In each of the past five years, hard drive capacities have doubled, keeping storage costs low and allowing technophiles and PC users to sock away more data. However, storage buffs believed the rate of growth could continue for only so long, and many asserted that the storage industry was about to hit the physical limit for higher capacities. But according to IBM, a new innovation will push back that limit. The company is first to massproduce computer hard disk drives using a revolutionary new type of magnetic coating that is eventually expected to quadruple the data density of current hard disk drive products -- a level previously thought to be impossible, but crucial to continue feeding the information-hungry Internet economy. For consumers, increased data density will help hasten the transition in home entertainment from passive analog technologies to interactive digital formats.
The key to IBM's new data storage breakthrough is a threeatom-thick layer of the element ruthenium, a precious metal similar to platinum, sandwiched between two magnetic layers. That only a few atoms could have such a dramatic impact caused some IBM scientists to refer to the ruthenium layer informally as "pixie dust". Known technically as "antiferromagnetically-coupled (AFC) media," the new multilayer coating is expected to permit hard disk drives to store 100 billion bits (gigabits) of data per square inch of disk area by 2003. Current hard drives can store 20 gigabits of data per square inch. IBM began shipping Travelstar hard drives in May 2001 that are capable of storing 25.7 gigabits per square inch. Drives shipped later in the year are expected to be capable of 33% greater density.
In information technology, the term "pixie dust" is often used to refer to a technology that seemingly does the impossible. In the past decade, the data density for magnetic hard disk drives has increased at a Pixie Dust phenomenal pace: doubling every 18 months and, since 1997, doubling every year, which is much faster than the vaunted Moore's Law for integrated circuits. It was assumed in the storage industry that the upper limit would soon be reached. The super paramagnetic effect has long been predicted to appear when densities reached 20 to 40 gigabits per square inch - close to the data density of current products. IBM discovered a means of adding AFC to their standard production methods so that the increased capacity costs little or nothing. The company, which plans to implement the process across their entire line of products, chose not to publicize the technology in advance. Many companies have focused research on the use of AFC in hard drives; a number of vendors, such as Seagate Technology and Fujitsu, are expected to follow IBM's lead.
AFC will be used across all IBM hard drive product lines. Prices of hard drives are unlikely to increase dramatically because AFC increases the density and storage capacity without the addition of expensive disks, where data is stored, or of heads, which read data off the disks. AFC will also allow smaller drives to store more data and use less power, which could lead to smaller and quieter devices.
Developed by IBM Research, this new magnetic media uses multilayer interactions and is expected to permit longitudinal recording to achieve a future data density of 100 gigabits/inch2 without suffering from the projected data loss due to thermal instabilities. This new media will thus delay for several years the impact of super paramagnetism in limiting future areal density increases. It also requires few changes to other aspects of the hard-disk-drive design, and will surely push back in time the industry's consideration of more complex techniques proposed for very high-density magnetic recording, such as, perpendicular recording, patterned media or thermally-assisted writing. Pixie Dust
II. CONVENTIONAL MEDIA
BASICS OF MAGNETIC RECORDING
Read-Rite's recording heads are the miniaturized hearts of disk drives and other magnetic storage devices. While they may appear to be simple components, their design and manufacture require leading-edge capabilities in device modeling, materials science, photolithography, vacuum deposition processes, ion beam etching, reliability testing, mechanical design, machining, air bearing design, tribology, and other critical skills. In general, recording heads function according to certain principles of magnetic recording which are based directly on four magnetic phenomena:
A. An electric current produces a magnetic field.
B. Some materials are easily magnetized when placed in a weak magnetic field. When the field is turned off, the material rapidly demagnetizes. These are called Soft Magnetic Materials.
C. In some magnetically soft materials the electrical resistance changes when the material is magnetized. The resistance goes back to its original value when the magnetizing field is turned off. This is called Magneto-Resistance or the MR Effect. Giant Magneto-Resistance, or the GMR Effect, is much larger than the MR Effect and is found in specific thin film materials systems.
D. Certain other materials are magnetized with difficulty (i.e., they require a strong magnetic field), but once magnetized, they retain their magnetization when the field is turned off. These are called Hard Magnetic Materials or Permanent Magnets. Pixie Dust
These four phenomena are exploited by Read-Rite in its design and manufacture of magnetic recording heads which read and write data (the source of the company's name) for storage and retrieval in computer disk drive memories, tape drives, and other magnetic storage devices.
Heads used for writing bits of information onto a spinning magnetic disk depend on phenomena A and B to produce and control strong magnetic fields.
Reading heads depend on phenomena A, B, and C, and are sensitive to the residual magnetic fields of magnetized storage media (D).
