Maximum areal densities in hard disk drives (HDD) are expected to more than double during the five-year period from 2011 to 2016, spurring continued growth for HDDs in storage-intensive applications, according to a report by market research firm IHS iSuppli.
HDD areal densities measuring data-storage capacities
are projected to climb to a maximum 1,800 Gigabits (Gb)
per square inch per platter by 2016, up from 744 Gb per
square inch in 2011, as shown in the figure below.
This means that from 2011 to 2016, the five-year
compound annual growth rate (CAGR) for HDD areal
densities will be equivalent to 19 percent. For this
year, HDD areal densities are estimated to reach 780 Gb
per square inch per platter, and then rise to 900 Gb per
square inch next year.
"The rise in areal density will pave the way for
continued growth of the HDD industry," said Fang Zhang,
analyst for storage systems at IHS. "Densities will
double during the next five years, despite technical
difficulties associated with the perpendicular magnetic
recording (PMR) technology now used to create
higher-areal-density hard disks. In particular, growth
opportunities will lie in applications associated with
mass enterprise storage requirements, gaming, and in
digital video recorders (DVRs) where massive capacity is
required to store high-definition video."
Areal density is the amount of data that can be
physically stored in a given amount of space on a
platter inside an HDD. Higher areal densities mean that
data can be packed more tightly onto the surface of a
disk, resulting in overall greater storage capacity.
Areal density equals bit density, or bits of information
per inch of a track (BPI), multiplied by tracks per inch
(TPI) on a platter.
This measure of density is distinct from actual HDD
capacity. This is because HDDs commonly use multiple
HDD areal density topped the 4-terabyte (TB) mark for
the first time in September 2011 with the introduction
of an external hard drive from Seagate Technology that
was designed for desktop applications. The Seagate HDD
had five platters each with an areal density of 625 Gb
per square inch, equivalent to more than 1 TB per
Just five years ago, HDD storage capacity per platter
was at a maximum of 180 gigabits per square inch.
Platters crossed the terabyte level for the first time
in 2007, with hard disk drives comprising two or more
platters becoming more common as HDD storage capacities
increased. Now with the 1 TB per-platter milestone
already reached, 5-TB hard disk drives using five
platters could be available on the market later this
Following Seagate's 4-TB external hard disk drive
product for the desktop HDD consumer market, a 4-TB
enterprise HDD suitable for business applications was
released in April by Hitachi GST; Hitachi GST has since
been acquired by Seagate archrival Western Digital.
HDDs with more than 1 TB in density per platter have
also been released by the industry for the mobile
market, with Toshiba's 2.5-inch 1-TB version boasting
the highest areal density for drives targeting the
portable PC and consumer electronics space.
All HDD manufacturers currently use PMR technology for
existing HDD products, but the industry consensus is
that existing PMR technology has two to three
generations left before reaching its areal density limit
at about 1-terabit (Tb) per square inch. In fact,
despite the solid five-year CAGR for higher-density
HDDs, growth rates could have been much higher were it
not for PMR technology nearing its limit.
Nonetheless, new developments are on the way, as they were outlined
latest International Magnetics Conference, INTERMAG
2012, held in in Vancouver from May 7th to May 11th,
Seagate in March announced it had achieved in its
research lab 1 Tb per square inch of areal density?30
percent higher than what could be achieved through PMR
technology - by using heat-assisted magnetic recording
(HAMR) technology, a promising approach to enable large
increases in the storage density of hard disk drives.
HAMR technology is likely to lead the way in creating
next-generation HDDs, even though satisfactory costs via
HAMR comparable to those of PMR have yet to be seen.
HAMR's challenges include solving a range of engineering
challenges such as how to integrate laser diodes and
recording heads. Patterned media proponents have yet to
demonstrate ways to cover a full disk with tiny magnetic
dots in a way that can be mass produced.
In theory, advanced technologies like HAMR could extend
HDD areal density to a range spanning 5 to 10 Tb per
The highest capacity for 3.5-inch HDDs could then reach
30 to 60 TB, while the smaller and thinner 2.5- inch
HDDs used in increasingly popular thinner notebooks
could reach 10 to 20 TB.
The trilemma/quadrilemma of magnetic recording
When researchers try to increase the amount of data in
the same physical area a problem arises. To retain sharp
bit transitions, where the boundary between the '0' and
'1' orientated areas of the media are well defined, the
grain size of the medium must be reduced.
"This then requires an increase in the magnetic
anisotropy of the grains, which ensures that the
magnetic state is stable for greater than 10 years,"
explains Dr Richard F. L. Evans, University of York.
However, the magnetic field generated by the
electromagnet (or write head) is limited by the material
and is theoretically around 2 Tesla. Now, the magnetic
field required to reverse the grains depends on the
magnetic anisotropy of the material, and so there are
conflicting requirements for 'thermal stability' and
'writability'. This problem is known as the magnetic
recording 'trilemma,' and this encapsulates the key
principles of magnetic media design.
Possible solutions to the trilemma involve changing the
properties of the material so that the writability and
thermal stability requirements are both satisfied (for
example using Exchange-Coupled-Composite Media),
changing the material type to a single large grain (Bit
Patterned Media), or changing the material properties
during the write process. The latter example is where
the heating part comes in. Magnetic properties such as
anisotropy and magnetisation are temperature dependent,
and so by applying heat it is possible to reduce the
magnetic anisotropy during the write process, while at
the storage temperature the data is stable for > 10
years. This is the principle behind Heat-assisted
magnetic recording, or HAMR.
An outstanding question for magnetic disk drives is what
is the maximum achievable data density, based on the
fundamental physical limitations of the recording
medium. As the grain size in magnetic media is reduced,
the thermal fluctuations in a magnetic system become
much more important in governing how the system behaves.
Thermal fluctuations in fine particle magnetic systems
are known to cause a loss of magnetic state over time,
which is the same phenomenon which gives magnetic
recording media its stability for around 10 years.
However, for very small systems (and high temperatures)
these fluctuations can also cause errors in the writing
process, where the applied field is insufficient to
guarantee that the individual grains are oriented in the
correct direction. This new requirement of 'thermal
writability' is then in addition to those of the usual
trilemma, requiring that the magnetic moment in the
recording media remains high, or for HAMR that the media
that the writing temperature is not too high. Hence the
trilemma is really a quadrilemma, requiring all four
components for a successful magnetic storage system.
Schematic of the quadrilemma of magnetic recording.
The quadrilemma has implications for the maximum
achievable storage density, as now 'thermal writability'
becomes the limiting factor in how small the grains can
"Using the ultimate storage medium with a single grain
per bit and HAMR to allow writing, the maximum
achievable density is around 25TBit/in2, which is
considerably lower than one would expect from the
trilemma alone," Evans says.