Manufacture of magnetic transducing heads

A process for the manufacture of pole structures for magnetic transducing heads is described in which the configuration of the non-magnetic gaps is formed by abrading the ferrite pole structure through a mask to erode recesses in the gap area which have inclined walls. The final dimension of the gap defining face of one or the magnetic elements is determined by removing material from the masked face of the structure until the dimension between the recesses has the desired value.

BACKGROUND OF THE INVENTION 
This invention relates to the manufacture of magnetic transducing heads. 
Magnetic transducing heads commonly have a pair of pole pieces separated by 
a non-magnetic operating gap. In use a recording medium is moved relative 
to the head and the gap sweeps out a track on which information is 
recorded or from which information is reproduced. 
It is desirable to manufacture the pole pieces and gap as accurately as 
possible in order to control the width and position of the track. It has 
been proposed, in British Patent Specification No. 1,164,754, to use a 
mask which covers part of the gap and the surrounding material of the pole 
pieces and exposes another part and the surrounding material. The exposed 
material is removed by, for example, sand blasting, and the mask is then 
removed. The parts which it had covered remain and form the pole pieces 
with the operating gap. The configuration of the head is thus determined 
by the form of the mask rather than the accuracy with which the 
constituent pieces of the pole pieces are assembled. 
We have, however, discovered that the edge of the mask tends to erode, 
making the size of the recesses formed by the abrasion rather larger than 
the corresponding openings in the mask as initially applied to the 
structure. And in addition it is difficult to make masks of a precisely 
determined size and shape. Consequently the size of the operating gap will 
not be precisely that of the corresponding part of the mask as initially 
formed. These inaccuracies can be significant with, for example, the small 
heads and high track packing densities currently used in digital 
recording. 
SUMMARY OF THE INVENTION 
This invention provides a process for the manufacture of a pole structure 
for a magnetic transducing head, the process comprising: providing a 
structure comprising a pair of members of magnetic material having gap 
defining faces separated by a gap filled with non-magnetic material and 
having a surface including said non-magnetic gap; masking said surface by 
masking means extending in abutting relationship with said surface, said 
masking means being so shaped and positioned as to mask a first region of 
the structure including a first part of the non-magnetic gap and to expose 
a second region of the structure including a second part of the 
non-magnetic gap neighbouring the masked first part of the gap; directing 
a stream of abrasive particles at the exposed second region of the 
structure to produce a recess in the structure having its wall so inclined 
as to provide, at the gap defining face on at least one of the members, an 
edge intersecting said surface and inclined thereto at an angle less than 
90.degree.; and thereafter removing the masking means and then removing 
material from the previously masked first region of the surface of the 
structure to produce a new surface having an intersection with said 
inclined edge at a desired position on the pole structure. 
Thus, the final size of the masked part of the gap is controlled by the 
removal step, which allows variations in the size of the recesses after 
the abrasion step to be compensated for. 
Preferably there are two exposed regions each including a part of the gap, 
these two parts of the gap being separated by the masked part of the gap, 
a recess being produced at each of the exposed regions, and each having 
its wall so inclined as to produce, on at least one of the side members, 
two edges inclined to one another with the masked part of the gap 
extending therebetween. 
There may be more than one non-magnetic gap in the original structure, each 
being treated by the process according to the invention. The head may thus 
be provided with one or more erase gaps, or with separate recording and 
reproducing gaps. The method of the invention allows them to be accurately 
disposed with respect to one another. 
Preferably the said two edges are inclined to one another, in the region of 
the gap at an included angle in the range 20 to 50 degrees. 
The removal of material from the part of the structure may be carried out 
in two stages, a first stage in which material is removed until the 
distance along the gap between the said two side walls reaches a 
predetermined value, and a second stage in which a predetermined depth of 
material is removed. 
The invention also provides a pole structure manufactured by the method of 
the invention, and a magnetic transducing head including a pole structure 
of the invention, the said gap forming a magnetic operating gap of the 
head.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, the head is manufactured from a structure 1 made up of 
a ferrite I-bar 2 and ferrite C-bars 3 and 4. The bars have their longest 
dimension extending perpendicular to the plane of the paper, as seen in 
FIG. 1, and during a later stage of the processing are sliced in spaced 
parallel planes parallel to the plane of the paper to yield a number of 
separate head cores. 
The bars 2, 3 and 4 are bonded together in a gap glassing step, by glassing 
at each end of each C-bar 3 and 4, as shown at 5a to 5d. The C-bar is 
separated from the I-bar 2 by a gap 6 at the front and a gap 7 at the rear 
of the bar 3 (the terms "front" and "back" are used in this description to 
refer to parts which are respectively nearer or further from the part of 
the head which in use faces the recording medium). The C-bar 4 is 
separated from the I-bar 2 by a gap 8 at the front and a gap 9 at the rear 
of the bar 4. 
