Title: Method and apparatus for precessional switching of the magnetization of storage medium using a transverse write field
Abstract: A magnetic recording process generally including the steps of determining an initial magnetization direction of a magnetic recording medium, and selectively applying a magnetic field to the magnetic recording medium along an axis substantially perpendicular to an axis of the initial magnetization direction of the recording medium. The magnetic field is selectively applied for a period of time sufficient to switch the magnetization of the magnetic recording medium from its initial magnetization direction to a final magnetization direction substantially anti-parallel to the initial magnetization direction. Typically, the initial and final magnetization directions will be along an easy axis of magnetization of the magnetic recording medium.
Patent Number: 6,985,318 Issued on 01/10/2006 to Clinton, et al.
| Inventors:
|
Clinton; Thomas W. (Pittsburgh, PA);
Crawford; Thomas M. (Pittsburgh, PA)
|
| Assignee:
|
Seagate Technology LLC (Scotts Valley, CA)
|
| Appl. No.:
|
454094 |
| Filed:
|
June 3, 2003 |
| Current U.S. Class: |
360/46; 360/55 |
| Current Intern'l Class: |
G11B 5/09 (20060101) |
| Field of Search: |
360/46,55,123-126
365/158,170-173
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Hudspeth; David
Assistant Examiner: Davidson; Dan I
Attorney, Agent or Firm: Buchanan Ingersoll PC
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of provisional patent application Ser. No.
60/386,774 entitled "Precessional Switching of the Magnetization of a Storage Medium
with a Transverse Write Field", filed on Jun. 6, 2002, the entire disclosure of
which is incorporated by reference herein.
Claims
We claim:
1. A method of magnetically recording information on a magnetic recording medium
having an easy axis of magnetization, the method of comprising the steps of:
providing a longitudinal magnetic recording medium having an easy axis of magnetization
parallel to a plane of the magnetic recording medium;
providing a magnetically soft underlayer adjacent the magnetic recording medium; and
applying a magnetic field to the magnetic recording medium substantially along
an axis perpendicular to the magnetic recording medium's easy axis of magnetization,
wherein the magnetic field is applied for a select period of time sufficient
to switch a magnetization of the magnetic recording medium from a first magnetization
direction to a second magnetization direction substantially anti-parallel to the
first magnetization direction.
2. The method of claim 1, wherein the first and second magnetization directions
are along the magnetic recording medium's easy axis of magnetization.
3. The method of claim 1, wherein the applying step comprises the step of applying
magnetic field pulses to the magnetic recording medium substantially along the
axis perpendicular to the magnetic recording medium's easy axis of magnetization,
wherein each magnetic field pulse is applied for the select period of time sufficient
to switch the magnetization of the magnetic recording medium from the first magnetization
direction to the second magnetization direction substantially anti-parallel to
the first magnetization direction.
4. A method of magnetically recording information on a magnetic recording medium
having an easy axis of magnetization, the method comprising the steps of:
determining a magnetization orientation direction of a bit previously recorded
in the magnetic recording medium along the easy axis of magnetization;
comparing the determined magnetization orientation direction of the previously
recorded bit with a magnetization orientation direction of a bit to be recorded; and
if the compared magnetization orientation directions of the previously recorded
bit and the bit to be recorded are different, applying a magnetic field to the
previously recorded bit in the magnetic recording medium substantially along an
axis perpendicular to the determined magnetization orientation direction of the
previously recorded bit,
wherein the magnetic field is applied for a select period of time sufficient
to switch the magnetization orientation direction of the previously recorded bit
from the determined magnetization orientation direction to the magnetization orientation
direction of the bit to be recorded which is substantially anti-parallel to the
determined magnetization orientation direction and along the magnetic recording
medium's easy axis of magnetization.
5. The method of claim 4, further comprising the step of:
if the compared magnetization orientation directions of the previously recorded
bit and the bit to be recorded are the same, not applying a magnetic field to the
previously recorded bit, such that the previously recorded bit becomes the bit
to be recorded.
6. The method of claim 4, wherein the determining step comprises the step of
DC erasing the magnetic recording medium prior to magnetically recording information
thereon, such that the magnetic recording medium has a uniform magnetization orientation direction.
7. The method of claim 4, wherein the magnetic recording medium is selected from
the group consisting of perpendicular and longitudinal magnetic recording mediums.
8. A magnetic recording device for magnetically recording information on a magnetic
recording medium having an easy axis of magnetization, the magnetic recording device comprising:
a main magnetic pole positionable adjacent the magnetic recording medium;
a coil magnetically coupled to the main magnetic pole for developing a magnetic
field in the main magnetic pole in a first magnetization direction substantially
perpendicular to the magnetic recording medium's easy axis of magnetization, wherein
the magnetic field developed in the main magnetic pole is selectively applied to
the magnetic recording medium in the first magnetization direction for a select
period of time sufficient to switch a magnetization of the magnetic recording medium
from a second magnetization direction to a third magnetization direction substantially
anti-parallel to the second magnetization direction; and
a controller operatively connected to the coil, the controller determining an
initial magnetization direction of the magnetic recording medium and selectively
energizing the coil, based on said determination, to selectively develop the magnetic
field in the main magnetic pole.
9. The magnetic recording device of claim 8, wherein the second and third magnetization
directions are along the magnetic recording medium's easy axis of magnetization.
