Manufacturing method for magnetic head

A method of manufacturing a magnetic head by bonding a pair of magnetic cores to each other, wherein a residual stress in the magnetic cores upon bonding them together is controlled so that a permeability of the magnetic cores in the vicinity of a magnetic recording medium sliding surface of the magnetic cores may become maximum. Accordingly, the residual stress in the magnetic cores is controlled so that the permeability of the magnetic cores may surely become maximum after forming the magnetic head, thereby ensuring a superior reproduction efficiency of the magnetic head.

BACKGROUND OF THE INVENTION 
The present invention relates to a manufacturing method for a magnetic 
head, and more particularly to a manufacturing method for a magnetic head 
using a magnetic core formed of monocrystal ferrite or combined 
monocrystal ferrite and polycrystal ferrite. 
A magnetic head to be mounted in a magnetic recording and reproducing 
device such as a VTR (video tape recorder) is constituted of a coil and a 
magnetic core formed of a magnetic material such as monocrystal ferrite. 
Such a magnetic head using a magnetic core formed of ferrite is called a 
ferrite head, and it is generally used in the art. 
FIG. 25 shows a structure of such a magnetic head in the related art. 
Referring to FIG. 25, reference numerals 207 and 208 designate a pair of 
magnetic cores bonded to each other. The magnetic cores 207 and 208 are 
formed with track width defining grooves 201 and 202, respectively, for 
defining a track width. The magnetic core 207 has a front gap forming 
surface 203 and a back gap forming surface 205, and the magnetic core 208 
has a front gap forming surface 204 and a back gap forming surface 206. 
The front gap forming surface 203 of the magnetic core 207 faces the front 
gap forming surface 204 of the magnetic core 208 to form a front gap 
g.sub.3 therebetween. Similarly, the back gap forming surface 205 of the 
magnetic core 207 faces the back gap forming surface 206 of the magnetic 
core 208 to form a back gap g.sub.4 therebetween. Further, the grooves 201 
of the magnetic core 207 face the grooves 202 of the magnetic core 208. 
Gap films 209 and 210 are formed on the opposed surfaces of the magnetic 
cores 207 and 208, respectively. A fusing glass 211 as a nonmagnetic 
material is filled in the vicinity of the front gap g.sub.3 and the back 
gap g.sub.4 to bond both the magnetic cores 207 and 208 each other. 
Further, the opposed surfaces of the magnetic cores 207 and 208 between 
the front gap g.sub.3 and the back gap g.sub.4 are formed with coil 
grooves 212 and 213 for receiving coils, respectively. 
The magnetic head shown in FIG. 25 is manufactured by the following method. 
Firsts as shown in FIG. 26, a substrate 214 formed of monocrystal ferrite 
or combined monocrystal ferrite and polycrystal ferrite is prepared, and a 
plurality of track width defining grooves 215 for defining a track width 
of the magnetic head are formed on an upper surface of the substrate 214 
so as to be arranged at a given pitch in a lateral direction of the 
substrate 214 and extend in a longitudinal direction of the substrate 214. 
Each groove 215 has a substantially semi-circular cross section. 
Then, as shown in FIG. 27, coil grooves 216 and 217 for receiving coils and 
glass grooves 218 and 219 for receiving glass are formed on the upper 
surface of the substrate 214 so as to be arranged at a given pitch in the 
longitudinal direction of the substrate 214 and extend in the lateral 
direction of the substrate 214. Each of the coil grooves 216 and 217 has a 
substantially trapezoidal cross section, and each of the glass grooves 218 
and 219 has a substantially U-shaped cross section. Thus, the coil grooves 
216 and 217 and the glass grooves 218 and 219 extend in orthogonal 
relationship to the track width defining grooves 215 on the upper surface 
of the substrate 214. 
Then, the substrate 214 is divided into a block 220 having the coil groove 
216 and the glass groove 218 as shown in FIG. 28 and a block 221 having 
the coil groove 217 and the glass groove 219 as shown in FIG. 29. 
Then, as shown in FIG. 28, a gap film 222 of a nonmagnetic material such as 
SiO.sub.2 is formed on the upper surface of the block 220 except the inner 
surfaces of the coil grooves 216 and the glass grooves 218 so as to have a 
thickness about half a gap length of the magnetic head by using a suitable 
thin film forming technique such as a magnetron sputtering process, thus 
forming a plurality of front gap forming surfaces 223 and a plurality of 
back gap forming surfaces 225. Similarly, a gap film 227 is formed on the 
upper surface of the block 221 to form a plurality of front gap forming 
surfaces 224 and a plurality of back gap forming surfaces 226 (see FIG. 
29). 
Then, as shown in FIG. 29, both the blocks 220 and 221 are matched with 
each other so that the front gap forming surfaces 223 and 224 face each 
other and the back gap forming surfaces 225 and 226 face each other. 
Then, glass rods formed of a fusing glass are inserted into a space defined 
by the coil grooves 216 and 217 and a space defined by the glass grooves 
218 and 219, and the glass rods are then fused at a given temperature 
under a pressure of about tens of MPa applied in opposite directions 
depicted by arrows P.sub.2 in FIG. 29. 
Accordingly, a fusing glass 231 is filled in the vicinity of a plurality of 
front gaps defined between the front gap forming surfaces 223 and 224, in 
the vicinity of a plurality of back gaps defined between the back gap 
forming surfaces 225 and 226, and in a plurality of spaces defined by the 
track width defining grooves 215 and 230. Thus, the front gaps and the 
back gaps functioning as recording and reproducing gaps are formed between 
the front gap forming surfaces 223 and 224 and between the back gap 
forming surfaces 225 and 226, respectively. 
Then, a tape sliding surface 232 of the combined blocks 220 and 221 is 
subjected to cylindrical grinding. 
Finally, the integrated body of the blocks 220 and 221 is cut into chips, 
and coils are located in the coil grooves 216 and 217 of each chip, 
thereby obtaining the magnetic head shown in FIG. 25. 
However, there occur various strains in the monocrystal ferrite in the 
above manufacturing process. The strains cause a great reduction in 
permeability of the magnetic cores to reduce a reproduction efficiency of 
the magnetic head. 
In the magnetic head to be manufactured by the above method for example, 
the pair of blocks 220 and 221 forming the pair of magnetic cores 207 and 
208 are bonded together by matching the blocks 220 and 221 and fusing the 
fusing glass 231 as a nonmagnetic material with heat and pressure to fill 
the fusing glass 231 between the blocks 220 and 221. In most cases, the 
blocks 220 and 221 before matching are curved. Accordingly, if the blocks 
220 and 221 are merely matched with each other, full-face contact between 
the front gap forming surfaces 223 and 224 and full-face contact between 
the back gap forming surfaces 225 and 226 cannot be obtained to cause 
opening of the magnetic gap. To cope with this problem, a given pressure 
(e.g., about tens of MPa) is applied to the blocks 220 and 221 in heating 
the fusing glass 231 and filling it between the blocks 220 and 221, 
thereby eliminating the curvature of the blocks 220 and 221 to form a 
desired magnetic gap. 
However, this pressure is concentrated at the magnetic gap forming portion 
formed of monocrystal ferrite or combined monocrystal ferrite and 
polycrystal ferrite as a magnetic material for forming the blocks 220 and 
221. As a result, even after bonding the blocks 220 and 221, an internal 
residual stress exists in the ferrite forming the magnetic gap forming 
portion. Therefore, a magneto-mechanical coupling effect due to the 
residual stress causes a reduction in permeability of the magnetic cores 
to reduce the reproduction efficiency of the magnetic head. 
