Bit Patterned Media (BPM) research

What is patterned media?

In a nutshell ...

Patterned bits in data storageIn conventional magnetic storage systems (hard disk drives - HDDs) information is recorded in the form of magnetic domains to a continuous magentic thin film covering the surface of a disc. Writing and reading data is performed by flying a read/write head in very close proximity to the disk as it spins (the spacing is sub-10nm in modern HDDs). As the demand for high-density digital storage capacity grows, magnetic HDDs have met the these demands by the continued increase in the areal storage capabilities of the continuous media, via the reduction in recorded mark (domain) size and the narrowing of track widths.

Current commercial magnetic disk drives offer storage densities in excess 300Gbits/in2, and are expected to offer storage capacities approaching 1Tbit/in2 using perpendicular recording media. However, it is difficult to foresee how far the present storage paradigms can be extended to meet increasing demand, as thermal effects become more rpominnt at smaller bit sizes leading to the imability to store data. Readout signal-to-noise ratios are determined by many factors, including transition noise and bit shifts due to strong inter-granular magnetic coupling and in particular, the number of grains in a recorded domain. In order to increase storage density the size of the recorded domains must be reduced, however, this results in a need to reduce the size of the magnetic grains that form the recording layer in order to maintain sufficient signal-to-noise ratio. However, a reduction in grain size can result in thermal magnetic instability, the so-called superparamagnetic limit. A solution is to increase the coercivity of tehe recording medium, however, this makes it difficult to write information to the medium due to the requirement for high write fields.

One solution to the problem of ultra-high-density recording on continuous media is the use of a patterned recording medium. Here, the continuous magnetic thin film is patterned to form an array of nanometre sized, isolated, magnetic islands, each storing a single bit of information. In order to attain a storage density of 1Tbit/in2 each island must occupy an area of approximately 25nm x 25nm ... which is very very small (approximately 1/4000 the width of a human hair). Since each island acts as a single bounded domain, the grains are strongly coupled and consequently the grain size need not be reduced. Furthermore, the effects of magnetic coupling between recorded marks, and hence transition noise, are effectively removed resulting in an improved signal-to-noise ratio.

There are a number of problems associated with the development of patterned media as a viable storage medium, the most important of which, is how do you fabricate small islands (typically 10-20nm in diameter) using the fabrication techniques currently available? How do you record to such a medium? and how do you recover the stored information?

The NEST group have had active research projects in the field of so-called "Bit Patterned Media" (BPM) for over 10 years, and we have investigated many aspects of BPM, including: fabricating BPM, characterising such media to understand the magnetic switching behaviour of magnetic nanostructures, writing simulations and data recovery studies. The following lists some of the funded work we have undertaken in these areas.

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