U.S. patent application number 12/182909 was filed with the patent office on 2008-11-27 for enhancement of magnetic media recording performance using ion irradiation to tailor exchange coupling.
This patent application is currently assigned to International Business Machines Corporation. Invention is credited to Manfred Albrecht, Charles Thomas Rettner, Bruce David Terris, Thomas Thomson.
Application Number | 20080292906 12/182909 |
Document ID | / |
Family ID | 34807855 |
Filed Date | 2008-11-27 |
United States Patent
Application |
20080292906 |
Kind Code |
A1 |
Albrecht; Manfred ; et
al. |
November 27, 2008 |
Enhancement of Magnetic Media Recording Performance Using Ion
Irradiation to Tailor Exchange Coupling
Abstract
Magnetic medium recording performance can be enhanced by
irradiating a magnetic medium with ions having an acceleration
voltage of between 10 keV and 100 keV to induce exchange coupling
between grains of the magnetic medium. The magnetic medium is
exposed to a cumulative ion dosage of between 10.sup.13
ions/cm.sup.2 and 10.sup.17 ions/cm.sup.2 using a non-patterned
exposure of the magnetic medium. The ions can be selected from the
group consisting of H.sup.+, He.sup.+, Ne.sup.+, Ar.sup.+,
Kr.sup.+, and Xe.sup.+. Alternatively, the ions can be selected
from the group consisting of Ga.sup.+, Hg.sup.+, and In.sup.+.
Inventors: |
Albrecht; Manfred;
(Isny-Rohrdorf, DE) ; Rettner; Charles Thomas;
(San Jose, CA) ; Terris; Bruce David; (Sunnyvale,
CA) ; Thomson; Thomas; (Mountain View, CA) |
Correspondence
Address: |
JOSEPH P. CURTIN, L.L.C.
1469 N.W. MORGAN LANE
PORTLAND
OR
97229-5291
US
|
Assignee: |
International Business Machines
Corporation
Armonk
NY
|
Family ID: |
34807855 |
Appl. No.: |
12/182909 |
Filed: |
July 30, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10768347 |
Jan 29, 2004 |
7425353 |
|
|
12182909 |
|
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|
Current U.S.
Class: |
428/800 ;
G9B/5.295 |
Current CPC
Class: |
G11B 5/84 20130101 |
Class at
Publication: |
428/800 |
International
Class: |
G11B 5/62 20060101
G11B005/62 |
Claims
1-16. (canceled)
17. A magnetic medium formed by irradiating the magnetic medium
with ions having an acceleration voltage of between 10 keV and 100
keV to induce exchange coupling between grains of the magnetic
medium, the ions being selected from the group consisting of
Hg.sup.+ and In.sup.+.
18. (canceled)
19. (canceled)
20. (canceled)
21. The magnetic medium according to claim 17, wherein the magnetic
medium has been exposed to an ion dosage of between 10.sup.13
ions/cm.sup.2 and 10.sup.17 ions/cm.sup.2.
22. The method according to claim 17, wherein an areal density of
magnetic bits that can be recorded on the medium is increased by
the irradiation of ions.
23-29. (canceled)
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to media for tape recording
systems and Hard Disk Drives (HDDs). More particularly, the present
invention relates to a technique for improving media recording
performance of tape recording systems and HDDs.
[0003] 2. Description of the Related Art
[0004] Magnetic recording disks are currently made having a
continuous magnetic layer that is deposited on a suitable aluminum
alloy or glass ceramic substrate. The materials, or media, used for
the magnetic layer are generally nickel, cobalt or iron alloys that
have been deposited by evaporation or by sputtering to form a
magnetic recording layer. Data is written to the magnetic layer by
applying a localized magnetic field using a recording head that
flies over the surface of the magnetic layer. In perpendicular
recording, the media is magnetized perpendicularly to the plane of
the disk and each magnetic bit consists of several hundred small
grains having a predominant magnetization direction. In
longitudinal recording, the magnetization is oriented in the plane
of the disk. More recently, other orientations of the magnetization
have been proposed to take advantage of the minimum in the
Stoner-Wohlfarth switching astroid for different magnetic recording
media.