In recent years, it has been demanded to reduce a track width of the 
magnetic head in response to a demand for high-density recording. The 
reduction in the track width results in an increase in the stress applied 
to the ferrite forming the magnetic gap forming portion. Accordingly, a 
large strain occurs in the magnetic gap forming portion to reduce the 
permeability of the ferrite and reduce the reproduction efficiency of the 
magnetic head. In particular, when the track width is about 20 .mu.m or 
less, the above problem becomes remarkable. 
Hitherto, there does not exist a method for quantitatively relating a 
reduction in permeability of a ferrite material due to a residual stress 
to a reduction in reproduction output of an actual magnetic head. 
Accordingly, it is necessary to actually make a magnetic head and measure 
a reproduction output of the magnetic head, so as to determine how the 
residual stress generating in forming the magnetic gap reduces the 
reproduction output. Thus, in the present circumstances, it is impossible 
to establish a definite guideline for designing in relation to the stress. 
Further, a stress dependency of permeability in monocrystal ferrite may 
change with a crystal orientation of the ferrite. That is, in designing a 
magnetic head employing monocrystal ferrite, it is necessary to decide a 
cutting face orientation of the monocrystal ferrite in consideration of an 
aspect of workability and wear resistance and another aspect of 
reproduction output and recording characteristic. It is further necessary 
to control the residual stress in the manufacturing process, so as to 
prevent a deterioration in magnetic characteristics of the ferrite. Thus, 
such a series of operations must be carried out by trial and error. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the present invention to provide a 
manufacturing method for a magnetic head which can control the residual 
stress in the magnetic cores so that the permeability of the magnetic 
cores may surely become maximum after forming the magnetic head, thereby 
ensuring a superior reproduction efficiency of the magnetic head. 
According to a first aspect of the present invention, there is provided a 
method of manufacturing a magnetic head by bonding a pair of magnetic 
cores to each other, wherein a residual stress in said magnetic cores upon 
bonding them together is controlled so that a permeability of said 
magnetic cores in the vicinity of a magnetic recording medium sliding 
surface of said magnetic cores may become maximum. 
According to a second aspect of the present invention, there is provided in 
a method of manufacturing a magnetic head by bonding a pair of magnetic 
cores to each other with a magnetic gap defined therebetween to form a 
closed magnetic circuit; the improvement comprising the steps of bonding a 
pair of magnetic gap forming portions of said magnetic cores to each other 
and then filling a nonmagnetic material in the vicinity of said magnetic 
gap. 
According to a third aspect of the present invention, there is provided a 
method of manufacturing a magnetic head, comprising the steps of forming a 
plurality of track width defining grooves at a given pitch on each of a 
pair of magnetic core substrates so that a flat portion having a width at 
least three times a track width remains on each of said magnetic core 
substrates; forming a coil groove for receiving a coil on each of said 
magnetic core substrates so that said coil groove extends in substantially 
orthogonal relationship to said track width defining grooves; bonding said 
pair of magnetic core substrates to each other to form an integral body so 
that said track width defining grooves of both said magnetic core 
substrates come into registration with each other; and cutting said 
integral body into a head chip. 
The present inventors have found that a reproduction output of a magnetic 
head can be improved by measuring a permeability of a magnetic core 
material (especially, monocrystal ferrite) under the stress applied 
condition, quantifying an influence of the stress on the permeability, 
obtaining a stress value corresponding to a maximum permeability of the 
ferrite in the vicinity of a magnetic recording medium sliding surface in 
a sliding direction before forming the magnetic head on the basis of the 
above influence, and setting a residual stress of the ferrite after 
forming the magnetic head to the above stress value, thus achieving the 
present invention. The present invention will now be described in detail. 
1. Relation between a permeability of a ferrite head in the vicinity of a 
magnetic gap and a reproduction efficiency of the ferrite head 
The permeability and the reproduction efficiency were analyzed by using a 
magnetic head as shown in FIG. 1. The magnetic head is constructed of a 
pair of monocrystal ferrite cores 1 and 2. A magnetic gap g is formed 
between the magnetic cores 1 and 2 on a magnetic recording medium sliding 
surface thereof. The magnetic head has the following dimensions. 
Magnetic gap length: 0.2 .mu.m 
Magnetic gap depth: 15 .mu.m 
Track width: 20 .mu.m 
Cutting depth: 65 .mu.m 
The analysis was carried out by using this ferrite head under the following 
conditions. 
(1) Program 
Hardware: CPU: NWS-1860 (manufactured by SONY) 
Software: three-dimensional magnetic field analysis 
software using a finite element method (Maxwell manufactured by Ansoft) 
(2) Calculation 
A magnetic flux density Bg in the magnetic gap was obtained by simulation, 
and a head efficiency .eta. was obtained from a maximum value of the 
magnetic flux density Bg. Further, a head inductance L was obtained from a 
magnetic flux density Bc and a sectional area Sc of the ferrite cores, and 
a head efficiency per unit inductance was calculated. This value 
corresponds to an actual level of a head output. The calculation was 
carried out in accordance with the following expressions. 
EQU .eta.=(Hg.times.Gap)/(I.times.N) 
EQU Hg=Bg/.mu.o 
where, 
.eta.: head efficiency 
Hg: magnetic field in the gap A/m! 
Bg: magnetic flux density in the gap T! 
.mu..sub.o : space permeability 
Gap: gap length m! 
I: current A! 
N: number of turns 
L=(BC.times.SC)/I.sub.A (per turn) 
where, 
L: head inductance H! 
Bc: magnetic flux density in the magnetic cores T! 
Sc: sectional area of the magnetic cores m.sup.2 ! 
I.sub.A : total Current AT! 
(3) Calculation conditions 
Sectional area of coils: Scoil=150.times.330 .mu.m.sup.2 ! 
Current: 10 mAT/10 turns for each coil 
Current density: 202020 A/m.sup.2 ! 
Boundary condition: fixed boundary condition, 
B=0 at infinity (using an infinite boundary element) 
(4) Permeability 
The permeability of the ferrite at a portion in the vicinity of the gap 
(see FIG. 1) was changed in the range of 100 to 500. 
The relation between the permeability in the vicinity of the magnetic gap 
and the head efficiency per unit inductance (.eta./L.sup.1/2) obtained by 
the simulation under the above conditions is shown in FIG. 2. As the 
inductance L changes with the number of turns, the head efficiency per 
unit inductance (.eta./L.sup.1/2) corresponds to an actual head output. 
(5) Other Dimensions 
The following dimensions are illustrated in FIGS. 1A through 1D: 
overall head height 1000 of 1900 .mu.m; 
overall head length 1001 of 1500 .mu.m; 
a magnetic gap depth 1002 of 15 .mu.m; 
a contact width 1003 of 60 .mu.m; 
a magnetic gap length 1004 of 0.2 .mu.m; 
a central contact area 1005 of 120 .mu.m; 
a cutting depth 1006 of 65 .mu.m; 
a track width 1007 of 20 .mu.m; and 
an overall core thickness 1008 of 200 .mu.m. 
It is understood from FIG. 2 that the head output is dependent upon the 
permeability .mu. in the vicinity of the magnetic gap. 
2. Stress and frequency dependencies of permeability 
As shown in FIG. 3, a measuring sample S1 in the form of a rectangular 
parallelepiped elongated in a sliding direction of a magnetic recording 
medium (e.g., magnetic tape) was prepared in consideration of a direction 
of a magnetic flux in a bulk head and an orientation of a stress to be 
generated in the bulk head. 