10. The magnetic recording device of claim 8, wherein the applied magnetic field
includes a magnetic field component along an axis parallel to the magnetic recording
medium's easy axis of magnetization, wherein the magnetic field component is applied
in a fourth magnetization direction substantially parallel to the third magnetization direction.
11. The magnetic recording device of claim 8, wherein the magnetic recording
medium is selected from the group consisting of perpendicular and longitudinal
magnetic recording mediums.
12. The magnetic recording device of claim 8, wherein the magnetic field in the
main magnetic pole comprises magnetic field pulses applied to the magnetic recording
medium in the first magnetization direction, wherein each of the magnetic field
pulses is applied for the select period of time sufficient to switch the magnetization
of the magnetic recording medium from the second magnetization direction to the
third magnetization direction substantially anti-parallel to the second magnetization direction.
13. The magnetic recording device of claim 8, wherein the magnetic recording
medium comprises a DC erased magnetic recording medium having a uniform magnetization direction.
14. The magnetic recording device of claim 8, wherein the controller comprises:
a magnetic read head for determining the initial magnetization direction of the
magnetic recording medium; and
a comparison circuit receiving the determined magnetization direction of the
magnetic recording medium as a first input and a magnetization direction of information
to be recorded as a second input and generating an output signal based on a comparison
of the first and second inputs, the output signal selectively energizing the coil
to selectively develop the magnetic field in the main magnetic pole based on the
comparison of the first and second inputs.
15. A magnetic recording device for magnetically recording information on a magnetic
recording medium having an easy axis of magnetization, the magnetic recording device comprising:
a main magnetic pole positionable adjacent the magnetic recording medium;
a coil magnetically coupled to the main magnetic pole for developing a magnetic
field in the main magnetic pole in a first magnetization direction substantially
perpendicular to the magnetic recording medium's easy axis of magnetization; and
a controller operatively connected to the coil for selectively energizing the
coil to selectively develop the magnetic field in the main magnetic pole, wherein
the controller comprises:
a magnetic read head for determining a magnetization direction of a previously
recorded bit in the magnetic recording medium; and
a comparison circuit receiving the determined magnetization direction of the
previously recorded bit as a first input and a magnetization direction of a bit
to be recorded as a second input and generating an output signal based on a comparison
of the first and second inputs, the output signal selectively energizing the coil
to selectively develop the magnetic field in the main magnetic pole based on the
comparison of the first and second inputs.
16. The magnetic recording device of claim 15, wherein if the compared magnetization
directions of the previously recorded bit and the bit to be recorded are different,
the output signal energizes the coil to apply the magnetic field developed in the
main magnetic pole to the previously recorded bit for a select period of time sufficient
to switch the magnetization direction of the previously recorded bit from the determined
magnetization direction to the magnetization direction of the bit to be recorded
which is substantially anti-parallel to the determined magnetization direction
and along the magnetic recording medium's easy axis of magnetization, and
wherein if the compared magnetization directions of the previously recorded bit
and the bit to be recorded are the same, the output signal de-energizes the coil
to not apply the magnetic field to the previously recorded bit, such that the previously
recorded bit becomes the bit to be recorded.
17. The method of claim 15, wherein the magnetic recording medium is selected
from the group consisting of perpendicular and longitudinal magnetic recording mediums.
18. A method of magnetically recording information on a magnetic recording medium,
the method comprising the steps of:
determining an initial magnetization direction of the magnetic recording medium; and
selectively applying, based on said determination, a magnetic field to the magnetic
recording medium along an axis substantially perpendicular to an axis of the initial
magnetization direction, wherein the magnetic field is selectively applied for
a period of time sufficient to switch a magnetization of the magnetic recording
medium from the initial magnetization direction to a final magnetization direction
substantially anti-parallel to the initial magnetization direction.
19. The method of claim 18, wherein the initial and final magnetization directions
are along an easy axis of magnetization of the magnetic recording medium.
20. The method of claim 18, wherein the magnetic recording medium is selected
from the group consisting of perpendicular and longitudinal magnetic recording mediums.
21. The method of claim 18, wherein the determining step comprises the step of
DC erasing the magnetic recording medium prior to magnetically recording information
thereon, such that the magnetic recording medium has a uniform initial magnetization
direction throughout.
22. The method of claim 18, wherein the determining step comprises the step of
reading the initial magnetization direction of the magnetic recording medium prior
to magnetically recording information thereon.
23. The method of claim 18, wherein the step of selectively applying, based on
said determination, comprises the steps of:
if the determined initial magnetization direction of the magnetic recording medium
and a magnetization direction of information to be recorded are different, applying
the magnetic field to the magnetic recording medium along an axis substantially
perpendicular to an axis of the initial magnetization direction, wherein the magnetic
field is selectively applied for a period of time sufficient to switch a magnetization
of the magnetic recording medium from the initial magnetization direction to a
final magnetization direction substantially anti-parallel to the initial magnetization
direction; and
if the determined initial magnetization direction of the magnetic recording medium
and the magnetization direction of information to be recorded are the same, not
applying the magnetic field to the magnetic recording medium, such that the initial
magnetization direction of the magnetic recording medium becomes the information
to be recorded.
Description
FIELD OF THE INVENTION
The present invention is directed toward magnetic recording processes and, more
particularly, toward a magnetic recording process utilizing a write field applied
transverse to the magnetization of the recording medium.
