U.S. patent application number 11/469808 was filed with the patent office on 2007-01-04 for tilted media for hard disk drives and magnetic data storage devices.
This patent application is currently assigned to Agency for Science, Technology and Research. Invention is credited to Ching Hian HEE, Jian Ping Wang, Haibao Zhao.
Application Number | 20070003794 11/469808 |
Document ID | / |
Family ID | 34375716 |
Filed Date | 2007-01-04 |
United States Patent
Application |
20070003794 |
Kind Code |
A1 |
HEE; Ching Hian ; et
al. |
January 4, 2007 |
Tilted Media for Hard Disk Drives and Magnetic Data Storage
Devices
Abstract
A digital storage medium for use in data storage devices has two
magnetic layers where the respective easy axes of the magnetic
moments in the two layers are perpendicular to each other. Exchange
coupling of the magnetic moments in the magnetic layers produces a
resultant magnetic moment that is tilted out of the plane of the
digital storage medium. The resultant magnetic moment of the medium
allows the use of either a ring head, a single pole head, or a head
that generates a field tilted at an angle for write operations.
Inventors: |
HEE; Ching Hian; (Menglembu,
MY) ; Wang; Jian Ping; (Minneapolis, MN) ;
Zhao; Haibao; (Singapore, SG) |
Correspondence
Address: |
HOGAN & HARTSON L.L.P.
1999 AVENUE OF THE STARS
SUITE 1400
LOS ANGELES
CA
90067
US
|
Assignee: |
Agency for Science, Technology and
Research
Singapore
SG
|
Family ID: |
34375716 |
Appl. No.: |
11/469808 |
Filed: |
September 1, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10632522 |
Aug 1, 2003 |
|
|
|
11469808 |
Sep 1, 2006 |
|
|
|
Current U.S.
Class: |
428/827 ;
428/828; G9B/5.241 |
Current CPC
Class: |
G11B 5/66 20130101; G11B
5/656 20130101 |
Class at
Publication: |
428/827 ;
428/828 |
International
Class: |
G11B 5/66 20060101
G11B005/66 |
Claims
1-23. (canceled)
24. A method of manufacturing a digital storage medium, the method
comprising depositing a first magnetic layer on a substrate
surface, wherein the first magnetic layer has a first magnetic
moment with a first easy axis; and depositing a second magnetic
layer on the first magnetic layer, wherein the second magnetic
layer has a second magnetic moment with a second easy axis; wherein
the second easy axis is perpendicular to the first easy axis; and
wherein exchange-coupling of the first magnetic moment and the
second magnetic moment forms tilted easy axes in the first and
second magnetic layers.
25. The method of claim 24, wherein the first magnetic layer has a
perpendicular anisotropy and the second magnetic layer has a
longitudinal anisotropy.
26. The method of claim 24, wherein the first magnetic layer has a
longitudinal anisotropy and the second magnetic layer has a
perpendicular anisotropy.
27. The method of claim 25, wherein the magnetic layer with the
perpendicular anisotropy includes a ferromagnetic material selected
from a group consisting of cobalt, iron, and alloys thereof.
28. The method of claim 26, wherein the magnetic layer with the
perpendicular anisotropy includes a ferromagnetic material selected
from a group consisting of cobalt, iron, and alloys thereof.
29. The method of claim 27, wherein the magnetic layer is formed
from a single layer of at least one alloy selected from a group
consisting of cobalt-platinum (CoPt), cobalt-palladium (CoPd),
cobalt-chromium-platinum (CoCrPt), cobalt-chromium-platinum-boron
(CoCrPtB), cobalt-chromium-platinum-tantalum (CoCrPtTa),
cobalt-chromium-platinum-niobium (CoCrPtNb), and iron-platinum
(FePt).
30. The method of claim 28, wherein the magnetic layer is formed
from a single layer of at least one alloy selected from a group
consisting of cobalt-platinum (CoPt), cobalt-palladium (CoPd),
cobalt-chromium-platinum (CoCrPt), cobalt-chromium-platinum-boron
(CoCrPtB), cobalt-chromium-platinum-tantalum (CoCrPtTa),
cobalt-chromium-platinum-niobium (CoCrPtNb), and iron-platinum
(FePt).
31. The method of claim 27, wherein the magnetic layer is formed
from multiple layers of ferromagnetic materials selected from a
group consisting of cobalt with palladium as a spacer layer
(Co/Pd), cobalt with platinum as a spacer layer (Co/Pt), a cobalt
alloy with palladium as a spacer layer, and a cobalt alloy with
platinum as a spacer layer.
32. The method of claim 28, wherein the magnetic layer is formed
from multiple layers of ferromagnetic materials selected from a
group consisting of cobalt with palladium as a spacer layer
(Co/Pd), cobalt with platinum as a spacer layer (Co/Pt), a cobalt
alloy with palladium as a spacer layer, and a cobalt alloy with
platinum as spacer layer.
33. The method of claim 29, wherein the alloys are doped with
non-ferromagnetic materials selected from a group consisting of
silicon oxide and silicon nitride.
34. The method of claim 30, wherein the alloys are doped with
non-ferromagnetic materials selected from a group consisting of
silicon oxide and silicon nitride.
35. The method of claim 24, wherein the first magnetic layer and
the second magnetic layer each have a thickness that is
variable.
36. The method of claim 24, wherein an angle is formed between the
tilted easy axes and a perpendicular plane of the digital storage
medium.
37. The method of claim 36, wherein the angle is adjustable by
varying the type of ferromagnetic materials in the first and second
magnetic layers.