[0005] Each grain of a magnetic recording medium ideally has a
single magnetization axis such that the magnetization can lie in
one of two directions separated by 180 degrees. The magnetization
transition between adjacent bits follows a line between the grains
of the respective bits. In a perfect medium, the grains on either
side of a transition would have opposite magnetization directions.
Such a condition would occur only when the grains are subjected to
weak magnetic coupling. When the magnetic coupling between two
adjacent bits is too great, though, the transition between adjacent
bits is poorly defined. Further, the magnetization vectors of each
grain within a bit should align in a single direction, which is a
state that is more easily obtained when the grains have strong
magnetic coupling and is in contrast to the requirement for a weak
magnetic coupling for a well-defined transition between adjacent
bits. Additionally, thermal fluctuations cause grain magnetization
to spontaneously reverse when the volume of the effective magnetic
unit is less than a certain minimum, causing loss of data. As the
volume of the effective magnetic unit increases, the coupling
between grains increases and thereby enhances stability.
[0006] It follows that, in view of the conflicting requirements
relating to the exchange coupling between adjacent bits and within
a given bit and the effects of thermal fluctuations, the degree of
coupling between grains is a key parameter that critically affects
the performance of a magnetic recording medium. In that regard, the
need for controlling exchange coupling between grains is well
known. See, for example, U.S. Pat. No. 5,834,085 to Lairson et
al.
[0007] Recent studies of perpendicular recording media have
suggested that the presence of some exchange coupling between
grains can lead to an enhancement of the recording performance and
thermal stability. Coupled granular/continuous (CGC) media have
been proposed as one technique for controlling exchange coupling in
a continuous medium. A typical CGC medium consists of a Pt-rich
CoCrPt layer having poor Co grain segregation and a thin Pt layer
improves the reversal nucleation field of a Co70Cr18Pt12 medium
from +420 to -600 Oe. The thermal decay of the signal amplitude
from recorded transitions is reduced from 2.23% to 0.10% per
decade. Unity squareness has been obtained for a thin Pt capping
layer with a magnetization decay rate of 0.21% per decade. See, for
example, Y. Sonobe et al., "Thermally Stable CGC Perpendicular
Recording Media with Pt-Rich CoPtCr and Thin Pt layers,"IEEE
Transactions On Magnetics, Vol. 38, No. 5, pp. 2006-2011, September
2002. CGC media having a Co/Pd multilayer capping structure has
also been shown to improve thermal stability without compromising
SNR. See, for example, Y. Sonobe et al., "Thermal Stability and SNR
of Coupled Granular/Continuous Media," IEEE Transactions
[0008] On Magnetics, Vol. 37, No. 4, pp. 1667-1670, July 2001.
Extra layers, however, are required, thereby increasing the medium
and deposition tool complexity.
[0009] Ion beam treatments have been described for the purpose of
improving tribological performance. See, for example, U.S. Pat. No.
6,368,425 to Segar et al. Additionally, ion beam treatments have
also been described as a technique for patterning recording media.
See, for example, U.S. Pat. No. 6,383,597 to Fullerton et al. Ion
beam treatments have also been described in JP 1110757A to K.
Akiysau as a technique for producing isolated magnetic grains by a
process that forms defects and "micronizes" (or pulverizes)
existing grains. Such a known process requires only 10.sup.10
ions/cm.sup.2 for H, He, Li, or Be, as compared to >10.sup.13
ions/cm.sup.2 for the technique of the present invention.
[0010] What is required is a technique for optimizing the exchange
coupling of a perpendicular magnetic recording medium that is
independent from the deposition process of the magnetic medium.