BACKGROUND OF THE INVENTION
The ability to increase the storage capacity of magnetic recording media is an
on going concern. As the bit areal densities of magnetic recording media continue
to progress in an effort to increase the storage capacity of hard disc drives,
the physical size of the sensors and writers designed to read and write data from
and to the magnetic recording media must correspondingly decrease. As a result
of this push to increase the storage capacity of hard disc drives, magnetic transition,
i.e., bit, dimensions and, concomitantly, recording head critical features are
being pushed below the 100 nm scale. In a parallel effort, in order to make the
magnetic recording medium stable at higher areal densities, magnetically harder
recording medium materials having a high coercivity are required. The high coercivity
of the recording medium helps to ensure the thermal stability of the data recorded
thereon. However, a problem with using high coercivity recording media is that
the magnetic field from the small recording pole needs to be sufficient to overcome
the coercivity of the magnetic recording medium in the disc in order to define
the recorded bits along the recording track in the recording medium.
Traditionally, writing to a harder recording medium has been achieved
by increasing the saturation magnetic flux density, i.e., 4πM
s
value, of the magnetic material which makes up the inductive write head, thus bolstering
the magnetic field applied to the recording medium. Although there has been some
success in the field of materials research to increase the saturation magnetization
M
s of write heads, the rate of increase that has been achieved is not
significant enough to sustain the annual growth rate of bit areal densities in
disc drive storage applications. Further, continued increases in the saturation
magnetization of write heads is likely unsustainable as the materials typically
used for write heads reach their fundamental limitations.
A consequence of higher areal densities in magnetic recording has been an increase
in the data rates at which the data is magnetically recorded. Data rates are advancing
toward a point where they will reach a giga-hertz (GHz) and beyond. At these high
data rates, it becomes increasingly difficult to switch the magnetization of the
recording medium using a conventional write field applied anti-parallel to the
magnetization direction of the recording medium, i.e., to the recording medium's
easy axis of magnetization. Thus, there is a need in the field of magnetic recording
for a recording process capable of switching higher coercivity recording media
at increasingly higher data rates.
The present invention is directed toward overcoming one or more of the above-mentioned problems.
SUMMARY OF THE INVENTION
A magnetic recording process is provided according to the present invention whereby
the write field is applied perpendicular to the recording medium magnetization
direction (easy axis of magnetization) in order to write a bit (magnetic transition)
in the recording medium. Specifically, a transverse write field, with a magnitude
exceeding a predetermined minimum value, is applied to the recording medium for
a duration of time less than a magnetic time scale of the medium, typically on
a nanosecond timescale, such that the magnetization of the recording medium switches
precessionally to its opposite state. The transverse write field applies the maximum
torque to the recording medium magnetization, thus minimizing the energy required
to write a magnetic transition (bit). The short time scale of the applied magnetic
field makes it possible to extend data rates well beyond present recording technology.
The inventive magnetic recording process may be utilized on both longitudinal and
perpendicular oriented recording media.
The inventive magnetic recording process generally includes the steps of determining
an initial magnetization direction of the magnetic recording medium, and selectively
applying a magnetic field to the magnetic recording medium along an axis substantially
perpendicular to an axis of the initial magnetization direction of the recording
medium. The magnetic field is selectively applied for a period of time sufficient
to switch the magnetization of the magnetic recording medium from its initial magnetization
direction to a final magnetization direction substantially anti-parallel to the
initial magnetization direction. Typically, the initial and final magnetization
directions will be along an easy axis of magnetization of the magnetic recording medium.
In one form, the initial magnetization direction of the magnetic recording medium
is compared with the magnetization direction of a bit to be recorded and, if the
compared magnetization directions are different, the magnetic field is applied
to the magnetic recording medium to precessionally switch the magnetization of
the magnetic recording medium from its initial magnetization direction to the final,
anti-parallel magnetization direction of the bit to be recorded. If, on the other
hand, the compared magnetization directions are the same, no magnetic field will
be applied to the magnetic recording medium, such that the magnetic recording medium
is left in its initial magnetization direction which is the magnetization direction
of the bit to be recorded. Thus, when the compared magnetization directions are
the same, no magnetic field is required by the inventive recording process to record
a bit.
In another form, the magnetic recording media is DC erased prior to magnetically
recording information thereon. DC erasing the recording medium ensures that the
medium is uniformly magnetized along the data path to be written, thus allowing
the initial magnetization direction of the magnetic recording medium to be determined.
A selectively applied magnetic field reverses the magnetization of the recording
medium where appropriate, and where the DC erased magnetization direction is desired,
no magnetic field is applied so that no magnetization switching occurs.
A magnetic recording device for magnetically recording information on a magnetic
recording medium is also provided according to the present invention. The magnetic
recording device includes a main magnetic pole positionable adjacent the magnetic
recording medium, and a coil magnetically coupled to the main magnetic pole for
developing a magnetic field in the main magnetic pole in a first magnetization
direction. In accordance with the present invention, the magnetic recording medium
has an easy axis of magnetization along which magnetic transitions, or bits, are
recorded. The first magnetization direction of the magnetic field is substantially
perpendicular to the magnetic recording medium's easy axis of magnetization. The
magnetic field developed in the main magnetic pole is selectively applied to the
magnetic recording medium in the first magnetization direction for a select period
of time sufficient to switch the magnetization of the magnetic recording medium
from an initial magnetization direction to a final magnetization direction substantially
anti-parallel to the initial magnetization direction. The magnetic recording device
may further include a controller operatively connected to the coil for selectively
energizing the coil to selectively develop the magnetic field in the main magnetic pole.