38. The method of claim 36, wherein the angle is adjustable by
varying an exchange-coupling constant between the first and second
magnetic layer.
39. The method of claim 36, wherein the angle is adjustable by
varying the thickness of the first and second magnetic layers.
40. The method of claim 24, wherein an interlayer is disposed
between the first and second magnetic layers.
Description
TECHNICAL FIELD
[0001] This invention relates to magnetic media used in data
storage devices, and more particularly to magnetic media that use
either a ring head or a pole head for write operations.
BACKGROUND OF THE INVENTION
[0002] A conventional magnetic storage medium includes a substrate
supporting an underlayer disposed under a magnetic layer protected
by an overcoating layer. These layers are usually deposited using a
sputtering process. The magnetic layer is typically formed from a
granular ferromagnetic material such as a cobalt alloy, in which
data bits are stored. Transitions of magnetization in the tiny
grains of the magnetic layer are used to store data and generate
signals, which may then be read or written by a read/write
head.
[0003] Magnetic layers are typically classified as being either
longitudinal or perpendicular. In a longitudinal magnetic storage
medium, the magnetic moments of the grains are aligned parallel to
the plane of the substrate surface of the storage medium. In
perpendicular magnetic media, the magnetic moments of the grains
are aligned perpendicular to this plane. A longitudinal recording
system uses a ring head for write operations while a perpendicular
recording system uses a single pole head for write operations.
[0004] As the demand for storage space grows, there is a constant
need to increase the areal density in magnetic storage media. To do
so, the number of grains in a medium is increased by decreasing the
size of the magnetic grains. This reduction in grain size also
leads to sharper transitions and a higher signal to noise ratio
(SNR), which improves data recording. Unfortunately, a major
problem with reducing grain size is that it leads to decreased
thermal stability.
[0005] Maintaining thermal stability of the magnetic grains is
critical to data retention, particularly over longer periods of
time. If the grains are unstable, then it is likely that data may
be lost through magnetization decay. The energy required to store
and maintain the stability of written bits of data is known as
magnetic-anisotropy energy, K.sub.uV, where K.sub.u is the
anisotropy constant and V is the volume of the grain. Below a
certain grain size, the magnetic layer will experience what is
known as a super-paramagnetic effect due to the decrease of this
magnetic-anisotropy-energy.
[0006] To ensure thermal stability of the grains and minimize the
tendency for magnetization decay, a thermal stability factor is
used as a reference to determine the stability of the grains With
respect to the life span of the media. This thermal stability
factor is defined by the magnetic-anisotropy energy over the
thermal energy of the magnetic media: K.sub.uV/k.sub.BT, where T is
the temperature and k.sub.B is the Boltzmann constant.
[0007] For example, a medium with thermal stability factor of 60,
the industry standard, has a life span of about 3.6.times.10.sup.9
years. If the thermal stability factor is reduced, the life span of
the medium is reduced accordingly. At a thermal stability factor of
40, for example, the life span would be approximately 7.5 years,
and at 25, the life span of the media is approximately 72 seconds).
It is therefore important to have this factor at or above 40 to
ensure that data stored in a medium resists magnetization decay
over a sufficiently long period of time.
[0008] While it would be ideal to use materials that have a higher
K.sub.u to maintain the thermal stability factor while V is
reduced, conventional longitudinal recording systems are limited by
the ring heads that are used. Currently, the ring heads have a
maximum head field of 2 Tesla. This head field limits the K.sub.u
of the digital storage medium to about 2.1.times.10.sup.6
erg/cm.sup.3. This limit thus imposes a highly undesirable minimum
threshold on grain size to maintain the thermal stability factor.
In contrast, perpendicular recording systems using a single pole
head are able to generate a higher field relative to that of the
ring head. Unfortunately, perpendicular recording systems are
subject to limitations and problems that have prevented the
technology from being more widespread. In particular, excessive
noise interference is caused by a soft underlayer used in a
perpendicular medium. Although perpendicular media have greater
thermal stability than longitudinal media, the coercivity in the
perpendicular medium is considerably higher. This high coercivity
requires higher switching field energy to perform write operations,
which may cause further noise interference. Thus, perpendicular
recording systems are not a commercially viable option.
[0009] In view of the foregoing, it is desirable to devise magnetic
recording disk media that will allow increases in areal density
while preventing magnetization decay. To overcome the limitations
of existing magnetic recording systems, there is a need to have a
magnetic recording medium that allows continued reduction of grain
size while preventing excessive interference from noise. In
addition, it is also desirable to have a magnetic recording medium
that maintains thermal stability as grain size is reduced.
SUMMARY OF THE INVENTION
[0010] The present invention provides a digital storage medium with
a substrate upon which two magnetic layers are disposed and
protected by an overcoat. The first magnetic layer is disposed on
the substrate while the second magnetic layer is disposed on the
fist magnetic layer. The first of the two magnetic layers has a
first magnetic moment with an easy axis that is tilted and the
second magnetic layer has a second magnetic moment with an easy
axis that is also tilted. The tilted axes are formed under the
influence of exchange-coupling between the first and second
magnetic layers. The tilted axes, which are biaxial, form acute
angles with a plane perpendicular to the surface of the digital
storage medium. The angles of the tilted easy axes in the two
magnetic layers may have magnitudes that are the same or different.
The tilted easy axes of the magnetic moments in the present
invention provide the advantage of writing data on the digital
storage medium with either a longitudinal ring head, a
perpendicular single-pole head, or any newly developed tilted field
head.
