U.S. patent number RE35,228 [Application Number 08/351,780] was granted by the patent office on 1996-05-07 for inductive thin film head having improved readback characteristics.
This patent grant is currently assigned to Seagate Technology, Inc.. Invention is credited to Gregory S. Mowry, Charles H. Tolman.
United States Patent |
RE35,228 |
Mowry , et al. |
May 7, 1996 |
Inductive thin film head having improved readback
characteristics
Abstract
An inductive thin film magnetic head carried on a substrate
having a reduced susceptibility to the "glitch" effect or "remote
read" effect. The inductive thin film head uses a three point
approach using hard axis only drive fields, narrow paddles and
shortened pole tips which are spaced apart from a saw alley when
the substrate is sliced.
Inventors: |
Mowry; Gregory S. (Burnsville,
MN), Tolman; Charles H. (Bloomington, MN) |
Assignee: |
Seagate Technology, Inc.
(Scotts Valley, CA)
|
Family
ID: |
24054381 |
Appl.
No.: |
08/351,780 |
Filed: |
December 8, 1994 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
516158 |
Apr 30, 1990 |
05170303 |
Dec 8, 1992 |
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Current U.S.
Class: |
360/125.51;
360/122; 360/123.39; 360/125.64 |
Current CPC
Class: |
G11B
5/3113 (20130101) |
Current International
Class: |
G11B
5/31 (20060101); G11B 005/147 () |
Field of
Search: |
;360/126,122-123 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0040994 |
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Feb 1981 |
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EP |
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57-203219 |
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Mar 1983 |
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JP |
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59-77715 |
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Oct 1984 |
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JP |
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1-098110 |
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Jul 1989 |
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JP |
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2-78006 |
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Mar 1990 |
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JP |
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Other References
EDN, vol. 22, No. 7, Apr. 5, 1977, pp. 32, 34 "Thin Film Magnetic
Recording Heads Promise to Increase Storage Densities" by Roscamp
et al. .
"IBM 3370 Film Head Design and Fabrication" by Jones, Jr..
|
Primary Examiner: Wolff; John H.
Attorney, Agent or Firm: Westman, Champlin & Kelly
Claims
What is claimed is:
1. A thin film magnetic recording head comprising:
a substrate;
a magnetic yoke deposited on the substrate including structure
defining a top paddle and a bottom paddle for defining a high
permeability magnetic flux path, the magnetic yoke deposited upon a
nonmagnetic substrate and having an axis of symmetry .[., the top
and bottom paddles having negative magnetostriction.].;
spaced apart paddle tips for the top and bottom paddles defining a
magnetic flux gap in the magnetic yoke for recording and reading
data proximate the flux gap, the paddle tips being normal to the
axis of symmetry;
the top and bottom paddles including their respective paddle tips
having a hard axis of magnetization generally parallel to the axis
of symmetry and an easy axis of magnetization normal to the hard
axis of magnetization in a plane of the top and bottom paddles;
a plurality of electrically insulated conductor windings, at least
one of the plurality of conductor windings lying in a single plane
and extending through the magnetic yoke between the top paddle and
the bottom paddle; and
the conductor windings are parallel to the easy axis of
.[.magnetorization.]. .Iadd.magnetization .Iaddend.and
perpendicular to a hard axis of magnetization of the magnetic
paddles for maximizing exposure of the magnetic yoke to hard axis
magnetic drive fields and minimizing exposure to easy axis magnetic
drive fields whereby drive fields in the easy axis direction are
less than local coercivity of the magnetic yoke.
2. The thin film magnetic recording head of claim 1 including a saw
alley in the substrate, the saw alley extending generally parallel
to the easy axis and spaced apart from the paddle tips.
3. The thin film magnetic recording head of claim 1 wherein the top
and bottom poles each comprise:
a paddle tip region having a paddle tip width;
a back region having a back region width and a back side which
defines a back region length, the back region width larger than the
paddle tip width; and
a middle region extending between the paddle tip region and the
back region and having a front edge with a front edge width which
contacts the paddle tip region and a back edge with a back edge
width which contacts the back region, the front edge width smaller
than the back edge width, the middle region including a middle side
extending between the back region and the paddle tip region,
wherein the back side and the paddle tips form a first angle and
the middle side and the paddle tips form a second angle.
4. The thin film magnetic recording head of claim 3 wherein the
first angle is between about 85.degree. and about 95.degree..