In one form, the controller includes a magnetic read head for determining the
initial magnetization direction of the magnetic recording medium, and a comparison
circuit receiving the determined initial magnetization direction and the magnetization
direction of a bit to be recorded. Based on a comparison of the magnetization directions,
the comparison circuits generates an output signal to selectively energize the
coil to selectively develop the magnetic field in the main magnetic pole to switch
the initial magnetization direction of the magnetic recording medium where appropriate.
The output signal, by selectively energizing the coil, generates an appropriate
sequence of magnetic field pulses in the main magnetic pole to reverse the initial
magnetization direction of the magnetic recording medium where appropriate and,
where the initial magnetization direction is desired, the main magnetic pole is
left in its quiescent state so that no magnetic switching of the magnetic recording
medium occurs.
It is an aspect of the present invention to increase the data rate of magnetic
recording processes.
It is a further aspect of the present invention to increase the storage capacity
of hard disc drives.
It is yet a further aspect of the present invention to utilize materials having
high coercivities as magnetic recording media in magnetic recording processes.
It is still a further aspect of the present invention to develop a magnetic recording
process capable of switching higher coercivity recording media at increasingly
higher data rates.
Other aspects and advantages of the present invention can be obtained from
a study of the specification, the drawings, and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of the precessional switching process according
to the present invention;
FIG. 2 is a perspective view of a magnetic recording head according to a first
embodiment of the present invention;
FIG. 3 is a timing diagram of current pulses in accordance with the precessional
switching process of the present invention;
FIG. 4 is a perspective view of a magnetic recording head according to a second
embodiment of the present invention;
FIG. 5 is a perspective view of a magnetic recording head according to a third
embodiment of the present invention;
FIG. 6 is a perspective view of a magnetic recording head according to a fourth
embodiment of the present invention;
FIG. 7 is a perspective view of a magnetic recording head according to a fifth
embodiment of the present invention;
FIG. 8 is a perspective view of a magnetic recording head according to a sixth
embodiment of the present invention; and
FIG. 9 is a perspective view of a magnetic recording head according to a seventh
embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention demonstrates that magnetization reversal can be achieved
in lithographically defined magnetic elements using sub-nanosecond magnetic field
pulses applied along the magnetization hard axis at right angles to the initial
magnetization direction of the magnetic elements. In fact, the present invention
reveals that the magnetization can be reversed from either of its bi-stable states
with a unidirectional, transverse magnetic field pulse. The field pulse need only
be applied with enough field strength that the precessional trajectory of the magnetization
overshoots the magnetic hard axis (goes beyond 90° from the easy axis of magnetization),
while the pulse duration should be short enough that the field turns off just before
the magnetization reaches the anti-parallel direction, i.e., Δt<τ
π,
where τ
π is the time required to precessionally switch the
magnetization 180°. The underlying physics of the present invention are expressed
by the following Landau-Lifshitz equation, which provides a simple model to describe
the dynamics of a single-domain magnetization {right arrow over (M)} in the presence
of a magnetic field {right arrow over (H)}.
##EQU1##
The constants in Eq. 1 are as follows: μ
o—the permeability
of free space; γ—the gyromagnetic ratio of the media; α—the
damping constant of the media. The first term of Eq. 1 describes the precessional
motion of the magnetization {right arrow over (M)} about the field {right arrow
over (H)}, while the second term of Eq. 1 represents the damping of the precessional
motion and ultimately will force the magnetization {right arrow over (M)} to relax
along the magnetic field {right arrow over (H)}. For timescales short enough, the
precessional motion term of Eq. 1 describes most of the dynamics, as there is no
time for significant damping to occur. In magnetic recording processes, the conventional
write process is quasi-static and damping term of Eq. 1 will describe the relevant
dynamics of the magnetization of the storage medium, where {right arrow over (M)}
ultimately relaxes along the effective direction of the write field, i.e., {right
arrow over (M)}∥{right arrow over (H)}
write, parallel to the
easy axis of magnetization of the storage medium. However, it has been found herein
that a writing process using a transverse magnetic field has the benefit of applying
the field with a maximum torque, T, applied to the magnetization where T=|{right
arrow over (M)}∥{right arrow over (H)}|sin θ=(|{right arrow over (M)}×{right
arrow over (H)}|), and θ is the angle between {right arrow over (M)} and
{right arrow over (H)}. Additionally, it has been found herein that if the magnetic
field is applied on a short timescale, such that the magnetization reverses precessionally,
then the switching speed will exceed current state-of-the-art data rates. Both
of these aspects associated with the precessional switching method described herein
minimize the energy needed to reverse the magnetization, and are conducive to extending
areal densities and data rates in the field of magnetic recording.
The precessional switching process of the present invention is schematically
depicted in FIG. 1. The magnetization of the recording medium is in an initial
state {right arrow over (M)}
o along the recording medium's easy axis
of magnetization, which is shown along the y-axis in FIG. 1. A magnetic field pulse
{right arrow over (H)}(Δt) is applied perpendicular to the initial magnetization
{right arrow over (M)}
o with a sufficient magnitude that the initial
magnetization {right arrow over (M)}
o overshoots its hard axis (goes
beyond 90° from the easy axis of magnetization). If the damping constant,
α, of the recording media is small enough, the precessional overshoot helps
to reduce the transverse magnetic field required for switching. If the magnetic
field is turned off just before the magnetization precessional trajectory {right
arrow over (M)}(t) passes the anti-parallel direction, the final magnetization
{right arrow over (M)}
f will be reversed from the initial magnetization
{right arrow over (M)}
o. While the field pulse {right arrow over (H)}(Δt)
is shown in FIG. 1 as applied along the x-axis, which is a magnetic hard axis of
the recording media, the magnetic filed pulse {right arrow over (H)}(Δt)
may also be applied along the z-axis, which is another magnetic hard axis of the
media, without departing from the spirit and scope of the present invention. The
inventive switching process, as outlined above, requires knowing the initial magnetization
state to achieve a particular, final magnetization state.
