Magnetic head with an improved inner core

In a magnetic head including an inner core of metallic magnetizable material sandwiched between a pair of outer reinforcing cores each having a coil winding insertion opening formed in it, the inner core includes a first core block which has a front gap defining surface and a back or rear gap defining surface respectively provided at one side and the other side of the first core block and a coil winding recess provided in a position adjacent to the front gap defining surface, and which is formed, between the front gap defining surface and the coil winding recess, with a space or groove for setting the lower end of the front gap defining surface and a brazing material layer for connecting the space or groove with the coil winding recess, and a second core block to be combined with the first core block so as to constitute the inner core.

BACKGROUND OF THE INVENTION 
The present invention generally relates to a magnetic head for use in 
magnetic recording and reproduction systems, for example, a video tape 
recorder and the like, and more particularly, to a magnetic head which can 
be applied to a high coercive tape such as the so-called metal tape, etc., 
of which coil force HC is more than 1000 Oe. 
Generally, for recording video information and the like on a high coersive 
tape such as a metal tape at a high density, it is necessary to employ a 
metallic magnetizable material such as sendust alloy (Fe-Al-Si alloy) 
formed into a configuration to suit to a narrow track width, for example, 
of 10 to 30.mu.. For the above purpose, the magnetic head therefor 
generally has a thin inner sendust core sandwiched between outer 
reinforcing cores. More specifically, the conventional inner sendust core 
shown, for example, in FIG. 1 is produced through the steps of filling a 
bonding or brazing material m such as silver alloy or the like into part 
of a coil winding recess h at the side of a back or rear gap Bg and also 
into a brazing material filling groove U which are formed in one sendust 
core block Sa, fitting said one sendust core block Sa onto the other or 
counterpart sendust core block Sb for brazing or welding therebetween, and 
subjecting the sendust core blocks Sa and Sb thus combined to a 
predetermined machining such as slicing or the like so as to obtain core 
chips of required size. However, the magnetic head which includes the 
known inner core as in FIG. 1 constituted by brazing together the two 
sendust core blocks Sa and Sb each made of thin metallic magnetizable 
material of 10 to 30.mu. in thickness, only at the back gap side Sb 
thereof, with the coil winding recess h provided at its intermediate 
portion, has a problem in mechanical strength owing to absence of any 
junction at its front gap side Fg, this giving rise to a possibility that 
a positional deviation may take place at said front gap side in the course 
of the machining or during use. Meanwhile, even if it is attempted to 
increase the mechanical strength by increasing the region to be occupied 
by the brazing material m in the coil winding recess h, the output is 
undesirably reduced due to decrease of the coil winding region in the 
recess h. 
It should be noted here that, in FIG. 1 and other figures referred to 
hereinbelow, although the edges at the front gap sides of the inner cores 
and of the magnetic heads constituted thereby, on which magnetic tapes are 
slidingly moved, are generally shown as squarish or angular for a 
schematic representation, such edges are, of course, curved into an 
arcuate configuration for smooth passage of the tapes in the actual 
products. 
In order to overcome the disadvantages as described above, there has 
conventionally been proposed an arrangement in which the two core blocks 
are joined together by brazing material disposed at approximately equal 
intervals on the confronting faces or edges of said core blocks, but such 
as arrangement as described above, although favorable for maintaining a 
sufficient mechanical strength, tends to impair the reproduction output 
characteristics of magnetic head in high frequency region, since the 
junctions provided in the vicinity of the coil winding recess give rise to 
magnetic loss with respect to magnetic flux of short wave lengths in the 
video region. 
In another known inner core, it is so arranged that brazing material 
filling grooves are provided in the first core block at its back gap and 
front gap sides, and also in a position between the back gap side and 
front gap side, while a coil winding recess is formed in the central 
groove after filling the brazing material into said brazing material 
filling grooves, and then, the first core block is brazed together with 
the second core block through gap distance defining layers formed on the 
front and back gap forming surfaces for subsequent machining of the inner 
core thus prepared into the predetermined shape. By the above known 
arrangement, although the mechanical strength may be improved owing to the 
provision of the junction also at the side of the front gap forming 
surface with respect to the coil winding recess, the magnetic 
characteristics of the resultant inner core show a marked deterioration at 
the high frequency region as shown in the dotted curve B in FIG. 8, which 
is attributable to the fact that the brazing material tends to flow into 
the diffusion prevention groove and coil winding groove during brazing to 
form a diffusion layer thereof to a large extent in the core block, thus 
adversely affecting the magnetic characteristics at the front gap portion. 
