• BPSI (Bits Per Square Inch) – areal density – amount of information which can be stored in one square inch of the magnetic disk.
• TPI (Tracks Per Inch) – value which shows the density of tracks stored in the magnetic plate. It is measured in tracks per inch.
• BPI (Bits Per Inch) – linear or recording density – value which shows the density of data stored in the track. It is measured in bits per inch of a track.

Impossibility of infinite decrease of a domain size is stipulated by the following factors:

• Size of a magnetic head. Nowadays it determines the size of the magnetized area – the domain.
• The decrease of the readout signal level and the increase of noise level in it.
• Spontaneous self-demagnetization of the domain caused by the thermal effect.

Besides the domain decrease technology, HDD manufacturers also utilize other methods of increasing the storage density, namely:

PRML (Partial Response Maximum Likelihood) – conversion scheme of the analog signal stored in the magnetic disk which is based on the range of concepts of the pattern recognition theory. In PRML, in order to decode a readout signal, this signal is compared to a number of patterns. That which is most similar to the original is selected. This method consists of two parts: Partial Response subsystem which converts analog signal to digital one and minimizes noises and Maximum Likelihood subsystem which carries out digital signal processing to restore its original form. This conversion scheme and its extension EPRML are used in most modern hard disk drives.

AFC (AntiFerromagnetically-Coupled) is a data storage technology which consists in covering Winchester disk with a 3-layer antiferromagnetic coating in which a special isolation layer of ruthenium is sandwiched between two magnetic layers. Since the top and the bottom domains have antiparallel orientation of the magnetic field, they form a coupled unity which is more resistant to spontaneous magnetization reversal than a single “flat” domain.

PMR (Perpendicular Magnetic Recording) is a data recording technology which delivers almost twice the storage density of traditional recording and reduces problems caused by magnetic interference. Contrary to the traditional recording technology, PMR uses magnetic domains with the magnetic field vector not parallel but perpendicular to the surface of the magnetic disk. Thus the neighboring and differing domains are aligned with similar poles (which as a rule push off from one another) directed not to one another but to the disk platter. Contrary to the traditional recording technology, PMR decreases the interdomain space, thus increasing storage capacity of hard disk drives.

Longitudinal magnetic recording

Perpendicular Magnetic Recording

HAMR (Heat Assistant Magnetic Recording) - is a data recording technology which takes advantage of high thermal stability magnetic compounds. To record data, HAMR uses laser thermal assistance to first heat a magnetic domain. Laser beam diameter determines the size of the area corresponding to one bit of information. When the magnetic domain is being heated, its coercivity reduces and the heated sites become capable of being magnetized. An effective heat sink technology (cable of cooling magnetic plates during data recording) is necessary to implement HAMR in mass production.

SOMA (Self-Organized Magnetic Array) is a technology which implies formation of the “self-organized magnetic arrays” of the smallest homogenous iron-platinum conglomerates of 3 nanometers (3 nanometers are 10-15 atoms of the solid material arranged in line). Implementation of this “nanotechnology” will contribute to the stability of magnetic grains and to the decrease in the domain size.