Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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23305-988
The invention relates to magnetic information recording
and playback technique, more particularly to a magnetic head usable
for information recording playback. The term "information record-
ing" covers sound recording on a magnetic carrier, video recording
technique, as well as digital information recording. For the sake
of simplicity the examples disclosed in the present specification
will relate to conventional tape recording applications, however,
any other recording and reading application is considered as
equivalent.
The development of information recording -technique has
created a small n~mber of head types.
A conventional and widely used head comprises a magnetic
core made of profiled soft-iron sheets in a laminated arrangement,
and there is provided an air gap between the sheets just across
the frontal zone which abuts the magnetic carrier. A coil is
wound around the core for connection with appropriate electronic
circuits. Such heads are held in a head support and have a
magnetic shielding around them. These heads proved to be popular,
they had fairly good electrical properties (at least in sound
recording applications) and their manufacture was not too expen-
sive. A basic drawback oE such heads was that; the soEt-core
material soon became worn by the Er.i.ctional efEects of the tape.
The development of the art has introduced chromium-
dioxide and metal tape materials which have been much harder than
normal tapes, and the laminated cores were unable to endure the
wear caused by such media.
In an improved technology a hard coating is provided on
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such heads, whereby the surface harness has become sufficiently
high. The problem with such heads lies in that the coating mater~
ial increases the effective air gap to twice its thickness, and
in most applications the high frequency response of the recording
has become much worse than without such coatings.
Another head structure family is based on the use of
ferrite or glass ceramic materials. These materials have suffi-
cient hardness to endure increased wear and they are also prefer-
able regarding their frequency response. A drawback with such
heads lies in the comparatively low value of permeability, whereby
their electric signals are at a lower level than in case of
permalloy cores. The greatest drawback of such heads lies in the
difficulties during manufacture. These hard materials are hard
to be formed and tooled, and their production is costly and it
requires much time and work. A further drawback of such heads
lies in their low heat conductivity. In operation, the effect
of friction might cause extreme temperatures in the vicinity of
the air gap, and at these elevated temperatures a recrystalliza-
tion might take place at the boundary surface of the glass and
the ferrite material, which virtually increases the air gap and
decreases Erequency response. The thermal stresses may often lead
to small cracks which mean the erld oE th~ir useiEul l.ife.
The object of thc i.nvention is to provide a method for
producing a magnetic head which not only has good electrical
properties, but can be manufactured with reasonable costs and has
improved wear-resistant properties.
The invention is based on the discovery that for
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23305-988
increasing the hardness of the soft-cored heads, the hard material
should be arranged between the soft-iron sheets rather than on the
head. If a hard material like e.g. titanium nitride is deposited
on the main surface of the iron sheets forming the core, then the
hardness of the so-obtained laminar sandwich structure will vary
along the width of the information carrier (the tape) as a comb-
like function. Since the tape has sufficient rigidity in the
stretched state regarding small distances, the tape is sliding
on the hard edges only and causing no wear to the soft-iron
material between the hard coatings.
This effect is similar to the passing of the wheels of
a car above the grating of a sewer or the like. The wheel cannot
get in between the iron rails if the rails are arranged in a
sufficient density~
It has been experienced that even as thin coatings as
a half micron or less could provide an increased resulting hard-
ness. If the thickness of the coating is above about 2 microns,
the resulting hardness will not increase significantly with
increasing coating thickness. The arrangement of the hard
material between the sheets results in a further advantage, i.e.
the structure remains unchanged after some wear. Whlle -the Eront-
deposited hard la~er was de~troyed Eollow:iny ~ w~r O:e about 1
micron, the sandwich structure according to the invention preserves
its hardness throughout the depth of the air gap.
The invention can be summarized as a method for producing
magnetic recording and/or playback heads with improved abrasion
resistance for use with a magnetic record medium having a
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magnetizable surface for retaining information in the form of a
pattern of magnetization, comprising the steps of: (a) providing
pole sheets of predetermined shape made of a material with high
magnetic permeability, each sheet having a frontal zone for slid-
ing engagement with said recording medium and a lateral surface
substantially normal to said frontal zone for defining a side of
a pole gap; (b) providing a hard layer on at least one surface of
said sheets extending to said frontal zone by means of high rate
reactive cathode sputtering, said layer having a substantially
uniform thickness of at least 0.5 and at most 5 micrometer, having
a Vickers hardness of at least 2000 kp/mm2 and being of an elec-
trically non-conductive and non-magnetizable material; (c)
assembling said sheets together to form a pair of half cores in
such a way that in each of said half cores said sheets are stacked
on each other and between each pair of sheets in said stack at
least one of said layers is arranged; and (d) mounting together
said cores to form said head.
The Vickers hardness of the surface can be as high as
HVlo=3000 kp/mm2 which is twenty tirnes as high as that of the
soft-iron material.
The resulting wear properties of the head according to
the invention are just as good as those oE Eerr.ite and ceramic
heads, while the h:igher re~ulting p~rmeclbility o the core
material results in higher signal levels, thus better signal to
noise ratios. The deposition step does not add significant costs
to the well-known manufacturing technology of soft-iron cored
heads, which means that these heads can be manufactured with
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reasonable costs.
