To record informations on surfaces a magnetic field is inducted by heads to the surface (changing the poles we change the binary value).

In order to increase data density and drive capacity the writing techniques has changed from longitudinal recording to perpendicular recording.

longitudinal-recording
perpendicular-recording

longitudinal recording

The common method of digital recording on a magnetic material. The bits are laid out end to end, and the direction of the magnetic charge is horizontal with respect to the medium.

longitudinal-vs-perpendicular

Longitudinal Vs. Perpendicular Recording
(Image courtesy of Hitachi Global Storage Technologies.)

Why Perpendicular Recording is Needed

One of the key challenges facing the hard drive industry is overcoming the constraints imposed by the super paramagnetic effect, which occurs when the microscopic magnetic grains on the disk become so tiny that ambient temperature can reverse their magnetic orientations. The result is that the bit is erased and thus, data is lost.


Perpendicular Recording

In longitudinal recording, the magnetic orientation of the data bits is aligned horizontally, as its name indicates, parallel to the surface of the disk.

By contrast, in perpendicular recording, the magnetic orientation of the data bits is aligned vertically, perpendicular to the disk. In this orientation, materials and smaller crystalline grains can be used wherein it is harder to reverse the magnetic orientation, resulting in smaller physical bits that are still stable at room temperature.

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