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Tuesday, May 22, 2012
 Hard Drive Densities To Double by 2016
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Message Text: 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 platters.

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 platter.

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 year.

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 at the latest International Magnetics Conference, INTERMAG 2012, held in in Vancouver from May 7th to May 11th, 2012.

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 square inch.

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 be.

"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.
 
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