Magnetic head which minimizes the effect of minute variations in the head gap

The magnetic head of the present invention has a core thickness set up in accordance with the range of the permeability drop starting frequency of the core material which is determined to reduce the influence imparted to the reproducing frequency characteristic of the head by any minute variation in the technique of making the head gap. The core thickness of the magnetic head may be attained by using a smaller number of superposed layers to form the core than in a conventional magnetic head, and in some cases, a single layer of the core may be used to provide the track width.

BACKGROUND OF THE INVENTION 
a. Field of the Invention 
This invention relates to a magnetic head having a good reproducing 
frequency characteristic. 
B. Description of the Prior Art 
Generally considered, the primary factors which affect the reproducing 
frequency charactristic of a magnetic head include the core loss resulting 
from the eddy current loss and the gap loss attributable to the operating 
gap width of the core. In the conventional method of making a magnetic 
head, a greater number of thin layers are used to form the core as a 
better frequency characteristic is desired and, thereby, the core loss can 
be reduced to a negligible extent. Thus, efforts have heretofore been 
devoted solely to reducing the gap loss in improving the reproducing 
frequency characteristic of the magnetic head. 
In making a magnetic head, it is a requisite to form the core with a 
uniform gap width which ensures the best reproducing frequency 
characteristic as noted above, but as a matter of course, errors occur in 
the operating gap width of the core depending on the manufacturing 
technique and it has been extremely difficult to to minimize such errors, 
say, to 0.5 .mu.m or less, even in the magnetic heads produced on a mass 
production scale by the best manufacturing method available at present. 
However, as the recording wavelength of the tape approaches the operating 
gap width of the magnetic head on which a magnetic tape slides, the 
reproducing frequency characteristic of the magnetic head is greatly 
aggravated by the gap loss Lg which is represented by the following 
##EQU1## 
WHERE G IS THE OPERATING GAP WIDTH AND .lambda. THE RECORDING WAVELENGTH. 
Therefore, the above-mentioned variation in the operating gap width 
seriously affects the reproducing frequency characteristic, and this has 
become a very serious problem in considering the characteristic of a tape 
recorder, such as especially a cassette tape recorder, in which the tape 
speed is slow but the required upper limit of the reproducing frequency 
has become 10 to 16 KHz, and it has thus become desirable to minimize the 
influence imparted to the reproducing frequency characeristic by the 
variation in the operating gap width. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a magnetic head which 
has a core thickness set up so as to prevent the irregularity of the gap 
width unavoidable in the manufacturing technique from seriously affecting 
the overall reproducing frequency characteristic of the head. 
It is another object of the present invention to provide a magnetic head 
which may be formed by a decreased number of superposed core layers, 
thereby improving the working yield and facilitating maintenance of the 
thickness precision of the core forming the track width, as well as 
reducing the working cost and enabling a hard material like Sendust to be 
used as the core material for the magnetic head. 
According to the present invention, the magnetic head comprises a highly 
permeable material having a DC specific permeability ranging from 8000 to 
50000, a magnetic core formed of the highly permeable material and 
provided with an operating gap of 1 .mu.m to 2.5 .mu.m to form a 
ring-shaped closed magnetic circuit, a core thickness dimension set up so 
that the permeability drop starting frequency of the magnetic core 
resulting from the core loss is 10 to 200 Hz, and a coil wound on the 
magnetic core having the core thickness dimension. The core thickness 
dimension is equal to the track width. 
The invention will become more fully apparent from the following detailed 
description thereof taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1 which is a pictorial perspective view of the magnetic 
head according to the present invention, core halves 11 and 11' are not 
superposed upon each other but opposed to each other to form a magnetic 
path 11-1 and gap 12 and a coil 13 is wound on the core halves. 
In the ensuing description of the present invention, analysis will be made 
of the influence of the core loss and gap loss imparted to the reproducing 
frequency characteristic. 