Several other properties and advantages of the present
invention will be described in connection with preferable embodi-
ments thereof, in which reference will be made to the accompanying
drawings, in which:
Figure 1 is an enlarged elevation view of a half core;
Figure 2 is a side view of Figure 1;
Figure 3 is a schematic arrangement of a head and a
tape during operation on an enlarged scale for visualization;
Figure 4 is an enlarged sectional view taken along
line IV-IV of Figure 3;
Figure 5 is a hardness versus width curve for the
structure of Figure 4;
Figure 6 is an enlarged top view of the frontal zone
of Figure 3 viewed across the tape;
Figure 7 is a schematic perspective view of a multi-
channel head assembly; and
Figure 8 is a hardness curve similar to Figure 5 for
the head of Figure 7.
Figures 1, 2 and 3 show schematically the magnetic poles
of a recording/reading head made according to the invention. The
head comprises a pair of half-poles 10, 11 each Gomp:ri~sing a
plurali.ty of prof ile sheet.s 12 o.E a soEt-iron material with high
magnetic permeability. The sheets 12 are stacked and fixed
together by means of an adhesive bondinq. The two half-poles 10,
11 are attached to each other as shown in Figure 3 and an air gap
13 is defined between them by inserting a non-magnetic foil
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23305-988
between their lateral side surfaces (surface 14 in Eigure 1).
The width of the air gap 13 is in the micron range and typically
falls between about 0.6 to 10 microns. The height of the lateral
surface 14 defines the full depth of the air gap 13 . A coil 15 is
arranged on the half-poles which serves as a pick up coil in play-
back mode and as a magnetizing coil in recording mode.
Figure 3 shows the head in operation when tape 16 is
pressed against frontal zone 17 and the tape is moving with a
predetermined speed in the direction of arrow A. The frontal zone
1 0 1 7 of the head is made preferably by a grinding operation and its
profile is tooled to provide an optimum guidance for the tape 16.
The head shown in Figures 1 to 3 resembles conventional
heads with laminated core, e.g. such as described in the book of
Dipl. Ing. Christian Scholz "Magnetband-speichertechnik" (VEB
Verlag Technik, Berlin, 1969, pp. 211-236) or used widely in
commercial tape recorders. A basic difference between the head
according to the invention and the conventional ones lies in that
each sheet 12 comprises a coating 18 on its surface at least in
the region defined by the depth of the air gap 13 and the frontal
zone 17. The coating 18 is made of a hard material which has an
increased resistivity against abrasion.
The coating can b~ mad~ by mQans of conv~ntlonal vapour
deposition technique such as ion plati.ng or cathode sputtering.
Since magnetic heads are manufactured by mass-production, it is
preferable iE the coating 18 :is made by means of high-rate cathode
sputtering technique which offers not only a high productivity
but also a uniform coating thickness, a perfect adhesion to the
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23305-988
substrate material and controllable coating properties. The
high-rate cathode sputtering technique is well-known in the art,
and it is widely used for various applications including the
hardening of cutting edges, coating watch-cases and bracelets and
thin-film technique. The high-rate cathode sputtering is described
in detail e.g. in the paper of W. D. Munz and G. Hessberger
entitled "Production of hard titanium nitride layers by means of
high-rate cathode sputtering" published by Leybold-Heraeus GmbH.
(FRG). In this technique a magnetic field of special distribution
is applied to the target, whereby the electrons are concentrated
in front of the target and high particle densities are obtained
that lead to a reduction in the discharge voltage and lead to
higher sputtering rate. For producing very hard layers the use
of reactive cathode sputtering is preEerable. This technique is
used when oxides, nitrides or carbides of a metallic basic material
should be deposited. The basic material, e.g. titanium is used
as target. The atmosphere in the discharge chamber is a mixture
of an inert gas like argon and a reactive gas, e.g. nitrogen.
During the sputtering process the reactive gas reacts with the
target, and is either resputtered from this, or becomes integrated
in the sputtered layer during the condensati~n o;E tll~ m~tal at~ms.
The hardness of the layer d~pends larg~ly on the ~artial pressure
of the reactive gas, and by means of an appropriate process
control optimum hardness can be achieved.
In making the coating 18, the sheets 12 are fed in the
discharge chamber and are used as substrates. The coating process
is facilitated if a pair of high-rate cathodes are ~ounted in an
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23305-988
opposite position and the substrates are placed in the middle zone
between the two cathodes. A preferable equipment for making such
coatings is the Modular In-Line Sputtering System Z 600 of the
Leybold-Hereaus GmbH.
~ preferable coating is titanium nitride which can have
a hardness HV1o about 3000 kp/mm2 if the partial pressure of the
nitrogen is about 5 to 10. 10 4 mbar. Similarly hard layers can
be obtained by using other kinds of films, such as chromium
nitride, silicon carbide, tantalum nitride, tungsten nitride and
other hard compounds. The hardness and oxidation behaviour of
such compounds is analyzed in the paper of W. D. Muntz and J.
