Electromagnetic transducer head

An electromagnetic transducer head for a tape recording and reproducing system comprises a pair of generally U-shaped single crystalline ferrite cores having their first legs secured together and their second legs spaced from each other to form a magnetic gap in which a wafer of glossy material is provided. Each of the ferrite cores has crystal planes (211), (110) and (111). The magnetic gap is aligned to the crystal plane (111) and the second legs of the cores are shaped to provide a tape contact surface which is parallel with the crystal plane (211). The crystal plane (110) is at right angles to the planes (211) and (111) and forms a passage for magnetic flux. The crystallographic axis <110> of each core extends at an angle to the magnetic gap so as to meet the axis <110> of the other core at a point aligned with the tape contact face.

BACKGROUND OF THE INVENTION 
The present invention relates to electromagnetic transducer heads for tape 
recording and reproduction, and more particularly to a single crystalline 
ferrite core magnetic head. 
Single crystalline ferrite cores are known to exhibit magnetic anisotropy 
and because of their electromagnetic properties such ferrite cores find 
extensive use in video tape recorders as recording and reproducing heads. 
Although the prior art ferrite core possesses a comparatively high 
resistance to wear, the available output signal level, the optimum 
recording level and signal-to-noise ratio are not satisfactory to permit 
high-quality video recording and reproduction. 
SUMMARY OF THE INVENTION 
According to the invention, the magnetic head comprises a pair of generally 
U-shaped single crystalline ferrite cores, the first legs of both cores 
being in abutment to each other with an air gap between the confronting 
surfaces of the second legs of the cores. A spacer is fused between the 
confronting surfaces of the second legs. Each core possesses crystal 
planes (211), (110) and (111) which are at right angles to each other. The 
air gap extends in a plane which is parallel with the crystal plane (111) 
and the second legs of both cores are shaped to provide a tape contact 
surface which is aligned to the crystal plane (211). The first and second 
legs of each core form a common surface parallel with the crystal plane 
(110) which is made flush with the common surface formed by the first and 
second legs of the other core. The crystalline axis &lt;110&gt; of each core 
extends at an angle to the gap so as to meet the axis &lt;110&gt; of the other 
core at a point aligned with the tape contact surface. 
Tests showed that the resistance to wear is comparable with that of the 
prior art head while the available output signal level, the recording 
signal level and signal-to-noise ratio are improved over the prior art 
device. 
An object of the present invention is therefore to provide a magnetic 
ferrite core head which is superior to prior art ferrite heads in terms of 
output signal level, recording level and signal-to-noise ratio with a 
satisfactory degree of resistance to wear.

DETAILED DESCRIPTION 
Before describing the present invention, reference is first made to FIG. 1 
in which an embodiment of a prior art magnetic head is illustrated. The 
prior art magnetic head 1 comprises a pair of generally U-shaped yokes or 
cores 2 and 3 formed of a single crystalline ferrite material with a 
positive magnetic anisotropic constant K.sub.1. Each of these cores has 
its crystalline axes so oriented that planes (100) lie in the tape-contact 
surfaces 4 and 4', planes (110) lie in the confronting faces 5 and 5' in 
which a spacer 9 is secured, and planes (110) run parallel with faces 6 
and 6' which are at right angles to both faces 4, 4' and 5, 5'. The easy 
axis of magnetization &lt;110&gt; extends normal to the plane (110). A coil 7 is 
wound on the cores 2, 3 as shown and connected to terminals 8. 
This prior art magnetic head is comparatively resistant to wear as 
indicated by curve M of FIG. 2 which was obtained as a result of a test 
wherein the tape-contact faces 5, 5' are lapped by a lapping tape No. 8000 
and the amount of wear was measured in terms of depth from the initial 
surface. However, the output voltage developed across termials 8 is 
comparatively small as indicated by a frequency response curve M.sub.1, 
FIG. 3, and the voltage required to record is relatively high as indicated 
by curve M.sub.2, FIG. 4, with a low signal-to-noise ratio. The low output 
voltage is accounted for by a relatively small magnetic resistance across 
the spacer 9, and hence the small leakage flux because of the orientation 
of the easy axis of magnetization &lt;110&gt; which is perpendicular to the 
spacer 9. 