The above-outlined inventive method is particularly useful in disc storage recording
processes, where the magnetization is that of the magnetic recording medium and
the write head delivers the transverse magnetic field pulse. The write field is
a spatial and temporal coordination of both a transverse (switching) field and
a field parallel to the recording medium's easy axis of magnetization (set field).
An inventive writing process is described herein whereby a transverse magnetic
field can be used exclusively to record data to a magnetic storage medium. The
duration of the transverse magnetic field pulse, Δt, has a similar role in
determining the final magnetization direction as that of the set field. The magnetic
pulse duration is a function of the storage medium used, its physical parameters,
as well as a function of the intensity of magnetic field pulse from the write head.
For exemplary purposes only, it is contemplated herein that a pulse duration Δt
on the order of 1 nanosecond may be sufficient to precessionally the magnetization,
however, other pulse durations are contemplated in accordance with the parameters
previously set forth. Described below are several detailed realizations of the
present invention that are by no means exhaustive, but are intended to convey the
general idea of the present invention to one of ordinary skill in the art.
FIG. 2 illustrates a single-pole inductive writer shown generally at
10,
which incorporates the inventive precessional writing process. The writer
10
includes a main magnetic pole
12, a magnetic return pole
14, and
a magnetic via
16 connecting the main
12 and return
14 magnetic
poles. An electrically conductive magnetizing coil(s)
18 is provided about
the magnetic via
16 and is magnetically coupled to the main pole
12
to generate a write flux
20 through the main pole
12. The write flux
20 flows into a recording medium
22 disposed adjacent the writer
10 at an air bearing surface thereof to write information onto the recording
medium
22. The return pole
14 and magnetic via
16 provide
a return path for the flux
20.
The writer
10 shown in FIG. 2 can deliver a largely perpendicular field
to the recording medium
22, which is a longitudinal media having an easy
axis of magnetization
24 parallel to a plane of the media
22. A magnetically
soft underlayer (SUL)
26 is provided underneath the recording medium
22
which has the effect of "pulling" magnetic field
20 through the recording
medium
22, such that the magnetic field
20 is largely perpendicular
as it passes through the recording medium
22.
The dashed arrow
28 shown in FIG. 2 represents the initial magnetization
direction {right arrow over (M)}
o associated with a data bit previously
recorded in the media
22. As shown in FIG. 2, the magnetic field
20
is applied to the medium
22 along a magnetic hard axis perpendicular to
the easy axis of magnetization
24. The perpendicular field
20 is
applied with a magnitude and duration appropriate to reverse the initial magnetization
direction
28 of the recorded data bit to the desired final state {right
arrow over (M)}
f represented by the solid arrow
30. As shown
in FIG. 2, the final magnetization direction
30 is substantially anti-parallel
to the initial magnetization direction
28, with both magnetization directions
28,
30 lying along the medium's easy axis of magnetization
24.
Although the magnetic field
20 generated by the writer
10
can be unidirectional for magnetization reversal, since either field polarity can
be generated by such a writer design it is proposed to utilize the write field
orientation depicted in FIG. 2, where the small, but non-zero, longitudinal field
component of the magnetic field
20 is parallel (as opposed to anti-parallel)
with the final magnetization direction
30 to further minimize the energy
required to write a magnetic transition (bit). For example, if the situation shown
in FIG. 2 were reversed and the initial magnetization direction of the medium
22
was shown at the solid arrow
30 with the final, desired magnetization direction
shown at the dotted arrow
28, the field polarity of the magnetic field
20
would be reversed from that shown in FIG. 2 (travel counter-clockwise), to ensure
that the small, non-zero longitudinal field component of the field
20 is
parallel with the final magnetization direction. In order for the magnetization
switching to be precessional, the write field
20 is required to be applied
on a short timescale, energized by a short timescale current pulse I(Δt),
shown at
32, to effectively create a magnetic footprint in the media
22.
Additionally, the media
22 should be properly engineered to have a small
damping constant, α, and to rotate coherently upon application of the transverse
switching field
20. In other words, the individual magnetic grains which
make up a recorded bit should all rotate along basically the same path upon application
of the transverse switching field
20.
The time dependence of the current pulses required to generate the switching
magnetic field is shown in FIG. 3. As shown in FIG. 3, the approximate time dependence
of the current pulses are realizable on a sub-nanosecond timescale with the inventive
technology. The current pulses, shown at
34, should not exceed the duration
Δt>τ
π, where τ
π is the
maximum time to precessionally switch the magnetization of the medium to a substantially
anti-parallel direction. As an example, a scenario of writing to a DC erased medium
is considered. In a DC erased medium, the initial magnetization of all bits is
known and is the same. The clock cycle time, τ
clock, which is
the inverse of the data rate (GHz), needs to be at least as long as the current
pulse duration Δt, as two current pulses
34 of opposite current polarity
will be generated every two clock cycles, and thus τ
clock≧Δt.
