Magnetic disc: what is it?

A magnetic disk is a disk covered with a magnetic layer where data is recorded. They are found especially in the HDDs, where several magnetic platters are superimposed in the case. The reading head induces a magnetic field that changes the orientation of thousands of small magnets present on the magnetic layer. When reading, the voltage difference is detected by the same reading head.

Magnetic disks of hard disks

Generally, a magnetic disk consists of a substrate in aluminum or glass, added with a magnetic layer composed of grains of alloys ferromagnetic (often made up of cobalt, chrome and platinum). A bit is made up of several tens of these magnetic grains and the information is stored in binary mode (0 and 1).

In hard disks, the magnetic disk is usually 2 to 7 centimeters in diameter and between 30 to 50 nanometers thick, 2,500 times thinner than a sheet of paper! The rotation speed determines either the access time to the data or the time the read head needs to get to where the data is. The standard is 5,400 rpm for an external hard drive and 7,200 rpm for an internal hard drive.

Storage capacity limits

The storage capacity of magnetic disks is limited by the surface density of the magnetic grains: when the grains are too close, their magnetism respective can influence the magnetic direction of the surrounding grains. It is then necessary to resort to new materials to improve the thermal stability grains, like some rare earth, from alloys iron/platinum or cobalt/samarium. It is also possible to create reliefs on the magnetic disk to separate the grains from each other using a process of photolithography.

Aside from discs, there are other magnetic recording media, such as audio and video cassettes, floppy disks or magnetic tapes on credit cards. The disadvantage of this technology is that the information can be altered by the passage of a magnet close to the magnetic track.