Actually, the sample S1 was attached to a crystallized glass plate, and a 
bending load was applied to the sample S1 in the longitudinal direction 
thereof so that the sample S1 on the upper or lower side of the glass 
plate may become convex on the upper side. In this stress applied 
condition, the permeability of the sample S1 in the longitudinal direction 
thereof was measured by using a network analyzer (41195A manufactured by 
Hewlett Packard). 
As shown in FIG. 6, the sample S1 actually corresponds to a portion of the 
magnetic head in the vicinity of the magnetic recording medium sliding 
surface. In this case, a face orientation (100) of the ferrite crystal 
forming the sample S1 appears in the longitudinal direction of the sample 
S1, that is, in the sliding direction of the magnetic recording medium. 
There is shown in FIG. 3 the result of measurement of the stress and 
frequency dependencies of the permeability of the sample S1 having the 
face orientation (100) appearing in the longitudinal direction of the 
sample S1. 
Similarly, there is shown in FIG. 4 the result of measurement of the stress 
and frequency dependencies of the permeability of a sample S2 having a 
face orientation (111) appearing in the longitudinal direction of the 
sample S2. As shown in FIG. 7, the sample S2 actually corresponds to a 
portion of the magnetic head in the vicinity of the magnetic recording 
medium sliding surface. Further, there is shown in FIG. 5 the result of 
measurement of the stress and frequency dependencies of the permeability 
of a sample S3 having a face orientation (211) appearing in the 
longitudinal direction of the sample S3. As shown in FIG. 8, the sample S3 
actually corresponds to a portion of the magnetic head in the vicinity of 
the magnetic recording medium sliding surface. 
3. Relation between a stress and a head efficiency 
The relation between a stress and a head efficiency as shown in FIG. 9 can 
be obtained by substituting the relation between the stress and the 
permeability of the monocrystal ferrite having a crystal orientation &lt;100&gt; 
in the longitudinal direction as shown in FIG. 3 for the relation between 
the permeability and the head efficiency as shown in FIG. 2. Similarly, 
the relation between a stress and a head efficiency in the crystal ferrite 
having a crystal orientation &lt;111&gt; in the longitudinal direction as shown 
in FIG. 4 and the crystal ferrite having a crystal orientation &lt;211&gt; in 
the longitudinal direction as shown in FIG. 5. 
Further, FIG. 11 shows the relation between a efficiency and a head 
frequency to be obtained under a given stress in the ferrite heads having 
the crystal orientations &lt;100&gt; and &lt;111&gt;. 
4. Relation to an actual head output 
(1) The case where a stress is changed 
Using monocrystal ferrite having an analytic composition of 55 mol % 
Fe.sub.2 O.sub.3, 22 mol % MnO, and 23 mol % ZnO and having a crystal 
orientation &lt;100&gt; in the longitudinal direction, two kinds of heads were 
prepared. That is, in one kind of head, compressive residual stress of 20 
MPa acts in the vicinity of a magnetic gap in a tape sliding direction, 
and in the other kind of head, a compressive residual stress of 50 MPa 
acts in the vicinity of a magnetic gap in a tape sliding direction. There 
compressive residual stresses were adjusted by controlling a pressure to 
be applied in forming the magnetic heads. That is, in forming the magnetic 
head as shown in FIG. 29 as previously mentioned, the blocks 220 and 221 
were matched with each other so that the front gap forming surfaces 223 
and 224 faced each other and the back gap forming surfaces 225 and 226 
faced each other. Then, the glass rods of fusing glass were inserted in 
the space defined between the coil grooves 216 and 217 and the space 
defined between the glass grooves 218 and 219, and were heated under the 
pressure of about tens of MPa in the opposite directions depicted by the 
arrows P.sub.2 in FIG. 29, thereby filling the fusing glass 231 in the 
vicinity of the front gap and the back gap and in the track width defining 
grooves 215 and 230. The pressure to be applied to the blocks 220 and 221 
in heating the glass rods was controlled to thereby adjust the compressive 
residual stress. 
Using these magnetic heads, a reproduction output at 5 MHz was measured. As 
the result, the output of the magnetic head with the residual stress of 20 
MPa was higher by 1.9 dB than that with the residual stress of 50 MPa. On 
the other hand, calculated values for the head efficiency 
(.eta./L.sup.1/2) at 5 MHz were plotted in respect of a change in stress 
to obtain the result shown in FIG. 9. 
It is understood from FIG. 9 that an output difference between 20 MPa and 
50 MPa is 2.2 dB, which almost agrees with the measured value. Therefore, 
an actual head output can be predicted by a stress value in forming a 
magnetic head, and the process conditions enabling a maximum head output 
can be decided according to the predicted value. 
(2) The case where a crystal orientation is changed 
Two kinds of heads having the crystal orientations &lt;100&gt; and &lt;111&gt; in the 
tape sliding direction were prepared with a compressive residual stress of 
50 MPa applied. A relative output of the head having the crystal 
orientation &lt;111&gt; to the head having the crystal orientation &lt;100&gt; is 
shown by circles in FIG. 11. 
On the other hand, the relation between a head efficiency (.eta./L.sup.1/2) 
and a frequency in each head was calculated from the data shown in FIG. 10 
by using the aforementioned magnetic head analysis software, and the 
calculated values were plotted to obtain the result shown in FIG. 11 (the 
data shown corresponds to the case where a compressive residual stress of 
47 MPa was actually applied, but it is almost the same as that in the case 
of 50 MPa). 
Further, calculated values for the relative output of the head having the 
crystal orientation &lt;111&gt; to the head having the crystal orientation &lt;100&gt; 
were also plotted with crosses to obtain the result shown in FIG. 11. It 
is understood that the calculated values for the relative output well 
agree with the found values indicated by the circles in FIG. 11. 
Therefore, an actual head output can be predicted from the permeability of 
the ferrite regardless of a difference in crystal orientation. When the 
crystal orientation is changed, the stress value improving the head output 
is changed. 
5. Method of controlling a stress 
Thus, the relation between changes in permeability and magnetic head output 
both depending upon a stress in monocrystal ferrite in each crystal 
orientation thereof can be made definite. In view of this definite 
relation, the steps of working and assembling the magnetic cores, the 
physical values of the materials, the crystal orientation of the 
monocrystal ferrite, etc. are suitably selected to thereby control a 
residual stress in the magnetic cores upon bonding them together so that 
the permeability of the magnetic cores (specifically, the permeability in 
a sliding direction of a magnetic recording medium) in the vicinity of a 
magnetic recording medium sliding surface of the magnetic cores may become 
maximum, whereby a reproduction efficiency of the magnetic head can be 
improved. 
In the conventional manufacturing method for the magnetic head as mentioned 
previously, a large stress is generated in the magnetic head in the step 
of bonding the magnetic cores to each other. That is, the magnetic cores 
are bonded together by a fusing glass as a nonmagnetic material in the 
presence of heat and pressure. At this time, a compressive stress remains 
as an internal stress in the magnetic cores, which is considered to reduce 
the permeability of the magnetic cores and therefore reduce the 
reproduction efficiency of the magnetic head. 
According to the present invention, there is provided measures for 
controlling a residual stress in the magnetic cores upon bonding them 
together so that the permeability of the magnetic cores (specifically, the 
permeability in the magnetic recording medium sliding direction) in the 
vicinity of the magnetic recording medium sliding surface of the magnetic 
cores may become maximum. As one specific example of the measures, in a 
manufacturing method for a magnetic head by bonding a pair of magnetic 
cores to each other with a magnetic gap defined therebetween to form a 
closed magnetic circuit, it is featured that the manufacturing method 
comprises the first step of bonding a pair of magnetic gap forming 
portions of the magnetic cores to each other and the second step of 
filling a nonmagnetic material in the vicinity of the magnetic gap after 
the first step. 