The zero-current time τ
o is given by the equation τ
0=τ
clock-Δt,
and the zero-current time To will be dictated by the media switching speeds and
data rate of the magnetic recording. Modeling results have indicated that there
is no lower bound to the magnetic field duration for precessional switching according
to the present invention, only an upper bound. It can thus be assumed that the
current pulse
34 duration Δt is at least a fraction of a nanosecond
and, likely, considerably less than the clock cycle (τ
clock>>Δt).
In this case, the magnetic write head would write by making a magnetic footprint
in the recording medium, where a recorded bit in the medium would be basically
a "snapshot" of the field distribution of the whole magnetic recording head where
the field exceeds the coercivity of the recording medium.
The inventive writing process described herein has the potential for very high
data rates, well in excess of a giga-hertz (GHz) as discussed previously. With
this in mind, a writer designed in accordance with the present invention must have
a high bandwidth capability. Presently, it is not known to what frequencies the
inductive writers shown and described herein can be extended and, thus, it is proposed
to use a writer that has the high frequency characteristics appropriate for the
inventive recording process described herein. There are various writer designs
for either longitudinal or perpendicular magnetic recording that have been proposed
and designed to have a very high bandwidth for writing, and would be appropriate
to use for the inventive precessional recording concept described herein at frequencies
in excess of a giga-hertz. However, for pedagogical purposes only, the present
invention described herein is illustrated as utilized in connection with inductive
writers, since their operation is well recognized in the field. However, by no
means is the present invention intended to be limited to only conventional writer
designs, and other writer designs may be utilized without departing from the spirit
and scope of the present invention.
FIG. 4 illustrates a longitudinal inductive writer, shown generally at
36,
which incorporates the inventive precessional writing process. The writer
36
includes a main magnetic pole
38, a magnetic return pole
40, and
a magnetic yoke, or via,
42 connecting the main
38 and return
40
magnetic poles. An electrically conductive magnetizing coil
44 is provided
about the magnetic via
42 and is magnetically coupled to the main pole
38
to generate a write flux
46 through the main pole
38. The write flux
46 flows into the magnetic recording medium
48 disposed adjacent
the writer
36 at an air bearing surface thereof to write information onto
the recording medium
48. The return pole
40 and magnetic via
42
provide a return path for the flux
46.
The recording medium
48 is a longitudinal recording media having an easy
axis of magnetization
50 which lies parallel to a plane of the recording
medium
48. The soft underlayer shown in FIG. 2 is not provided, and the
magnetic field
46 flowing through the recording medium
48 to write
a magnetic transition (bit) includes both longitudinal
52 and perpendicular
54 field components. The peak magnitudes of the perpendicular
54
and longitudinal
52 field components are comparable, but the perpendicular
field component
54 applies the largest torque to the media
48. If
the magnetic field pulse duration is short enough, the longitudinal field component
52 will not effect the magnetization of the media
48 significantly,
and the writing will be precessional as the perpendicular field component
54
will dominate the process. As shown in FIG. 4, the perpendicular write field is
the perpendicular field component
54 at the trailing edge
56 of the
main magnetic pole
38. The dashed arrow
58 represents the initial
magnetization direction associated with a data bit recorded in the medium
48,
and the perpendicular field component
54 is applied with a magnitude and
duration appropriate to reverse its direction to the desired final magnetization
state represented by the solid arrow
60. It is proposed herein to use a
write field
46 orientation as depicted in FIG. 4, where the longitudinal
field component
52 is parallel (as opposed to anti-parallel) with the final
magnetization direction
60 to further minimize the energy required to write
a magnetic transition (bit). For example, if the situation shown in FIG. 4 were
reversed and the initial magnetization direction of the medium
48 was shown
at the solid arrow
60 with the final, desired magnetization direction shown
at the dotted arrow
58, the field polarity of the magnetic field
46
would be reversed from that shown in FIG. 4 (travel counter-clockwise), to ensure
that the longitudinal field component
52 is parallel with the final magnetization
direction. However, other write field orientations may be utilized without departing
from the spirit and scope of the present invention. Additionally, in order for
the magnetization switching to be precessional, the write field
46 is required
to be applied on a short timescale, energized by a short timescale current pulse
I(Δt), shown at
61, to effectively create a magnetic footprint in
the media
48.
FIG. 5 illustrates a single-plane yoke (SPY) inductive writer, shown generally
at
62, for applying a transverse field to a longitudinal recording media
in accordance with the precessional recording method according to the present invention.
The SPY writer
62 includes a main magnetic pole
64, a magnetic return
pole
66, and a magnetic via
68 connecting the main
64 and
return
66 magnetic poles. An electrically conductive magnetizing coil
70
is provided about the magnetic via
68 and is magnetically coupled to the
main pole
64 to generate a write flux
72 through the main pole
64.
The write flux
72 flows into the recording medium
74 disposed adjacent
the SPY writer
62 at an air bearing surface thereof to write information
onto the recording medium
74. The return pole
66 and magnetic via
68 provide a return path for the flux
72.
