In 1980, Seagate Technology introduced the world’s first 5.25-inch hard drive, bringing HDDs to a broader audience; prior to 1980 only large and well funded companies could afford the technology.

HDD capacity grew as much as 30% each year in the 1980s before accelerating to more than 60% per year in the 1990s. By 1999, HDD capacity was doubling every nine months [26]

The SPE Barrier–HDD Innovation
To achieve the HDD’s nearly exponential density growth, scientists and engineers miniaturized the magnetic grains or bits on the platter’s surface, squeezing more bits into the same or even smaller physical space [27]. These same researchers also developed more sensitive read/write heads (the giant-magnetoresistive head introduced in 1997, for example), capable of detecting faint magnetic fields [28].

Since its inception, HDDs have faced a density-growth challenge in the form of the superparamagnetic effect (SPE). “Superparamagnetism occurs when the microscopic magnetic grains on the disk become so tiny that random thermal vibrations at room temperature cause them to lose their ability to hold their magnetic orientations. What results are ‘flipped bits’ – bits whose magnetic north and south poles suddenly and spontaneously reverse – that corrupt data, rendering it and the storage device unreliable.” [29]

Temperature plays a role in the SPE since another way to describe the effect is to say that when the ambient thermal energy equals the amount of energy needed to change a bit’s polarity, that bit can flip and lose the data it was storing.

As bits are compressed, they become more susceptible to SPE, meaning that larger and faster HDDs have the potential to become less reliable [30]. For several decades, HDD developers have searched for ways to stave off the eventuality of reaching the density and reliability limits of HDDs.

One of the chief ideas proffered was to align bits perpendicularly rather than longitudinally. Famed inventor Valdemar Poulsen, who is sometimes called the Danish Edison, was one of the first researchers to experiment with perpendicular recording nearly 100 years ago [31], but it took modern engineers at leading HDD makers to actually produce HDDs with perpendicular bits like Hitachi Global Storage Solutions first introduced in 2006.

In longitudinal magnetic recording, each bit is oriented horizontally on the platter, whereas perpendicular recording orients bits vertically on the platter and actually increases the number of bits that can be aligned on the disk [32]. Perpendicular recording is also inherently more stable across temperature ranges [33] because its poles are arranged south pole to south pole and north pole to north pole. In this way, bits naturally repel each other, reducing the likelihood of the SPE occurring [34]

Several of the world’s leading HDD makers now offer perpendicularly aligned HDDs.

Perpendicular Recording

Heat-Assisted Magnetic Recording
Heat-assisted magnetic recording (HAMR) is a hybrid of magnetic and optical technology that represents the latest innovation in HDD development. HAMR has the potential to increase HDD density by an order of magnitude while still avoiding the SPE ‘s limitations [35].

With HAMR, engineers use a laser to briefly heat an area of an HDD’s platter. The heat lowers that area’s coercivity so it is below the coercivity of the magnetic field that the recording head is producing, essentially making it easier to flip a given bit’s magnetic orientation in a stable magnetic material and allowing for “smaller thermally stable grains.” [36]

Notes:

[26] Zeytinci, page 7.
[27] Kim Nguyen, “Perpendicular Recording: A Boon for Consumer Electronics,” Hitachi Global Storage Technologies, San Jose, Calif. (April 2005): Page 1.
[28] Zeytinci, page 10.
[29] Nguyen, page 1.
[30] “Hard Disk Superparamagnetics,” Dataclinic: downloaded from http://www.dataclinic.co.uk/hard-disk-superparamagnetic-effect.htm on October 12, 2008.
[31] Nguyen, page 3.
[32] Nguyen, page 3.
[33] Yochiro Tanaka, “Fundamental Features of Perpendicular Magnetic Recording and the Design Consideration for Future Portable HDD Integration,” IEEE (January 2005).
[34] John Best, “Perpendicular Recording: Opening the Boors for 10-fold Hard Drive Capacity Expansion,” Computer Technology Review (June 1, 2005).
[35] Mark H. Kryder, Edward C. Gage, Terry W. McDaniel, William A. Challener, Robert E. Rottmayer, Ganping Ju, Yiao-Tee Hsia, and M. Faith Erden, “Heat Assisted Magnetic Recording,” Proceedings of the IEEE, Volume 96, No. 11: (November 2008): page 1,810.
[36] Kryder et al: page 1,810.