The magnetic cores may be formed of a known magnetic material generally 
used in manufacture of a magnetic head, such as monocrystal ferrite or 
combined monocrystal ferrite and polycrystal ferrite. It is preferable 
that the magnetic gap forming portions are bonded together by a fusing 
glass. Further, it is preferable that the nonmagnetic material to be 
filled in the vicinity of the magnetic gap is a fusing glass having a 
glass transition point lower than a flexure point of the fusing glass to 
be used for bonding of the magnetic gap forming portions. 
While the fusing glass to be used for bonding of the magnetic gap forming 
portions may be a known fusing glass generally used for manufacture of a 
magnetic head, it is preferable that the fusing glass has a bonding 
ability and a relatively high flexure point. Further, a base layer such as 
an SiO.sub.2 film may be interposed between the fusing glass and the 
magnetic cores. Further, while the nonmagnetic material to be filled in 
the vicinity of the magnetic gap may be a known nonmagnetic material such 
as SiO.sub.2 generally used for manufacture of a magnetic head, it is 
preferable that the nonmagnetic material has a relatively low glass 
transition point. 
Further, the bonding of the magnetic gap forming portions may be effected 
by thermal diffusion of a metal film at low temperatures. The metal film 
may be formed of Au, Ag, Pt, Pb, etc. Further, a base layer such as an 
SiO.sub.2 film or a Cr film may be interposed between the metal film and 
the magnetic cores. For example, a composite thin film composed of an 
SiO.sub.2 film and an Au film or composed of a Cr film and an Au film may 
be used for the bonding of the magnetic gap forming portions. The 
composite thin film may be formed by a known thin film forming technique 
such as a sputtering process. 
As another specific example of the measures to more easily control the 
stress by reducing the pressure to be applied to the magnetic cores upon 
bonding them together to thereby reduce the stress remaining in the 
magnetic cores, it is featured that the manufacturing method for the 
magnetic head according to the present invention comprises the steps of 
forming a plurality of track width defining grooves at a given pitch on 
each of a pair of magnetic core substrates so that a flat portion having a 
width at least three times a track width remains on each magnetic core 
substrate, forming a coil groove for receiving a coil on each magnetic 
core substrate so that the coil groove extends in substantially orthogonal 
relationship to the track width defining grooves, bonding the pair of 
magnetic core substrates to each other to form an integral body so that 
the track width defining grooves of both the magnetic core substrates come 
into registration with each other, and cutting the integral body into a 
head chip. 
As described above, in the manufacturing method for the magnetic head by 
bonding a pair of magnetic cores to each other, the residual stress in the 
magnetic cores upon bonding them together is controlled so that the 
permeability of the magnetic cores (specifically, the permeability in the 
magnetic recording medium sliding direction) in the vicinity of the 
magnetic recording medium sliding surface of the magnetic cores. 
Accordingly, the permeability of the magnetic cores after forming the 
magnetic head can be made surely maximum, and the reproduction efficiency 
of the magnetic head can be greatly improved. 
Further, after the magnetic gap forming portions of the magnetic cores are 
bonded to each other, the nonmagnetic material is filled in the vicinity 
of the magnetic gap. Accordingly, the pressure to be applied to the 
magnetic gap forming portions of the magnetic cores can be made very 
small, and a stress maximizing the permeability can be left in the 
magnetic gap forming portions of the magnetic cores, thereby ensuring a 
superior reproduction efficiency of the magnetic head. 
In the preferred mode where a fusing glass is used for bonding of the 
magnetic gap forming portions, and a fusing glass having a glass 
transition point lower than a flexure point of the fusing glass to be used 
for bonding of the magnetic gap forming portions is used as the 
nonmagnetic material to be filled in the vicinity of the magnetic gap, it 
is possible to prevent that the magnetic gap will be opened in filling the 
nonmagnetic material in the vicinity of the magnetic gap, thus ensuring a 
sufficient bonding strength in the magnetic ahead. 
Further, in the preferred mode where the bonding of the magnetic gap 
forming portions is effected by thermal diffusion of a metal film at low 
temperatures, a sufficient bonding strength in the magnetic head can be 
ensured, and the manufacture of the magnetic head can be performed at low 
temperatures to thereby improve the productivity and accordingly increase 
the industrial value. 
Further, in the preferred mode where the track width defining grooves are 
formed on each magnetic core substrate at a given pitch so as to leave a 
flat portion having a width at least three times a track width on each 
magnetic core substrate, the pressure to be applied to each magnetic core 
substrate upon bonding the substrates together can be reduced by the flat 
portion, thereby easily controlling a stress to the magnetic core 
substrates. As a result, the magnetic characteristics of the magnetic core 
substrates can be optimally controlled. 
Other objects and features of the invention will be more fully understood 
from the following detailed description and appended claims when taken 
with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
There will now be described some preferred embodiments of the present 
invention with reference to the drawings. 
Using Mn--Zn monocrystal ferrite as a magnetic core material, the present 
inventors investigated the relation between an internal residual stress in 
the material and a permeability of the material. Typical examples of the 
Mn--Zn monocrystal ferrite may include so-called .beta.-ferrite having 
face orientations as shown in FIG. 6 and so-called J-ferrite having face 
orientations as shown in FIG. 7. The face orientations of the 
.beta.-ferrite and the J-ferrite are shown in Table 1. 
TABLE 1 
______________________________________ 
Gap forming Sliding Chip side 
surface surface surface 
______________________________________ 
.beta. 
(100) (110) (110) 
J (111) (211) (110) 
______________________________________ 
Using the .beta.-ferrite and the J-ferrite, the present inventors measured 
a permeability of the .beta.-ferrite by applying a stress thereto in a 
direction perpendicular to the gap forming surface of the .beta.-ferrite, 
that is, in the direction &lt;100&gt;, and also measured a permeability of the 
J-ferrite by applying a stress thereto in a direction perpendicular to the 
gap forming surface of the J-ferrite, that is, in the direction &lt;111&gt;. The 
results of measurement are shown in FIG. 18, wherein the relation between 
the permeability of the .beta.-ferrite and the stress applied is shown by 
a solid line, and the relation between the permeability of the J-ferrite 
and the stress applied is shown by a broken line. As apparent from FIG. 
18, the permeability in each case is dependent upon the stress applied. 
That is, the permeability tends to decrease with an increase in absolute 
value of the stress applied. 
The stress dependency of the permeability of the Mn--Zn monocrystal ferrite 
may be explained as follows: 
In general, the Mn--Zn monocrystal ferrite forming the magnetic cores has a 
relatively large magnetostriction constant of about 10.sup.-6. Therefore, 
apparent magnetic anisotropy changes with an internal residual stress 
remaining in the ferrite after applying a pressure thereto. 
In general, an apparent anisotropic energy K is expressed as follows: 
EQU K=K1-(3/2).lambda..sub.abc .multidot..sigma..sub.abc (1) 
where K1 represents magnetic anisotropy; .lambda..sub.abc represents a 
magnetostriction constant in a direction &lt;abc&gt; perpendicular to the gap 
forming surface; and .sigma..sub.abc represents a stress in the direction 
&lt;abc&gt;. 
In general, the relation between a permeability .mu. and an anisotropic 
energy K in a frequency region dominated by a rotating magnetization stage 
is expressed as follows: 
EQU .mu..alpha.1/K (2) 
That is, the permeability .mu. is in reverse proportion to the anisotropic 
energy K. 
