Magneto-optical extinction trend inversion in ferrofluids

• Ferrofluid columnar aggregates have been observed in a homogeneous magnetic field.
• Magneto-optical extinction trend inversion is related to the Mie light scattering.
• Crucial role of columnar aggregates growth and lateral coalescence has been revealed.
• Residual extinction trend inversion was observed after the field switch off.

Effects of pulse magnetic field on the optical transmission properties of thin ferrofluid (FF) layers were experimentally investigated. It was observed that, under an influence of an external uniform magnetic field, pulses applied to the samples surfaces in normal direction decrease the optical transmission with further returning it to its original state, even before the end of the field pulse. The dependencies of the observed effects on the magnetic pulse magnitude and the samples thickness were investigated. The experimental results are explained using FF columnar aggregates growth and lateral coalescence under influence of a magnetic field, leading to a light scattering type Rayleigh-to-Mie transition. Further evolution of this process comes to a geometrical optics scale and respective macroscopic observable opaque FF columnar aggregates emergence. These changes of optical transmission are non-monotonic during the magnetic field pulse duration with minimal value in the case of Mie scattering, which is known as a magneto-optical extinction trend inversion. The residual inversion was detected after the external magnetic field pulse falling edge. Using molecular dynamics simulation, we showed that a homogeneous external magnetic field is enough for the formation of columnar aggregates and their fusion. The results clarify the known Li theory ([20] and [54]), implying an inhomogeneous field as a required prerequisite for the magneto-optical extinction trend inversion phenomenon.