IBM has found a way to quadruple hard disk sizes by coating disks with a special compound.

The company is adding a new type of magnetic coating, ruthenium, to its disk drives. Ruthenium is layered between the two magnetic layers of the disk platter. IBM refers to ruthenium as 'pixie dust', but it is also known as AFC (antiferromagnetically coupled) media.

The technology can be implemented without redesigning current disk drive plants, IBM claims. By 2003, each disk drive made with AFC technology will be able to store 100 million bits per square inch, the company says.

The technology is most likely to be used first in notebook hard disks, although it should eventually enter all IBM products.

IBM claims that within two years 100Gb density would allow 400GB desktop drives, 200GB notebook drives and 6GB - 13 hours of MPEG-4 compressed digital video - in handheld devices.

Magnetic hard drive density has doubled every 18 months through to 1996. Since then it has doubled every 12 months.

The company says it will be sharing the technology with other vendors.