In a completed head the gap 6 forms the read-write gap and the gap 8 forms 
a pair of erase gaps which, when the head is recording, erase a track on 
each side of the written track. This ensures that any earlier recorded 
information is removed and not detected during a subsequent read operation 
if the head is slightly displaced from the writing position. The gap 6 
will be called the read-write gap and the gap 8 the erase gap after their 
eventual functions. 
The structure 1 may be assembled from its constituent parts by the 
following method. The bars 2, 3 and 4 are originally made to extend to 
include parts 10, 11 and 12, as shown in FIG. 1 by the dotted lines. 
Metallic spacers, for example of gold, are deposited on the faces of the 
C-bars 3 and 4 that will form walls of the gaps at positions on the C-bars 
which will be discarded in a subsequent operation in which the assembled 
structure is sliced into individual cores. 
The bars 2, 3 and 4 are clamped together, their separation at the gaps 6 to 
9 being determined by the thickness of the spacers. In the gap glassing, a 
slurry paste of glass is applied to the regions 5a to d, the paste is 
allowed to dry, and the assembly is heated until the glass softens and 
flows to fill the gaps 6 to 9 around the spacers by capilliary action. The 
assembly is then cooled in a controlled manner and released. It is now 
bonded by the glass. 
For a reason which will be explained, the glass in the gaps is chosen to 
have a relatively high softening temperature. 
The distance from the front to the rear of the erase gap 8 is made slightly 
larger (as a result of the initial shaping of the C-bars 3 and 4) than the 
corresponding dimension of the read-write gap 6. 
Referring to FIG. 2, after the structure 1 has been assembled its front 
face 13 is polished and a mask 15 is applied to it. The mask 15 has a 
series of groups of five holes, each group corresponding to a separate 
head. In each group there are two holes 16 over the read-write gap 6 and 
three holes over the erase gap 8, a central hole 17 and two side holes 18. 
To locate the mask correctly it is provided with notches 19 (of which only 
one is shown) accurately positioned with respect to the holes 16 to 18. 
The structure 1 is positioned on a jig with the side of the read-write 
C-bar 3 resting against pins 20 which project from the jig. The notches 19 
are also positioned against the pins 20, so that they are aligned with the 
side of the C-bar 3, which serves as a datum. The holes 16 to 18 are thus 
correctly positioned with respect to the gaps 6 and 8. 
As an example of a suitable material the mask may be made of stainless 
steel. The holes and notches are formed by photo-etching, but could 
alternatively be formed for example by punching. 
Before being applied to the structure the mask 15 is coated with a thin 
layer 14 of adhesive. After the mask has been applied to the structure the 
assembly is cured to bond the mask 15 to the structure 1. It is important 
for the subsequent abrasion process that the mask should lie in abutting 
relationship with the surface of the ferrite structure and that there 
should be good adhesion between the ferrite and the material of the mask 
in those areas overlying the non-magnetic gaps such as those between the 
holes 17 and 18. There should also be no significant amount of adhesive in 
the holes 16, 17 and 18. 
The structure 1 with its mask 15 is then subjected to an abrasion process. 
It is placed in an air abrasion machine having a slot-like nozzle 21 
positioned with its long dimension transverse to the gaps 6 and 8, and 
spanning the holes 16 to 18. The nozzle 21 is caused to oscillate along 
the length of the structure 1, parallel to the gaps 6 and 8. As it does so 
it ejects a fan-shaped stream of abrasive particles which remove material 
of the structure 1 from the holes 16 to 18 to form recesses 22 to 24 (see 
the left-hand group in FIG. 2). 
The abrasion process also erodes the mask 15 at the edges of the holes. The 
holes therefore increase in size so that they have the final contours 16' 
to 18' as shown dotted in the left-hand group of FIG. 2. The bridges 
between the holes that mask parts of the transducing gaps 6 and 8 are 
correspondingly reduced. 
The abrasion process is continued until the distances between the holes, 
taken along the transducing gaps, as determined for example by optical 
examination, fall within relatively broad limits. It is an advantage that 
the subsequent steps of the invention allow this distance to be controlled 
more precisely, because it is difficult to achieve high accuracy at this 
stage, because of variations in the mask and the difficulty of determining 
the precise position of the edge of the gap without polishing to remove 
the particulate structure of the abraided surface. 
