How does Hard Drive Work?

 

How does a hard drive store so much information in such a small space? At the heart of every hard drive is a stack of high-speed spinning discs with a recording head flying over each surface. Each disc is coated with a film of microscopic magnetised metal grains, and our data doesn't live there in a form you can recognize. Instead, it is recorded as a magnetic pattern formed by groups of those tiny grains. In each group also known as a bit, all of the grains have their magnetizations aligned in one of the two possible states, which corresponds to zeros and ones. Data is written onto the disc by converting strings of bits into electrical current fed through an electromagnet. This magnet generates a field strong enough to change the direction of the metal grain's magnetization. Once this information is written onto the disc,  the drive uses a magnetic reader to turn it back into a useful form, which is much like a phonograph needle. 

But how can you get so much of information out of zeros and ones? Well by putting lots of them together. For example, a letter is represented in one byte or eight bits, and your average photo takes up several megabytes, each of which is 8 million bits. Because each bit must be written onto a physical area of the disc, we are always seeking to increase the disc's areal density, or how many bits can be squeezed into one square inch. The areal density of a modern hard drive is about 600 Gbits per square inch. 300 million times greater than that of IBM's first hard drive from 1957. 

This amazing advance in storage capacity was not just a matter of making everything smaller, but it involved multiple innovations. A technique called the thin-film lithography process allowed engineers to shrink the reader and writer, and despite its size, the reader became more sensitive by taking advantage of new discoveries in magnetic and quantum properties of matter. Bits could also be packed closer together, the mathematic algorithms filter out noise from magnetic interference and find the most likely bit sequences from each chunk of the read-back signal. The thermal expansion control of the head, enabled by placing a heater under the magnetic writer, allowed it to fly less than five nanometers above the disc's surface. 

For the past several decades the exponential growth in computer storage capacity and processing power has followed a pattern known as Moore's Law. This law predicted in 1975 that information density would double every two years. But at around 100 gigabits per square inch, shrinking the magnetic grains further or cramming them closer together posed a new risk called the superparamagnetic effect. When a magnetic grain volume is too small, its magnetization is easily disturbed by heat energy and can cause all the bits to switch unintentionally, leading to data loss.

Scientists resolved this limitation in a remarkably simple way, by just changing the direction of recording from longitudinal to perpendicular, allowing areal density to approach one terabit per square inch. Recently, the potential limit has been increased yet again through the heat-assisted magnetic recording. This uses an even more thermally stable recording medium, whose magnetic resistance is momentarily reduced by heating up a particular spot with a laser and allowing data to be written. 

So thanks to the combined efforts of generations of engineers, material scientists, and quantum physicists that this tool of incredible power and precision can spin in the palm of our hand.