5. The thin film magnetic recording head of claim 3 wherein the
second angle is between about 45.degree. and about 90.degree..
6. The thin film magnetic recording head of claim 3 wherein the
middle region includes a middle region length between the back
region and the paddle tip region and the middle region length is
less than about 35 microns.
7. The thin film magnetic recording head of claim 1 wherein the
back region length plus the middle region length is between about
135 microns and about 165 microns.
8. The thin film magnetic recording head of claim 3 wherein:
the first angle is between about 85.degree. and 95.degree.;
the second angle is between about 45.degree. and about
90.degree.;
the middle region includes a middle region length between the back
region and the paddle tip and the middle region length is less than
about 35 microns; and
the back region length plus the middle region length is between
about 135 microns and about 165 microns.
9. A thin film magnetic head recording head comprising:
a magnetic yoke including top and bottom poles for defining a high
permeability magnetic flux path, the top and bottom poles having a
hard axis of magnetization; and
a flattened, generally elliptical coil laying in a single plane and
extending through the magnetic yoke including a flattened portion
having a plurality of substantially parallel conductors extending
through the magnetic yoke and a substantially elliptical portion
extending beyond the magnetic yoke wherein the plurality of
substantially parallel conductors are perpendicular to the hard
axis of magnetization and parallel to the easy axis for maximizing
exposure of the magnetic yoke to hard axis magnetic drive fields
and minimizing exposure of the magnetic yoke to easy axis magnetic
drive fields whereby magnetic drive fields in an easy axis
direction are less than local coercivity of the magnetic yoke.
10. A thin film magnetic head, comprising:
a substrate;
a magnetic lower pole piece deposited over the substrate including
a lower pole tip;
an insulating layer deposited over the lower pole piece;
a magnetic gap layer deposited over the lower pole tip;
a magnetic upper pole piece deposited over the magnetic lower pole
piece and including an upper pole tip deposited over the magnetic
gap layer, the magnetic upper and lower pole pieces forming a
magnetic flux path to the upper and lower pole tips which form a
magnetic flux gap across the magnetic gap layer, the upper and
lower pole pieces having a hard axis of magnetization and an easy
axis of magnetization which is perpendicular to the hard axis of
magnetization, and the magnetic upper and lower pole pieces have a
coercivity; and
a plurality of electrical conductors, at least one of the plurality
of electrically conductors lying in a single plane and extending
through the insulating layer and between the magnetic upper pole
piece and the magnetic lower pole piece, the plurality of
electrical conductors being oriented parallel with the easy axis of
magnetization and perpendicular to the hard axis of magnetization
of the magnetic upper and lower pole pieces for maximizing exposure
of the magnetic upper and lower pole pieces to hard axis magnetic
drive fields produced by the plurality of electrical conductors and
minimizing exposure of the magnetic upper and lower pole pieces to
easy axis magnetic drive fields produced by the plurality of
electrical conductors, whereby an electrical current passed through
the plurality of electrical conductors provides a magnetic field
parallel to the hard axis of magnetization and perpendicular to the
easy axis of magnetization of the magnetic upper and lower pole
pieces so that magnetic drive fields in an easy axis direction are
less than local coercivity of the pole pieces.
11. The thin film head of claim 10 wherein the substrate includes a
saw alley which extends generally parallel with the easy axis and
spaced apart from the upper and lower pole tips.
12. The thin film head of claim 10 wherein the magnetic upper and
lower pole pieces have and overall length between a back gap which
is opposite the upper and lower pole tips and the upper and lower
pole tips, and a middle region length and a ratio between the
middle region length and the overall length is less than about
0.21.
Description
BACKGROUND OF THE INVENTION
The present invention relates to inductive thin film magnetic
read/write heads. In particular, the invention relates to an
improved thin film head design which provides fewer erroneous
output spikes due to "glitches" during the read back operation.
Thin film magnetic read/write heads are used for magnetically
reading and writing information on a magnetic storage medium such
as a magnetic disk which are in motion relative to one another. A
thin film magnetic head comprises a pair of "yokes" or "paddles"
which form the magnetic core of the head. Electrical conductors
pass between the two paddles and are used for both reading and
writing information on the magnetic storage medium. During a write
operation, electrical current is caused to flow through the
conductors which generates a magnetic field in the paddle. The
structure includes a gap region which comprises a small space
between the two tips of the two paddles. The write current in the
coils causes magnetic flux to span the gap region. This magnetic
flux is then used to impress a magnetic field upon a storage medium
during the write operation which results in a magnetic transition
being recorded. During the read operation the magnetic head and the
storage medium also move relative to each other causing a changing
magnetic field to be induced in the coil and the coils link the
magnetic circuit. Electric signals in the conductors may be sensed
with electric circuitry which allows recovery of information stored
on the magnetic medium.
