Magneto-optical extinction trend inversion in ferrofluids

TL;DR

This study investigates how pulsed magnetic fields affect the optical transmission of ferrofluids, revealing a non-monotonic extinction trend inversion caused by aggregate growth and scattering transitions, supported by experiments and molecular dynamics simulations.

Contribution

It demonstrates that homogeneous magnetic fields can induce columnar aggregate formation and extinction trend inversion in ferrofluids, clarifying the role of field uniformity and aggregate dynamics.

Findings

Optical transmission decreases and recovers during magnetic pulses.

Extinction trend inversion occurs due to scattering transition and aggregate growth.

Homogeneous magnetic fields suffice for aggregate formation and trend inversion.

Abstract

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 (Li et al., J. Phys. D: Appl. Phys. 37 (2004) 3357, and Sci. Technol. Adv. Mate. 8 (2007) 448), implying an inhomogeneous field as a required prerequisite for the magneto-optical extinction trend inversion phenomenon.