since a small coil pattern / magnetic core having excellent dimensional accuracy can be obtained, it is possible to obtain a thin-film magnetic head having low magnetic resistance and good performance.

Further, the joint surface of the upper core 9 other than the core sandwiching the gap 7 does not appear in the medium facing surface 6. Therefore, peeling does not occur unlike the conventional thin film magnetic head. In addition, as shown in FIGS. 2A and 2B, the joint surface of the lower core 4 and the intermediate core 8 a that sandwich the gap 7 and the joint surface of the upper core 9 other than the core that sandwich the gap 7 face the medium. Since the surfaces 6 do not face each other and the influence of the pseudo gap due to the misalignment of the cores is eliminated, the resolution does not decrease.

[Embodiment 2] FIG. 3 shows a thin film magnetic head 14 in which a (magnetic) gap 7 is set between an upper core 9 and an intermediate core 8a. The thin film magnetic head 14 has a structure in which the upper core 4 and the intermediate core 8a are projected (exposed) on the medium facing surface 6, and the lower cores 4 other than the core sandwiching the gap 7 are retracted from the medium facing surface 6. There is.

[Embodiment 3] FIG. 4 shows two layers of coil patterns 10-1, 10-2, two layers of lower intermediate core 8a-1,
8b-1, upper intermediate cores 8a-2, 8b-2,
The thin film magnetic head 15 has the (magnetic) gap 7 set between the lower core 4 and the lower intermediate core 8a-1. The thin film magnetic head 15 has a structure in which the lower core 4 and the lower intermediate core 8a-1 are projected (exposed) on the medium facing surface 6, and the upper core 9 and the upper intermediate core 8a-2 other than the core sandwiching the gap 7 are provided. Has a structure retracted from the medium facing surface 6.

[Embodiment 4] FIG. 5 shows two layers of coil patterns 10-1, 10-2, two layers of lower intermediate core 8a-1,
8b-1, upper intermediate cores 8a-2, 8b-2,
The thin film magnetic head 16 has a (magnetic) gap 7 set between the lower intermediate core 8a-1 and the upper intermediate core 8a-2. The thin film magnetic head 16 has a structure in which the lower intermediate core 8a-1 and the upper intermediate core 8a-2 are projected (exposed) from the medium facing surface 6, and the upper core 9 and the lower core 4 other than the core sandwiching the gap 7 are provided. Has a structure retracted from the medium facing surface 6.

[Embodiment 5] FIG. 6 shows two layers of coil patterns 10-1, 10-2, two layers of lower intermediate core 8a-1,
8b-1, upper intermediate cores 8a-2, 8b-2,
The thin film magnetic head 17 has a (magnetic) gap 7 set between the upper core 9 and the upper intermediate core 8a-2. The thin-film magnetic head 17 has a structure in which the upper core 9 and the upper intermediate core 8a-2 are projected (exposed) on the medium facing surface 6, and the lower core 4 and the lower intermediate core 8a-1 other than the core with the gap 7 sandwiched therebetween. Has a structure retracted from the medium facing surface 6.

The thin film magnetic head 14 shown in Examples 2-5.
Also in # 17, since flat insulating layers are stacked and magnetic layers formed at predetermined locations in these insulating layers are connected to form a magnetic circuit, photolithography is possible on each insulating layer surface without steps. Becomes Therefore, since a small coil pattern / magnetic core having excellent dimensional accuracy can be obtained, it is possible to obtain a thin-film magnetic head having low magnetic resistance and good performance.

Further, the joint surface of the core other than the core with the gap sandwiched does not appear in the medium facing surface. Therefore, unlike the conventional thin film magnetic head, the influence of the pseudo gap due to the peeling or the misalignment of the core is eliminated, so that the resolution is not lowered.

[0021]

In the thin-film magnetic head according to the present invention, the lower core, the upper core, and the intermediate core connecting them are made of a magnetic material in the insulating layer, and the insulating layer including the connecting surface of each core is formed. In a thin film magnetic head having a substantially flat surface and forming a gap at the connecting portion of the core, only the core sandwiching the gap among the cores faces the medium facing surface, and the cores other than the core sandwiching the gap are formed. Since it is separated from the medium facing surface, the bonding surface of the core that sandwiches the gap and the core bonding surface other than the core that sandwiches the gap do not face each other in the medium facing surface, and no peeling or pseudo gap occurs. . Therefore, unlike the conventional thin film magnetic head, the influence of the pseudo gap due to the peeling of the core and the misalignment of the core is eliminated, the resolution is not deteriorated, and the thin film magnetic head having high performance and high reliability is provided.

Claims (1)

[Claims] 1. A lower core, an upper core, and an intermediate core connecting them are made of a magnetic material in an insulating layer, and a surface of each insulating layer including a connecting surface of each core is substantially flat. In a thin film magnetic head in which a gap is formed at a connecting portion of cores, only the cores sandwiching the gap among the cores face the medium facing surface, and the cores other than the cores sandwiching the gap are separated from the medium facing surface. Characteristic thin film magnetic head.