ft3 HEWLETT
.:7'.... PACKARD
Roadmap for 10 Gb/in2 Media: Challenges
Edward S. Murdock
Ralph F. Simmons*, Robert Davidson*
Computer Peripherals Laboratory
HPL·92·70
June,1992
media,recording
media, thin film disks,
magnetic recording,
media noise, thin
magnetic films,
density trends, ultra
high density media
The history of rigid disk magnetic recording
shows a sustained growth in areal density by a
factor of ten per decade for forty years. Recent
technology demonstrations by mM and Hitachi
promise a continuation of that growth rate for the
next ten years. This paper applies informed
speculation to the question of what advances in
media technology will be necessary to continue
the trend to 10 Gb/in 2 • We apply a read/write
model to examining the media properties required
for this density in a practical drive application.
We also consider the trends in media
microstructure and noise, some physical
limitations of present materials and a brief look
at candidate materials systems for 10 Gb/in 2
recording. Some revolutionary changes in media
technology appear to be necessary to support this
recording density.
Internal Accession Date Only
"'Disk Memory Division, Boise, Idaho
To be published in IEEE Transactions on Magnetics, vol. 28, September 1992
© Copyright Hewlett-Packard Company 1992
------------------------------------_.._-_._ _..
1 Motivation
The march of areal density in magnetic recording has continued at a pace of one order of
magnitude per decade for nearly forty years. Recently introduced high-end products are
recording at approximately 150 Mb/ii. With the recent technology demonstrations of 1
Gb/ii [1] and 2 Gb/ii [2], it appears that the established pace will continue and by the
year 2010 magnetic memory products with 10 Gb/ii will be available. However, it is
becoming harder to simultaneously provide much higher He, much thinner films, smaller
grains and lower media noise. Analysis of the requirements for media to support 10 Gb/in2
in a practical disk drive strongly suggests that evolutionary improvements in present media
technologies will not do the job.
It is likely that continued progress requires development of a family of new "ultra-thin"
materials: layers that, for longitudinal recording, are only 5-15 nm thick and for
perpendicular recording are under 100 nm thick and whose magnetic and microstructural
properties will support the requirements of 10 Gb/in' and beyond. Because of this, such
work needs to begin within the next few years rather than waiting for 10-15 years until the
product needs force industry's attention.
What lessons can we infer from history regarding the next technology shift that will be
necessary in order to continue to increase areal recording density at near the historical rate
of lOX per decade? History suggests, first, that the follow-on technology should be
available in incipient form for many years before it is required by demands. This was the
case for the transition from particulate oxide media to thin metal film media, which had
existed since the 1950's. Second, the basic follow-on technology may be embodied in
competing types of processes and material systems during this incipient and developmental
period. This was again the case over the last ten years as plated and sputtered media
technologies competed for the market.
This paper will focus on the challenges faced by the recording media developers, taking
into account the triad of head, media and channel technologies. Our goal has been to
identify some of the properties required by 10 Gb/ii media and where the key challenges
lie in meeting those required properties. We will calculate the magnetic properties needed
and some aspects of the anticipated microstructures using a combination of models and
observed trends in media. Finally, some observations will be made regarding known
materials systemswhich show some promise now as candidates for the media of the future.
The change in media technology may be as profound as the change from particulate to thin
metal film media.
1
2
2.1
Definition and Technology Requirements
State of the Art and Definition of 10 Gb/in2 Recording
Present day high performance media operate at about 130-150 Mb/In', with linear and
track densities typically near 60 kbpi and 2200-2500 tpi, respectively. The media used are
typically in the range of He = 1400-1600 De, ~ =2-3 memu/cm', and S* =0.85-0.9.
Average grain sizes of the Co-based recording films vary from 20-45 nm [3-5], with larger
grain size associated with higher coercivity [5,6]. There are, of course, wide variations from
product to product.
The work reported by mM and Hitachi on 1-2 Gb/irr recording has used media with
somewhat different properties, especially the Hitachi medium. In mM's case, the media
had H e= 1800 De, Mp== 0.7 memu/cnr, and S*== 0.75. Its grain structure had well-isolated
grains for low noise and average grain size of 15-20 nm [1]. In Hitachi's case, the media
was a multiple magnetic layer media with different alloys in the two magnetic layers. The
net H e= 2100 De, Mp= 1.3 memu/crrr, and S*= 0.75. They did not report their grain size
[2].