Referring to FIGS. 5-8, embodiments of the present invention are 
illustrated. Single crystalline ferrite bodies with a positive magnetic 
anisotropic constant K.sub.1 are shaped to provide two parallelpiped 
blocks 10a and 10b as shown in FIG. 5, whose crystalline axes &lt;110&gt; are 
skewed at an angle .alpha.(-55.degree.) to the longitudinal axis of each 
body and parallel with the plane (110), with the crystal axes &lt;100&gt; being 
at right angles to the axes &lt;110&gt;. The ferrite blocks 10a and 10b have 
parallel opposite faces A, B and G, H, respectively which lie in the 
crystal plane (211), faces C, D and I, J lying in the plane (111) and 
faces E, F and K, L lying in the plane (110). 
The blocks 10a and 10b are respectively shaped to provide generally 
U-shaped half cores 12a and 12b as shown in FIG. 6 which are secured 
together with their side faces 16, 16' lying in the crystal planes (111). 
A thin wafer or spacer 16 of glass is fused between the interfaces 16 to 
act as a magnetic gap. The easy axes of magnetization &lt;110&gt; extend from 
the inside of each leg of the U-shaped cores at an angle of 55.degree. to 
the magnetic gap 16 and meet at a point on a tape contact face defined by 
faces 14a, 14b which are parallel with the planes (211), with the crystal 
axis &lt;211&gt; extending normal to the plane (211). The legs of each core 
defines a surface 15 which is made flush with the face 15 of the other 
core and parallel with the crystal plane (110). 
It has been recognized in the past that there is a tendency of each of the 
glass and ferrite materials toward diffusing into the adjacent region of 
the other material during the fusion process if the crystallographic 
structure of such regions has been distorted by the previous surface 
polishing process. It is, however, found that the crystallographic 
structure on the plane (111) remains substantially unchanged in the 
presence of mechanical shocks, the confronting faces of the core legs 
across the spacer 16 are less liable to damage during such polishing 
process, so that diffusion of adjacent material is less likely to occur. 
Therefore, the crystallographic structure of the core adjacent to the 
magnetic gap 16 remains unchanged so that the effective permeability of 
the adjacent regions is not reduced from the value it has previously 
possessed. This ensures that the effective gap length can be made 
relatively small with the result that a relatively high output voltage is 
developed when the head is used to pick up signals from the magnetic tape, 
as indicated by a dot-line curve N.sub.1 of FIG. 3. The optimum recording 
voltage is also small as indicated by a dot-line curve N.sub. 2 in FIG. 4. 
The video frequency signal-to-noise ratio and the voltage output 
performance of the magnetic head 11 are respectively 2dB greater than 
those available with the prior art head 1. 
The easy axis of magnetization is known to correspond to the crystalline 
axis &lt;100&gt;. In the embodiment of FIG. 6 the axis &lt;110&gt; is the easy axis of 
magnetization. This is contrary to the known fact. The FIG. 6 embodiment 
is based on the assumption that the easy axis of magnetization has changed 
from the axis &lt;100&gt; to the axis &lt;110&gt; during the process of shaping the 
bodies 10a, 10b into the magnetic head. This assumption is verified by 
experimentally fabricating a magnetic head as shown in FIG. 7 which is 
similar to the magnetic head of FIG. 6 except that the axes &lt;110&gt; extend 
at right angles to the axes &lt;110&gt; of the FIG. 6 embodiment so that the 
axes &lt;100&gt;, which are the easy axes of magnetization prior to the 
fabrication, correspond to the axes &lt;110&gt; of the FIG. 6 embodiment. It 
appears at a first glance that, since the easy axis of magnetization &lt;100&gt; 
converges at the gap, the effective gap length may be smaller than the 
embodiment of FIG. 6 and the voltage developed may be higher than with the 
latter. However, tests showed that the output voltage derived from the 
magnetic head of FIG. 7 is smaller than with the FIG. 6 magnetic head as 
indicated by a chain-dot curve 0.sub.1, FIG. 3. The optimum recording 
voltage is relatively large as indicated by a chain-dot curve O.sub.2 in 
FIG. 4. This is an indication that the new easy axis of magnetization is 
the axis &lt;110&gt;. Therefore, in the embodiment of FIG. 6 the axes &lt;110&gt; are 
the new easy axes of magnetization, and the axes &lt;100&gt; are the previous 
axes of easy magnetization. 