The magnetic recording medium
74 is longitudinal recording media having
an easy axis of magnetization
76 which is parallel with the plane of the
longitudinal media
74. The SPY writer
62 has the benefit of applying
a largely transverse magnetic field
72 to the magnetization of the media
74 using a low complexity writer design. The magnetic field
72 is
applied perpendicular to the magnetization direction of the magnetic transitions
recorded along the medium's easy axis
76, but with a magnetic field
72
that is largely in the plane of the medium
74. The dashed arrow
78
represents the initial magnetization direction associated with a data bit previously
recorded in the medium
74. The magnetic field
72 is applied with
a magnitude and duration appropriate to reverse the initial magnetization direction
78 to the desired final state magnetization direction represented by the
solid arrow
80, which is substantially anti-parallel to the initial magnetization
direction
78. In order for the media switching to be precessional, the perpendicular
write field
72 is applied on a short timescale, energized by a short timescale
current pulse I(Δt), shown at
82, effectively creating a magnetic
footprint in the media
74.
FIG. 6 illustrates the SPY writer
62 shown in FIG. 5 utilized for precessional
recording in accordance with the present invention to a perpendicular magnetic
recording medium
84. The perpendicular medium
84 includes an easy
axis of magnetization
85 which is substantially perpendicular to the plane
of the medium
84. As shown in FIG. 6, the SPY writer
62 has the benefit
of applying a largely transverse magnetic field
72 to the magnetization
of the media
84 using a low complexity writer design. The magnetic field
72 applied by the SPY writer
62 is applied transverse to the magnetization
direction of the magnetic transitions recorded in the media
84, but with
a magnetic field that is largely in the plane of the media
84. The dashed
arrow
86 represents the initial magnetization direction associated with
a data bit previously recorded in the medium
84. The perpendicular magnetic
field
72 is applied with a magnitude and duration appropriate to reverse
the initial magnetization direction
86 to the desired final state magnetization
direction represented by the solid arrow
88. In order for the media switching
to be precessional, the write field
72 is applied on a short timescale,
energized by the short timescale current pulse I(Δt), shown at
82,
effectively creating a magnetic footprint in the media
84.
In using the SPY writer
62 to record magnetic transitions in a perpendicular
media
84, there is a field component applied to the initial magnetization
direction
86 that is parallel to the magnetization easy axis
85 of
the media
84. The peak magnitudes of the transverse and parallel field components
are comparable, but the transverse field component applies the largest torque to
the media
84. If the field pulse duration is short enough, the parallel
field component will not effect the magnetization significantly and the writing
will be precessional as the transverse field component dominates the process.
FIG. 7 illustrates the longitudinal inductive writer
36 shown in FIG.
4 utilized to record magnetic transitions to a perpendicular magnetic recording
medium
90 in accordance with the precessional recording method of the present
invention. The perpendicular magnetic medium
90 has an easy axis of magnetization
92 which is perpendicular to the plane of the medium
90. The magnetic
field
46 is applied transverse to the magnetization direction of the magnetic
transitions recorded in the media
90, but with a field that is largely in
the plane of the media
90. The dashed arrow
94 represents the initial
magnetization direction associated with a data bit previously recorded in the medium
90. The perpendicular magnetic field
46 is applied with a magnitude
and duration appropriate to reverse the initial magnetization direction
94
to the desired final state magnetization direction represented by the solid arrow
96. The magnetic field
46 is applied transverse to the magnetization
direction of the recorded bits, but with a field that is largely in the plane of
the media
90. In order for the media switching to be precessional, the write
field
46 is applied on a short timescale, energized by the short timescale
current pulse I(Δt), shown at
61, effectively creating a magnetic
footprint in the media
90.
It should be noted that there is a field component applied parallel to the magnetization
easy axis
92 of the media
90, as well. The peak magnitudes of the
transverse and parallel field components are comparable, but the transverse field
component applies the largest torque to the media
90. If the field pulse
duration is short enough, the parallel field component will not effect the magnetization
significantly, and the writing will be precessional as the transverse field component
will dominate the process. It is proposed to use the write field orientation depicted
in FIG. 7, where the parallel component of the field at the trailing edge
56
is aligned, as opposed to anti-parallel, with the final magnetization direction
96 to further minimize the energy required to write magnetic transitions
(bits). For example, if the situation shown in FIG. 7 were reversed and the initial
magnetization direction of the medium
90 was shown at the solid arrow
96
with the final, desired magnetization direction shown at the dotted arrow
94,
the field polarity of the magnetic field
46 would be reversed from that
shown in FIG. 7 (travel counter-clockwise), to ensure that the small, non-zero
longitudinal field component of the field
46 is parallel with the final
magnetization direction. However, any write field orientation may be utilized without
departing from the spirit and scope of the present invention.
As previously discussed, the present invention for precessional writing requires
knowledge of the initial magnetization orientation of the recording medium to achieve
the desired final magnetization direction. This is unlike traditional magnetic
recording where an overwrite process is essentially independent of the initial
magnetization condition. FIGS. 8 and 9 illustrate two ways to precessionally write
according to the present invention when the initial magnetization conditions need
to be established.
FIG. 8 illustrates the single-pole inductive writer
10 shown in FIG.
2 utilized with a controller, shown generally at
98, for determining the
initial magnetization direction of the recording medium
22 and selectively
energizing the coil
18 to selectively develop the magnetic field
20
in the main magnetic pole
12. While the controller
98 is depicted
in FIG. 8 as utilized with the single-pole inductive writer
10, the controller
98 may be utilized with any writer design for precessionally recording magnetic
transitions according to the present invention.