2.2
Technology Challenges
In order to see
what the trends in
areal
density
mean, Figure 1
compares the size
of a bit at 100
Mb/irr, 1 Gb/irr,
and 10 Gb/irr'.
The
traditional
increase in areal
density in rigid
disk products has
been achieved by
scaling both the
track width and
the bit length.
The ratio of the
track width to the
bit length has
been increasing
over the years,
2
lOOMbnn~ 0.71 um
35 kfci X2200 I~
90
• urn
1Gbnn
2
V\N\MW.NIJW\N\M
175 kfci I 6600 Ip/
3.0 um
-
2
10 Gbfln
0.15 um
IilII
1.6 x0.032 urn 0.8 x0.064 urn 0.42 X0.12 urn
Case 1
Figure 1: Comparison of bit sizes.
2
Case 3
Case 5
e.g. in 1980 the ratio was 10 to 1, in 1990 the ratio was 20 to 1. If the trends are
extrapolated to 10 Gb/iJr, the track density and linear density are the ones shown in Case 1
of Table 1. For the inner track of a 25 mm diameter disk spinning at 10000 RPM, the
transfer rate is 225 Mbit/sec,
The trends will not continue if they lead to transfer rates which are beyond the capability of
the electronics. Furthermore, magnetoresistive (MR) head technology promises to provide
revolutionary improvements in head technology which will favor increasing the track
density and, from velocity independent sensitivity, will favor decreasing the disk size; both
of which will tend to decrease the transfer rate for a given linear density. Hence, it appears
that the future head technology and future product requirements will tend to accelerate the
increase in track density and decrease the growth rate of linear density. Cases 2 through 5
provide examples of 10 Gbit/irr areal density using very aggressive track densities and
somewhat conservative projections of linear densities.
In order to minimize the degree of
speculation involved in discussing a
100X increase in areal density, we
have simulated each of the recording
systems listed in Table 1. The results
of the simulations shown were
computed self-consistently using a
Preisach's model for the hysteresis
[7], finite element models of the
recording head [8] for writing and
Karlqvist's model of the MR read
head field. The readback waveform
is computed using superposition; the
read gap, g, is the shield-to-MR
spacing. The discussion is focused on
a 25 mm diameter disk, spinning at
10,000 rpm with an inner track at 6
mm. At 10 Gb/in' the unformatted
capacity is 900 MBytes per disk. The
transfer rate for each system is shown
in Table 1.
2.3
Assumptions
In this section, the improvements
which are believed to be aggressive
but realistic for head, mechanical and
electronic technologies to support 10
TABLEt
Modeled Case Studies in Achieving 10 Gb/in
2
Case I
Case 2
Case 3
Case 4
Case 5
ktni
II
16.7
25
31.6
40
kbpi
909
600
400
316
250
Read Gap
40
60
100
100
100
20
30
40
60
70
70
107
157
197
244
a, (Oe)
4500
3500
2500
2500
2500
Mp
0.35
0.4
0.5
0.6
1.0
36.9
37.2
37.1
37.0
36.8
225
150
100
78
62
1.1
1.7
2.5
3.2
4.1
(nm)
HlMSep
(nm)
PW so
(nm)
(memuj
2
cm )
SNR (dB)
(Hd +
Elec)
Transfer
Rate
(Mbit/ sec)
Transition
Jitter (nm)
N.B. Transition jitter is 4% of minimum transition snaciOlI:
3
Gb/irr' recording media will be described. In order to achieve the track densities shown in
Table 1, it is assumed that track following capability on the order of that used in present
optical drive technology will be available such that signal-to-noise requirements are
determined by on-track performance only.
It is assumed that sophisticated electronic equalization techniques will be available so that
the linear densities can be achieved by recording 2.5 bits/PWso, i.e., we assume 2.5 flux
changes per PWso•
In addition to improvements in electronics, there will also be improvements in write and
read head technologies. For the write head, we assume that the magnetic material will
have at least 1.6 times the saturation magnetization of the presently used permalloy. The
write head used in the modeling is a thin film head with a gap length of 0.4 p. m and it
records at the head/media separations shown in Table 1.
