Particulate dispersions for magnetic recording data

Abstract

The study of a magnetic recording media dispersion has been carried out using hindered settling analysis, the purpose of which was to probe the equilibrium state structure in zero applied magnetic field. The system chosen for the main study was based on a commercial formulation of a y-Fe 203 suspension of 0.3 j.tm length acicular particles, with aspect ratio 5: 1, that was used to manufacture a back-up/archive tape.
From the sedimentation analysis, a large fraction of fluid (- 86 % by volume of the settling unit) was estimated to be trapped within the clusters of particles that make-up the overall dispersion structure. Confidence in the result was given by correction of the classical hindered settling models to account for the trapped fluid. In this manner, the range of Steinour and Kozeny-Carman parameters evaluated reduced near to, or agreed with, the fundamental values found by the original authors on simple model systems that contained little to no trapped fluid.
Near- Newtonian fluid measurements resulted in an equivalent Stokes particle diameter of - 6.7 Jim.
A computer simulation of hindered settling has been developed, based on a laminae concept and the Kozeny-Carman equation, in order to complement the experimental investigation. The concentration profiles over columii height compared well with those obtained using a scanning column magnetometer to give an enhanced description of the hindered settling phenomena.
This showed that the descending 'plug' of constant solids concentration falls down into, and is continually subsumed by, the rising layers of the lower compression region. The plug effectively contracts about its mid-point and the initial constant rate of fall is only maintained up to the point where the plug is destroyed. It was also found that the model only gave a reasonable comparison with experiment when the effective ratio of trapped fluid was large; giving further confidence in the trapped fluid estimate.
The theory of the scanning column magnetometer technique has been developed beyond that already known. This resulted in good agreement between theory and experiment in characterising the main detection coil response, or 'form factor' and means that future design work will be enhanced.
The effects of magnetic interaction in the magnetic media dispersion have been investigated by varying the magnetic y-Fe 203 content by dilution with cz-Fe 203 (produced from the same particles) whilst holding the total particle concentration constant. The resultant increase in sedimentation with magnetic interactions has been interpreted in terms of hindered settling theory as being due to the formation of clusters by magnetostatic attraction. Extension of the experiment should give a strong indication of the extent that magnetic particles trap fluid in comparison to their non-magnetic depletion flocculated precursor. This could be significant in the context of the drying stage of the magnetic recording media production process.