Each hard disk drive platter is made of aluminum or glass and on it a magnetic material layer is applied, usually iron oxide mixed with other elements. We saw in the previous page that the read/write head magnetizes the magnetic particles found on the disk surface according to the applied current. We also saw that a sequence of magnetized particles represent a data bit.

Under longitudinal recording technology, found in virtually all hard disk drives found on the market today, the magnetic particles are horizontally aligned, i.e., they are found side by side on the disk surface, as you can see in Figure 2.

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Figure 2: Longitudinal recording technology.

During several years the most common method engineers used to increase storage capacity (storage density) was to decrease the size of the magnetic particles on the hard disk surface. The smaller the particles, the more data can be stored on the hard disk drive. Shrinking the magnetic particles, however, leads to a problem called superparamagnetism, which compromises data integrity. Superparamagnetism occurs when the particles are so small that temperature variations can reverse the magnetic fields from the particles, what would corrupt stored data.

Superparamagnetism prevents manufactures from building hard disk drives with higher capacities.

On perpendicular recording technology magnetic particles are vertically aligned, as you can see in Figure 3.

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Figure 3: Perpendicular recording technology.

With perpendicular recording technology more data can be stored on the hard disk and less superparamagnetism problems occur.