Abstract

The first-order hyperpolarizability $\beta$ of magnetite nanoparticles in colloidal dispersion was measured in the presence and absence of an external magnetic field of magnitude $H=800\mathrm{G}$, applied with permanent neodymium magnets. For that, the (linear) attenuation spectrum was measured, and the nonlinear properties were obtained through the hyper-Rayleigh scattering technique. The attenuation spectrum is the same regardless of the external magnetic field, indicating that large aggregates of nanoparticles were not formed on our system. The first hyperpolarizability, on the other hand, increased when the incident beam polarization was parallel to the magnetic field lines and decreased when the directions were orthogonal, due to the alignment of crystallographic planes of the material when nanoparticles rotate in order to align their individual magnetic momentum, with respect to the external field. In the absence of a magnetic field, the hyperpolarizability ${\beta }_{H=0}=8.5(1)\times {10}^{-28}{\mathrm{cm}}^5/\mathrm{esu}$. For the parallel case, ${\beta }_{\parallel }=9.8(2)\times {10}^{-28}{\mathrm{cm}}^5/\mathrm{esu}$, while for the perpendicular configuration, ${\beta }_{\perp }=8.1(1)\times {10}^{-28}{\mathrm{cm}}^5/\mathrm{esu}$. Defining the $x$ axis of the particle reference frame parallel to the $⟨111⟩$ crystallographic direction, which corresponds to the direction of easy magnetization, ${\beta }_{\parallel }={\beta }_{xxx}$, and ${\beta }_{\perp }$ corresponds to an average from ${\beta }_{yyy}$ and ${\beta }_{zzz}$. When there is no external field applied, the nanoparticles are randomly oriented, and the measured hyperpolarizability corresponds to an average over the three orthogonal directions; that is, $⟨{\beta }_H⟩=\frac{1}{3}({\beta }_{\parallel }+2{\beta }_{\perp })=8.6(1)\times {10}^{-28}{\mathrm{cm}}^5/\mathrm{esu}$, which is compatible with the measured value for the system without a magnetic field ${\beta }_{H=0}$.

© 2018 Optical Society of America

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