In the Mn--Zn monocrystal ferrite used for the measurement of the 
permeability, the magnetic anisotropy K1 is positive, and the 
magnetostriction constant .lambda..sub.100 in the direction &lt;100&gt; is 
negative. Assuming that the stress in the direction of extension is 
positive, it is understood from Expression (1) that when the stress 
.sigma..sub.100 in the direction &lt;100&gt; increases in the direction of 
extension (i.e., in the positive direction), the anisotropic energy K in 
the direction &lt;100&gt; increases with the increase in the stress 
.sigma..sub.100. Accordingly, it is understood from Expression (2) that 
the permeability .mu. decreases with the increase in the anisotropic 
energy K. Further, it is understood from Expression (1) that when the 
stress .sigma..sub.100 increases in the direction of compression (i.e., in 
the negative direction), the anisotropic energy K decreases until the 
stress .sigma..sub.100 reaches a certain value, then inverting the sign to 
start increasing. Accordingly, it is understood from Expression (2) that 
when the stress .sigma..sub.100 increases in the direction of compression, 
the permeability .mu. increases to a certain value and then decreases. 
These results agree with the relation between the change in the 
permeability of the .beta.-ferrite and the change in the stress in the 
direction &lt;100&gt; as shown by the solid line in FIG. 18. 
On the other hand, the magnetostriction constant .lambda..sub.111 in the 
direction &lt;111&gt; is positive. Assuming that the stress in the direction of 
extension is positive, it is understood from Expression (1) that when the 
stress .sigma..sub.111 in the direction &lt;111&gt; increases in the direction 
of extension (i.e., in the positive direction), the anisotropic energy K 
in the direction &lt;111&gt; decreases until the stress .sigma..sub.111 reaches 
a certain value, then inverting the sign to start increasing. Accordingly, 
it is understood from Expression (2) that when the stress .sigma..sub.111 
increases in the direction of extension, the permeability .mu. increases 
to a certain value and then decreases. Further, it is understood from 
Expression (1) that when the stress .sigma..sub.111 increases in the 
direction of compression (i.e., in the negative direction), the 
anisotropic energy K in the direction &lt;111&gt; increases with the increase in 
the stress .sigma..sub.111 Accordingly, it is understood from Expression 
(2) that the permeability .mu. decreases with the increase in the 
anisotropic energy K. These results agree with the relation between the 
change in the permeability of the J-ferrite and the change in the stress 
in the direction &lt;111&gt; as shown by the broken line in FIG. 18. 
The present inventors prepared two kinds of magnetic heads using the 
.beta.-ferrite and the J-ferrite having the face orientations shown in 
Table 1 by the conventional manufacturing method, and evaluated the 
reproduction efficiencies of these magnetic heads. As the result, it was 
confirmed that the reproduction efficiency of the magnetic head using the 
.beta.-ferrite having the face orientation (100) as the gap forming 
surface is higher by 10 dB than the reproduction efficiency of the 
magnetic head using the J-ferrite having the face orientation (111) as the 
gap forming surface. This result indicates that the permeability of the 
.beta.-ferrite is higher than that of the J-ferrite. Further, this result 
agrees with the relation in permeability between the .beta.-ferrite and 
the J-ferrite shown in FIG. 18 in the case where the stress in the 
direction perpendicular to the gap forming surface is applied in the 
direction of compression, and it was confirmed that the stress in the 
direction perpendicular to the gap forming surface remains in each 
magnetic head. 
Now, a first preferred embodiment of the manufacturing method for the 
magnetic head according to the present invention will be described with 
reference to FIGS. 12 to 17. 
First, a substrate 3 formed of ferrite as shown in FIG. 12 is prepared, so 
as to form a pair of magnetic cores. Then, a plurality of track width 
defining grooves 4 for defining a track width of each magnetic core are 
formed on an upper surface of the substrate 3 so as to be arranged at a 
given pitch in a lateral direction of the substrate 3 and extend in a 
longitudinal direction of the substrate 3. Each track width defining 
groove 4 has a substantially semi-circular cross section. 
Then, as shown in FIG. 13, coil grooves 5 and 6 for receiving coils and 
glass grooves 7 and 8 for receiving glass are formed on the upper surface 
of the substrate 3 so as to be arranged at a given pitch in the 
longitudinal direction of the substrate 3 and extend in the lateral 
direction of the substrate 3. Each of the coil grooves 5 and 6 has a 
substantially trapezoidal cross section, and each of the glass grooves 7 
and 8 has a substantially U-shaped cross section. Thus, the coil grooves 5 
and 6 and the glass grooves 7 and 8 extend in orthogonal relationship to 
the track width defining grooves 4 on the upper surface of the substrate 
3. Then, a plurality of gap forming surfaces 3a formed as a residual 
portion of the upper surface of the substrate 3 are mirror-finished. 
Each of the coil grooves 5 and 6 is formed at a position such that a front 
gap depth will become zero, and each of the glass grooves 7 and 8 is 
formed at a position such that a backgap depth will become a given value. 
Then, the substrate 3 is divided into a block 9 having the coil groove 5 
and the glass groove 7 as shown in FIG. 14 and a block 10 having the coil 
groove 6 and the glass groove 8 as shown in FIG. 15. 
Then, as shown in FIG. 14, a gap film 11 formed of a fusing glass is formed 
on the upper surface of the block 9 except the inner surfaces of the coil 
grooves 5 and the glass grooves 7 so as to have a thickness about half a 
gap length of the magnetic head by using a suitable thin film forming 
technique such as a magnetron sputtering process, thus forming a plurality 
of front gap forming surfaces 12 and a plurality of back gap forming 
surfaces 13. Similarly, a gap film 14 is formed on the upper surface of 
the block 10 to form a plurality of front gap forming surfaces 15 and a 
plurality of back gap forming surfaces 16 (see FIG. 15). 
Then, as shown in FIG. 15, both the blocks 9 and 10 are matched with each 
other so that the front gap forming surfaces 12 and 15 face each other and 
the back gap forming surfaces 13 and 16 face each other. Then, a pressure 
is applied to the blocks 9 and 10 in opposite directions depicted by 
arrows P1 in FIG. 15 in the presence of heat under predetermined 
conditions. 
As a result, the front gap forming surfaces 12 and 15 of the blocks 9 and 
10 are bonded together through the gap films 11 and 14, and the back gap 
forming surfaces 13 and 16 of the blocks 9 and 10 are also bonded together 
through the gap films 11 and 14. Thus, both the blocks 9 and 10 are bonded 
together at the gap forming portions only. 
Then, as shown in FIG. 16, glass rods formed of a fusing glass are inserted 
into a space defined by the coil grooves 5 and 6 and a space defined by 
the glass grooves 7 and 8, and the glass rods are then fused at a given 
temperature to be filled as a fusing glass 18 in the vicinity of a 
plurality of front gaps defined between the front gap forming surfaces 12 
and 15, in the vicinity of a plurality of back gaps defined between the 
back gap forming surfaces 13 and 16, and in a plurality of spaces defined 
by the track width defining grooves 4. Then, a tape sliding surface 19 of 
such an integrated body of the blocks 9 and 10 is subjected to cylindrical 
grinding, and the integrated body is then cut into chips as the magnetic 
cores. Thereafter, coils are located in the coil grooves 5 and 6 of each 
chip, thereby obtaining the magnetic head as shown in FIG. 17. 
As shown in FIG. 17, a pair of magnetic cores 20 and 21 are bonded to each 
other so that the front gap forming surfaces 12 and 15 face each other to 
define a front gap g.sub.1 therebetween and the back gap forming surfaces 
13 and 16 face each other to define a back gap g.sub.2 therebetween. 
