1. How a hard disk stores 0s and 1s with tiny magnets
Inside a hard disk, there are circular disks. The drive remembers information by magnetizing small areas on the surface in different directions [2]. For example, a magnet pointing right might mean 0, and pointing left might mean 1. The first commercial hard disk was the IBM 350, released in 1956. It used 50 disks, each 24 inches (about 61 cm) in diameter, spinning at 1,200 revolutions per minute. It could store about 3.75 megabytes, which is less than a single photo on a modern smartphone [1]. How did such small capacity grow by leaps and bounds? The key was making the magnets smaller and smaller. But as they shrank, an unexpected wall appeared.
2. Why shrinking magnets lets heat scramble their direction
To increase capacity, you need to pack more magnetic particles into the same area by making them smaller. However, if they get too small, they hit a limit. Smaller magnets hold less magnetic energy. Eventually, this energy is weaker than the surrounding thermal energy, causing the magnet's direction to become easily disordered by heat. This is called thermal fluctuation [4]. In modern hard drives, one bit is supported by roughly dozens of tiny magnetic grains. When these grains shrink to their limit, the risk of random reversal due to heat becomes real, meaning 0s and 1s can no longer be read reliably [4]. In 2005, reports stated that the old method—laying magnets flat parallel to the disk surface—had a practical limit: above 120 gigabits per square inch, reading data became difficult [5]. How was this wall crossed?
3. The 1977 idea of standing magnets upright on the disk
In 1977, a team led by Professor Shun-ichi Iwasaki at Tohoku University proposed a new recording principle: perpendicular magnetic recording, where magnets stand vertically against the disk surface. Practical use began in 2005 [3]. This method combined three parts: a recording layer of thin cobalt alloy film, a soft magnetic underlayer beneath it, and a single-pole head that concentrates the magnetic field at one point. It was shown that this setup could record at densities unreachable by the old flat method [3]. The writing process also changed. Previously, ring-shaped heads applied magnetic fields parallel to the disk. Perpendicular recording uses a single-pole head to apply fields perpendicular to the recording layer [5]. Why does standing them up increase density? According to summaries from Tohoku University materials, when tiny magnets are oriented vertically, they attract each other and become more stable as they are packed closely. In the flat method, facing magnets repel each other, causing instability [6].
4. Why it took 28 years to turn the idea into a product
Perpendicular recording was proposed in 1977 and implemented in 2005. Tohoku University’s Institute of Electrical Communication notes that the invention was made by Professor Iwasaki in 1977 and put into practical use in 2005, a 28-year gap [4]. In April 2005, Hitachi Global Storage Technologies announced plans to release a 2.5-inch hard disk with about 120 gigabytes using perpendicular recording in late 2005. Prototypes achieved 230 gigabits per square inch, with a vision of reaching 1 terabit in the future [5]. Note that the 1 terabit figure was a goal at the time, not a proven result. The 230 gigabits in prototypes was about 1.9 times the 120 gigabit limit of the flat method, showing that the barrier could be crossed at the prototype stage. In October 2023, IEEE named perpendicular recording an IEEE Milestone for its important role in increasing storage capacity [3]. Research continues today, with bit-patterned media, where one bit is carried by a single dot, being studied as the next generation technology [4].
5. How GMR heads read very weak magnetic signals
As bits got smaller, the magnetic changes coming from the disk became very weak. New technology was needed not just for writing, but for reading. In 1988, Albert Fert from France and Peter Grünberg from Germany independently discovered the Giant Magnetoresistance (GMR) effect. This is a phenomenon where very weak magnetic changes cause large differences in electrical resistance [2]. If electrical resistance changes significantly, the magnetic information on the disk can be extracted as changes in current. The first reading heads using GMR appeared in 1997 and soon became standard. Fert and Grünberg were awarded the Nobel Prize in Physics in 2007 [2]. It took 9 years from discovery to product. By the end of the 1990s, the barrier on the reading side had also been broken.
6. Why the head floats on air above the spinning disk
If the head touched the disk while it was spinning, the recording surface could be damaged. So, the head floats just above the disk. Even in the 1956 IBM 350, the head floated on a cushion of air, just above the surface [1]. As bits became smaller, the head had to move closer to the smaller magnetic regions, so its flying height was reduced. In the RAMAC era the head flew about 20 micrometers above the disk, and that height has been reduced a great deal since.
7. Experiments you can try yourself with two magnets
You can try this at home: get two small magnets and bring them close together on a desk, and notice when they attract and when they repel as you change their orientation. Do not bring magnets close to computers or smartphones. Be careful not to let small magnets go into your mouth. If you go out, look for displays of old memory devices at science museums. Comparing the size and number of disks in old machines with modern hard drives helps you realize the meaning of the 28 years it took to put this technology into practical use. For official information, you can read explanations from the Computer History Museum, IEEE Milestone certificates, or research introductions from Tohoku University’s Institute of Electrical Communication.