Magneto-Optical Properties of Ferromagnetic Suspensions

Observations on the change of light transmission through magnetite suspensions under the influence of alternating magnetic fields show that the orientation of the particles is not exclusively of dipolar origin, as was assumed by Heaps and Elmore. Fields of frequencies between 500 and 15,000 cycles per sec. produce a steady change of absorption. In fields of lower frequencies ω the change consists of a steady and a fluctuating part. The wave form of the latter is studied with a photo-cell amplifier and an oscillograph. In general, it is given by an equation of the form: Acosωt+Bcos2ωt. By varying the field intensity, the frequency or the viscosity, either A or B can be made to vanish. This behavior is shown to be characteristic of polar anisotropic particles, for which the torque on the induced dipole is of the same order of magnitude as that due to the permanent dipole. Included is a survey of the work done on the magneto- and electro-optical properties of numerous other colloids and suspensions.