[0011] A nickel phosphorous seed layer is coated onto the
substrate, an underlayer is disposed on the seed layer, an
intermediate layer is disposed over the underlayer, and the first
magnetic layer is disposed over the intermediate layer. A soft
underlayer may also be disposed between the underlayer and the
interlayer.
[0012] In one embodiment, an interlayer is disposed between the
first and second magnetic layers. The interlayer can be cobalt, a
cobalt alloy, or a non-magnetic material such as ruthenium (Ru),
rhodium (Rh), chromium (Cr), copper (Cu), iridium (Ir), or alloys
of any of these materials. With a non-magnetic material as the
interlayer, the tilted easy axes of the first and second magnetic
moments are anti-parallel where cobalt or alloys of cobalt are used
for the interlayer, the easy axis of the first and second magnetic
layers are parallel. The magnetic thickness of the digital storage
medium is the combined thickness of the first and second magnetic
layers, in addition to the thickness of the interlayer, if one is
disposed between the two magnetic layers.
[0013] In one embodiment of the present invention, the first
magnetic layer has a perpendicular magnetic anisotropy in which the
easy axis of the magnetic moment in the first magnetic layer is
perpendicular to the plane of the digital storage medium, and the
second magnetic layer has a longitudinal anisotropy in which the
easy axis of the magnetic moment in the second magnetic layer is
parallel to the plane of the digital storage medium. Alternatively,
the first magnetic layer has a longitudinal magnetic anisotropy and
the second magnetic layer has a perpendicular magnetic anisotropy
with respect to the plane of the digital storage medium. The
magnetic layer with longitudinal anisotropy is formed with a
ferromagnetic material selected from a group consisting of cobalt
(Co), iron (Fe), nickel (Ni), or alloys of these ferromagnetic
materials while the magnetic layer with the perpendicular
anisotropy is formed with ferromagnetic materials like cobalt,
iron, and their alloys. The magnetic layer with perpendicular
anisotropy is a single layer of an alloy like
cobalt-chromium-platinum (CoCrPt), cobalt-chromium-platinum-boron
(CoCrPtB), cobalt-chromium-platinum-tantalum (CoCrPtTa),
cobalt-chromium-platinum-niobium (CoCrPtNb), or iron-platinum
(FePt). The single layer can also be formed from alloys of cobalt
that are doped with non-magnetic materials like silicon oxide and
silicon nitride.
[0014] The perpendicular magnetic layer may also be formed from
multiple layers of the same ferromagnetic materials with spacer
layers between the multiple layers of ferromagnetic material. The
ferromagnetic material forming the multiple layers can include
cobalt, cobalt oxides, and cobalt alloys, while the spacer layer
can include palladium or platinum. The cobalt alloys include
cobalt-silver (Co--Ag), cobalt-boron (Co--B), cobalt-chromium
(Co--Cr), cobalt-copper (Co--Cu), cobalt-palladium (Co--Pd), and
cobalt-tantalum (Co--Ta).
[0015] In another aspect of the invention, a method of
manufacturing the digital storage medium of the present invention
is provided. The method provides for deposition of a first magnetic
layer on a substrate followed by a second magnetic layer, where the
first magnetic layer has a first magnetic moment with a first easy
axis, and the second magnetic layer has a second magnetic moment
with a second easy axis such that the two easy axes are
perpendicular to each other. Exchange-coupling of the magnetic
moments of the two magnetic layers result in the easy axes
effectively tilting out of plane of the digital storage medium. The
tilted easy axes of the present method form acute angles with a
plane perpendicular to the plane of the digital storage medium. The
magnitude of the acute angles depends on the exchange coupling
strength, as well as the anisotropy energies of the respective
magnetic layers. The anisotropy energies of the respective magnetic
layers compete to attain a stable tilted configuration of the easy
axes. The competition of the anisotropy energies is influenced by
the exchange-coupling constant, anisotropy constant which can be
changed by varying the type of ferromagnetic materials in the
magnetic layers as well as the thickness of the respective magnetic
layers. The thicknesses of the first and second magnetic layers are
variable. The exchange-coupling between the first and second
magnetic layers increases the activation volume, V, through the
combined thickness of the two magnetic layers, and hence increases
the thermal stability of the digital recording or storage medium
according to the thermal stability factor, K.sub.uV/k.sub.BT. The
exchange-coupling constant between the first and second magnetic
layers is variable.
[0016] The method of the present invention provides for one
magnetic layer having longitudinal anisotropy, where the easy axis
of the magnetic moment in the magnetic layer is parallel to the
plane of the digital storage medium, and another magnetic layer
having perpendicular anisotropy, where the easy axis of the
magnetic moment is perpendicular to the plane of the digital
storage medium. The magnetic layer with perpendicular anisotropy is
disposed over the magnetic layer with longitudinal anisotropy, or
alternatively, the magnetic layer with the longitudinal anisotropy
is disposed over the magnetic layer with perpendicular anisotropy.
