Some physical properties of magnetic fluids (ferro-fluids) are well-known. Magnetic fluids are colloids. It means that solid magnetic nanoparticles (usually Fe3O4 particles with diameter ~10nm) are uniformly distributed in liquid carrier (water or kerosene or other). Usually oleic acid is added as a surfactant to prevent spontaneous particle aggregation. So properties of magnetic fluids depend on external magnetic fields, and magnetic fluids can have either properties of fluids or properties of solid bodies. But MF can have many interesting optical properties. These properties have attracted great interest in the fabrication of magneto-optical devices such as optical switches, tunable gratings and magnetic field sensors. However it is necessary to keep in mind nonlinear optical properties of MF and its dependence on external magnetic fields. Due to the diffusion of heat, the spatial temperature profile can differ significantly from the laser intensity profile; hence the mechanism is nonlocal. Hence there is a need to investigate magnetic fluids using Z-scan technique with external magnetic field, various concentrations and various magnetic fluids.