Magnetic Disks

Recording data onto a disk has obvious advantages with respect to access times, as the head can readily be moved to the appropriate place on the disk whereas a tape would need to be rewound or advanced. There are two types of disk: floppy and hard. The principles of manufacturing and recording on floppy disks are very similar to that of particulate magnetic tape, i.e. the same particulate materials on a plastic substrates.

Hard disk drives are formed on a rigid substrate, usually aluminium, which is around 2mm thick. On to the substrate are deposited several layers: an under layer to help adhesion (~10nm nickel phosphide); a layer of chromium (5-10nm) to control orientation and grain size of magnetic layer; the magnetic layer (50nm PtCo with various additions of Ta, P, Ni, Cr); a protective overcoat (e.g. 10-20nm zirconia) and finally lubricant to reduce friction and wear of the disk (e.g. a monolayer of long chain fluorocarbons). The magnetic layer forms a cellular structure of Co-rich magnetic cells in a non-magnetic matrix. These cells act just like particulate recording media but on a much finer scale.