Reverse field orientation effects on rigid disk coating

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Abstract
A reverse magnetic field usually exists for orienting electromagnets used to align particles in magnetic media. Disk disorientation caused by this reverse field, known as the "dorf effect", was found to be a strong function of coating rheology, actual process steps, and reverse field magnitude. Experiments were done on 8-inch and 14-inch rigid disks with coatings of different viscosity varying the most sensitive orientation process parameter, spin-off time. In addition, more controlled experiments were done using 1/2-inch-diameter hand-coated aluminum disks exposed in the gap of an electromagnet to an orienting field followed by varying magnitudes of disorienting fields. Finally, a modified orientation electromagnet with a reduced reverse field was constructed. Our conclusions are that reverse field magnitudes, required to produce significant disorientation are strong functions of coating viscosity, coating formulation, and spin-off times. A 100 Oe reverse field creates a disorienting problem only when using very short spin-off times for coatings of low viscosity, containing a significant portion of high boiling point solvents. Reverse fields larger than 200 Oe, resulting from either high orienting current or a particular electromagnet design, will cause serious disorientation for most magnetic coatings.