SUMMARY OF THE INVENTION 
Accordingly, an essential object of the present invention is to provide an 
improved inner core of a magnetic head which is capable of increasing 
mechanical strength at its front gap portion, with a sufficient coil 
winding region provided. 
Another important object of the present invention is to provide an improved 
inner core of a magnetic head of the above described type which is capable 
of reducing passing loss of high frequency magnetic flux, while 
maintaining a sufficient mechanical strength, without deterioration of 
magnetic characteristics thereof. 
A further object of the present invention is to provide an improved inner 
core of a magnetic head of the above described type which is simple in 
construction and stable in functioning at high reliability, and can be 
manufactured on a large scale at low cost. 
In accomplishing these and other objects, according to one preferred 
embodiment of the present invention, there is provide a magnetic head 
including an inner core of metallic magnetizable material sandwiched 
between a pair of outer reinforcing cores each having a coil winding 
insertion opening formed therein, and the inner core comprises a first 
core block which has a front gap defining surface and a back gap defining 
surface respectively provided at one side and the other side of said first 
core block and a coil winding recess provided in a position adjacent to 
the front gap defining surface, and which is formed, between the front gap 
defining surface and the coil winding recess, with a space or groove for 
setting the lower end of the front gap defining surface and a brazing 
material layer for connecting the space or groove with the coil winding 
recess, and a second core block to be combined with the first core block 
so as to constitute the inner core. 
By the arrangement according to the present invention as described above, a 
magnetic head having an improved inner core is advantageously presented, 
with substantial elimination of disadvantages inherent in the conventional 
magnetic heads of this kind.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings, there is shown in FIGS. 2 and 3 the 
construction of a magnetic head to which an improved inner core directly 
related to the present invention may be applied. In FIGS. 2 and 3, the 
magnetic head generally includes the improved inner core according to the 
present invention, for example, a core C1 to be described in detail later 
with reference to FIG. 1 and sandwiched between side cores or reinforcing 
cores P1 and P2, each of which is constituted by a rectangular lower 
portion Pb, for example, of ferrite material suitably combined with an 
upper portion Pa of glass material and the like and formed with a coil 
winding insertion opening Po in a position corresponding to a coil winding 
opening 6 of the inner core C1 as shown. 
Referring particularly to FIGS. 4 to FIG. 6(g), the inner core C1 of FIG. 4 
according to the present invention is produced by a process comprising the 
steps of firstly forming, in one surface of a sendust core block C1-a, a 
wide groove 1, a set of spaced narrow grooves 2a and 2b, and a bevelling V 
at the corner adjacent to a back gap defining side 9 of said core block 
C1-a for filling bonding or brazing material therein, which are provided 
in parallel relation to each other and extending over the entire width of 
the core block C1-a (FIG. 6(a)), subsequently forming, in the wide groove 
1 provided adjacent to a front gap defining side 7 of the core block C1-a, 
in the set of spaced narrow grooves 2a and 2b, and on the bevelling V at 
the gap defining side 9, plated or deposited copper layers l1, l2, l3 and 
l4 each having a thickness of approximately more than 10.mu. (FIG. 6(b)), 
applying the brazing material onto the copper layers l1 to l4 as shown at 
1a, 4a, 4b and Va (FIG. 6(c)), subsequently forming, in the wide groove 1, 
a brazing material diffusion preventing groove 5 and a coil winding recess 
6 having a depth more than three times that of the groove 1 (FIG. 6(d)) so 
as to provide layers 8 and 3 of the brazing material between the groove 5 
and one edge of the recess 6, and also at the other edge of said recess 6 
respectively, polishing and flattering the surface of the core block C1-a 
thus processed (FIG. 6(e)), providing a spacer (not shown), for example, 
of SiO.sub.2 on the front and back gap defining surfaces 7 and 9 for 
restricting the gap distance, brazing the core block C1-a thus prepared 
with a counterpart core block C1-b (FIG. 6(f)) to consititute the inner 
core, and subjecting said inner core to machining such as slicing for 
obtaining core chips of predetermined dimensions. (FIG. 6(g)). 
