Rheomagnetic measurements on suspensions of magnetic recording particles

Abstract

Measurement of magnetic particle flow orientation properties is of considerable practical and fundamental interest as it relates to the particulate recording tape and disk manufacturing industry. For production of high-quality magnetic tapes and disks, the suspension must be well dispersed. Previous study focused on rodlike single-domain magnetic particles for longitudinal magnetic recording. In contrast, platelike particles are considered here for use in perpendicular recording. Both theory and experiment have demonstrated that a magnetic sensing technique employed to measure the hydrodynamic orientation provides a measure of the flocculation state for rodlike magnetic particle suspensions. Here the previous theory is extended and new experimental results are shown to demonstrate that the hydrodynamic orientation measurement apparatus can also be used to measure the flocculation state of platelike particles. The key new experimental result is the contrast between the flow orientation properties of the rod versus the platelike particles. Rodlike single-domain particles orient parallel to the flow direction causing the suspension magnetic permeability parallel to the flow direction to decrease with increasing flow rate. In the case of the platelike Ba-ferrite particles, the opposite behavior is observed. As the flow rate is increased, the particles orient with their flat side parallel to the flow direction causing the suspension magnetic permeability parallel to the flow direction to increase.