One of the design advantages of MR heads is that the output voltage depends on the ratio
of the flux available from the transition to the flux required to saturate the MR element.
The flux available from the media should be in the neighborhood of 20 % of the saturation
flux of the MR element. The thickness of the MR element can be varied in order to obtain
this ratio for any given media and head/media separation (h/m sep.). With recent reports
of giant magnetoresistance, it is realistic to assume that a material with five times the
anisotropic resistivity of permalloy will be available. Therefore, it is assumed that the read
head sensitivity can be optimized for any recording system shown in Table 1 such that the
output per unit track width is 250 p. V /p.m for an isolated pulse.
One of the supporting technologies which will contribute to future increases in areal
density is the read preamp technology. It is anticipated that the preamp technology will be
capable of providing a preamp optimized for an MR head which has 0.1 nV,lVHz of input
equivalent noise, 0.3 pA,lVHz noise current and negligible sense current noise. Of course
this low noise can provide an advantage only if the Johnson noise from the resistance of the
MR read sensor is comparable to this low input equivalent noise. It is assumed that the
MR sensor for the 11 ktpi system has a total resistance of 10 ohms. At room temperature
the thermal noise density of a 10 ohm resistance is 0.39 nV,lVHz. Under these conditions,
the head/preamp noise is dominated by the head. Since the resistance of the MR head is
proportional to the track width, the head noise is smaller for all other cases shown in Table
1. However, even at 40 ktpi where the head resistance is approximately 3 ohms, the head
noise density of 0.2 nV,lVHz is still the dominant term in the head/preamp total noise
density. The rms noise is obtained by integrating the square of the total noise density over
the effective noise bandwidth. The effective noise bandwidth is that of a matched filter for
a Lorentzian pulse shape, i.e, BW=0.6366/PWso, see [12].
4
2.4
Signal-to-Noise Requirements
The fundamental requirement of a memory device is the error free recollection of
previously stored data. Given that the head is accurately centered on the target track, the
error rate is determined by the intersymbol interference and the signal to noise at the
detector. The intersymbol interference at 2.5 bits per PWso will be controlled by the use of
pulse slimming equalization. The required signal to noise is determined by the raw error
rate requirement. It is expected that error detection and correction codes will come into
general use in future rigid disk products, in which case the acceptable raw error rate can be
as high as l(J4. In order to have 10% margin in an amplitude detection channel at 10"4
error rate, the signal to noise at the output of the equalizer must be 18 dB, see e.g. [13].
Moon and Carley [14] estimate the SNR net loss for a partial response equalizer with a
Viterbi detector in the vicinity of 2.5 bits per PWso is 8 dB when the noise is Gaussian.
When the noise includes nongaussian media noise as pulse position jitter, the SNR loss is
as much as 15 dB. Assuming that media noise is the dominant noise source, the SNR net
loss for the equalizer and maximum likelihood detector is taken as 15 dB. Therefore, the
zero to peak signal for isolated pulses to rms noise (head, preamp and media within the
matched filter bandwidth) at the input to the equalizer in order to have 10 % margin at 10"4
raw error rate is 33 dB. If the head/preamp noise is to be less than or equal to the
amplitude of the media noise but statistically independent of media noise then the
head/preamp minimum SNR requirement is 36 dB. Note that the SNR shown in Table 1 is
the calculated head/preamp SNR based on improvements in MR head sensitivity and
improvements in preamp noise performance as described above.
Since the MR head noise dominates the head/preamp noise and since the MR head signal
and resistance scale with track width, we find that the signal to noise ratios available with
the five cases shown in Table 1 are all very similar. Under these conditions, the
requirements on media noise, i.e, position jitter, will scale with the aspect ratio of the bit.
The transition jitter in Table 1 is 4% of the minimum transition spacing.
The variations in track density and linear density to reach 10 Gbit/irr shown in Table 1 not
only affect the improvements needed in heads, preamps and mechanics but significantly
impact the properties of the media needed. In Case 1, the linear density requires that the
coercivity be very large and the h/m sep. be very small (20 nm h/m sep. is considered to be
contact recording). In contrast, Case 5 requires relatively modest coercivity and large h/m
sep.
