DE3931374C2 - Thin film magnetic head for writing and / or reading information on or from a magnetic disk - Google Patents

Description

The invention relates to a thin film magnetic head with the Features specified in the preamble of claim 1.

Such one Thin film is known from U.S. Patent 4,219,855.

One of the main factors affecting the recording density determine a magnetic disk, is the design of the top and the lower magnetic pole piece of the magnetic head to which the Magnetic disk facing ends.

The aforementioned US patent is concerned in this context with the result of inaccuracies in production Problem of partial wrap-around which the upper pole piece is shifted from the lower and this partially encompasses, which leads to disturbances in the magnetic field leads the rims. To avoid this problem beats the publication, the upper magnetic pole piece by about 10% to make it narrower than the lower one.

The cited document shows that the effective Track width on the magnetic disc from the width of the top, narrower magnetic pole piece is determined by the dimensions the wider, lower magnetic pole piece, however, relative is independent. It can also be seen that this status the technology deals with pole piece widths down to 25 µm, which has a magnetic track density of around 1000 TPI (tracks per inch) or 40 tracks / mm.

From Japanese Patent Application Laid-Open No. 62-212910 is also a thin film magnetic head is known in which the lower magnetic pole piece beveled at its lateral edges towards the magnetic gap is and therefore a smaller along the magnetic gap Width than the upper magnetic pole piece.

The invention has for its object a magnetic head to be specified for high track densities from around 1800 TPI or 70 Tracks / mm has an improved signal-to-noise ratio.  

The solution to this problem is in the characterizing part of the claim 1 specified. Advantageous developments of the invention are marked in the subclaims.

The drawing represents preferred embodiments of the Invention represents

Fig. 1 is a schematic representation of a magnetic disk apparatus according to an embodiment of the invention,

Fig. 2A and 2B, the design of the contour of the surface facing the magnetic disk side seen from an embodiment of the magnetic pole pieces, as used in the thin film magnetic head,

Fig. 3 shows the design of the contour of the magnetic pole pieces according to another embodiment of the invention,

FIGS. 4A, 4B and 4C, the magnetic field intensity on the given by the extension of the gap axis,

Fig. 5 shows the relationship of the signal intensity and the background intensity to the width difference of upper and lower magnetic pole piece,

Fig. 6 shows the dependence of the signal-to-background ratio of the difference in width of the upper and lower magnet pole piece,

Fig. 7 shows the relationship between the reciprocal of the track density and the limit value for the width difference of upper and lower magnetic pole of the thin film magnetic head,

Fig. 8 shows a further embodiment of the design of the contour of the magnetic pole pieces according to the invention,

Fig. 9 is a front view showing a common configuration of the contour of the magnetic pole of a thin film magnetic head,

Fig. 10 is a partial illustration of the position of the track on the magnetic disk and the thin film magnetic head to each other,

Fig. 11 is a perspective view of the construction of a thin film magnetic head.

Fig. 9 shows, by way of example, in enlarged form on the free surface facing the recording medium, contours of magnetic pole pieces of a thin-film magnetic head, as used in a magnetic disk device, and a magnetic gap between the magnetic pole pieces.

An upper magnetic pole piece 21 has an edge with the length C _WU (μm) along a magnetic gap 22 . A lower magnetic pole piece 23 has an edge along the magnetic gap 22 with the length C _WD (μm). The terms "top" and "bottom" are used as seen when the magnetic head is held with its base plate down.

On the condition that the track density (Tr) of the magnetic disk is 1800 TPI or more, the difference (ΔC _W ) between C _WU and C _WD satisfies the following relationship:

0 <ΔC _W ≦ 5000 / Tr (µm).

In addition, the larger of the two values C _WU and C _{WD must be} smaller than the track spacing of the magnetic disk and larger than the track width.

Assuming that C _{WU is} the smaller value, the track pitch and track width of the magnetic disk are shown in Fig. 10.

