  In this paper we performed steady shear and oscillatory magnetorheological
(MR) studies in magnetic fluids containing CoNi nanoclusters of 450 nm in
diameter. Co-rich nanoclusters were synthesized by conventional homogeneous
nucleation without any external surfactant or reducing agent in liquid polyol
at elevated temperature. The x-ray diffraction, energy dispersive X-Ray
analysis, scanning and transmission electron microscopy studies were done for
analyzing the sample composition and morphology. Two variants of fluid samples
were prepared by dispersing 15 vol% and 20 Vol% of CoNi powders in castor oil.
Room temperature steady magnetoshear studies indicate viscoplastic behavior
with stronger dependence of static yield stress on magnetization than a dipolar
coupling that was operational in the dynamic yield stress. Magnetosweep
measurements at constant shear rate showed interesting relaxation at high
magnetic fields. We also explored dynamical elastic behavior through
oscillatory magnetorheological studies under both strain sweep and frequency
sweep modes, and showed glass transition like phenomenon occurring in them
above critical shear amplitudes.