One of the design options available with MR heads will provide the media designer with a
trade-off which has not been available with inductive heads. As mentioned above, the
output of the MR sensor can be optimized for the flux available from the media.
Therefore, the head designer has the freedom to choose the geometry of the MR sensor to
achieve 250 p.V/p.m for an isolated pulse for any given flying height. (Variations in flying
5
height in the neighborhood of the nominal will of course lead to variations in output.) If
the head designer were then given a different nominal flying height and different media
properties, the designer can modify the write head design and the read head design such
that again the output at the new nominal flying height is 250 p. VIp.m for an isolated pulse.
Given this design opportunity, the system designer can trade him sep. for media transition
length without affecting the signal level. Figure 2 is a plot of PWso versus him sep. for
Cases 1,3 and 5 of Table 1. The pulse width requirement for Case 3 is indicated on the
graph, 160 nm. The requirement can be met not only with the system shown for Case 3 in
the table but also
by using very low
300.....- - - - - - - - - - - - - . , . . -.....
him sep., 15 nm,
and fairly modest
250
coercivity media,
E
Case 5
2500 Oe and 1.0
memu/cm'. This ..5 200
design would put
o
tremendous
."
~ 150
pressure on the
mechanical
a.
properties of the
100
media since the
head would be
50............- 0 1 _......- . 1 . _.................._ _........._ ...........____
operating
in
contact, with a
100
o
60
80
120
20
40
thin
wear
(protective)
Head/Media Separation (nm)
overcoat.
In
contrast to this
Figure 2: Pulse width vs. head/media separation for three cases from Table 1,
design, the pulse
showingtrade-off of him separation and media properties.
width requirement
can be met by
using a large him sep., 70 nm, and a high coercivity media, 4500 Oe and 0.35 memu/cm'.
Traditional air bearing technology could be used but revolutionary changes to the magnetic
properties of the recording layer would be necessary. This design option provided by MR
heads will have a significant impact on the trade off of mechanical and magnetic properties
required of the media.
-
3
3.1
Media Materials: Challenges and Candidate Systems
General Properties
As the calculations in the preceding section show, the magnetics required for longitudinal
media to support recording at 10 Gb/in' vary considerably depending on the trade-offs
6
structure. Finally, the optimum orientation of the magnetocrystalline easy axis has not yet
been established. It may be that the medium should be perpendicular, with the c-axis
normal to the film plane, or longitudinal orientation may be best. There are proponents
for both views and the experimental answer is not yet in. Nonetheless, careful control of
the media's anisotropy will be essential to obtain high quality 10 Gb/irr media.
3.3
Medium Noise
One of the essential properties in media useful for 10 Gb/irr is low medium noise. With a
spacing between transitions in the range of 25-100 nm the transition jitter cannot be more
than about 1-4 nm (4% of the transition spacing, based on the mM 1 Gb/itt
demonstration [1] and channel modeling as described above); for sampled voltage
channels (like PRML) this can be expressed as requiring that the rms medium noise added
to the signal voltage at the sample points cannot be more than 1-2%. Since transition jitter
is caused by variations in local magnetization within the written track, there will be a
calculable relationship between transition jitter and signal amplitude jitter at the points
away from the transition center where a sampled voltage channel takes samples. For
instance, it should be possible to derive jitter values from Mallinson's recent model for
noise in densely packed particulate media [15].
Bertram [16] has derived a general model for the transition position jitter based on noise
clusters. The result is a function of cross-track correlation length, s, head read gap, g, and
track width, w. The cross-track correlation length would be a few grain diameters. The
factor of (0.37) comes from assuming that a=h=g/4, where a=transition parameter and
h =total him sep. The transition jitter is, then,
J - (0.37) g
r;-
{3d
V-; - (0.37) g V-;-
(1)
For Cases 1-5 in Table 1, d =8-10 nm and s/d- 3 grains, this expression predicts jitter in the
range 1.5-8 nm, which is within a factor of 2 of the required values listed in Table 1 (an
adequate estimate for an approximate model).
