U.S. patent application number 10/727703 was filed with the patent office on 2005-06-09 for magnetic recording system with patterned multilevel perpendicular magnetic recording.
Invention is credited to Albrecht, Manfred, Hellwig, Olav, Hu, Guohan, Terris, Bruce David.
Application Number | 20050122612 10/727703 |
Document ID | / |
Family ID | 34465770 |
Filed Date | 2005-06-09 |
United States Patent
Application |
20050122612 |
Kind Code |
A1 |
Albrecht, Manfred ; et
al. |
June 9, 2005 |
MAGNETIC RECORDING SYSTEM WITH PATTERNED MULTILEVEL PERPENDICULAR
MAGNETIC RECORDING
Abstract
A magnetic recording system, such as a magnetic recording disk
drive, uses a patterned perpendicular magnetic recording medium
where each magnetic block or island contains a stack of individual
magnetic cells to provide multilevel recording. Each cell in an
island is formed of a material or set of materials to provide the
cell with perpendicular magnetic anisotropy and is a single
magnetic domain. Each cell is magnetically decoupled from the other
cells in its island by nonmagnetic spacer layers. Thus each cell
can have a magnetization (magnetic moment) in one of two directions
(into or out of the plane of the layer making up the cell), and
this magnetization is independent of the magnetization of the other
cells in its island. This permits multiple magnetic levels or
states to be recorded in each magnetic island.
Inventors: |
Albrecht, Manfred;
(Isny-Rohrdorf, DE) ; Hellwig, Olav; (Berlin,
DE) ; Hu, Guohan; (Campbell, CA) ; Terris,
Bruce David; (Sunnyvale, CA) |
Correspondence
Address: |
THOMAS R. BERTHOLD
18938 CONGRESS JUNCTION COURT
SARATOGA
CA
95070
US
|
Family ID: |
34465770 |
Appl. No.: |
10/727703 |
Filed: |
December 3, 2003 |
Current U.S.
Class: |
360/59 ; 360/135;
360/44; 360/48; G9B/5.306 |
Current CPC
Class: |
B82Y 10/00 20130101;
G11B 5/743 20130101; G11B 2005/0021 20130101; G11B 5/855 20130101;
G11B 5/66 20130101 |
Class at
Publication: |
360/059 ;
360/048; 360/135; 360/044 |
International
Class: |
G11B 005/02; G11B
005/09; G11B 005/82 |
Claims
What is claimed is:
1. A magnetic recording system comprising a multilevel magnetic
recording medium comprising a substrate and a plurality of
spaced-apart magnetic islands on the substrate, each island
comprising at least two stacked magnetic cells, each cell in an
island being separated from the other cells in its island and
having a magnetic moment oriented in one of two opposite directions
substantially perpendicular to the substrate; and an inductive
write head including an electrical coil for generating magnetic
fields generally perpendicularly to the substrate, the head being
capable of switching the orientation of the moment of one cell in
an island without switching the orientation of the moments of the
other cells in that island.
2. The system of claim 1 further comprising a current source
coupled to the coil and capable of generating current in two
directions.
3. The system of claim 2 wherein the current source is capable of
generating current in two directions and with at least two
different values in each direction.
4. The system of claim 1 wherein the head is a longitudinal head
having fringe fields oriented generally perpendicularly to the
substrate.
5. The system of claim 1 wherein the head is a perpendicular
head.
6. The system of claim 1 wherein the head is a cantilever probe
having a probe tip, the probe tip being formed of magnetic
material.
7. The system of claim 2 further comprising a heater for heating an
island.
8. The system of claim 7 wherein the inductive head has a write
pole and wherein the heater is located adjacent the write pole.
9. The system of claim 7 wherein the inductive head has two poles
and wherein the heater is located between the two poles.
10. The system of claim 7 wherein the heater is an electrically
resistive heater.
11. The system of claim 1 wherein the system is a disk drive and
the medium is a rotatable disk, and wherein the islands are
arranged on the substrate in generally concentric tracks.
12. The system of claim 8 further comprising an actuator coupled to
the head for moving the head across the tracks.
13. The system of claim 9 wherein the head is a magnetic force
microscopy probe having a cantilever and a probe tip at one end of
the cantilever, and wherein the other end of the cantilever is
attached to the actuator.
14. The system of claim 1 wherein the system is a scanning probe
system and wherein the islands are arranged on the substrate in an
x-y array and the head is a cantilever probe having a probe tip,
the probe tip being formed of magnetic material, the probe tip and
array of islands being movable relative to one another in x and y
directions.
15. The system of claim 11 wherein the islands in the x-y array are
grouped into array sections and further comprising a plurality of
cantilever probes, each probe being associated with an array
section and each probe and its associated array section being
movable relative to one another in x and y directions.
16. The system of claim 1 wherein each island includes a layer of
nonmagnetic material between the stacked cells for separating the
cells.