As shown in FIG. 8, the controller
98 includes a magnetic read head
100
for sensing the initial magnetization orientation direction of the recording medium
22 prior to writing to it. The determined magnetization direction of a previously
recorded bit in the magnetic recording medium
22 is sensed by the read head
100 and fed back into the writing process. The reader
100 is positioned
at the leading edge of the writer
10 to sense the magnetization orientation
of the bit. The reader output, shown at
102, is fed to a comparison circuit
104 which also receives the data
106 to be recorded in the magnetic
recording medium
22. The reader output data
102 and the to-be-written
data
106 are compared by the comparison circuit
104, which in turn
generates an output signal
108 which selectively energizes the coil
18
using current pulses
32 to selectively develop the magnetic field
20
based on the comparison of the reader output
102 and to-be-written data
106. The recording medium
22 may be magnetized in either of two bi-stable
states along the easy axis of magnetization
24. The two bi-stable states
of magnetization represent either logic "1" or logic "0" recorded bits.
Basically, three unique outcomes are possible based on the possible initial
and final magnetization orientations of the magnetic recording media
22
(±M), where +M represents logic "1" and -M represents logic "0". If the initial
and final magnetizations are determined to be the same, no magnetic field is applied
and the sensed magnetization orientation of the previously recorded bit becomes
the to-be-written data (initial (+M)=final (+M) or initial (-M)=final (-M), no
magnetic field applied). If the initial magnetization is determined to be positive,
and the final magnetization is required to be negative, a positive magnetic field
pulse is applied as shown in FIG. 8 (initial (+M)≠final (-M), positive magnetic
field pulse applied). Finally, if the initial magnetization is determined to be
negative and the final magnetization is required to be positive, a negative field
pulse is applied which would be in the direction oppose that shown in FIG. 8 (initial
(-M)≠final (+M), negative magnetic field pulse applied).
Since the reader on a conventional head is inactive during the write process,
the reader is available during writing to function as the above-described read
sensor
100. Thus, this embodiment of the present invention does not require
an additional field sensor, and the level of complexity of the magnetic recording
head for precessional recording according to the present invention is simplified.
It should be noted, however, that the reader
100 should be properly shielded
from the write head
10 so that it can continue to perform during the entire
writing process.
A further embodiment of the present invention is to precessionally write to a
DC
erased media. FIG. 9 illustrates the longitudinal inductive writer
36 shown
in FIGS. 4 and 7 utilized for precessionally writing to a DC erased longitudinal
medium
110. The DC erased longitudinal medium
110 includes an easy
axis of magnetization
112 which is parallel with the plane of the medium
110. As shown in FIG. 9, the medium
110 is initially uniformly magnetized
along the data path to be written, i.e., DC erased. The initial magnetization states
are shown by the dotted arrows
114. An appropriate sequence of magnetic
pulses provided by the writer
36 will reverse the magnetization where appropriate,
and where the DC erased orientation
114 is desired, the writer
36
will be left in its quiescent state so that no switching occurs and the initial
magnetization
114 becomes the final magnetization, as shown at arrow
116.
It should be noted that the embodiment shown in FIG. 9 requires a magnetic recording
head which can generate a large enough field parallel to the media magnetization
to DC erase it.
The present describes a method and apparatus for magnetic recording based on
precessional switching of the magnetization of the media, which is in contrast
to the quasi-static switching employed in conventional magnetic recording. The
magnetization of the storage medium can be reserved using a transverse magnetic
field applied for a duration of time that is short compared to the clock cycle.
A transverse magnetic field applies the maximum torque to the medium magnetization,
minimizing the energy required to write a magnetic transition (bit), while the
short timescale makes it possible to extend data rates well beyond present recording
technology. Additionally, the inventive precessional writing technique and apparatus
described herein should make it possible to extend areal densities of hard disc
drives well beyond the present state-of-the-art technology.
Both the magnitude of the applied transverse magnetic field and the pulse duration
Δt can be determined, or calculated, theoretically using the equations provided
herein. Alternately, they can be determined using a trial and error approach which
will be readily appreciated by one of ordinary skill in the art. For example, the
pulse duration Δt may be determined by bringing the write head in contact
with the recording media and initially applying a magnetic field to the media for
the shortest duration possible. The magnetic field should be at a fixed magnetic
field strength starting with the maximum field available from the write head. The
duration of the applied field is then increased until the write head writes to
the recording media. The pulse duration Δt is then continually increased
until the write process is no longer optimum (the write head stops writing or writes
the wrong bit, or the writing process takes too long to be consistent with the
desired data rate, etc.). This will give a pulse window (minimum and maximum field-pulse
time duration) in which to work. The optimum pulse duration Δt should be
within this pulse window.
Similarly, and for exemplary purposes only, the magnitude of the transverse
magnetic field can be determined using the experimental process previously described
at different magnetic field strengths (different write currents, different write
head designs, different write head materials, etc.). In this manner, both the pulse
duration Δt and the magnetic field strength can be optimized for a given
recording system.
While the present invention has been described with particular reference to
the drawings, it should be understood that various modifications can be made without
departing from the spirit and scope of the present invention. For example, the
current pulse duration to develop the magnetic field pulses may vary depending
upon the particular physical parameters of the recording media utilized and the
magnetic field intensity from the magnetic recording head. Additionally, the recording
medium should be chosen to have a small damping constant, α, and rotate coherently
upon application of the transverse magnetic field. However, based on the teachings
herein, these particular variables and materials are readily ascertainable by those
of ordinary skill in the art.
*
