Magneto-optical properties of binary ferrocolloids

Abstract

A new method for increasing the optical anisotropy of a ferrocolloid by introducing polymer molecules in the coil state or nanosized non-magnetic particles of elongated shape is realized in this work. The dimensions of the structural elements entering into such a binary colloidal solution are small in comparison with the wavelength, so the ferrocolloid remains optically homogeneous. Binary ferrocolloids of the first type are obtained by introducing polybutadiene molecules into a magnetic fluid (magnetite + kerosene + oleic acid). In this case, the birefringence enhancement (DLB) is due to the deformation and stretching of the polymer coils along the magnetic field. In weak fields, double amplification of the signal was detected at a concentration of polymer molecules of about 0.5%. A further increase in the concentration of impurity molecules weakens DLP due to a violation of the sedimentation stability of the solution and the precipitation of colloidal particles into the precipitate. A binary solution of the second type is synthesized on the basis of a magnetic fluid and rod-shaped impurity nanoparticles of goethite (a-FeOOH). The transverse size of the impurity particles (10-30 nm) was close to the average diameter of single-domain magnetite particles, and the longitudinal one was an order of magnitude larger. The amplification of DLP occurs due to the orientation of long axes of impurity particles along the magnetic field due to the difference in the "demagnetizing" coefficients along and across the axis of the particle. Magnetic birefringence has been studied depending on the concentration of magnetite and impurity particles and the strength of the magnetic field. For the first time, experimental confirmation of multiple amplification of the DLP signal by impurity particles was obtained. In fields (up to 10 kA / m) and the volume fraction of impurity particles of the order of one percent, the DLP signal is amplified by more than an order of magnitude. In stronger fields, the additive to the signal, due to the influence of impurity particles, reaches saturation and, with a further increase in the field, remains practically unchanged, while the total anisotropy of the solution continues to increase due to the orientation of the magnetite particles.