17. The system of claim 1 wherein the islands are spaced apart by
voids.
18. The system of claim 17 wherein the substrate is patterned into
a plurality of pillars and wherein the islands are formed on the
pillars.
19. The system of claim 1 wherein the islands are spaced apart by
spacing material formed on the substrate between the islands and
having substantially no perpendicular magnetic anisotropy.
20. The system of claim 19 wherein the spacing material is
nonmagnetic.
21. The system of claim 1 wherein there are only two cells in each
island.
22. The system of claim 1 wherein each cell is a multilayer of
alternating layers of a first material selected from the group
consisting of Co and Fe and a second material selected from the
group consisting of Pt and Pd, said multilayer having magnetic
anisotropy substantially perpendicular to the substrate.
23. The system of claim 1 wherein each cell is formed of a
ferromagnetic material comprising one or more of Co, Ni, Fe and
alloys thereof.
24. The system of claim 23 wherein each cell is formed of a
ferromagnetic material comprising an alloy of Co and Cr having a
magnetocrystalline anisotropy substantially perpendicular to the
substrate.
25. The system of claim 24 wherein each cell is formed directly on
a growth enhancing sublayer.
26. The system of claim 25 wherein the growth enhancing sublayer is
formed of a material selected from the group consisting of Ti,
TiCr, C, NiAl, SiO.sub.2 and CoCr, where Cr is about 35-40 atomic
percent in the CoCr sublayer.
27. The system of claim 1 wherein the cell closest to the substrate
in each island has a magnetic coercivity greater than the magnetic
coercivity of the other cells in its island.
28. The system of claim 1 further comprising an underlayer on the
substrate beneath the islands.
29. The system of claim 28 wherein the underlayer is a soft
magnetically permeable underlayer of material selected from the
group consisting of NiFe, FeAlSi, FeTaN, FeN, CoFeB and CoZrNb.
30. A magnetic recording disk drive comprising: a multilevel
magnetic recording disk comprising a substrate and a plurality of
spaced-apart magnetic islands on the substrate, each island
comprising at least two stacked magnetic cells and a nonmagnetic
spacer layer between said at least two cells, each cell in an
island having a magnetic moment oriented in one of two opposite
directions substantially perpendicular to the substrate, the cell
closer to the substrate in each of the islands having a coercivity
greater than the other cells in the islands; and an inductive write
head including an electrical coil for generating magnetic fields
generally perpendicularly to the substrate, the head being capable
of switching the orientation of the moment of one cell in an island
without switching the orientation of the moments of the other cells
in that island.
31. The disk drive of claim 30 further comprising a current source
coupled to the coil and capable of generating current in two
directions.
32. The disk drive of claim 31 wherein the current source is
capable of generating current in two directions and with at least
two different values in each direction.
33. The disk drive of claim 30 wherein the head is a longitudinal
head having fringe fields oriented generally perpendicularly to the
substrate.
34. The disk drive of claim 30 wherein the head is a perpendicular
head.
35. The disk drive of claim 30 further comprising a heater for
heating an island.
36. The disk drive of claim 35 wherein the inductive head has a
write pole and wherein the heater is located adjacent the write
pole.
37. The disk drive of claim 35 wherein the inductive head has two
poles and wherein the heater is located between the two poles.
38. The disk drive of claim 35 wherein the heater is an
electrically resistive heater.
Description
RELATED APPLICATIONS
[0001] This application is related to the following concurrently
filed co-pending applications, all of which are based on a common
specification:
[0002] "PATTERNED MULTILEVEL PERPENDICULAR MAGNETIC RECORDING
MEDIA" (Applicants' Docket HSJ920030213US1)
[0003] "METHOD FOR MAGNETIC RECORDING ON PATTERNED MULTILEVEL
PERPENDICULAR MEDIA USING VARIABLE WRITE CURRENT" (Applicants'
Docket HSJ920030215US1)
[0004] "METHOD FOR MAGNETIC RECORDING ON PATTERNED MULTILEVEL
PERPENDICULAR MEDIA USING THERMAL ASSISTANCE AND FIXED WRITE
CURRENT" (Applicants' Docket HSJ920030246US1)
TECHNICAL FIELD
[0005] This invention relates to magnetic recording media and
systems, such as magnetic recording hard disk drives, and more
particular to media and systems with patterned perpendicular
magnetic recording media.
BACKGROUND OF THE INVENTION
[0006] Patterned magnetic recording media have been proposed to
increase the bit density in magnetic recording data storage, such
as hard disk drives. In patterned media, the magnetic material is
patterned into small isolated blocks or islands such that there is
a single magnetic domain in each island or "bit". The single
magnetic domains can be a single grain or consist of a few strongly
coupled grains that switch magnetic states in concert as a single
magnetic volume. This is in contrast to conventional continuous
media wherein a single "bit" may have multiple magnetic domains
separated by domain walls. U.S. Pat. No. 5,820,769 is
representative of various types of patterned media and their
methods of fabrication. A description of magnetic recording systems
with patterned media and their associated challenges is presented
by R. L. White et al., "Patterned Media: A Viable Route to 50
Gbit/in.sup.2 and Up for Magnetic Recording?", IEEE Transactions on
Magnetics, Vol. 33, No. 1, January 1997, 990-995.
