Magnetic disk recording was invented in 1953 and has undergone intensive development ever since. As a result of this 38 years of development, the cost per byte and the areal density have halved and doubled respectively every 2-2 1/2 years. Today, the cost per byte is lower than 10(exp -6) dollars per byte and area densities exceed 100 10(exp 6) bits per square inch. In this talk, the recent achievements in magnetic disk recording are first surveyed briefly. Then, the principal areas of current technical development are outlined. Finally, some comments are made about the future of magnetic disk recording

Mallinson, John C.

English

NASA Technical Reports Server

MAGNETIC DISK
John C. Mallinson
MALLINSON MAGNETICS, INC.
7618 Reposada Drive
Carlsbad, CA 92009
N93-13134
Magnetic disk recording was invented in 1953 and has under gone intensive
development ever since. As a result of this 38 years of development, the cost per byte and the
areal density have halved and doubled respectively every 2-2 1/2 years. Today, the cost per byte
is lower than 10 -6 dollars per byte and areal densities exceed 100 106 bits per square inch.
In this talk, the recent achievements in magnetic disk recording will first be surveyed
briefly. Then the principal areas of current technical development will be outlined. Finally,
some comments will be made about the future of magnetic disk recording.
pRESENT ACHIEVEMENTS
High end disk drives today operate at areal densities of between 50 and 100 106 bits per
square inch, with, typically, 2500 tracks per inch and 30,000 flux reversals per inch. When
"run length limited" coding is used, the effective linear bit density is 40,000 bits per inch. Areal
densities tend to be higher the smaller the diameter of the disk.
Data rates run as high as 6 Megabytes per second (48 Megabits per second) per single
head-disk channel. Parallel access disk systems, with as many as 10 heads in parallel have
been manufactured which provide the full CTIR 4:2:2 component digital video output rate (216
Megabits per second).
Since as many as 6 disks can be fitted in the standard 5 1/4" full height form factor
package, 5 1/4" drive data capacities exceeding 2 Gigabytes are now available from several
manufacturers.
In summary, it may be said that the magnetic disk products being manufactured today
offer access times, data rates and drive bit capacities considerably in excess of those offered by
optical disk drives. Areal density is the only parameter which currently falls below that of
optical disks, by a factor of 3-4.
OF TE_H_q_L DEVELOPMENT
The overwhelming success of magnetic disk products over the last three or four decades
has led to the establishment of a $50 billion per year world wide business in disk drives. This
enormous business support research and development into every conceivable aspect of disk
recording technology In order to permit continuing increases in performance. Only the major
areas of such research and development cam be discussed below.
IMPROVED RECORDING MEDIA
Virtually all modern disk drives now use thin film metallic media with coercivities
close to 1000 Oe. It may be expected that coercivities exceeding 2000 Oe will be used in the next
few years. Higher coercivities lead to both sharper output pulses of greater amplitude and also
to improved signal-to-noise ratios.
IMPROVED WRITING HEADS
As the medium coercivity increase, it is necessary to increase the saturation induction
of the writing head pole tip materials. Presently, Alfesfl and Permalloy with maximum
1-69
https://ntrs.nasa.gov/search.jsp?R=19930003946 2020-03-17T10:15:01+00:00Z
inductions of I0-12,000 G are used. Materials such as Co-Ru and Fe-N with maximum
induction of 16,000 and 19,000 G may be expected to be introduced.
NARROWER TRACKWIDTHS
It has been realized for two decades that, when seeking higher areal densities, it is better
to use narrower trackwidths than higher linear densities. Operation with trackwidths
substantially narrower than normal (10pm) leads to a number of very fundamental questions
concerning the operation of the track following servo system. In particular, the outstanding
question is 'Xvhat is the source of the tracking error signal?". In magnetic disks today the