BRIEF SUMMARY OF THE INVENTION
[0011] The present invention provides a technique for optimizing
the exchange coupling of a perpendicular magnetic recording medium
that is independent from the deposition process of the magnetic
medium.
[0012] The advantages of the present invention are provided by a
method for controlling exchange coupling of grains of a magnetic
medium, such as a magnetic medium having granular magnetic
particles on a tape or a disk. First, a magnetic medium having
magnetic grains is formed. The magnetic medium can have a
perpendicular magnetization, a longitudinal magnetization, or some
intermediate direction of magnetization (i.e., a magnetization
between a perpendicular magnetization and a longitudinal
magnetization). Then, the magnetic medium is irradiated with ions
to induce exchange coupling between grains of the magnetic medium.
The magnetic medium is irradiated with ions having an acceleration
voltage of between 10 keV and 100 keV so that the magnetic medium
is exposed to an ion dosage of between 10.sup.13 ions/cm.sup.2 and
10.sup.17 ions/cm.sup.2 using a non-patterned exposure of the
magnetic medium. Alternatively, the magnetic medium is irradiated
with ions having an energy that substantially affects an entire
thickness of the magnetic medium. According to one aspect of the
present invention, the ions can be selected from the group
consisting of H.sup.+, He.sup.+, Ne.sup.+, Ar.sup.+, Kr.sup.+, and
Xe.sup.+. According to another aspect of the present invention, the
ions can be selected from the group consisting of Ga.sup.+,
Hg.sup.+, and In.sup.+.
[0013] The present invention also provides a magnetic medium that
is formed by irradiating the magnetic medium with ions to induce
exchange coupling between grains of the magnetic medium using a
non-patterned exposure of the magnetic medium. The magnetic medium
can be a magnetic medium having granular magnetic particles on a
tape or a disk. Further, the magnetic medium can have a
perpendicular magnetization or a longitudinal magnetization.
According to the present invention, the magnetic medium has been
exposed to an ion dosage of between 10.sup.13 ions/cm.sup.2 and
10.sup.17 ions/cm.sup.2. Additionally, the magnetic medium has been
irradiated with ions having an acceleration voltage of between 10
keV and 100 keV. Alternatively, the magnetic medium has been
irradiated with ions having an energy that substantially affects an
entire thickness of the magnetic medium. According to one aspect of
the present invention, the ions can be selected from the group
consisting of H.sup.+, He.sup.+, Ne.sup.+, Ar .sup.+, Kr.sup.+, and
Xe.sup.+. According to another aspect of the present invention, the
ions can be selected from the group consisting of Ga.sup.+, Hg
.sup.+, and In.sup.+.
[0014] The present invention further provides a method of
irradiating a magnetic medium with ions in a non-patterned (e.g.,
uniform) fashion to increase the areal density of magnetic bits
that can be recorded on the medium. The magnetic medium is
irradiated with ions having an acceleration voltage of between 10
keV and 100 keV, so that the magnetic medium is exposed, in a
non-patterned fashion, to an ion dosage of between 10.sup.13
ions/cm.sup.2 and 10.sup.17 ions/cm.sup.2. Alternatively, the
magnetic medium is irradiated with ions having an energy that
substantially affects an entire thickness of the magnetic medium.