Single crystalline ferrite material tends to develop distortion in the 
surface area of its crystallographic structure as a consequence of the 
sliding contact with magnetic tape. This crystallographically modified 
region of the ferrite core, while it becomes hardened so as to contribute 
to increase the resistance to wear, tends to degrade the magnetic 
properties of the core material and results in a so-called "spacing loss" 
which reduces its electromagnetic transducing efficiency. 
Wear tests were conducted to ascertain the wear resistance of the magnetic 
head 11 of FIG. 6 by lapping the surfaces 14a and 14b with the lapping 
tape No. 8000. As indicated by curve N of FIG. 2 the wear resistance is 
substantially comparable to that of the prior art magnetic head of FIG. 1. 
In an electron diffraction analysis a distorted crystallographic mosaic 
pattern was observed on the surfaces 14a, 14b, rather than a complete halo 
pattern and even if a portion of the surface is etched away the distorted 
mosaic pattern was observed until the etched portion reaches a substantial 
depth from the original surface plane. The surfaces 14, 14b, even if they 
have been lapped over time during use, will not distort their adjacent 
crystal structures, so that trunsducing efficiency remains unchanged. 
The ferrite bodies may also be cut so that their faces 21a, 21b are at an 
angle of .theta. (which is less than 10.degree.) to the surfaces 15a, 15b. 
This results in a magnetic head 20, shown in FIG. 8, with the gap 22 being 
skewed (90.degree.-.theta.) relative to the path of magnetic flux. The 
surface planes 27a, 27b thus make an angle .theta. to the crystal plane 
(110) and the side faces 28 are no longer parallel with the plane (111). 
Observation of an etchpit diagram on the faces 27 indicates that, when the 
angle .theta. is within 0.degree. to 10.degree., the faces 27 do not lose 
their magnetic properties as the crystal plane (110) so that they can 
still serve as a principle magnetic flux path. 
In order to ascertain the favorable characteristics of the present 
invention, three other magnetic heads are experimentally fabricated from 
single crystalline ferrite bodies in a similar manner to the embodiment of 
FIG. 6 with the following exceptions: 
(1) The first experimental head has its tape contact face parallel to the 
crystal plane (111) with the plane (110) as a magnetic flux path. The wear 
resistance characteristic of this head is indicated by curve Q of FIG. 2, 
which is unfavorably compared with the characteristic of the present 
invention. 
(2) The second experimental model was made with its tape contact face 
parallel with the plane (110), with the crystal plane (111) serving as a 
magnetic flux path. The wear characteristic is indicated by curve R, FIG. 
2, which is compared unfavorably with the present invention. 
(3) The third model was made with its tape contact face parallel with the 
crystal plane (211), with the crystal plane (111) serving as a magnetic 
flux path; the wear characteristic being indicated by curve S which is 
close to that of the present invention. However, the electromagnetic 
characteristics of the third model are indicated by curves S.sub.1 and 
S.sub.2 shown respectively in FIGS. 3 and 4, which are compared 
unfavorably with the present invention. 
The magnetic head 11 of FIG. 6 can also be formed of a ferrite material 
with a negative magnetic anisotropic constant to achieve the same 
characteristics as mentioned previously in so far as the tape contact face 
is parallel with the crystal plane (211) with the crystal plane (111) 
parallel with the gap and with the plane (110) serving as a magnetic path. 
In summary, the electromagnetic transducer 11 of the present invention 
comprises a pair of single crystalline generally U-shaped ferrite cores 
each having crystal planes (211), (111) and (110) which extend at right 
angles to each other. A coil is wound on the cores. The first legs of the 
U-shaped ferrite cores are in abutment with each other, the second legs of 
both cores being spaced from each other to form a gap between them. The 
gap extends in a plane which is parallel with the crystal plane (111) and 
the second leg of each core forms a tape contact face parallel with the 
crystal plane (211). The first and second legs of each core form a common 
surface which is flush with the common surface formed by the first and 
second legs of the other core, the common surfaces being parallel with the 
crystal plane (110). A spacer is fused between the confronting surfaces of 
the second legs of the cores. The crystallographic axis &lt;110&gt; of each core 
extends at an angle to the magnetic gap so as to meet the axis &lt;110&gt; of 
the other core at a point on the tape contact face.