The performance of a thin film magnetic head is degraded by a
phenomenon known as the "glitch" effect or "remote read" effect.
The remote read effect is characterized by a voltage noise spike
generated by the head in the coils after a random delay following
completion of a write process. The voltage spike is of sufficient
magnitude to be decoded as valid data which causes a system error.
In prior art thin film magnetic heads, the glitch typically has a
probability of occurring about once in every 10.sup.3 to 10.sup.5
write operations.
The occurrence of a glitch causes incorrect data to be read back
from the magnetic medium and is highly undesirable. The prior art
does not explain the interrelation of both design and material
properties used a thin film head on the remote read effect. An
improved thin film head in which the glitch effect or remote read
effect is statistically less likely would be a significant
improvement to the art.
SUMMARY OF THE INVENTION
The subject invention relates top the elimination or minimization
of an effect known variously as the "glitch" effect or "remote
read" effect in inductive thin film head transducers. The remote
read effect is characterized by a voltage noise spike induced in
the coil which was generated by the head after a random delay
following completion of a write process to a magnetic disk. The
effect probably results from the relaxation or sudden change of the
magnetic domain structure of the paddle. Such a voltage noise spike
can be read (incorrectly) as data during a write verification step.
When this occurs, correct write data can be interpreted by the
system as being in error.
The invention provides for configuration of the coil and the paddle
of an inductive thin film head so that the pole (paddle) structure
is subjected primarily to hard axis ("HA") magnetic write fields.
In particular, the head/coil configuration is such that the
magnetic field in the easy axis direction does not exceed the local
coercivity, H.sub.c, of the paddles. Further provided is a pole
(paddle) having a large length-to-width ratio, where elongated
shapes support domain stability. The ferromagnetic pole itself is
formed using a negative magnetostriction material (typically
permalloy) to reduce magnetic distortion of the pole domain
structure. The overall pole tip length prior to machining is small
to minimize interaction observed between slider machining and pole
domain structure variation.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a top sectional view of a thin film magnetic read/write
head in accordance with the present invention.
FIG. 2 is a cross sectional view of the thin film head of FIG. 2
taken along line 2--2.
FIG. 3 shows the magnetic domain pattern for the paddle of the thin
film head of FIG. 1.
FIG. 4 is a top view of a thin film magnetic head in accordance
with the present invention.
FIG. 5 is a perspective view of a wafer carrying many thin film
magnetic read/write heads made in accordance with the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
As the track width of inductive thin film heads decreases, certain
noise phenomena become more and more frequent. One such phenomena,
called a glitch, is typified by the occurrence of a noise spike
during a read operation in the output voltage of the inductive thin
film head after a write process has been completed. The noise
spike, which occurs after a short, random, time lag following
writing, results in a read noise voltage of sufficient amplitude to
cause a system error in the information decoding process of a disk
drive. Since it is a desirable to have a system bit error rate of
less than 1.times.10.sup.-10, and since the glitch phenomena is
observed to occur in prior art designs on the order of
1.times.10.sup.-3 to 1.times.10.sup.-5 for every write, the glitch
effect limits system performance.
The origin of the glitch is related to the output voltage of the
thin film head (V.sub.out) as follows: ##EQU1##
When an inductive thin film head is reading information, H=O, and
dH/dt=0. If the domain structure of the inductive thin film head
changes after a write process has terminated, dM/dt has a
relatively large value, and a glitch voltage proportional to dM/dt
results due to the relationship of Equation 2. Hence the source of
glitch can be identified with sudden, thermodynamically
irreversible changes in the domain structure of an inductive thin
film head after writing. The glitch is probably triggered either by
stress relaxation due to cooling after writing, or by the weak
magnetic read fields that the inductive thin film head senses from
recorded data.
Three experimental observations have lead to the present invention.
First, in the course of studying magnetoresistive head (MRH) domain
structure, it has been determined that any axis (EA) drive field
tends to leave the domain structure of a paddle in a highly
unstable and unpredictable state which can suddenly change.
Furthermore, it has also been found in these studies that the
domain structure is significantly more reproducible and stable for
hard axis (HA) drive fields.
