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We have measured microrheological and microstructural properties of a superparamagnetic ferrofluid made of Mn0.75Zn0.25Fe2O4 (MZF) nanoparticles, using passive microrheology in a home-built inverted microscope. Thermal motion of a probe microsphere was measured for different values of an applied external magnetic field and analysed. The analysis shows anisotropy in magneto-viscous effect. Additional microrheological properties, such as storage modulus and loss modulus and their transition are also seen. We have also obtained microstructural properties such as elongational flow coefficient [Formula: see text] , relaxation time constant [Formula: see text] , coefficient of dissipative magnetization [Formula: see text] , etc., using the analysis given in Oliver Muller et al., J. Phys.: Condens. Matter 18, S2623, (2006) and Stefan Mahle et al., Phys. Rev. E 77, 016305 (2008) over our measured viscosity data. Our values for the above parameters are in agreement with earlier theoretical calculations and macro-rheological experimental measurements. These theoretical calculations consider an ideal situation of zero-shear limit, which is best approximated only in the passive microrheology technique described here and a first time measurement of all these parameters with passive microrheology.

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The nanoparticles and ferrofluids of spinel ferrites are useful in bio-sensors, transducers, storage devices, optical devices, and so on. The Mn-Zn ferrite (MZF) is generalized soft spinel ferrite having high saturation magnetization at low applied magnetic field. This chapter covers the synthesis of nanoparticles of various sizes and compositions of Mn 1-x Zn x Fe 2 O 4 with x = 0-1 by co-precipitation method. The structural and magnetic properties of the nanoparticles are discussed. The ferrofluids of superparamagnetic and ferromagnetic MZF nanoparticles were synthesized. The magneto-viscosity of ferrofluids with the dispersion of nanoparticles in different colloidal was studied. The Herschel-Bulkley model is applied to analyse the data for low viscosity ferrofluids.