The main challenge in designing magnetic information storage media is retaining the magnetization of the medium despite thermal fluctuations. If the thermal energy is too high, there may be enough energy to reverse the magnetization in a region of the medium, destroying the data stored there. The energy required to reverse the magnetization of a magnetic region is proportional to the size of the magnetic region (where a larger magnetic region is more stable), as well as to the magnetic coercivity of the material, there is a minimum size for a magnetic region at a given temperature and coercivity. If it is any smaller it is likely to be randomly de-magnetized. Perpendicular recording uses higher coercivity material. This is possible due to the fact that the head's write field penetrates the medium more efficiently in the perpendicular geometry.
The popular explanation for the advantage of perpendicular recording is that it achieves higher storage densities by aligning the poles of the magnetic elements, which represent bits, perpendicularly to the surface of the disk platter, as shown in the illustration. In this not quite accurate explanation, aligning the bits in this manner takes less platter than what would have been required had they been placed longitudinally. So they can be placed closer together on the platter, thus increasing the number of magnetic elements that can be stored in a given area. The true picture is a bit more complex, having to do with the use of a magnetically "stronger" (higher coercivity) material as the storage medium. This is possible due to the fact that in a perpendicular arrangement the magnetic flux is guided through a magnetically soft (and relatively thick) underlayer underneath the hard magnetic media films (considerably complicating and thickening the total disk structure). This magnetically soft underlayer can be effectively considered a part of the write head, making the write head more efficient, thus making it possible to produce a stronger write field gradient with essentially the same head materials as for longitudinal heads, and therefore allowing for the use of the higher coercivity magnetic storage medium. A higher coercivity medium is inherently thermally more stable, as stability is proportional to the product of bit (or magnetic grain) volume times the uniaxial anisotropy constant Ku, which in turn is higher for a material with a higher magnetic coercivity.
in short perpendicular technology stores data perpendicular to disk than the longitudinal metho, parallel to disk.
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