The magnetic layer with longitudinal anisotropy is formed with a
ferromagnetic material selected from a group consisting of cobalt
(Co), iron (Fe), nickel (Ni), and the alloys of these ferromagnetic
materials while the magnetic layer with the perpendicular
anisotropy is formed with ferromagnetic materials like cobalt, iron
and their alloys. The magnetic layer with perpendicular anisotropy
can take the form of a single layer of alloys of ferromagnetic
materials like cobalt-chromium-platinum (CoCrPt),
cobalt-chromium-platinum-boron (CoCrPtB),
cobalt-chromium-platinum-tantalum (CoCrPtTa),
cobalt-chromium-platinum-niobium (CoCrPtNb) and iron-platinum
(FePt). The single layer can also be formed from alloys of cobalt
which are doped with non-magnetic materials like silicon oxide and
silicon nitride. Alternatively, the magnetic layer with
perpendicular anisotropy is formed from multiple layers of the same
ferromagnetic materials with spacer layers between the multiple
layers of ferromagnetic material. The ferromagnetic material can be
cobalt, cobalt oxides and cobalt alloys and the spacer layer
includes palladium and platinum. The cobalt alloys includes
cobalt-silver (Co--Ag), cobalt-boron (Co--B), cobalt-chromium
(Co--Cr), cobalt-copper (Co--Cu), cobalt-palladium (Co--Pd) and
cobalt-tantalum (Co--Ta).
[0017] An underlayer can be disposed between the substrate and the
first magnetic layer, and a seed layer can be disposed on the
substrate before the underlayer. An additional soft underlay can be
disposed on the underlayer. An intermediate layer can then be
disposed between the underlayer and the first magnetic layer. An
interlayer can be disposed under the first magnetic layer. The
interlayer is selected from a group of high saturation
magnetization materials like pure cobalt, alloys and compounds of
cobalt, nickel, iron, alloys of nickel and alloys of iron to
enhance exchange-coupling at the interface of the first and second
magnetic layers. Alternatively, the interlayer is selected from a
group of nonmagnetic materials consisting of ruthenium (Ru),
rhodium (Rh), chromium (Cr), copper (Cu), iridium (Ir), and their
alloys such that the first magnetic layer is anti-ferromagnetically
exchange-coupled to the second magnetic layer. The interlayer is of
a variable thickness generally less than 1 nm.
[0018] The present invention has a recording direction that is
independent of the types of head used for write operations due to
the natural formation of the bi-axial easy axis in the system.
Thus, the present invention provides a method of manufacturing a
recording medium that can be used with any read/write head
independent of the field direction of the read/write head.
[0019] Furthermore, due to the titled direction of the magnetic
moment to writing field of either a ring head or single pole bead,
a much higher anisotropy constant, K.sub.u, can be used to increase
the thermal stability while maintaining the writeable switching
field value. The magnetic medium also enables sharper magnetic
transitions due to the reduction of the switching fields in the
titled transition. This results in a higher linear bit density for
the medium.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The details of the present invention will become apparent by
reference to the following description and accompanying drawings,
in which:
[0021] FIG. 1 is a perspective view of a data storage disc
embodying the present invention lying in a plane.
[0022] FIG. 2 is a cross-sectional view of the structure of an
embodiment of a digital medium of the present invention.
[0023] FIG. 3 is a cross-sectional view of the structure of another
embodiment of a digital medium of the present invention.
[0024] FIG. 4 is a schematic illustration of an orientation of the
easy axes of the two magnetic layers of the embodiment of FIG.
2.
[0025] FIG. 5 is a schematic illustration of another orientation of
the easy axes of the two magnetic layers of the embodiment of FIG.
3.
[0026] FIG. 6 is a schematic sectional view of the interaction of
magnetic moments of the two magnetic layers of the present
invention.
[0027] FIG. 7 is a graph illustrating the relationship of the
tilted angle with the coupling constant.
[0028] FIG. 8 is a perspective view illustrating the relationship
of the applied field, the easy axis and the plane of the magnetic
medium.
[0029] FIG. 9 is a graph based on a Stoner-Wolfarth model of the
normalized switching field in relation to the angle of the tilted
magnetic moment in a magnetic medium having a single magnetic
layer.
[0030] FIG. 10 is a graph of differential switching in relation to
the angle of the tilted magnetic moment in a magnetic medium having
a single magnetic layer.
[0031] FIG. 11 is an illustration of the effective bi-axial easy
axes due to exchange-coupling in each of the magnetic layers in the
present invention.
[0032] FIG. 12 is a schematic sectional view of the interaction of
magnetic moments of the two magnetic layers, where an interlayer is
disposed between the two layers.
[0033] FIG. 13 is a schematic sectional view of the resultant
magnetic moments at a recorded magnetic transition of the present
invention, where an interlayer disposed between the top and bottom
magnetic layer results in anti-parallel magnetic moments in the
respective layers.
[0034] FIG. 14 is a graph showing the time required for switching
the magnetic moments in a magnetic medium having tilted magnetic
moments when a field is applied to the medium.
[0035] FIG. 15 is a graph of coercivity field in relation to angle
dependence for the structure of FIG. 2 with Pd interlayer
thicknesses of 0.6 nm, 1.2 nm, and 2.5 nm.
[0036] FIG. 16 is a graph of magnetization squareness and angle
dependence for the structure of FIG. 2 with Pd interlayer layer
thicknesses of 0.6 nm, 1.2 nm, and 2.5 nm.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0037] FIG. 1 shows a digital storage medium 10 in the form of a
disk lying in a plane 11. The digital storage medium 10 typically
includes a substrate 12, upon which a seed layer 13 is disposed
followed by an underlayer 14 as illustrated in FIG. 2 where the
cross section of the digital medium is shown. An intermediate layer
16 is disposed on top of the underlayer as in a typical disk
storage medium. A first magnetic layer 18 is then deposited on the
intermediate layer 16. The seed layer 13, underlayer 14, and
intermediate layer 16 are represented jointly in subsequent figures
as a combined layer denoted as 100. The first magnetic layer 18
forms the bottom magnetic layer upon which a second magnetic layer
20 is deposited. The second magnetic layer includes a series of
alternating layers of ferromagnetic materials and spacer layers. A
protective overcoat 22 is then deposited on the second magnetic
layer 20.
