The Z-scan technique is used to measure the nonlinear refractive index n 2 and the nonlinear absorption coefficient  of ionic magnetic colloids as a function of the nanoparticles' diameter ͑D rx ͒ in the femto-and millisecond time-scale regimes. A change from a convergent-lens type (at a femtosecond time scale) to a divergent-lens type behavior (at a millisecond time scale) is observed. From the values of n 2 and  measured in the femtosecond time-scale regime, the third-order susceptibility ͑3͒ is calculated and is shown to depend on D rx : the smaller the D rx , the bigger the ͑3͒ . A possible origin of the high values of ͑3͒ found may be confining effects related to the physical nanoscale of the magnetic particles.
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