Data Recording Technologies in the HDD
Separate sites of a magnetic disk can be magnetized by one of two possible ways which designate a zero or one, i.e. 1 bit. Such magnetized area is named the magnetic domain and it is a miniature magnet on a disk surface with a certain orientation of southern and northern magnetic poles. In order to record one bit the magnetic head creates definitely directed magnetic field which orients the domain; the domain's vector of magnetization remains unchanged for a long time after the head has stopped affecting the magnetic surface. A bit density and volume of the information which can be recorded to the unit of the plate surface depend on the sizes of domains. Commonly used values of a bit density are:
- BPSI (areal density) - the volume of information which can be recorded to one square inch of a magnetic disk.
- TPI (track density) - the value showing how close to each other the tracks on the plate are located. It is measured in quantity of tracks per inch.
- BPI (linear or recording density) - the value showing how tightly the data on the track is "packed". It is measured in bits per inch of a track.
The main causes of impossibility of infinite reduction of the domain size are:
- The size of a magnetic head. Nowadays it is the magnetic head that defines the size of the minimum magnetized area - the domain.
- The decrease of the read-out signal level and the increase of noise level in it.
- Spontaneous self-demagnetization of the domain caused by temperature effect.
Besides the reduction of the size of domains manufacturers of hard disk drives also use other technologies of increasing the recording density:
PRML (Partial Response Maximum Likelihood). It is the algorithm of conversion of the analog signal recorded on a magnetic disk, based on a number of concepts of the theory of pattern recognition. In PRML method, the set of samples to which read signal is compared is applied to decoding, and the most similar one is accepted as a result. The method consists of two parts - partial response subsystem translates a signal from an analog form to a digital one, minimizing noises, and maximum likelihood subsystem carries out the digital processing of a signal in order to restore its most verisimilar form. The given algorithm and its development EPRML is applied in practically all the modern hard drives.
AFC (AntiFerromagnetically-Coupled). The idea is to lay on the disk a three-layer antiferromagnetic covering in which the pair of magnetic layers is separated by a special isolating layer made of ruthenium. As the magnetic domains located opposite each other have an antiparallel orientation of a magnetic field, they form a pair which appears to be steadier against spontaneous magnetic reversal than a single "flat" domain.
PMR (Perpendicular Magnetic Recording). This technology enables a twofold increase of the density of data recording and reduces problems with magnetic influence (interference). Unlike classical technology of recording, magnetic domains with the vector of a magnetic field perpendicular, not parallel, to a surface of a disk are used. Thus, the neighboring and differing domains are not placed over against each other with similar poles which, as it is known, push off from each other. In comparison with the classical technology of recording, it allows reducing the size of interdomain space, thus increasing the capacity of storage devices as well.
HAMR (Heat Assistant Magnetic Recording). The idea is to use the magnetic materials providing high thermostability of recorded sites of a surface. For data recording the magnetic domain is preliminary warmed up by means of the focused laser beam. The diameter of a beam also defines the size of the area corresponding to one bit of the information. At domain increase in temperature, there is an essential change of its magnetic properties (coercive force decreases), and, thus, the heated sites become capable of magnetization. Working out an effective heat-conducting path taking the heat away from the magnetic plates during data recording is necessary for mass introduction of HAMR in batch production.
SOMA (Self-Organized Magnetic Array). The given technology implies the formation of a monodisperse layer of "self-organized magnetic arrays" consisting of the smallest homogeneous iron-platinum conglomerates with the size about 3 nanometers (3 nanometers are 10-15 atoms of the firm substance which have been laid out abreast) on a disk surface. Application of this "nanotechnology" will provide an essential lowering of the level of instability of separate magnetic grains; it will also reduce the size of the domain.