source is a previously written magnetic disk servo track and it is only possible to operate the
tracking servo when reading but not when writing. In optical disks, which operate at 5-6 times
the track density, the source is always some physical feature (pits, grooves, bumps, etc.) and the
tracking servo system is then operable during both reading and writir¢ a. This leads to another
question: '_Vfll magnetic disks eventually use optical tracking servo systems?".
IMPROVED READING I_FAD_
As trackwidths decrease, it becomes increasingly difficult to keep the channel signal-
to-noise ratio media-noise limited because the output voltage of an inductive head falls
proportionally with the trackwidth. It is anticipated that the next generation of high end disk
drives will use magneto-resistive (M-R) reading heads where the magnetic fields from the
medium changes the electrical resistance of a thin film M-R element. Considerably higher
output voltages are available with M-R heads and they are independent of head-medium
relative velocity.
IN-CONTACT OPERATION
Today's disk drives operate with a deliberate head-to-disk spacing of, typically, 6-8
microinches (0.15Bin). It is known that both the writing and reading processes on magnetic
disks improve when the spacing is reduced. All disks today are overcoated (Ag-Sn, ArO2
amorphous C, ZiO2, etc.) in order to control friction and wear and it seems very likely that,
together with redesigned heads of significantly lower mass, continuous operation in contact
may become possible. This is particularly true at low head-to-disk relative velocities.
SMALLER DISK DIAMETERS
An interesting sequence of design changes becomes possible following a reduction in
the head-to-disk spacing. First, a higher linear density may be written. Second, because the
data rate has now become too high, the disk diameter or spindle RPM must be reduced. Third,
at the reduced head-to-disk velocity, it now becomes possible to reduce the head-to-disk
spacing even further because any mechanical impact now transfers less energy. Fourth, ff a
smaller disk diameter has been chosen, the mechanical tolerances (flatness, areal runout, etc.)
are reduced which again permits the head-to-disk spacing to be reduced even further. This
sequence has led the drive industry from 5 1/4" to 3 I/2" to 2 1/2" to 1 1/2" diameters with
increasing areal density. Still smaller diameters and higher areal densities are anticipated.
As an example of the levels of performance attainable when many of these
developments are combined, consider the 1989 IBM _ (1100 Megabit) per square inch
technology demonstration:
Medium coercivity - Cobalt-Platinum - 1700 Oe
Write Head-thin film-trackwidth 41an
Read Head - magneto-resistive - trackwidth 2-3_m
Head-to-disk spacing - about 1 microinch
Linear density - about 160,000 bpi
Track density - about 7,000 tpi
1-70
With this demonstration,IBM showedthat magneticdisk recordinghas the potential to
exceedtoday'sopticaldiskarealdensitiesby abouta factorof 2.
TheIBM 1989demonstrationprovedI. 1Gigabitper squareinch feasibility. Today's
researchpapers(see,for exampleIntermag'91 paperMA-01)discussdemonstrationsof 2
Gigabitper squareinch (at 17,000tpi and 120,000frpi). It seemsto beabundantlyclearthat the
magneticdisk recorderscouldrangefromthe 50-I00 Megabitper squareinch of today's
manufacturedhardwareto future productswith arealdensitiesperhapsashigh as 16 times
greater.
It usedto be saidthat the greatadvantageof optical (versusmagnetic)recordingwasthat
it wasnot necessaryto fabricateanythingwith dimensionscomparableto the wavelengthof
light in orderto achieveveryhigh arealdensitiesbecausethe lenscouldfocusthe light downto
Lord Rayleigh's'diffraction limit.
Nowadays,it seemsthat a veryfundamentalchangein philosophyhasoccurred.
Indeed,it is frequentlystatedthat the real advantageof magneticversusoptical recordinglies
in the fact that the only effectivelimits operatingtodayconcemJust how small can certain
featuresand objectsbemadeand that their dimensionsare/loXJll]Iggl_]_merephysical
diffractionof light! For example,the gap-lengthin mass-produced8 mm VCRheadsis 10