The track width is provided with the reference symbol 50 and stands for the width over which information is recorded on the magnetic disk and which is determined by the smaller of the two lengths C _WU and C _WD . In Fig. 10, 56 denotes a center line of a strip between two adjacent tracks, which is called a security strip. The two parts labeled 55 in FIG. 10 are each halves of the security strip. The track spacing of the magnetic disk is denoted by 51 and stands for a width which is composed of the track width 50 and the width of the security strip which is provided as a protective zone in order to prevent information from overlapping on adjacent tracks.

The upper magnetic pole piece is designated in FIG. 10 with 52 , the lower one with 53 and the magnetic gap with 54 .

Under the above condition for the dimensions the signal-to-background ratio turns out to be high, and the secondary track properties are improved.

The contours of the magnetic pole pieces must be in with high accuracy Agreement with the track density, to increase the signal-to-background ratio and the secondary tracking properties also in the event that the track density increases will improve.

The optimal range of ΔC _W that satisfies the above relationship may be valid even if the magnetic disk has a track density that is not larger than 1800 TPI.

When the track density is low, the signal-to-background ratio of the magnetic disk device is largely determined by factors other than ΔC _W , and there has been no need in the past to prescribe values for ΔC _W.

For a track density of 1800 TPI or more, however, it has been found that the value of ΔC _W influences the signal-to-background ratio of the magnetic disk memory very strongly.

The track spacing of a magnetic disk in a magnetic disk device with high recording density and a track density of 1800 TPI or more is about 14 µm.

In accordance with this, the contours of the magnetic pole pieces of the thin film magnetic head facing the magnetic disk free space to be small for information recording to be adapted. Otherwise it would be between neighboring ones Traces of crosstalk occur, the signal-to-background Reduce ratio and accurate reproduction of information prevent.  

In particular, because of the security strip, the track width, which corresponds to the smaller value of C _WU and C _WD and is an important factor for information recording, must be smaller than the track spacing of the magnetic disk.

The manufacturing process, however, lays down the reduction of Size of the magnetic pole pieces restrictions on, and too small Magnetic pole pieces endanger the reading of the information.

In view of the importance of the dimensions of the contour of the magnetic core under these circumstances and the positioning accuracy of the magnetic disk device, the value C _{WU should} differ from C _WD and the edge zone magnetic field which arises due to this difference should be used to advantage.

The values C _WU and C _WD must be precisely defined in order to generate a precisely controlled edge zone magnetic field. The boundary zone magnetic field that arises due to their difference is used to write information over a large area and thereby improve the secondary track properties.

The effect achieved here is based on magnetic edge zone fields in the areas adjacent to the two lateral ends of the two magnetic pole pieces. Therefore, the difference (ΔC _W ′) between each of the lateral edges of one side of the upper (C _WU ) and lower (C _WD ) magnetic pole piece along the magnetic gap satisfies the following relation:

0 <ΔC _W ′ ≦ 2500 / Tr (µm).

As in the case of the relationship previously described for the Dimensions, it turns out that this relationship is the signal to underground ratio increased and the secondary track properties improved.

Considering the manufacturing process, the lower limit of ΔC _W in practice can preferably be 0.2 µm and the lower limit of ΔC _W 'can preferably be 0.1 µm.

It was also demonstrated that the upper magnetic pole piece preferably essentially symmetrical with respect to the central,  axis of symmetry running in the writing and reading direction of the lower magnetic pole piece is to be arranged to be uniform generate magnetic fields in the areas run into the lateral ends of the two magnetic pole pieces.

On the other hand, both the track density and the linear recording density can be increased to the total recording density to improve a magnetic disk device.

Because the signal-to-background ratio decreases with increasing linear recording density, precise control of the values of ΔC _W or ΔC _W ′ is of great importance.

It is found that the linear recording density is preferred Should be 12 or more kilo bits per centimeter.

Furthermore, it turns out that the surface recording density preferably 8.4 or more mega bits per square centimeter should be.