Second, theoretical calculations and pole structure variation
experiments indicate that a narrower pole structure further
stabilizes the inductive thin film head permalloy domain structure
for the negative magnetostriction plating compositions currently
employed in inductive thin film head manufacturing.
Third, these studies have also shown that slider machining
processes can significantly alter the domain structure if attention
is not given to the relation between where machining occurs
relative to the magnetic pole structure. Specifically, the slicing
of the substrate can damage the pole structure if the slicing
operation comes near to or cuts through the pole tip.
The nature of this invention is to utilize these experimental
observations in a new approach which minimizes the glitch rate.
More specifically, by attempting to minimize the total magnetic
energy in the domains of a thin film head, thereby establishing a
domain structure with improved stability, the present invention
reduces the remote read effect. The curve of total domain wall
energy versus number of magnetic domains for a given thin film head
pole design and magnetic material properties has a "U" shape. The
most stable magnetic domain state being found at the minimum of
this curve. The present invention attempts to place the thin film
head pole design at the base of this "U" curve in order to optimize
domain stability by minimizing domain energy. The aspects of this
invention are:
1) A new coil design which subjects the magnetic pole structure to
only hard axis drive fields.
2) A narrow pole design which, when used in conjunction with a
negative magnetostriction bath, improves the stability of the
inherent domain structure.
3) A reduced pole tip length to minimize the interaction between
pole tip machining and pole domain structure variations.
FIG. 1 shows a thin film magnetic head 10 in accordance with the
present invention. Inductive thin film head 10 includes a pair of
yokes, poles or paddles 12, of which only the top pole is seen,
conductors 14 formed in a coil and upper pole tip 16. Saw alley 18
extends parallel to the front edge of pole tip 16. The terms yokes,
poles and paddles are used interchangeably.
FIG. 2 is a cross sectional view of thin film head 10 taken along
line 2--2 in FIG. 1 which is an axis of symmetry of head 10. Lower
yoke; pole or paddle 20 of thin film head 10 and upper pole or
paddle 12 sandwich conductors 14 and conductors 22. Lower paddle 20
includes lower pole tip 24. Upper pole tip 16 and lower pole tip 24
are separated by an insulating gap layer 26 used for reading and
writing information on a magnetic storage medium. Upper paddle 12
and lower paddle 20 are in contact at a back gap via 28. (This
contact is not necessarily required.) Thin film head 10 is
deposited upon substrate 30 adjacent to saw alley 18. Saw alley 18
or kerf is the area where substrate 30 is sliced, explained below
in more detail.
FIG. 3 shows a typical magnetic domain structure of inductive thin
film head 10 in accordance with the present invention. FIG. 3 shows
horizontal (or "180.degree.") walls 31 and "closure" walls 32,
sometimes referred to as "90.degree. walls." The arrows within
domains formed by walls 31 and 32 indicate the direction of the
material's magnetization in a relaxed state, when no external
magnetic fields are applied. (This is also true if all the arrows
pointed in the opposite direction.)
As stated above, the present invention uses a three point approach
to increase the domain stability and reproducibility in thin film
heads. First, the drive field used during the write operation
subjects the magnetic head to drive fields pointed only along the
hard axis direction. This is achieved as shown in FIG. 1 by forming
coil 14 at a perpendicular angle with respect to the magnetic hard
axis of paddle 12 of FIG. 2 (which points from back gap 28 toward
pole tip regions 16 and 24 along an axis of symmetry through the
paddle 12). Coil 14 is formed in a flattened egg or elliptical
shape. The windings of coil 14 curve around paddles 12 and 20 but
the windings are straight and parallel where they pass between
paddles 12 and 20. The straight portion of coil 14 provides a
magnetic drive field in the direction of the hard axis of paddles
12 and 20. In comparison, a prior art thin film head includes a
coil which is wrapped at a substantially curved angle through the
core of the prior art thin film head. By subjecting thin film head
10 of the present invention exclusively to hard axis drive fields,
the magnetic domain structure of paddles 12 and 20 are more stable
and more reproducible than in prior art thin film paddles, and
therefore less susceptible to the glitch or remote read effect. In
particular, the head/coil configuration of the present invention
limits the drive field applied in the easy axis direction so that
it does not exceed the local coercivity (H.sub.c) of the magnetic
material.
