August 03, 2020
The interesting property of ferromagnetic materials, which are used to make permanent magnets, is we can actually change their permant magnetic field orientation by applying a large magnetic field on them to force the orientation of the magnetic orientations inside the material.
Let's imaging we can arrange on a line a series of ferromagnets separated by some distance. Let's align their magnetic fields in the same direction. Now, take a strong magnetic field generator, say a strong electro-magnet. If we want to encode information into our system, we can approach the electro-magnet to the ferromagnets representing positions where we want to encode bit 1 and switch it on to change the orientation of these magnets while keeping the others to the initial orientation (representing bit 0).
This is the idea behind a magnetic hard drive. In reality, the ferromagnets are tiny regions of a coating ferromagnetic material that is spread over a circular disk, called a platter. This disk rotates around its center at a given speed, usually around 5000 rotations per minute. A fixed reading head is placed just above the surface of the disk and reads the data while the disk is rotating. An electronic controller converts the requests from the computer in order to move the read /write head for reading / writing the correct portion of information.
Magnetic hard drives are still the predominant form of mass storage on computers and servers. It is the cheapest of the persistent memories per gigabyte of storage. Nevertheless, due to the mechanical parts that have to move inside the hard disk drive to place the head at the correct locations, these drives are slower than flash / SSDs.