It should be noted here that the brazing material diffusion preventing 
groove 5 defines, at its edge 5a (FIG. 5), the lower end of the front gap 
defining surface 7, and the width of said groove 5 which is formed, for 
example, by etching or the like with less processing strain is selected to 
be in such an extent that, during the brazing of the core block C1-a to 
the counterpart core block C1-b, the front gap defining surface 7 is not 
affected by the fused brazing material 8. More specifically, even when a 
diffusion layer (not shown) tends to be formed from the fused brazing 
material 8 towards the front gap defining surface 7 during the brazing, 
such a diffusion layer does not reach the surface 7 having a large 
influence on the magnetic characteristics by the presence of the groove 5, 
and thus, the deterioration in the magnetic characteristics is 
advantageously prevented. 
It should also be noted that, in the embodiment of FIG. 4, since the 
brazing material layer 8 connects the core block C1-a with the mating core 
block C1-b at the side at the coil winding recess 6 adjacent to the front 
gap defining surface 7, the mechanical strength at said portion may be 
markedly improved. Furthermore, during the connection of the core blocks 
C1-a and C1-b, the layers of the brazing material 8 and 3 are fused to the 
corresponding portions of the core block C1-b without flowing into the 
coil winding recess 6, and therefore, said recess 6 is provided with a 
sufficient space, without reduction of the number of turns of the coil 
winding and without consequent reduction of the reproduction output. 
It should further be noted that, although the manufacturing process of 
FIGS. 6(a) to 6(g) may be so modified that the brazing material is 
directly applied onto the portions to be formed with the brazing material 
layers such as 8, 3, 4a, 4b and Va for subsequent heating, the brazing 
material tends to be partially solidified into granular shape in that case 
due to poor wetting characteristics between said brazing material and the 
metallic magnetizable material (i.e. sendust) of the core blocks, with 
consequent difficulties in perfectly filling such portions. According to 
the process of FIGS. 6(a) to 6(g), since the copper layers l1 to l4 
superior in wetting characteristics are preliminarily applied to the 
portions 8, 3, 4a, 4b and Va as described earlier, the brazing material 
placed on these copper layers l1 to l4 for melting is prevented from being 
formed into granular form by the copper layers so as to be uniformly fused 
in the directions in which said portions or grooves extend for perfect 
filling thereof. 
Referring to FIG. 7, there is shown a modification of the inner core C1 of 
FIG. 4. In the modified inner core C2 in FIG. 7, the grooves 5, 2a and 2b 
and the silver alloy brazing material layers 8, 3, 4a and 4b are replaced, 
for example by welding or brazing spots w1, w2, w3, w4, w5, w6 and w7 of 
silver alloy brazing material, provided between the core blocks C2-a and 
C2-b as shown, while the distribution of the brazing spots w1 to w7 is 
arranged to be sparse in the vicinity of the coil winding recess 6, and to 
be dense towards the bottom side of the back gap defining surface 9, with 
the ratio of the brazing spot intervals being, for example, 7:5:3:1:1 as 
viewed from the side of the brazing spot w2 at the lower portion of the 
recess 6. In this modification, the gap depth d is set to be 40.mu. and 
the gap length or distance between the confronting core blocks C2-a and 
C2-b is set at 0.3.mu.. 
Referring also to FIG. 8 showing the relation between the maximum 
reproduction output and frequency characteristics with respect to the 
modified inner core C2 of FIG. 7 according to the present invention and 
the conventional inner core having the equal brazing spot interval ratio, 
it is seen that the characteristics of the inner core C2 of the present 
invention as represented by a solid line curve A are superior especially 
in the high frequency region as compared with those of the conventional 
inner core denoted by a dotted line curve B. The above superiority of the 
inner core C2 of the present invention is considered to be attributable to 
the fact that the magnetic loss due to the brazing spots present in the 
vicinity of the coil winding recess 6 as observed in the conventional 
inner core is reduced according to the arrangement of the present 
invention. Furthermore, as a result of a mechanical load resistance test 
carried out by the present inventors on the conventional inner cores and 
the inner core C2 according to the present invention, the inner core C2 of 
the present invention showed a larger resistance than the conventional 
inner cores partly due to the fact that the silver alloy brazing material 
is employed. 
As is seen from the foregoing description, in the modified inner core C2 of 
FIG. 7, since the brazing spots for brazing together the core blocks C2-a 
and C2-b are arranged to be sparsely distributed in the vicinity of the 
coil winding recess and densely distributed at the bottom side of the back 
gap defining surface, the magnetic loss at the high region can be reduced, 
with consequent improvement of the high frequency region characteristics 
as shown by the solid curve A in FIG. 8 as well as that of the mechanical 
strength. 