[0007] Patterned media with perpendicular magnetic anisotropy have
the desirable property that the magnetic moments are oriented
either into or out of the plane, which represent the two possible
magnetization states. It has been reported that these states are
thermally stable and that the media show improved signal-to-noise
ratio (SNR) compared to continuous (unpatterned) media. However, to
achieve patterned media with a bit density of 1 Terabit/in.sup.2, a
nanostructure array with a period of 25 nm over a full 2.5 inch
disk is required. Even though fabrication methods supporting bit
densities of up to 300 Gbit/in.sup.2 have been demonstrated, large
area ultra-high density magnetic patterns with low defect rates and
high uniformity are still not available.
[0008] The use of multiple level (multilevel) magnetic storage has
been proposed, as described in U.S. Pat. No. 5,583,727, but only
for continuous (unpatterned) magnetic films and not patterned
magnetic islands. However, in multilevel continuous magnetic films
the number of magnetic grains, and hence the signal and noise, is
divided into the multiple levels, and hence the SNR is
degraded.
[0009] What is needed is a magnetic recording media and system that
takes advantage of both patterned media and multilevel
recording.
SUMMARY OF THE INVENTION
[0010] The invention is a magnetic recording system, such as a
magnetic recording disk drive, that uses a patterned perpendicular
magnetic recording medium where each magnetic block or island
contains a stack of individual magnetic cells. Each cell in an
island is formed of a material or set of materials to provide the
cell with perpendicular magnetic anisotropy and is a single
magnetic domain. Each cell is magnetically decoupled from the other
cells in its island by nonmagnetic spacer layers. Thus each cell
can have a magnetization (magnetic moment) in one of two directions
(into or out of the plane of the layer making up the cell), and
this magnetization is independent of the magnetization of the other
cells in its island. Therefore the total magnetization integrated
over the different cells per island permits multiple magnetic
signal levels or states to be recorded in each magnetic island.
Because each cell in each island is a single magnetic domain, there
is no increase in noise due to the multiple magnetic levels. The
number n of magnetic cells stacked in the islands give rise to
2.sup.n different readback signal levels. The recording density is
thus increased by a factor of 2.sup.(n-1).
[0011] Each cell in an island has a magnetic coercivity different
from the coercivity of the other cells in its island. The magnetic
cells can be written (have their magnetizations switched) by an
inductive write head capable of writing with multiple write
currents, each write current providing a different magnetic write
field. Application of a write field greater than the coercivity of
only some of the cells but less than the coercivities of the other
cells writes just those selected cells in the island. Application
of a write field greater than the coercivity of the highest
coercivity cell writes all of the cells in the island. The magnetic
cells can also be written with thermal assistance by an inductive
write head with a fixed write current that provides only a single
magnetic write field. Application of the write field without
thermal assistance writes only the lower coercivity cell.
Application of the same write field but with thermal assistance
will write all the cells in the island that have had their
temperature raised to close to their Curie temperature because the
coercivity of those cells will be below the write field.
[0012] The magnetic islands are spaced apart on the substrate by
voids or material that does not affect the magnetic properties of
the cells and that does not adversely affect writing to the cells.
The substrate can be a magnetic recording disk substrate with the
islands patterned in concentric tracks or a substrate of the type
used in probe-based array storage systems with the islands
patterned in an x-y pattern of mutually perpendicular rows.
[0013] For a fuller understanding of the nature and advantages of
the present invention, reference should be made to the following
detailed description taken together with the accompanying
figures.
BRIEF DESCRIPTION OF THE DRAWING
[0014] FIG. 1 is a schematic sectional view of the patterned
multilevel perpendicular magnetic recording medium according to the
present invention.
[0015] FIG. 2 is a magneto-optical Kerr effect (MOKE) hysteresis
loop of an unpatterned section of the magnetic recording medium
showing schematic representations of the four possible
magnetization levels (labeled as A, B, C, and D) of the cells in a
magnetic island.
[0016] FIG. 3 is a readback signal of the patterned multilevel
media after dc magnetizing the sample (top) and after applying a
square wave write pattern (middle), and showing a schematic
representation of the cell magnetizations corresponding to the
readback signal (bottom).
[0017] FIG. 4 is a schematic illustrating the method in which an
inductive write head writes the four different magnetization levels
in a magnetic island of the patterned multilevel media.
[0018] FIG. 5 is a schematic illustrating the method in which an
inductive write head with an electrically resistive heater writes
the four different magnetization levels in a magnetic island of the
patterned multilevel media.
