  We have measured micro-rheological and micro-structural properties of a super
paramagnetic ferrofluid made of Mn$_{0.75}$Zn$_{0.25}$Fe$_2$O$_4$ (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
magnetoviscous effect. Additional microrheological properties, such as storage
modulus and loss modulus and their transition is seen. Following the analysis
given in \cite{Oliver Muller, Stefan Mahle}, we have obtained microstructural
properties such as elongational flow coefficient $\lambda_2$, relaxation time
constant $\tau$, coefficient of dissipative magnetization $\alpha$ etc. From a
single set of viscosity data, all the above parameters could be obtained. Our
values for the above parameters are in agreement with earlier theoretical
calculations and macro-rheological experimental measurements.