Further, the fusing glass 18 as the nonmagnetic material is filled in the 
track width defining grooves 4 and in the vicinity of the front gap 
g.sub.1 and the back gap g.sub.2. Further, the coil grooves 5 and 6 for 
receiving the coils are formed on the opposed surfaces of the magnetic 
cores 20 and 21 so as to be opposed to each other. 
Some examples of the manufacturing method mentioned above will now be 
described. 
(EXAMPLE 1) 
A substrate formed of Mn--Zn monocrystal .beta.-ferrite was prepared, and a 
plurality of track width defining grooves were formed on the (100) surface 
of the substrate as a gap forming surface so that a track width may become 
20 .mu.m. Then, a plurality of coil grooves and glass grooves were formed 
on the (100) surface of the substrate so as to cross the track width 
defining grooves. Then, the substrate was divided into a pair of blocks 
each having every unit of the coil groove and the glass groove. 
Then, a gap film having a thickness (e.g., 1000 angstroms) half a gap 
length was formed on each block under the conditions of a gas pressure of 
0.8 Pa and a power density of 1 W/cm.sup.2, thus forming front gap forming 
surfaces and back gap forming surfaces. A fusing glass mainly composed of 
SiO.sub.2 and having a flexure point of 583.degree. C. and a glass 
transition point of 638.degree. C. was used as the material of the gap 
film. 
Then, both the blocks were matched with each other so that the front gap 
forming surfaces of both the blocks faced each other and the back gap 
forming surfaces faced each other. In this conditions heat and pressure 
were applied to the blocks at 640.degree. C. under tens of MPa for one 
hour. Then, Pb glass rods as a fusing glass having a glass transition 
point (560.degree. C.) lower than the flexure point of the fusing glass 
used for the gap film were located in the coil grooves and the glass 
grooves formed on the opposed surfaces of the blocks, and the Pb glass 
rods were heated at 560.degree. C. for one hour to be fused and filled in 
the vicinity of the magnetic gaps and in the track width defining grooves. 
At this time, the fusing glass forming the gap film was not fused again, so 
that the magnetic gaps were not opened. Then, a tape sliding surface of an 
integrated body of both the blocks was subjected to cylindrical grinding, 
and the integrated body was then cut into chips. Thereafter, coils were 
located in the coil grooves of each chip to obtain a magnetic head 
employing the Mn--Zn monocrystal .beta.-ferrite as the magnetic cores. 
(EXAMPLE 2) 
A substrate formed of Mn--Zn monocrystal J-ferrite was prepared, and a 
plurality of track width forming grooves were formed on the (111) surface 
of the substrate in the same manner as that in Example 1. Thereafter, the 
same process as that in Example 1 was carried out to obtain a magnetic 
head employing the Mn--Zn monocrystal J-ferrite as the magnetic cores. 
(Comparison 1) 
For the purpose of comparisons a magnetic head employing the Mn--Zn 
monocrystal .beta.-ferrite as the magnetic cores was made by the 
conventional manufacturing method as mentioned previously. That is, a 
plurality of track width defining grooves, coil grooves and glass grooves 
were formed on a substrate formed of Mn--Zn monocrystal .beta.-ferrite to 
prepare a pair of blocks. Then, a gap film of SiO.sub.2 was formed on each 
block to form gap forming surfaces, and both the blocks were matched with 
each other so that the gap forming surfaces of both the blocks faced each 
other. Then, Pb glass rods as a fusing glass having a glass transition 
point of 560.degree. C. were located in the coil grooves and the glass 
grooves of the blocks, and heat and pressure were applied to the blocks at 
560.degree. C. under tens of MPa for one hour to fuse the Pb glass rods 
and fill the same in the vicinity of the magnetic gaps and in the track 
width defining grooves. Then, a tape sliding surface of an integrated body 
of both the blocks was subjected to cylindrical grinding, and the 
integrated body was then cut into chips. Thereafter, coils were located in 
the coil grooves of each chip to obtain a magnetic head employing the 
Mn--Zn monocrystal .beta.-ferrite as the magnetic cores. 
(Comparison 2) 
Using Mn--Zn monocrystal J-ferrite rather than Mn--Zn monocrystal 
.beta.-ferrite as the material of the magnetic cores, a magnetic head was 
made by the same method as that in Comparison 1. 
The electromagnetic conversion characteristics of the magnetic heads 
obtained in Examples 1 and 2 and Comparisons 1 and 2 were evaluated. 
Comparing a reproduction output of the magnetic head employing the 
.beta.-ferrite in Example 1 with that of the magnetic head employing the 
.beta.-ferrite in Comparison 1, it was confirmed that the reproduction 
output in Example 1 was higher by 2 dB than that in Comparison 1. This 
result is considered to be due to the fact that the internal residual 
stress in the magnetic cores in Example 1 was reduced more greatly than 
that in Comparison 1 to improve the permeability of the magnetic cores. 
Further, comparing a reproduction output of the magnetic head employing 
the J-ferrite in Example 2 with that of the magnetic head employing the 
J-ferrite in Comparison 2, it was confirmed that the reproduction output 
in Example 2 was higher by 10 dB than that in Comparison 2. This result is 
similarly considered to be due to the fact that the internal residual 
stress in the magnetic cores in Example 2 was reduced more greatly than 
that in Comparison 2 to improve the permeability of the magnetic cores. 
In the above examples, Mn--Zn monocrystal ferrite is used as the magnetic 
material for forming the magnetic cores. However, the magnetic cores may 
be formed of combined monocrystal ferrite and polycrystal ferrite so that 
a front end portion only of each magnetic core forming the front gap is 
formed of the monocrystal ferrite and the other portion is formed of the 
polycrystal ferrite. 
Further, in the above examples, the gap film is formed as a single-layer 
film of fusing glass. However, the gap film may be formed as a 
double-layer film consisting of an SiO.sub.2 film as a base layer and a 
fusing glass film formed on the base layer. For example, the double-layer 
film may be obtained by forming the SiO.sub.2 film having a thickness of 
600 angstroms and then forming the fusing glass film having a thickness of 
400 angstroms by a suitable thin film forming method such as magnetron 
sputtering. 
Further, the gap film may be formed as a composite film consisting of a Cr 
film as a base layer and an Au film formed on the base layer. For example, 
the composite film may be obtained by forming the Cr film having a 
thickness of 600 angstroms and then forming the Au film having a thickness 
of 400 angstroms by a suitable thin film forming method such as 
sputtering. In this case, the bonding of the gap films formed on the pair 
of blocks may be carried out at 200.degree. to 300.degree. C. under tens 
of MPa. The Cr film as the base layer may be replaced by an SiO.sub.2 film 
as required. 
According to the manufacturing method in the above preferred embodiment, 
the magnetic gap forming portions of both the blocks are first bonded 
together, and the fusing glass is then filled in the vicinity of the 
magnetic gaps without applying a pressure to the blocks. Therefore, the 
internal stress remaining in the magnetic cores during the manufacture can 
be greatly reduced, so that no strain is generated in the magnetic gap 
forming portions of the magnetic cores to thereby improve the permeability 
of the magnetic cores. As a result, the reproduction efficiency of the 
magnetic head to be obtained can be improved. Further, the glass 
transition point of the fusing glass to be filled in the vicinity of the 
magnetic gaps is lower than the flexure point of the fusing glass to be 
used for the bonding of the magnetic gap forming portions. Therefore, even 
when the filling of the fusing glass in the vicinity of the magnetic gaps 
is carried out after the bonding of the magnetic gap forming portions, 
there is no possibility of the magnetic gaps opening, thus ensuring a 
sufficient bonding strength of the magnetic head. 
A second preferred embodiment of the manufacturing method according to the 
present invention will now be described with reference to FIGS. 19 to 24. 