The frequency characteristic of the permeability of a magnetic material may 
generally be known by making the magnetic material into a predetermined 
thickness d, forming therein a closed magnetic circuit into a ring-like 
shape, winding a coil thereon and measuring the inductances thereof for 
various frequencies, and such characteristic is usually such as shown in 
FIG. 2. 
In FIG. 2, curve A (solid line) represents the frequency characteristic of 
the specific permeability of a core material which is 78% Permalloy having 
a thickness of 0.15 mm and curve B (broken line) represents the frequency 
characteristic of the specific permeability of a core material which is 
Sendust having a thickness of 0.6 mm. In the graph of FIG. 2, fcA and fcB 
represent the frequencies at which the drop of permeability attributable 
primarily to the eddy current loss and the skin effect of the magnetic 
flux resulting therefrom starts to take place for the core materials and 
core thicknesses represented by the curves A and B, and such frequencies 
become lower as the core thicknesses are increased. (Even if the portion 
of the Sendust having a core thickness of 0.6 mm is substituted for by 78% 
Permalloy having a core thickness of 0.6 mm, no variation will occur in 
the characteristics and the difference in characteristic between the two 
materials depends primarily on the core thickness. Thus, in FIG. 2, the 
difference between fcA and fcB, namely, the difference between the curves 
A and B, results from the difference in core thicknesses and does not mean 
the difference in quality of material.) Now, let fc generally represent 
the frequency at which such drop of permeability starts to take place. 
Usually, at the frequencies below fc, the specific permeability .mu.s is 
nearly constant and substantially equal to DC specific permeability 
.mu.so, and at the frequencies above fc, the specific permeability .mu.s 
is decreased in proportion to f.sup.-.alpha. (.alpha..div.0.6 to 0.7). 
This may be formulated as follows: 
EQU When f&lt;fc, .mu.s .div. .mu.so (2) 
.mu.so . . . DC specific permeability 
EQU When f&gt;fc, .mu.s .div. .mu.so (f/fc).sup.-.alpha. (2)' 
where .alpha.=0.6 to 0.7 
Now, the reproducing efficiency resulting from core loss will be 
considered. 
The relation between the reproducing magnetic flux .PHI..sub.1 of a usual 
ring-shaped magnetic head and the constant magnetic flux .PHI..sub.0 
imparted to the operating gap portion of the head may be represented by 
the use of an equivalent circuit as shown in FIG. 3. 
In FIG. 3, Rc represents the magnetic resistance of the core decreased by 
the core loss, Rg the magnetic resistance of the operating gap, and 
.phi..sub.0 the recording residual flux on the tape but, for simplicity of 
illustration, various leakage fluxes are not shown. 
From FIG. 3, the relation between .PHI..sub.1 and .PHI..sub.0 is given as: 
##EQU2## 
From this, the reproducing efficiency .eta.=I.sub.1 /I.sub.0 of the head 
may be deduced thus: 
##EQU3## 
On the other hand, the specific permeability .mu.s of the core may be 
represented by the relation of equation (2)' for f when f&gt;fc, as already 
noted, but the magnetic resistance Rc and the specific permeability .mu.s 
are inversely proportional to each other and usually, Rc is expressed as: 
##EQU4## 
Thus, where the magnetic resistance of the core when f&lt;fc (when .mu.s = 
.mu.so) is Rco, there is the following relation: 
##EQU5## 
Therefore, the reproducing efficiency .eta. of the head is given by the 
equations (2)', (4), (4)' and (3)', as follows: 
##EQU6## 
From this, it is seen that Rg is substantially proportional to the width of 
the operating gap, so that an increase in the width of the operating gap 
brings about an increase in the reproducing efficiency .eta., thus 
decreasing the core loss. 
A specific example will now be mentioned. In the construction of a commonly 
available cassette stereo head (gap width g=1.3 .mu.m, 78% Permalloy, 
thickness 0.15 mm, four layers superposed), m.sub.o is in the range of 
0.02 to 0.04 and fc ranges from 500 to 2000 Hz, and the typical core loss 
characteristic thereof is such as shown by curve A-1 in FIG. 4, while in 
the case of Sendust having a thickness of 0.6 mm, a single layer and gap 
width of 1.3 .mu.m, m.sub.o equals 0.04 and fc ranges from 20 to 40 Hz and 
the core loss characteristic thereof is such as shown by curve B-1 in FIG. 