It is needless to say that, the mechanical strength of the inner core C1 of 
FIG. 4 may further be improved, if the brazing spots such as w5, w6 and w7 
of FIG. 7 are further provided in the vicinity of the bottom side of the 
back gap defining surface 9 between the groove 2b and the bevelled portion 
V in the arrangement of FIG. 4. 
Referring to FIGS. 9(a) to 9(c), there is shown a further modification of 
the inner core C1 of FIG. 4. In the modified inner core C3 in FIGS. 9(a) 
to 9(c), there are provided a first groove 5B formed in the surface of the 
core block C3-a between the front gap defining surface 7 and the coil 
winding recess 6 for setting said front gap defining surface, and a second 
groove 8g formed at the side of the coil winding recess 6 in a position 
spaced by a predetermined distance from said first groove 5B, and thus it 
is intended to prevent the deterioration of the characteristics by 
arranging so that the front gap portion is not magnetically affected by 
diffusion layer during brazing of the one set of core blocks C3-a and C3-b 
to each other by utilizing said second groove 8g, and simultaneously to 
increase the mechanical strength at the front gap side 7 by providing a 
brazed portion at said front gap side. 
The modified inner core C3 of FIGS. 9(a) to 9(c) is produced generally in 
the similar manner as in the inner core C1 of FIG. 4, but in this 
modification, the brazing materials 8 and 3 are arranged to be filled in 
grooves 8g and 3g formed adjacent to the edges of the coil winding recess 
6, while the brazing material diffusion preventing groove 5B, and a 
brazing material filling groove g in which the brazing material 10 is 
filled, are formed in parallel and spaced relation with respect to said 
groove 8g as shown. 
The brazing material diffusion preventing groove 5B is the first groove 
which sets by its edge 5Ba, the lower end or depth end of the front gap 
defining surface 7 between the grooves g and 5B, while the groove 8g is 
the second groove which is formed in the core surface between the front 
gap defining surface 7 and coil winding recess 6 at a position spaced by a 
predetermined distance l from said first groove 5B so that the diffusion 
due to the brazing material 8 does not reach the front gap defining 
surface 7. The distance m between the second groove 8g and the one edge of 
the coil winding recess 6 is also set to be approximately equal to said 
distance l. In this modification, the above distance l and m are each 
arranged to be approximately 10 to 40.mu.. 
During brazing of the core block C3-a prepared in the above described 
manner with the counterpart core block C3-b (FIG. 9(c)) and subsequent 
slicing of the core C3 thus produced into core chips of the required size, 
non-magnetizable layer n integral with the sendust core material due to 
the diffusion of the brazing material is formed in the regions between the 
groove 5B and second groove 8g and between the second groove 8g and coil 
winding recess 6, and thus, magnetic short-circuiting at the above regions 
is prevented. On the other hand, adverse effect to the front gap defining 
surface 7 due to the diffusion is perfectly eliminated by the first groove 
5B serving for the preventing of such diffusion, while there is not any 
possibility that the silver alloy blazing material flows into the coil 
winding recess 6. Accordingly, in the arrangement of FIGS. 9(a) to 9(c), 
the deterioration in the magnetic characteristics due to the diffusion 
layer is advantageously prevented, and the improvement of the 
characteristics at the high frequency region as represented by the curve B 
in FIG. 8 referred to earlier has been achieved. 
After completion of the inner core C3, one end portion thereof including 
said groove g is cut off during the machining as shown by a chain line in 
FIG. 9(c) at a position adjacent to the front gap defining surface 7. 
As is clear from the foregoing description, in the inner core C3 of FIGS. 
9(a) to 9(c), since the core block C3-a is arranged to be secured to the 
counterpart core block C3-b at the side of the front gap defining surface 
of the coil winding recess, there is no possibility of opening or 
deviation in the gap due to warping or curving of the inner core, even 
when the core is formed into very thin core chips, with a sufficient 
mechanical stability, while the adverse effect due to diffusion of the 
brazing material is eliminated by preventing entry of the brazing material 
onto the front gap defining surface from the second groove through 
provision of the first or brazing material diffusion preventing groove 
which sets the lower end of said front gap defining surface, and thus, the 
undesirable deterioration of the magnetic characteristics has also been 
prevented. 
Although the present invention has been fully described by way of example 
with reference to the attached drawings, it is to be noted that various 
changes and modifications will be apparent to those skilled in the art. 
Therefore, unless otherwise such changes and modifications depart from the 
scope of the present invention, they should be construed as included 
therein.