Rheo-optical study of magnetic particle orientation under external fields

Abstract

The orientation distribution of magnetic particle suspensions subjected to external magnetic and hydrodynamic fields is of considerable practical interest as it relates to the manufacture of particulate recording media. Polarization modulated linear dichroism, a non-invasive optical technique developed in recent years for studying suspension microrheology, is adopted in this paper to investigate the orientational ordering in ethylene glycol suspensions of rod-like γ-Fe 2O 3, and plate-like barium ferrite particles subjected to applied magnetic fields up to 2000 Oe. Particle concentrations range from a volume fraction φ of 10 −5 to 10 −4, which is hydrodynamically but not magnetically dilute. Linear dichroism measurements <Δ n″>/LD max (where LD max is the dichroism at uniaxial alignment), which reflect the degree of particle alignment about the field axis, are shown to be sensitive indicators of particle magnetic interactions (Δ n″ vs. φ), of the effects of particle intrinsic magnetic properties (Δ n″ vs. the coercivity H c), of the effects of particle shape (γ-Fe 2O 3 vs. barium ferrite), and of qualitative microstructural changes during suspension aging. A scaling theory for uniaxial single domain systems is presented, which qualitatively reproduces the observed φ, H c and shape dependences of the order parameter deduced from the <Δ n″> data. The theory modifies the classical Debye—Langevin approach to paramagnetism by including magnetic anisotropy. Finally, we report preliminary observations which suggest an empirical equivalence between the magnitude of independently applied hydrodynamic and magnetic fields required to produce a suspension order for these particle systems.