FIG. 3 is a section, taken after abrasion, through the mask 15 and 
structure 1 along the face of the C-bar 3 which bounds the read-write gap 
6 (the line X--X of FIG. 1). The walls of the recesses 22 to 24 formed 
during abrasion are inclined to the normal. Therefore, to take the case of 
the recesses 22 shown in FIG. 3, their walls on either side of the part of 
the C-bar 3 in the region of the gap 6 are inclined to one another so that 
the face of the C-bar 3 bounding the gap 6 has inclined edges 26 and 
tapers towards the front edge 27 of the gap. The abrading machine is 
adjusted to give the desired angle for the edges 26 (the criteria for the 
angle are discussed below). The adjustment may be achieved by varying the 
velocity of the nozzle along the workpiece or the distance between the 
nozzle 21 and the work piece. 
After the abrasion process the mask 15 is stripped off the structure 1 and 
the front face 13 cleaned. It is then glassed in a first glassing step. 
The glass is applied as a slurry paste, care being taken to fill the 
recesses 22 to 24 well. The paste is allowed to dry and the structure 
subjected to a heat-treatment cycle. The temperature is raised to cause 
the glass to soften and flow, and the temperature is maintained at a high 
value for long enough for the bubbles which tend to form in small ferrite 
pockets to escape. The temperature is then lowered. 
The glass is chosen to soften at a sufficiently low temperature for the 
glass to be able to reach a low enough viscosity for the bubbles to escape 
at a temperature below that at which the glass in the gaps softens and 
puts the accurate dimensions of gaps at risk. The glass applied to the 
face 13 must also form a good bond with the ferrite and match its 
expansion characteristics. It must be hard enough to protect the edges of 
the ferrite during subsequent machining, but will generally not be hard 
enough to provide good wear characteristics when the head is in use. 
Suitable glasses can for example be chosen from the zince borate family. 
The glassed front face 13 is now machined or lapped to remove first the 
surplus glass and secondly part of the material of the head. This stage is 
carried on until the distance between the inclined edges 26 is increased 
from the length of the edge 27 to a desired value, as measured for example 
by a travelling microscope, achieved when the front edge of the gap 
reaches a position 28. 
The structure 1 is now sliced into individual pole structures 29 by 
machining away the material between the individual groups of holes with a 
sawing wheel, the material being removed between pairs of lines such as 
the lines 30a and 30b and 31a and 31b of FIGS. 2 and 3. If the groups of 
holes are spaced from one another by a constant amount it is found that 
once the correct position for the first cut has been established the 
remaining cuts may be made simply by advancing the workpiece with respect 
to the wheel by the pitching distance. The cut sides of the individual 
pole structures 29 are lapped and polished and the material at the rear of 
the structure 29 removed back to a rear face 32 as shown in FIG. 1. 
Side cheeks 34 and 35 are now attached to each pole structure 29 to 
strengthen it and protect the edges. The side cheeks 34 and 35 and the 
pole structure 29 are clamped in a location jig and tacked temporarily by 
a heat resistant cement applied round the edges of the side cheek. The 
side cheeks 34 and 35 are non-magnetic and may for example be barium 
titanate ceramic. They and the cement are chosen to match the expansion 
characteristics of the ferrite. 
The side cheeks 34 and 35 are then bonded permanently by a second glassing 
step. The glass is a mixture of the glass used in the first glassing step 
(in which the recesses were filled with glass) and a second glass of a 
higher softening temperature. The softening point of the mixture is chosen 
to be between that of the glass of the first glassing and that of the gap 
glass (it will therefore be seen why the gap glass is chosen to have a 
relatively high softening point). The second glass is hard and provides 
good wear characteristics, it may for example be selected from the lead 
silicate family, whereas the first glass is relatively soft and does not 
provide good wear characteristics. 
The glassing mixture is applied as a thin slurry paste on the front face 13 
of the pole structure 29 and adjacent front faces of the side cheek. The 
paste is allowed to dry and then the temperature raised to the higher 
softening temperature of the second glass to cause the glass to soften and 
flow. The first glass in the recesses 22 to 24 flows at a lower 
temperature. At this higher temperature the first glass has a lower 
viscosity than the second glass and the mixture diffuses into it and 
partially replaces it in the recesses, but without the bubbles at the 
interface with the ferrite that would be liable to exist if the mixture 
were introduced into the recesses initially. 
The assembly is allowed to cool in a controlled fashion. The second 
glassing provides protection for the side walls of the recesses and the 
ends of the gap when the head is in use. 
The assembly is now inserted in a plastic housing 36 and held in place with 
epoxy resin. To complete the head a read-write coil 37 and an erase coil 
38 are slipped over the appropriate limbs 3 and 4 and attached to 
terminals in the housing (not shown). A keeper bar 39 is then bonded with 
epoxy resin to the central limb 2 and read-write limb 3 to complete the 
magnetic circuit, and another keep (not visible in FIG. 4) joins the far 
side of the central limb to the erase limb 4. 
