Rheo-optical measurements on suspensions of magnetic recording particles (abstract)

Abstract

New rheo-optic measurements of the orientation of magnetic particles (used in particulate recording media) in suspension under imposed external fields (hydrodynamic and/or magnetic) are presented. The apparatus consists of a rheo-optical device1 employing polarization modulation technology, and the external magnetic and flow field generating equipment (plane Poiseuille flow cell and quadrupole electromagnet). The polarization modulation dichroism measurement technique, which has the advantage of improved sensitivity and precision over conventional optical measurements, is able to characterize uniaxial single domain magnetic particle suspensions under external magnetic and hydrodynamic fields. Linear dichroism measurements, which reflect the degree of particle alignment about the field axis, are shown to be sensitive indicators of particle magnetic interactions, of effects of particle intrinsic magnetic properties, and of effects of particle shape. In this paper we focus on the comparison between effects of imposed magnetic and hydrodynamic fields on particle orientation for rodlike γ-Fe2O3, CrO2, and plate-like Ba ferrites. Relationships between the particle orientation due to the two external fields are established through dimensional analysis, and the ‘‘equivalent magnetic field,’’ which yields the same magnitude of dichroism obtained under a given flow field, is measured. The flow field and the equivalent magnetic field show a nearly linear relationship. Once the equivalent magnetic field plot is obtained experimentally, one can conveniently transform hydrodynamic field to applied magnetic field and then determine the particle orientation. Preliminary results indicate that this device represents the best technique for systematic investigation of magnetic particle orientation during simultaneous imposition of magnetic and hydrodynamic fields.