FIG. 4 shows an embodiment of the present invention which yields a
significantly reduced glitch rate. Thin film head 34 comprises pole
36 and coil 44. Paddle dimensions A and B and angles C and D of
pole 36 are shown in FIG. 4. Paddle 36 includes back region 38,
middle region 40 and paddle tip region 42. Dimension A is the
length of middle region 40 plus the length of back region 38.
Dimension B is the length of middle region 40. Angle C is formed
between a side of back region 38 and a line perpendicular to the
axis of symmetry of paddle 36. Angle D is formed between a side of
middle region 40 and a line perpendicular with the axis of symmetry
of paddle 36. Through experimental observation on the thin film
heads manufactured using standard plated permalloy, suitable values
for these dimensions and angles to reduce the glitch rate are as
follows:
______________________________________ Parameter Value
______________________________________ A 135-165 .mu.m B Less than
35 .mu.m C 85.degree.-95.degree. D 45.degree.-90.degree.
______________________________________
These values were determined where pole 36 had an anisotropy
(H.sub.K) between about 3 Oersteds and about 5 Oersteds, a
coercivity (H.sub.c) of about 0.8 Oersteds, a saturation
magnetization (M.sub.s) of about 10,000 Gauss, a thickness
(.delta.) between about 1 micron and about 4 microns and a negative
magnetostriction. Paddle width at backgap region 38 is calculated
using the formula: width=pole tip width+(2.times.B/Tan (D)). As the
present invention is for use at high recording densities, pole tip
width is relatively small and can be substantially ignored in
calculating paddle width.
In FIG. 4, thin film head 34 made in accordance with the present
invention also uses a reduced paddle width-to-length ratio which is
narrower than a prior art paddle. This eliminates the paddle "ears"
characteristic of prior art thin film heads. This narrow
configuration with respect to the length of paddle 36 in FIG. 4
further stabilizes the inductive thin film head domain structure
when negative magnetostriction material is used to form paddle 36.
Plating a ferromagnetic alloy with negative magnetostriction is the
method currently used in fabricating inductive thin film magnetic
heads to produce the desired characteristic magnetic domain
structure.
FIG. 5 is a perspective view of a portion of a substrate 30
carrying thin film magnetic heads 10. During fabrication, many thin
film heads 10 are deposited across the surface of substrate 30 as
shown in FIG. 5. After heads 10 are deposited, substrate 30 is
sliced along "saw alleys" 18 into a plurality of bars, each
carrying thin film heads 10 arranged linearly along the bar. The
saw alley or kerf is necessary because the process of slicing the
substrate removes a swath of material which creates and
"alley."
Finally, the present invention uses an improved pole tip design to
reduce stress during the sawing process. Inductive thin film head
10 of the present invention includes pole tips 16 (and 24) of FIG.
2 having a shorter length than a pole tip used in a prior art thin
film head. The purpose of the reduced pole tip length is to provide
an increased distance between pole tips 16 and 24 and saw alley 18.
In a prior art thin film head, the pole tips are often very long
relative to the head. The dimension was not considered important
because the tips were simply cut when the wafer was sliced. Large
separation between the end of the pole tip 42 (of FIG. 4) and the
saw alley is preferred. In the present invention, however, the pole
tip length should be less that about 10 microns. The spacing
between the pole tips and the saw alley provides a buffer region
which limits induced thin film head stress during the sawing
process. Final pole tip dimensions are reached using a standard
lapping process in which the pole tips are applied to a mild
abrasive such as a diamond slurry. This lapping process is far
gentler than the sawing process. By limiting the stress to the thin
film head, the domain structure of the thin film head of the
present invention remains substantially intact in the final
product, further increasing magnetic domain stability and
reproducibility over prior art thin film heads.
The inductive thin film magnetic head of the present invention
offers improved stability an reproducibility in the magnetic domain
pattern thereby limiting the glitch effect or remote read effect.
The present invention uses a three point approach to deliver an
improved overall thin film head package. First, the coils are
formed to produce magnetic drive fields in only the magnetic hard
axis direction. Second, a narrower pole design is used which
eliminates the "ears" of prior art thin film head paddles and
further improves domain structure stability and reproducibility.
Finally, the head is subjected to less stress during the
fabrication process by limiting the interaction of the sawing
process with the structure of the thin film head.
Although the present invention has been described with reference to
preferred embodiments, workers skilled in the art will recognize
that changes may be made in form and detail without departing from
the spirit and scope of the invention. For example, some designs
may only use a bottom pole of ferrite made in accordance with the
present invention.
* * * * *