[0038] The second magnetic layer 20 can be switched with the first
magnetic layer 18 such that the bottom layer is the second magnetic
layer 20 and the top layer is the first magnetic layer 18 as shown
in FIG. 3. An interlayer 20a may be included between the first and
second magnetic layers in either of the arrangements shown in FIG.
2 and FIG. 3. This interlayer will be discussed in detail with
reference to FIG. 12 and FIG. 13.
[0039] The relationship between the first magnetic layer 18 and the
second magnetic layer 20 of the arrangement illustrated in the
embodiment of FIG. 2 is further discussed with reference to FIG. 4.
The first magnetic layer 18 has a first magnetic moment having a
first easy axis 19. Similarly, the second magnetic layer 20 has a
second magnetic moment with a second easy axis 21. These easy axes
are uniaxial in that the moments of the magnetic grains are
orientated in one direction of the easy axes. In FIG. 4, the first
easy axis 19 is parallel to the plane 11 of the surface of the
digital storage medium 10 and the second easy axis 21 is
perpendicular to the first easy axis 19. In the embodiment of FIG.
3, the top magnetic layer 18 has a first easy axis 19 parallel to
the surface of the digital storage medium 10 while the bottom
magnetic layer 20 has a second easy axis 21 perpendicular to the
first easy axis 19 as shown in FIG. 5.
[0040] In this invention, the easy axes for the magnetic layers are
deliberately designed such that each is perpendicular to the other.
When the two magnetic layers are magnetically isolated or not
exchange-coupled, the remnant moments at the respective layers
align in the directions of the easy axes of the respective layers.
Thus, in one layer the easy axis is perpendicular to the plane of
the magnetic layers while the other easy axis lies in the same
plane as the magnetic layers. When exchange-coupling is introduced
at the interface between the two magnetic layers, a new energy is
introduced and the magnetic moments tend to align together. The two
exchange-coupled moments pull towards each other, but this pull is
restricted by the anisotropy energy that exists in each respective
magnetic layer. As illustrated in FIG. 4 and FIG. 5, the two easy
axes which are perpendicular to each other have exchange-coupling
linking together the magnetic moments from the two magnetic layers.
At the same time, though, a competition is created between the
anisotropy energies from the respective magnetic layers. When a
balance of the competing anisotropy energies is achieved, a new
easy axis with a different configuration is generated. The new easy
axes tilt at an angle out of the plane 11 in which the magnetic
layers 18 and 20 lie. This phenomenon of exchange-coupling at the
interface of the magnetic layers will become apparent from FIG. 6
and FIG. 7.
[0041] FIG. 6 illustrates the interaction between the respective
magnetic moments of the first 20 and second 18 magnetic layers. The
interaction of the magnetic moments results in remnant magnetic
moments, M.sub.1 (15) and M.sub.2 (17). The respective remnant
magnetic moments are: M.sub.1=M.sub.r1t.sub.1 and
M.sub.2=M.sub.r2t.sub.2, where M.sub.r1 and M.sub.r2 are the
remnant magnetization from the respective magnetic layers due to
exchange coupling at the interface of the two magnetic layers, and
t.sub.1 and t.sub.2 are the respective thicknesses of the magnetic
layers. The exchange-coupling in the present design is utilized for
forming remnant magnetic moments 15 and 17 which tilt at an acute
angle to the plane 11. The tilting of the magnetic moments at an
acute angle provides an advantage over existing prior art, in which
the magnetic moments are either longitudinal or perpendicular with
respect to the plane of the medium. The magnitudes of the acute
angles, .theta..sub.1 and .theta..sub.2, at which the remnant
magnetic moments tilt depends on the exchange-coupling strength
between the magnetic moments of the two magnetic layers. The graph
of FIG. 7 illustrates the effect of the exchange-Coupling constant
on the angles .theta..sub.1 and .theta..sub.2 of the remnant
magnetic moments M.sub.1 and M.sub.2. In this example, both
magnetic layers have the same anisotropy constant and thickness. As
the exchange-coupling constant increases, the angle .theta..sub.2
of the second easy axis 21 decreases while the angle .theta..sub.1
of the first easy axis 19 increases. Therefore, an
exchange-coupling constant has to be selected to have the remnant
magnetic moments tilted at a desired angle. Depending on the
exchange-coupling strength, the angles of the easy axes in the two
magnetic layers will tilt at different angles (i.e.
.theta..sub.1.noteq..theta..sub.2). The tilting angle is influenced
by the competition of the anisotropy energies between the magnetic
layers in an exchange-coupled system. The anisotropy energy for the
respective magnetic layer can be varied to adjust the tilting
angle. The strength of the anisotropy energy or the anisotropy
density in each magnetic layer is determined by the thickness and
anisotropy constant of the magnetic layer. The principle in this
design is to control the competing energies between the two
anisotropy energies in the perpendicular direction to each other
when both are exchange coupled. As such, varying the anisotropy
constants, K.sub.u1 and K.sub.u2, of the respective magnetic layers
will affect the competition of the anisotropy energies in the
exchange-coupled system and the moment will tilt in the direction
of the dominant anisotropy. Therefore, the tilting angle is also
dependent on the anisotropy constant, K.sub.u1, of the first
magnetic layer 18 and K.sub.u2, of the second magnetic layer 20.
