Effect of thermal fluctuations on magneto-optic rotation in CuZnFe2O4 ferrofluids

We present results of a magneto optical rotation (MOR) of polarization of light by a suspension of Cu0.8Zn0.2Fe2O4 nanoparticles in water. Conventional MOR in a dielectric medium arises due to magnetic field induced anisotropy which, in turn leads to a birefringence. On the other hand, magneto optic effects in colloidal supsensions of magnetic nanoparticles arise due to different scattering of different polarizations-a form of dichroism. The differential scattering happens because the nanoparticles align along the magnetic field and polarizations along and perpendicular to field are scattered differently. However, thermal fluctuations affect the alignment of the nanoparticles and hence the resulting anisotropy. We show that an increased thermal fluctuation reduces the effective MOR. This sample also exhibits a hysteresis behaviour, which is consistent with the viscosity measurement-clearly indicating that the MOR effect is due to the same anisotropy caused by alignment of magnetic nanoparticles, which also affects the viscosity and all other parameters.