Storage Media (e.g., computer disks)
Magnetic storage media are permanently magnetized in a direction (North or South) determined by the writing field. Storage media exploit phenomenon D.
Writing Magnetic Data
Simplified sketches of a writing head are shown in Figure1. The
view from the top of the writing head (left) shows a spiral coil wrapped
between two layers of soft magnetic material; on the right is a cross-section
of this head as viewed from the side. Note two things in this figure: at the
lower end, there is a gap between these layers, and at their upper end these
layers are joined together. The top and bottom layers of magnetic material
are readily magnetized when an electric current flows in the spiral coil, so
these layers become North and South magnetic poles of a tiny
electromagnet. [In a real head, the distance from the gap to the top of the coil
is about 30 microns (or 0.0012 inch).] Pixie Dust
FIGURE 1: A WRITING HEAD
The N-S poles at the gap end of the writing head further
concentrate the field to make this region the business end, which is the area
where the writing field leaks into space outside the head. When a magnetic
storage medium (a spinning computer disk, for example) is put in close
proximity with the writing head, the hard magnetic material on the disk
surface is permanently magnetized (written) with a polarity that matches the
writing field. If the polarity of the electric current is reversed, the magnetic
polarity at the gap also reverses.
Computers store data on a rotating disk in the form of binary
digits, or bits transmitted to the disk drive in a corresponding time sequence
of binary one and zero digits, or bits. These bits are converted into an electric
current waveform that is delivered by wires to the writing head coil. This
process is sketched in Figure2. In its simplest form, a one bit corresponds to
a change in current polarity, while a zero bit corresponds to no change in
polarity of the writing current. A moving disk is thus magnetized in the
positive (North) direction for positive current and is magnetized in the
negative (South) direction for negative current flow. In other words, the stored
ones show up where reversals in magnetic direction occur on the disk and
the zeroes reside between the ones.Pixie Dust
FIGURE 2: WRITING DATA ON A STORAGE MEDIUM
A timing clock is synchronized to the turning of the disk and bit
cells exist for each tick of the clock; some of these bit cells will represent a
one (a reversal in magnetic direction such as N going to S or S going to N)
and others represent zeroes (constant N or constant S polarity). Once
written, the bits at the disk surface are permanently magnetized in one
direction or the other until new data patterns are written over the old. A fairly
strong magnetic field exists directly over the location of ones and fades
rapidly in strength as the recording head moves away. Moving significantly in
any direction away from a one causes a dramatic loss of magnetic field
strength, thus, to reliably detect data bits, it is extremely important for reading
heads to fly very close to the surface of a magnetized disk.
Reading Magnetic Data
In the case of Read-Rite's leading edge products, recording
heads read magnetic data with magnetically sensitive resistors called Spin
Valves which exploit the GMR Effect. These GMR/Spin Valve heads are
placed in close proximity to a rotating magnetized storage disk, thereby
exposing the GMR element to magnetic bit fields previously written on the
disk surface. If a GMR head is moved only slightly away from the disk Pixie Dust
(perhaps 2 to 3 millionths of an inch) the field strength drops below a useful
level, and magnetic data cannot be faithfully retrieved.
FIGURE 3: READING DATA FROM A STORAGE MEDIUM
When a current is passed through the GMR element, changes
in resistance (corresponding to changes of magnetic states arising from
written N and S bits) are detected as voltage changes. These voltage
fluctuations -- referred to as the signal-- are conducted to the GMR sensor
terminals. Electrical noise, however, is present in all electrical circuits (GMR
heads are no exception) so the combined signal and noise from a GMR
reader are sent via wires to the disk-drive electronics for decoding the time
sequence of pulses (and spaces between pulses) into binary ones and
zeroes. The reading process, including the undesired but ubiquitous noise, is
sketched in Figure 3.
Storing more information on a computer disk or other medium is
a function of squeezing as many pulses as possible onto a data storage
track. However, when pulses are very close to one another, electronic Pixie Dust
decoders suffer in their ability to separate ones from zeroes in the presence
of electrical noise. This problem is alleviated somewhat by placing the GMR
element between two layers of soft magnetic material to shield the element
from the influence of bit fields of adjacent ones. These shields, also shown in
Figure 3, have the effect of slimming down the data pulses significantly,
allowing more information to be stored and faithfully retrieved. Pixie Dust
III. SUPERPARAMAGNETIC EFFECT
Computers get better and better, faster and faster; and, of all
computer components, probably the greatest rate of evolution belongs to the
stalwart hard drive. On a daily basis, the storage capacity and speed of hard
drives increases, while their cost just keeps on shrinking. This is one of those
rare situations in which both consumers and companies profit; but something
called superparamagnetic effect may soon bring an end to this golden age.