microinches,which is but onethird the wavelengthof red light.
Thesteadyincreasein arealdensity, by a factor of 2 every 2-2 i/2 years, has been
mentioned already. By this criterion alone, it appears then that magnetic disk recording
technology can sustain another 20 years of growth ( a factor of 16 = 24; 4 x 2.5 = 10 years) on the
basis of demonstrables which exist in the laboratories today.
To move from scientific extrapolation to the realm of technical speculation, it seems to
be very likely that 1 Gigabit (109) per square inch areal densities will appear in disk (and video
tape) drives in considerably less than 20 years. Indeed some industry observers have opined
that 5 1/4" full height drives with 100 Giga-byte capacity will appear before the year 2000: this
represents a doubling of the historic rate of increase. Given the magnitude of the research and
development activities in magnetic disk recording being undertaken worldwide, even such
surprising estimates do not appear, to this writer, to be unduly optimisticl
1-71
AREAL DENSITY
= TRACK DENSITY x BIT DENSITY
CI'PI) (BPI)
VOLUME DENSITY
= AREAL DENSITY x 1
tHICKNESS
TYPICAL AREAL DENSITIES,
CAPACITIES AND COSTS TODAY
HI-END (LARGE)
HI-END (SMALL)
HI-END (SMALL)
AREAL CAPACITY COST
60 106 10 109 $100K
100 106 2 109 $2000
20 106 0.1 109 $400
NOTE: 10-4 CENTS/BYTE
10-5 CENTS/BIT
AREAL IS IN BITS/SQUARE INCH
CAPACITY IS IN BYTES
1-72
EFFECT OF RAISING H¢
÷M
t
I
n
t_- -A_
MAGNETIZATION
I
/, I
-M
DISTANCE
GI_ O__f Hc
DATE
1980
1986
1989
Now
Future
Hc (OE)
3O0
600
900
1200-1400
2800 Reported in Literature
1-73
Ho = 2.5 HC
H o < 0.6 BS
(TO WRITE PROPERLY)
(TO AVOID POLE TIP SATURATION)
WITH FERRITE POLE TIPS BS = 5000 G
0.6 BS = 3000 G
:. MAX HC= 1200 OE
METAL-IN-GAP HEAD_
V///FERRITE _,
V/_._: _ooo_ "
GAP
1-74
MAGNETO-RESISTIVE HEADS
I
SINGLE DOMAIN THIN FILM
RESISTANCE VARIES AS (COSINE 0) 2
V (MRH) = 20,000 V (INDUCTIVE)
SPEED x NO OF TURNS
SUPPOSE DISK SPEED IS 200 IPS:
= 20,000 x V (INDUCT) FOR N = 1
= 2,000 N = 10
= 200 N = 100
= 20 N = 1000
V (MRH)
MRH ARE THE KEY TO HIGHER TPI's WITH SMALL DIAMETER DISKS.
"IN CONTACT" DISKS
1-75
THE TRACK FOLLOWING ISSUE
ATTAIN 10-15,000 TPI
ALL USE DIFFRACTION OF LIGHT OFF
MECHANICAL FEATURES
MAGNETIC DISKS CURRENTLY ATTAIN 2-3,000 TPI
ALL USE A F_..__U_BfJ___ SURFACE
WITH _ SERVO TRACKS
IDEALLY, THE SOURCE OF THE TRACK FOLLOWING SERVO SIGNALS SHOULD BE INDEPENDENT OF
THE RECORDED DATA, SO THAT THE TRACK FOLLOWING SERVO CAN OPERATE AT ALL TIMES.
WILL MAGNETIC DISK_; ADOPT OPTICAL TRACKING?
A)
B)
C)
D)
E)
OR F)
G)
SMALL DISK EVOLUTION
MAKE DISK SMOOTHER
LOWER FLYING HEIGHT
INCREASE LINEAR DENSITY
REDUCE SPEED
LOWER RPM
SMALLER DISK
SMALL DISK IF FLATTER
.'.LOWER FLYING HEIGHT AGAIN
ACCELS LOWERR AT LOWER RPM
.'.LOWER FLYING HEIGHT AGAIN
REPEAT CYCLE (C)
1-76
_ =
Metallic Thin Film Disk
Inductive Thin Film Write Head
Magnetoreslstlve Read Head
50% Roll-Off Density
Linear Bit Density
Data Rate
Error Rate
Signal-to-Noise Ratio
Areal Density
IBM'S 1 GIGABITIINCH 2 DEMONSTRATION
DECEMBER, 1989
HC = 1800 Os
G = 300A Co-Pt-Cr
W = 4pm
6-7000 TPI
W = 3pm
= 110,000 FRPI
= 160,000 BPI
28 Mbs
= 10 .8 . 10-9
= 23 dB
= 1.18 109 Bits/inch 2
SIGNAL POWER
........................ = No OF GRAINS/BIT CELL
NOISE POWER
IBM'S DEMO HAD 500 A (2p. INCH) METALLIC GRAINS
THE BIT CELL WAS ABOUT 6p. " LONG X 120_
.'. No OF GRAINS/BIT CELL = 200
.'. SNR = 200 OR 10 LOG 10 200 = 23 dB
" WIDE
This demonstrates that the basic laws governing recording hold to Giga-bit densities.
Note that a doubling In areal density costs a halving (-3 dB) in SNR with thin media.
1-77
THE FUTURE
SUPPOSE 2X AREAL DENSITY
EVERY 2 1/2 YEARS HOLDS
DATE AREAL DENSITY
1991 100 106
1994 200 106
1996 400 106
1999 800 106
2OO2 1600 106
This evolution appears to be reasible on the basle of today's laboratory demonstrations.
BJCtL_CJIEg_Q
A PROFOUND CHANGE IN DESIGN PHILOSOPHY
The advantage of optical recording Is that dimensions comparable to optical
wavelengths do not have to be manufactured.
The advantage of magnetic recording Is that the effective limits are governed by
how small can objects be made and this limit far exceeds optical diffration.
1-78


Magnetic disk

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server