[0019] FIG. 6 is a top view of a disk drive embodiment of the
multilevel magnetic recording system showing the magnetic islands
as dots on concentric tracks of the recording disk.
[0020] FIG. 7 is a sectional view of one type of magnetic force
microscopy (MFM) probe as an inductive write head for the
multilevel magnetic recording system.
[0021] FIGS. 8(a) and 8(b) show the two different magnetizations of
the probe tip of the MFM probe type inductive head shown in FIG.
7.
[0022] FIG. 9 is a view of a scanning probe embodiment of the
multilevel magnetic recording system showing an x-y array of MFM
probe inductive write heads and the media substrate with the
magnetic islands arranged as an x-y array, the islands being
represented as dots.
DETAILED DESCRIPTION OF THE INVENTION
[0023] Patterned Multilevel Perpendicular Magnetic Recording
Media
[0024] FIG. 1 is a schematic of the patterned magnetic recording
medium according to the present invention. The medium includes a
substrate 12, a multilevel perpendicular magnetic recording layer
50, an optional underlayer 14 and optional protective overcoat 16.
The recording layer 50 includes a plurality of islands, such as
representative islands 52, 54, 56, 58, spaced-apart by spaces 60.
Each island is formed of a first layer 20 of magnetic material with
perpendicular magnetic anisotropy, a second layer 40 of magnetic
material with perpendicular magnetic anisotropy and a spacer layer
30 that separates and magnetically decouples the two magnetic
layers 20, 40 in each island. Each island is thus a multilevel
magnetic island with at least two stacked magnetically decoupled
cells, such as cells 22, 32 in island 52. Each cell is a single
magnetic domain and is separated from the other cell in its island
by the spacer layer 30 and from the cells in other islands by the
regions depicted as spaces 60.
[0025] The spaces 60 define the regions between the magnetic
islands and are typically formed of nonmagnetic material, but may
be formed of ferromagnetic material provided the material does not
adversely affect the signal recording and detection from the
magnetic islands that they separate. The magnetic islands can be
formed by first lithographically patterning the substrate,
depositing the layers making up recording layer 50 over the
patterned resist and then removing the resist, leaving the magnetic
islands. Alternatively, the magnetic islands can be formed by first
depositing the layers making up recording layer 50 on the
substrate, lithographically patterning the recording layer, etching
the recording layer through the lithographic mask, and then
removing the resist, leaving the magnetic islands. In both
examples, the spaces 60 in the regions between the islands are
voids that may be filled with nonmagnetic material, such as alumina
or spin-on glass. A substantially planar surface topography can
then be formed. The process would involve first forming the
magnetic islands, then depositing alumina to a thickness greater
than that required to fill the spaces 60, and then polishing the
alumina with a chemical-mechanical polish (CMP) process until the
magnetic islands were just exposed. This leaves the alumina in the
spaces 60 and the tops of the magnetic islands approximately
coplanar.
[0026] Patterned media may also be fabricated by ion irradiation
through a mask to alter the properties of the irradiated regions.
In one example of the patterned media ion irradiation fabrication
process, the spaces are formed of magnetic material that does not
affect the perpendicular magnetic properties of the magnetic
islands. For example, the strong perpendicular magnetic anisotropy
of Co/Pt multilayers can be destroyed by ion irradiation through
holes in a mask to create regions of magnetic material with
in-plane magnetization that serve as the spaces between the
magnetic islands of non-irradiated Co/Pt multilayers. Ion
irradiation methods of fabricating patterned magnetic recording
media are described in the following references: C. Chappert, et
al., "Planar Patterned Magnetic Media Obtained by Ion Irradiation,"
Science, Vol. 280, Jun. 19, 1998, pp. 1919-922; A. Dietzel et al.,
"Ion Projection Direct Structuring for Patterning of Magnetic
Media", IEEE Transactions on Magnetics, Vol. 38, No. 5, September
2002, pp. 1952-1954; U.S. Pat. Nos 6,331,364 and 6,383,597.
[0027] As shown by the representative letters A, B, C, D and the
arrows in the cells in FIG. 1 there are four possible magnetic
levels or states in each island, each magnetic state depending on
the direction of magnetization (magnetic moment) in each magnetic
cell. Each magnetic state in the two-layer embodiment of FIG. 1 can
thus be represented as a two-bit byte or word. If the cells in the
lower layer 20 are selected as the first bit in the byte or word
and magnetization in the up direction is considered a 0, then the
magnetic states are as follows:
[0028] A: [1,1]
[0029] B: [0,1]
[0030] C: [0,0]
[0031] D: [1,0]
[0032] FIG. 1 is depicted with two magnetic layers, but 3 or more
magnetic layers are possible. The total readback signal integrated
over the n different magnetic layers gives rise to 2.sup.n
different signal levels, which can be used for magnetic recording.
The recording density is thus increased by a factor of
2.sup.(n-1).