As hard drives become capable of storing more information and
accessing it at faster speeds, their data becomes more susceptible to
corruption. This data-density barrier is known as the superparamagnetic
effect (or SPE). Before going on to say more about SPE, though, it might be
helpful (and scenic) to take a brief detour to examine the technology at the
hub of your average hard drive.
Today's hard drive resembles a small record player that's
capable of stacking its disks, or platters, to hold up to eight of them at a time.
Each platter is covered with a magnetic film that is ingrained with tiny
particles called bits. When a read-write head (looking like the needle of a
record player) passes over the bits, it either magnetically aligns the particles
to record information (turning them into series of 1's and 0's), or it reads them
in order to access previously-stored data. These operations take place at
phenomenal speeds; the platters spin around thousands of times per minute,
and both sides of them are scanned simultaneously by read-write heads.
Advances in hard drive technology continue to increase the
number of bits that fit onto each platter. Bits are getting smaller and smaller,
making for greater storage capacity, but also bring the SPE barrier closer and
closer. So what exactly does SPE do? Basically, SPE destabilizes the 0 or 1-
orientation of magnetic bits, resulting in corruption of stored data. When the
energy in the bits' atoms approaches the thermal energy around them, the Pixie Dust
bits start randomly switching between 0's and 1's. In layman's terms, SPE
makes bits flip out.
The superparamagnetic effect originates from the shrinking
volume of magnetic grains that compose the hard-disk media, in which data
bits are stored as alternating magnetic orientations. To increase data-storage
densities while maintaining acceptable performance, designers have shrunk
the media's grain diameters and decreased the thickness of the media. For
media limited noise signal/noise ratio is proportional to square root of N,
where N is the number of media grains per bit.
Signal/Noise ~ N
At samller grain volumes, grains can randomly reverse thair
magnetisation direction, resulting in an exponential decay whose rate
strongly depends on temperature. The resulting smaller grain volume makes
them increasingly susceptible to thermal fluctuations, which decreases the
signal sensed by the drive's read/write head. If the signal reduction is great
enough, data could be lost in time to this superparamagnetic effect.
Fig. 4. TEM of the grain structures in magnetic media. (magnification
= 1 million)
In Figure 4 are transmission electron micrographs (TEM) for
two different disk media illustrating how the grain structure has changed over
time. The TEM on the left is a magnetic media that supports a data density of
about 10 gigabits/inch2 with an average grain diameter of about 13 Pixie Dust
nanometers. The magnetic media on the right supports a data density of 25
gigabit/inch2 with an average grain diameter of about 8.5 nanometers.
Historically, disk drive designers have had only two ways to
maintain thermal stability as the media's grain volume decreases with
increasing areal density:
1) Improve the signal processing and error-correction codes
(ECC) so fewer grains are needed per data bit.
2) Develop new magnetic materials that resist more strongly
any change to their magnetization, known technically as higher coercivity.
But higher coercivity alloys also are more difficult to write on.
While improvements in coding and ECC are ongoing, IBM's new AFC media
is a major advancement because it allows disk-drive designers to have their
cake and eat it too: It is easy to write at very high areal densities but is much
more stable than conventional media. Pixie Dust
IV. AFC MEDIA
Antiferromagnetically Coupled (AFC) media (synthetic
ferrimagnetic media (SFM) or Laminated Antiferromagnetically Coupled
(LAC) media) technology is expected to extend the lifetime of longitudinal
magnetic recording technology. LAC media differ from the conventional
media by their structure and functionality. Conventional recording media have
one or more magnetic layers, which may be coupled ferromagnetically to
each other. In AFC media, there are at least two magnetic layers, but the
magnetic layers are coupled antiferromagnetically. In comparison to
conventional media, AFC media exhibit similar or better recording
performance. But, at the same time, AFC media show much improved
thermal stability, which makes them attractive.
The principle of AFC media is based on adding extra energy in
the form of antiferromagnetic coupling to stabilize the bits. Conventional disk
media stores data in only one magnetic layer, typically of a complex magnetic
alloy (such as coblat-platinum-chromium-boron, CoPtCrB). AFC media is a
multi-layer structure in which two magnetic layers are separated by an
extraordinarily thin -- just three atoms thick -- layer of the nonmagnetic metal,
ruthenium. This precise thickness of the ruthenium causes the magnetization
in each of the magnetic layers to be coupled in opposite directions -- antiparallel -- which constitutes antiferromagnetic coupling. A schematic
representation of this structure is shown in Figure5.