Referring to FIG. 19, a plate-like magnetic core substrate 22 formed of an 
oxide magnetic material such as Mn--Zn monocrystal ferrite or Ni--Zn 
monocrystal ferrite is prepared, and an upper surface 22a of the substrate 
22 to be formed into a magnetic gap forming surface is mirror-finished. 
Then, a plurality of track width defining grooves 23 (according to the 
number of magnetic heads to be manufactured; two in this preferred 
embodiment) are formed on the upper surface 22a of the substrate 22 so as 
to be arranged at a given pitch P in a lateral direction of the substrate 
22 and extend in a longitudinal direction of the substrate 22. Each of the 
track width defining grooves 23 has a substantially arcuate cross section. 
More specifically, the track width defining grooves 23 are formed on the 
upper surface 22a of the substrate 22 in such a manner that a flat portion 
24 having a width at least three times a track width Tw of a magnetic gap 
g (see FIG. 21) is left in one pitch P on the upper surface 22a. 
Then, a plurality of track width defining grooves 25 are similarly formed 
on the upper surface 22a of the substrate 22 so as to be arranged at a 
given pitch in adjacent relationship to the track width defining grooves 
23 in the lateral direction of the substrate 22 and extend in the 
longitudinal direction of the substrate 22. Each of the track width 
defining grooves 25 also has a substantially arcuate cross section. 
Accordingly, a magnetic gap forming portion 26 having the track width Tw is 
formed between the track width defining grooves 23 and 25 adjacent to each 
other. 
Then, a coil groove 27 for receiving a coil and a glass groove 28 for 
receiving a glass rod are formed on the upper surface 22a of the substrate 
22 so as to cross the track width defining grooves 23 and 25. 
More specifically, the coil groove 27 has a substantially trapezoidal cross 
section such that it is formed by a bottom surface 27a as a coil mounting 
surface substantially parallel to the upper surface 22a and a pair of 
inclined side surfaces 27b and 27c continuing upwardly from the opposite 
side edges of the bottom surface 27a so as to be inclined with respect to 
the upper surface 22a. The coil groove 27 extends in the lateral direction 
of the substrate 22 so as to cross the track width defining grooves 23 and 
25. The inclined side surface 27b of the coil groove 27 also serves to 
define a depth of the magnetic gap g. 
On the other hand, the glass groove 28 has a substantially U-shaped cross 
section such that it is formed by a bottom surface 28a substantially 
parallel to the upper surface 22a and a pair of vertical side surfaces 28b 
and 28c continuing upwardly from the opposite side edges of the bottom 
surface 28a so as to be perpendicular to the upper surface 22a. The glass 
groove 28 extends in the lateral direction of the substrate 22 in parallel 
relationship to the coil groove 27 so as to cross the track width defining 
grooves 23 and 25. 
Then, the above process is similarly repeated to prepare another magnetic 
core substrate 32 having a plurality of track width defining grooves 33 
and 35, magnetic gap forming portions 36, a flat portion 34, a coil groove 
37 and a glass groove 38. 
Then, both the substrates 22 and 32 are matched with each other so that the 
magnetic gap forming portions 26 of the substrate 22 face the magnetic gap 
forming portions 36 of the substrate 32 as shown in FIG. 20. 
Before matching both the substrates 22 and 32 as mentioned above, a gap 
spacer (not shown) having a thickness corresponding to a predetermined gap 
length is interposed between the opposed surfaces of the substrates 22 and 
32 except the magnetic gap forming surfaces. Alternatively, gap films (not 
shown) such as SiO.sub.2 films may be formed on the opposed surfaces of 
the substrates 22 and 32. 
Then, glass rods (not shown) are inserted into a space defined by the coil 
grooves 27 and 37 and a space defined by the glass grooves 28 and 38, and 
the glass rods are then heated to be fused under the pressure applied to 
the lower surface of the substrate 22 and the upper surface of the 
substrate 32 as viewed in FIG. 20. 
Since the flat portions 24 and 34 each having a width at least three times 
the track width Tw are formed on the opposed surfaces of the blocks 22 and 
32, the pressure to be applied to the opposed surfaces to be bonded 
together can be reduced by the wide flat portions 24 and 34 to thereby 
greatly reduce a stress in the magnetic core substrates 22 and 32. 
Accordingly, the magnetic characteristics of the substrates 22 and 32 are 
not be deteriorated, and it is expected that the reproduction efficiency 
of the magnetic head to be obtained can be improved. Furthermore, since 
the pressure is reduced, there is no possibility of the magnetic gap 
forming portions 26 and 36 being deformed. 
After the glass rods are fused as mentioned above, both the substrates 22 
and 32 are bonded together by a fusing glass 29 as shown in FIG. 20. The 
fusing glass 29 is also filled in the track width defining grooves 23 and 
25 of the substrate 22 and the track width defining grooves 33 and 35 of 
the substrate 32, thereby ensuring a contact characteristic with respect 
to a magnetic recording medium. 
Then, an integrated body of the substrates 22 and 32 is cut into a 
plurality of head chips (two chips in this preferred embodiment) (not 
shown). Then, a magnetic recording medium sliding surface of each head 
chip is subjected to cylindrical grinding. Further, as shown in FIG. 21, a 
pair of recesses 42 and 52 for receiving coils are formed by cutting the 
substrates 22 and 32, thus obtaining a magnetic head. 
In the magnetic head thus manufactured, the pressure to be applied to the 
opposed surfaces of the substrates 22 and 32 in the glass fusing step can 
be reduced by the flat portions 24 and 34. Accordingly, the magnetic 
characteristics of the substrates 22 and 32 are prevented from being 
deteriorated, thereby obtaining a high reproduction efficiency. 
Furthermore, the magnetic gap forming portions 26 and 36 forming the 
magnetic gap g are not deformed by the pressure applied the glass fusing 
step, thereby maintaining the accurate track width Tw. 
FIG. 23 shows a conventional manufacturing method for a magnetic head, in 
which a plurality of track width defining grooves 302 are formed on a 
magnetic core substrate 301 at a given pitch P1, and a plurality of track 
width defining grooves 402 are similarly formed on a magnetic core 
substrate 401 at the same pitch P1. Both the substrates 301 and 401 are 
bonded together so that a magnetic gap g having a track width Tw is 
defined by the adjacent ones of the track width defining grooves 302 and 
the adjacent ones of the track width defining grooves 402. In this case, a 
bonding area of each of the substrates 301 and 401 in one pitch P1 is 
calculated as (Track width Tw).times.(Depth of a front gap FG+Depth of a 
back gap BG). Accordingly, a large pressure is applied to the magnetic gap 
forming surfaces of the substrates 301 and 401 to cause a deterioration in 
magnetic characteristics of the substrates 301 and 401 and a reduction in 
reproduction efficiency of the magnetic head. Furthermore, as shown in 
FIG. 24, the magnetic gap forming surface of the substrate 401 is deformed 
at its opposite ends 303 and 304 so as to penetrate into the substrate 301 
by the large pressure. 
There will now be described in more detail the reason why the stress damage 
of the substrates 22 and 32 can be reduced by forming the flat portions 24 
and 34 each having a width at least three times the track width Tw on the 
substrates 22 and 32. In the following description, the reduction in the 
stress damage of the substrate 22 only having the front portion 24 will be 
examined because the other substrate 34 has the same structure as that of 
the substrate 22. 
It is assumed that the pitch P of the grooves 23 formed on the substrate 22 
is set to be N times the pitch P1 (=Cw+Tw) of the grooves 302 shown in 
FIG. 23, wherein Tw represents the track width which is the width of each 
magnetic gap forming surface, and Cw represents the width of each track 
width defining groove 23 in the same plane as the magnetic gap forming 
surface. 