Gobel "Oxidation behaviour of high rate sputtered TiN, TiC, TiCN,
CrN and WN films" published during the 1lth ICMC conference in San
Diego (Calif.) 1984. There are also a number of publications
which deal with the deposition of hard films on metal substrates,
therefore the invention cannot be limited to any particular
compound.
It should be mentioned that the high-rate reactive
cathode sputtering technlque provides for an extremely good
cohesion between the substrate and the coating, and this cohesion
can surely endure the force and temperature conditlons that
prevail in the engagement ~one 17 between the telpe and the head.
ReEerence will be made now to Figure 4 which shows the
enlarged sectional view along line IV-IV of Figure 3. The scale
is distorted for the sake of better illustration. The tape 16
moves normal to the plane of the drawing and presses the head with
pressure P. The support surface of the head consists of a laminar
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sandwich-like structure of the iron sheets 12 and the coatings 18
thereon. The thickness of the sheets is between about 0.1-0.15
mm, while that of the coating is in the micron range, preferably
at least 1 micron. The Vickers hardness HV10 of the structure
measured along the tape width _ can be seen in Figure 5 in kp/mm2
units. The resulting hardness versus width curve is a comb-like
formation with about 3000 kp/mm2 peak and 150 kp/mm2 basic hardness
values. It has been experienced that due to the rigidity of the
tape material in the short distance between adjacent spaced hard
coatings, the resulting hardness of the structure is defined by
the coating material and the soft-iron sheets 12 have practically
no functional role in determining the surEace abrasion. The
significant increase in hardness is experienced even if the thick-
ness of the coating was a fragment of a micron, for safety reasons,
however, it is preferable if the coating 18 is at least about 0.5
to 1 micron, preferably two microns thick. There is no further
hardening effect if the coating thickness is increased beyond this
value, however, one can produce thicker coatings, too. The soft-
iron sheets 12 provide for a good support for the thin coating,
therefore this latter is definitely fixed between the sheets.
Since the sheets 12 are made of a heat-conducting metal, the heat
generated by the Eriction hetween l:he kap~ and the head w:ill be
transported away r thereEore remarkable local temperature gradients
cannot occur. If the head structure becomes abraded, the sandwich
structure remains unchanged and the Eull depth of the head in the
region of the air gap 13 can be utilized. These factors explain
why the expected life-time oE the head according to the invention
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23305-988
has increased substantially compared to that of conventional heads
of laminated iron sheets.
Figure 6 shows the top view of the engagement area
viewed through the tape in the case of a double track head. In
the engagement zone 17 between the head and the tape there are two
sandwich structures 19 and 20, respectively and each of them
consists of a plurality of sheets 12 and associated coatings 18.
The tape 16 is wider than the tracks and a magnetic shielding 21
is arranged between the tracks. It is preferable if the shielding
21 is also made of the high permeability iron sheet and it is
covered by a coating either on one face or on both. By providing
a hard coating on the shield, the hazard of a magnetic short-
circuit between the tracks is eliminated, since the coating is
made of a non-magnetic material. In addition to this advantage,
the coating on the shield 21 can provide a further support for the
tape. Figure 6 shows further two optional coated sheets 22, 23
on both sides of the structures 19 and 20, which can be located
in the support material of the head made generally of copper. The
use of coated sheets 22 and 23 can provide further supports for
the tape 16.
While the coating 18 was mentioned as a l~yer depoqited
on the surface of the pole sheets, It can well be understood, that
separate foils, e.g. tungsten Eoils or Eoils of any other hard
material can be used instead of the deposited coating. With
present technologies, however, the deposited coating seems to be
far more favourable than separate foils.
The sheets can be coated on both sides or on one side
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23305-988
only, or one might use coated and uncoated sheets alternatingly.
The important thing is to arrange the hard edges with such
spacings that the resulting surface hardness of the frontal zone
be defined decisively by them. It is often preferable if the full
surfaces of the sheets 12 are coated.
The plane of the sheets 12 and of the coatings 18 need
not be normal to the direction of the tape movement. A tilting
on either directions or a tilting relative to the plane of the
tape is possible. There are several magnetic head arrangements,
in which tilted heads are used for obtaining increased channel
separation. The presence of the coating 18 does not limit the
conventional possibilities for arranging the head relative to the
tape.
Figure 7 shows a simplified perspective view of a multi-
channel head with a number of pole structures each comprising
sheets coated with a hard film. The magnetic shieldings between
the channels comprise also hard coatings. The hardness versus
width curve is shown in Figure 8, in which the dashed lines
correspond to the shield plates between the channels.
According to the invention the hardness of soft-iron
heads has been increased, whereby the liEetirne has been lncreased
by a factor oE ~ to 8 and such head~ can as we~ll be use~d with
chromium dioxide and metal tapes as with Eerrite and glass ceramic
heads. The good magnetic properties of permalloy-cored heads
remain unchanged, since the small amount of non-magnetic coatings
cannot decrease the eactive iron mass to a noticable extent. The
insulating properties of the coating can decrease the eddy-current
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losses, since any current flow between the sheet is safely blocked.
A further advantage lies in the preservation of the high produc-
tion rate of conventional sheet-cored heads for manufacturing
high quality, long life magnetic heads.
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