Study of Magnetoviscosity of Ferromagnetic MnZn-Ferrite Ferrofluid

Abstract

Mn0.75Zn0.25Fe2O4 (MZF) ferrofluid was synthesized with oleic acid-coated MZF nanoparticles dispersed in toluene (less viscous fluid). The Rietveld refinement for the X-ray diffraction pattern of MZF nanoparticles showed the single phase spinel structure formation with Fd-3m space group. The nanophase and the particle size distribution of MZF nanoparticles (7–15 nm) were observed on transmission electron micrograph. The soft ferrimagentic behavior can be observed from the magnetization versus applied magnetic field ( $H$ ) plots at 325 and 5 K. The power law behavior is verified for shear rate versus viscosity plots at various magnetic fields. The power index $(n)$ value decreases from 0.1 to 0.04 with increase in magnetic field from 0 to 1.33 T. The low $n$ -value indicates non-Newtonian behavior shear thinning for the ferrofluid at various applied magnetic fields. The Herschel–Bulkley model fit is verified for shear stress versus shear rate. The yield stress values were obtained by extrapolating the experimental shear stress versus shear rate plots to zero shear rate. The ferromagnetic nanoparticles (magnetic dipole particles) of ferrofluid rotate in the direction of magnetic field with the increase in field. Above certain magnetic field $(>0.2~{\rm T})$ , the magnetic field effect dominates other effects present in the fluid. This is due to long chain formation perpendicular to the shear flow field in the ferrofluid. We also discuss the dependence of viscosity and shear stress on applied magnetic field from magnetoviscosity plots at constant shear rates.