[0033] For experimentation, a magnetic thin film was
sputter-deposited at room temperature onto an hexagonal array of
SiO.sub.2 pillars with a diameter of 150 nm and a height of 80 nm.
The spacing between the center of the pillars was 300 nm. The
pillars were formed by lithographically patterning a SiO.sub.2 film
formed on a Si substrate. The structure had two perpendicular Co/Pd
multilayers separated by a 5 nm thick Pd layer to magnetically
decouple the upper and lower multilayers. The composition of the
film was as follows:
[0034] C(40 .ANG.)/Pd(10 .ANG.)/[Co(3.3 .ANG.)/Pd(8.3
.ANG.)].sub.6/Pd(50 .ANG.)/[Co(2.5 .ANG.)/Pd(6.5
.ANG.)].sub.10/Pd(20 .ANG.)/SiO.sub.2
[0035] Comparing this experimental structure to the schematic of
FIG. 1, the multilayer of 10 Co/Pd pairs is the lower magnetic
layer 20, the multilayer of 6 Co/Pd pairs is the upper magnetic
layer 40, and the 5 nm thick Pd layer is the spacer layer 30. The
strength of the magnetic anisotropy and coercivity of the magnetic
layers can be easily altered by changing the Co and Pd thicknesses.
In this structure all of the layers making up layers 20, 30 and 40
were also deposited into the regions or "trenches" in the SiO.sub.2
between the pillars of SiO.sub.2. However, because of the depth of
these layers relative to the magnetic islands on top of the
pillars, the magnetic properties of the islands are not affected by
the magnetic material in the trenches and there are thus voids
between the magnetic islands.
[0036] Magneto-optical Kerr effect (MOKE) hysteresis measurements
on a continuous unpatterned section of this structure revealed the
distinct switching of each Co/Pd multilayer at different applied
fields, as shown in FIG. 2. To confirm that magnetic interaction
through magnetostatic coupling between the two magnetic layers was
negligible, minor loops were also measured to determine the
coupling field, but no indication of coupling was found. Therefore
the hysteresis loop shape can be simply understood as a
superposition of the hysteresis of two independent magnetic layers.
FIG. 2 also shows that the [Co(2.5 .ANG.)/Pd(6.5 .ANG.)].sub.10
multilayer (lower layer) has a coercivity of approximately 350 Oe
and the [Co(3.3 .ANG.)/Pd(8.3 .ANG.)].sub.6 multilayer (upper
layer) has a coercivity of approximately 700 Oe.
[0037] A magnetic recording experiment was also performed on this
island array structure. The structure was fixed on a x-y stage,
controlled by piezoelectric drivers with a resolution of less than
2 nm, and scanned at low velocity (approximately 5 .mu.m/s) while
in physical contact with the recording head. A conventional
longitudinal recording giant magnetoresistive (GMR) read/write head
was used with write and read head widths of about 240 nm and 180
nm, respectively. The structure was first dc magnetized in an
external perpendicular field of 20 kOe. The recording head was then
aligned parallel to the rows of magnetic islands. Although a
conventional longitudinal inductive write head generates a write
field between its poles that is generally in the plane of the
media, in this experiment the perpendicular components of the
fringing field from the poles were used to change the magnetization
of the perpendicularly magnetized cells in the islands.
[0038] In contrast to writing on conventional continuous media,
where the bits can be written everywhere on the medium, writing on
patterned media requires the synchronization of the square wave
write pattern with the island pattern. The island locations can be
easily retrieved from the readback signal of the dc-erased
magnetized islands, where the minima indicate the trenches or
spaces separating the islands, as shown by the top signal in FIG.
3. The read head width of about 180 nm enables the signal to be
read back from an individual island in the array. In this
experimental example, a horizontal write head was used with a fixed
write current of 40 mA. Even though a fixed write current was used,
by proper timing of the write pulses it was possible to apply the
fringing field with a strength less than maximum to an island so
that only the cell with the lower coercivity in that island had its
magnetization switched. Timing of the write pulses so that the
maximum fringing field was applied resulted in switching the
magnetizations of both cells in an island. All four magnetic states
(A,B,C,D) were able to be written in this manner. The bottom signal
in FIG. 3 shows the readback waveform generated by dragging the GMR
read head across the island patterns and reveals the four different
magnetization levels (A,B,C,D).
[0039] The experimental results described above were for a
multilevel magnetic recording medium wherein the magnetic cells
with perpendicular magnetic anisotropy were multilayers of
alternating Co/Pd layers. Co/Pt multilayers may also be used. The
invention is also fully applicable with other types of magnetic
recording materials and structures that provide perpendicular
magnetic anisotropy.
[0040] The magnetic cells can be formed of a granular
polycrystalline cobalt-chromium (CoCr) alloy grown on a special
growth-enhancing sublayer that induces the crystalline C-axis to be
perpendicular to the plane of the layer, so that the layer has
strong perpendicular magnetocrystalline anisotropy. Materials that
may be used as the growth-enhancing sublayer for the CoCr granular
layer include Ti, TiCr, C, NiAl, SiO.sub.2 and CoCr, where Cr is
about 35-40 atomic percent.
