Advances in hard disk drive technology and the essential introduction of a few approaches are expected to allow the rapid increasment of the future HDD areal density - the amount of digital storage that can be stored on a given surface area of a HDD. According to the latest roadmap for HDD areal density released by the Advanced Storage Technology Consortium (ASTC) - a forum for collaborative joint R&D efforts related to future HDD technologies such as Heat Assisted Magnetic Recording (HAMR) or Bit Patterned Media (BPM) - Heat Assisted Magnetic Recording (HAMR) should be introduced into HDDs by 2017, increasing the average annual areal density growth rate to 30% .

As you see below, the roadmap shows that the Bit Pattern Media technology will be introduced about 2021 combined with SMR or an extension to SMR called Two Dimensional Magnetic Recording (TDMR). Combined with HAMR, will result in up to 10 Terra-bit per square inch (Tbpsi) areal densities by 2025. Note that today’s HDDs have an areal density as high as 0.86 Tbpsi. So expect a 3.5-inch HDD built with that technology to have about 10X the capacity of the 10 TB HDDs in 2025, or 100 TB.

Both technologies – HAMR and BPM – hold promise for driving areal density far beyond about a terabit per square inch, and ultimately, ASTC expects the industry to use both techniques in tandem to drive areal density beyond 10 Tbits/square inch.

A key technical challenge in scaling to higher areal density is the thermal stability of the media. There are two approaches to pushing the thermal limits of the media to higher areal density. One direction is to enable scaling to smaller data bits without having to reduce the physical grains size in the magnetic media. The other is to increase the magnetic stiffness of the media materials.

BPM technology addresses the first pathway through intentional patterning of the bits into 'islands' rather than isolated grains. It records data in magnetic islands (one bit per island), as opposed to current hard disk drive technology where each bit is stored in 20-30 magnetic grains within a continuous magnetic film. The islands would be patterned from a precursor magnetic film using nanolithography.

In patterned media, the thin magnetic film is first deposited so that there is strong exchange coupling between the grains. That means that energy barrier is proportional to the island volume, rather than the volume of individual grains within the island. Therefore storage density increases can be achieved by patterning islands of increasingly small diameter, whilst maintaining thermal stability.

Of course, there are still lithographic challenge to be overcome.

HAMR addresses how to write higher coercivity media to enable the second pathway. It allows hard drives to grow in size without using either shingle magnetic recording (SMR) which is dense but has speed considerations for random write access, or helium-filled drives which rely on helium.

In HAMR, a small laser is used to temporarily spot-heat a tiny area being written to at any given time. When the temperature of the area being written is raised in this way above the Curie temperature, the magnetic medium effectively loses much of its coercivity, so a realistically achievable magnetic write field can write data to the medium. As only a tiny part of the disk is heated at a time, the heated part cools very quickly, and comparatively little power is needed.

HAMR could eventually increase the limit of magnetic recording by more than a factor of 100. This could result in storage capacities as great as 50 terabits per square inch.

Seagate and TDK have demonstrated HAMR drives but the technology could appear in commercial hard drives by 2016.

To achieve the highest possible areal densities, both BPM and HAMR technologies will likely be necessary.

But what about Two Dimensional Magnetic Recording (TDMR)?

When disk track density is getting so high, track pitch is becoming so small, that the magnetic read heads of a HDD have become wider than the actual data track width. Because of this, read heads are starting to pick up more inter-track noise and it’s getting more difficult to obtain a decent signal to noise ratio (SNR) off of a high-density disk platter with a single read head.

TDMR read heads can be used to counteract this noise by using multiple read heads per data track and as such, help to create a better signal to noise ratio during read back.

TDMR heads are any configuration of multiple read heads used in reading a single data track. They could be either used in series, where one head is directly behind another head, or in-parallel (side by side), where three heads were configured in-parallel across the data track and the two inter-track bands. In the latter case one head is configured directly over the data track with portions spanning the inter-track gap to each side, one head was half way across the data track and the next higher track, and a third head was placed half way across the data track and the next lower track.

But until then, HDDs with up to 10 TB of storage capacity have been set for release during next year. The development of 8 and now 10 TB drives involves the use of technologies such as shingled magnetic recording (SMR) and hermetically sealed He-filled hard disk drives.

Shingled recording works by writing a set of tracks closely together in parallel on the hard disk, similar to roof shingles, and unlike traditional hard disk design where there is space between the tracks.

Because of the parallel tracks, writing can be slower because all affected overlapping tracks can be affected when trying to write only one track, meaning that all will have to be rewritten. There problems could be overcome with firmware adjustments of the HDD (device-managed) or by depending on the operating system to know how to handle the drive, and only write sequentially to certain regions of the drive (host-managed devices).

Seagate has already started shipping sample SMR hard drives with 8TB capacity to its select customers.

But the SMR technology probably cannot add too much more to areal density growth. In addition, putting helium in a hard drive can only allow adding so many more disks to a drive.