Study of magneto-optical properties in magnetic liquids from ac measurements

摘要

We present an AC original polarimetric method to study "Faraday rotation" (FR) of ferrofluids. The studied magnetic liquids are colloidal suspensions of Fe_(2)CoO_(4) ferrite particles dispersed in various solvents. This method is based on an experimental device involving a Faraday modulator positioned between a polarizer and an analyzer with an angular uncrossing of a few degrees. From the detected light intensity we extract the various frequency components V_(O),V_(f) and V_(2f) where f is the field frequency (h(t)=H_(O).cos2πft). We consider the origin of the Faraday effect, with AC magnetic fields, in magnetic fluids, as the result of the orientation of the particles towards the field, producing a macroscopic magnetization which can be described by Langevin theory. The theoretical harmonic components V_(O),V_(f) and V_(2f) are calculated. We show that they depend on the physical caracteristics of the magnetic liquids (concentration Φ, saturation magnetization M_(d), mean diameter d of the magnetic particles). They also depend on the amplitude H_(O) and the frequency f of the AC magnetic field h(t). We also relate the theoretical expression of V_(f) with the time constant of the Brownian relaxation in the ferrofluids. We compare the theoretical and experimental variations of V_(O),V_(f) and V_(2f) with H_(o) and f for various ferrofluids. Finally we mention some applications concerning particles mean diameter and susceptibility measurement and magnetization curves determination.