[0041] The magnetic cells can also be formed any of the known
amorphous materials that exhibit perpendicular magnetic anisotropy,
such as CoSm, TbFe, TbFeCo, and GdFe alloys.
[0042] The magnetic cells can also be formed of chemically ordered
CoPt, CoPd, FePt, FePd, CoPt.sub.3 or CoPd.sub.3.
Chemically-ordered alloys of CoPt, CoPd, FePt or FePd, in their
bulk form, are known as face-centered tetragonal (FCT)
L1.sub.0-ordered phase materials (also called CuAu materials). They
are known for their high magnetocrystalline anisotropy and magnetic
moment. The c-axis of the L1.sub.0 phase is the easy axis of
magnetization and is oriented perpendicular to the substrate, thus
making the material suitable for perpendicular magnetic recording
media. Like the Co/Pt and Co/Pd multilayers, these layers have very
strong perpendicular anisotropy.
[0043] While Pd was used as the spacer layer material in the
example described above, essentially any nonmagnetic material can
be used, provided it is thick enough to assure that the magnetic
cells in the islands are magnetically decoupled. Cu, Ag, Au and Ru
are examples of other materials that may be used for the spacer
layer.
[0044] In perpendicular magnetic recording systems that use pole
heads for reading and writing, a "soft" magnetically permeable
underlayer is often used on the substrate beneath the magnetic
layer to provide a flux return path for the field from the
read/write pole head. In perpendicular magnetic recording systems
that use ring heads for reading and writing, a soft underlayer may
not be necessary. Alloy materials that are suitable for the soft
underlayer include NiFe, FeAlSi, FeTaN, FeN, CoFeB and CoZrNb.
[0045] Method for Recording on the Multilevel Media using Variable
Write Current
[0046] FIG. 4 illustrates the manner in which the inductive write
head 100 records each of the four possible magnetic states in a
magnetic island having two single domain magnetic cells. The head
100 is a perpendicular head and has a coil 102 connected to a
current source 104. The current source 104 is part of the write
driver circuitry that also includes switching circuitry to generate
bidirectional write pulses with at least two different current
levels, I.sub.1 and I.sub.2. The write current generates a
generally perpendicular magnetic field from the write pole 105 that
returns back to the return pole 107. In the preferred embodiment of
a two-layer medium, the lower magnetic layer 20 has a higher
coercivity than the upper magnetic layer 40. Current level I.sub.1
generates a magnetic write field greater than the coercivity of the
lower layer 20. Thus, as shown in FIG. 4(a) a positive I.sub.1
changes the magnetization direction in both layers 20, 40 and
generates the A state. Similarly, as shown in FIG. 4(b), a negative
I.sub.1 write pulse changes the magnetization direction in both
layers 20, 40 and generates the C state. To generate the D state,
the island must first be in the A state, after which a second
current pulse with a value of I.sub.2 in the "negative" direction
is applied, as shown in FIG. 4(c). This negative I.sub.2 current
pulse generates a magnetic write field greater than the coercivity
of the upper layer 40 but less than the coercivity of the lower
layer 20 so only the magnetization of only the upper layer 40 is
switched. Similarly, to generate the B state, the island must first
be in the C state, after which a second current pulse with a value
of I.sub.2 in the "positive" direction is applied, as shown in FIG.
4(d). This positive I.sub.2 current pulse generates a magnetic
write field greater than the coercivity of the upper layer 40 but
less than the coercivity of the lower layer 20 so only the
magnetization of the upper layer 40 is switched. A substantial
difference in coercivity between the magnetic layers assures that
only the upper cell in the magnetic island is switched when the
I.sub.2 pulse is applied. However, the coercivities in the two
cells in an island can be very close or the coercivity of the lower
cell only slightly greater than the coercivity of the upper cell if
the media is designed so that the lower cell is sufficiently far
from the upper cell. For example, by appropriate selection of the
thickness of the spacer layer 30, the lower layer 40 will be
exposed to a much lower write field than the upper layer 20 when
the I.sub.2 pulse is applied.