4. In FIG. 4, the cases of g=1.3 .mu.m (curves A-1, B-1) and g=1.9 .mu.m 
(curves A-2, B-2) are comparatively shown (A refers to the case of 
Permalloy having a thickness of 0.15 mm and four layers while B refers to 
the case of Sendust having a thickness of 0.6 mm and a single layer) and 
it can be seen that the smaller the core thickness and the greater the 
operating gap width g, the better the core loss characteristic. 
The gap loss will now be considered. The gap loss represented by equation 
(1) shown in the Description of the Prior Art is determined by the 
operating gap width g of the core and the recording wavelength .lambda. 
for the tape independently of the material of the core, as is clear from 
equation (1). Incidentally, if g=1.5 .mu.m and .lambda..div.3.4 .mu.m, Lg 
at f=14 KHz is thus: 
EQU Lg = -2.98dB .div. -3.0dB 
if the operating gap width is increased by 0.5 .mu.m, 
EQU g = 2.0 .mu.m 
EQU Lg' = -5.67dB 
Thus, Lg is increased by 2.67dB and the reproducing frequency 
characteristic is correspondingly decreased. 
FIG. 5 illustrates the gap loss characteristics at various frequencies when 
the gap width g=1.3 .mu.m (curve C-1) and g=1.9 .mu.m (curve C-2). From 
this graph, it is seen that an increased gap width g results in aggravated 
gap loss. 
While the individual factors for the core loss and the gap loss have been 
discussed, the overall reproducing frequency characteristic is determined 
by the sum of these two factors. FIG. 6 illustrates the composite loss 
curve which is the sum of the core loss and the gap loss curve in FIG. 5. 
In the graph of FIG. 6, solid line A-1 indicates the composite loss curve 
in the case of Permalloy having a thickness of 0.15 mm, four layers and 
g=1.3.mu., dot-and-dash line A-2 indicates the composite loss curve in the 
case of Permalloy having a thickness of 0.15 mm, four layers and 
g=1.9.mu., solid line B-1 indicates the composite loss curve in the case 
of Sendust having a thickness of 0.6 mm, a single layer and g=1.3.mu., and 
broken line B-2 indicates the composite loss curve in the case of Sendust 
having a thickness of 0.6 mm, a single layer and g=1.9.mu.. 
In addition to these two types of loss, there is the clearance loss 
resulting from the clearance between the tape and the head, but such loss 
is not appreciable in a sufficiently well finished head and may be 
neglected. 
FIG. 7 shows the deviation of the composite loss curve for g=1.9.mu. with 
the composite loss curve for g=1.3.mu. as the reference. In the graph of 
FIG. 7, the composite loss curves, for g=1.3.mu., of Sendust having a 
thickness of 0.6 mm and a single layer and Permalloy having a thickness of 
0.15 mm and four layers are brought into accord with the O-axis. In the 
graph, the curve A refers to the case of 78% Permalloy having a thickness 
of 0.15 mm and four layers, and the curve B refers to the case of Sendust 
having a thickness of 0.6 mm and a single layer. As seen, in the Sendust 
having the thickness of 0.6 mm and a single layer (curve B), the increase 
in gap loss resulting from the increase in gap width is more greatly 
offset by the decrease in core loss and therefore, the use of this 
material can more greatly reduce the influence imparted to the reproducing 
frequency characteristic by the variation in gap width which is 
unavoidable in the manufacturing process, than the use of the Permalloy 
core having the thickness of 0.15 mm and four layers (curve A). 
FIG. 8 clearly shows the permeability drop starting frequency of the 
present invention in the frequency characteristic graph of FIG. 2 
concerning the specific permeability. In Permalloy or Sendust having a DC 
specific permeability of 40000, the shaded portion defined between the 
curve B1 for the permeability drop starting frequency 10 Hz and the curve 
B2 for the permeability drop starting frequency 200 Hz represents the 
range over which the core thickness of the present invention is provided. 