As described above, the increased signal-to-background Ratio and the improved secondary track property for ensure a high recording density of the magnetic disk device, by making the length difference between the sides along the magnetic Gaps between the upper and lower magnetic pole piece exactly controlled and kept in an optimal range.

On the other hand, the magnetic boundary zone field changes depending on the width of the sides of the magnetic pole pieces and depending on the angles on the transverse sides.

The angle (Θ _S ) between the end edge of the magnetic pole pieces and the side facing the magnetic gap should preferably have a value in the range

45 ° ≦ Θ _S ≦ 150 °

are observed in order to form an effective magnetic boundary zone field. This applies to that of the upper or lower magnetic pole piece, the side (C _WU or C _WD ) of which is shorter along the magnetic gap.

For Θ _S <45 °, the magnetic core and its edge become magnetically saturated, and as a result, an effective magnetic field cannot be generated during writing, and the signal-to-background ratio during reading is reduced.

For Θ _S <150 °, the track width is increased considerably, and an effective magnetic field cannot be generated either.

It was also shown that the angle (Θ _L ) between the end edge of the magnetic pole piece and its side along the magnetic gap is preferably a value in the range

91 ° ≦ Θ _L ≦ 150 °

should be observed in order to obtain an effective magnetic boundary zone field. This applies to that of the upper or lower magnetic pole piece that has the longer edge (C _WU or C _WD ) along the magnetic gap.

It has also been shown that the effect can be increased if the most favorable ranges of the angles Θ _S and Θ _L are observed at the same time.

In this way, an effective magnetic fringe field generated to the secondary track property and that Improve crosstalk behavior when reading.

Although in the arrangement of the magnetic cores shown in FIG. 9, an example was chosen in which C _{WU is} shorter than C _WD , the effect is in no way impaired if C _{WU is} longer than C _WD .

In the practical manufacturing process of a magnetic head with a length C _WU longer than C _WD, however, non-magnetic insulating elements with the same thickness as the lower magnetic pole piece have to be arranged adjacent to its two lateral ends in order to obtain parallel, flat surfaces and to encompass the state with an outside encompassing Avoid magnetic field.

A complicated manufacturing step is required to form the non-magnetic insulating members. Therefore, C _{WU is} preferably shorter than C _WD .

In a thin film magnetic head, a conductive coil and the insulation film form a step with a height of 10 to 20 µm, and the upper magnetic pole piece is usually structured and machined on its lower part adjacent to the step so that the side length (C _WU ) along the magnetic Gap results.

The value of C _WU can be controlled by means of this profile formation method so that it interacts well with the value C _{WD of} the side length along the magnetic gap of the lower magnetic pole piece. However, in the practical manufacturing process, it is difficult to manufacture the magnetic core with high accuracy at its lower part adjacent to the step portion.

Under these circumstances, as an exemplary embodiment of the invention, a magnetic pole piece with a contour as shown in FIG. 3 was invented, in which the magnetic core produces a dimensionally accurate track width and a high track density can thus be achieved.

In this exemplary embodiment, an upper magnetic pole piece 31 has a central section (denoted by a in FIG. 3) with a side length C _WU which is shorter by ΔC _W than the side length C _{WD of} a lower magnetic pole piece 33 along a magnetic gap 32 , and lateral sections (denoted by b in Fig. 3) extending in a direction away from the lower magnetic pole piece.

How the thin film magnetic head having the shape shown in Fig. 3 is manufactured will now be described.

First the lower magnetic pole piece is manufactured and then a non-magnetic insulating layer that both sides and covered both side edges, so deposited that top a recess remains on the lower magnetic pole piece.

Then the recess is closed with a magnetic gap Film covered and the upper magnetic film deposited on this.

In the case of a thin-film magnetic head with the contour shown in FIG. 3, the track width of the magnetic pole piece is determined by the value C _WU . In this arrangement, the height of the step can be minimized and the length C _{WU can} be controlled with high accuracy.