According to one aspect of the present invention, the ions can be
selected from the group consisting of H.sup.30 , He.sup.+,
Ne.sup.+, Ar .sup.+, Kr.sup.+, and Xe.sup.+. According to another
aspect of the present invention, the ions can be selected from the
group consisting of Ga.sup.+, Hg .sup.+, and In.sup.+.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The present invention is illustrated by way of example and
not by limitation in the accompanying figures in which like
reference numerals indicate similar elements and in which:
[0016] FIGS. 1A-1D show magnetic force microscopy (MFM) images of
the AC-demagnetized state for 5 .mu.m .times.5 .mu.m area of
Co.sub.70Cr.sub.18Pt.sub.12 granular perpendicular recording media
having varying doses of Ar.sup.+ ions/cm.sup.2;
[0017] FIG. 2 is a graph respectively showing data for coercivity
(curve 201) and S*(curve 202), a measure of the slope of the
hysteresis loop close to the coercivity, obtained from polar Kerr
rotation measurements on granular Co.sub.70Cr.sub.18Pt.sub.12
perpendicular media as a function of 20 keV Ar.sup.+ ion
dosage;
[0018] FIG. 3 shows D50 data, the linear density at which signal
amplitude is reduced to 50% of its peak value, obtained using a
Guzik spinstand measurement for a series of exemplary disks;
[0019] FIG. 4 is a graph showing PW50, the width of a
differentiated pulse at half its maximum amplitude, from the
readback signal of a perpendicular magnetic medium as a function of
increasing Ar.sup.+ ion dosage;
[0020] FIGS. 5A and 5B are graphs respectively showing changes in
coercivity H.sub.C and remanance M.sub.R/M.sub.s resulting from
irradiation of a perpendicular magnetic medium with 30 keV Ga+
(square data points) and 20 KeV Ar+ (round data points);
[0021] FIG. 6 shows a graph of domain size of a perpendicular
magnetic medium measured with MFM as a function of 30 keV Ga+ ion
dosage (square data points) and of 20 KeV Ar+ ion dosage (round
data points);
[0022] FIG. 7 is a graph showing variations of S* as a function of
30 keV Ga+ and 20 keV Ar+ ion dosage; and
[0023] FIG. 8 shows simulation results of ion beam damage that are
produced in Co.sub.70Cr.sub.18Pt.sub.18 by 10 keV Ne.sup.+, 20 KeV
Ar.sup.+, and 30 keV Ga.sup.+.
DETAILED DESCRIPTION OF THE INVENTION
[0024] The invention provides a technique for improving the
recording performance of magnetic media formed by conventional
techniques, such as sputter deposition, by exposing the media to
ion beam radiation. In particular, the present invention provides a
technique of irradiating a Co.sub.70Cr.sub.18Pt.sub.12 granular
perpendicular recording media without a soft underlayer using
Ar.sup.+ and Ga.sup.+ ions at acceleration voltages between 10-100
keV (preferably between 20-30 keV). The performance improvement
provided by the present invention can be observed based on readback
signal measurements of D50, which is the linear density at which
signal amplitude is reduced to 50% of its peak value, and based on
measurements of PW50, which is the width of a differentiated pulse
at half its maximum amplitude.
[0025] FIGS. 1A-1D are magnetic force microscopy (MFM) images
showing that as ion irradiation dose increases, the exchange
coupling increases, as shown by an increase in domain size and
contrast. More specifically, FIGS. 1A-1B show MFM images of an
AC-demagnetized state for a 5 .mu.m.times.5 .mu.m area of a
Co.sub.70Cr.sub.18Pt.sub.12 granular perpendicular recording media
having varying doses of Ar.sup.+ ions/cm.sup.2. FIG. 1A shows an
MFM image for a dosage of no Ar.sup.+ ions. FIG. 1B shows an MFM
image for an Ar.sup.+ ion irradiation dosage of 9.4.times.10.sup.14
ions/cm.sup.2. FIG. 1C shows an MFM image for an Ar.sup.+ ion
irradiation dosage of 18.8.times.10.sup.14 ions/cm.sup.2. FIG. 1D
shows an MFM image for an Ar.sup.+ ion irradiation dosage of
37.7.times.10.sup.14 Ar.sup.+ ions/cm.sup.2. Evolution of the
domain structure as a function of increasing level of Ar.sup.+ ion
irradiation is apparent as greater coupling leads to larger
domains. At the highest Ar.sup.+ dose (FIG. ID), the MFM image
becomes less clear because the coercivity is sufficiently reduced,
thereby allowing the MFM tip to interact with the medium and change
the magnetic structure.
