The discs inside of a hard drive, often referred to as platters, are made out of non-magnetic metals like aluminum, glass, or ceramic. Hard drives store data on these spinning platters which are coated with a thin magnetic film. Read/write heads float just above the platters on an actuator arm, allowing data to be written to and read from the platters as they spin.
Hard Drive Basics
Hard drives have been the dominant form of long-term data storage in computers for decades. At its most basic, a hard drive consists of one or more circular platters stacked on top of each other and spinning at very high speeds inside an enclosed, sealed chamber.
The platters are made of non-magnetic material, usually aluminum or glass, which is then coated with a very thin layer of magnetic material, typically 10-20 nanometers thick. This magnetic coating allows data to be stored on the platters in binary form, as regions of positive and negative magnetization.
An actuator arm inside the hard drive enclosure contains read/write heads that can detect and change the magnetization of these regions on the platter surfaces. By changing the polarization of a small area on the platter (a magnetic domain), a 1 or 0 can be recorded. These 1s and 0s make up the binary data stored on the drive.
The substrate material that platters are constructed from has evolved over time:
- Aluminum: Early hard drives used aluminum, which was lightweight and inexpensive. However, aluminum platters can warp or bend over time due to temperature changes and high RPM speeds.
- Glass: Glass platters were developed to be more rigid and resist warping under stress. Glass is amorphous and isotropic, meaning it has a consistent structure in all directions, giving it strength.
- Ceramic: Newer platters are made of ceramic or a glass-ceramic composite, which combines the strength of glass with heat resistance and low density of ceramics.
The substrate material must have a very smooth and consistent surface for the magnetic coating to be applied evenly across the entire platter surface. Any variation in height on the nanometer scale can affect the ability of the heads to read/write data.
The magnetic coating applied to the platters is what actually stores the data. There are two main types of magnetic coatings used:
- Longitudinal recording – An older technique where magnetic domains were oriented longitudinally or horizontally along the platter surface. Limits storage density.
- Perpendicular recording – More modern technique where magnetic domains are arranged perpendicularly or vertically standing up from the platter surface. Allows for greater storage density.
The magnetic coating is applied through a process called sputtering. The platter is placed into a vacuum chamber containing the magnetic material to be deposited as a target. Argon gas is then introduced into the chamber and an electrical current ionizes the gas, creating plasma. The positively charged ions accelerate toward the negatively charged target, ejecting atoms that coat the platter in an extremely thin and uniform layer.
Common magnetic materials used in sputter coating include:
- Cobalt-based alloy like cobalt-chromium-platinum
- Iron-platinum alloys
- Various oxides like barium ferrite or iron oxide
The thickness and magnetic properties of the film can be precisely controlled during sputtering to optimize performance. The coating allows the magnetic orientation of tiny regions on the platter to be flipped to store data.
Data is written to the magnetic platters using write heads on the end of actuator arms. These use electromagnetic coils to generate strong magnetic fields that can flip the magnetization of a domain on the platter. Each domain can store a 1 or 0 bit.
The read head on the arm contains sensors that can detect the orientation of the magnetic domains by their poles as they pass under the head. In this way, the 1s and 0s encoded as magnetic regions can be read. Combinations of 1s and 0s make up the data.
The platters rotate at high speeds, up to 15,000 RPM in some drives. This allows the heads to access data anywhere on the platters in just milliseconds. The actuator arm can move in and out so the heads can access data in circular tracks on the platters.
Platter Size and Density
Platter diameters in hard drives typically range from 1-3.5 inches for consumer drives. Enterprise and high capacity drives may use larger platters up to 5 inches. More platters can be stacked vertically to increase total capacity.
Areal density is a measure of how much data can be crammed onto a platter surface, measured in gigabits per square inch. Densities have skyrocketed in the past decades from less than 0.5 Gb/in2 in the early 90s to over 1 Tb/in2 today.
Higher areal densities allow HDDs with the same size platters to have more capacity. Density increases by using thinner magnetic films, smaller magnetic domains, and read heads that are more sensitive.
In summary, the platters that store data inside hard drives are made from non-magnetic aluminum, glass, or ceramic materials. These are coated with an extremely thin layer of magnetic film, as little as 10 nanometers thick. The magnetic coating allows 1s and 0s to be stored in tiny domains on the platter surfaces.
Read/write heads on actuator arms can magnetize these domains to write data, and then detect their magnetic orientation to read the data back. Faster platter rotation and more dense data recording enables hard drives with incredible storage capacity compared to their physical size.
While solid state drives are taking over as the newest data storage technology, hard disk drives continue improving through innovations in magnetic platter engineering and recording techniques allowing them to remain a viable, affordable mass storage option.
- Mee, C. D., & Eric, D. (1996). Magnetic Recording Handbook: Technology and Applications. New York: McGraw-Hill.
- Grochowski, E., & Halem, R. D. (2003). Technological impact of magnetic hard disk drives on storage systems. IBM Systems Journal, 42(2), 338-346.
- Fontana Jr, R. E., & Hetzler, S. R. (1996). The physics of magnetic recording. IBM Journal of Research and Development, 40(1), 3-21.
- Shiroishi, Y., Fukuda, K., Tagawa, I., … & Yoshikawa, M. (2009). Future options for HDD storage. IEEE Transactions on Magnetics, 45(10), 3816-3822.