As in the case of the thin film magnetic head with the arrangement shown in Fig. 9, it has also been shown here that for a magnetic disk device with a track density of 1800 TPI or more, the value ΔC _{W of} the condition

0 <ΔC _W ≦ 5000 / Tr (µm)

should be enough.

It was also shown that the difference ΔC _W 'between the central portion a of the upper magnetic pole piece 31 and each of the two lateral ends of the lower magnetic pole piece 33 is preferably the condition

0 <ΔC _W ′ ≦ 2500 / Tr (µm)

should be enough.

In practice, considering the manufacturing process, the lower limit of ΔC _{W should} preferably be 0.2 µm and that of ΔC _W 'should preferably be 0.1 µm.

Furthermore, considering the desire that the upper magnetic pole piece 31 and the lower magnetic pole piece 33 should be arranged symmetrically to the axis in the read / write direction, and that the linear recording density is 12 or more kilo-bits per centimeter and the areal recording density is 8.4 or should be more mega-bits per square centimeter, shown that the angle at the end edge of the upper magnetic pole piece (denoted by Θ _S in Fig. 3) and the angle at the end edge of the lower magnetic pole tip (denoted by Θ _L in Fig. 3) are preferred the following condition should be met:

45 ° ≦ Θ _S ≦ 150 °

91 ° ≦ Θ _L ≦ 150 °

In order to achieve a high recording density, the magnetic part, i.e. the recording medium on the  Magnetic plate has a high coercive force and a small thickness have.

In view of the effect of the invention, the magnetic Layer has a coercive force of 48,000 A / m or more and have a thickness of 0.35 µm or less.

With the aim of a sufficiently high, effective magnetic field to generate with the magnetic head, the magnetic pole pieces can each preferably a single layer film with a saturation magnetic flux density from a Tesla or more or from one multilayer film of the same saturation magnetic flux density, with one or more non-magnetic films interposed be made, and the number of turns of the conductor coil can be 18 or more.

To the efficiency of writing and reading on the magnetic disk to increase and so a high recording density reach, the flying height of the magnetic head must be minimized. According to the invention, it can preferably be 0.25 μm or be less.

In the thin film magnetic head described above is the difference between the widths of the upper and lower magnetic pole piece optimized to the best combination of fringe magnetic field when writing and underground reduced zone to achieve when reading. This ensures that the Signal-to-background ratio of the magnetic disk device increased and the lane behavior can be improved.

Accordingly, the effect be improved if the thin-film magnetic head works inductively and read and write head are identical.

However, even if the thin film magnetic head, for example is realized as a mixed type, which is inductive when writing and acts as a magneto-resistance head when reading (MR- Type), the above measures can at least apply to the writing part of the thin film magnetic head can be applied.  

As for the structure, the location of the ends of the magnetic pole pieces precisely, so this should preferably by dry or wet etching one by sputtering shaped magnetic device happen, as in M. Hanazono et al., "Design and Fabrication of Thin-Film Heads Based on a Dry Process ", J. Appl. Phys., Vol. 61, No. 8, pp. 4157-4162 (1987), or by selective metallization under Use a photoresist mask. As in R.E. Jones Jr., "IBM 3370 Film Head Design and Fabrication", IBM Disk Storage Technology, February 1980, pp. 6-9.

In addition, in a thin film magnetic head for use in a magnetic disk device of this type, the value ΔC _{W of} the relationship should be

0 <ΔC _W ≦ 5000 / Tr

are sufficient, where Tr denotes the track density of the magnetic disk.

A recording system using the magnetic disk device can be connected to a primary device, such as a computer, be connected to a system with large storage capacity to provide.

As previously shown, it is the conditions the design of the contour of the magnetic pole pieces Thin film magnetic head on the storage medium facing must be made available for use in a free area Magnetic disk device with high storage density and a magnetic disk with a track density of 1800 or more TPI.

Specifically, the difference between the length C _{WU of} the upper magnetic pole piece and C _{WD of} the lower magnetic pole piece along the magnetic gap is optimized in accordance with the track density of the magnetic disk of the magnetic disk device in order to increase the signal-to-background ratio and to improve the secondary tracking properties, even if the recording density is high.