[0026] The S* parameter obtained from polar Kerr measurements show
an initial increase as a function of Ar.sup.+ ion irradiation. The
S* parameter is a measure of the slope of the hysteresis loop close
to the coercivity. The steeper the slope of the hysteresis loop,
the larger the S* parameter which indicates a greater exchange
coupling. FIG. 2 is a graph respectively showing data for
coercivity and S* obtained from polar Kerr rotation measurements on
granular Co.sub.70Cr.sub.18Pt.sub.12 perpendicular media as a
function of 20 keV Ar.sup.+ ion dosage. Data points for coercivity
are shown as circles. Data points for S* are shown as squares.
Curve 201 indicating coercivity and curve 202 indicating S* in FIG.
2 respectively show the tendencies of coercivity and S* as a
function of increasing Ar.sup.+ ion dosage. In FIG. 2, S* (curve
202) goes through a maximum as a function of ion dose, whereas
coercivity (curve 201) monotonically decreases as a function of ion
dose. Both of the MFM measurements (FIGS. 1A-1D) and S*
measurements (FIG. 2) indicate increase exchange coupling for low
ion doses.
[0027] Spinstand measurements of signal as a function of linear
density provide an indication of the relative recording performance
of a medium. The roll off curve can be parameterized using D50,
which is the linear density at which the output signal is reduced
to half its peak value. FIG. 3 shows D50 data in kilo flux changes
per inch (kfci) obtained using a Guzik spinstand measurement for a
series of exemplary disks having a granular
Co.sub.70Cr.sub.18Pt.sub.12 perpendicular magnetic medium. An
increase in D50 of approximately 10% is shown in FIG. 3 for low
Ar.sup.+ dose, with a maximum occurring for a dose of
3.times.10.sup.14 ions/cm.sup.2. A further increase in Ar.sup.+
irradiation leads to a reduction in D50. Curve 301 in FIG. 3 shows
the tendency of the spinstand measurement as a function of
increasing Ar.sup.+ ion dosage.
[0028] FIG. 4 is a graph showing PW50, which is the width of the
differentiated pulse at half its maximum amplitude, from the
readback signal of a granular Co.sub.70Cr.sub.18Pt.sub.12
perpendicular magnetic medium as a function of increasing Ar.sup.+
ion dosage. Curve 401 in FIG. 4 indicates an improvement for PW50
that is similar to the improvement of D50 as a function of
increasing Ar.sup.+ ion dosage shown in FIG. 3. In particular, PW50
exhibits a minimum for an Ar.sup.+ ion dose that corresponds to the
Ar.sup.+ ion dose for a maximum in D50. The data shown in FIG. 4
were obtained using a Guzik spinstand measurement that was
processed on digital oscilloscope. The procedure used resulted in
larger error bars than are normally reported, with .sigma.=2
ns.
[0029] The peak in exchange coupling (shown in FIG. 2) occurs for
greater Ar.sup.+ dose than for the maximum in D50 (shown in FIG. 3)
and the minimum in PW50 (shown in FIG. 4). Accordingly, the
differences in the dosage amount between the respective maxima and
minima suggests that excess exchange coupling leads to a reduction
in recording performance. It should, however, also be noted that
the coercivity is changing significantly over the dosage range of
FIGS. 2-4 and it may be equally valid to hypothesize that a
reduction in coercivity leads to a reduction in D50 and increase in
PW50 at higher Ar.sup.+ doses.
[0030] Regardless, given either mechanism, a recording medium
performance advantage is obtained when the medium is subjected to
Ar.sup.+ ion irradiation. Thus, the present invention provides a
technique for controlling exchange coupling in magnetic media,
thereby optimizing recording medium performance properties, based
on subjecting the magnetic recording medium to an accumulated dose
of Ar.sup.+ ion irradiation. Ar gas can be ionized to create
Ar.sup.+ ions for irradiating a magnetic medium according to the
present invention. Other gases that can be used for the present
invention and that are commonly used to make ion beams include
H.sup.+, He.sup.+, Ne.sup.+, Kr.sup.+, and Xe.sup.+. With the
exception of H.sup.+, these gases are chemically inert and do not
react with the ionizer or the sample.
