Patterned Media for High Density Magnetic Recording

Areal recording densities in hard-disk systems have been at least doubling every three years since the technology was first introduced in 1955. To continue this trend, substantial advances in magnetic recording media will be needed to allow for even higher density storage with sufficiently high read-back signal-to-noise ratios. Two factors which currently limit the densities of recording systems are the problem of transition noise, which arises from the irregular zig-zag domain walls between bits, and the superparamagnetic limit which imposes a lower bound on the sizes of the magnetic grains making up the recording medium. To address these problems, we have proposed a novel form of recording medium consisting of a closely packed regular array of single domain particles, each of which is suitable for storage of a single bit of information. In order for such a scheme to be practical, we require magnetic islands with well defined magnetic properties, including predictably oriented easy axes of magnetization and high switching fields. To investigate the properties of small magnetic islands, we have developed a lithographic patterning procedure to define islands with feature sizes down to 0.1 mum. We have applied this patterning process to both longitudinally oriented polycrystalline cobalt films and to single crystal iron films with a strong in -plane uniaxial surface anisotropy. Although sufficiently small polycrystalline cobalt islands do tend to be uniformly magnetized, these single domain particles do not have very predictable magnetic properties, primarily because of the inherent magnetic inhomogeneities in the polycrystalline film. In contrast, the single crystal iron islands were uniformly magnetized over a wider range of shapes and sizes, and all of the islands had their easy axes aligned with the surface anisotropy of the film. Our ability to simultaneously control both the crystallographic orientation and shapes of the islands allowed a comparison of the relative strengths of magnetocrystalline and shape anisotropy in the islands we examined.