Using the thin film magnetic head having a lower and an upper magnetic pole piece whose contours on the side facing the recording medium are shaped as shown in Fig. 4A, the intensity of the magnetic field generated by the magnetic pole pieces was examined. The results of measuring the longitudinal magnetic field of the recording medium at a point on the extension line of the magnetic gap in the Z direction ( Fig. 4A) in the height at which the head hovers over the recording medium ( Fig. 4B) that the intensity decreases as the distance in the Z direction increases. The results also show that the distance in the Z direction, at which a certain value Ha of the magnetic field intensity is assumed, increases as the difference ΔC _W between C _WU and C _WD increases, as shown for the lengths W am, W₁ and W₂ . However, the value converges to W₂₀₀ as shown in Fig. 4C. The value of ΔC _W is indicated by the index 0, 1, 2 and 200 in FIG. 4C. The graphic representation shows that the scattering magnetic field is limited in its intensity, even if the difference ΔC _W increases.

The values W₀, W₁ and W₂₀₀ for the distance in Z- Direction correspond to lengths for scattering the magnetic pole pieces generated magnetic field.

In this way, the relationship between the value of ΔC _W and the width of the peripheral magnetic field can be determined to enable the relationship between the signal intensity and ΔC _{W to also be} determined when a thin film magnetic head with a certain track width is used. The results are shown by curve S in FIG. 5.

On the other hand, as the value of ΔC _{W increases,} the intensity of the background signal, in particular the background generated by crosstalk from neighboring tracks and the sub-track background when reading, increases, as shown in curve N of FIG. 5.

Correspondingly, the relationship between the signal-to-background ratio (signal-to-noise ratio) as a typical feature of the performance of a magnetic disk device and the value ΔC _{W is shown} in FIG. 6. It has been shown that the signal-to-background ratio is greater in a range 0 <ΔC _W <ΔC _Wlim than in ΔC _W = 0.

Fig. 6 also shows that the signal-to-background ratio decreases and the value ΔC _{Wlim decreases} to ΔC _Wlim 'when the track density of the magnetic disk is further increased.

Fig. 7 shows the relationship between the track density (Tr) of the magnetic disk and ΔC _Wlim . It proves that the value of ΔC _Wlim is substantially proportional to the reciprocal of the track density (1 / Tr), with a proportionality constant of approximately 5000.

It has also been shown that the value ΔC _Wlim , measured as a function of the difference ΔC _W ′ between the side C _WU and the side C _WD , at each of the lateral ends of the magnetic pole pieces is also substantially proportional to the reciprocal of the track density (1 / Tr ) and the proportionality constant is approximately 2500.

Accordingly, in a high density magnetic disk apparatus using a magnetic disk having a track density of 1800 TPI or more, the signal-to-background ratio may be larger than ΔC _W = 0 or ΔC _W ′ = 0 if the values are in the range

0 <ΔC _W ≦ 5000 / Tr

0 <ΔC _W ′ ≦ 2500 / Tr

to get voted.  

The following are practical examples described.

As shown in Fig. 1, a magnetic disk device has in its structure the components identified by reference numerals 1 to 8 and a voice coil controller.

1 denotes a base plate and 2 denotes a spindle.

A plurality of circular magnetic disks 4 are carried by the single spindle as shown in FIG. 1.

In the example shown, the spindle carries five magnetic disks. However, the number of magnetic disks is in no way limited to five.

Alternatively, a plurality of spindles can be provided, each of which in turn carries a plurality of magnetic disks in the manner shown in FIG. 1.

3 with a motor is named, which drives the spindle 2 to rotate the magnetic plates.

5 denotes magnetic heads for data information and 5 a a magnetic head for positioning.

6 is a carriage, 7 is a voice coil and 8 is a magnet.

The voice coil 7 and the magnet 8 form the voice coil drive.

The positioning of the head is carried out by means of the carriage 6 of the coil 7 and the magnet 8 .