[0047] Method for Recording on the Multilevel Media using Thermal
Assistance and Fixed Write Current
[0048] FIG. 5 illustrates the manner in which a thermally-assisted
inductive write head 100' records each of the four possible
magnetic states in a magnetic island having two single domain
magnetic cells. The head 100' is a perpendicular head and has a
coil 102 connected to a current source 104 that provides a fixed
write current I.sub.0. The current source 104 is part of the write
driver circuitry that also includes switching circuitry to generate
bi-directional write pulses with plus or minus current levels,
I.sub.0. The head 100' also includes an electrically resistive
heater 103 located between the write pole 105 and the return pole
107. The heater 103 is connected to circuitry for applying current
pulses that enable the heater 103 to generate heat pulses to the
magnetic islands on the medium. U.S. Pat. No. 6,493,183 describes
an inductive write head for generating the magnetic write field and
a heater for heating the media. The write current I.sub.0 generates
a generally perpendicular magnetic field from the write pole 105
that returns back to the return pole 107. In the preferred
embodiment of a two-layer medium, the lower magnetic layer 20 has a
higher coercivity than the upper magnetic layer 40. Current level
I.sub.0 generates a magnetic write field greater than the
coercivity of the upper layer 40 but less than the coercivity of
the lower layer 20. Thus without thermal assistance from heater
103, only the magnetization of the cells in upper layer 40 will be
switched by the magnetic write field. However, when heat is applied
to the medium from heater 103 the temperature of the lower layer 20
is raised close to the Curie temperature of the ferromagnetic
material in the lower layer 20, which reduces the coercivity of the
lower layer 20 to below the write field generated by current
I.sub.0. Thus, as shown in FIG. 5(a) a positive I.sub.0 current
pulse in combination with a heat pulse from heater 103 changes the
magnetization direction in both layers 20, 40 and generates the A
state. Similarly, as shown in FIG. 5(b), a negative I.sub.0 current
pulse in combination with a heat pulse from heater 103 changes the
magnetization direction in both layers 20, 40 and generates the C
state. To generate the D state, the island must first be in the A
state, after which a negative I.sub.0 current pulse is applied, as
shown in FIG. 5(c). This negative I.sub.0 current pulse generates a
magnetic write field greater than the coercivity of the upper layer
40 but less than the coercivity of the lower layer 20 so only the
magnetization of the upper layer 40 is switched. Similarly, to
generate the B state, the island must first be in the C state,
after which a positive I.sub.0 current pulse is applied, as shown
in FIG. 5(d). This positive I.sub.0 current pulse generates a
magnetic write field greater than the coercivity of the upper layer
40 but less than the coercivity of the lower layer 20 so only the
magnetization of the upper layer 40 is switched. A substantial
difference in coercivity between the magnetic layers assures that
only the upper cell in the magnetic island is switched when the
I.sub.0 pulse is applied. However, the coercivities in the two
cells in an island can be very close or the coercivity of the lower
cell only slightly greater than the coercivity of the upper cell if
the media is designed so that the lower cell is sufficiently far
from the upper cell. For example, by appropriate selection of the
thickness of the spacer layer 30, the lower layer 40 will be
exposed to a much lower write field than the upper layer 20 when
the I.sub.0 pulse is applied. As one example of this embodiment,
the lower layer can be formed of a 6 [Co(4 .ANG.)/Pd(10 .ANG.)]
multilayer and have a coercivity of approximately 3000 Oe, and the
upper layer can be formed of a 6 [Co(2.5 .ANG.)/Pd(5 .ANG.)]
multilayer and have a coercivity of approximately 2000 Oe. A fixed
write current pulse of 10 mA will generate a magnetic write field
of approximately 3000 Oe. A heat pulse of a few milliwatts will
increase the temperature of both layers by approximately 40 K,
which will reduce the coercivity of the lower layer to
approximately 1000 Oe.
[0049] Although FIG. 5 shows the heater 103 located between the
poles 105, 107, the heater may also be located on either side of a
pole. Also, the heater may be formed as part of the coil of a
longitudinal write head, as described in published U.S. patent
application Ser. No. 2003/0021191A1, in which case portions of the
coil serve as the electrical leads to the heater. In addition, the
heater does not need to be an electrically resistive heater and may
be a separate element not directly associated with the inductive
write head, such as a laser that directs a light spot to the
medium, provided the heat pulse and the magnetic write field can be
localized to assure that only the desired cells have their
magnetizations switched.
[0050] FIG. 6 is a top view of a disk drive embodiment of the
multilevel magnetic recording system according to the present
invention. The drive 200 has a housing or base 212 that supports an
actuator 230 and a drive motor for rotating the multilevel magnetic
recording disk 214. The disk 214 substrate may be any suitable
substrate, such as the glass or aluminum-magnesium (AlMg)
substrates used in conventional disk drives. The actuator 230 may
be a voice coil motor (VCM) rotary actuator that has a rigid arm
234 and rotates about pivot 232 as shown by arrow 224. A
head-suspension assembly 220 includes a suspension 221 that has one
end attached to the end of actuator arm 234 and a head carrier 222,
such as an air-bearing slider, attached to the other end of
suspension 221. The magnetic islands 215 on disk 214 are arranged
in radially-spaced circular tracks 218. As the disk 214 rotates,
the movement of actuator 230 allows the head 100 on the trailing
end of head carrier 222 to access different data tracks 218 on disk
214 for the recording of multilevel data in the magnetic islands
215. As previously mentioned, the writing on patterned media
requires the synchronization of the write pulses with the island
pattern. A patterned media magnetic recording system that uses the
magnetic islands to clock the writing is described in published
application US20030107833A1 titled "Patterned media magnetic
recording disk drive with timing of write pulses by sensing the
patterned media" published Jun. 12, 2003 and assigned to the same
assignee as the present application.