[0031] Experiments using 30 keV Ga.sup.+provide results that are
very similar to the results obtained using 20 KeV Ar.sup.+, as
shown by FIGS. 5-8. Liquid Ga can be used for generating Ga.sup.+
ions. Other liquid metal sources that can be used for the present
invention and that are suitable for producing focused beams include
Hg.sup.+, and In.sup.+. FIGS. 5A and 5B are graphs respectively
showing changes in coercivity H.sub.C and remanance M.sub.R/M.sub.S
resulting from irradiation of a perpendicular magnetic medium with
30 keV Ga.sup.+ and 20 KeV Ar.sup.+. Data for Ga.sup.+ is shown as
square data points. Data for Ar.sup.+is shown as round data points.
Note that 1 nC/.mu.m.sup.2=6.25.times.10.sup.17ions/cm.sup.2.
[0032] FIG. 6 shows a graph of domain size of a perpendicular
magnetic medium measured with MFM as a function of 30 keV Ga.sup.+
ion dosage and of 20 KeV Ar.sup.+ ion dosage. Data for Ga.sup.+is
shown as square data points. Data for Ar.sup.+ is shown as round
data points. As with coercivity and M.sub.ra/M.sub.s, shown in
FIGS. 5A and 5B, the changes in domain size are very similar for
both Ga.sup.+ and Ar.sup.+ irradiation. Note that 0.005
nC/.mu.m.sup.2 =3.1.times.10.sup.15 ions/cm.sup.2.
[0033] FIG. 7 is a graph showing variations of S* as a function of
30 keV Ga.sup.+ and 20 keV Ar.sup.+ ion dosage. Data for Ga.sup.+is
shown as round data points. Data for Ar.sup.+ is shown as square
data points.
[0034] FIG. 8 shows simulation results of ion beam damage that are
produced in Co.sub.70Cr.sub.18Pt.sub.18 by 10 keV Ne.sup.+, 20 KeV
Ar.sup.+, and 30 keV Ga.sup.+. The simulations were carried out
using a Monte-Carlo based code known as SRIM. See, for example, J.
F. Ziegler et al., "The Stopping and Range of Ions in Solids,"
Pergamon Press, New York, 1985. The SRIM software that was used for
the simulations is available from http://www.srim.org/. The extent
of the damage is similar for each of these different ions. The
difference in size and mass of the different ions shown in FIG. 8
is compensated by changing the energy. The dose required for the
same amount of damage, however, decreases with increasing ion mass.
For example, 10 keV Ne.sup.+ produces only about 1/3as much damage
(i.e., defects) per ion as 30 keV Ga.sup.+.
[0035] The advantage of magnetic recording medium performance
enhancement is not dependent on the level of ion current, only the
accumulated dose. In one series of experiments using Ar.sup.+,
results obtained using currents differing by more than a factor of
three agreed well when magnetic recording mediums were exposed at
the same dose, i.e., the low current case involved an exposure that
was over three times longer.
[0036] Up to this point in the disclosure of the present invention,
the focus has been on a magnetic recording medium having a
perpendicular magnetization. It should be understood that the
present invention is applicable to magnetic recording mediums
having a longitudinal magnetization.
[0037] Although the foregoing invention has been described in some
detail for purposes of clarity of understanding, it will be
apparent that certain changes and modifications may be practiced
that are within the scope of the appended claims. Accordingly, the
present embodiments are to be considered as illustrative and not
restrictive, and the invention is not to be limited to the details
given herein, but may be modified within the scope and equivalents
of the appended claims.
* * * * *
References