The voice coil 7 is connected to the magnetic heads 5 and 5 a via a voice coil control.

In Fig. 1, the primary device includes, for example, a computer system.

The thin-film magnetic head 5 in FIG. 1 has magnetic pole pieces with a contour on the free surface facing the magnetic plate 4 , as is shown enlarged in FIGS. 2A and 2B.

The thin film magnetic head is constructed as shown in FIG. 11.

In Fig. 11, reference numeral 100 denotes the base plate and 200 a base film for forming a flat surface.

Stacked on the backing film 200 are a lower magnetic pole piece 300 made of magnetic material, an insulating film 600 to define a magnetic gap G, and an upper magnetic pole piece 400 also made of magnetic material, in that order.

Coil conductors 500 are embedded in the insulating film 600 and are electrically excited to induce a magnetic field.

Reference numeral 700 denotes a protective film which is deposited on the upper magnetic pole piece 400 .

The hatched portion in FIG. 11 is specially designed as shown in FIG. 2A or in FIG. 2B.

A thin-film magnetic head used in the magnetic disk device with a track density of 2500 TPI has magnetic cores with a contour configuration on the free surface facing the recording medium, as shown in Fig. 2A or 2B.

Particularly in the case of a thin-film magnetic head, the magnetic pole pieces of which were produced by means of magnetron sputtering of a magnetic material and by means of shaping by means of an ion beam etching process, the magnetic pole pieces have a contour configuration on the free surface facing the magnetic disk, as shown in FIG .

In the magnetic pole in Fig. 2A, an upper magnetic pole piece 11 has the width _WU C of 6.9 microns and a lower magnetic pole piece 13 has a width of 8.7 microns (C _WD), in each case along a Magnetpolspaltes 12th

The upper and lower magnetic pole pieces 11 and 13 are arranged so that their contours, viewed from the side facing the magnetic disk, are symmetrical with respect to a common axis lying in the read / write direction.

The properties of the thin film magnetic head described above when writing and reading with the magnetic disk device were examined, and the results showed that the signal-to-background ratio compared to a magnetic head with C _WU = 6.9 µm and C _WD = 9.1 µm, was 4.3% larger.

On the other hand, in a thin film magnetic head having a magnetic core in which a lower magnetic pole piece 16 was made by magnetron sputtering a magnetic material and patterning by an ion beam etching process and an upper magnetic pole piece 14 by patterning by a selective metallization process using a photoresist mask, the magnetic pole pieces have one Contour as shown in Fig. 2B.

In the magnetic pole tip shown in FIG. 2B, the upper magnetic pole piece 14 has a width (C _WU ) of 7.0 μm and the lower magnetic pole piece 16 has a width (C _WD ) of 8.6 μm, each along a magnetic gap 15 .

The difference between a lateral end of the upper magnetic pole piece 14 and the corresponding end of the lower magnetic pole piece 16 is 0.9 μm, whereas the difference between the other lateral end of the upper magnetic pole piece and the corresponding end of the lower magnetic pole piece is 0.7 μm.

The properties of the thin film magnetic head described above when writing and reading with the magnetic disk device were examined, and the results showed that the signal-to-background ratio was 8.6% compared to a magnetic head with C _WU = 7.0 µm and C _WD = 9.5 µm, was increased.

The thin film magnetic head for use in a magnetic disk device with a track density of 3000 TPI has a magnetic pole arrangement with a contour configuration on the surface facing the recording medium as shown in FIG. 8.

In particular, in a thin film magnetic head using a magnetic pole assembly made by magnetron sputtering a magnetic material and patterning by an ion beam etching process, the magnetic core assembly has a contour as shown in FIG. 8.

To form this magnetic pole structure, a lower magnetic pole piece 83 is first produced, then a non-magnetic insulating material 84 is deposited on both sides and on the lateral end regions of the surface of the lower magnetic pole piece such that a cutout 85 remains on top of the lower magnetic pole piece 83 , and then deposited a magnetic gap film 82 to cover the non-magnetic insulating material and the recess.