In the preferred embodiment, a bonding area S1 of the substrate 22 in one 
pitch P assuming that a length in the depth direction is 1 is expressed as 
follows: 
EQU S1=(Tw+Cw).times.N-2.times.Cw (3) 
On the other hand, in the related art shown in FIG. 23, a bonding area S2 
of the substrate 301 in one pitch P1 assuming that a length in the depth 
direction is 1 is expressed as follows: 
EQU S2=Tw.times.N (4) 
Accordingly, the ratio between the bonding areas S1 and S2 is expressed as 
follows: 
EQU S1/S2=(Tw+Cw).times.N-2.times.Cw!/(Tw.times.N)=(1+Cw/Tw)-Cw/Tw.times.2/N(5 
) 
When the track width Tw is reduced from 20 .mu.m to 10 .mu.m, it is 
understood from Expression (4) that the bonding area S2 in the related art 
becomes half that in the case of Tw=20 .mu.m, and accordingly the pressure 
in the bonding area S2 becomes twice that in the case of Tw=20 .mu.m. 
Further, in the case of Tw=10 .mu.m and Cw=190 .mu.m, the ratio of the 
bonding areas S1 and S2 is calculated from Expression (5) to give: 
EQU S1/S2=20-38/N (6) 
That is, the bonding area S1 in the preferred embodiment is (20-38/N) times 
the bonding area S2 in the related art. Further, when N=3 is inserted into 
Expression (6) (i.e., the pitch P is three times the pitch P1), the 
following value is given. 
EQU S1/S2-22/3 (7) 
That is, the bonding area S1 in the preferred embodiment is 22/3 times 
(i.e., a little over seven times) the bonding area S2 in the related art. 
In other words, the pressure in the bonding area S1 in the preferred 
embodiment is 3/22 times the pressure in the bonding area S2 in the 
related art. 
Further, in the case of Tw=20 .mu.m and Cw=190 .mu.m, the ratio of the 
bonding areas S1 and S2 becomes as follows: 
EQU S1/S2=25/6 (8) 
That is, the bonding area S1 in the preferred embodiment is 25/6 times 
(i.e., a little over four times) the bonding area S2 in the related art. 
In other words, the pressure in the bonding area S1 is 6/25 times the 
pressure in the bonding area S2. 
It is understood from Expression (6) that the ratio of the pressure F1 in 
the bonding area S1 to the pressure F2 in the bonding area S2 in the case 
of Tw=10 .mu.m and Cw=190 .mu.m becomes as follows: 
EQU F1/F2=(20-38/N).sup.-1 (9) 
For example, if the following inequality is given (i.e., if the pressure F1 
in the preferred embodiment is not greater than half the pressure F2 in 
the related art), 
EQU (20-38/N).sup.-1 .ltoreq.1/2 (10) 
then the value of N is calculated as follows: 
EQU N.gtoreq.38/18 (11) 
On the other hand, the width Fw of the flat portion 24 is expressed as 
follows: 
EQU Fw=(N-1).times.Tw+(N-2).times.Cw (12) 
In the case of Tw=10 .mu.m, Cw=190 .mu.m and N.gtoreq.38/18, it is 
understood from Expression (12) that the width Fw of the flat portion 24 
becomes about 32 .mu.m or more, which is at least three times the track 
width Tw=10 .mu.m. Accordingly, by setting the width Fw of the flat 
portion 24 to a value at least three times the track width Tw, the 
pressure F1 in the bonding area S1 can be reduced to a value not more than 
half the pressure F2 in the bonding area S2. Thus, the pressure applied to 
the opposed surfaces of both the magnetic core substrates in the glass 
fusing step according to the preferred embodiment can be greatly reduced 
to thereby easily control a stress in the magnetic core substrates. 
The present inventors actually prepared a ferrite head in accordance with 
the above preferred embodiment under the conditions of Tw=10 .mu.m, Cw=190 
.mu.m and N=3, and evaluated the reproduction efficiency of the ferrite 
head by using an 8-mm video tape recorder (NTSC). In this case, a gap 
length was set to 0.25 .mu.m and an inductance was 0.7 .mu.H (5 MHz). As 
the result of evaluation, an increase in reproduction output by 1.5 dB as 
compared with the case of N=1 in the related art was confirmed. This 
result is considered to be due to the fact that the pressure applied to 
the magnetic cores in the glass fusing step was controlled. 
In the manufacturing method according to the second preferred embodiment 
mentioned above, a closed magnetic circuit is formed in the magnetic head 
by the monocrystal ferrite used as the material of the magnetic cores. 
However, the present invention may be applied to a manufacturing method 
for a so-called metal-in-gap type magnetic head employing a metal magnetic 
thin film as the material for the magnetic cores. In this case, as shown 
in FIG. 22, metal magnetic thin films 30 and 40 are formed on the opposed 
surfaces of the substrates 22 and 32, respectively, after forming the 
track width defining grooves 23, 25, 33 and 35, the coil grooves 27 and 37 
and the glass grooves 28 and 38. Then, both the substrates 22 and 32 are 
matched with each other so that the metal magnetic thin films 30 and 40 
face each other. Thereafter, the same process as that of the second 
preferred embodiment is carried out to obtain the metal-in-gap type 
magnetic head employing the metal magnetic thin films 30 and 40 as main 
cores. 
The metal magnetic thin films 30 and 40 are formed of a known ferromagnetic 
alloy material having a high saturation magnetic flux density and a 
superior soft magnetic characteristic. Such a ferromagnetic alloy material 
may be crystalline or noncrystalline. Examples of the ferromagnetic alloy 
material may include crystalline alloy materials such as Fe alloys, Co 
alloys, Fe--Ni alloys, Fe--C alloys, Fe--Al--Si alloys, Fe--Ga--Si alloys, 
Fe--Al--Ge alloys, Fe--Ga--Ge alloys, Fe--Si--Ge alloys, Fe--Co--Si 
alloys, Fe--Ru--Ga--Si alloys and Fe--Co--Si--Al alloys, and may also 
include amorphous alloys such as Co--Zr--Nb alloys and Co--Zr--Nb--Ta 
alloys. Other amorphous alloys for general use may, of course, be used in 
the present invention. Such amorphous alloys may include metal-metalloid 
amorphous alloys such as alloys composed of at least one of Fe, Ni and Co 
and at least one of P, C, B and Si, or alloys mainly composed of these 
elements and further containing Al, Be, Sn, In, Mo, W, Ti, Mn, Cr, Zr or 
Hf, and may also include metal-metal amorphous alloys such as alloys 
mainly composed of transition elements such as Co--Zr or Co--Hf, or alloys 
mainly composed of these elements and further containing rare earth 
element. 
Further, in order to further increase the output of the magnetic head and 
avoid eddy current loss in a high band, the metal magnetic thin film may 
be formed in a multilayer structure consisting of multiple metal thin 
layers laminated together through insulating films. The insulating films 
may be formed of SiO.sub.2, Ta.sub.2 O.sub.5, Al.sub.2 O.sub.3, ZrO.sub.2 
or Si.sub.3 N.sub.4. The formation of each metal thin layer and each 
insulating film may be performed by a vacuum thin film forming technique 
such as vacuum deposition, sputtering, ion plating, or cluster ion beam 
processing. 
While the invention has been described with reference to specific 
embodiments, the description is illustrative and is not to be construed as 
limiting the scope of the invention. Various modifications and changes may 
occur to those skilled in the art without departing from the spirit and 
scope of the invention as defined by the appended claims.