Patterned media

Patterned media (also known as bit-patterned media or BPM) is a potential future hard disk drive technology to record data in magnetic islands (one bit per island), as opposed to current hard disk drive technology where each bit is stored in 20-30 magnetic grains within a continuous magnetic film. The islands would be patterned from a precursor magnetic film using nanolithography. It is one of the proposed technologies to succeed perpendicular recording due to the greater storage densities it would enable. BPM was introduced by Toshiba in 2010.

In existing hard disk drives, data is stored in a thin magnetic film. This film is deposited so that it consists of isolated (weakly exchange coupled) grains of material of around 8 nm diameter. One bit of data consists of around 20-30 grains that are magnetized in the same direction (either "up" or "down", with respect to the plane of the disc). One method of increasing storage density has been to reduce the average grain volume. However, the energy barrier for thermal switching is proportional to the grain volume. With existing materials, further reductions in the grain volume would result in data loss occurring spontaneously due to superparamagnetism.

In patterned media, the thin magnetic film is first deposited so there is strong exchange coupling between the grains. Using nanolithography, it is then patterned into magnetic islands. The strong exchange coupling means that the energy barrier is now proportional to the island volume, rather than the volume of individual grains within the island. Therefore, storage density increases can be achieved by patterning islands of increasingly small diameter, whilst maintaining thermal stability. Patterned media is predicted to enable areal densities up to 20-300 Tb/in² as opposed to the 1 Tb/in² limit that exists with current HDD technology.

In existing HDDs data bits are ideally written on concentric circular tracks. This process is different in bit patterned media recording where data should be written on tracks with predetermined shapes, which are created by lithography (c.f. the next section) on the disk. The trajectories that are required to be followed by the servo system in patterned media recording are characterized by a set of "servo tracks" existing on the disk. Deviation of a servo track from an ideal circular shape is called "repeatable runout" (RRO). Therefore, the servo controller in bit patterned media recording has to follow the RRO which is unknown in the time of design, and as a result the servo control methodologies used for conventional drives cannot be applied. Patterned media recording has some specific challenges in terms of servo control design which are briefly listed here:

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