Finally, the front face 13 of the pole structure and the surrounding face 
of the keepers 34 and 35 and housing 36 are spherically ground, lapped and 
polished to provide the final working surface of the head. During this 
stage a predetermined depth of material is removed to bring the front 
surface of the head back to a line 40 as shown in FIG. 3. It is found that 
the inclination of the side walls 26 can be made sufficiently accurate for 
the change in distance between the side walls 26 from the line 28 (where 
the distance is assured by measurement) to the line 40 to be a 
predetermined amount within the required accuracy. This distance d along 
the line 40 is the track width of the head. 
The angle of inclination of the side walls is selected in conjunction with 
the changes expected in the dimensions of the gap during processing so 
that when the desired track width d is achieved the dimension from the 
front to rear of the gap (the line 40 to the glass 5c) has a desired 
value. 
The following are details of a specific head which has proved satisfactory. 
The read-write gap 6 is about 0.00008 inches and the erase gap about 
0.0001 inches. They are separated by 0.027 inches. The original dimension 
from the front face 13 to the back of the read-write gap is about 0.005 
inches, with the same dimension for the erase head being 0.001 to 0.004 
inches larger. The length of the ferrite structure, i.e. in a direction 
parallel with the gaps 6 and 7 may be of the order of one inch. 
The mask is produced by photo-etching and the distance between the holes 16 
of the mask is originally between about 0.011 and 0.0115 inches 
(photo-etching tends to produce slight variations from one mask to the 
next. The abrasion nozzle is shaped to give a fan tail emission and has an 
aperture 0.007.times.0.125 inches. The abrading material is alumina powder 
of 9/um particle size, and the flow rate of the powder typically 10 to 15 
grams/minute at an airline pressure of 100 pounds per square inch. The 
nozzle is traversed back and forth at a uniform rate of 3 to 4 inches a 
second over the structure and to ensure that all parts of the structure 
are subjected to the same degree of abrading, the traverse of the nozzle 
extends beyond the structure so that during any non-uniform movement in 
reversal of the movement of the nozzle the structure is not abraded by 
particles issuing from the nozzle. The spacing of the nozzle from the 
surface of the mask is set in the range 0.030 to 0.080 inches. 
It is found that the inclination of each side wall 26 to the normal to the 
pole face 13 (in its original flat form), with the above mentioned flow 
rate of powder and arrangement of nozzle, is about 21 degrees, giving an 
inclination of 42 degrees between the two side walls on either side of the 
gap, which is of course preserved when the final spherical surface is put 
on the pole face 13. Variations of the nozzle distance within the range 
given tend to alter the rate of removal of material rather than the angle, 
but the angle can change sharply if the distance is brought outside this 
range. 
During the first removal material is removed until the distance along the 
line 28 is about 0.0094 inches, at a depth of about 0.002 inches below the 
original surface. During the final removal another 0.001 inches is removed 
to give a track width d of 0.0104 inches. 
The gap glass may soften at about 800.degree. C. The glass used in the 
first glassing may be a Corning 7570 or 1417. In the first glassing the 
temperature may be raised to 600.degree. C. at a rate of 40.degree. C. per 
minute, held for 15 to 20 minutes and lowered at a rate not exceeding 
15.degree. C. per minute. The glass used in the second glassing may be a 
50:50 (by weight) mixture of the first glass and a Ramsden RE2852 or a 
Corning 8875, 9013 or 8161. The glassing is carried out by raising the 
assembly to 700.degree. C. at a rate of not more than 30.degree. C. per 
minute, maintaining the temperature for 20 minutes and lowering it at a 
rate not exceeding 10.degree. C. per minute. To enhance the outgassing in 
the high temperature stage the pressure may be reduced to 100 Torr and 
then released to atmospheric several times. 
Various modifications may be made. The final track width may be achieved by 
a single removal stage after the abrasion, with no significant material 
being removed from the pole structure when the spherical shape is given to 
the final assembled housing and head. Alternatively, the track distance 
along the read-write gap may be measured at this stage and grinding 
continued until the desired track width is achieved. This is particularly 
useful if, though the holes of any one group are consistent in size, there 
is a gradual variation from one group to the next (as is possible with 
photo-etching techniques) because it allows the track width of individual 
heads to be controlled. 
The glass used in the second glassing need not include the first glass 
provided it mixes satisfactorily with the first glass during the glassing. 
The recesses need not extend into the glass menisci at the rear of the 
gaps, although this may reduce the performance of the read-write gap in 
particular. Instead of a spherical housing the side cheeks may be of 
unequal proportions and a central air-relief groove machined to form a 
catamaran-type slider.