In FIG. 8, it is seen that the curve B of FIG. 7 is represented by the 
curve B and this curve B is contained within the range of the present 
invention. Of course, the curve A which is representative of the 
conventional core comprising superposed layers of small core thickness is 
not contained within the range of the present invention. Thus, the 
magnetic head having the core thickness of the present invention can more 
greatly reduce the influence imparted to the reproducing frequency 
characteristic by the variation in gap width than the conventional head. 
In the foregoing, the difference in characteristic resulting from the 
difference in core thickness and the difference in gap width has been 
shown. As regards the core material, the aforementioned portion formed of 
78% Permalloy may be substituted for by Sendust without any particular 
difference resulting therefrom, or vice versa, and both of 78% Permalloy 
and Sendust are thus suitable as the core material. In the past, however, 
Permalloy has been the most popular core material for magnetic head. The 
reason is that Permalloy not only has a high permeability but also is rich 
in malleability and ready to be sheeted for the formation of a laminated 
core. On the other hand, Sendust has a high permeability comparable to 
that of Permalloy but is poor in malleability and this has made it 
difficult for Sendust to be used as the core material. Nevertheless, as 
noted above, the present invention enables a greater core thickness that 
the conventional one to be adopted for the magnetic head and thus, permits 
the use of Sendust as the core material for the magnetic head. 
The DC specific permeability of the core used in the magnetic head of the 
present invention is in the range of 8000 to 50000, because any specific 
permeability below 8000 cannot provide a magnetic property suited for the 
head core and any specific permeability above 50000 encounters 
difficulties in manufacturing. Further, the head gap in the magnetic head 
of the present invention is in the range of 1.mu.m to 2.5.mu.m. This is 
because a head gap less than 1.mu.m not only involves difficulties in 
manufacturing but also increases the influence of irregularity and a head 
gap greater than 2.5.mu.m adversely affects the recording frequency 
characteristic and is undesirable in practice. Furthermore, in the 
magnetic head of the present invention, the permeability drop starting 
frequency ranges from 10 to 200 Hz. This range is lower than that of the 
conventional head and would unavoidably suffer from a greater core loss 
than before but nevertheless, this is a range which can achieve the 
intended reduction of the influence on the frequency characteristic 
resulting from irregularity of the head gap. 
The present invention determines the above-described DC specific 
permeability and head gap and further sets up the core thickness such that 
the aforementioned permeability drop starting frequency ranges from 10 to 
200 Hz (see the shaded portion in FIG. 8). The reason why any frequency 
below 10 Hz is not used is that such a low frequency reduces the effective 
permeability. Thus, the present invention can reduce the deviation of the 
composite loss as shown by the curve B in FIG. 7 and minimize the 
influence imparted to the reproducing frequency characteristic by the 
irregularity of the operating gap width of the core. 
In the foregoing, of course, the various losses in the reproducing head 
should desirably be as small as possible and although there are obtained 
the effects as described above, the core represented by the curve B in 
FIG. 7, namely, the core comprising a single layer of Sendust having the 
thickness of 0.6 mm is never superior to the core represented by the curve 
A, namely, the core comprising four superposed layers of Permalloy each 
having the thickness of 0.15 mm, inasmuch as the former core suffers from 
a greater core loss as seen in FIG. 4. 
However, if the reproducing output need only be in a range sufficient to 
secure S/N, it is not particularly necessary to discuss the magnitude of 
the loss alone but this problem may be compensated for by the 
characteristic of the reproduce amplifier and where such a point of view 
can be relied on, the reduction of the range of irregularity which forms a 
problem in the manufacture of tape recorders will become very important in 
respect of the quality and cost of the product. 
In this sense, the present invention is effective to reduce by half the 
influence imparted to the reproducing frequency characteristic of the head 
by any minute technical fluctuation in the manufacture of head gap and 
this is highly useful in practice.