The Magnetic and Optical Anisotropy of Selected Microscopic Crystals and the Linear Magneto-Optical Anisotropy of Their Suspensions

Abstract

Superposition of magnetic and orthogonal hydrodynamic orientation is used for measuring the relative magnitude of magnetic anisotropy in paramagnetic and diamagnetic, organic and inorganic, microscopic anisometric crystals, dispersed in liquids. Some magnetically biaxial crystals show a "two-step" orientation. The optical anisotropy of the crystals is determined and compared with the magnetic anisotropy. The criteria as regards the probable magnitude of magnetic double refraction and of magnetic dichroism in suspensions of the crystals are discussed. The sign and a series of complications in sign of magnetic double refraction and of magnetic dichroism in such suspensions are deduced. Magnetic double refraction and magnetic dichroism, as actually observed in dispersed systems of ultramicroscopic ${\mathrm{V}}_2$ ${\mathrm{O}}_5$ crystals and of $\alpha -\mathrm{FeOOH}$ crystals are in agreement with the deductions. Provided their size is ultramicroscopic, $\alpha -\mathrm{FeOOH}$ crystals are able to orient themselves either parallel or perpendicular to the lines of force, depending on their size, axial ratio, and magnetic anisotropy, and depending on temperature. The resulting magneto-optical complications in suspensions of the crystals, as observed, are briefly reviewed. A competition between morphic and intrinsic magnetic anisotropy is made responsible for the phenomenon of switching orientation. The colloidal phenomenon of "thixotropy" and the optical effect of "morphic conservative" dichroism are used for detecting the switching of orientation from perpendicular to the lines of forces at low temperature, to parallel above 25°C.