This can be achieved by varying the magnetic properties of the two
magnetic layers. Once the magnetic materials for the respective
magnetic layers are selected, the anisotropy constants of the
magnetic layers are constant. However, while the anisotropy
constants K.sub.u1, and K.sub.u2, of the respective magnetic layers
18 and 20 are unchanged, the interaction through exchange-coupling
of the two magnetic layers gives rise to an effective anisotropy
constant, K.sub.u,eff for the combined magnetic layers. Thus the
exchange coupling between the two magnetic layers brings about a
new magnetic property of the digital storage medium 10. The overall
magnetic property of the digital storage medium 10 is thus
determined by the exchange-coupling of the selected magnetic
properties of the respective magnetic layers. The overall effective
anisotropy constant, K.sub.u,eff, takes on a new value that is
different from either of the anisotropy constants, K.sub.u1 and
K.sub.u2, of the respective magnetic layers.
[0042] The magnitude of the tilting angle can also be controlled by
adjusting the thickness of each magnetic layer since varying the
thickness varies the anisotropy energy of that layer. The overall
magnetic properties may tend towards the magnetic layer having
stronger magnetic properties in view of the competing energies from
the two magnetic layers in the exchange-coupled system.
[0043] The overall remnant magnetic moment is:
M.sub.r=M.sub.1+M.sub.2. It follows from the above that if the
remnant magnetic moment of one magnetic layer dominates the other,
the overall remnant magnetic moment will tend towards the dominant
magnetic moment under the influence of the competing energies in
the exchange coupling system. The exchange-coupling system provides
flexibility in enabling the design of a medium with particular
magnetic properties by selection of ferromagnetic materials to form
the magnetic layers in the digital storage medium.
[0044] The exchange-coupling of the magnetic moments between the
two magnetic layers provides an advantage through increasing the
activation volume, V, of the digital storage magnetic medium 10.
The activation volume is: V=Dt, where D is the diameter of the
grain and t is the magnetic thickness of the digital storage
medium. The magnetic thickness for the exchange coupled system is:
t=t.sub.1+t.sub.2, where t.sub.1 is the thickness of the magnetic
layer 18 and t.sub.2 is the thickness of the magnetic layer 20 as
shown in FIG. 2 and FIG. 3. From this relationship, V=Dt, an
increase in the thickness of the magnetic layers is directly
proportional to the effective activation volume, V, of the
exchange-coupled digital storage medium 10. The present exchange
coupled system enables a decrease in grain size in the magnetic
layers while increasing the overall activation volume since the
effective thickness of the medium is the combined thickness of the
two magnetic layers. By increasing the thickness of the medium,
vertical expansion of the medium is achieved, which leads to
improved thermal stability of the digital storage magnetic medium
10.
[0045] The increase in this coupled magnetic activation volume, V,
is important for determining thermal stability of the
exchange-coupled system. The magnetic-anisotropy energy of the
exchange-coupled system is: K.sub.u,effV, while the thermal energy
is: k.sub.BT. Therefore, the thermal stability factor of the
exchange-coupled system is: K u , eff .times. V kBT . ##EQU1## An
increase in the magnetic-anisotropy-energy, K.sub.u,effV of the
exchange-coupled system provides greater thermal stability as
compared to a prior art magnetic medium which is not
exchange-coupled. The thermal stability of the tilted medium in the
present invention can be increased through increasing the magnetic
anisotropy energy, which can be achieved by increasing the overall
effective anisotropy constant, K.sub.u,eff, or the activation
volume, V. The activation volume, V, when increased vertically,
would contribute to an increase in the thermal stability of the
tilted medium. However, if the activation volume is increased
through horizontal expansion, it can be easily seen from the
relation V=Dt that a horizontal expansion of the medium indicates
increase in D, which means a horizontal increase in domain size.
This would translate into a decrease in areal density, which is not
desired. Hence, the selection of the magnetic materials for the
respective layers is important to ensure that the activation volume
is increased only vertically through the thickness, t, of the
medium.
[0046] FIG. 8 shows the relationship of the direction of the
applied field 50, the easy axis 52 and the plane 11 in which the
magnetic medium lies. The easy axis forms an angle .theta. with the
plane 11, while the applied field forms an angle .alpha. with the
easy axis. The sum of these two angles is .beta.(i.e.,
.beta.=.alpha.+.theta.). From this illustration, the effects of
having a medium with magnetic moments tilted at an angle .theta. is
discussed in connection with FIG. 9.
[0047] FIG. 9 shows the ratio of switching field and anisotropy
field (H.sub.s/H.sub.k) in relation to the angle, .alpha., between
the tilted easy axis and the applied field direction. It is
observed that the ratio of the switching field and anisotropy field
decreases as the angle .alpha. of the tilted easy axis increases
from 0.degree. to 45.degree. and then increases as .alpha.
increases further to 90.degree.. If the anisotropy field H.sub.k is
kept constant, the switching field H.sub.s reaches a minimum when
the magnetic moment is tilted at an angle, .alpha.=45.degree. to
the applied field direction. This means that for the same magnitude
of applied field, the anisotropy constant, K.sub.u, of the tilted
magnetic medium can be increased while the writability of the
medium is maintained. Therefore, the thermal stability of a tilted
medium can be further improved by using materials of high
anisotropy constant, K.sub.u. So long as the easy axis of the
medium is angled between .alpha.=0.degree. and .alpha.=90.degree.,
the switching field, H.sub.s, required for performing write
operations by a transducer head will be less than that required for
write operations in a longitudinal or perpendicular recording
medium. This shows that the present invention provides an advantage
in that the tilted medium can be paired with either a ring head, a
single pole head, or a tilted head for write operations. The
anisotropy constant, K.sub.u, is related to the anisotropy field
such that K u = H k .times. M s 2 , ##EQU2## where M.sub.s is the
saturation magnetization, which is kept constant. From FIG. 9, the
relationship: H.sub.s/H.sub.k=X is illustrated, where X=f(.alpha.)
and .alpha. is the angle between the easy axis and the applied
field. Therefore K.sub.u can also be expressed in relation to the
switching field, H.sub.s: K u = H s .times. M s 2 .times. X .
