Neutrons effective in probing magnetic cluster sizes in magnetic recording disks. (General Developments).

The equivalent of Moore's law for magnetic recording is that the areal density of magnetically written bits increases at 60% to 100% per year. To achieve this, the noise in the magnetic media must be reduced continuously, which is accomplished by decreasing the size of the magnetic clusters--regions of the media that are magnetically coupled. The cluster size must decrease, since the media noise primarily results from the finite cluster size; the bit transitions are narrower when the clusters are smaller. Presently, cluster sizes are believed to be about 10 nm to 15 nm, but accurate knowledge of the size distribution and even the average size is lacking. A common assumption is that the cluster size is identical to the crystalline grain size of the media, but this has not been demonstrated.

Recently, by carrying out small-angle neutron scattering (SANS) measurements on actual recording disks, scientists from a private company, working with researchers in NIST's Center for Neutron Research (NCNR), have demonstrated that this is indeed the case. The advantage that SANS provides is the ability to quantitatively separate the scattering due to the crystalline grains from the scattering that originates from the interaction of the neutron's magnetic moment with the net moment of the magnetic clusters. This was accomplished by first collecting data from disks that had been prepared to have the magnetic moments of the clusters randomly oriented. In this state, the SANS data contain contributions from the physical film structure (grains) and the magnetic film structure (magnetic clusters). Data then were collected while a large magnetic field was applied parallel to the disk surface. The field aligns the cluster moments (the sample effectively becomes one macroscopic cluster), and the remaining SANS is d ue to physical grain structure only. Subtracting the zero field and high field spectra leaves the desired magnetic SANS.

The SANS measurements showed that the magnetic cluster size scales with the grain size. Moreover, the intergranular magnetic interactions are not very strong, especially for the most advanced media tested where the cluster size was on average only about 10 % larger than the grain size. The measured cluster size distributions, however, were rather broad. The implication is that further reducing grain size to reduce magnetic cluster size will lead to higher recording densities if the size distribution of cluster sizes can be narrowed as well.

CONTACT: Charles Glinka, [phone]; [e-mail].