Large magneto-optical birefringence of colloidal suspensions of α-FeOOH goethite nanocrystallites

A large magnetic-field induced birefringence (reduced value of approximately 10−2), was observed at 40 kA/m field strength for colloid suspensions of spherical goethite nanocrystallite. We ascribe this behavior of these suspensions to long-range orientation of optically equivalent ellipsoids, rather than long-ranged spatial ordering of building blocks (as liquid crystals) or interparticle interaction (as ferrofluids) from the magnetic field. Comparing experimental and theoretical results based on bulk properties, goethite nanocrystallites have refractive index approximately 103 times larger than that of the bulk; accordingly, dielectric constant is approximately 106 times larger in the optical-frequency regime.