[0051] The inductive write head used to record the signal shown in
FIG. 3 was a conventional longitudinal inductive write head, and
the inductive write head depicted in FIG. 4 is a perpendicular head
with a write pole and a return pole. Another type of inductive
write head for use in the present invention is based on a
magnetic-force-microscopy (MFM) probe comprising a cantilever with
a nanometer-sharp magnetic tip at the cantilever end. One type of
MFM probe is described in U.S. Pat. No. 5,900,729 and shown in FIG.
6. FIG. 7 is a side sectional view of the probe 300 showing the
probe body 310 attached to cantilever 350. The probe body 310 has a
pair of poles 340, 342 and an inductive coil 311. The coil 311 and
poles 340, 342 are formed using conventional lithographic
techniques, as is well known in the manufacture of disk drive thin
film inductive write heads in which the coil and poles are formed
on the trailing end of a conventional disk drive air-bearing
slider. The poles 340, 342 are spaced apart by a nonmagnetic gap
314. The poles 340, 342 are interconnected to form a yoke through
which the coil 311 passes. The sectioned ends of the coil windings
are shown in end view as coil 311. When current passes through coil
311, a magnetic field is induced in the yoke and magnetic flux is
generated between the poles 340, 342 just as in a conventional
longitudinal thin film inductive write head. A probe tip 320, which
is formed in contact with at least one of the poles 340 or 342 and
preferably also in contact with the end surface of the gap 314,
extends from the ends of the poles. The probe tip 320 has at least
one surface or side 322 which is in contact with one of the poles
and is formed of a magnetic material. The probe tip 320 is shown as
having a generally conical shape, but its actual shape can vary. As
an alternative to the yoke structure depicted in FIG. 7, the coil
may be wrapped in a helical manner around probe tip 320, the probe
body 310, or the cantilever 350 provided these structures are
formed of material that allows the magnetic field generated by the
coil to be directed to the magnetic probe tip 320. This type of MFM
probe is described in U.S. Pat. No. 5,436,448. In any such
arrangement using an MFM probe as the inductive write head, the
coil receives current I.sub.1 or I.sub.2 from the write driver,
which causes probe tip 320 to be magnetized in one direction with a
field strength determined by the value of I.sub.1 or I.sub.2. When
the current direction is switched through the coil, the direction
of magnetization of the probe tip is reversed. These two
magnetization directions are shown schematically in FIGS.
8(a)-8(b). These two possible magnetization directions and two
possible magnetic field values enable the four possible magnetic
states (A, B, C, D) to be written in the magnetic islands in the
same manner as described with respect to FIG. 4.
[0052] In the disk drive embodiment of the present invention with
the MFM probe as the inductive write head, the cantilever 350 with
probe tip 320 is attached to the actuator arm 234 (FIG. 5). Another
type of actuator that enables the MFM probe to be used in a disk
drive is described in U.S. Pat. No. 5,804,710. However, the MFM
probe type of inductive write head also permits multilevel magnetic
recording in a scanning probe system. A scanning probe system is
described in "Millipede-A MEMS-Based Scanning-Probe Data-Storage
System", IEEE Transactions on Magnetics, Vol. 39, No. 2, March
2003, pp.938-945. The "Millipede" system is a thermomechanical
system in which the data is recorded by heating the probe tips to
cause pits in a polymeric storage medium. The scanning probe
embodiment of the multilevel magnetic recording system according to
the present invention is shown in FIG. 9. The multilevel magnetic
recording medium 400 is as described with respect to FIG. 1 and
includes the substrate 401 and the magnetic islands 402. The
islands 402 are arranged as an x-y array of mutually perpendicular
rows on the substrate 401. The substrate 401 is supported on a
platform 402 of an xyz scanner. An array of MFM type probe tips 410
with associated cantilevers 411 is fabricated on a chip 420. The
chip 420 and medium 400 are movable relative to one another in the
x-y directions by the xyz scanner. Thus each probe is associated
with only a section of the total island array and addresses only
the islands in that section. Multiplex drivers (MUX) 430, 432 allow
write currents I.sub.1, I.sub.2 to be delivered to each MFM probe
individually.
[0053] The scanning probe system described above and depicted in
FIG. 9 has an array of probes. However, the scanning probe
multilevel magnetic recording system according to the present
invention is also possible with only a single probe in cooperation
with an xyz scanner, in the manner of a conventional MFM
system.
[0054] While the present invention has been particularly shown and
described with reference to the preferred embodiments, it will be
understood by those skilled in the art that various changes in form
and detail may be made without departing from the spirit and scope
of the invention. Accordingly, the disclosed invention is to be
considered merely as illustrative and limited in scope only as
specified in the appended claims.
* * * * *