This manufacturing process is suitable for controlling the value of C _WU very precisely.

The upper magnetic pole piece 81 has a width (C _WU ) of 6.0 μm and the lower magnetic pole piece 83 has a width (C _WD ) 7.2 μm, in each case along the magnetic gap.

The properties of the above thin film magnetic head when writing and reading with the magnetic disk device were examined, and the results showed that the signal-to-background ratio was 8.6% compared to a magnetic head with C _WU = 6.0 µm and C _WD = 8 , 0 µm was increased.

Claims (10)

1. Thin-film magnetic head ( 5 ) for writing and / or reading information on or from a magnetic disk ( 4 ) with a track density Tr, specified in TPI ("tracks per inch", corresponding to tracks per 25.4 mm), with an upper magnetic pole piece ( 400; 52; 21 ) and a lower magnetic pole piece ( 300; 53; 23 ), which forms a magnetic gap (G; 54; 22 ) with the upper magnetic pole piece and has a width C _WD measured along the air gap that of the width C _{WU of} the upper magnetic pole piece is different, characterized in that
that the smaller of the widths C _WU , C _{WD of} the two magnetic pole pieces ( 300, 400; 52, 53; 21, 23 ) is less than 14 µm,
that the difference ΔC _W , given in µm, between the widths C _WU and C _WD fulfills the relation 0 <ΔC _W ≦ 5000 / Tr, preferably 0.2 ≦ ΔC _W ≦ 5000 / Tr, and
that in the wider magnetic pole piece ( 300; 53; 23 ) the angle Θ _L between the edge running along the magnetic gap (G; 54; 22 ) and the side edge is between 91 ° and 150 °. 2. Magnetic head according to claim 1, characterized in that the difference ΔC _W 'in µm between the respective corresponding ends of the two magnetic pole pieces ( 300, 400; 52, 53; 21, 23 ) the relation 0 <ΔC _W ' ≦ 2500 / Tr, preferably 0.1 ≦ ΔC _W ′ ≦ 2500 / Tr. 3. Magnetic head according to claim 1 or 2, characterized in that in the narrower magnetic pole piece ( 400; 52; 21 ) the angle Θ _s between the along the magnetic gap (G; 54; 22 ) and the side edge between 45 ° and 150 ° lies. 4. Magnetic head according to one of claims 1 to 3, characterized in that the upper magnetic pole piece ( 31 ) on its end facing the magnetic plate ( 4 ) has a substantially parallel to the end face of the lower magnetic pole piece ( 33 ) extending central region and of the lower magnetic pole piece has more distal end portions. 5. Magnetic head according to one of claims 1 to 4, characterized in that the end faces of both magnetic pole pieces ( 400, 300; 52, 53; 21, 23 ) are substantially symmetrical with respect to the center line running in the read / write direction. 6. Magnetic head according to one of claims 1 to 5, characterized in that the upper magnetic pole piece ( 400; 52; 21 ) is narrower than the lower ( 300; 53; 23 ). 7. Magnetic head according to one of claims 1 to 6, characterized in that both magnetic pole pieces ( 400, 300; 52, 53; 21, 23 ) consist of a magnetic material with a saturation magnetic flux density of 1 T and have a single-layer film structure. 8. Magnetic head according to one of claims 1 to 6, characterized in that both magnetic pole pieces ( 400, 300; 52, 53; 21, 23 ) consist of a magnetic material with a saturation magnetic flux density of at least 1 T and a multilayer film structure with have one or more intermediate layers of non-magnetic films. 9. Magnetic head according to one of claims 1 to 8, characterized in that it contains a conductor coil ( 500 ) with at least 18 turns. 10. Magnetic head according to one of claims 1 to 9, characterized in that on both side surfaces and the lateral regions of the end face of the lower magnetic pole piece ( 83 ) a non-magnetic insulating layer ( 84 ) is arranged, which on the top of the end face one Has recess ( 85 ) defining the magnetic gap.