##EQU3## For optimum writing operations, the external writing
field, H.sub.g=1.5H.sub.s and so K u = H g .times. M s 3 .times. X
. ##EQU4## In a longitudinal or perpendicular system where the
magnetic moments are not tilted at an acute angle, (i.e. when
.alpha.=0.degree. or 90.degree.), X=f(.alpha.)=1, leaving K u = H g
.times. M s 3 . ##EQU5## When the magnetic moments are angled such
that .alpha.=45.degree., X=f(.alpha.)=0.5, which gives K u = 2
.times. H g .times. M s 3 . ##EQU6## It is evident that given the
same writing field, H.sub.g, and saturation magnetization, M.sub.s,
when the magnetic moments are tilted at an angle of 45.degree., the
anisotropy constant K.sub.u is doubled as compared to having
magnetic moments at 0.degree. or 90.degree.. Therefore, a lower
switching field also enables an increase in the anisotropy constant
for the same writing field. To illustrate this, it is assumed that
saturation magnetization, M.sub.s=400 emu/cm.sup.3 and the writing
gap field, H.sub.g=16 kOe, where the head field, B.sub.o=2 Tesla
and the optimum writing field is at 80% of B.sub.o. Using the
relationship above, the value of K.sub.u=2.1.times.10.sup.6
erg/cm.sup.3 when X=1 (i.e., when the angle .alpha.=0.degree. or
90.degree.); and the value of K.sub.u=4.3.times.10.sup.6
erg/cm.sup.3 when X=0.5 (i.e. when the angel .alpha.=45.degree.).
It is shown in this illustration that the anisotropy constant,
K.sub.u, is increased when the magnetic moments tilt at an angle
.theta. between 0.degree. and 90.degree. in the same applied
field.
[0048] It is further observed from FIG. 10 that the differential
switching field over angle is minimal at the angle ranging from
10.degree. to 80.degree.. This indicates that the magnetic moment
with a tilted easy axis varying within this range is less sensitive
to dispersion of the applied field. Hence the magnetic moments with
an easy axis tilted at an angle in this range are able to reverse
more uniformly and generate less noise as compared to the magnetic
moments in a longitudinal (0.degree.) or perpendicular (90.degree.)
system. In actual application of the tilted medium, it is
preferable that the magnetic moments tilt at an angle ranging from
10.degree. to 45.degree. when using a ring head and tilt at an
angle ranging from 45.degree. to 80.degree. when using a single
pole head. This indicates that when the magnetic medium has an easy
axis tilted at an angle that falls within these ranges, the medium
is less sensitive to the dispersion in the applied field, thereby
improving the performance of the medium.
[0049] One characteristic of the exchange-coupled system is that
the remnant magnetic moments, M.sub.1 and M.sub.2, have effective
easy axes 102 that are biaxial in character as illustrated in FIG.
11. The tilting remnant magnetic moments, M.sub.1 and M.sub.2,
which have the directions of the remnant magnetic moments 15 and
17, are allowed to fall in either direction of the biaxial easy
axes. The biaxial character of the effective easy axes of the
remnant magnetic moments, M.sub.1 and M.sub.2, provides an
advantage over existing prior art where the magnetic moments lie in
effective easy axes that are uniaxial in character. When compared
to prior art storage media that have a tilted uniaxial easy axis
which restricts the direction of the magnetic moment, the biaxial
character provides flexibility to the writing of data on the
magnetic medium without compromising quality. The grains of a
magnetic medium with a uniaxial tilted easy axis tilt either in the
down-track or cross track direction, which provides a
highly-oriented tilted medium. Although such highly-oriented media,
particularly those with grains having easy axes tilted in the
down-track directions, are acceptable for data storage
applications, they are not achievable with current sputter
deposition methods. In addition to this limitation, concentration
of magnetic moments at the edges of tracks will expand the track
width limiting the track density and areal density of such a
medium. In a magnetic medium of the present invention with a
biaxial easy axis, data bits can fall in an alternative direction
avoiding concentration of dipole charge at the track edge. This
reduces the interference with neighboring data bits, which reduces
the overall transition and track edge noise providing an advantage
over magnetic media with uniaxial easy axes.
[0050] In some embodiments, the exchange-coupling constant can be
varied by inserting a thin interlayer of a high saturation
magnetization material 27 between the first magnetic layer 18 and
the second magnetic layer 20. FIG. 12 shows the structure of one
preferred embodiment of the present invention with a thin
interlayer 27 selected from a group of cobalt compounds, including
pure cobalt and cobalt alloy. The function of the interlayer 27 is
to enhance the exchange-coupling at the interface between the first
magnetic layer 18 and the second magnetic layer 20. As such, the
tilting angle of the respective layer can be tuned as desired for a
specific recording system head. In the presence of the interlayer
27 a strong exchange-coupling can be achieved so that both magnetic
moments from the first and second magnetic layers switch coherently
and form a tilted angle in the same magnetization remnant state.