3.4
Intrinsic Limitations: Superparamagnetism
If it is correct that the grain size of this media must be approximately 8-10 nm diameter by
10-15 nm thick, then attention must be given to phenomena which may limit how small
these grains can be. One such limitation to consider is superparamagnetism. Any
sufficiently small single domain ferromagnetic particle is subject to thermal
demagnetization because of thermal agitation of the spins of the individual atoms in the
particle. Spontaneous demagnetization is inhibited by the anisotropy energy of the entire
particle, which is proportional to particle volume and which forms a potential energy
8
barrier to thermal demagnetization [17]. This leads to the well-known phenomenon of
superparamagnetism and means that recording media whose grains are too small cannot
maintain a stable magnetization for a long time.
Following Cullity [18] the minimum size cobalt particle that would be stable for, say, at
least ten years can be shown to be slightly more than 10 nm. Similar results are obtained
from consideration of the anisotropy energy based on the saturation magnetization and the
particle coercivity [18]. Increasing 1\, M, or both decreases the size at which particles
become superparamagnetic. To reduce dmin to below 8 nm requires coercivity in excess of
3000 Oersteds for moderate ~ - 300 emul em'. In either case, advances in materials will
be required in order to ensure that a very fine-grained ferromagnetic thin film for
recording avoids the superparamagnetic limit in minimum grain size.
3.5
Candidate Materials Systems
Investigations aimed at developing media suitable for at least 10 Gb/irr' need not start in a
vacuum. There are a number of existing materials systems which show promise as media
for ultra high density. Of course, none of them at present is without performance
problems, so much further work will be needed to find which candidate media can be
developed into practical commercial 10 Gb/in' media.
One candidate is high coercivity perpendicular recording media. The highest linear density
reported to date in any media has been reported in CoCr perpendicular media. That
highest density was approximately 650 kfci [19] The importance of the measurement is that
it demonstrated the ability of this type of medium to support magnetization reversals with a
bit length (390 A) about equal to twice the diameter of the columns, 210 A. On the other
hand, the output signal was extremely low at the highest density, which was a combination
of a head limitation and of the medium. This low output signal severely limits the track
density which would be achievable in a disk drive. Finally, despite the evident promise of
perpendicular recording media and intense research efforts for many years no rigid disk
products are on the market. This naturally leads to questions about the real advantages of
perpendicular recording. Nonetheless, this recording mode cannot be ignored as a
candidate for ultra-high density recording.
Another possibility is the development of new cobalt alloys for longitudinal (or inclined
easy axis) recording. For instance, Cobalt-Rare Earth alloys such as Co-Gd and Co-Sm are
known to have very high anisotropies. Although other properties, such as poor crystallinity,
presently make these materials unsuitable due to high medium noise, this might be
overcome. In conjunction with new alloys the introduction of multilayer or laminated
media [2,20,21] could improve noise performance while contributing to control of the
magnetics and grain microstructure of the media. Also, in an extension of this approach
perhaps some form of superlattice media with the right properties may be developed.
9
Even more exotic departures from present approaches may prove fruitful. A number of
researchers are studying sputtered granular films, made by co-sputtering iron and an
insulator such as Si~ [22-24]. The resulting films (for Fe content less than about 40 wt-%)
consist of tiny Fe spheres in an insulating, non-magnetic matrix. The spheres are often so
small that they are superparamagnetic, but under some conditions even 4 nm spheres
exhibit ferromagnetism in the bulk film [24]. The main problem to date is that the
coercivity and magnetization are extremely sensitive functions of temperature, with He
decreasing rapidly near room temperature. It is not known if this can be overcome,
perhaps with other magnetic materials than pure iron. An additional potential advantage
of this type of film is the possibility that the non-magnetic matrix material may be its own
wear layer. In other words, if an Si~ or C matrix containing ferromagnetic spherules can
be lubricated then the carbon overcoat can be dispensed with, bringing the recording head
that much closer to the medium.
4 Summary
We have examined the requirements for 10 Gb/in' media, taking guidance from a
recording model and known trends in high performance media. We have seen that the
required magnetic and microstructural properties with the concomitant noise and
superparamagnetic limits demand significant improvements over existing media, to the
extent that revolutionary improvements will likely be required. Several candidate media
systems exist now that show some promise for this density, but much work remains before
they can be made practical.
5 Acknowledgments
We especially wish to thank Dr. Tadashi Yogi of IBM for giving us a copy of the
micrographs in Figure 3.
6
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12