The interlayer 27 may also be used in embodiments where the easy
axes of the magnetic layers are switched as shown in FIG. 3. It is
preferable to have the interlayer as thin as possible (below 1 nm)
to maintain the recording performance. If the thickness of the
interlayer 27 is beyond 1 nm, it is likely that the interlayer may
contribute to horizontal expansion in the domain size of the
magnetic medium. The thickness of the interlayer greater than 1 nm
may also contribute additional signal to the overall recording
which will add to the transition noise in a high recording density
medium.
[0051] In other embodiments of the present invention, the selection
of the type of non-magnetic material for the interlayer 44 and its
thickness is used to fabricate a digital storage medium that has
resultant magnetic moments, M.sub.1 40 and M.sub.2 42,
anti-parallel to each other, as shown in FIG. 13, such that the
remnant magnetic moment is low but non-zero. An example of the type
of material that can be used to achieve this uses ruthenium (Ru) as
part of the interlayer. The ruthenium can be disposed between two
layers of cobalt, which are disposed between the first 18 and
second 20 magnetic layers. The interaction of the exchange-coupling
results in anti-parallel magnetic moments that are tilted at an
angle to the plane 11 of the digital storage medium. Other types of
non-magnetic materials that can be used for anti-ferromagnetic
exchange-coupling include rhodium (Rh), chromium (Cr), copper (Cu),
iridium (Ir), and their alloys.
[0052] FIG. 14 compares the switching time required at tilted
moments of various angles in two different normalized applied
fields, H.sub.z/H.sub.k=2, and H.sub.z/H.sub.k=3, Where H.sub.z is
the applied external field, which is also known as the Zeeman
field. It is observed from the plots that at .theta.=0.degree., the
reversal or switching time is 220 ps (picoseconds). Similarly, it
can be observed from the trend of the plots that when
.theta.=90.degree. the switching time is comparably high. When the
angle, .theta. ranges between 0.degree. and 90.degree., the
switching time is considerably lower at below 120 ps for the
applied field at H.sub.z/H.sub.k=3, and below 160 ps for the
applied field at H.sub.z/H.sub.k=2. This result shows that when
magnetic moments are tilted at an acute angle, the time required to
switch the magnetic moments is shorter. A shorter transition time
also enables the write operations to be faster as compared to a
longitudinal or perpendicular recording media. Hence, the faster
transitions would serve extremely high recording density media that
require very fast writing operations.
[0053] In an example of the present invention, the substrate 12 is
formed from an aluminum magnesium (AIMg) or glass disk. The seed
layer 13 is disposed on the substrate 12. The underlayer may be a
chromium alloy deposited onto the seed layer 13. The intermediate
layer 16 may include cobalt, chromium, and tantalum (CoCrTa). The
first magnetic layer 18 disposed on the intermediate layer 16
includes cobalt (Co), chromium (Cr), platinum (Pt) and boron (B),
to form an alloy, CoCrPtB. The second magnetic layer 20 includes a
series of alternating cobalt/palladium (Co/Pd) layers, where the
first contact layer 20a with the first magnetic layer can be a
palladium or cobalt. Three samples, with different thickness for a
Pd interlayer 20a interfaced with the CoCrPtB film were compared
and the coercivity in relation to the field angle were measured at
room temperature for each sample. These measurements are shown in
FIG. 15. The field angle, .beta. at 0.degree. indicates that an
external field was applied in the direction of the sample plane
during measurement. At the field angle .beta.=90.degree., the field
is applied at a direction perpendicular to the plane during
measurement. Varying the thickness of the first Pd layer 20a
modifies the exchange coupling strength between the two
ferromagnetic layers, 18 and 20.
[0054] The symmetry of the two peaks in FIG. 15 indicates a
formation of the biaxial easy axes in all three examples where the
thickness of the palladium is 0.6 nm, 1.2 nm, and 2.5 nm. The peaks
are observed at angles occurring between 0.degree. and 90.degree.
to the magnetic plane. In the sample where the thickness of
palladium is 0.6 nm, two peaks occur at angle
.theta.=.+-.60.degree.. As the thickness of the palladium layers
increases further, the peaks shift to .theta.=.+-.45.degree. at the
thickness of 1.2 nm, and .theta.=.+-.30.degree. at the thickness of
2.5 nm, and the coercivity is observed to increase with the
thickness of the palladium layer. A similar angle dependent trend
was observed for the magnetization squareness measurement, given by
the ratio of the remnant magnetic moment and the saturation
magnetization: M.sub.r/M.sub.s, for the three samples as shown in
FIG. 16.
[0055] This observation re-enforces the conclusion that the
exchange-coupling constant is enhanced by adjusting the thickness
of the interlayer between the two magnetic layers 18 and 20.
[0056] Thermal stability is enhanced in the current invention
because a higher anisotropy constant, K.sub.u, can be achieved in
the magnetic grains without affecting writing operations. The
present invention allows the use of a longitudinal ring head, a
perpendicular single pole head, or a transducer head that is
capable of applying a writing field at an angle for recording
purposes. This enables an increase in areal density with an
increase in the coupled magnetic activation volume while at the
same time thermal stability is increased.
[0057] It is to be noted that the above description is only
illustrative of the present invention. Modification of the examples
without departing from the scope and spirit of the present
disclosure is possible to provide different versions of the
invention.
* * * * *